Series-parallel multi-machine centrifugal heat pump control system

Abstract

The application belongs to the technical field of automatic control, and particularly relates to a hybrid multi-machine centrifugal heat pump control system, which comprises a hybrid unit group, the hybrid unit group comprises multiple centrifugal heat pump units connected through a chilled water side or a cooling water side in a series connection, parallel connection or series-parallel connection combination mode; and further comprises a common DC bus module, a signal acquisition module, a hot gas bypass module and an edge controller; according to the total cold and heat load demand of the hybrid unit group and the bus voltage value, the speed change slope of the speed-up unit and the speed-down unit is dynamically coordinated to enable the regenerated electric energy to be absorbed through the common DC bus, and when the surge margin value of any one centrifugal heat pump unit is lower than the first margin threshold value, the corresponding high-speed electromagnetic pilot valve is driven to open the corresponding hot gas bypass main valve to cooperatively perform load transfer with the remaining units. The application can reduce the instantaneous power-on and power impact on the AC power grid during the process of adding and reducing loads of multiple machines, and can inhibit the surge precursor and ensure the stability of the user-side water supply temperature in a millisecond-level response.

Description

Technical Field

[0001] This invention belongs to the field of automatic control technology, and relates to industrial control software, specifically to a hybrid multi-unit centrifugal heat pump control system, which is particularly suitable for the control of heat pumps in energy-saving air conditioning systems. Background Technology

[0002] Centrifugal heat pump units, with their advantages of large single-unit capacity and high volumetric efficiency, have been widely used in centralized cooling and heating systems for large public buildings, data centers, and industrial parks. In engineering practice, the rated capacity of a single centrifugal heat pump unit is often insufficient to cover the peak cooling and heating loads of the entire building or process. Therefore, multi-unit systems combining parallel or series connections of multiple centrifugal heat pump units have become the mainstream configuration. Using a hybrid topology combining parallel and series connections on the water side can simultaneously address flow sharing and temperature cascading, resulting in significant operational economics when there is a large temperature difference between the supply and return chilled water.

[0003] For group control of multi-unit centrifugal heat pumps, the conventional approach is to use a two-tier architecture consisting of a host group controller and individual unit distributed controllers. The group controller estimates the total cooling and heating load demand based on the temperature difference between the supply and return water on the user side and the water flow rate, and distributes the load to each unit according to proportional allocation, load sharing, or equal incremental rate methods. The individual unit controllers use inlet guide vanes and variable frequency speed control to maintain the outlet water temperature. Regarding surge protection for centrifugal compressors, engineering practices generally rely on a fixed surge boundary curve obtained from bench tests at the factory, combined with measured pressure ratio and flow rate to determine a hard threshold. If necessary, the operating point is moved away from the surge zone by closing the inlet guide vanes, reducing the speed, or opening the hot gas bypass valve.

[0004] However, the above solutions still have significant shortcomings in multi-unit mixed-unit scenarios. First, the variable frequency drives of multiple units draw power independently from the AC grid, and the regenerative energy during load increase and decrease is directly consumed in the braking resistor or fed back to the grid, resulting in wasted energy interaction among multiple units. Second, the fixed surge boundary only reflects the critical pressure ratio under thermodynamic conditions and does not take into account the impact of electrical coupling factors such as bus voltage fluctuations and current distribution changes generated by multiple variable frequency drives in a shared electrical system on the surge boundary, causing the operating margin near the boundary to be overestimated or underestimated. Third, the execution delay of the inlet guide vanes and simple speed regulation is usually hundreds of milliseconds, making it difficult to cope with sudden surge precursors. Furthermore, the cooling capacity gap after a single unit surge is triggered lacks a cross-unit collaborative compensation mechanism, affecting the stability of the outlet water temperature on the user side. Summary of the Invention

[0005] The main objective of this invention is to provide a hybrid multi-unit centrifugal heat pump control system. This invention can reduce the instantaneous power consumption and power surge to the AC power grid during the loading and unloading of multiple units, track surge boundary drift caused by electrical coupling to avoid margin misjudgment under fixed boundaries, and suppress surge precursors with millisecond-level response while ensuring the stability of the user-side water supply temperature.

[0006] To address the aforementioned technical problems, this invention provides a hybrid multi-unit centrifugal heat pump control system, comprising a hybrid unit group, wherein the hybrid unit group includes multiple centrifugal heat pump units connected in series, parallel, or a series-parallel combination via the chilled water side or the cooling water side; and further comprising: The common DC bus module includes a positive DC copper busbar and a negative DC copper busbar that connect the DC sides of the frequency converters of each centrifugal heat pump unit to form a common DC busbar, as well as an active front-end rectifier module connected to the input end of the common DC busbar. The signal acquisition module is used to acquire the bus voltage value of the common DC bus, the branch current value of each frequency converter driver, and the thermodynamic operating parameters of each centrifugal heat pump unit. A hot gas bypass module is configured in each centrifugal heat pump unit and includes a hot gas bypass pipeline, a hot gas bypass main valve installed on the hot gas bypass pipeline, and a high-speed electromagnetic pilot valve installed at the pilot port of the hot gas bypass pipeline. The edge controller dynamically coordinates the speed change slope of the deceleration and acceleration units based on the total cooling and heating load demand and bus voltage value of the hybrid unit group, so that the regenerated power is absorbed through the common DC bus. It uses a reference thermodynamic model and a recursive least squares method with a forgetting factor to identify the equivalent surge boundary offset caused by the electrical coupling of the common DC bus and outputs the surge margin value of each centrifugal heat pump unit. When the surge margin value of any centrifugal heat pump unit is lower than the first margin threshold, it drives the corresponding high-speed electromagnetic pilot valve to open the corresponding hot gas bypass main valve to coordinate with the other units to perform load transfer.

[0007] Furthermore, the signal acquisition module includes: a bus voltage sensor installed between the positive and negative DC copper busbars of the common DC bus; branch current sensors installed at the access nodes of each inverter driver on the positive DC copper busbar; and exhaust pressure sensors, return gas pressure sensors, evaporation temperature sensors, condensation temperature sensors, and inlet guide vane opening sensors installed on each centrifugal heat pump unit to collect thermal operating parameters. The thermal operating parameters include the exhaust pressure value output by the exhaust pressure sensor, the return gas pressure value output by the return gas pressure sensor, the evaporation temperature value output by the evaporation temperature sensor, the condensation temperature value output by the condensation temperature sensor, the inlet guide vane opening value output by the inlet guide vane opening sensor, and the real-time speed value and inverter output power value reported by the corresponding inverter driver. The edge controller is connected to the signal acquisition module and each inverter driver via an industrial Ethernet bus.

[0008] Furthermore, the common DC bus module also includes branch contactors and branch fuse protection units connected in series between each frequency converter and the common DC bus; the active front-end rectifier module is used to convert AC grid power into DC power and output it to the common DC bus, and the active front-end rectifier module has the four-quadrant operation capability that allows regenerative power from the DC side to be fed back to the AC grid side.

[0009] Furthermore, when performing dynamic coordination of speed change slope, the edge controller is further used to: determine the centrifugal heat pump units that need to reduce speed and load and the centrifugal heat pump units that need to increase speed and load based on the total cooling and heating load demand; when the bus voltage value is higher than the first bus voltage threshold, increase the loading slope of the speed-up unit or decrease the load reduction slope of the speed-down unit so that the speed-up unit absorbs the regenerative power released by the speed-down unit from the common DC bus; when the bus voltage value is lower than the second bus voltage threshold, under the premise of meeting the total cooling and heating load demand, the minimum stable speed of each unit, and the surge margin constraint, decrease the loading slope of the speed-up unit or increase the load reduction slope of the speed-down unit with load reduction margin; when the second bus voltage threshold is lower than the first bus voltage threshold, the feedback start threshold of the active front-end rectifier module is higher than the first bus voltage threshold and lower than the bus overvoltage protection threshold; when the bus voltage value is higher than the feedback start threshold, excess regenerative power is fed back to the AC grid side through the active front-end rectifier module.

[0010] Furthermore, the edge controller stores a reference thermodynamic model for each centrifugal heat pump unit. The reference thermodynamic model includes a reference operating pressure ratio sub-model and a reference surge boundary sub-model. The reference operating pressure ratio sub-model takes the evaporation temperature, condensation temperature, real-time speed, inlet guide vane opening, and inverter output power as inputs, and outputs the reference operating pressure ratio value of the corresponding centrifugal heat pump unit under the condition of not considering the electrical coupling effect of the common DC bus. The reference surge boundary sub-model outputs the reference critical pressure ratio value of the corresponding centrifugal heat pump unit under the current thermodynamic conditions with the same input as the reference operating pressure ratio sub-model. The edge controller obtains the pressure ratio observation value by dividing the exhaust pressure value by the return gas pressure value at fixed control intervals.

[0011] Furthermore, the edge controller constructs an electrical coupling correction vector for identifying the equivalent surge boundary offset. The electrical coupling correction vector includes the following components: the deviation between the bus voltage value and the rated bus voltage in the current fixed control cycle, the rate of change of the bus voltage value relative to the previous fixed control cycle, the branch current value in the current fixed control cycle, the rate of change of the branch current value relative to the previous fixed control cycle, and the speed tracking error between the speed command value and the real-time speed value of the strain frequency drive.

[0012] Furthermore, the edge controller maintains the identification parameter vector and covariance matrix for each centrifugal heat pump unit. When the hot gas bypass main valve of any centrifugal heat pump unit is closed and the sensor data meets the validity criteria, the edge controller performs one recursive update of its identification parameter vector and covariance matrix in each fixed control cycle, using the electrical coupling correction vector as the regression input, the deviation between the pressure ratio observation value and the reference operating pressure ratio value as the observation output, and the recursive least squares method with forgetting factor. When the hot gas bypass main valve of any centrifugal heat pump unit is open or the sensor data does not meet the validity criteria, the edge controller suspends the recursive update of its identification parameter vector and covariance matrix.

[0013] Furthermore, the edge controller also includes a boundary register. The edge controller multiplies the updated identification parameter vector with the current electrical coupling correction vector to obtain the electrical coupling safety correction amount. After limiting the electrical coupling safety correction amount, it is superimposed on the reference critical pressure ratio value to obtain the equivalent safety critical pressure ratio value of the corresponding centrifugal heat pump unit under the current operating conditions and writes it into the boundary register. The equivalent safety critical pressure ratio value is then subtracted from the pressure ratio observation value to obtain the surge margin value of the corresponding centrifugal heat pump unit.

[0014] Furthermore, when coordinating with other units to perform load transfer, the edge controller is further used to: open the corresponding hot gas bypass main valve through the triggered high-speed electromagnetic pilot valve, allowing a portion of the high-pressure gaseous refrigerant on the compressor exhaust side of the corresponding centrifugal heat pump unit to flow back to the compressor suction side via the opened hot gas bypass main valve, thereby increasing the effective refrigerant flow rate and suction pressure on the compressor suction side; and calculate the reduced effective cooling and heating output of the corresponding centrifugal heat pump unit based on the evaporator water side flow rate, chilled water specific heat capacity, chilled water density, and the change in evaporator inlet and outlet water temperature difference within the preset sampling window before and after the hot gas bypass is triggered, and then convert the effective cooling output into a variable value. The heat output is used as the load to be compensated. For each of the remaining centrifugal heat pump units, the first allowable increment is calculated based on its surge margin value, the second allowable increment is calculated based on the difference between its current real-time speed and the maximum allowable speed, and the third allowable increment is calculated based on its inverter current margin. The minimum value among the first, second, and third allowable increments is taken as the increase in heat and cold output. The units to be compensated are selected in descending order of surge margin value until the sum of the increase in heat and cold output of the selected units covers the load to be compensated. A compensation speed-up command is then sent to the inverter drive of the selected units.

[0015] Furthermore, the edge controller continuously monitors the surge margin value of the centrifugal heat pump unit that triggered the high-speed electromagnetic pilot valve. When the number of control cycles in which the surge margin value is continuously maintained above the second margin threshold reaches the preset recovery cycle number, the edge controller sends a shutdown signal to the triggered high-speed electromagnetic pilot valve to close the corresponding hot gas bypass main valve, gradually withdraws the compensation speed-up command according to the preset unloading slope, and re-executes the dynamic coordination of the speed change slope to allow the hybrid unit group to transition to a new load distribution state. The second margin threshold is greater than the first margin threshold, and a hysteresis interval is formed between the second margin threshold and the first margin threshold.

[0016] The hybrid multi-unit centrifugal heat pump control system of the present invention has the following beneficial effects: This invention proposes an overall control scheme for the control system of hybrid multi-unit centrifugal heat pumps, which combines common DC bus energy mutual assistance, online identification of electrical coupling surge boundary and high-speed anti-surge of hot gas bypass, and can achieve beneficial effects of mutual support in many aspects.

[0017] First, by adopting a common DC bus topology, the regenerative power generated by each frequency converter during the load increase and decrease process is preferentially absorbed by the speed-up unit inside the bus. Only when the bus voltage exceeds the feedback start threshold is it fed back to the AC grid through the active front-end rectifier module. This significantly reduces the instantaneous power consumption and power impact on the AC grid during the multi-machine coordinated load increase and decrease process, and improves the energy utilization rate among multiple machines.

[0018] Secondly, based on the recursive least squares method with forgetting factor, the reference critical voltage ratio is corrected online with the electrical coupling correction vector as input. This can dynamically track the surge boundary offset caused by the common DC bus electrical coupling, so that the edge controller can obtain a reliable equivalent safety critical voltage ratio in each fixed control cycle, avoiding the problem of overestimation or underestimation of the margin in the multi-machine electrical coupling scenario of the fixed surge boundary.

[0019] Third, the series connection structure of the high-speed electromagnetic pilot valve and the pneumatic diaphragm hot gas bypass main valve enables millisecond-level surge precursor response. At the moment of triggering, the surge margin, speed margin and current margin are combined to perform cross-unit cooling capacity compensation for other units, ensuring the stability of the user-side water supply temperature.

[0020] The three types of effects work together to simultaneously improve the overall system's energy efficiency, safety margin, and load adaptability. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the water-side topology of a hybrid unit group provided in an embodiment of the present invention; Figure 2 This is a schematic diagram of the common DC bus electrical topology of a multi-machine frequency converter driver provided in an embodiment of the present invention; Figure 3This is a schematic diagram of the branch current time-domain response curve provided in an embodiment of the present invention; Figure 4 A schematic diagram of the surge margin time-domain response curve of any one centrifugal heat pump unit provided in an embodiment of the present invention; Figure 5 This is a schematic diagram of the time-domain response curve of the common DC bus voltage provided in an embodiment of the present invention. Detailed Implementation

[0022] The method of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0023] A hybrid multi-unit centrifugal heat pump control system includes a hybrid unit group, comprising multiple centrifugal heat pump units connected in series, parallel, or a series-parallel combination via the chilled water side or the cooling water side; and further includes: The common DC bus module includes a positive DC copper busbar and a negative DC copper busbar that connect the DC sides of the frequency converters of each centrifugal heat pump unit to form a common DC busbar, as well as an active front-end rectifier module connected to the input end of the common DC busbar. The signal acquisition module is used to acquire the bus voltage value of the common DC bus, the branch current value of each frequency converter driver, and the thermodynamic operating parameters of each centrifugal heat pump unit. A hot gas bypass module is configured in each centrifugal heat pump unit and includes a hot gas bypass pipeline, a hot gas bypass main valve installed on the hot gas bypass pipeline, and a high-speed electromagnetic pilot valve installed at the pilot port of the hot gas bypass pipeline. The edge controller dynamically coordinates the speed change slope of the deceleration and acceleration units based on the total cooling and heating load demand and bus voltage value of the hybrid unit group, so that the regenerated power is absorbed through the common DC bus. It uses a reference thermodynamic model and a recursive least squares method with a forgetting factor to identify the equivalent surge boundary offset caused by the electrical coupling of the common DC bus and outputs the surge margin value of each centrifugal heat pump unit. When the surge margin value of any centrifugal heat pump unit is lower than the first margin threshold, it drives the corresponding high-speed electromagnetic pilot valve to open the corresponding hot gas bypass main valve to coordinate with the other units to perform load transfer.

[0024] In large-scale centralized cooling and heating systems, the capacity of a single centrifugal heat pump unit is often insufficient to cover the full load demand of buildings or industrial users. Therefore, a parallel configuration of multiple units is typically adopted in engineering practice. However, a simple parallel multi-unit system can easily force all units into inefficient or even surge-prone conditions when the user-side load is lower than the minimum stable output of a single unit. Furthermore, it cannot achieve temperature cascading when there are large temperature differences on the water side. This embodiment uses a hybrid unit group to connect multiple centrifugal heat pump units. This allows each unit to share the flow load with other units in the same stage on the water side and to form a series temperature relay with units above and below it, thus maintaining a relatively stable single-unit operating point even when the user-side load fluctuates significantly.

[0025] There are several implementation methods for the water-side topology of hybrid unit groups. In a typical implementation, two parallel branches are used, each containing two water-side centrifugal heat pump units connected in series. The four units are connected to the supply and return water headers on the user side via an external main pipe, forming a hybrid topology of "two parallel branches and two units in series on each branch". The rated cooling capacity of each centrifugal heat pump unit is 1758 kW (approximately 500 refrigeration tons), and the rated input power is 305 kW. The total rated cooling capacity of the four units reaches 7032 kW, which can cover the peak cooling load demand of a medium-sized data center or a large commercial complex. At the same time, because the branches are connected in parallel, when one branch is shut down for maintenance, the other branch can still output about 50% of the rated capacity, avoiding a complete cooling outage on the user side. In another implementation, a topology of 3 parallel lines with 2 units in series per line for a total of 6 units can be adopted, or a topology of 2 parallel lines with 3 units in series per line for a total of 6 units can be adopted. The former is more inclined to flow sharing, while the latter is more inclined to temperature cascading. The choice can be made based on the design temperature difference between the supply and return water on the user side. Empirically, the former is preferred when the design temperature difference between the supply and return water is no greater than 6 degrees Celsius, the latter is preferred when the design temperature difference exceeds 8 degrees Celsius, and both topologies are acceptable when the temperature difference is between the two.

[0026] refer to Figure 1 The entire hybrid unit group consists of four centrifugal heat pump units, labeled A1, A2, B1, and B2. Units A1 and A2 are connected in series on the water side to form one parallel branch (labeled Parallel Branch 1, located on the left side of the attached diagram); Units B1 and B2 are connected in series on the water side to form another parallel branch (labeled Parallel Branch 2, located on the right side of the attached diagram). The water-side inlet and outlet ends of the two parallel branches are connected to each other via the supply water header located below the attached diagram and the return water header located above the attached diagram, forming a hybrid topology of two parallel branches, each with two units connected in series. The supply water header and return water header are led to the user load located in the very center of the attached diagram via two longitudinal branch pipes, forming a complete hydraulic closed loop for water flow.

[0027] The direction of water flow within each parallel branch is indicated by arrows on the longitudinal pipe sections. Water flows from the supply main pipe through the longitudinal branch pipes, passing sequentially through the lower and upper units before returning to the return main pipe. The intermediate temperature between the front and rear units in a series circuit is denoted as... The water supply side temperature is recorded as The return water temperature is recorded as .

[0028] Each centrifugal heat pump unit is represented in the attached diagram by a rectangular frame with three parallel short lines inside, which indicate the tube bundle array inside the shell-and-tube heat exchanger. Each unit has an independent bypass line on its right side, equipped with an electrically operated bypass valve. When any unit is shut down for maintenance, the corresponding bypass valve opens, allowing water to bypass the shut-down unit and continue flowing in the series loop, preventing sudden hydraulic changes.

[0029] Figure 1 The topology shown causes the water flow to undergo two series heat exchanges within each parallel branch. The upstream unit bears a relatively high evaporation temperature and a corresponding low pressure ratio, while the downstream unit bears a relatively low evaporation temperature and a corresponding high pressure ratio. This results in overall pressure sharing, bringing the operating point of each unit closer to its region of high isentropic efficiency. The two parallel branches also share the flow load and allow for large-scale capacity regulation by starting and stopping one of the parallel branches when the user-side load fluctuates significantly. Optionally, the number of parallel branches can be selected from 2, 3, or more, and the number of series units on each parallel branch can be selected from 2 to 3.

[0030] The two units connected in series on the water side are directly coupled via an insulated water pipe. The evaporator outlet of the upstream unit becomes the evaporator inlet of the downstream unit (in cooling mode), while the opposite is true on the cooling water side. To prevent water flow interruption when the series circuit stops, each unit is equipped with an electric bypass valve at its water-side inlet and outlet. When a single unit stops, the bypass valve opens, allowing water to bypass the stopped unit and continue flowing in the series circuit.

[0031] In addition to the water-side topology, centrifugal heat pump units can also form a slight coupling on the refrigerant side, such as sharing a set of economizers or a set of condensing pressure regulating valves. To facilitate fault isolation and maintenance, it is preferable that each centrifugal heat pump unit operates its own independent closed refrigerant circuit.

[0032] The inverter drivers for each centrifugal heat pump unit utilize a common DC bus module on the electrical side to form an energy-sharing structure. The original diode-controlled uncontrolled rectifier front-end of each inverter driver is eliminated. The positive DC terminal of the inverter unit is directly led out via a short copper busbar and uniformly connected to a single positive DC copper busbar running across the electrical control cabinet. The negative DC terminal of the inverter unit is uniformly connected to a single negative DC copper busbar arranged parallel to the positive DC busbar. Physically, the positive and negative DC busbars are tin-plated pure copper busbars with a cross-section of 80 mm × 10 mm, maintaining an electrical clearance of no less than 50 mm between the busbars and equipped with epoxy resin support insulators. Considering four units and a peak DC current of approximately 600 amps per unit, the long-term allowable current of the busbars is no less than 1200 amps.

[0033] In one specific implementation, the rated voltage of the positive and negative DC copper busbars is selected as 750V DC, corresponding to the steady-state value of the rectified AC side line voltage of 380V. In a 6kV medium-voltage drive scenario, the rated voltage of the positive and negative DC copper busbars can be selected as 1100V DC, corresponding to the AC side line voltage of 690V. The specific values ​​need to match the DC bus operating range of the selected centrifugal compressor's matching frequency converter. During engineering implementation, it should be ensured that the rated voltage is not lower than the rated DC bus voltage indicated on the nameplate of the frequency converter and not higher than 90% of its maximum allowable DC bus voltage.

[0034] An active front-end rectifier module is connected to the power input terminals of both the positive and negative DC copper busbars. The active front-end rectifier module is constructed using six insulated-gate bipolar transistors in a three-phase full-bridge topology, and connected in series with an LCL filter network consisting of three filter reactors and six filter capacitors before being connected to the AC grid. The active front-end rectifier module converts AC grid power into DC power and outputs it to the common DC bus. It also has four-quadrant operation capability, allowing excess regenerative energy on the common DC bus to be fed back to the AC grid. In an embodiment with four generating units and a common DC bus rated voltage of 750V, the rated power of the active front-end rectifier module is set to 800 kW, with approximately 30% instantaneous overload capacity to withstand the power surge during simultaneous acceleration of multiple units. The active front-end rectifier module stabilizes the common DC bus voltage at around 750V DC through dual closed-loop control (outer voltage loop and inner current loop), with a steady-state deviation not exceeding ±1.5%. When the common DC bus voltage rises to the feedback start-up threshold, it switches to feedback mode to invert excess power and inject it back into the AC grid at unity power factor. In this embodiment, the feedback start-up threshold is set to 800V DC, between the steady-state voltage of 750V and the bus overvoltage protection threshold of 850V, leaving a 50V electrical margin for the speed-up unit to preferentially absorb regenerated power. If the load at the project site fluctuates drastically, the feedback start-up threshold can be increased to 810V or 820V, but a safety margin of at least 30V should be maintained between it and the bus overvoltage protection threshold.

[0035] Each inverter drive is also equipped with a branch contactor, a branch fuse protection unit, and a branch pre-charge circuit between itself and the common DC bus. The branch contactor is a double-pole DC contactor with a DC switching capacity of not less than 1500 amps, used to disconnect the inverter drive from the common DC bus during single-unit maintenance or fault isolation. The branch fuse protection unit uses a fast-acting fuse with a breaking time of not more than 5 milliseconds and a fusing current of 1000 amps DC, serving as the last line of defense against branch short circuits. The branch pre-charge circuit consists of a 30-ohm power resistor and a small-capacity DC contactor connected in series, used to limit the charging current to the DC support capacitor inside the inverter drive at the moment of closing, preventing bus voltage drops and disturbances to other units. The pre-charge time is approximately 200 milliseconds. After the pre-charge is completed, the branch contactor opens and the main branch contactor closes.

[0036] refer to Figure 2 The leftmost part of the attached diagram is the symbol for an AC power grid, consisting of three vertically arranged circles representing the three phases. Three horizontal conductors extend from the right side of the three circles and converge to connect to an LCL filter. To the right of the LCL filter is a rectangle labeled as an active front-end rectifier module. Inside the rectangle are six small rectangular blocks representing six insulated-gate bipolar transistors.

[0037] The middle and right parts of the attached diagram represent the common DC bus region. The common DC bus consists of a positive DC copper bus located at the top of the diagram and a negative DC copper bus located at the bottom, with the two copper buses running across the entire diagram. The right side of the active front-end rectifier module is connected to both the positive and negative DC copper buses via two short leads, thus forming the power input terminal of the common DC bus.

[0038] Located immediately to the right of the active front-end rectifier module, a bus voltage sensor is installed between the positive and negative DC busbars; further to the right, a set of bus support capacitors is installed between the positive and negative DC busbars. The capacitor is represented by two short, thick horizontal lines indicating the plates, and is labeled... .

[0039] The attached diagram shows four variable frequency drive (VFD) branches side-by-side on the right, labeled VFD 1 to VFD 4 from left to right. Each branch connects to a fuse, a DC contactor, and a Hall current sensor in series from the top of the positive DC busbar downwards, with the rectangular frame of the VFD below. Three-phase output wires are then led downwards from the bottom of each VFD to a motor symbol, labeled Compressor 1 to Compressor 4 below each symbol. The negative terminal of each branch connects from the bottom of the VFD downwards to the negative DC busbar.

[0040] The attached diagram shows the direction of regenerative power flow at the top with a horizontal line bearing an arrow, pointing from the branch containing the self-decelerating unit to the branch containing the speed-up unit. Optionally, a branch pre-charging circuit, insulation monitoring device, or busbar energy storage element can be added near each branch connection point between the positive and negative DC busbars to improve overall reliability.

[0041] To collect the operating status of the common DC bus, a Hall effect bus voltage sensor is installed between the positive and negative DC copper busbars, with a range of 0 to 1000 volts DC and an accuracy better than ±0.2%. A Hall effect branch current sensor is connected in series at the access node of each frequency converter driver on the positive DC busbar, with a range of bidirectional DC 0 to 1500 amperes (bidirectional means able to distinguish between inflow and outflow directions) and an accuracy better than ±0.5%. The output signals of the bus voltage sensor and each branch current sensor are connected to the remote input / output slave station in the electrical control cabinet via shielded twisted-pair cables, and then aggregated to the edge controller via an industrial Ethernet bus. An insulation monitoring device is also installed at the midpoint of the common DC bus to continuously monitor the impedance to ground of the positive and negative DC copper busbars. An insulation warning is issued when the impedance value is below 100 kΩ, and the grid-side circuit breaker of the active front-end rectifier module is immediately tripped when it is below 30 kΩ.

[0042] Each centrifugal heat pump unit's thermodynamic operating parameters include seven variables: The exhaust pressure is collected by a piezoelectric pressure sensor at the compressor exhaust pipe flange, with a range of 0 to 4.0 MPa (absolute pressure), an accuracy better than ±0.1% of full scale, and a sampling frequency of no less than 1 kHz to accommodate both steady-state measurement and pressure pulsation capture; the return gas pressure is collected by a pressure sensor of the same model at the compressor return gas pipe flange, with a range of 0 to 1.0 MPa (absolute pressure), and the remaining parameters are consistent with the exhaust pressure sensor; the evaporation temperature and condensation temperature are measured by a 4-wire platinum resistance thermometer (PT100) in the evaporator shell-side saturated refrigerant region and the condenser shell-side saturated refrigerant region, respectively. The PT100 has an allowable deviation level of A, corresponding to a deviation of no more than ±0.15 degrees Celsius at 0 degrees Celsius. The inlet guide vane opening value is measured by a non-contact angular displacement sensor mounted on the compressor inlet guide vane actuator, with a range of 0 to 90 degrees and a resolution better than 0.1 degrees. The real-time speed value and inverter output power value are calculated by the internal sensing module of the inverter driver. The former is obtained through a motor flux linkage observer or encoder (for magnetic levitation compressors, the speed feedback from the magnetic levitation controller is directly read), with a resolution better than 1 rpm. The latter is directly calculated from the DC bus voltage, DC bus current, and modulation ratio inside the inverter driver, and is updated once per control cycle. The inverter driver transmits the real-time speed value and inverter output power value to the edge controller in real time via the industrial Ethernet communication interface on its control board.

[0043] The physical layer of the industrial Ethernet bus uses 100 Mbps Ethernet, and the link layer uses a protocol stack that supports Time-Sensitive Networking (IEEE 802.1AS and IEEE 802.1Qbv subsets). The synchronization jitter between nodes is controlled within 1 microsecond to ensure strict alignment of the multi-machine collaborative control cycle. Considering the inherent dynamic time scale of centrifugal compressors, which ranges from tens to hundreds of milliseconds, a fixed control cycle of 10 milliseconds is taken as a typical value. When there are many units (no less than 8), it can be relaxed to 20 milliseconds. For high-speed magnetic levitation types with strict surge protection requirements, it can be encrypted to 5 milliseconds. Within each fixed control cycle, the edge controller first completes the synchronous sampling of all signals, then sequentially performs hydraulic balance determination, single-machine thermal calculation, common DC bus coordination, and surge margin update, and finally issues speed commands to each inverter driver.

[0044] The edge controller uses an industrial-grade embedded controller with a quad-core ARM Cortex-A series processor as the main control chip, with a main frequency of no less than 1.2 GHz and a memory of no less than 2 Gigabytes. It is equipped with a field-programmable gate array (FPGA) coprocessor board for hard real-time signal processing. Within each fixed control cycle, the edge controller performs a 5th-order finite impulse response (FIR) digital filter on the acquired bus voltage value with a cutoff frequency of 200 Hz to filter out disturbances to the control loop caused by the 4 kHz switching frequency of the active front-end rectifier module on the common DC bus and its harmonic components. The same filter structure is used for the branch current value with a cutoff frequency of 100 Hz. Before calculating the input pressure ratio observation value, the exhaust pressure and return pressure values ​​of each centrifugal heat pump unit are first taken as the arithmetic mean of 5 consecutive sampling points to suppress random disturbances introduced by refrigerant airflow pulsation, corresponding to a data smoothing window of approximately 5 milliseconds. Due to the inherent thermal inertia of the PT100, the evaporation temperature and condensation temperature values ​​are already relatively slow, so only a 1st-order low-pass filter is needed, with a time constant of 100 milliseconds.

[0045] For any centrifugal heat pump unit, let its exhaust pressure be... The return pressure value is Both are expressed in absolute pressure, so the observed pressure ratio is ;in, This represents the pressure ratio observation value for the current fixed control cycle. This represents the exhaust pressure value collected during the current fixed control cycle. This represents the return gas pressure value collected during the current fixed control cycle. The pressure ratio observation value uses absolute pressure instead of gauge pressure because the surge boundary of the centrifugal compressor is defined in the pressure ratio dimension. When the field instrument outputs gauge pressure, the local atmospheric pressure (taken as 0.1013 MPa, corrected according to the local measured atmospheric pressure at higher altitudes) should be added inside the edge controller before being substituted into the calculation.

[0046] The branch current value of the frequency converter driver is uniformly defined by the following formula for the reference direction: positive is the direction of current flow from the frequency converter driver to the common DC bus, and negative is the opposite. Therefore, the branch current value is positive during deceleration regeneration and negative during acceleration absorption. This reference direction definition allows the edge controller to directly calculate the algebraic sum of all branch current values ​​to obtain the net injected power direction on the common DC bus: an algebraic sum greater than 0 indicates excess energy on the bus and a tendency for the bus voltage to rise; an algebraic sum less than 0 indicates insufficient energy on the bus and a tendency for the bus voltage to fall, thus enabling a simpler way to coordinate subsequent load increases and decreases based on the bus voltage window.

[0047] Optionally, the signal acquisition module can also install a temperature sensor (arranged close to the surface of the positive DC busbar) on the common DC busbar to monitor the heating of the copper busbar. The temperature threshold is 70 degrees Celsius. When the temperature exceeds the threshold, the allowable power limit of the active front-end rectifier module is reduced. Optionally, an ultrasonic flow sensor that directly measures the main inlet air flow or refrigerant flow can also be installed next to the inlet guide vane actuator of each unit to improve the accuracy of operating condition identification. However, since the inlet guide vane opening value can indirectly reflect the flow information, it can be omitted for cost-sensitive projects.

[0048] For surge protection of centrifugal compressors, relying solely on the movement of the inlet guide vanes or speed adjustment to address sudden surge precursors has insufficient response time: the servo actuator of the inlet guide vanes typically requires 200 to 500 milliseconds from receiving the command to the guide vanes reaching their position; speed adjustment requires a multi-layer closed-loop response from the frequency converter driver, and the compressor's rotational inertia is large, so relying on deceleration to pull the operating point back to the stable region also requires hundreds of milliseconds; while the time scale for a centrifugal compressor to enter rotational stall and develop into full surge is often only tens to 100 milliseconds. Therefore, this embodiment configures one hot gas bypass channel on each centrifugal heat pump unit as a rapid anti-surge actuator.

[0049] For a single centrifugal heat pump unit, the hot gas bypass channel consists of a separately laid bypass water pipe, a hot gas bypass main valve installed on the bypass water pipe, and a high-speed electromagnetic pilot valve installed at the pilot port of the hot gas bypass main valve. The inlet of the bypass water pipe is connected to the horizontal section of the compressor exhaust pipe (typically, the pressure tap is located 5 to 8 times the pipe diameter downstream of the compressor exhaust flange, avoiding the area near the flange where the exhaust vortex is most intense), and the outlet is connected to the horizontal section of the compressor suction pipe located 6 to 10 times the pipe diameter upstream of the suction flange. This exhaust gas intake and suction gas return connection method allows high-pressure gaseous refrigerant to directly bypass the condenser, throttling element, and evaporator and flow back to the compressor suction side when the bypass is open, establishing a short-circuit channel independent of the main cycle at both ends of the compressor, effectively increasing the refrigerant flow on the suction side. The outlet of the bypass water pipe should avoid being directly opposite the compressor intake port. Instead, it should be obliquely connected to the main intake direction at a 30- to 45-degree angle. Downstream of the oblique connection, there should be a mixing straight pipe section with a diameter of no less than four times the pipe diameter, so that the bypass gas and the main intake gas can be fully mixed before entering the compressor.

[0050] The diameter of the bypass water pipe is determined based on the compressor's rated suction volume flow rate. For the specific implementation method described above, with a rated cooling capacity of 1758 kW per unit, the compressor's rated suction volume flow rate at R134a refrigerant and an evaporation temperature of 5 degrees Celsius is approximately 0.42 cubic meters per second. Based on the engineering experience that "one bypass area can bypass 30% to 40% of the rated flow rate," an inner diameter of 80 mm is suitable for the bypass water pipe, corresponding to a flow cross-section of approximately 5024 square millimeters. When the compressor's capacity level increases or decreases, the inner diameter of the bypass water pipe can be selected between 50 mm and 125 mm, ensuring that the maximum bypass flow rate does not exceed 50% of the rated suction flow rate (an excessively large bypass will cause excessively rapid loss of cooling capacity in the main circulation, leading to a sudden rise in evaporator-side water temperature). The bypass water pipe is fully insulated with a rubber-plastic insulation layer with a thickness of not less than 25 mm to prevent condensation from forming on the pipe wall due to heat loss through a cold bridge after the bypass is opened.

[0051] The hot gas bypass main valve adopts a pneumatically driven diaphragm angle seat valve structure. The valve core of the angle seat valve contacts the valve seat at a 45-degree angle, and the full-open to full-close stroke is only about 1 / 4 of the valve core diameter, resulting in a shorter stroke time. In one specific embodiment, the nominal diameter of the hot gas bypass main valve is matched with the inner diameter of the bypass water pipe at 80 mm, the valve core lift is 20 mm, and the full-stroke switching time of the pneumatic diaphragm actuator at a 0.6 MPa air source pressure does not exceed 80 milliseconds. The hot gas bypass main valve adopts a fail-safe structure with air source-driven opening and spring-reset closing. When the pilot air source loses pressure or the control signal is disconnected, the main valve automatically returns to the closed position by relying on the pre-compression spring, preventing the hot gas bypass channel from remaining open under fault conditions. The body material of the hot gas bypass main valve is ductile iron, the sealing element is made of polytetrafluoroethylene filled with aramid fiber, the upper limit of the operating temperature is not less than 150 degrees Celsius, and the operating differential pressure range is 0 to 2.5 MPa.

[0052] The high-speed electromagnetic pilot valve is responsible for switching the compressed air source, determining when to introduce pilot air pressure into the diaphragm actuator of the hot gas bypass main valve. The high-speed electromagnetic pilot valve adopts a direct-acting structure; the electromagnetic coil directly drives the valve core without relying on medium pressure difference. The valve core stroke is only about 0.5 mm. With the aid of a short-circuit ring to accelerate magnetic circuit establishment, the time from the control signal input to the valve core reaching its position can be controlled between 1.5 milliseconds and 3 milliseconds. In one specific embodiment, the rated operating voltage of the high-speed electromagnetic pilot valve is 24 volts DC, with a pull-in power of 8 watts. The holding power is reduced to 2 watts via a peak-holding circuit to reduce coil temperature rise; the valve body diameter is selected as 1 / 4 inch (nominal 6 mm). Under normal conditions, the high-speed electromagnetic pilot valve is in the closed position, the pilot air source is cut off, and the main valve diaphragm chamber is connected to the atmosphere through the pilot valve's exhaust port. The main valve is maintained in the closed state by a spring. When the electromagnetic coil is energized, the pilot valve switches to the open position, the pilot air source enters the main valve diaphragm chamber through the pilot valve, and the main valve opens. The pilot air source is provided by a centralized compressed air supply system, with the air source pressure stable between 0.5 and 0.7 MPa. The front end of the air source is equipped with a refrigerated dryer and a precision filter to ensure that the air source dew point is not higher than -20 degrees Celsius and the oil mist content is not higher than 0.01 mg / m³.

[0053] Optionally, the hot gas bypass channel can use two sets of high-speed electromagnetic pilot valves arranged in parallel to improve redundancy. The two pilot valves are respectively connected to two independent digital output channels of the edge controller. Optionally, the valve core position feedback signal of the hot gas bypass main valve is connected to the edge controller via a feedback line to facilitate real-time monitoring of the actual opening and closing status of the main valve. If no feedback on the main valve opening is received within a preset time window (typically 100 milliseconds) after the command is issued, the system switches to the backup surge protection strategy. Optionally, a manual ball valve can also be connected in series in the middle of the bypass water pipe as a maintenance isolation valve.

[0054] The goal of dynamic load adjustment coordination is to match the overall output of the hybrid unit group with the total cooling and heating load demand, while prioritizing the absorption of regenerated electricity by the speed-up units within the common DC bus, reducing the amount of electricity fed back to the AC grid by the active front-end rectifier modules. The total cooling and heating load demand is estimated by multiplying the supply and return water temperature difference on the user side by the total water flow rate. In the formula, This represents the total cooling and heating load demand of the current fixed-control-cycle hybrid unit group, in kilowatts; The density of chilled water is expressed as 1000 kg / m³. The specific heat capacity of chilled water is expressed as 4.18 kJ / kg·Kelvin. This indicates the total water flow rate on the user side, measured by an electromagnetic flow meter on the user side water supply main pipe, with units of cubic meters per second. Indicates the return water temperature on the user side; This represents the user-side water supply temperature. The above four field-measured variables are filtered using a first-order low-pass filter (time constant 30 seconds) to avoid load estimation fluctuations caused by short-term disturbances. The same load estimation formula is used in heating operation mode, only the supply and return water temperatures are in the opposite direction.

[0055] Edge controller will The sign of the difference between the actual output load of the mixed-unit group and the load determines the direction of action: when the difference is greater than the set load dead zone (typically 2% of the rated total capacity), it enters the speed-up loading branch; when the difference is less than the negative load dead zone, it enters the speed-down unloading branch; when the difference is within the dead zone, each centrifugal heat pump unit maintains its current speed. In the speed-up loading branch, the edge controller further determines the load-bearing capacity of each unit based on its current surge margin, speed margin from the maximum allowable speed, and current margin from the inverter's rated current. One or more units are selected to undertake the speed-up task in descending order of load-bearing capacity. Similarly, in the speed-down unloading branch, units are selected to reduce speed in descending order of speed margin from the minimum stable speed.

[0056] The dynamic adjustment of the speed change slope is based on the bus voltage value. When the bus voltage value is higher than the first bus voltage threshold (DC 770V), it means that the regenerative power released by the deceleration unit on the common DC bus has not yet been fully absorbed by the speed-up unit. At this time, the edge controller multiplies the loading slope of the speed-up unit by an acceleration factor of 1.3 to 1.5 times and the load reduction slope of the deceleration unit by a deceleration factor of 0.7 to 0.9 times. When the bus voltage value is lower than the second bus voltage threshold (DC 730V), it means that the energy demand of the speed-up unit on the common DC bus exceeds the release capacity of the deceleration unit. At this time, the edge controller, under the premise of confirming that the total cooling and heating load requirements, the minimum stable speed of each unit, and the surge margin constraints are met, multiplies the loading slope of the speed-up unit by a deceleration factor of 0.7 to 0.9 times and the load reduction slope of the deceleration unit by an acceleration factor of 1.1 to 1.3 times. The voltage thresholds for the second bus (730V), the voltage threshold for the first bus (770V), the feedback start-up threshold (800V), and the bus overvoltage protection threshold (850V) are arranged from low to high, so that the excess regenerated energy on the common DC bus is processed in sequence through three priority levels: absorption by the speed-up unit, accelerated absorption, and grid-side feedback, so as to maximize the recycling of regenerated energy within the common DC bus.

[0057] The specific values ​​of the acceleration factor and deceleration factor are obtained by the edge controller through interpolation based on the degree to which the current bus voltage value deviates from the steady-state voltage (DC 750V) according to a pre-stored lookup table. The lookup table covers the range of deviation from -50V to +100V. Alternatively, the acceleration factor and deceleration factor can also be calculated in real time by the proportional-integral controller.

[0058] The equivalent surge boundary identification is based on the fact that, under the electrical coupling of a common DC bus, the surge critical pressure ratio of a centrifugal compressor will deviate from the reference value when there is no electrical coupling. This deviation can be identified by the relationship between measured electrical quantities and the operating pressure ratio. This identification mechanism consists of two sub-models and one recursive algorithm.

[0059] The baseline operating pressure ratio sub-model, without considering the electrical coupling effect of the common DC bus, uses evaporation temperature, condensing temperature, real-time speed, inlet guide vane opening, and inverter output power as inputs to output the operating pressure ratio that the centrifugal compressor should exhibit under the current thermodynamic conditions. During the engineering commissioning phase, the baseline operating pressure ratio sub-model can be used to perform multi-condition bench tests or field scanning tests on each centrifugal heat pump unit, recording the correspondence between the above five variables and the stable operating pressure ratio. After multivariate polynomial fitting, the fitting coefficients are stored in the non-volatile memory of the edge controller. Alternatively, the stable operating pressure ratio can be analytically calculated using a physical model based on the compressor manufacturer's performance curves. The input to the baseline surge boundary sub-model is the same as that of the baseline operating pressure ratio sub-model, but the target quantity fitted is the critical pressure ratio. The critical pressure ratio sample is obtained through bench surge scanning tests. During the scan, the inlet guide vane is slowly closed or the condensing pressure is slowly increased until a sudden increase in pressure pulsation or a sudden drop in exhaust flow is detected. The pressure ratio value at that instant and the corresponding operating condition input are recorded.

[0060] The electrical coupling correction vector is denoted as It consists of 5 components: ; In the formula, Represents the electrical coupling correction vector; This represents the difference between the current bus voltage value during a fixed control cycle and the rated bus voltage (750 volts DC), expressed in volts. It represents the rate of change of the bus voltage value relative to the previous fixed control cycle. It is obtained by subtracting the bus voltage value of the previous cycle from the bus voltage value of the current cycle and then dividing by the length of the fixed control cycle. The unit is volts per second. This indicates the branch current value corresponding to the centrifugal heat pump unit during the current fixed control cycle. It is defined according to the direction of "current flowing from the frequency converter to the common DC bus is positive", and the unit is amperes. This represents the rate of change of the branch current value relative to the previous fixed control cycle, expressed in amperes per second. This represents the difference between the speed command value issued by the variable frequency drive of the centrifugal heat pump unit under the current fixed control cycle and the real-time speed value collected, in revolutions per minute (rpm). This indicates the matrix transpose.

[0061] Bus voltage deviation and bus voltage change rate reflect the transient state of energy interaction among multiple units on the common DC bus; branch current value and branch current change rate reflect the intensity of energy exchange between the unit and the common DC bus; speed tracking error reflects whether the current loop and torque loop margins of the frequency converter driver are momentarily squeezed under the influence of bus voltage disturbance. These five components together characterize the intensity and direction of the slight offset caused by the electrical state on the common DC bus side to the mechanical operating point of the centrifugal compressor.

[0062] Figure 3 This figure shows the time-domain response curve of the branch current in a specific embodiment of the present invention, corresponding to the measured changes in branch current of three typical units in a four-channel frequency converter driver during load increase and decrease. The horizontal axis of the figure represents time, denoted as... The unit is seconds, with the scale divided into 1-second intervals from 0 to 6 seconds; the vertical axis represents the branch current, denoted as... The unit is ampere, and the scale is evenly distributed in 30-ampere increments from -60 amperes to +60 amperes.

[0063] The three curves in the attached diagram correspond to the branch current time series of the three generating units: the solid line represents unit 1, the dashed line represents unit 2, and the dotted line represents unit 3. The directions of the three curves are defined as follows: the current flowing from the frequency converter driver to the common DC bus is positive, and vice versa.

[0064] Unit 1 at approximately At a certain second, an active deceleration occurs. The compressor motor converts part of the rotor's kinetic energy into DC electrical energy via the inverter bridge of the variable frequency drive and injects it into the common DC bus. Therefore, the branch current of unit 1 exhibits a distinct positive pulse at this moment, with a pulse peak value of approximately [value missing]. Ampere, pulse width approximately 0.4 seconds. Unit 2 and Unit 1 simultaneously undergo speed-up loading during deceleration and absorb electrical energy from the common DC bus to drive the compressor motor. At the same moment, the branch current of Unit 2 experiences a negative pulse with a peak value of approximately... An ampere, with a pulse width of approximately 0.45 seconds, together with the positive pulse of Unit 1, constitutes a pair of energy release-absorption mutual aid actions.

[0065] Unit 3 in approximately Another acceleration loading occurs at the second mark, presenting a negative pulse with a peak value of approximately Ampere, pulse width approximately 0.4 seconds. This negative pulse occurs simultaneously with another positive pulse (peak value approximately...) appearing at unit 1. (Ampere) corresponds to.

[0066] Depend on Figure 3 It can be seen that when the unit enters the deceleration stage, the branch current direction becomes positive, and the corresponding energy flows from the unit to the common DC bus; during the same period, the branch current direction of the unit that accelerates becomes negative, and the corresponding energy flows from the common DC bus to the unit. Thus, the internal mutual assistance of regenerated power is realized among multiple units through the common DC bus, reducing the amount of power fed back to the AC grid by the active front-end rectifier module.

[0067] Let the identification parameter vector for the maintenance of each centrifugal heat pump unit be denoted as The covariance matrix is ​​of the same dimension as the electrical coupling correction vector (both are 5-dimensional column vectors), and its initial value is all 0; each unit simultaneously maintains one 5th-order covariance matrix. The initial value is taken as ,in It is a 5th order identity matrix, and 1000 is the initial covariance magnitude (a large value is chosen to make the recursive algorithm more sensitive to new data in the initial stage).

[0068] The recursive least squares method with a forgetting factor performs the recursion less than once within each fixed control period: ; ; In the formula, the subscript Indicates the current fixed control cycle number. Indicates the previous fixed control cycle; Represents the recursive gain vector for the current fixed control cycle; and These represent the covariance matrices of the previous fixed control cycle and the current fixed control cycle, respectively. This represents the electrical coupling correction vector for the current fixed control cycle; This represents the forgetting factor, set to 0.97 (the value ranges from 0.95 to 0.99; the closer to 1, the longer the memory of historical data; the closer to 0.95, the faster the tracking of sudden changes in operating conditions. In engineering, a value of 0.97 achieves a good balance between tracking speed and noise resistance). and These represent the identification parameter vectors for the previous fixed control cycle and the current fixed control cycle, respectively. The observation residual for the current fixed control cycle is given by the difference between the observed pressure ratio and the benchmark operating pressure ratio output by the benchmark operating pressure ratio sub-model under the same thermodynamic conditions. Represents a 5th-order identity matrix; superscript This indicates the matrix transpose.

[0069] The reason for using the difference between the observed pressure ratio and the baseline operating pressure ratio as the observation residual is that the baseline critical pressure ratio describes the boundary condition for the centrifugal compressor to enter surge. During normal operation, the operating point almost never falls on this boundary. If the baseline critical pressure ratio is used as the observation target, the recursive algorithm will fail to obtain samples near the boundary for a long time and will not converge. The baseline operating pressure ratio is the operating point that the centrifugal compressor passes through in each fixed control cycle, providing sufficient residual signal, allowing the identification algorithm to converge stably. Within the small-signal disturbance range, the identification parameter vector describes the influence coefficient of electrical quantity changes on the operating pressure ratio. This embodiment further assumes that the influence of electrical quantities on the operating pressure ratio is consistent with the influence of electrical quantities on the surge boundary in sign and order of magnitude, thus ensuring that the identified... Multiplying this by the electrical coupling correction vector yields the correction amount for the reference critical pressure ratio, known as the electrical coupling safety correction amount. In the formula, This represents the electrical coupling safety correction amount for the current fixed control cycle. To prevent the identification algorithm from outputting excessively large correction amounts during data drift, causing excessive shift in the surge boundary, [this is applied]. Amplification is performed within the range of [-0.1, +0.1] (i.e., within ±10% of the reference critical pressure ratio). If the amplitude exceeds the limit range, the boundary value is used. The electrical coupling safety correction after limiting is added to the reference critical pressure ratio to obtain the equivalent safety critical pressure ratio, which is then written to the boundary register.

[0070] To avoid contamination of the identification parameter vector under abnormal operating conditions, the recursive update is subject to strict validity criteria: the identification parameter vector and covariance matrix are updated only once when the hot gas bypass main valve of the centrifugal heat pump unit is closed, all sensor signals are within their range and there is no timeout alarm, the frequency converter is not in a current limiting fault state, and the rate of change of the pressure ratio observation does not exceed the preset upper limit (typically 5 units / second); if any condition is not met, the update for this cycle is skipped.

[0071] The surge margin value is given by the difference between the equivalent critical pressure ratio and the observed pressure ratio: ; In the formula, This represents the surge margin value for the current fixed control cycle; This represents the equivalent safety critical pressure ratio for the current fixed control cycle; This represents the observed pressure ratio during the current fixed control cycle. A positive value, and the larger the value, the farther the operating point is from the surge region; A value approaching 0 indicates that the operating point is nearing the surge region; A negative value indicates that the operating point has crossed the equivalent surge boundary, and surge protection should be triggered immediately. The first margin threshold is set to 0.15, and the second margin threshold is set to 0.25, forming a hysteresis interval with a width of 0.10. The value of the first margin threshold is based on allowing at least 5% to 8% of the rated pressure ratio as a response margin. For a centrifugal compressor with a rated pressure ratio of approximately 3.0, 0.15 corresponds to a relative margin of approximately 5%.

[0072] Figure 4 This is the surge margin time-domain response curve of any centrifugal heat pump unit in one specific embodiment of the present invention, corresponding to the surge margin. The trajectory of change during a typical load disturbance. The horizontal axis in the attached figure represents time, denoted as... The unit is seconds, with the scale divided into 1-second increments from 0 to 6 seconds; the vertical axis represents surge margin. Dimensionless, with graduations evenly distributed in increments of 0.1 from 0.0 to 0.5.

[0073] The attached diagram shows it as a single thick solid line. Real-time values ​​throughout the entire time period. Within a time interval of seconds, the curve maintains a stable, small-amplitude periodic fluctuation between 0.40 and 0.50, reflecting that the unit has sufficient surge margin under steady-state operation; the fluctuations correspond to the small fluctuations in evaporation temperature, condensation temperature, and real-time speed under steady-state conditions. The small signal changes obtained from the transmission, among which This represents the equivalent critical safety pressure ratio. This represents the observed pressure ratio.

[0074] At approximately Around 0.03 seconds, the curve experiences a significant dip, with the lowest point at approximately 0.03 seconds and a half-width of approximately 0.4 seconds. This corresponds to a sudden load disturbance that shifts the operating point towards lower flow rates and higher pressure ratios. Approaching rapidly During the concave process, the curve crosses two horizontal reference lines on the vertical axis: the upper one corresponds to the second margin threshold of 0.25, and the lower one corresponds to the first margin threshold of 0.15.

[0075] The moment the curve first crosses the first margin threshold of 0.15 from top to bottom corresponds to the moment the edge controller outputs an opening signal to the high-speed electromagnetic pilot valve of the unit. Afterwards, the hot gas bypass channel opens, and some high-pressure gaseous refrigerant flows back to the compressor suction side via the hot gas bypass main valve, causing... decline, As the price recovers, the curve rises and successively crosses the first margin threshold of 0.15 and the second margin threshold of 0.25, entering the margin recovery phase. After a few seconds, the curve stabilized again between 0.40 and 0.50, which is comparable to the steady-state level before the disturbance.

[0076] The 0.10 width between the first and second margin thresholds constitutes the hysteresis interval. Hot gas bypass is triggered when the curve crosses the first margin threshold downwards, and is only allowed to close after crossing the second margin threshold upwards and maintaining this position for a preset number of recovery cycles. Optionally, the specific values ​​of the first and second margin thresholds can be adjusted appropriately during the engineering commissioning phase based on the surge boundary margin requirements of the selected centrifugal compressor, but the second margin threshold should always be greater than the first margin threshold. .

[0077] The execution process of hot gas bypass triggering and load transfer is as follows. The instant the surge margin value of any centrifugal heat pump unit first falls below the first margin threshold, the edge controller immediately outputs a 24V DC pull-in signal to the corresponding high-speed electromagnetic pilot valve of that unit. The pilot valve opens the pilot port within 3 milliseconds, and the pilot gas source enters the diaphragm chamber of the hot gas bypass main valve through the pilot port. The hot gas bypass main valve fully opens within approximately 80 milliseconds; some high-pressure gaseous refrigerant flows back to the compressor suction side through the hot gas bypass main valve, causing the suction pressure to rise, the pressure ratio observation value to decrease, and the compressor operating point to move away from the surge region. Simultaneously, the edge controller estimates the effective cooling and heating output lost by the unit due to the opening of the hot gas bypass as the load to be compensated. In the formula, This indicates the reduction in effective cooling and heating output of the unit due to the opening of the hot gas bypass, in kilowatts. and Same meaning as before; This indicates the water-side flow rate of the evaporator in this unit; This represents the arithmetic mean of the temperature difference between the inlet and outlet water of the evaporator within the sampling window 5 seconds before the hot gas bypass is triggered. This represents the arithmetic mean of the temperature difference between the inlet and outlet water of the evaporator within a 5-second sampling window after the hot gas bypass is triggered.

[0078] The load to be compensated is allocated according to a strategy of decreasing surge margin: the edge controller calculates three allowable increments for each of the remaining centrifugal heat pump units that have not triggered hot gas bypass—the first allowable increment is calculated based on the increase in cooling and heating output of the unit while maintaining a surge margin of at least 0.10; the second allowable increment is calculated based on the increase in cooling and heating output of the unit without exceeding the maximum allowable speed (specifically calculated from the difference between the current real-time speed and the maximum allowable speed using the compressor performance curve); the third allowable increment is calculated based on the increase in cooling and heating output of the unit without exceeding 95% of the inverter's rated current (calculated from the difference between the current inverter output current and the rated current using the compressor performance curve); the minimum value among the three allowable increments is taken as the increase in cooling and heating output of the unit; the increase in cooling and heating output of each unit is accumulated in descending order of surge margin until the cumulative value is not less than the load to be compensated; the selected unit receives the compensation speed-up command from the inverter driver and increases the speed and cooling and heating output accordingly.

[0079] If the surge margin value of the unit triggered by the hot gas bypass remains above the second margin threshold (0.25) for a preset number of recovery cycles (typically 50 fixed control cycles, corresponding to 500 milliseconds), it indicates that the operating point has stabilized and returned to the safe region. The edge controller outputs a shut-off signal to the high-speed electromagnetic pilot valve, which cuts off the pilot gas supply. The hot gas bypass main valve closes under the action of spring reset. Simultaneously, the edge controller gradually withdraws the compensation speed-up command for the compensated unit according to a preset unloading slope (typically 50 rpm·second) to avoid water-side temperature fluctuations caused by load jumps in the compensated unit. After unloading is completed, dynamic load increase / decrease coordination is re-executed to transition the mixed unit group to a new steady-state load distribution. The exhaust temperature sensor, motor winding temperature sensor, and intake superheat sensor of the triggered unit are connected to the high-priority monitoring channel of the edge controller. When any temperature exceeds the protection threshold, the hot gas bypass channel immediately reopens and reduces the speed command of the triggered unit until the temperature returns to the normal range.

[0080] Optionally, the hot gas bypass trigger can also be coordinated with the slow-motion operation of the inlet guide vanes: Simultaneously with the high-speed electromagnetic pilot valve being energized, the edge controller sends an auxiliary command to the inlet guide vane actuator of the triggered unit to close by 5 to 10 degrees, causing the hot gas bypass and the inlet guide vanes to jointly pull the operating point away from the surge region. However, the angle change of the auxiliary command must be strictly limited to prevent further reduction in intake flow. Optionally, for implementations using a magnetically levitated compressor, the surge margin determination can be superimposed with the compressor shaft displacement signal as auxiliary evidence: when the energy of the shaft displacement signal spectrum near the 1x speed frequency suddenly increases by more than 3 times in a short period, the hot gas bypass can be triggered in advance even if the surge margin value has not yet fallen below the first margin threshold.

[0081] Figure 5 This figure shows the time-domain response curve of the common DC bus voltage according to a specific embodiment of the present invention. The horizontal axis of the figure represents time, denoted as... The unit is seconds, with the scale divided into 1-second intervals from 0 to 6 seconds; the vertical axis represents the bus voltage, denoted as... The unit is volt, and the scale is evenly distributed in 20-volt increments from 720 volts to 820 volts.

[0082] The attached diagram shows it as a single thick solid line. The real-time value. During the steady-state phase, It exhibits an amplitude of approximately 750 volts. A small periodic fluctuation of volts with a frequency of about 0.6 Hz.

[0083] The attached diagram spans three reference lines: one dashed line corresponds to the first bus voltage threshold of 770 volts, another dashed line corresponds to the second bus voltage threshold of 730 volts, and a dotted line corresponds to the feedback activation threshold of 800 volts. These three thresholds, arranged from low to high, form three voltage ranges: 730 volts to 770 volts is the normal energy mutual assistance range, 770 volts to 800 volts is the enhanced energy mutual assistance range, and 800 volts to 850 volts is the grid-side feedback range.

[0084] At approximately At the specified time, the attached curve shows a distinct positive pulse with a peak value of approximately 788 volts and a pulse width of approximately 0.4 seconds. This corresponds to a sudden deceleration of one generator unit, injecting regenerative energy into the common DC bus. The pulse peak value exceeds the first bus voltage threshold of 770 volts but is below the feedback activation threshold of 800 volts, indicating that the regenerative energy is sufficient to initiate the regenerative braking process. It enters the energy mutual assistance enhancement zone, but has not yet reached the grid-side feedback condition. Based on this, the edge controller increases the loading slope of the speed-up unit to accelerate its absorption of regenerated power.

[0085] At approximately At the second mark, the curve shows a negative dip, with a minimum of approximately 720 volts and a half-width of approximately 0.45 seconds. This corresponds to a state where the power demand of the speed-up unit on the common DC bus exceeds the release capacity of the speed-down unit, causing... When the voltage drops below the second bus voltage threshold of 730 volts, the edge controller reduces the loading slope of the booster unit to alleviate bus pressure.

[0086] At approximately A second positive pulse appeared at the specified time, with a peak value of approximately 778 volts. This pulse remained within the enhanced energy exchange range, further verifying that the regenerated power between multiple units was effectively exchanged within the common DC bus, and that the active front-end rectifier module did not trigger feedback action throughout the entire timeframe. Optionally, the specific values ​​of the three voltage thresholds can be adjusted appropriately based on the AC grid voltage level, the rated DC bus voltage of the frequency converter, and the load fluctuation range at the construction site. However, the second bus voltage threshold should always be kept lower than the first bus voltage threshold, and the feedback activation threshold should be higher than the first bus voltage threshold but lower than the bus overvoltage protection threshold.

[0087] While specific embodiments of the present invention have been described above, those skilled in the art should understand that these specific embodiments are merely illustrative. Those skilled in the art can omit, substitute, and modify the details of the above methods and systems in various ways without departing from the principles and essence of the present invention. For example, combining the above method steps to perform substantially the same function and achieve substantially the same result according to substantially the same method falls within the scope of the present invention. Therefore, the scope of the present invention is defined only by the appended claims.

Claims

1. A hybrid multi-unit centrifugal heat pump control system, comprising a hybrid unit group, wherein the hybrid unit group includes multiple centrifugal heat pump units connected in series, parallel, or a series-parallel combination via a chilled water side or a cooling water side; characterized in that, Also includes: The common DC bus module includes a positive DC copper busbar and a negative DC copper busbar that connect the DC sides of the frequency converters of each centrifugal heat pump unit to form a common DC busbar, as well as an active front-end rectifier module connected to the input end of the common DC busbar. The signal acquisition module is used to acquire the bus voltage value of the common DC bus, the branch current value of each frequency converter driver, and the thermodynamic operating parameters of each centrifugal heat pump unit. A hot gas bypass module is configured in each centrifugal heat pump unit and includes a hot gas bypass pipeline, a hot gas bypass main valve installed on the hot gas bypass pipeline, and a high-speed electromagnetic pilot valve installed at the pilot port of the hot gas bypass pipeline. The edge controller dynamically coordinates the speed change slope of the deceleration and acceleration units based on the total cooling and heating load demand and bus voltage value of the hybrid unit group, so that the regenerated power is absorbed through the common DC bus. It uses a reference thermodynamic model and a recursive least squares method with a forgetting factor to identify the equivalent surge boundary offset caused by the electrical coupling of the common DC bus and outputs the surge margin value of each centrifugal heat pump unit. When the surge margin value of any centrifugal heat pump unit is lower than the first margin threshold, it drives the corresponding high-speed electromagnetic pilot valve to open the corresponding hot gas bypass main valve to coordinate with the other units to perform load transfer.

2. The system as described in claim 1, characterized in that, The signal acquisition module includes: a bus voltage sensor installed between the positive and negative DC copper busbars of the common DC bus; branch current sensors installed at the access nodes of each inverter driver on the positive DC copper busbar; and exhaust pressure sensors, return gas pressure sensors, evaporation temperature sensors, condensation temperature sensors, and inlet guide vane opening sensors installed on each centrifugal heat pump unit to collect thermal operating parameters. The thermal operating parameters include the exhaust pressure value output by the exhaust pressure sensor, the return gas pressure value output by the return gas pressure sensor, the evaporation temperature value output by the evaporation temperature sensor, the condensation temperature value output by the condensation temperature sensor, the inlet guide vane opening value output by the inlet guide vane opening sensor, and the real-time speed value and inverter output power value reported by the corresponding inverter driver. The edge controller communicates with the signal acquisition module and each inverter driver via an industrial Ethernet bus.

3. The system as described in claim 1, characterized in that, The common DC bus module also includes branch contactors and branch fuse protection units connected in series between each frequency converter and the common DC bus; the active front-end rectifier module is used to convert AC grid power into DC power and output it to the common DC bus, and the active front-end rectifier module has the four-quadrant operation capability to allow regenerative power from the DC side to be fed back to the AC grid side.

4. The system as described in claim 1, characterized in that, The edge controller, in performing dynamic coordination of the speed change slope, is further used for: determining the centrifugal heat pump units that need to reduce speed and load and the centrifugal heat pump units that need to increase speed and load based on the total cooling and heating load demand; when the bus voltage value is higher than the first bus voltage threshold, increasing the loading slope of the speed-up unit or decreasing the load reduction slope of the speed-down unit to allow the speed-up unit to absorb the regenerative energy released by the speed-down unit from the common DC bus; when the bus voltage value is lower than the second bus voltage threshold, under the premise of meeting the total cooling and heating load demand, the minimum stable speed of each unit, and the surge margin constraint, decreasing the loading slope of the speed-up unit or increasing the load reduction slope of the speed-down unit with a load reduction margin; when the second bus voltage threshold is lower than the first bus voltage threshold, the feedback start threshold of the active front-end rectifier module is higher than the first bus voltage threshold and lower than the bus overvoltage protection threshold; when the bus voltage value is higher than the feedback start threshold, excess regenerative energy is fed back to the AC grid side through the active front-end rectifier module.

5. The system as described in claim 2, characterized in that, The edge controller stores a reference thermodynamic model for each centrifugal heat pump unit. The reference thermodynamic model includes a reference operating pressure ratio sub-model and a reference surge boundary sub-model. The reference operating pressure ratio sub-model takes the evaporation temperature, condensation temperature, real-time speed, inlet guide vane opening, and inverter output power as inputs and outputs the reference operating pressure ratio of the corresponding centrifugal heat pump unit under the condition of not considering the electrical coupling effect of the common DC bus. The reference surge boundary sub-model outputs the reference critical pressure ratio of the corresponding centrifugal heat pump unit under the current thermodynamic conditions with the same input as the reference operating pressure ratio sub-model. The edge controller obtains the pressure ratio observation value by dividing the exhaust pressure value by the return gas pressure value at fixed control intervals.

6. The system as described in claim 5, characterized in that, The edge controller constructs an electrical coupling correction vector for identifying the equivalent surge boundary offset. The electrical coupling correction vector includes the following components: the deviation between the bus voltage value and the rated bus voltage in the current fixed control cycle, the rate of change of the bus voltage value relative to the previous fixed control cycle, the branch current value in the current fixed control cycle, the rate of change of the branch current value relative to the previous fixed control cycle, and the speed tracking error between the speed command value and the real-time speed value of the strain frequency drive.

7. The system as described in claim 6, characterized in that, The edge controller maintains the identification parameter vector and covariance matrix for each centrifugal heat pump unit. When the hot gas bypass main valve of any centrifugal heat pump unit is closed and the sensor data meets the validity criteria, the edge controller performs one recursive update of its identification parameter vector and covariance matrix in each fixed control cycle, using the electrical coupling correction vector as the regression input, the deviation between the pressure ratio observation value and the reference operating pressure ratio value as the observation output, and the recursive least squares method with forgetting factor. When the hot gas bypass main valve of any centrifugal heat pump unit is open or the sensor data does not meet the validity criteria, the edge controller suspends the recursive update of its identification parameter vector and covariance matrix.

8. The system as described in claim 7, characterized in that, The edge controller also includes a boundary register. The edge controller multiplies the updated identification parameter vector with the current electrical coupling correction vector to obtain the electrical coupling safety correction amount. After limiting the electrical coupling safety correction amount, it is superimposed on the reference critical pressure ratio value to obtain the equivalent safety critical pressure ratio value of the corresponding centrifugal heat pump unit under the current operating condition and writes it into the boundary register. The equivalent safety critical pressure ratio value is then subtracted from the pressure ratio observation value to obtain the surge margin value of the corresponding centrifugal heat pump unit.

9. The system as described in claim 1, characterized in that, When coordinating with other units to perform load transfer, the edge controller is further used to: open the corresponding hot gas bypass main valve through the triggered high-speed electromagnetic pilot valve so that part of the high-pressure gaseous refrigerant on the compressor exhaust side of the corresponding centrifugal heat pump unit flows back to the compressor suction side through the opened hot gas bypass main valve, thereby increasing the effective refrigerant flow on the compressor suction side and increasing the suction pressure. Based on the evaporator water-side flow rate, chilled water specific heat capacity, chilled water density, and the change in evaporator inlet and outlet water temperature difference within the preset sampling window before and after the hot gas bypass trigger, the effective cooling and heating output of the corresponding centrifugal heat pump unit is calculated, and the effective cooling and heating output is taken as the load to be compensated. For each of the remaining centrifugal heat pump units, the first allowable increment is calculated based on its surge margin value, the second allowable increment is calculated based on the difference between its current real-time speed and the maximum allowable speed, and the third allowable increment is calculated based on its inverter current margin. The minimum value among the first, second, and third allowable increments is taken as its increaseable cooling and heating output. The units to be compensated are selected in descending order of surge margin value until the sum of the increaseable cooling and heating output of the selected units covers the load to be compensated, and a compensation speed-up command is issued to the inverter driver of the selected unit.

10. The system as described in claim 9, characterized in that, The edge controller continuously monitors the surge margin value of the centrifugal heat pump unit that triggered the high-speed electromagnetic pilot valve. When the number of control cycles in which the surge margin value is continuously maintained above the second margin threshold reaches the preset recovery cycle number, the edge controller sends a shut-off signal to the triggered high-speed electromagnetic pilot valve to close the corresponding hot gas bypass main valve, gradually withdraws the compensation speed-up command according to the preset unloading slope, and re-executes the dynamic coordination of the speed change slope to allow the hybrid unit group to transition to a new load distribution state. The second margin threshold is greater than the first margin threshold, and a hysteresis interval is formed between the second margin threshold and the first margin threshold.

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