Gravity energy storage system with net forming capability

Abstract

The utility model relates to a gravity energy storage system with a net forming capability, which comprises a heavy object storage yard with an upper stacking area with a high potential energy position and a lower stacking area with a low potential energy position, and heavy object blocks are stacked in the heavy object storage yard; one or more weight blocks are driven between the upper stacking area and the lower stacking area through a gravity turbine, the weight blocks are connected with synchronous motors through conveying devices, and each synchronous motor is connected with a bidirectional converter; the system comprises a plurality of gravity energy storage units, wherein each gravity energy storage unit consists of a weight block, a gravity turbine, a synchronous motor and a bidirectional converter; all the gravity energy storage units are connected with a power grid through a medium-voltage transformer; all the bidirectional converters are in control connection through an energy storage coordination controller; adjustments capable of being carried out through the system comprise primary frequency modulation, inertia response and transient support, the short-circuit current level of the system can be effectively improved, and the system strength is improved.

Description

Technical Field

[0001] This utility model patent belongs to the field of gravity energy storage systems, and in particular relates to a gravity energy storage system with network-building capabilities. Background Art

[0002] As the installed capacity of new energy sources increases, the local power grid becomes weaker and weaker (the short-circuit ratio decreases), resulting in a series of wide-band oscillation problems in voltage-oriented grid-following converters. In order to improve stability and explore new orientation methods, power synchronization can greatly improve the stability of weak networks.

[0003] With the continuous increase of renewable energy sources, the operating mode based on current source leads to weak voltage control ability of the power grid. There is no unit to establish and support voltage. This is unacceptable for the power system that still operates on voltage source. Therefore, in order to build a new power system based on renewable energy, these renewable energy sources also need to show voltage source characteristics to the power grid.

[0004] At present, electrochemical energy storage has also introduced networking technology in recent years. Its fundamental reason is the control strategy of the converter inverter. The battery itself does not have the inertia support capability, and the converter is used to build a virtual synchronous machine to provide support for the system. However, in terms of storage energy sources, a large number of backup batteries and converters with at least 2 times the overload capacity are required. Because transient networking does not often occur in the power system, electrochemical energy storage needs to increase a lot of additional costs to achieve networking capabilities, which is not economical at all. Utility Model Content

[0005] This patent proposes a gravity energy storage system and regulation method with grid-forming capabilities. By utilizing its grid-forming control technology in new power systems, it can effectively increase the system's short-circuit current level and improve system strength; provide damping and inertia for the system to improve system frequency stability; respond quickly when the system loses step and is disconnected, improve the stability of the system's first swing cycle, and actively support system recovery; and weaken the impact of inter-harmonics and unbalanced voltage in the power system.

[0006] The technical means adopted by the utility model are as follows:

[0007] A gravity energy storage system with network-building capability, comprising:

[0008] A heavy object storage yard, which includes an upper storage area with a high potential energy level and a lower storage area with a low potential energy level, wherein heavy object blocks are stored in the upper storage area and the lower storage area;

[0009] One or more weight blocks are driven between the upper stacking area and the lower stacking area by a gravity turbine, and the weight blocks are connected to synchronous motors through a conveyor, and each synchronous motor is connected to a bidirectional converter;

[0010] The system includes several gravity energy storage units consisting of weight blocks, gravity turbines, synchronous motors and bidirectional converters, and the bidirectional converters of all gravity energy storage units are connected to the grid through medium voltage transformers;

[0011] And all bidirectional converters are connected and controlled by an energy storage coordination controller.

[0012] Compared with the prior art, the advantages of this solution are:

[0013] Since the gravity energy storage system uses the weight and height difference of heavy objects, this natural physical process is more stable than other forms of energy storage technology (such as battery energy storage or compressed air energy storage). The movement of heavy objects over the height difference is a slow and continuous process that is not easily affected by the external environment, so the energy storage system can provide a more stable energy output. The system has almost no energy loss during the energy conversion process because its core principle is the mutual conversion of potential energy and mechanical energy. Gravity energy storage systems with grid-building capabilities can be seamlessly integrated with the power grid. This means that such a system can not only provide energy when the grid demand is high, but also store energy when the grid demand is low or no demand. This technology can smooth out voltage changes. In areas where new energy is concentrated, if phase commutation failure occurs, transient overvoltage will occur in the AC system. Grid-building energy storage can smooth out this rapid voltage change by responding in a timely manner, solving the response lag and reverse adjustment problems of traditional grid-following dynamic reactive compensation devices. The system can improve the short-circuit ratio of multiple renewable energy stations, which is equivalent to a parallel voltage source on the main grid side, indirectly changing the short-circuit impedance on the system side, increasing the short-circuit current, and then increasing the short-circuit capacity to support more renewable energy access. With the continuous access of renewable energy, the grid-connected renewable energy units show current source characteristics, and there are problems such as insufficient short-circuit capacity during faults. By configuring a grid-connected gravity energy storage system, the proportion of voltage source equipment such as synchronous generators in the system can be equivalently increased. The gravity energy storage system will become an ideal choice for balancing the grid load and improving the stability and reliability of the grid. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 It is a structural schematic diagram of the gravity energy storage system with network-building capability of the utility model.

[0015] In the figure:

[0016] 1Heavy goods storage yard;

[0017] 2 heavy blocks;

[0018] 3 gravity turbine;

[0019] 4Electrically excited synchronous motor;

[0020] 5 two-way communicator;

[0021] 6. Control unit;

[0022] 7. Energy storage coordination controller;

[0023] 8 Medium voltage transformer;

[0024] 9Power grid. DETAILED DESCRIPTION

[0025] like Figure 1 As shown, a gravity energy storage system with network-building capability is proposed, which is constructed by using height difference and includes:

[0026] A heavy object storage yard 1, which includes an upper stacking area with a high potential energy level and a lower stacking area with a low potential energy level. A large number of heavy object blocks 2 are stacked in the upper stacking area and the lower stacking area. The heavy object blocks 2 are solid-state energy storage media for storing energy and realizing potential energy and electrical energy conversion, and their positions can be changed in energy storage and energy release modes;

[0027] One or more weight blocks 2 are driven between the upper stacking area and the lower stacking area by a gravity turbine 3, and the weight blocks 2 are connected to the synchronous motor 4 by a transmission device, and can use their own gravity to provide motive force for the synchronous motor 4. Each synchronous motor 4 can have an excitation system, and each synchronous motor 4 is connected to a bidirectional converter 5;

[0028] The system includes several gravity energy storage units consisting of weight blocks 2, gravity turbines 3, synchronous motors 4 and bidirectional converters 5, all of which constitute a gravity energy storage power station, and the bidirectional converters 5 of all gravity energy storage units are connected to a power grid 9 via a medium voltage transformer 8;

[0029] And the control units 6 of all bidirectional converters 5 are connected to the energy storage coordination controller 7, and are all controlled and connected by an energy storage coordination controller 7 for unified coordination control; the energy storage coordination controller 7 measures the parameters of the grid connection points of multiple gravity energy storage units, generates power increase and decrease instructions according to the measurement results, and distributes and notifies them in all converters, completes the collaboration between converters, and realizes the consistency of the overall response of the gravity energy storage power station.

[0030] The grid-type gravity energy storage system designed by the utility model mainly has two operating states: energy storage and discharge. When storing energy, the heavy block is lifted from a low place to a high place for storage, and the electrical energy is finally converted into gravitational potential energy; when discharging, the heavy block is dropped from a high place to a low place, and the gravitational potential energy is first converted into kinetic energy, which is then driven by the transmission device to generate electricity and finally converted into electrical energy. The grid-type gravity energy storage system can work in grid-connected mode and off-grid mode. The system uses the gravitational potential energy of the heavy block to provide a powerful motive force for the synchronous motor by controlling the operating speed. At the same time, the system has the inertia support and excitation regulation system unique to the synchronous motor, and then the voltage source that supports the stable operation of the large power grid is constructed through the energy storage inverter, which can play the role of rapid frequency and voltage regulation, increase inertia and short-circuit capacity support, and suppress broadband oscillation, thereby enhancing the stability of the power system.

[0031] Specific implementation plan: In power systems with a high proportion of renewable energy, the reduction in synchronous generators leads to reduced system strength and lack of damping and inertia, which makes the voltage and frequency stability of the new power system face challenges. The gravity energy storage system using grid-building technology can solve the stability problems of voltage and frequency in the power system and improve the power supply quality of the power grid and its ability to absorb renewable energy. The network-building technology is the control algorithm plus hardware. The hardware capability supports the control algorithm to achieve the required special performance and indicators. The gravity energy storage system of this scheme adopts a gravity turbine to drive the heavy block to fall and generate electricity. The heavy block needs to undergo acceleration, uniform speed, and deceleration during the falling power generation process. This operation process causes the gravity turbine, synchronous motor, and bidirectional converter to have an overload capacity of more than 2 times (of the heavy block). At the same time, the gravity energy storage system is based on the synchronous motor. The synchronous motor itself has a strong overvoltage and overcurrent capacity, and the long-term allowable frequency deviation range is wide, which can directly provide inertial support for the system. At the same time, the system is based on the synchronous motor excitation regulation function, which can effectively control the motor's current, voltage and power output. The system's bidirectional converter adopts an air-supply voltage source converter, which adjusts the active and reactive power by adjusting the voltage phase angle and amplitude respectively, which can simulate the synchronous motor, provide inertia and damping, and achieve synchronization by generating a phase angle through power control. All bidirectional converter control units in the gravity energy storage power station are connected to the energy storage coordination controller. The energy storage coordination controller measures the parameters of multiple gravity energy storage power station grid connection points, generates power increase and decrease instructions based on the measurement results, and distributes and notifies them in all converters to complete the collaboration between converters and achieve consistency in the overall response of the gravity energy storage power station. At the same time, it can also realize seamless switching between the grid-connected mode and the off-grid mode of the gravity energy storage system.

[0032] The grid-forming gravity energy storage system, through the energy storage inverter, constructs a voltage source to support the stable operation of the large power grid, which can play roles such as rapid frequency modulation and voltage regulation, increasing inertia and short-circuit capacity, and suppressing broadband oscillations. When the load in the power system suddenly withdraws or is put into operation, the gravity energy storage system will instantaneously share this part of the power, thus ensuring the real-time balance of the active power in the power system. Due to the imbalance between the shaft power and the output electric power of the generator, the rotational speed of the generator will change up and down. Relying on the inertia of the generator rotor, the change in rotational speed will be controlled within a reasonable range. When the change in rotational speed exceeds a certain limit, the gravity energy storage system will automatically issue an instruction to increase or decrease the shaft power, and utilize the advantage of the rapid adjustment of the power electronic control system of the converter to quickly change the operating speed of the gravity turbine and pull the rotational speed of the generator back to near the rated value, thereby ensuring that the frequency of the entire power grid is within a reasonable range. When the power grid is disturbed, such as a short-circuit fault occurs, the grid-forming gravity energy storage system can keep the amplitude of the internal electromotive force in the synchronous motor basically unchanged and instantaneously output reactive current. After the short-circuit fault is removed, the gravity energy storage system can instantaneously adjust its reactive power output according to the recovery of the power grid voltage, and the automatic excitation control system will then automatically issue an instruction to increase or decrease the excitation to control the voltage within a reasonable range. When the grid-forming gravity energy storage power station is in the power system, whether it is a load change or a short-circuit fault, the gravity energy storage system, through the combination of its own inherent characteristics and speed regulation and excitation control, controls the frequency and voltage of the power grid within a reasonable range.

[0033] The gravity energy storage system of the present invention has the control characteristics of a grid-forming converter, can operate continuously without disconnecting from the grid within the range of 0 < U < 110% Un of the converter AC port voltage, and at the same time has a certain overload capacity. The AC side current can operate continuously for a long time at 110% of the rated current; at 120% of the rated current, the sustainable operation time is not less than 2 minutes. The system has the ability to operate in a power grid with a low short-circuit ratio, the static voltage regulation rate of the AC side is less than 1%, its primary frequency modulation droop rate is 0.5% - 3%, and the active power regulation deviation when reaching stability does not exceed ±2% Pn. It has an inertia response function within the active power range, has an oscillation suppression function, can receive and respond to the AGC instructions sent by the dispatching, and the adjustable range includes -100% - 100% Pn; the steady-state deviation of the active power is not greater than 2% Pn; the system can receive and respond to the AVC instructions sent by the dispatching, and the steady-state deviation of the reactive power is not greater than 2% Pn.

[0034] The gravity energy storage system has the control characteristics of a grid-forming converter and also has the overload characteristics of a traditional synchronous generator, which not only meets the requirements of the short-circuit capacity but also can avoid the short-circuit current exceeding the standard. The specific adjustment methods of the system include:

[0035] (1) Primary frequency modulation:

[0036] The grid-type gravity energy storage system releases or stores energy by changing the speed of the weight lifting and lowering, and performs auxiliary frequency modulation. The torque acting on the single weight block includes the electromagnetic torque of the synchronous motor and the load torque. Its motion equation is:

[0037]

[0038] Where, T e is the electromagnetic torque, T L is the load torque, J is the moment of inertia, ω m is the mechanical angular velocity, B is the rotor resistance coefficient;

[0039] The difference between the output power of the gravity energy storage and the reference value of the total output power of the gravity energy storage device system and the increment of the difference between the actual value of the grid frequency and the rated value of the grid frequency are used as the control input of the gravity energy storage system to obtain the output / input power of the gravity energy storage system. The control process can be expressed as:

[0040] P f =P ref +K g (ff ref )

[0041] Where P f is the output / input power of the gravity energy storage system; P ref is the reference value of the total output power of the system; K g is the unit adjustment power of the gravity energy storage system; f is the actual value of the grid frequency; f ref is the rated value of the grid frequency, which is 50Hz.

[0042] (2) Inertia response:

[0043] The inertia response process of the grid-type gravity energy storage system is supported by synchronous inertia and virtual inertia. The frequency response process of the system can be described by the following equivalent rotor motion equation:

[0044]

[0045] Where: H sys is the equivalent inertia constant; f N and Δf(t) are the system rated frequency and frequency fluctuation value respectively; D is the load frequency response coefficient; PG(t) is the system primary frequency response power; P mec (t) is the mechanical power of the system; P ele (t) is the electromagnetic power of the system;

[0046] Specifically, when an accident occurs in the power system, inertia can be used to buffer unbalanced power and slow down the frequency change speed, thus gaining more reaction time for a frequency modulation response. During the control process of the gravity energy storage system generator set, the converter adjusts the active power output of the unit according to the change in the speed of the heavy object, so that the generator has time to adjust the power generation power and rebuild the power balance; when there is an active power disturbance in the power grid, the gravity energy storage generator set shares the active power change of the system, and when the frequency suddenly changes, it injects or absorbs the sudden active power into the system instantaneously through rapid power control, and then releases or absorbs the kinetic energy of the heavy object and the generator rotor by controlling the speed change. The gravity energy storage generator set controlled by the power electronic converter has flexible and controllable active power regulation capabilities. The gravity energy storage unit can switch the weight of the heavy object to run at a variable speed, release or store the rotational kinetic energy of the unit, and virtualize the inertial response.

[0047] (3) Excitation control can also be performed if an excitation system is available:

[0048] The synchronous motor excitation control system mainly includes voltage measurement and comparison, comprehensive amplification, power unit and other main links. According to the network characteristics, the mathematical model of the generator excitation circuit is established, and the transfer function of the synchronous generator excitation circuit is obtained as follows:

[0049]

[0050] In the formula, K D is the synchronous generator gain factor, T d0 is the time constant of magnetic saturation neglected;

[0051] Specifically: the excitation automatic control system composed of the excitation system and the generator maintains the terminal voltage at a given level by adjusting the excitation current. When a large disturbance fault occurs in the power system, the system is controlled by the excitation system to transition to a new stable state or restore the original stable operating state. That is, the excitation system improves the transient stability of the power system and ensures the reliability of power supply to the power grid.

[0052] (4) Transient stability support through damping control under large system disturbances:

[0053] The damping control of the gravity energy storage system unit not only suppresses the system power oscillation caused by small disturbances, but also supports transient stability under large disturbances. The utility model has the ability to quickly adjust, can quickly absorb or release active and reactive power, support node voltage, smooth system frequency fluctuations, alleviate system safety issues caused by high proportion of renewable energy grid connection, enhance the ability of the power grid to resist sudden events and failures, and avoid the occurrence of large-scale chain failures. When the system power oscillates, the variable speed unit adjusts the speed according to the frequency change:

[0054] Δω r =-2πkP Δf

[0055] In the formula, Δω r k is the motor angular velocity increment; p is the unit active damping control coefficient;

[0056] When the unit is under maximum power tracking control, the small disturbance of electromagnetic power can be expressed as:

[0057]

[0058] When adjusting the speed change of the unit, the generated power change ΔPe is made to meet the requirements of the above formula, thereby improving the system damping and achieving transient stability support through damping control.

Claims

1. A gravity energy storage system with network-building capability, characterized in that: include: A heavy object stacking yard (1), the heavy object stacking yard (1) comprising two areas, an upper stacking area with a high potential energy level and a lower stacking area with a low potential energy level, wherein heavy object blocks (2) are stacked in the upper stacking area and the lower stacking area; One or more weight blocks (2) are driven between the upper stacking area and the lower stacking area through a gravity turbine (3), and the weight blocks (2) are connected to a synchronous motor (4) through a transmission device, and each electrically excited synchronous motor (4) is connected to a bidirectional converter (5).

2. A gravity energy storage system with network-building capability according to claim 1, characterized in that: The system comprises a plurality of gravity energy storage units consisting of weight blocks (2), gravity turbines (3), synchronous motors (4) and bidirectional converters (5), and the bidirectional converters (5) of all gravity energy storage units are connected to a power grid (9) via a medium voltage transformer (8).

3. A gravity energy storage system with network-building capability according to claim 1, characterized in that: Furthermore, all bidirectional converters (5) are connected and controlled by an energy storage coordination controller (7).

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