Uninterruptible power supply with autonomous under-frequency detection and load shedding functions

Abstract

An uninterruptible power supply (UPS) (10) is disclosed, the uninterruptible power supply comprising: an alternating current (AC) mains power supply input (12) for connecting the UPS to an AC mains power supply; an AC power supply output (14) for supplying power to at least one load connected to the AC power supply output; a direct current De link (16); a rectifier (18) for converting AC received via the AC mains power supply input (12) into DC and supplying the DC to the DC link (16); an inverter (20) for converting DC received from the DC link (16) into AC to supply the AC via an AC power supply output (14); at least one storage (22) for electrical energy, connected to the DC link (16) via a controllable DC / DC converter (24); a frequency measurement unit (26) provided for measuring a frequency of AC supplied via the AC mains power supply input (12); and a controller (28) configured to detect an underfrequency condition of AC supplied via the AC mains power supply input (12), and is configured for regulating the power demand via the AC mains supply input (12) by controlling the supply of power from the at least one storage (22) for electrical energy and / or from the AC mains supply input (12) to at least one load connected to the AC supply output (14) depending on the detected underfrequency condition.

Description

Technical Field

[0001] This specification relates to an uninterruptible power supply (UPS) with autonomous underfrequency detection and load shedding capabilities. Background Art

[0002] International patent application WO2022 / 233457A1 discloses a system for frequency regulation in an alternating current (AC) power system. The system includes a UPS electrically connected to the AC power system and including a phase-locked loop and / or frequency-locked loop (PLL) for monitoring the frequency of an AC input voltage received from the AC power system. The phase-locked loop (PLL) and / or frequency-locked loop (FLL) are designed to operate at an operating frequency higher than the nominal grid frequency and are selected to detect a deviation of the monitored frequency from the nominal grid frequency within a predetermined time span after a frequency dip occurs in the monitored frequency. A control signal is generated based on the detected deviation of the monitored frequency from the nominal grid frequency. The generated control signal is used to regulate power flow between the uninterruptible power supply and the grid until the monitored frequency is within a predefined range around the nominal grid frequency. The system supports AC power systems (e.g., power systems powered by low-inertia generators) by enabling the UPS control system to react to frequency deviations from the nominal grid frequency. The system specifically implements frequency regulation using fast sub-cycle frequency measurements from the PLL or FLL provided by an internal measurement system in the UPS. By using a PLL or FLL, a fast frequency measurement can be provided, which makes it possible to monitor the frequency of the AC input voltage even in the event of an initial frequency drop and thus allows the UPS to react almost immediately to detected frequency deviations. Summary of the Invention

[0003] This specification describes a UPS with autonomous underfrequency detection and load shedding capabilities.

[0004] According to one aspect of the present specification, a UPS includes the following components: an AC mains power input for connecting the UPS to an AC mains power supply; an AC power output for supplying power to at least one load connected to the AC power output; a direct current (DC) link; a rectifier for converting AC received via the AC mains power input into DC and supplying the DC to the DC link; an inverter for converting DC received from the DC link into AC to supply the AC via the AC power output; at least one storage for electrical energy connected to the DC link via a controllable DC / DC converter; a frequency measuring unit provided for measuring the frequency of AC supplied via the AC mains power input; and a controller configured to detect an underfrequency condition of the AC supplied via the AC mains power input, and configured to adjust the power demand via the AC mains power input by controlling the supply of power from the at least one storage for electrical energy and / or the AC mains power input to at least one load connected to the AC power output according to the detected underfrequency condition.

[0005] In an embodiment, regulating the power demand via the AC mains power input may comprise controlling a DC / DC converter to control power flow from the at least one storage for electrical energy to the at least one load connected to the AC power supply output.

[0006] In an embodiment, detecting an underfrequency condition of the AC supplied via the AC mains power input comprises detecting a frequency anomaly of the measured frequency, wherein the frequency anomaly comprises the measured frequency falling below one or more predefined frequency limits.

[0007] In a further embodiment, the UPS may include an output power measuring unit provided for measuring the output power supplied to at least one load, wherein regulating the power demand via the AC mains input further comprises controlling the supply of power from at least one storage and / or the AC mains input to at least one load connected to the AC power output based on the detected underfrequency condition and the measured output power.

[0008] In yet another embodiment, controlling the supply of power from at least one storage and / or the AC mains power input to at least one load connected to the AC power output based on the detected underfrequency condition may include allocating the supply of power from the at least one storage for protecting a critical load among the at least one load connected to the AC power output.

[0009] In an embodiment, allocating the supply of power from the at least one storage for protecting a critical load in at least one load connected to the AC power output may be based on defined parameter settings that determine limits for discharging the at least one storage.

[0010] In an embodiment, the controller may be further configured to disconnect one or more of the at least one load connected to the AC power output and / or disconnect the entire load connected to the AC power output depending on the detected underfrequency condition.

[0011] In a further embodiment, the controller may be configured to regulate power demand via the AC mains power input by disconnecting one or more of at least one load connected to the AC power output if the detected underfrequency condition exceeds one or more predefined thresholds, wherein the one or more predefined thresholds include one or more of: a maximum duration of the underfrequency condition; a maximum deviation of the detected underfrequency condition from the desired frequency; a maximum rate of change of the detected underfrequency condition.

[0012] According to another aspect of the present specification, a redundant power distribution topology includes several distribution paths for supplying power to one or more loads, wherein each load is supplied with power from at least two distribution paths, wherein each distribution path includes a UPS having an autonomous underfrequency detection function according to any one of the preceding claims.

[0013] The details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features and advantages will be apparent from the description and drawings, and from the claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 A block diagram illustrating an embodiment of a UPS with autonomous underfrequency detection and load shedding capabilities is shown;

[0015] Figure 2 shows an example of a redundant power distribution topology including a traditional UPS and UFLS relays; and

[0016] Figure 3 An embodiment of a redundant power distribution topology is shown that includes a UPS with autonomous underfrequency detection and load shedding the AC mains supply by reducing its own AC mains input demand. DETAILED DESCRIPTION

[0017] In the following, functionally similar or identical elements may have the same reference numerals.Absolute values ​​are shown below only by way of example and should not be interpreted as limiting.

[0018] Underfrequency load shedding (UFLS) is commonly used by operators of electricity transmission systems (grids) as a last resort to restore power balance in the system after a major emergency and when primary methods (frequency regulation) have failed to control grid frequency within prescribed limits. In the European Union, UFLS is mandated by Commission Regulation (EU) 2017 / 2196 of 24 November 2017 laying down a network code on electricity emergency response and restoration.

[0019] In practice, if the grid frequency drops to a certain level (i.e., the limits specified in the European synchronization area regulations), the system operator is obliged to disconnect electricity consumers (demand) from the grid. This demand can consist of large energy consumers directly connected to the high-voltage (HV) grid and the medium-voltage (MV) distribution network. Demand disconnection is performed using UFLS relays and can potentially lead to (large-scale) blackouts in the system.

[0020] For large energy consumers connected to the HV grid, such as data centers, a complete disconnection from the power supply and resulting in a blackout could pose a serious operational risk. In these installations, there are usually one or more backup generators to start and supply the load, but this is not necessarily the case if these are not needed under normal circumstances. And the situations in which UFLS needs to be activated may be very rare, for example never occurring in Northern Europe, and therefore providing backup generators only in such cases increases technical complexity. However, obtaining permission to connect to the HV network is a technical requirement, and the likelihood of UFLS events is likely to increase due to the energy transition and the increasing number of asynchronous generation sources (such as wind and solar) being introduced into the grid.

[0021] The implementation of the UFLS functionality with a UPS as described herein allows asset owners to meet the required demand reduction by shifting some of the demand to the UPS energy storage (e.g., batteries) without causing a power outage. Since the required demand reduction may not be 100%, only part of the load (demand) can be shifted to the UPS energy storage until the frequency of the AC mains power input to the UPS is restored. Compared to the traditional approach of tripping the main feeder in the HV connection and moving 100% of the load to the batteries and operating the entire site in battery mode, the technical solution proposed herein can be safer for mission-critical operations and can allow for significantly reduced operational risks since the partially discharged batteries can be allowed to operate in this mode for a longer period of time. Furthermore, after the main HV connection is tripped, the connection may not be restored after the AC mains power frequency is restored before the batteries are fully discharged and the critical loads are shut down. Therefore, in short, the technical solution described herein can provide UFLS in systems that do not have backup generation without the need to shut down (critical) loads.

[0022] Since the UFLS feature may be a requirement for connection to the HV grid, the UFLS feature may have to be running whenever the site (demand / load) is connected to the HV grid. Therefore, integrating the UFLS functionality into the UPS, as provided by the solution described herein, is the preferred approach whenever the UPS is running. Relying on an external device to control the UPS UFLS response is subject to communication / signal failures, etc. In addition, the UFLS response is preferably proportional to the actual UPS load. As the demand changes, the response also changes. This is especially beneficial for systems with built-in redundancy, such as data centers. When one part of the system is under maintenance and the load is supported by other parts of the system (site), the equipment supporting the load adjusts its response as its load increases (increases response), and vice versa, and the system (site) provides the correct amount of UFLS response when needed ("response follows load").

[0023] Figure 1 A block diagram of an embodiment of a UPS 10 is shown having a circuit for connecting the UPS 10 to an AC mains power source ( Figure 1 AC mains power input 12 in MAINS.

[0024] The UPS 10 further includes an AC power output 14 for supplying power to at least one load ( Figure 1 The AC power output 14 may include one or more sockets to connect one or more loads (e.g., one or more information technology (IT) devices, such as computers in a data center) and supply power to the one or more loads via the UPS 10.

[0025] The AC mains power input 12 is connected to the AC power output 14 via a direct current (DC) link 16; a rectifier 18 converts AC power received via the AC mains power input 12 into DC power and supplies the DC power to the DC link 16; an inverter 20 converts DC power received from the DC link 16 into AC power to supply the AC power via the AC power output 14; at least one storage 22 for electrical energy (particularly one or more (rechargeable) batteries) is connected to the DC link 16 via a controllable DC / DC converter 24. The AC mains power input 12 can be directly connected to the AC power output 14 via a bypass switch 32, particularly during normal operation of the UPS 10, for example when no power outage has occurred and / or no abnormality has occurred at the AC mains power input 12.

[0026] The UPS 10 includes a frequency measurement unit 26 provided for measuring the frequency of the AC supplied via the AC mains supply input 12 and a controller 28 configured for detecting an underfrequency condition of the AC supplied via the AC mains supply input 12 and for regulating the power demand via the AC mains supply input 12 by controlling the supply of power from the storage for electrical energy 22 and / or the AC mains supply input 12 to a load connected to the AC power output 14 in accordance with the detected underfrequency condition.

[0027] Regulating the power demand via the AC mains power input 12 may include controlling the DC / DC converter 24 to control the power flow from the storage 22 to the load connected to the AC power output 14, thereby allowing control of the power demand from the AC mains power input 12 and the storage 22. The power flow from the AC mains power input 12 and the storage 22 to the AC power output 14 within the UPS 10 is controlled in Figure 1 Indicated by thick arrows.

[0028] The controller 28 may be implemented, for example, by a processor configured with a program stored in a non-volatile memory to perform assigned tasks, or configured as an application specific integrated circuit (ASIC) or a (field) programmable gate array ((F)PGA).

[0029] The controller 28 implements an algorithm that processes the frequency of the AC supplied via the AC mains input 12 and measured by the frequency measurement unit 26. The processing includes detecting an underfrequency condition of the AC supplied via the AC mains input 12. Specifically, this includes detecting a frequency anomaly in the measured frequency. The frequency anomaly may, for example, include the measured frequency falling below one or more predefined frequency limits (e.g., one or more of 48.8 Hz, 48.6 Hz, 48.4 Hz, 48.2 Hz, 48.0 Hz in an AC mains supply having a nominal frequency of 50 Hertz (Hz).

[0030] The UPS 10 may further comprise an output power measurement unit 30, which is provided for measuring the output power supplied to the load. The measured output power may then be used together with the detected frequency anomaly as a parameter of a function for controlling the supply of power from the storage 22 and / or the AC mains power input 12 to the load connected to the AC power output 14 to regulate the power demand via the AC mains power input 12.

[0031] Controlling the supply of power from storage 22 and / or AC mains power input 12 to loads connected to AC power output 14 based on a detected underfrequency condition can include allocating the supply of power from storage 22 to protect a critical load among at least one load connected to AC power output 14. For example, when a detected underfrequency condition predicts potential damage to a critical load (e.g., a load with a specific AC power frequency requirement), controller 28 can control the power supply to the critical loads so that these loads are only supplied with power from storage having the required AC power frequency. This can be achieved, for example, by controller 28 switching the outlets of AC power output 14 to which the critical loads are connected to receive power from storage 22. This allocation can be based on defined parameter settings, such as determining limits for discharging the storage. For example, a parameter setting can be defined for a maximum discharge current in amperes from the storage, or a maximum discharge duration of 5 minutes at a specific discharge current (e.g., 10 amperes). A number of limits for discharging can be defined, particularly different combinations of discharge current and discharge duration. Defined parameter settings protect the memory specifically against overload and high discharge currents.

[0032] The controller 28 may also be configured to disconnect one or more of the loads connected to the AC power output 14, and / or disconnect the entire load connected to the AC power output 14, depending on the detected frequency underfrequency condition. This disconnection may be achieved by shutting off one or more outlets to which the loads are connected to the AC power output 14. For example, the controller 28 may shut off all outlets to which heavy loads, such as electric motors, are connected, or the controller may shut off all outlets to which loads that are highly sensitive to underfrequency conditions, such as IT equipment, are connected.

[0033] Controller 28 can be specifically configured to regulate power demand via AC mains power input 12 by disconnecting one or more loads connected to AC power output 14 if a detected underfrequency condition exceeds one or more predefined thresholds, the predefined thresholds specifically including one or more of the following: a maximum duration of the underfrequency condition; a maximum deviation of the detected underfrequency condition from the desired frequency; a maximum rate of change of the detected underfrequency condition. For example, controller 28 may disconnect one or more loads only if the duration of the detected underfrequency condition exceeds a predefined (maximum) duration and / or only if the deviation (in time and / or frequency) of the detected underfrequency condition from the desired frequency exceeds a predefined maximum value. The maximum rate of change can, for example, determine that rapid frequency changes may result in load disconnection, while slow frequency changes (i.e., frequency changes below the maximum rate of change) do not result in load disconnection. Several predefined thresholds may be combined depending on the needs.

[0034] The method described herein of regulating the power demand via the AC mains supply input 12 by controlling the supply of power from at least one storage 22 for electrical energy and / or the AC mains supply input 12 to at least one load connected to the AC power output 14 in dependence on a detected underfrequency condition may be based on a defined ramp-down curve and in particular may use a linear ramp or discrete steps to obtain a power response from the storage and regulate the storage discharge to provide the "correct" amount of power from the storage for the demand connected to the UPS.

[0035] Next, an example is given to describe the application of the UPS with autonomous underfrequency detection function described in this article in a redundant power distribution topology.

[0036] Figure 2 A redundant power distribution topology including conventional UPS (UPS A to UPSn) and UFLS relays is shown. Several loads AB to An and Bn are connected to the AC power output of one or more of the UPSs. Each of the loads is powered by two UPSs: load AB is powered by UPS A and B, load An is powered by UPS A and n, and load Bn is powered by UPS B and n. Therefore, at least two active power distribution paths are provided for critical loads, and each load is shared between these paths. When one of the UPSs, such as UPS A (shown in dotted lines), has a fault or is put into service that causes its AC power output to lose power, the demand on that path (i.e., the distribution path via the failed or in-service UPS A) will automatically move to the other healthy paths, in Figure 2 UPS A through n are connected to the AC mains power input of the grid via UFLS relays. When an underfrequency condition is detected, a conventional UFLS approach is used that uses UFLS relays to disconnect demand by opening the circuit breakers in each path to create discrete steps, so that once the energy storage of the UPS in the disconnected path is depleted, the demand from that path can be transferred to other paths or immediately transferred through automatic load management. Therefore, the demand reduction from the grid providing UFLS can be net zero. In addition, if a redundant distribution path is in operation, its demand will be zero, and tripping the UFLS relays and circuit breakers will not result in any demand reduction.

[0037] Figure 3 Shows the application of UPS A to n Figure 2 An embodiment of a redundant power distribution topology wherein each UPS includes the autonomous underfrequency detection functionality described herein. Figure 2In contrast to the topology shown, applying the UPS described in this article to provide a UFLS response proportional to the actual load by moving the demand to the UPS energy storage in a linear ramp or discrete steps can ensure that the required UFLS response can be provided to support the grid in the most critical emergency events under various operating conditions of mission-critical facilities. Due to the implementation of the autonomous underfrequency detection function in the UPS described in this article, Figure 3 The topology does not need to be Figure 2 The topology is like a separate UFLS relay.

[0038] Reference numerals and abbreviations

[0039] 10 UPS

[0040] 12 AC mains power input

[0041] 14 AC power outputs

[0042] 16 DC Link

[0043] 18 Rectifier

[0044] 20 Inverter

[0045] 22 Energy storage (e.g., one or more batteries)

[0046] 24 DC / DC converters

[0047] 26 Frequency measurement unit

[0048] 28 Controller

[0049] 30 Output power measurement unit

[0050] 32 Bypass switch

[0051] AC

[0052] DC

[0053] FLL Frequency Locked Loop

[0054] HV High Voltage

[0055] IT Information Technology

[0056] MV Medium Pressure

[0057] PLL Phase-Locked Loop

[0058] UFLS Under Frequency Load Shedding

[0059] UPS Uninterruptible Power Supply

Claims

1. An uninterruptible power supply (UPS) (10), comprising: An AC mains power input (12) for connecting the UPS (10) to an AC mains power source; an AC power output (14) for supplying power to at least one load connected to the AC power output; Direct current (DC) link (16); a rectifier (18) for converting AC received via the AC mains power input (12) into DC and supplying the DC to the DC link (16); an inverter (20) for converting DC received from the DC link (16) into AC to supply the AC via the AC power output (14); at least one storage (22) for electrical energy, connected to the DC link (16) via a controllable DC / DC converter (24); a frequency measurement unit (26) provided for measuring the frequency of the AC supplied via the AC mains power input (12); an output power measuring unit (30) provided for measuring the output power supplied to the at least one load; as well as a controller (28) configured to detect an underfrequency condition of the AC supplied via the AC mains supply input (12), and configured to regulate power demand via the AC mains supply input (12) by controlling the supply of power from the at least one store for electrical energy (22) and from the AC mains supply input (12) to the at least one load connected to the AC supply output (14) in dependence on the detected underfrequency condition and the measured output power.

2. The UPS (10) according to claim 1, wherein: Regulating the power demand via the AC mains power input (12) includes controlling the DC / DC converter (24) to control power flow from the at least one storage for electrical energy (22) to the at least one load connected to the AC power output (14).

3. The UPS (10) according to claim 1 or 2, wherein: Detecting an underfrequency condition of the AC supplied via the AC mains power input (12) includes detecting a frequency anomaly of the measured frequency, wherein the frequency anomaly includes the measured frequency falling below one or more predefined frequency limits.

4. A UPS (10) according to any preceding claim, wherein: Controlling the supply of power from at least one reservoir (22) and / or the AC mains power input (12) to the at least one load connected to the AC power output (14) based on the detected underfrequency condition includes allocating the supply of power from the at least one reservoir (22) to protect a critical load among the at least one load connected to the AC power output (14).

5. The UPS (10) according to claim 4, wherein: Distributing the supply of power from the at least one reservoir (22) to protect a critical load among the at least one load connected to the AC power output (14) is based on defined parameter settings that determine limits for discharging the at least one reservoir (22).

6. A UPS (10) according to any preceding claim, wherein: The controller (28) is further configured to disconnect one or more of the at least one load connected to the AC power output (14) and / or disconnect the entire load connected to the AC power output (14) depending on the detected frequency underfrequency condition.

7. A UPS (10) according to any preceding claim, wherein: The controller (28) is configured to regulate the power demand via the AC mains power input (12) by disconnecting one or more of the at least one load connected to the AC power output (14) if the detected underfrequency condition exceeds one or more predefined thresholds, wherein the one or more predefined thresholds include one or more of the following: a maximum duration of the underfrequency condition; a maximum deviation of the detected underfrequency condition from a desired frequency; a maximum rate of change of the detected underfrequency condition.

8. A redundant power distribution topology comprising a plurality of power distribution paths for supplying power to one or more loads, wherein: Each load is powered by power from at least two power distribution paths, wherein each power distribution path comprises a UPS with autonomous underfrequency detection according to any one of the preceding claims.

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