Control method, computer program product, control system and use

Abstract

A method for controlling the output voltage of at least one transformer (10) in a power grid. The method comprises the following steps: providing the at least one transformer (10) having a power electronic converter (12) and an output voltage controller (16); determining a grid frequency of the power grid; comparing the determined grid frequency with a reference value; generating an error signal corresponding to a difference between the determined grid frequency and the reference value; applying the error signal as feedback to the output voltage controller (16); and generating a control action to cause the output voltage controller (16) to change the output voltage of the at least one transformer (10) including the power electronic converter (12) so as to limit the difference between the grid frequency and the reference value. The power electronic converter (12) is an AC-AC converter, and the method comprises the following steps: connecting the AC-AC converter to the primary side or the secondary side of the transformer (10). The AC-AC converter has a first AC side and a second AC side, and the method comprises the steps of: connecting two terminals on the first AC side in parallel to a winding or a part of a winding of a transformer (10); and connecting the second AC side directly to an external circuit line or in series with the external circuit line.

Description

Technical Field

[0001] The present invention relates to a method, a computer program product and a system for controlling the output voltage of at least one transformer in an electrical grid. The invention also relates to the use of such a method, computer program product or system for controlling active power consumption within an electrical grid, i.e. any interconnected network (such as a transmission and distribution network) for delivering electricity from at least one producer to at least one consumer. Background Art

[0002] The electric grid consists of generating stations (often located near energy resources), substations (used to step up voltage for transmission or step down voltage for distribution), high-voltage transmission lines (carry electricity from distant energy sources to demand centers), and distribution lines (supply electricity to individual customers such as residential and commercial buildings and industrial parks).

[0003] Centralized power stations (such as coal, gas or nuclear power plants, or hydroelectric dams) often require the transmission of electricity over long distances. However, electricity can be generated by distributed energy resources (DER), i.e., decentralized and modular technologies, which can be located close to the loads they serve. As renewable energy sources (such as solar, wind, biogas, biomass, and geothermal) become more integrated, more electricity is injected into the alternating current (AC) grid using power electronic converters. This type of power injection is called non-synchronous generation (NSG), whereby renewable energy sources are connected to the grid asynchronously, rather than providing system inertia like traditional generation based on large synchronous generators.

[0004] In some grids, the penetration of NSGs can be very high during certain parts of the year. However, high levels of NSGs in a grid present challenges with respect to frequency stability, voltage stability, short circuit power levels and harmonic stability.

[0005] In order to understand the frequency control problem in a grid with high NSG penetration, it is instructive to review the dynamics of synchronous machines. By rough approximation, a synchronous grid can be considered as a single synchronous machine with a turbine in which mechanical power is being generated and which is connected to an electrical load. The electromechanical equations for a single synchronous machine (SM) can be written as:

[0006]

[0007] Where J is the moment of inertia [kg*m 2 ],ω m is the speed of the machine [rad / s], P m is the mechanical power supplied from the turbine [W], and P e is the output electrical power [W].

[0008] In the power grid, it is necessary to calculate the instantaneous load (P e )Adjust the power generation (P m ) to constantly control the grid frequency. If the mechanical power supplied from the turbine is greater than the output electrical power (P m >P e ), the grid frequency will increase. If the mechanical power supplied from the turbine is less than the output electrical power (P m <P e ), the grid frequency will decrease. The grid frequency is usually maintained within a narrow frequency range around the nominal grid frequency (50 Hz for European grids and 60 Hz for the United States) by automatically controlling the amount of power generated.

[0009] In grids with high NSG penetration, synchronous generation is disconnected from the grid and the moment of inertia in the grid is reduced. This means that the effective moment of inertia J in the equation given above is reduced. This in turn leads to faster frequency changes in case of a mismatch between generated and consumed power (e.g. due to a tripping event of a large generator in the grid). In order to keep the grid frequency in the same range for low inertia grids, a faster adjustment of the active power balance is required.

[0010] To address the challenges of frequency control, which is associated with the low amount of rotational inertia in NSG-rich grids, transmission system operators are using several mitigation methods. One such mitigation method utilizes fast frequency control units that are able to react very quickly when the grid frequency deviates outside the dead band (i.e., the band of input values ​​in the control system where the output is zero (the output is 'dead' - no action occurs)). Examples of units that can provide fast frequency response (FFR) are battery energy storage units (BESS) and HVDC links.

[0011] Another mitigation method that can be used to control frequency in the grid is demand response, which the U.S. Federal Energy Regulatory Commission (FERC) defines as: "a change in end-customer electricity use from its normal consumption pattern in response to changes in electricity prices over time or to incentive payments designed to induce a reduction in electricity use during times of high wholesale market electricity prices or when system reliability is threatened." Controlled load reduction is widely adopted as a last alternative to save the grid from blackouts after severe and unexpected faults in the grid.

[0012] Rapid control of loads in the grid (i.e., fast demand response) is an alternative solution to provide fast frequency response to improve frequency suppression in a grid with increasing NSG levels and decreasing inertia levels. For example, Yue Guo's doctoral thesis entitled "Evaluation of the Capacity of Flexible Tap-changing Technology in Distribution Networks for Transmission Network VAr Support and Demand Response" (published in 2017 and available from the Manchester University Library) discusses flexible tap-changing technology that utilizes parallel transformers in the distribution network to provide reactive power absorption and demand response services for the transmission system.

[0013] The authors of this PhD thesis found that demand reductions can be achieved without violating voltage constraints in the distribution network and suggest voltage drops in the range of 1-4%. The impact on the load is directly related to the voltage dependence of the load in the distribution network. As part of this PhD work, different load characteristics were investigated and it was found that the change in active power demand caused by a voltage drop is roughly between 0 (constant active power load) and 5% (constant impedance load). In many cases, the load can be reduced by several percentage points without violating the voltage constraints in the distribution network under study.

[0014] Additionally, the article by A. Ballanti and L. F. Ochoa entitled "Preliminary Evaluation of Voltage-Led Demand Response for Residential Loads in the United Kingdom" (published at the IEEE Power & Energy Society Innovative Smart Grid Technologies Conference (ISGT), 2015, pp. 1-5) presents two load modeling approaches (time-independent and time-varying) and applies them to a single primary substation, and then extends these approaches to the entire distribution network operator domain. The results for a time-varying model, pure residential loads, and a 3% voltage drop during a winter day indicate that it is possible to achieve a total peak reduction of more than 150 MW (i.e., more than 3%).

[0015] These prior art documents show that a certain degree of slow control of the load in the distribution network can be achieved by controlling the on-load tap changer (OLTC) of the distribution transformer to adjust the voltage without violating the voltage control requirements of the power grid. However, considering the slow time response of the OLTC, the standard distribution transformer with OLTC cannot provide fast demand response and therefore cannot participate in the fast frequency control action of the power grid, which is increasingly required in today's power grid.

[0016] An article titled “Smart Transformer Modeling and Semi-physical Verification” by Tu Yuxiaoying et al. (2019 IEEE 10th International Symposium on Power Electronics for Distributed Generation Systems (PEDG), June 3, 2019, pp. 1019-1025) discloses a solid-state transformer with fully rated AC / DC, DC / DC, and DC / AC stages to change the voltage on the load side, thereby achieving a change in load power, which is then controlled to support the main grid frequency.

[0017] An article by Marius Langwasser et al. titled “Frequency support based on smart transformers under variable inertia conditions” (IEEE 13th International Conference on Compatibility, Power Electronics and Power Engineering, April 23, 2019, pp. 1-6) discloses a solid-state transformer with fully rated AC / DC and DC / AC stages to control downstream load voltage in order to support grid frequency.

[0018] An article by Carlos R. Baier et al. titled "Hybrid Transformer with Virtual Inertia for Future Distribution Networks" (45th Annual IEEE Industrial Electronics Society Conference, Vol. 1, October 14, 2019, pp. 6767-6772) discloses a fractional rated AC / DC-DC / AC converter arrangement connected across a main transformer in a parallel-like manner. Additional DC side energy storage (super-large DC capacitors) is used in the converter to provide active power / frequency support. The hybrid transformer described in the article is a combination of a dc link (ac-dc-ac) connected in parallel with a transformer. The converter is connected to both sides of the transformer.

[0019] The article titled "Power System Voltage Regulation Frequency Control Using SVC Devices" by Mohammed Ahsan Adib Murad et al. (IEEE Power and Energy Society General Meeting (PESGM), August 4, 2019, pp. 1-5) focuses on voltage-based frequency control (VFC) of static VAR compensators (SVCs) (shunt compensation). The proposed VFC scheme exclusively utilizes local voltage and frequency measurements. The authors find that simulation results indicate that even a small number of SVCs can effectively support primary frequency control without making the voltage response of the large network worse. Summary of the invention

[0020] It is an object of the present invention to provide an improved method for controlling the output voltage of at least one transformer in a power grid.

[0021] This object is achieved by a method comprising the method steps recited in claim 1. The method comprises the steps of providing said at least one transformer having a power electronic converter and an output voltage controller; determining (i.e. measuring, estimating, calculating or receiving) a grid frequency of the grid; comparing the determined grid frequency with a reference value; generating an error signal corresponding to the difference between the determined grid frequency and the reference value; applying the error signal as feedback to the output voltage controller; and generating a control action to cause the output voltage controller to change the output voltage of said at least one transformer comprising the power electronic converter so as to limit the difference between the grid frequency and the reference value (i.e. limiting the maximum frequency deviation), whereby the power electronic converter is an AC-AC converter, and the method comprises the steps of connecting the AC-AC converter to the primary side or the secondary side of the transformer, whereby the AC-AC converter has a first AC side and a second AC side, and the method comprises the steps of connecting two terminals on the first AC side in parallel to a winding or a part of a winding of the transformer; and connecting the second AC side directly to an external circuit line or in series with an external circuit line.

[0022] Such a topology makes it possible to use power electronic converters with a (fractional) rating less than the total throughput power of the at least one transformer. This is in contrast to the topologies disclosed in the prior art, which either include fully rated converters (i.e. converters with a rating equal to the throughput power of the transformer) or disclose converters with a fractional rating, but which are connected to both sides of the transformer. Thus, the topology described above will result in improvements in implementation, control and cost compared to known topologies.

[0023] The method uses a measurement of grid frequency to control an output voltage of a power electronic enhanced transformer (PEET) such that the output voltage of the PEET is a function of the grid frequency. When the grid frequency drops below a predetermined reference value or outside a predetermined range of predetermined frequency values ​​(such as a predefined dead band), the output voltage of the at least one transformer may be reduced, and when the grid frequency increases above a predetermined reference value or a predetermined range of grid frequency values, the output voltage of the at least one transformer may be increased.

[0024] That is, the goal of the control method according to the present invention is to develop a model or algorithm that governs the application of inputs to the grid system to drive the system to a desired state while minimizing any delays, overshoots, or steady-state errors and ensuring a degree of control stability in order to achieve a certain degree of optimality. To do this, a control method or control system with the necessary corrective behavior is required. The control method and control system according to the present invention use feedback control to monitor the process variable (i.e., grid frequency), for example by using a sensor to obtain a measurement of the grid frequency or by receiving a grid frequency estimate from a remote location. A local controller can be used to estimate the relationship between the output voltage of a transformer in the grid and the active power drawn by the load downstream of the transformer, so as to know in advance the demand response active power changes that can be achieved through voltage changes.

[0025] According to an embodiment of the present invention, a rating (fractional rating) of the power electronic converter is smaller than a total throughput power of one transformer of the at least one transformer, such as a main transformer of the at least one transformer.

[0026] According to an embodiment of the invention, the AC-AC inverter comprises a DC bus.

[0027] According to an embodiment of the invention, the transformer comprises a plurality of windings (such as two, three, four or more windings) which are isolated from each other. The method thus comprises the following steps: connecting two terminals on a first AC side of an AC-AC converter in parallel to a winding or a part of one of the windings of the transformer.

[0028] According to an embodiment of the invention, the step of determining a grid frequency of the electrical grid comprises the step of estimating the local grid frequency using a phase locked loop (PLL) of the power electronic converter.

[0029] Alternatively, the step of determining the grid frequency of the grid comprises the step of: the output voltage controller receiving a grid frequency estimate via a communication channel from a remote location such as a control center of a transmission service operator. That is, the grid frequency estimate is received from a remote location rather than being measured locally.

[0030] According to an embodiment of the invention, at least one method step or the entire method may be implemented at a remote location, whereby control actions are sent to the output voltage controller via a communication channel. That is, a single control system may be used to control the output voltage of multiple transformers located in different locations in a power grid.

[0031] According to an embodiment of the invention, the method comprises the step of continuously determining the active throughput power and the output voltage of the at least one transformer in order to estimate the voltage dependency of downstream loads in the power grid.

[0032] According to an embodiment of the present invention, the method comprises the following steps: controlling a plurality of transformers connected in parallel, whereby control actions for changing the output voltages of the plurality of transformers are simultaneously sent to the plurality of transformers. Depending on the communication between the output voltage controller and the local power electronic converters of the plurality of transformers, the method may comprise the following steps: sending a synchronization signal to synchronize the clocks of the local output voltage controllers of the power electronic converters of the plurality of transformers.

[0033] According to an embodiment of the invention, the step of determining the grid frequency comprises the step of determining the grid frequency at a plurality of different locations in the grid in order to improve the accuracy of the grid frequency determination.

[0034] The invention also relates to a computer program product comprising a computer program containing computer program code means arranged to cause a computer or a processor to perform all the steps of the method according to any embodiment of the invention, the computer program being stored on a computer readable medium or carrier wave.

[0035] The invention further relates to a control system for controlling the output voltage of at least one transformer in a power grid. The at least one transformer comprises a power electronic converter, and the control system comprises an output voltage controller and a sensor, the sensor being configured to determine a grid frequency of the power grid (i.e., the sensor being configured to measure, estimate or receive the grid frequency of the power grid). The control system is configured to: compare the determined grid frequency with a reference value; generate an error signal corresponding to the difference between the determined grid frequency and the reference value and apply the error signal as feedback to the output voltage controller; and cause the output voltage controller to generate a control action to change the output voltage of the at least one transformer comprising the power electronic converter, thereby limiting the difference between the grid frequency and the reference value, whereby the power electronic converter is an AC-AC converter connected to the primary side or the secondary side of the transformer, whereby the AC-AC converter has a first AC side and a second AC side, and two terminals on the first AC side are connected in parallel to a winding or a part of a winding of the transformer, and the second AC side is directly connected to an external circuit line or connected in series with the external circuit line.

[0036] The reference value may be a nominal grid frequency or a value indicative of a maximum or minimum allowed grid frequency, such as an upper or lower limit of a deadband frequency range.

[0037] According to an embodiment of the invention, the rating of the power electronic converter is smaller than the total throughput power of a main transformer of the at least one transformer, such as the main transformer of the at least one transformer.

[0038] According to an embodiment of the invention, the AC-AC inverter comprises a DC bus.

[0039] According to an embodiment of the invention, the transformer comprises a plurality of windings (such as two, three, four or more windings) which are isolated from each other. The two terminals on the first AC side of the AC-AC converter of the control system are thereby connected in parallel to one winding or a part of one of the windings of the transformer.

[0040] According to an embodiment of the invention, the power electronic converter comprises a phase locked loop (PLL) configured to determine a grid frequency of the grid by estimating a local grid frequency.

[0041] According to an embodiment of the invention, the control system comprises a communication channel configured to receive a grid frequency estimate from a remote location such as a control center of a transmission service operator, and the control system is configured to determine a grid frequency of the grid using the grid frequency estimate.

[0042] According to an embodiment of the invention, the control system comprises a communication channel configured to receive the generated control action from a remote location such as a control center of a transmission service operator.

[0043] According to an embodiment of the invention, the control system comprises an instrument configured to continuously measure the active throughput power and the output voltage of the at least one transformer, and the control system is configured to use these measurements to estimate the voltage dependency of downstream loads in the power grid.

[0044] According to an embodiment of the present invention, the control system is configured to control the output voltage of a plurality of transformers, the plurality of transformers are connected in parallel and include power electronic converters, and the output voltage controller is configured to send control actions for changing the output voltage of the plurality of transformers to the plurality of transformers simultaneously. Depending on the communication between the output voltage controller and the local power electronic converters of the plurality of transformers, the control system may be configured to send a synchronization signal to synchronize the clocks of the local output voltage controllers of the power electronic converters of the plurality of transformers.

[0045] The invention also relates to the use of a method or a computer program product or a control system according to any of the embodiments described herein for controlling active power consumption within an electrical network. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] The invention will be further explained hereinafter by means of non-limiting examples with reference to the accompanying drawings, in which:

[0047] Figure 1 Four topologies of PEET are shown,

[0048] Figure 2A system for controlling the output voltage of a PEET using local grid frequency determination according to an embodiment of the present invention is schematically shown.

[0049] Figure 3 A system for controlling the output voltage of a PEET using remote grid frequency determination according to another embodiment of the present invention is schematically shown, and

[0050] Figure 4 is a flow chart showing the steps of a method according to an embodiment of the present invention.

[0051] It should be noted that the drawings are not necessarily drawn to scale and that the dimensions of certain features may have been exaggerated for the sake of clarity. DETAILED DESCRIPTION

[0052] Figure 1 Four topologies of transformers 10 having power electronic converters 12 (i.e., four power electronic enhanced transformers (PEETs)) are shown, whereby the power electronic converter 12 is an AC-AC converter, which optionally includes a DC bus. The AC-AC converter is connected to the primary side or the secondary side of the transformer 10. The AC-AC converter has a first AC side and a second AC side, and two terminals on the first AC side are connected in parallel to a portion of the winding of the transformer 10, and the second AC side is directly connected to an external circuit line or connected in series with the external circuit line.

[0053] Figure 1 a shows an AC-AC converter topology, which includes a DC bus. The voltage terminal on the first AC side is connected in parallel to one of the windings of the transformer 10, whereby the transformer 10 has two windings on the PEET side of the transformer 10, which are isolated from each other. The voltage terminal on the second AC side of the AC-AC converter is connected in series with the external circuit line.

[0054] Figure 1 b shows an AC-AC converter topology, which includes a DC bus. The voltage terminals on the first AC side are connected in parallel to a part of the winding of the transformer 10. The voltage terminals on the second AC side of the AC-AC converter are directly connected to the external circuit lines.

[0055] Figure 1 The topology shown in a-1d preferably comprises a fractional rated converter arrangement connected across the transformer 10.

[0056] Figure 1c shows an AC-AC converter topology, in which the voltage terminal on the first AC side is connected in parallel to one of the windings of the transformer 10, whereby the transformer 10 has two windings on the PEET side of the transformer 10, the two windings are isolated from each other, and the voltage terminal on the second AC side of the AC-AC converter is connected in series with the external circuit line.

[0057] Figure 1 d shows an AC-AC converter topology, wherein the voltage terminals on the first AC side are connected in parallel to a part of the winding of the transformer 10, and the voltage terminals on the second AC side of the AC-AC converter are directly connected to the external circuit line.

[0058] Depending on the application requirements, one or more filters may be included or omitted.

[0059] exist Figure 1 a) and Figure 1 In the topology shown in b), the present invention uses a combination of series active / reactive compensation and parallel reactive compensation. This means that both types of compensation are combined in a single device.

[0060] Figure 1 c) and Figure 1 The topology shown in d) allows the voltage to be changed only by injecting it in series and keeping it consistent with the transformer voltage.

[0061] The power electronic converter 12 as described herein may include any suitable power electronic components (such as SCR, TRIAC, IGBT) to control and convert power. Silicon-based semiconductor devices and / or wide bandgap semiconductor devices (e.g., silicon carbide) may be used in the power electronic converter 12. The advantage of the latter type of semiconductor device is that it can be switched faster, which is beneficial to the response time of the power electronic converter 12 and the size of the filter that can be used. That is, faster switching can result in a shorter response time, and therefore faster frequency regulation and a more compact filter.

[0062] According to an embodiment of the present invention, the power electronic converter 12 is an AC to AC converter, which connects an AC power source to an AC load by controlling the amount of power supplied to the load. The power electronic converter 12 converts an AC voltage at one level to another level by changing the amplitude and frequency of the supply voltage. For AC voltages with the same frequency, the power electronic converter 12 can also change the phase of the AC voltage.

[0063] Such a power electronic converter 12 may be used in an uninterruptible power supply, high power AC to AC transmission, or renewable energy conversion system.

[0064] By replacing the transformer's on-load tap changer (OLTC) with a power electronic converter 12 to regulate the voltage on the secondary side of the transformer 10, the inventors have found that it is possible to control the output voltage of the transformer 10 much faster than using an OLTC solution.

[0065] The power electronic enhanced transformer (PEET) used in the method according to the present invention has a much faster response time when it comes to adjusting the voltage on the low voltage (LV) side of the transformer 10 compared to a transformer with an OLTC. This enables the PEET to provide very fast demand response when needed. The output voltage on the LV side of the PEET can be adjusted very quickly to achieve demand response in active power loads. For example, when the grid frequency in the grid decreases below a predetermined reference value (such as a threshold value according to the transmission system operator's requirements for FFR services), a voltage reduction resulting in load reduction can be quickly applied.

[0066] It should be noted that if the power electronic converter 12 is connected to the high voltage (HV) side of the transformer 10, the power electronic enhanced transformer (PEET) used in the method according to the invention will also have a much faster response time when it comes to regulating the voltage on the high voltage (HV) side of the transformer 10. Figure 1 In all the topologies shown in , therefore, the power electronic converter 12 can be connected either to the LV side of the transformer 10 or to the HV side of the transformer 10. From the perspective of compactness, it is preferred to connect the power electronic converter 12 to the LV side of the transformer 10.

[0067] Furthermore, the control system according to the present invention may be cheaper to implement compared to a conventional battery energy storage unit (BESS) since no energy storage is required.

[0068] Figure 2 A control system 14 for controlling the output voltage of a PEET using a local grid frequency determination according to an embodiment of the present invention is schematically shown. The system 14 comprises an output voltage controller 16 (which may be regarded as an FFR controller) and a controller for measuring the local grid frequency f 测量 Sensor 18. Local grid frequency f 测量 It can be estimated, for example, by a phase-locked loop (PLL) of the power electronic converter 12 of the PEET 10. The grid frequency f 测量 It may also be determined at a plurality of different locations in the grid and an average value may be determined in order to improve the accuracy of the grid frequency determination.

[0069] The transformer 10 having a power electronic converter 12 as described herein may be part of a power grid including distributed energy resources (DERs), such as distributed renewable energy resources. The power grid may serve the needs of one or more customers and may include at least one controllable load.

[0070] The control system 14 is configured to convert the measured grid frequency f 测量 The frequency of the power grid is compared with a reference value of the power grid frequency. For example, the reference value may be the nominal frequency of the power grid (such as 50 Hz). The system 14 is configured to generate a frequency corresponding to the determined power grid frequency f 测量 and applying the error signal as feedback to the output voltage controller 14. If the grid frequency f 测量 If the frequency of the PEET 10 exceeds the predefined deadband (e.g., below 49.9 Hz or above 50.1 Hz), indicating that an event such as a trip event of a large generator in the grid may have occurred, the control system 14 is configured to generate a control action to cause the output voltage controller 14 to change the output voltage of the PEET 10, thereby limiting the grid frequency f 测量 The primary control can be used to stabilize the grid frequency f 测量 , the secondary control can be used to make the grid frequency f 测量 The tertiary control can be used to restore the power reserve of the generator for the secondary frequency control and to restore the power reserve of the generator for the primary frequency control.

[0071] If the measured grid frequency f 测量 In the range of 49.9 Hz to 50.1 Hz, no control action will be generated. Thus, “control action” as used in this document may be “no action”, ie there is no action to change the output voltage of the transformer 10 .

[0072] If a control action to change the output of the transformer 10 is necessary, the output voltage on the low voltage side of the PEET 10 (with a reference value U 参考 ) to achieve demand response in active power loads.

[0073] Control actions from other controllers, such as frequency controllers of generators and possibly other demand response schemes, may be used to bring the grid frequency to the same value as the reference value or to within a predetermined deviation of the reference value.

[0074] Figure 3 A schematic diagram of a method for utilizing a remotely determined grid frequency f according to another embodiment of the present invention is shown. 系统The control system 14 includes a communication channel 20 configured to receive a grid frequency estimate f from a remote location such as a control center of a transmission service operator. 系统 , and the control system 14 is configured to use the grid frequency estimate to determine the grid frequency of the grid. That is, the grid frequency is the same throughout the grid, except for local short-term oscillations. At each moment, the amount of electricity generated by the power plants of the grid must be equal to the amount of energy drawn from the grid by the consumers.

[0075] The control system 14 according to the invention may comprise a device for utilizing the local grid frequency f 测量 A sensor 18, and a sensor for utilizing a remote grid frequency f 系统 The communication channel 20 for both.

[0076] The communication channel 20 may also be used to receive control actions from a remote location, whereby the steps of the control method according to the invention are implemented at the remote location.

[0077] It should be noted that the components of the control system 14 according to the invention do not necessarily need to be located at the same location. The control system 14 may comprise a single unit or a plurality of units located in one or more locations for implementing the steps of the method according to the invention.

[0078] It would be important to estimate the voltage dependency of the loads in the grid to which the transformer 10 with the power electronic converter 12 is connected in order to know how much demand response is available at any given time. According to an embodiment of the invention, the control system 14 comprises an instrument for continuously determining the active throughput power and the output voltage of the at least one transformer 10 in order to estimate the voltage dependency of the downstream loads in the grid. For example, the purpose of such method steps may be to determine the exponent np in the following equation based on the measured values ​​of the output voltage of the transformer 10 and the throughput power P of the transformer 10,

[0079]

[0080] Where V0 is the nominal voltage and P0 is the base power.

[0081] P0 will change continuously in the grid as loads are continuously switched on and off. Therefore, it is most reliable to use the step response in active power obtained when controlling the transformer 10 to change the output voltage as the main input for estimating the voltage dependence of the load. In many cases, P0 can be considered as a constant during a short period of time before and after the transformer output voltage changes. The response in voltage and power recorded during this time can then be used to estimate the load characteristics.

[0082] There will always be uncertainty in the predictive assessment of available demand response capacity that can be obtained by the method according to the present invention. The amount of active power response (e.g., load reduction) will depend on the load of the transformer at the time of FFR activation and the voltage dependency of the downstream load. The load pattern in the grid is usually continuously monitored and predicted. This information can be used together with previous knowledge of the range of load voltage dependency characteristics and potentially also online estimation data thereof to obtain a good estimate of the available demand response capacity released by the transformer 10.

[0083] According to an embodiment of the present invention, a control system 14 according to the present invention may be used to control a plurality of transformers 10 connected in parallel, whereby control actions for changing the output voltages of the plurality of transformers 10 are simultaneously sent to the plurality of transformers.

[0084] In the case where a PEET is connected in parallel to one or more conventional transformers equipped with an OLTC, the method according to the present invention may have limitations. In such cases, it may not be suitable to quickly change the output voltage of the PEET, because this will cause the secondary side voltage of the PEET to differ compared to the conventional transformer connected in parallel (because the OLTC cannot be controlled as quickly as the PEET). This may cause circulating currents between the transformers, which is unacceptable in some cases. However, in the case of multiple PEETs connected in parallel, since the secondary side voltages of all PEETs can be changed simultaneously, such problems should not occur.

[0085] Figure 4 The steps of a method for controlling the output voltage of at least one transformer 10 in a power grid are shown. A computer program product can be used to cause a computer or processor to perform all the steps of the method. The method provides a decentralized control strategy for power grid components, which utilizes changes in local or remote grid frequency to control active power consumption within the grid. Using a transformer 10 with a power electronic converter 12 controlled using the method according to the invention allows limiting the active power flow to the grid to a desired value.

[0086] Further modifications of the invention within the scope of the claims will be apparent to the skilled person.

Claims

1. A method for controlling the output voltage of at least one transformer (10) in a power grid, wherein: The method comprises the steps of: providing the at least one transformer (10) having: a power electronic converter (12), thereby providing at least one power electronic enhanced transformer (PEET), and an output voltage controller (16); determining a grid frequency of the grid; comparing the determined grid frequency with a reference value; generating an error signal, the error signal corresponding to a difference between the determined grid frequency and the reference value; applying the error signal as feedback to the output voltage controller (16); and generating a control action to cause the output voltage controller (16) to change the grid frequency of the grid comprising the power electronic converter (12). The method comprises the steps of: connecting the AC-AC converter to the primary side or the secondary side of the transformer (10); wherein the AC-AC converter has a first AC side and a second AC side; and connecting two terminals on the first AC side in parallel to a winding or a part of a winding of the transformer (10); and connecting the second AC side directly to an external circuit line or in series with an external circuit line, Characterized in that the at least one PEET transformer has one of the following topologies: a) the voltage terminal on the first AC side is connected in parallel to one of the windings of the transformer (10), wherein the transformer (10) has two windings on the PEET side of the transformer (10), the two windings being isolated from each other, and the voltage terminal on the second AC side of the AC-AC inverter is connected in series with the external circuit line, b) The voltage terminal on the first AC side is connected in parallel to a portion of the winding of the transformer (10), and the voltage terminal on the second AC side of the AC-AC converter is directly connected to the external circuit line.

2. The method according to claim 1, characterized in that The rating of the power electronic converter (12) is less than the total throughput power of the main transformer in the at least one transformer (10).

3. The method according to claim 1, characterized in that: The AC-AC inverter includes a DC bus.

4. The method according to claim 1, characterized in that: The transformer (10) includes a plurality of windings isolated from each other.

5. The method according to claim 1, characterized in that The step of determining the grid frequency of the grid comprises the steps of estimating a local grid frequency (f 测量 ).

6. The method according to any one of claims 1 to 4, characterized in that The step of determining the grid frequency of the grid comprises the steps of: the output voltage controller (16) receiving a grid frequency estimate (f 系统 ).

7. The method according to any one of claims 1 to 4, characterized in that The method is implemented at a remote location and the control actions are sent to the output voltage controller (16) via a communication channel (20).

8. The method according to any one of claims 1 to 4, characterized in that The method comprises the steps of continuously determining the active throughput power and the output voltage of the at least one transformer (10) in order to estimate the voltage dependency of downstream loads in the power grid.

9. The method according to any one of claims 1 to 4, characterized in that The method comprises the following steps: controlling a plurality of transformers connected in parallel, wherein the control action for changing the output voltages of the plurality of transformers is sent to the plurality of transformers simultaneously, and optionally, the method comprises the following steps: sending a synchronization signal to synchronize the clocks of local output voltage controllers (16) of the power electronic converters (12) of the plurality of transformers.

10. The method according to any one of claims 1 to 4, characterized in that Determine the grid frequency (f 测量 , f 系统 ) comprises the following steps: determining the grid frequency at a plurality of different locations in the grid.

11. A computer program product, characterized in that The computer program product comprises a computer program containing computer program code means arranged to cause a computer or a processor to perform all the steps of the method according to any one of claims 1 to 10, the computer program being stored on a computer readable medium or a carrier wave.

12. A control system for controlling the output voltage of at least one transformer (10) in a power grid, wherein the at least one transformer (10) comprises a power electronic converter (12) and thereby constitutes at least one power electronic enhanced transformer (PEET), and the control system comprises an output voltage controller (16) and a sensor (18), the sensor being configured to determine a grid frequency of the power grid, and the control system being configured to: compare the determined grid frequency with a reference value; generate an error signal corresponding to the difference between the determined grid frequency and the reference value and apply the error signal as feedback to the output voltage controller (16); and causing the output voltage controller (16) to generate a control action to change the output voltage of the at least one transformer (10) including a power electronic converter (12) so as to limit the difference between the grid frequency and the reference value, wherein the power electronic converter (12) is an AC-AC converter connected to the primary side or the secondary side of the transformer (10), wherein the AC-AC converter has a first AC side and a second AC side, and two terminals on the first AC side are connected in parallel to a winding or a part of a winding of the transformer (10), and the second AC side is directly connected to an external circuit line or is connected in series with an external circuit line, It is characterized in that The at least one PEET transformer has one of the following topologies: a) the voltage terminal on the first AC side is connected in parallel to one of the windings of the transformer (10), wherein the transformer (10) has two windings on the PEET side of the transformer (10), the two windings being isolated from each other, and the voltage terminal on the second AC side of the AC-AC inverter is connected in series with the external circuit line, b) The voltage terminal on the first AC side is connected in parallel to a portion of the winding of the transformer (10), and the voltage terminal on the second AC side of the AC-AC converter is directly connected to the external circuit line.

13. The control system according to claim 12, characterized in that: The power electronic converter (12) has a rating that is less than the total throughput power of one of the at least one transformer (10).

14. The control system according to claim 12, characterized in that: The AC-AC inverter includes a DC bus.

15. The control system according to claim 12, characterized in that: The transformer (10) includes a plurality of windings isolated from each other.

16. The control system according to claim 12, characterized in that: The power electronic converter (12) comprises a phase locked loop (PLL) configured to estimate the local grid frequency (f 测量 ) to determine the grid frequency of the grid.

17. The control system according to any one of claims 12 to 16, characterized in that: The control system includes a communication channel (20) configured to receive a grid frequency estimate (f 系统 ), and the control system is configured to use the grid frequency estimate (f 系统 ) to determine the grid frequency of the grid.

18. The control system according to any one of claims 12 to 16, characterized in that: The control system includes a communication channel (20) configured to receive the generated control action from a remote location such as a control center of a transmission service operator.

19. The control system according to any one of claims 12 to 16, characterized in that: The control system comprises an instrument configured to continuously measure the active throughput power and the output voltage of the at least one transformer (10), and the control system is configured to use the measured values ​​to estimate the voltage dependency of a downstream load in the power grid.

20. The control system according to any one of claims 12 to 16, characterized in that: The control system is configured to control the output voltage of a plurality of transformers, the plurality of transformers being connected in parallel and comprising power electronic converters (12), and the output voltage controller (16) being configured to simultaneously send the control action for changing the output voltage of the plurality of transformers to the plurality of transformers, and optionally, the control system is configured to send a synchronization signal to synchronize the clocks of the local output voltage controllers (16) of the power electronic converters (12) of the plurality of transformers.

21. Use of a method according to any one of claims 1 to 10, a computer program product according to claim 11, or a control system according to any one of claims 12 to 20 for controlling active power consumption in a power grid.

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