Method for suppressing subsequent commutation failure of back-to-back direct current system

Abstract

The invention discloses a method for suppressing subsequent commutation failure of a back-to-back direct current system. A direct current receiving end converter station turn-off angle deviation signal is added under the constant voltage control of the STATCOM through additional turn-off angle control, and the function of suppressing the subsequent commutation failure of the system is achieved, and the method comprises the steps that the direct current operation state is judged according to the alternating current bus voltage, monitored in real time, of the direct current receiving end converter station; when the direct current commutation failure detection logic judges that the direct current has a commutation failure fault, a turn-off angle error signal of the direct current converter station is transmitted to an additional turn-off angle controller of the STATCOM in the near-region AVC substation through a channel; and the additional turn-off angle controller gains the signal and inputs the gained signal into a constant voltage for control, so that the reactive power output by the STATCOM is changed. In addition, on the basis of the output characteristics of the STATCOM under different working conditions, a setting principle for adding the upper limit of the gain coefficient of the turn-off angle controller is provided. The method can effectively inhibit the subsequent commutation failure of the back-to-back DC system, and has economical and practical values.

Description

Technical Field

[0001] The present invention relates to the technical field of power systems, and in particular to a method for suppressing subsequent commutation failure of a back-to-back DC system. Background Art

[0002] my country's western region is rich in wind power, photovoltaic and hydropower resources. By building a high-voltage direct current transmission project based on grid-commutating converters, the problem of the inverse geographical distribution of electric energy and the eastern load center has been solved to a certain extent.

[0003] In recent years, with the large-scale grid connection of new energy sources such as wind and solar, the proportion of new energy units in the DC receiving power grid has increased, and the number of thermal power units has decreased. The receiving power grid has shown the phenomenon of low grid voltage support capacity with "heavy load, small amount of thermal power, and high proportion of new energy". If a DC phase change failure occurs, on the one hand, the transmission power will be lost, and on the other hand, it will bring a greater impact to the weak AC power grid at the receiving end. If it is not properly adjusted, affected by the change of the commutation bus voltage and DC control, the reactive power impact caused by the converter station to the AC system is likely to cause subsequent phase change failures, and even cause successive phase change failures in the multi-feed DC system. Seriously endanger the reliability of new energy power electronic devices such as wind power and photovoltaics. For the receiving power grid with a relatively low short-circuit ratio and insufficient reactive compensation, phase change failure may lead to "N-1" failure, large-scale grid disconnection accidents, and thus trigger chain failures of the power grid. Therefore, it is urgent to take necessary measures to improve the dynamic reactive support capacity of the receiving system to reduce the hidden dangers of system instability caused by phase change failure.

[0004] At present, the research on suppressing the subsequent commutation failure of back-to-back DC systems focuses on optimizing DC control and supplementing the reactive power of the receiving end. According to the paper "Chao Wujie, Deng Chaoping, Huang Junwei, Li Yuanqi, Wang Yuhong, Chen Liwei, "A variable slope current deviation control method for suppressing subsequent commutation failure", Electric Power Construction", the larger the slope of the ramp function in the current deviation control, the more sensitive it is to the input current deviation and the larger the increment of the commutation margin, but due to the limitation of the variable range of the slope, the role that this method can play is relatively limited. According to the paper "Wang Juanjuan, Zheng Ruina, Fu Chuang, Wu Qiumei, "Subsequent commutation failure suppression method based on dynamic reactive power control of inverter station", Transactions of China Electrotechnical Society", the parameters of the low-voltage current limiting unit on the rectifier side are adjusted, and the shutdown angle command is calculated in real time on the inverter side according to the measured AC bus voltage and DC current, so as to control the reactive power of the inverter station during the fault. However, for a DC system that is already in transient operation, there are errors in collecting the instantaneous values ​​of various electrical quantities when calculating and modifying the shutdown angle command value, and the calculation is cumbersome, which does not have good economic and practical value.

[0005] In actual engineering sites, the configuration of reactive power compensation equipment is the most widely used solution to suppress commutation failure, including the configuration and commissioning of reactive power compensation equipment such as fixed capacitors, static synchronous compensators, and synchronous condensers. The grid source combined voltage regulation scheme that coordinates power supply and capacitance can increase the reactive output capacity of the generator by adjusting the reactive output of the main power supply and the reactive compensation capacity of the grid capacitor. However, this scheme requires more static capacitor compensators to be equipped in each power plant, which reduces the power factor of the system during steady-state operation. Synchronous condensers have excellent spontaneous response reactive power, can provide short-circuit capacity, and have stronger applicability in the scenario of weak receiving-end systems. Additional configuration of distributed condensers can realize targeted planning of reactive power compensation capacity in weak grid areas, and the optimized coordinated configuration scheme of condensers for emergency control can improve the transient voltage stability of the system. However, the investment cost required for the synchronous condenser configuration scheme is no less than that of the static capacitor compensator, and the synchronous condenser needs to be maintained all year round, and the economy and equipment stability are poor. Equipping STATCOM in DC converter stations or new energy stations has good flexibility.

[0006] The control mode of STATCOM can be switched between constant voltage control and constant reactive power control. When working in constant voltage control mode, the reactive power output of STATCOM may not be optimal due to different working conditions and the reactive power interaction between the converter station and the receiving system, which increases the probability of commutation failure in the receiving system. Therefore, it is necessary to improve the control strategy of STATCOM so that it can not only ensure reactive power output under normal operation, but also quickly respond to the reactive power demand of the system under large disturbances.

[0007] Based on the above research, the present invention proposes a method for suppressing the subsequent commutation failure of a back-to-back DC system. By real-time monitoring of the AC bus voltage level of the DC receiving-end converter station, the STATCOM additional shutdown angle control is implemented to respond in real time to the system reactive power shortage caused by the DC receiving-end grounding fault, thereby effectively improving the voltage stability of the back-to-back DC receiving-end power grid system. This method does not affect the reactive output of the STATCOM during normal DC operation, and can coordinate and suppress the subsequent commutation failure of the receiving-end system when the DC commutation fails, which has economic and practical value. Summary of the invention

[0008] The technical problem to be solved by the present invention is to propose a method for suppressing subsequent commutation failure of a back-to-back DC system, so as to solve the problem of insufficient grid voltage support of a DC receiving-end grid constructed by the prior art under some working conditions.

[0009] The present invention is achieved by adopting the following technical solutions:

[0010] A method for suppressing subsequent commutation failure of a back-to-back DC system. Commutation failure detection logic determines whether a DC commutation failure fault occurs. When a DC commutation failure fault occurs, a shutdown angle error signal of a DC receiving-end converter station is transmitted through a channel. The STATCOM additional shutdown angle control responds to system voltage fluctuations caused by the DC commutation failure fault in real time, thereby effectively improving the voltage stability of the receiving end of the back-to-back DC system.

[0011] As a preferred embodiment of the present invention, the scheme comprises the following steps:

[0012] S1: Obtaining the operating status of the high-voltage DC transmission system

[0013] According to the AC bus voltage level monitored in real time by the DC converter station, whether a DC commutation failure occurs is determined through the commutation failure detection logic;

[0014] S2: Channel transmission signal

[0015] According to the operating status of the HVDC transmission system, the specific signal of the DC converter station is transmitted to the STATCOM additional shutdown angle controller in the AVC substation through the channel;

[0016] S3: STATCOM additional shutdown angle controller function

[0017] The additional shutdown angle controller of STATCOM changes the voltage set value, thereby changing the reactive power output by STATCOM, and realizing the function of STATCOM to further improve the voltage support capability of the receiving end.

[0018] Preferably, step S2 transmits the turn-off angle error signal of the inverter side of the DC receiving end converter station, and the specific strategy is:

[0019] S21: When the DC commutation failure logic determines that the DC is in a normal operating state or a commutation failure fault state, the DC receiving-end converter station only transmits a digital 1 signal to the additional shutdown angle controller of the STATCOM through the channel;

[0020] S22: When the DC commutation failure detection logic determines that the DC is in a commutation failure fault state, the DC receiving-end converter station transmits a DC receiving-end converter station shutdown angle error signal to the STATCOM additional shutdown angle controller through a channel;

[0021] The DC receiving end converter station turn-off angle error signal is defined as the difference between the DC receiving end converter station turn-off angle reference value and the actual measured minimum value, and its formula is:

[0022] Δγ=γ*-γ (1)

[0023] Where Δγ is the turn-off angle error signal of the DC converter station, which is also the input signal of the additional turn-off angle controller, γ* is the turn-off angle reference value of the DC receiving-end converter station, and γ is the minimum value of the turn-off angle measurement of the DC receiving-end converter station.

[0024] Preferably, in step S3, an additional STATCOM cut-off angle controller is put into use, and the control link of the additional control is a proportional link. The specific strategy is:

[0025] S31: When the input signal of the additional turn-off angle controller of STATCOM is digital 1, it is equivalent to that the additional turn-off angle controller does not work. In this scenario, the input system voltage error signal is defined as the difference between the voltage reference value under the STATCOM constant voltage control and the actual effective value of the voltage at the STATCOM grid connection point. The formula is:

[0026] U err =U ref -U s (2)

[0027] Where U err is the voltage error signal under constant voltage control, U ref is the STATCOM constant voltage reference value, U s is the actual effective value of the voltage at the STATCOM grid connection point;

[0028] S32: When the input signal of the STATCOM additional turn-off angle controller is the turn-off angle error signal of the back-to-back DC converter station, the additional turn-off angle controller takes effect. The input voltage error signal in this scenario is defined as the difference between the voltage reference value under the STATCOM constant voltage control and the actual effective value of the voltage at the STATCOM grid connection point, plus the product of the turn-off angle error signal on the inverter side of the DC converter station and the gain coefficient of the additional turn-off angle controller. The formula is:

[0029] U err =K·Δγ+U ref -U s (3)

[0030] Where K is the additional turn-off angle controller gain coefficient.

[0031] Preferably, the step S3 invests in an additional STATCOM cut-off angle controller, including the output characteristics of the STATCOM under different working conditions and the setting principle of the additional cut-off angle controller gain coefficient K:

[0032] A1: For the STATCOM output expression, different working conditions and output quantity have a certain functional relationship. By simulating and comparing STATCOM under different working conditions, the output characteristic curve can be used to estimate the STATCOM reactive power that can be discovered under a certain working condition:

[0033] A2: Based on equation (3) and the STATCOM output characteristic curve, the selection principle and variation range of the gain coefficient K can be verified through power system time domain simulation.

[0034] The beneficial effects of the present invention are as follows: when the DC is in a normal operating state, the additional shutoff angle controller of STATCOM does not play a role. When the DC is in a commutation failure fault state, the shutoff angle error signal of the DC receiving-end converter station is transmitted through the channel, and the additional shutoff angle controller acts on the constant voltage control of STATCOM, so that STATCOM has the ability to directly respond to DC faults and effectively and flexibly suppress the subsequent commutation failure of the system. Based on the STATCOM output expression and different working conditions, the output characteristics of STATCOM under different working conditions are obtained, so that the role of STATCOM in improving the voltage support capacity of the power grid is fully utilized in parameter configuration. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 A control flow chart of a method for suppressing subsequent commutation failure of a back-to-back DC system provided by the present invention.

[0036] Figure 2 This is a block diagram of the additional shutdown angle control of the STATCOM provided by the present invention.

[0037] Figure 3 A schematic diagram of a reactive power additional controller based on the inverter-side shutdown angle deviation provided by the present invention.

[0038] Figure 4 The structural diagram of a back-to-back DC receiving system example is used to verify the method of suppressing subsequent commutation failure of the back-to-back DC system.

[0039] Figure 5 This is a comparison diagram of the dynamic response of the turn-off angle and reactive power before and after the method of suppressing the subsequent commutation failure of the back-to-back DC system is configured when a commutation failure occurs in the back-to-back DC receiving end system. Among them, Figure 5 (a) is a comparison diagram of the shutdown angle of the converter station. Figure 5 (b) is the comparison diagram of STATCOM output reactive power.

[0040] Figure 6 This is a comparison diagram of the dynamic response of reactive power and DC current before and after configuring a method to suppress subsequent commutation failure of the back-to-back DC system when a single-phase grounding fault occurs in the DC receiving end system. Figure 6 (a) is the reactive power comparison waveform sent from the DC receiving end to the converter station. Figure 6 (b) is the DC current comparison waveform.

[0041] Figure 7This is a comparison diagram of the dynamic response of the turn-off angle and DC current before and after the method of suppressing the subsequent commutation failure of the back-to-back DC system is configured when a three-phase grounding fault occurs in the DC receiving end system. Among them, Figure 7 (a) is the DC receiving end turn-off angle comparison waveform, Figure 7 (b) is the DC current comparison waveform.

[0042] Figure 8 To set the output characteristic curve of STATCOM under different working conditions. Figure 8 (a) is the output characteristic curve when a single-phase grounding fault occurs at the DC receiving end. Figure 8 (b) is the output characteristic curve when a three-phase grounding fault occurs at the DC receiving end. DETAILED DESCRIPTION

[0043] The present invention provides a method for suppressing subsequent commutation failure of a back-to-back DC system. To make the purpose, technical solution and effect of the present invention clearer, the specific implementation scheme of the present invention is described in detail below with reference to the accompanying drawings and examples. The specific examples described in the present invention are only used to explain the present invention and are not used to limit the present invention.

[0044] 1. Introduction to the specific embodiments of the invention

[0045] Figure 1 A flow chart of a method for suppressing subsequent commutation failure of a back-to-back DC system provided by the present invention. Figure 1 The specific steps of the balanced order reduction method described in the present invention are as follows:

[0046] S1: Get the operating status of DC;

[0047] S2: Transmits the turn-off angle error signal of the DC sending-end converter station;

[0048] S3: Enable STATCOM additional shutdown angle controller.

[0049] Further, the method comprises the following specific steps:

[0050] S1: Get the running status of DC

[0051] Figure 2 The additional shutdown angle control block diagram of STATCOM provided by the present invention is as follows: Figure 3 This is a schematic diagram of a reactive power additional controller based on the inverter side turn-off angle deviation provided by the present invention. Figure 2 and Figure 3 The DC commutation failure logic monitors the DC state in real time to determine whether a DC commutation failure occurs.

[0052] S11: When DC is operating normally, the power system voltage remains stable, STATCOM operates normally, and there is no need to invest in an additional shutdown angle controller.

[0053] S12: When a DC commutation failure occurs, the system's reactive power support capacity is insufficient in this scenario, and STATCOM needs to use an additional shutdown angle controller to suppress subsequent commutation failures in the system.

[0054] S2: Transmitting the shutdown angle error signal of the DC sending end converter station

[0055] S21: When the DC commutation failure logic determines that the DC is in a normal operating state or a commutation failure fault state, the DC receiving-end converter station only transmits a digital 1 signal to the additional shutdown angle controller of the STATCOM through the channel;

[0056] S22: When the DC commutation failure detection logic determines that the DC is in a commutation failure fault state, the DC receiving-end converter station transmits a DC receiving-end converter station shutdown angle error signal to the STATCOM additional shutdown angle controller through a channel;

[0057] The DC receiving end converter station turn-off angle error signal is defined as the difference between the DC receiving end converter station turn-off angle reference value and the actual measured minimum value, and its formula is:

[0058] Δγ=γ*-γ (4)

[0059] Where Δγ is the turn-off angle error signal of the DC converter station, which is also the input signal of the additional turn-off angle controller, γ* is the turn-off angle reference value of the DC receiving-end converter station, and γ is the minimum value of the turn-off angle measurement of the DC receiving-end converter station.

[0060] S3: Enable STATCOM additional shutdown angle controller.

[0061] S31: When the input signal of the additional turn-off angle controller of STATCOM is digital 1, it is equivalent to that the additional turn-off angle controller does not work. In this scenario, the input system voltage error signal is defined as the difference between the voltage reference value under the STATCOM constant voltage control and the actual effective value of the voltage at the STATCOM grid connection point. The formula is:

[0062] U err =U ref -U s (5)

[0063] Where U err is the voltage error signal under constant voltage control, U ref is the STATCOM constant voltage reference value, U s is the actual effective value of the voltage at the STATCOM grid connection point;

[0064] S32: When the input signal of the STATCOM additional turn-off angle controller is the turn-off angle error signal of the back-to-back DC converter station, the additional turn-off angle controller takes effect. The input voltage error signal in this scenario is defined as the difference between the voltage reference value under the STATCOM constant voltage control and the actual effective value of the voltage at the STATCOM grid connection point, plus the product of the turn-off angle error signal on the inverter side of the DC converter station and the gain coefficient of the additional turn-off angle controller. The formula is:

[0065] U err =K·Δγ+U ref -U s (6)

[0066] Where K is the additional turn-off angle controller gain coefficient.

[0067] Preferably, the step S3 invests in an additional STATCOM cut-off angle controller, including the output characteristics of the STATCOM under different working conditions and the setting principle of the additional cut-off angle controller gain coefficient K:

[0068] A1: For the STATCOM output expression, different working conditions and output quantity have a certain functional relationship. By simulating and comparing STATCOM under different working conditions, the output characteristic curve can be used to estimate the STATCOM reactive power that can be discovered under a certain working condition:

[0069] A2: Based on equation (6) and the STATCOM output characteristic curve, the selection principle and variation range of the gain coefficient K can be verified through power system time domain simulation.

[0070] 2. Technical feasibility verification of the present invention

[0071] The system model of the engineering simulation example is as follows Figure 4 The simulation results are shown in Figure 5 , Figure 6 , Figure 7 As shown, the output characteristics of STATCOM are as follows: Figure 8 As shown, the effectiveness of the present invention is verified.

[0072] Figure 5 , 6 7 is a comparison diagram of the dynamic response of various electrical quantities before and after configuring a method to suppress subsequent commutation failure of the back-to-back DC system when a commutation failure occurs in the back-to-back DC receiving end system. Figure 5 (a) is a comparison diagram of the shutdown angle of the converter station. Figure 5 (b) is the comparison diagram of STATCOM output reactive power, Figure 6 (a) is the reactive power comparison waveform sent from the DC receiving end to the converter station. Figure 6 (b) is the DC current comparison waveform, Figure 7(a) is the DC receiving end turn-off angle comparison waveform, Figure 7 (b) is the DC current comparison waveform.

[0073] from Figure 5 (a) It can be seen that in the case of a system without STATCOM and a system equipped with STATCOM but without a coordinated control strategy, the inverter-side shutdown angle of the HVDC system without additional control drops to 0° for two consecutive times, resulting in a continuous valve-level commutation failure, while the shutdown angle decline trend of the HVDC system with coordinated control is suppressed and only occurs once, thus avoiding subsequent commutation failures.

[0074] from Figure 5 (b) It can be seen that the coordinated control dynamically adjusts the current inner loop command according to the turn-off angle error and the voltage error, thereby dynamically increasing the reactive power generated by the STATCOM. The reactive power generated by the STATCOM equipped with the coordinated control strategy is 340Mvar when the AC side fails, which is close to twice the reactive power without additional control, providing better reactive support. After the fault is eliminated, the coordinated control is locked, and the voltage reference value drops stepwise, causing the reactive power characteristics of the STATCOM output to change significantly.

[0075] like Figure 6 As shown in (a), during the period of bus voltage reduction, the reactive power Q (taking the direction of the converter station flowing to the receiving end AC system as positive) exchanged by the AC and DC systems in the converter station increases when the voltage decreases. As the reactive power emitted by STATCOM increases, the reactive support provided by the converter station of the receiving end system also becomes stronger, causing the receiving end system voltage to drop more during the fault. In the fault recovery stage, after the coordinated control strategy is equipped, the reactive power flowing from the converter station to the JY station becomes smaller, reducing the system reactive power surplus, and the degree of overvoltage and recovery of the receiving end voltage are improved. Figure 6 (b) is a comparison of the DC current before and after the fault occurs in the receiving AC system. Before and after the configuration of the STATCOM coordinated control strategy, the change amplitude of the DC current is significantly suppressed. When the system is at the stable operating point under the fault, the DC current under the traditional control model drops to 0kA (0p.u.). After the STATCOM is equipped with a coordinated control strategy, the commutation bus line voltage drops to 1.15kA (about 0.23pu), which improves the power transmission capacity of the DC system and provides a higher stability margin for the inverter-side AC system.

[0076] Depend on Figure 7 (a) It can be seen that when STATCOM is not installed and STATCOM is installed and traditional control is used, the valve group has a total of 4 valve-level phase-change failures within 71ms after the fault. After the additional control strategy is equipped, only one phase-change failure occurs, and the duration of the phase-change failure is 60ms, which is shorter than the 72ms of the other two cases. Figure 7(b) It can be seen that when coordinated control is not configured, the DC current has a brief rise at 0.816s. Before and after the STATCOM coordinated control strategy is configured, the DC current does not have a brief rise at 8.16s, the valve-level commutation failure occurs only once, and the DC system recovers quickly after the fault is cleared.

[0077] In summary, the method of suppressing subsequent commutation failure of a back-to-back DC system described in the present invention can be used to activate an additional cut-off angle controller in STATCOM when a commutation failure occurs in DC, and the constant voltage control of STATCOM is quickly converted from a single control object of the voltage at the PCC to a dual control object of the cut-off angle of the coordinated DC receiving-end converter station, thereby increasing the reactive power output by STATCOM. This strategy fully explores the reactive potential of STATCOM when the STATCOM capacity allows, improves the supporting capacity of the receiving-end power grid voltage, and has economic and practical value. .

[0078] Finally, it should be noted that the above examples of the present invention are merely examples for illustrating the present invention, and are not intended to limit the embodiments of the present invention. Although the applicant has described the present invention in detail with reference to the preferred embodiments, for those of ordinary skill in the art, other different forms of changes and modifications can be made based on the above description. It is impossible to list all the embodiments here. Any obvious changes or modifications derived from the technical solution of the present invention are still within the scope of protection of the present invention.

Claims

1. A STATCOM coordinated control strategy method for suppressing subsequent commutation failure of a back-to-back DC system, characterized in that: The following steps are involved: S1: Obtaining the operating status of the high-voltage DC transmission system According to the AC bus voltage level monitored in real time by the DC converter station, whether a DC commutation failure occurs is determined through the commutation failure detection logic; S2: Channel transmission signal According to the operating status of the HVDC transmission system, the specific signal of the DC converter station is transmitted to the STATCOM additional shutdown angle controller in the AVC substation through the channel; S3: STATCOM additional shutdown angle controller function The additional shutdown angle controller of STATCOM changes the voltage set value, thereby changing the reactive power output by STATCOM, and realizing the function of STATCOM to further improve the voltage support capability of the receiving end.

2. A method for suppressing subsequent commutation failure of a back-to-back DC system according to claim 1, characterized in that: Step S2 transmits the turn-off angle error signal of the inverter side of the DC receiving end converter station. The specific strategy is: S21: When the DC commutation failure logic determines that the DC is in a normal operating state or a commutation failure fault state, the DC receiving-end converter station only transmits a digital 1 signal to the additional shutdown angle controller of the STATCOM through the channel; S22: When the DC commutation failure detection logic determines that the DC is in a commutation failure fault state, the DC receiving-end converter station transmits a DC receiving-end converter station shutdown angle error signal to the STATCOM additional shutdown angle controller through a channel; The DC receiving end converter station turn-off angle error signal is defined as the difference between the DC receiving end converter station turn-off angle reference value and the actual measured minimum value, and its formula is: Δγ=γ*-γ (1) Where Δγ is the turn-off angle error signal of the DC converter station, which is also the input signal of the additional turn-off angle controller, γ* is the turn-off angle reference value of the DC receiving-end converter station, and γ is the minimum value of the turn-off angle measurement of the DC receiving-end converter station.

3. A method for suppressing subsequent commutation failure of a back-to-back DC system according to claim 1, characterized in that: Step S3: The STATCOM additional cut-off angle controller is put into operation. The control link of the additional control is the proportional link. The specific strategy is: S31: When the input signal of the additional turn-off angle controller of STATCOM is digital 1, it is equivalent to that the additional turn-off angle controller does not work. In this scenario, the input system voltage error signal is defined as the difference between the voltage reference value under the STATCOM constant voltage control and the actual effective value of the voltage at the STATCOM grid connection point. The formula is: IN err =U ref -IN s (2) Where U err is the voltage error signal under constant voltage control, U ref is the STATCOM constant voltage reference value, U s is the actual effective value of the voltage at the STATCOM grid connection point; S32: When the input signal of the STATCOM additional turn-off angle controller is the turn-off angle error signal of the back-to-back DC converter station, the additional turn-off angle controller takes effect. The input voltage error signal in this scenario is defined as the difference between the voltage reference value under the STATCOM constant voltage control and the actual effective value of the voltage at the STATCOM grid connection point, plus the product of the turn-off angle error signal on the inverter side of the DC converter station and the gain coefficient of the additional turn-off angle controller. The formula is: IN err =K·Δγ+U ref -IN s (3) Where K is the additional turn-off angle controller gain coefficient.

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