Modeling and simulation of extra high voltage magnetically controlled shunt reactor based on PSCAD

For the two typical topologies of the extra high voltage magnetically controlled shunt reactor (MCSR), this study introduces its specific wiring methods and working principles. It conducts an analysis of the change characteristics and response characteristics of the core magnetic flux under different wiring modes of the controlled shunt reactor body. Based on PSCAD, two typical structures are used to model the body of the first 500 kV MCSR put into operation in a provincial power grid and perform steady-state and dynamic simulations. The results of field experiments are compared and analyzed. The winding connection mode of the device’s body electromagnetic transient model is determined, which lays a foundation for the follow-up research.


Introduction
Extra high voltage magnetically controlled shunt reactors (referred to as MCSR) [1][2][3], due to their flexible control and fast response time, can smoothly regulate the reactive power of the system [4][5], achieving true flexible transmission.It can also suppress power frequency and operating overvoltage, reduce line losses, and greatly improve the stability and safety of the system [6][7].Therefore, MCSRs are important reactive power compensation devices in ultra-high voltage and ultra-high voltage power grids.
In order to accurately grasp the working principle and operating characteristics of MCSR, it is necessary to build an electromagnetic transient model for simulation research.There are two typical topological structures of controllable high impedance, and the external response characteristics vary depending on the different wiring methods of the body.This article introduces the different wiring methods and working principles of two typical body structures of controllable high impedance.It deeply analyzes the changes and response characteristics of the magnetic flux of the iron core under different wiring methods.The first 500 kV MCSR put into operation in a provincial power grid is taken as an example.Two different body structures were built based on PSCAD for simulation analysis and compared with on-site measured data to determine the winding connection method of the equipment's electromagnetic transient model, laying a foundation for subsequent research.

MCSR body structure and working principle
MCSR primary wiring method mainly refers to the three-phase wiring method of two branches of each phase of the grid side winding, control winding, and compensation winding [8][9][10].The main function of a general compensation winding is to provide a path for the third-order zero sequences harmonic current.Therefore, the two branches of each phase of the compensation winding are in forward series, and the three phases are in a triangular connection.The grid side winding and control winding have the following two wiring combinations: (1) The wiring method is shown in Figure 1 L , respectively, are the branch windings of the three-phase winding on the grid side, with two branch windings in parallel for each phase and Y formed by three-phase connection; ap L , aq L , L , q c L , respectively, connected end-to-end, and then connected in reverse parallel with an open triangle.This connection method is also known as the "three series and two parallel".(2) Wiring method 2 is shown in Figure 2, where the two branch windings on each phase of the grid side in Figure 2 are "parallel changed to series", and the three phases are connected to Y form.The structure of the control winding in Figure 2 is changed from "three series and two parallel" to "two series and three parallel".Two control branch windings of each phase are connected in reverse polarity series to form a control branch, and the three-phase control branch is connected in parallel.

Simulation and experimental verification
The 500 kV BZ substation of an actual provincial power grid is located at the end of the system, and the regional power grid structure is weak.The randomness of new energy output leads to frequent voltage fluctuations in the busbars of each station in the system, seriously affecting power quality.As a hub station, voltage stability should be ensured.Therefore, the first 500 kV MCSR was put into operation at the BZ station.
In PSCAD, the controllable high impedance ontology models of the station are respectively constructed as connection mode 1 and connection mode 2. Through steady-state and dynamic simulations, their waveforms are analyzed and compared with the on-site measured data results to determine the ontology structure of the controllable high impedance of the station.

Steady-state characteristics
We simulate the excitation current mode regulation of the controllable high impedance electromagnetic transient model of BZ substation in connection modes 1 and 2. The target output current of the system in the excitation current mode is adjusted, with 5% of the rated excitation current of the controllable high impedance as the level difference.The excitation current is gradually increased from the initial state until the output capacity of the controllable high impedance rises to 70 Mvar.The increase or decrease of the excitation current of the self-excitation system is recorded.The corresponding changes in controllable high reactive power, as well as the relationship between simulated and measured controllable high reactive power and excitation current, are shown in Figure 3. From the comparison in Figure 3, it can be seen that there is a significant difference between the simulation results of the controllable high-impedance excitation current mode regulation test and the on-site measurement results.In the simulation, the relationship between the controllable highimpedance reactive power and the excitation current changes slowly.In comparison, in the actual measurement, the relationship between the controllable high-impedance reactive power and the excitation current shows a linear relationship under the unsaturated state of the controllable high impedance, and the changing trend is obvious.The simulation results of the excitation current mode regulation of the controllable high impedance self-excited system under wiring method 2 are similar to the experimental results measured on site, and the trend of change is basically consistent.

Dynamic characteristics
In the connection mode, the controllable high-impedance electromagnetic transient model of the BZ substation is simulated for reactive closed-loop regulation mode step response.The controllable high impedance operates in the reactive closed-loop regulation mode, and the reactive reference value is 30 Mvar.At 6 s, the reactive reference value steps up to 37 Mvar, and at 16 s, the reactive reference value returns to the initial value of 30 Mvar.The simulation waveform is shown in Figure 4 (the green line in the figure is the measured result, and the blue line is the simulation result).The simulation step response indexes are shown in Table 1.Next step 0.38 6.9 0.5 Comparing the simulation results of the controllable high impedance under the wiring mode with the step response measured on site, it can be seen that under the wiring mode, the step response speed of the controllable high impedance is slow.The rise time of the upper and lower steps is longer, which is quite different from the field experimental results.

Wiring method 2.
Under wiring mode II, the controllable high impedance is simulated for step response of reactive closed-loop regulation mode.The step amount and step time are the same as those of wiring mode I.The simulation waveform is shown in Figure 5 (the black line in the figure is the measured result, and the red line is the simulation result), and the step response index is shown in Table 3.  Next step 0.32 0 0 From the above simulation results, it can be seen that in connection mode 2, the step response waveform of reactive power, excitation current, and DC voltage simulated by controllable high reactance for reactive closed-loop regulation mode is basically consistent with the step response waveform measured on site.Compared with the simulation results of connection mode 1, the step response speed is faster, and the rise time is shorter.
The transient electromagnetic simulation was conducted on the main winding of the first controllable high-impedance transformer put into operation at the BZ station using wiring method 1 and wiring method 2, respectively.Compared with the on-site measurement results, it can be concluded that the main winding wiring of the station adopts the structure of wiring method 2. Under this structure, the relationship between the reactive power of the controllable high-impedance transformer and the excitation current is linear under the unsaturated state of the controllable highimpedance transformer, and the system responds quickly to external characteristics.

Concluding remarks
This article introduces two typical topological structures of controllable high impedance based on the working principle of magnetically controlled controllable high impedance, as well as the varying characteristics of iron core magnetic flux.A simulation model is established for the first magnetic controlled controllable high impedance put into operation in a provincial power grid in PSCAD/EMTDC.The steady-state and dynamic characteristics of controllable high impedance under different winding connection methods are simulated and analyzed.It is proved that the main body is connected in parallel with each phase branch winding on the grid side.The response characteristics of controllable high impedance will be relatively slow.By comparing with the on-site measured results, the main winding wiring method for the newly added controllable high impedance in transient electromagnetic modeling of the power grid has been determined.

L
branch windings of the control side three-phase winding ap L

Figure 3 .
Figure 3. MCSR Controlled high anti-excitation current mode regulation test simulation and field measurement comparison diagram (a) Reactive power step response waveform (b) Excitation current step response waveform (c) DC voltage step response waveform Figure 4. MCSR Wiring method one,Step response waveform of reactive power 30 Mvar-37 Mvar in reactive mode (a) Reactive power step response waveform (b) Excitation current step response waveform (c)DC voltage step response waveform Figure 5. MCSR Wiring method two, Step response waveform of reactive power 30 Mvar-37 Mvar in reactive mode

Table . 1
. MCSR Wiring method one, reactive command 30-37 Mvar step response index According to the step test waveform of controllable high impedance in reactive mode, the measured step response indexes are shown in Table2.Table2.MCSR reactive command 30-37 Mvar step response index