A New Control Strategy of Hybrid Reactive Power Compensation System in Distribution Network

Reactive power compensation is an important measure to improve the power quality of distribution networks, especially with the increasing connection of distribution transformers, heavy industrial electrical equipment, and asynchronous motors, which generate a large amount of reactive power demand. Traditional reactive power compensation devices, such as fixed capacitors (FC) and thyristor switched capacitors (TSC), have some drawbacks, such as low response speed, harmonic generation, and switching shock. A static synchronous compensator (STATCOM) is a flexible AC transmission system (FACTS) device that can provide fast and continuous reactive power compensation, but it also has some limitations, such as high cost, large capacity requirement, and low utilization rate. Therefore, hybrid reactive power compensation systems that combine FC or TSC with STATCOM have attracted much attention in recent years. Many researchers have proposed and studied different topologies and control methods of hybrid systems to overcome the disadvantages of single devices and achieve better performance. However, most of the existing hybrid systems are designed for low or medium-voltage distribution networks, and their performance under high-voltage distribution networks is still unclear. Moreover, the coordination control strategy of the hybrid system and its subsystems is also a key issue that affects the effectiveness and efficiency of reactive power compensation. In view of this, this paper aims to propose a TSC+STATCOM hybrid reactive power compensation system for high-voltage distribution networks, which optimizes the shortcomings of traditional reactive power compensation devices. The system uses the synergistic effect of TSC and STATCOM to achieve large-scale static reactive power and small-scale dynamic reactive power compensation and adopts a switching strategy based on an expert decision system. This paper introduces the working principle of each subsystem of the hybrid reactive power compensation system in detail and builds a simulation model of the hybrid reactive power compensation system under a high voltage distribution network on Matlab/Simulink platform. The simulation results verify the feasibility and excellent performance of the system control method in reactive power compensation and improving power quality.


INTRODUCTION
With the advancement of technology, a considerable number of devices are connected to the grid, and clean energy sources are integrated into the grid, generating a substantial reactive power demand, which significantly impacts the traditional transmission and distribution network [1].Relying solely on reactive power compensation on the generation side is no longer sufficient to meet the power quality requirements.Therefore, maintaining voltage, frequency, and waveform has become a crucial research topic.
At present, there are three types of reactive power compensation methods: shunt, series, and hybrid.Traditional capacitors are extensively employed in distribution networks, but their limitations have led to the development of thyristor switched capacitors (TSC).Generators bear a significant amount of reactive power load but are constrained by transmission losses and rated power.Static synchronous compensators (STATCOM) possess rapid response and precise compensation capabilities, but their high cost limits widespread adoption [2][3].
Consequently, the research direction has shifted towards hybrid reactive power compensation methods.Widely used hybrid reactive power compensation methods for power systems include UPFC, traditional capacitor + TCR [4][5], and TCR + TSC.In TSC + STATCOM compensation, TSC excels in stabilizing reactive loads, while STATCOM has an advantage in handling rapidly changing reactive loads.However, current studies often treat them as independent components, which may lead to errors and disturbances in the control and regulation process.Future research should focus on better integrating these two compensation devices.
This paper proposes a hybrid reactive power compensation strategy based on TSC and STATCOM and optimizes the existing system.A unified controller based on expert decision-making is employed for real-time regulation of reactive power compensation tasks.The study includes: detailing the principles of STATCOM and TSC, building a mathematical model of the hybrid system, comparing topologies, selecting the optimal solution, designing a unified control strategy, allocating tasks, and adopting a coordinated control system based on expert decision-making.

Principle of t thyristor-switched capacitor (TSC)
TSC is a reactive power compensation device, which realizes the input and output of the capacitor branch in the power network by means of a semi-controlled device, thyristor, and current limiting reactor.The capacitor is responsible for reactive power compensation, and the thyristor controls the on/off and throwing moment [6].The capacitor is responsible for the reactive power compensation, the thyristor controls the on/off and the switching moment, and the current limiting reactor prevents inrush current and reduces the inrush current [7].Compared with the traditional mechanically thrown FC, TSC has high accuracy, short response delay, and fast response.When the capacitor is put in, there is a U-I relationship as shown in Equation (1).

Principle of Static Synchronous Compensator (STATCOM)
The static synchronous compensator (STATCOM) can be equated to an adjustable voltage source, which can be seen as a voltage source connected in parallel in the line during operation.[8] The output of STATCOM is equivalent to a controllable voltage source Ug, and the system is regarded as an ideal voltage source Us. δ is the angle by which the latter leads the former.
In the actual application process, there is always a certain degree of loss, and the resistance R reflects the active power loss and transformer copper consumption caused by the static synchronous compensator.When R is present, the controlled voltage source Ug and the ideal voltage source Us of the system are related.The angle of δ and the presence of R enable the static synchronous compensator to absorb active power from the system, compensate for various internal losses during the operation of the STATCOM, and perform the process of charging and discharging the capacitor to maintain a constant voltage at the capacitor terminals.
During the dynamic process, by changing δ, the capacitor C is charged or discharged, so that the voltage amplitude output by STATCOM will also change correspondingly, and the purpose of regulating reactive power can be achieved.

Principle of hybrid reactive power compensation system
2.1 and 2.2 investigate the operating principles of TSC and STATCOM, respectively, by connecting them in parallel to form a hybrid reactive power compensation system as shown in Figure 1 below.
TSC and STATCOM are connected in parallel in the grid so that the hybrid reactive power compensation system can improve the compensation response speed, prevent system reactive power deficiency and reduce the cost at the same time [9][10].
Figure 1.Cable connection of the hybrid reactive power compensation system

Simulation model construction and parameter setting
In order to truly simulate the compensation ability of the hybrid reactive power compensation system under the high-voltage distribution network, a model of a 35kV power compensation system as shown in Figure 2 was built in Simulink.The model includes controllable voltage sources, three-phase RL branches, three-phase transformers, lines, constant loads, controllable loads, etc., which basically cover various parts from the power generation end, and transmission end to the power consumption end.Measuring points from m1 to m7 are set up to measure the parameters of each node in real time, which can simulate the compensation effect of the hybrid reactive power compensation system under various working conditions.

Figure 2. Simulation design diagram of hybrid reactive power compensation system in HV distribution network
This design uses a programmable voltage source to simulate the operation of the entire power grid when the amplitude and phase of the power supply change.We enter the Parameters tab, and set the voltage source parameter to 1.05 pu, phase to 21°, and frequency to 50 Hz (this paper sets 35 kV, 100 MVA as the reference value, the same as below).Under normal operation, the voltage source does not produce large fluctuations, and the voltage instability is more due to the sudden increase of resistive and capacitive loads.Therefore, the voltage source in this design does not change with time, which is considered to be a programmable voltage source that does not change with time.
After setting the basic power supply parameters, it is also necessary to add the Powergui module to the simulation for further setting of the power system parameters.This simulation model is a solver model based on the ode23t rigid model variable step algorithm with simulation time 0.5 s, frequency 50 Hz, maximum iteration 50 times, and maximum step 1e-05.

Overall design and verification of the hybrid reactive power compensation system
In this section, based on the power system established in Section 3.1, an inner-loop reactive current coordination allocation unit based on expert decision-making, a static synchronous compensator control and the driving unit, a thyristor-switched capacitor control and the driving unit, and a capacity of ±3Mvar for the static synchronous compensator and a capacity of 1Mvar, 2Mvar, and 3Mvar for the thyristorswitched capacitor in accordance with the "K+1" unequal capacity switching principle were added.

Design of inner-loop reactive current coordination distribution unit based on the expert decision
The Controller system is the most important part of the logic implementation and control implementation of the whole hybrid reactive power compensation system for the high-voltage distribution network based on expert decisions.The central control unit collects the real-time data of voltage and current in the grid and outputs the reactive power commands to each sub-control unit through the inner loop current distribution rule.The sub-control units include STATCOM and TSC, which output or absorb reactive power to the grid according to the instructions of the upper-level system through their own control links.The inner loop current distribution rule is implemented by if-else statements, and considers the transient reverse output generated by TSC during the switching process.In order to improve the system response speed, a feedforward control link is introduced, which is superimposed on the system control quantity through the feedforward channel.The feedforward channel model is shown in the upper part of Figure 3.

Thyristor-throwing capacitor internal design
The control link of TSC mainly focuses on the measurement module of system current and voltage, the judgment module of throwing logic, the reactive power command selection unit, and the trigger device.The control system takes the reactive current assigned to the TSC from the expert decision control system as input and controls the number and sequence of capacitor banks according to the synchronization signal, input current magnitude, square wave voltage, switching signal, etc.The overall control architecture of the TSC and the internal design of each group of thyristor banks are shown in Figures 5 and 6.

Simulation and verification of the inner-loop current distribution rules based on the expert decision
To verify the correctness and reliability of the TSC+STATCOM hybrid reactive power compensation system and its reactive current inner loop distribution rule based on the expert decision, the experimental model built in sections 3.1 and 3.2 is used to switch on and off inductive loads at three-time intervals of 0.31 s, 0.32 s, and 0.34 s, simulating the load fluctuation of the distribution network, and calculating in per unit value (see the reference value described in section 3.1).

Figure 7. Changes of TSC, STATCOM, and reference current over time
As shown in Figure 7, before 0.305 s, the system reactive power demand was small, and STATCOM alone provided voltage support and inductive reactive power compensation; at 0.31 s and 0.32 s, the system connected 2 Mvar and 3.5 Mvar of inductive loads respectively, and the expert decision system allocated most of the reactive power to the TSC group, taking into account the transient reverse output generated by TSC during the switching process; at 0.34 s, the system disconnected 4.5 Mvar of inductive load, leaving only 1 Mvar of additional load connected at the end of the distribution network, and all the extra reactive power was borne by the TSC group, while STATCOM returned to its initial state.Figure Figure 8. Switching instructions of three SETS of TSC in three working conditions

Simulation and verification of compensation capacity of hybrid reactive power compensation system for high-voltage distribution network
Based on the experimental platform of the hybrid reactive power compensation system and the verification of the expert decision system built in section 3.2, this section compares and analyzes the voltage and current waveforms, power factor, and other parameters when a sudden inductive load is added, and verifies the improvement effect of the hybrid reactive power compensation system on the power quality of the distribution network.In order to simulate the worst working condition, that is, the situation of grid parameters when an inductive load is added at the end of the distribution network under a three-phase unbalanced condition.This section simulates and analyzes according to the situation when 2Mvar is input at 0.3s.
(a) Before compensation (b) After compensation Figure 9. Current waveform It can be found in Figure 9 that due to the addition of resistive loads, the current amplitude is unequal and the difference is large before the hybrid reactive power compensation device is added, which will have an adverse effect on the stability of the grid.After the intervention of the hybrid reactive power compensation system, the current amplitude can basically be kept consistent, and by observing the change of the power factor, it can be found that the power factor can quickly recover to near 1 after continuous fluctuation, which proves the effectiveness.Due to the addition of inductive load, the current amplitude is unequal without a hybrid reactive power compensation device, affecting the grid stability.With a hybrid reactive power compensation system, the current amplitude is consistent and the power factor quickly recovers to near 1 after fluctuation, proving the effectiveness of a hybrid reactive power compensation system.In order to nearly confirm the improved hybrid reactive power compensation method as well as to judge whether the harmonic content meets the national standard, FFT analysis is performed for three cases before three-phase current compensation, compensation using the traditional method, and compensation using this improved method, respectively, to verify the feasibility of the compensation method by comparison, and to compare the difference between the traditional method and the improved method in terms of compensation accuracy by THD content.The results of the FFT analysis are shown in Figures 11, 12, and 13 below.and 16, it can be seen that the THD of phases A, B, and C before compensation were 16.11%, 15.19%, and 22.23%, respectively.After using this hybrid reactive power compensation, the THD of phases A, B, and C were 1.39%, 1.24%, and 1.28%, respectively.After using the improved hybrid reactive power compensation, the THD of phases A, B, and C were 0.99%, 0.94%, and 0.97%, respectively.It can be seen that the harmonic content has significantly decreased after compensation, which verifies the effect of compensation.Moreover, it can be found by comparison that both the hybrid reactive power compensation before and after improvement can meet the requirements of harmonic

Summary and Outlook
This paper proposes and verifies a coordinated control strategy of a hybrid reactive power compensation system based on expert decision-making for high-voltage distribution networks.The strategy can dynamically adjust the reactive current distribution between TSC and STATCOM subsystems according to different operating conditions, optimize the capacity allocation and switching methods of the two subsystems to reduce the switching frequency, improve the utilization rate, and extend the service life of the devices.The simulation results on Matlab/Simulink platform show that the proposed strategy can achieve better reactive power compensation performance than conventional methods, effectively suppress harmonics, and meet national standards.The paper contributes to the field of reactive power compensation by presenting a detailed analysis of the hybrid system and its subsystems.
However, the research is not comprehensive due to time and ability constraints.Future work should explore the following aspects: 1) The performance of the system under different voltage levels and transient loads.
2) The improvement of the control system and the optimization of the hybrid reactive power compensation system based on practical engineering.

Figure 3 .
Figure 3.The Internal structure of the inner ring reactive current coordination distribution unit based on the expert decision

Figure 4 .
Figure 4. Interior design model of the static synchronous compensator Six HBSM half-bridge sub-modules are set up with a DC-side voltage of 1000V, a connection reactance of 0.01H, an opening resistance of 0.001 Ohm, a DC-side capacitor of 10 mF, and a compensation capacity of 3 Mvar.The simulation model of the static synchronous compensator established by the direct current control method is shown in Figure 4.

Figure 10 .
Figure 10.Power factor variation diagram Figure 11.A-phase voltage and current waveform (a) Before compensation (b) Traditional method (c) Improved method Figure 14.Current FFT analysis before and after a-phase compensation (a) Before compensation (b) Traditional method (c) Improved method Figure 15.Current FFT analysis before and after PHASE B compensation (a)Before compensation (b) Traditional method (c) Improved method Figure 16.Current FFT analysis before and after C phase compensation From Figures 14, 15 , .1088/1742-6596/2625/1/012024 11 content, and the improved hybrid reactive power compensation can improve the accuracy of compensation, which verifies the correctness of the improved hybrid reactive power compensation method.