1. Introduction
With the development of the entire social economy, the requirements for power quality are becoming increasingly stringent. Reactive loads in the system, especially impulsive reactive loads and the application of numerous power electronic devices, not only increase various losses but also seriously affect power quality. Therefore, real-time and rapid reactive power compensation is of great significance for optimizing power grid flow distribution and improving power quality. The novel Static Var Generator (ASVG) is an important component of Flexible AC Transmission Systems (FACTS). It enables flexible and rapid control of the power system's network parameters and structure, continuously regulating reactive power across the entire range from inductive to capacitive, achieving rapid compensation for the system's reactive power demand, thereby suppressing voltage fluctuations and enhancing system stability.
2. FACTS and various FACTS devices
Over the years, power industry workers have reached a consensus: improving the safe operation and power quality of the power grid requires not only the grid structure itself and its exceptions, but also advanced regulation and control methods. The safe and economical operation of the power grid largely depends on its "controllability." Since American scholar L. Gyugyi proposed the theory of reactive power compensation using semiconductor converters in 1976, experts from around the world have been actively researching and applying high-power novel static synchronous compensators in both theory and engineering.
Currently, ASVG has made rapid progress in engineering applications. In theoretical research, the main research results on ASVG focus on main circuit wiring methods and topology selection, system model establishment, controller design, and the role of the device in power systems. Most ASVG systems currently in operation employ a multi-phase structure in their main circuits, using multiple inverters and appropriately connecting them through the secondary windings of transformers. This allows the waveforms output from multiple inverters to be phase-shifted and superimposed to obtain a three-phase symmetrical output voltage that approximates a sinusoidal waveform.
While studying the nonlinear dynamic modeling of ASVG, the design of ASVG control strategies and controllers is also a key research focus. An ASVG controller typically consists of two parts: an inner-loop controller and an outer-loop controller. The basic task of the inner-loop controller is to generate a synchronous drive signal, thereby establishing a linear relationship between the converter's output current and reactive power command.
3. Basic Principles of ASVG Devices
3.1 Working principle of ASVG
Figure 1 shows a schematic diagram of the working principle of ASVG. The DC side is an energy storage capacitor, which provides DC voltage support for ASVG. The inverter is usually composed of multiple inverter bridges connected in series or in parallel. Its main function is to convert DC voltage into AC voltage. The magnitude, frequency and phase of the AC voltage can be controlled by controlling the drive pulses of the turn-off devices in the inverter.
Figure 1. Schematic diagram of the principle of ASVG device regulating reactive power.
Figure 2 Single-phase bridge circuit
Since the single-phase bridge circuit is the foundation of the ASVG device, and the working principle of the three-phase bridge is similar, this section will start with the working principle of the single-phase bridge circuit to analyze the ASVG device.
Figure 3. Trigger pulses and output voltages of the turn-off devices in a single-phase bridge circuit.
Figure 4 Single-phase bridge circuit
(1) The equivalent circuit diagram of the single-phase bridge circuit connected to the system shown in Figure 4. Figure 3 shows the trigger pulses and timing relationships of each turn-off device in the single-phase bridge circuit. From Figure 4, we can analyze the conduction status of each turn-off device and the operating state of the single-phase bridge circuit within one cycle.
3.2.2 Time-domain mathematical model of the ASVG device
Figure 5 shows the schematic diagram of the ASVG device, which can be built using the input-output modeling method.
Figure 5. Schematic diagram of the ASVG device
Figure 6. Block diagram of ASVG system control
In a PID controller, the derivative action is mainly used for controlled objects with large inertia to improve their dynamic performance and increase stability, but it is sensitive to disturbances and its ability to suppress external disturbances is weakened. In an ASVG, high-frequency interference is introduced due to the modulation of the base frequency signal, which makes it unsuitable for PID control. Therefore, a Fuzzy-PI controller is used in this device.
4. Simulation Results
4.1 Simulation Scheme and Simulation Results
An AC motor was used to simulate an impulsive reactive load on the power grid. The dynamic regulation performance of the Active Var Generator (ASVG) was tested by measuring the effective voltage change at the connection point, including its response speed and regulation effect on the connection point voltage. The simulation tested the dynamic characteristics of the ASVG by detecting voltage changes at three points: phases a, b, and c. In this simulation, five 5kW motors were connected to a 380V power grid to simulate an impulsive reactive load. The control scheme adopted was the current indirect control described earlier. Multiple motors were connected and operated, and some of the operating motors were connected or disconnected sequentially. Based on the bus and connection point voltage waveforms, the rapid dynamic reactive power regulation performance of the ASVG and its effect on improving the power quality of the entire power grid were tested. The actual measurements show that after adopting the dynamic reactive power compensation device, voltage fluctuations and flicker on the power supply bus were effectively suppressed, improving the power grid environment. The application comparison results are shown in Figure 9.
(a) ASVG output current profile
(b) ASVG output reactive power
(c) System power supply bus voltage waveform
(d) Voltage fluctuations before ASVG is put into operation
(e) Voltage waveform after ASVG is put into operation
Figure 9 shows the suppression effect of ASVG on voltage fluctuations and flicker.
5. Conclusion
This control scheme effectively compensates for reactive power in the system, improves the power factor of the power grid, and can dynamically and continuously adjust reactive power of various types, stabilizing the voltage at the operating point, effectively improving power quality and reducing grid losses. The system operates normally in harsh environments, demonstrating high reliability and meeting long-term operational needs. This paper elucidates the reactive power compensation principle of ASVG (Automatic Var Compensator), and compared with conventional static var compensators, ASVG has outstanding advantages: large capacity, fast response, high precision, low output voltage harmonic content, and advanced control methods, making it the preferred static var compensator. A steady-state model of ASVG is established, and a dynamic mathematical model is built using the switching function method, laying the foundation for a better understanding of the ASVG principle and the implementation of effective control.
For more information, please visit the Power Electronics channel.
