An extensible configuration is proposed for static var generator (SVG) with advanced controller included for reactive power compensation of grid. Compared with the traditional configurations, the major advantage of such system configuration is that the power modules are very flexible and easy to extend or reduce without changing the main equipment of SVG under the different voltage levels. Furthermore, in order to solve the problems of modeling uncertainty, nonlinearities, and outside disturbance by using proportion integration (PI) controller, an advanced controller is proposed based on auto disturbance rejection control (ADRC). By controlling the amount and direction of reactive current, the reactive power is generated or absorbed from SVG into power grid with fast response, which can realize the excellent dynamic compensation for both the internal and external interferences. Simulations results show that the proposed controller has better performance of the transient and steady state than PI controller. Moreover, the verification tests are executed in 380 V, 6.5 kVA experiment systems, suggesting that the excellent dynamic performance and strong robustness are achieved.
As a member of flexible alternate current transmission system (FACTS), static var generator (SVG) is being widely applied to compensate the reactive power with fast response time by generating or absorbing reactive power continuously in power system [
In the past few years, a great deal of research has been done to deal with these problems. The most widely used control technique is proportion integration (PI) controllers [
One of the most serious problems of the existing SVG is that the protection control unit (PCU) of the SVG generally is the fixed structure and cannot be adapted [
To overcome these obstacles, an advanced SVG control system is proposed in this paper, in which the reactive current is commutated into power grid reliably and efficiently, which can accomplish a successful reactive compensation. Improvements are achieved as follows: First, a new configuration of ADRC controller is first applied for reactive current control in SVG and it realizes excellent dynamic compensation for the internal and external interferences. Second, to achieve the compatibility of PCU on the different voltage levels, the configuration of the extensible SVG is constructed. Simulations show that the adaptability of the parameters is better and the robustness and stability of ADRC controller are stronger than PI controller. Finally, a series of verification tests is achieved in the 380 V 6.5 kVA experimental system. The viability and effectiveness of the proposed SVG are verified.
Figure
(a) Configuration of SVG. (b) Actual hardware view of SVG.
As shown in Figure
The carriers phase-shifted sinusoidal pulse-width modulation switching (CPS-SPWM) technology is adopted to operate the switches, which can generate sinusoidal wave voltage with least harmonics. This technology can be briefly explained by two H-bridge pulse-width modulation (PWM) converters. Figure
Two H-bridge cascaded PWM converters.
The SPWM pulses are generated through sine wave compared with triangular wave. Two triangle carrier wave signals have the same frequency and they are phase shifted by
The sequence diagram of cascaded 2 H-bridge PWM converters.
The driving signal If initial phase of triangular wave is zero, the driving signal If initial phase of triangular wave is zero, the driving signal of If initial phase of triangular wave is one, the driving signal If initial phase of triangular wave is one, the driving signal
Figure
Sequence diagrams of the actual H-bridge PWM converters.
Figure
Control block diagram for the 10 kV 2 MVA SVG.
The proposed controller involves the current inner loop control and dc voltage external loop control, triangular wave generator, and PWM generator. The first three parts are achieved in the ARM, while the last three parts are achieved in the FPGA.
In addition, the DC voltage external loop control can provide reference currents
Referring to Figure
By applying the transformation
And, then, we design ADRC for the active current
Figure
Block diagram of current inner loop control based on ADRC.
As shown from Figure
The design processes of the active current
The reactive current
ESO is the core part of ADRC, which can observe and estimate the actual reactive current and unknown disturbances dynamically. Whether the actual reactive current and the unknown disturbances can be estimated accurately with ESO directly influences the control effect of ADRC.
The ESO for the reactive current
Nonlinear state error feedback (NLSEF) unit can be used to calculate the control variable of the reactive power adjustment for the current inner loop control. But the selection of NLSEF unit parameters is very difficult in the practical application. Thus, we will propose a simplified and linear method to achieve NLSEF:
Finally, by combining NLSEF with the observed disturbances, the simplified ADRC can be realized. As a result, the current inner control of SVG is achieved.
Three main parts is included in the SVG control system involving protect and control unit (PCU), valve control unit (VCU), and power modules. The PCU is the core part. While SVG system is protected and controlled by sending and receiving the control commands, the VCU can execute the commands and manage the power modules directly. The power module is viewed as the PWM converter.
Figure
Control block of SVG control system.
As shown in Figure
Figure
Structure drawing of PCU and VCU in SVG control system.
In Figure
Figure
Structure drawing of PCU and the extensible VCU in SVG control system.
In order to verify the performance of the proposed ADRC controller, the comparative simulations are constructed based on the conventional PI and the proposed ADRC, respectively. The model of 380 V 6.5 kVA SVG is established in PSCAD environment. Figure
(a) Dynamic response of
Figure
In addition, the experimental platform of 380 V 6.5 KVA SVG is constructed in laboratory environment based on the proposed configuration and control system. The 380 V 6.5 KVA SVG consists of 12 H-bridge pulse-width modulation (PWM) converters, and 4 converters are involved in each phase.
To further verify the proposed SVG control system, the var generator (VG) is applied in this experiment, which can generate the set reactive current. Simultaneously, the proposed SVG can generate the compensating current that prevents the reactive current from flowing into the grid. In current inner loop control, the experimental waveforms of the current of A-phase cluster are observed, recorded by the oscilloscope.
The current inner loop control consists of four parts involving startup process, stopping process, steady-state process, and dynamic process.
Figure
(a) The experimental result in the startup process. (b) The experimental result in stopping process. (1) Yellow line: reactive current. (2) Green line: compensating current. (3) Blue line: residual current of grid.
Figure
(a) Experimental results verify the effect of current inner control in steady-state process. (b) Experimental results verify the effect of current inner control in overload state. (1) Yellow line: reactive current. (2) Green line: compensating current. (3) Blue line: residual current of grid.
Figure
Dynamic performance of SVG in the dynamic process. (1) Yellow line: reactive current. (2) Green line: compensating current. (3) Blue line: residual current of grid.
As a result, the SVG can track the reference command current accurately and compensate the reactive current rapidly when the reactive current changes suddenly by the current inner loop control. The compensation precision of SVG is high and the dynamic response of SVG is fast.
This paper has analyzed the configuration of SVG and designed an advanced control system. The improvement is achieved in three aspects: To solve the problems of nonlinear and strong coupling in electrical power system, the active disturbance rejection controller (ADRC) is first applied for current feedback decoupling control strategy. To guarantee the adaptability and robustness, the control strategy regards the uncertainties and disturbances from the circumstance outside as the total disturbance that can be evaluated and compensated automatically. By comparing the ADRC with the proportion integration differentiation (PID), better dynamic performance and steady performance are achieved by using ADRC. In order to solve the problem of poor compatibility for SVG under different voltage grade, an extended control and protection system of SVG is designed. By increasing and decreasing the number of valve control unit, the control scale can be adjusted without changing other equipment. Thus, original system can satisfy different needs for different control object. On the basis of the control and protection system, an actual SVG is constructed with reduced proportion in the laboratory environment. A series of verifications tests are executed in experimental system for verifying modulation technology, control strategy, and software and hardware of control system. These verifications establish the foundation for the perfect prospect in industry application of SVG.
The authors declare that they have no competing interests.