The static VAR generator (SVG) is an important device in flexible AC transmission systems (FACTS) for the development of smart grids. Based on the basis principle of SVG and instantaneous reactive power theory, the conventional _{p}_{q}

As low-voltage distribution networks are becoming more and more popular and electronic devices attract much more attention, the quality of power systems is becoming an increasingly greater issue of concern. With the rapid development of high-voltage high-power power electronic equipment, flexible AC transmission systems (FACTS) are typically applied to the intelligent power grid [

SVGs play an important role in improving the voltage quality and ensuring the normal operation of FACTS [_{p}_{q}_{p}_{q}

Based on the working principle of SVG direct current control, considering the common situation of three-phase grid asymmetry, the phase-locked loop (PLL) in the detection method based on instantaneous reactive power theory is improved, and the single-phase fundamental reactive current is used to replace the phase-component of the single-phase grid voltage to lock the phase and eliminate detection bias. In addition, the existence of low-pass filter (LPF) in the detection algorithm also has great influence on the steady-state and dynamic performance of the system, and the sliding-averaging filter (SMF) is designed to improve the detection compensation performance of SVG in consideration of both the detection accuracy and the response speed. Finally, experimental results verify the correctness and effectiveness of the proposed scheme.

The structure of the SVG is shown in _{1}–_{6} in the main circuit, the inverter absorbs a small amount of active current from the AC system to charge the DC bus capacitor

In order to realize the active and negative decoupling control, the AC signal in a three-phase stationary coordinate system is transformed into the two-phase rotation

The direct current control method is generally used in active filter medium and low voltage SVG, the compensation principle is shown in _{L}_{C}

Reactive power compensation mainly includes harmonic and fundamental phase shifts. Within the allowable range of the device, an SVG should compensate for both harmonics and fundamental reactive power except the fundamental active components. In practical engineering applications, fundamental reactive power accounts for the majority of the proportion in all reactive power loss. In order to reduce the switching frequency and improve system capacity, the reactive power can be compensated to a certain range only with fundamental reactive power compensation.

The basic principle is to separate the reactive components in the fundamental load current. Based on the matrix transformation, the fundamental components of the input three-phase load current is converted into a DC component in the corresponding coordinates. The DC component containing a fundamental reactive current is separated through the Low Pass Filter (LPF) [

In most power grids, three-phase voltage asymmetry is more common, which is assumed as:

Based on the symmetric component theory, the three-phase distortion asymmetric current can be decomposed into:_{n}_{+}, _{n}_{−}, _{n}_{0} (_{n}_{+}, _{n}_{−}, _{n}_{0} denote the positive order, negative order, and zero sequence initial phase angle respectively.

The signals sin_{a}_{a}_{a}_{a}_{a}

Three-phase distortion asymmetric current can be converted to the Formula (5) through

The DC component in the above equation is extracted by the LPF as shown in Equation (6):

Then, the inverse of the _{q}

It can be seen from the above equation that when the three-phase voltage is asymmetric, the fundamental positive sequence reactive current deduced from the _{q}

Based on the power quality standards, the asymmetry of the three-phase voltage at the Point of Common Coupling (PCC) of the system is less than 2% and less than 4% for a short time [_{p}_{q}

The three-phase fundamental positive sequence reactive current is obtained through the reverse

Therefore, the improved output of the three-phase grid fundamental positive sequence reactive current _{a}_{1q+} is symmetrical and has the same frequency with grid voltage, so the grid voltage _{a}_{a}_{1q+} to lock the phase to obtain sin

In addition, on the basis of improving the PLL, the response speed and detection accuracy of the

When SVG is working for reactive compensation, the main purpose of LPF is to separate the DC component of the signal, which is the key link of the fundamental reactive current detection algorithm. Since the DC component of

In order to reflect the dynamic real-time compensation performance of SVG, according to the repeatability of each cycle of the AC signal, considering sampling the _{ld}_{ld}

When the input signal amplitude and frequency are not changed, the sliding average output does not change. Once the signal changes, the sliding average output also changes, approaching the true value. The moving average algorithm reflects the speed of data updating in terms of the true value of a periodic average method after a cycle.

In order to achieve the data update, there must be

The expression in

The model of the SMF is established according to (10) in

Analyze the comparison of the SMF with the common Butterworth LPF, by Setting the sampling frequency to 10 kHz and

As shown in

In the detection of reactive current based on instantaneous power theory, SMF can greatly reduce the amount of calculation. It is easy to realize the programming of a DSP chip in engineering practice. The program avoids mathematical floating-point operations of the traditional digital LPF, reduces latency, and improve the real-time performance of reactive power compensation, also has better real-time performance with better dynamic response.

The experimental prototype of 100 kVA SVG is shown in

The RL load is used in this experiment,

Dynamic response test waveform is shown in

For the RC load, SVG can also offer capacitive reactive power compensation as shown in

So SVG can compensate for both inductive reactive power and capacitive reactive power, and continuously adjust the system reactive power, and has faster dynamic adjustment.

In this paper, based on the principle of direct current compensation of SVG, the phase-locked loop in the instantaneous reactive current detection method is improved for the case of an asymmetric three-phase power grid, and the moving average LPF is used instead of the commonly used LPF. The goal of achieving the detection accuracy and dynamic response performance is realized, and the algorithm is small, so it is easy to achieve in the DSP programming. The experimental results show that the SVG system can effectively compensate the reactive power on the three-phase grid side, and have better dynamic response performance compared with the widely used Butterworth LPF, which has certain reference value and applicability.

This research was supported by the project National Natural Science Foundation of China (Grant No: 51777146). Xueliang Wei conceived and designed the study. Jianghua Lu, Wenjing Li and Erjie Qi were responsible for the simulations and experiments. This work was performed under the advisement and regular feedback from Guorong Zhu.

The authors declare no conflict of interest.

The diagram of SVG main circuit structure.

The schematic of direct current control for SVG.

The method of current detection based on _{p}_{q}

Block diagram of reactive current detection for SVG.

The model of a moving average low-pass filter.

The frequency response and step response of the moving average filter and Butterworth low-pass filter.

Experimental prototype of 100kVA SVG.

Waveform of DC side voltage and reactive current.

SVG Waveforms of U–I relation before and after compensation (resistive–inductive load).

Magnitude spectrum of current waveform when SVG working on.

SVG Waveforms of U–I relation when load changing.

SVG Waveforms of U–I relation before and after compensation (resistive-capacitive load).