Shicheng Zheng ,Ying Shu ,Mengmeng Qi ,Jiahong Lang and Xuefeng Hu

(1.School of Electrical and Information Engineering,Anhui University of Technology,Ma' anshan 243002,China;2.Key Lab of Power Electronics &Motion Control,Anhui University of Technology,Ma anshan 243002,China)

the inverter and grid is small,and there is a larger capacitor on the DC side,so a large starting surge current will be generated when the equipment is directly launched without control.As a result,there will be starting failure for the electrical and mechanical shocks caused by the starting current.More seriously,the equipment could also be damaged.Soft-start is a key technology to restrain unfavorable factors such as current impact and DC side voltage overcharge during the starting process,ensuring that the equipment can be started stably and put into operation smoothly.Therefore,soft-start technology is significant and difficult in the design of low-voltage SVG systems[13].

Hence,this paper proposes a new open-loop soft-start method based on the D-axis orientation,which not only realizes the non-impact soft-start of SVG,but also reduces the difficulty of debugging,while being easily implemented.

2 Topology and principle of SVG

The topology of the system is shown in Fig.1.The voltage voltage-source type three-phase bridge converter circuit is composed of IGBTs installed in parallel on the load side through series reactors,and the dynamic compensation of reactive power could be realized according to the actual load[14-15].Its principle can be illustrated by the single-phase equivalent circuit and vector diagram,which are shown in Fig.2.

Fig.1 System topology

Fig.2 Equivalent circuit and vector diagrams

3 Soft-start strategy

3.1 Traditional strategy

In order to reduce the starting inrush current,many typical control methods have been employed to improve the system performance.But these methods are based on the following premise:the soft-start resistor is removed when the pre-charge ends and the PWM rectification method based on the closed-loop control is used to complete the soft-start of the DC side voltage.The equivalent schematic diagram of the device is shown in Fig.3,where the output of the inverter is connected to the power grid through a reactor,a soft-start resistor,a contactor,and a circuit breaker.The conventional starting method is to keep the soft-start contactor closed at the end of the uncontrolled rectification phase,then the soft-start resistor is shorted out,and the DC side voltage can reach the given value by using the closed-loop voltage control method,completing the soft-start process of the system[18-20].

Fig.3 Equivalent diagram of SVG

However,it can be seen from Fig.3 that when the soft-start contactor is closed,the inverter starts to send out the PWM signal,due to the influence of dead zone and other factors,the output voltage of the inverter is smaller than the grid voltage.The soft-start resistor has been removed at this time,so that a voltage difference will be generated on the connected reactor.As the inductive impedance of the reactor is very small,this voltage difference will generate a large surge current at the moment of switching,which will threaten the safety of power devices and eventually lead to the soft-start failure of the system.In addition,parameter tuning using the closed-loop PI control strategy is also difficult,which is not conducive to soft-start debugging.

3.2 Principle of D-axis orientation

In this paper,the open-loop soft-start control method with resistor based on D-axis orientation of the grid voltage is proposed to solve the problem that has been discussed above.The schematic diagram of coordinate transformation (abc/αβ/dq) is shown in Fig.4.

Fig.4 Schematic diagram of coordinate transformation()

The transformation process is as follows:first,the instantaneous values of the three-phase grid voltage and current are transformed from the three-phaseabcstatic coordinate system to the two-phase orthogonalαβstatic coordinate system through Clark transformation,and then further transformed to the synchronously rotatingcoordinate system by Park transformation.

The Clark transformation matrix and Park transformation matrix are respectively given by

3.3 Analysis of the PWM boost rectification process

The open-loop voltage control is applied in the D-axis orientation soft-start stage.The equivalent schematic diagram of the AC side with a soft-start resistor for PWM rectification is shown in Fig.5,whereRis the sum of the soft-start resistor and the circuit equivalent resistor,XLrepresents the equivalent impedance of the reactor,UIrepresents the output voltage of the three-phase inverter side,andUSrepresents the grid voltage.At the soft-start stage,the soft-start resistor is still connected in series in the circuit,therefore the amplitude of the grid voltage is higher than the inverter side,and the currentIflows out of the power grid,which indicates the power grid is delivering power to the inverter side.

Fig.5 Equivalent schematic diagram of the AC side

Fig.6 shows the vector diagram of the AC side voltage with a soft-start resistor for PWM rectification,whereUIrepresents the output voltage of the three-phase inverter side,USrepresents the grid voltage,URis the voltage drop across the circuit resistor,ULis the voltage drop across the reactor,andθis the angle between the current and the grid voltage,that is,the power factor angle.

Fig.6 Vector diagram of the AC side voltage at the soft-start stage

The expression for the power factor angleθcould be written as

Simultaneously,when the modulation ratio is set to a maximum value close to 1 (such as 0.99),the phase and amplitude of the output voltage of the inverter sideUIis consistent with the grid voltageUs.Even if when the output voltage of the inverter sideUIis slightly smaller than the grid voltageUSdue to factors such as device voltage drop and dead zone,the existence of the soft-start resistor and the resistor is much greater than the equivalent impedance of the connected reactor,therefore,there is no current surge in the system.whereUuvis the output line voltage of the three-phase inverter bridge andUdcis the DC side capacitor voltage.

It can be seen from Eqs.(12) and (13) that the voltage variation process of the DC side capacitor can be expressed as follows

With the periodic decrease of the modulation ratioM,the DC side capacitor voltageUdcis gradually increased until its value is equal to the rated voltage valueUgiv.

This paper adopts the soft-start method based on D-axis orientation.After the uncontrolled rectification stage finishes,coordinate transformation is performed on the grid voltage signal.Through the D-axis orientation of the grid voltage,a sinusoidal PWM signal synchronized with the grid voltage signal is generated and used as a modulation wave.After modulation,the six-PWM waves are generated to drive the power devices,and then the inverter bridge will output the AC voltage synchronized with the grid voltage,it is superimposed on both ends of the soft-start resistor with the grid voltage.By using the open-loop control,the modulation ratio of the PWM wave is gradually reduced until the voltage of capacitor on the DC side reaches the rated value.At this time,the soft-start resistor is cut off,and the entire soft-start process is completed.

4 Experimental results

Based on the above theoretical analysis,a±100 kVar SVG device is developed.The main and rear views of the SVG are shown in Fig.7,where the soft-start resistor is shown in Fig.7a.

Fig.7 Experimental prototype platform of the SVG

The main parameters of the SVG are shown in Tab.1.

Tab.1 Main parameters of the SVG

Fig.8 shows the soft-start waveform under the conventional soft-start method.Fig.9 shows the soft-start waveform obtained under the method proposed in this paper.It can be seen from the experimental results that under the proposed method,the soft-start waveform rises more smoothly and has no overshoot.

Fig.8 Soft-start waveform under the conventional soft-start method

Fig.9 Soft-start waveform obtained under the proposed soft-start method

In order to further verify the dynamic and steady performance of the SVG,a closed-loop grid-connected experiment is carried out.The voltage and current waveforms of phase A of the grid side at steady state is shown in Fig.10.

Fig.10 Voltage and current waveforms of phase A of the grid side

As shown in Fig.10,the phases of the voltage and current are consistent,and it can be concluded that the SVG has achieved unity power factor control.Fig.11 shows the experimental waveforms under different load conditions.It can be seen from the figure that the system can quickly track the change of the reactive current signal on the load side,and the phases of voltage and current are consistent during the entire soft-start process.

Fig.11 Experimental waveforms under different load conditions

5 Conclusions

This paper discusses a soft-start strategy used in SVG.The reason for the failure of traditional soft-start is analyzed and a novel SVG soft-start control strategy based on D-axis orientation is proposed.Through experiments,the soft-start waveforms under two methods are obtained,which are consistent with the theoretical analysis.The experimental results verify the validity and feasibility of the proposed scheme.