2.1 how SVG works

Figure 2-1 SVG compensation schematic diagram

2.1.1 principles of SVG function

Users can set the parameters so that the equipment can be equipped with functions such as harmonic filtering, dynamic reactive power compensation, three-phase unbalance compensation and voltage drop compensation.

Principle of harmonic filtering

SVG collects current signal in real time through external current transformer, separates the harmonic part through internal detection circuit, and generates compensation current equal to and opposite to the harmonic in the system through IGBT power converter, so as to achieve the function of filtering harmonic.

The output compensation current of SVG changes dynamically and accurately according to the harmonic quantity of the system, so there is no problem of over-compensation. In addition, SVG has an overload protection function. When the harmonic quantity of the system is greater than the filter capacity, the device can automatically limit the output at 100% rated capacity without overload.

Compensating reactive power principle

Based on the reactive power of the system, SVG generates capacitive or inductive fundamental wave current through IGBT power converter, so as to achieve the purpose of dynamic reactive compensation. The target value of compensation can be set through the operation panel, and there will be no overcompensation, and the compensation will be smooth, and no inrush shock to load and power grid will be generated.

Compensate three - phase unbalance principle

SVG rotates the dq coordinate according to the current of the system, extracts the unbalanced component, and the three-phase sends out the current equal to the unbalanced component and opposite to the phase. By compensating the unbalanced part to zero, the three-phase unbalanced current can be corrected to the three-phase balanced current.

2.1.2 principles of SVG internal control

Figure 2-2 schematic diagram of SVG internal control

As shown in figure 2-2, after the isolation switch is closed, in order to prevent the instantaneous impact of the grid on the dc bus capacitor when the power is on, SVG first charges the capacitor of the dc bus through soft resistance. This process will last for several seconds. When the bus voltage Udc reaches a predetermined value, the main contactor closes. As an energy storage device, dc capacitor provides energy to output compensating current through IGBT inverter and internal reactor. SVG collects current signals in real time through external CT and sends them to signal conditioning circuit, and then to the controller. Controller will fundamental component separation, extract all of the harmonic current, reactive current and three-phase unbalance current, will be collected to compensate current components and SVG has difference, compare with the compensation current as a real-time compensation signal output to the driving circuit, trigger the IGBT converter will offset current injected into the power grid, realize closed loop control, complete compensation function.

2.2 Dynamic and steady state characteristics of SVG

SVG is particularly notable for its dynamic, steady-state characteristics. The following figure shows the dynamic behavior of SVG when the load changes and the steady-state behavior when the load is full.

Yellow: mains voltage; Green: mains current (compensation target); Red: load current; Blue: device current
Dynamic characteristics during load transition: response time <10ms	Steady-state characteristics at full load: THDi compensates from >70% to <3%

Figure 2-3 dynamic steady-state characteristics

2.3 SVG product features

1.Modular design, any module failure will not affect the normal work of other modules, greatly improving the reliability of the whole machine;

2. Reactive power compensation can make the power factor reach 1; Three - phase current imbalance can be corrected to complete balance;

3.Filtering, compensating reactive power and compensating three-phase unbalance can be selected single or multiple, and the priority of functions can be set

4.The sliding window iterative DFT detection algorithm is adopted, with fast calculation speed, instantaneous response time less than 0.1ms, and full compensation response time less than 10ms

5.The CT wiring position in the field can be sampled at either the load side or the grid side

6.It can be connected in parallel with any LC passive device in the field without resonance

7.Adopt reliable current limiting control link. When the current to be compensated in the system is larger than the capacity of the device of static reactive power generator, the device can automatically limit the current at 100% capacity output, maintain normal operation, and will not be overloaded and burnt out.

8.The main circuit adopts three-level structure of three-bridge arm, with high output waveform quality and low switching loss

Adopt high definition 7-inch touch screen, easy to operate, screen real-time display system and device operation parameters, with fault alarm and recall function

To save space for users, the maximum power of 600mm wide cabinet is 450A/300kvar, and the maximum power of 800mm wide cabinet is 900A/600kvar, which is the first in the industry.

9.The input end of the static reactive power generator is designed in accordance with the second-level lightning protection and equipped with reliable surge protection device, which can protect the equipment in case of lightning strike without damaging the equipment;

Adopt layered design, dust and rain will not adhere to the circuit board, adapt to the use of harsh conditions.  

2.3 SVG product features

1.Modular design, any module failure will not affect the normal work of other modules, greatly improving the reliability of the whole machine;

2. Reactive power compensation can make the power factor reach 1; Three - phase current imbalance can be corrected to complete balance;

3.Filtering, compensating reactive power and compensating three-phase unbalance can be selected single or multiple, and the priority of functions can be set

4.The sliding window iterative DFT detection algorithm is adopted, with fast calculation speed, instantaneous response time less than 0.1ms, and full compensation response time less than 10ms

5.The CT wiring position in the field can be sampled at either the load side or the grid side

6.It can be connected in parallel with any LC passive device in the field without resonance