Analysis of SVG Function with PV Inverter

Author: Haijun

2022-05-25 17:01

As the main clean energy, solar energy is widely used in photovoltaic power stations. However, because the output power of PV systems will be affected by factors such as weather and temperature, resulting in changes in the active power output to the grid connection point, the reactive power adjustment of the system is required to stabilize the voltage of the grid-connected point. During the operation of the power grid, the output power that changes over time will not only affect the stability of the power grid, but also affect the quality of electrical energy. With the increase in the application of renewable energy power generation, its impact on electric energy and the grid will become greater and greater. Therefore, it is necessary to rationally deploy reactive power compensation devices based on the actual conditions of the power station to provide reliable guarantees for the safe and stable operation of the power station and power support for the grid.

1. Reactive power trend direction of photovoltaic power station

The main equipment that affects the reactive power of the ground power station is the step-up transformer, the step-up line and the line of the collection station in the power station. In the daytime, the main factors affecting the reactive power of utility power station are the reactive power loss caused by the step-up transformer and lines in the station. At this time, the photovoltaic power station absorbs reactive power from the grid. At night, the main reactive power influencing factors are the excitation reactive power of the step-up transformer in no-load operation and the capacitive reactive power on the line. At this time, the reactive power is returned to the grid in the capacitive state.

2. Introduction to existing SVG compensation schemes

At present, most photovoltaic power plants adopt the scheme of installing SVG reactive power compensation devices. Because the reactive power compensation adjustment device of SVG has smooth voltage control ability and short response time. Even in the case of undervoltage, the compensation capability is very strong, which can improve the performance of photovoltaic power plants, guarantee the quality of electric energy, and effectively improve the stability of the power grid.

2.1 SVG principle

SVG type reactive power compensation device is an active reactive power generator using IGBT. Compared with the SVC that uses large-capacity capacitors and reactors, SVG realizes the conversion of reactive energy through the switch function of power electronic devices, and can dynamically emit and absorb reactive power. The SVG power module is a bridge circuit composed of multiple IGBT

and parallel connected to the grid in parallel through a reactor. The reactor can effectively suppress the harmonics generated by the SVG switching circuit, make the non-step fluctuation of the reactive power output by the SVG smoother, and prevent the current impact from malfunctioning. The schematic diagram of SVG principle is as follows:

2.2. SVG equipment composition and advantages

（1） Main equipment composition

SVG equipment is mainly composed of the linking groups of reactors (the linking groups of transformers), starting device, IGBT valve set and control system.

·Linking groups of reactors: to achieve electrical isolation, increase system reliability, and inhibit current mutations.

·Starting device: buffer the starting circuit to reduce the impact of grid connection.

·IGBT valve set: core component, for real-time power conversion.

·Control system: real-time acquisition of current and voltage signals, calculating and analyzing the reactive power and power quality.

（2） SVG advantages

SVG has been widely used in all aspects of power generation, transmission and distribution, such as new energy power generation, power systems, electrified railways, urban rail transit, airports, ports, metallurgy, chemical industries and other industries. Compared with the traditional compensation device, it has the following advantages:

·Fast response speed: SVG can effectively suppress voltage fluctuation and flicker.

·Good low-voltage characteristics: the output current is not affected by the bus voltage, and it can effectively support the bus voltage.

·Good compensation performance: two-way adjustable reactive power, can quickly adjust reactive power output, and ensure that the power factor of the assessment point meets the standard.

·The operating loss is small: Compared with the traditional compensation device, the power electronic device has high operating efficiency and low loss.

·Good harmonic characteristics: low output voltage and current harmonic distortion rate. (Please refer to the attachment for more details.)