The Impact of Grid-connected Photovoltaic Power Generation on Power Quality and Solutions

Abstract：Photovoltaic power generation, as a new and pollution-free power generation method, has greatly alleviated the demand for traditional electric energy. However, for the grid-connected photovoltaic power generation system, due to its inherent randomness, volatility, and intermittent characteristics, and the grid-connected photovoltaic power generation system contains a large number of nonlinear power electronic components, compared with traditional power generation methods, photovoltaic power generation has The power quality of the grid has a great impact. This article analyzes the harmonics, voltage fluctuations and flickers, DC injection, islanding effect and other problems caused by grid-connected photovoltaic power generation on the power grid, and studies and discusses feasible measures to improve power quality.

0 Introduction

With the acceleration of the process of internationalization, the rapid development of the world economy, energy consumption has also increased, and the gradual depletion of traditional energy and environmental problems are increasingly serious, solar energy as a clean, pollution-free renewable energy has been paid close attention by people. In recent years, the installed capacity of photovoltaic power generation continues to expand, and the on-grid electricity has also increased year by year, but because of the characteristics of its installed capacity is generally small, the site layout is relatively dispersed, and the output power fluctuation is large, it has also caused a great impact on the power quality of the grid. Therefore, it is of great significance to study the influence of photovoltaic power generation on power quality to promote power production and the safe and stable operation of power grid.

1 Basic Principle of Photovoltaic Power Generation

Photovoltaic power generation uses the photovoltaic effect present on the surface of the semiconductor to send a direct current through the light at both ends of the semiconductor material. When the sun shines on the semiconductor P-N node, a new electron-hole pair is formed, and after the photon excites the electron from the covalent bond, the electron flows to the N region and the hole flows to the P region, resulting in a potential difference between the two ends of the semiconductor. Once the circuit at both ends of the PN junction is connected, a current will be formed, flowing from the P zone to the N zone through the external circuit, and the electrical power will be output to the load.

2 Structure and Classification of Grid-connected Photovoltaic Power Generation

Grid-connected photovoltaic power generation system is mainly composed of solar panel (module), high power tracking (MPPT) controller, DC-AC inverter several parts, using insulated gate bipolar transistor (IG-BT) as the switching element of photovoltaic inverter. The DC output of the solar cell is raised by the DC-DC converter to increase the voltage level, and then the DC is converted into alternating current with the same amplitude, frequency and phase of the voltage of the power grid through the DC-AC inverter, so as to realize the integration into the power grid or supply power to the AC load. The structure of the photovoltaic power generation system is shown in Figure 1.

Figure 1 Grid-connected photovoltaic power generation system structure

According to the grid-connected operation mode, the photovoltaic power generation system can be divided into three forms: countercurrent grid-connected, no countercurrent grid-connected and switching grid-connected. Grid-connected photovoltaic power generation system is directly connected to the power grid, does not require energy storage batteries, saves the floor area, greatly reduces the configuration cost, and the load power deficit is supplemented by the power grid. Therefore, grid-connected photovoltaic power generation system is the main development direction of solar power generation, and it is also a potential new energy power generation method at this stage.

• The influence of grid-connected photovoltaic power generation on power quality of the grid

Photovoltaic power generation as a new energy generation, lighting, temperature and other external conditions of randomness, volatility, intermittent changes are the main factors of photovoltaic power generation impact on the grid. Among them, DC-AC inverter is one of the main devices of grid-connected photovoltaic power generation system, and the quality of photovoltaic inverter determines whether the power quality of photovoltaic power generation can meet the requirements of grid-connected to a certain extent. When photovoltaic power generation is connected to the grid, problems such as harmonics, voltage fluctuations and flickers, DC injection, and island effect will occur, which will reduce the power quality of the grid and cause adverse effects on the grid. In serious cases, it will disrupt the safe and stable operation of the power supply system and photovoltaic power generation equipment itself.

3.1Harmonic Influence

Photovoltaic power generation is to convert solar energy into direct current through photovoltaic modules, and then through the grid-connected inverter to convert direct current into alternating current to achieve grid-connected. In the photovoltaic power generation system, the inverter is the main equipment to produce harmonics. The large number of applications of power electronic components in grid-connected inverters has improved the information and intelligent processing of the system, but it also increases a large number of nonlinear loads, causing waveform distortion and bringing a large number of harmonics to the system. The delay of the switching speed of the inverter will also affect the output of the overall dynamic performance within the power system, resulting in a small range of harmonics. If the weather (irradiance, temperature) changes greatly, the fluctuation range of harmonics will also become larger. Although the output current harmonics of a single grid-connected inverter are small, the output current harmonics of multiple grid-connected inverters will be superimposed after they are connected in parallel, resulting in the phenomenon of output current harmonics exceeding the standard. In addition, the parallel connection of inverters is easy to produce parallel resonance, which leads to the coupling resonance phenomenon, resulting in the expansion of specific harmonic current and the problem of excessive harmonic content of grid-connected current.

Aiming at the power quality problem after photovoltaic access, the effective methods of suppressing harmonics are put forward:

• Starting from the source of harmonic generation, the harmonic source is reformed to reduce harmonic injection.
• Device active or passive filters to absorb some specific number of harmonic currents.
• Install additional harmonic compensation devices.

3.2 Voltage fluctuations and flickers

In the traditional distribution network, the change of active power and reactive power with time will cause the system voltage fluctuation. For photovoltaic power generation, the change of active power of photovoltaic power generation system is the main factor that causes voltage fluctuation and flicker of access point. The high power point of photovoltaic panels, the core components of photovoltaic power generation system, is closely related to radiation intensity, weather, season, temperature and other factors, and the random changes of these natural factors cause the output power to change greatly, resulting in frequent changes in the load power within a certain range, resulting in voltage fluctuations and flickers at the load end of the grid-connected user.

At present, the solutions to the photovoltaic voltage fluctuation and flicker problems are as follows:

• Optimize the control strategy of photovoltaic grid-connected inverters to improve the voltage stability.

2) Increase the short circuit capacity of the substation bus.

3) When the capacity of the photovoltaic power station is determined, its power factor is increased to increase the total active power, thereby reducing the amount of reactive power change and meeting the limit requirements of voltage fluctuations.

3.3 Dc injection problem

Another key problem to be solved in grid-connected photovoltaic power generation system is DC injection. Dc injection affects the power quality of the grid, and also brings adverse effects to other equipment in the grid. IEEEStd929-2000 and IEEEStd547-2000 clearly stipulate that the DC current component injected into the grid by the grid-connected power generation device cannot exceed 0.5% of the rated current of the device. The main reasons for DC injection are:

The dispersion of the power electronic device itself and the inconsistency and asymmetry of the drive circuit; 2) Zero drift and nonlinearity of measuring devices in high-power controller; 3) Asymmetry of line impedance of each switching device, influence of parasitic parameters and parasitic electromagnetic fields, etc.

At present, the main methods to suppress DC injection include: 1) detection compensation method; 2) Optimize and design the grid-connected structure of the inverter; 3) Capacitor straight separation; 4) Virtual capacitance method; 5) Device isolation transformer.

3.4 The effect of island effect

The islanding effect refers to the phenomenon that the grid power supply is interrupted due to human factors or natural factors, but each grid-connected photovoltaic power generation system fails to detect the power grid blackout state in time, so that the photovoltaic power generation system and the connected load still operate independently. With the continuous expansion of the access penetration rate of grid-connected photovoltaic power generation, the probability of island effect is gradually increasing. The formation of the island effect has adverse effects on the power quality of the entire distribution network, mainly including:

• at the location where the island effect occurs, the voltage and frequency fluctuate greatly, which reduces the power quality, and the voltage and frequency in the island are not controlled by the power grid, which may cause damage to the system electrical equipment and reclosing faults, and may also cause personal safety hazards to the power grid maintenance personnel.
• In the process of power supply recovery, inrush current will be generated due to the asynchronism between voltage phases, which may cause the grid waveform to drop instantaneously.
• After the island effect of photovoltaic power generation system, if the original power supply mode is single-phase power supply mode, it is possible to cause the problem of three-phase load asymmetry in the distribution network, and then reduce the overall quality of electricity consumption of other users.
• When the distribution network switches to the island mode and only relies on the photovoltaic power generation system to supply electricity, if the power supply system capacity is too small or no energy storage device is installed, it may cause voltage instability and flicker problems in the user load.

For the impact of island effect, there are mainly the following solutions:

• Optimize the island detection method of grid-connected photovoltaic power generation system, analyze the influence of photovoltaic power generation on the size, direction and distribution of fault current in distribution network, and improve the selection technology of load cutting speed and island division under fault conditions.
• Improve the reliability of island detection technology, configure fast and effective anti-island protection function, accurately judge the island status under abnormal circumstances and quickly and effectively interrupt the grid connection.

4 Solution

4.1Power Quality Online Monitoring

The APView500 power quality online monitoring device adopts a high-performance multi-core platform and embedded operating system, and measures the power quality indicators according to the measurement methods specified in IEC61000-4-30 "Test and Measurement Technology - Power Quality Measurement Methods". It integrates harmonic analysis, waveform sampling, voltage dip/rise/interrupt, flicker monitoring, voltage unbalance monitoring, event recording, measurement control and other functions. The device has reached the IEC61000-4-30A-class standard in the standardization of power quality index parameter measurement methods, the measurement accuracy of index parameter, clock synchronization, event marking function and other aspects, and can meet the requirements of power quality monitoring of 110kV and below power supply systems.

4.2 Anti-island protection device

When the anti-island protection device detects that there are abnormal data such as reverse power, frequency mutation, etc., that is, when the island phenomenon occurs, the device can cooperate with the circuit breaker to quickly cut off the node, so that the station and the power grid side are quickly separated, and ensure the life safety of the entire power station and related maintenance personnel.

4.3 Product Introduction

5 Conclusion

With the rapid development of China's photovoltaic power generation industry, the installed capacity and quantity of grid-connected photovoltaic are increasing, which has greatly affected the power quality of the grid. Therefore, it is necessary to study the influence of grid-connected photovoltaic power generation on the power quality of the grid. This paper analyzes the basic principle and structural characteristics of photovoltaic power generation, expounds the causes of harmonic, voltage fluctuation and flicker, DC injection and island effect in grid-connected photovoltaic power generation, and puts forward feasible measures to improve power quality, which has certain reference significance for further improving the power quality of photovoltaic power generation.

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