From the interaction between distributed photovoltaic power plants and distribution networks - talk about inverter technology

December 27, 2019

Background introduction

Today, all walks of life in China, from policy makers to market practitioners, have reached a consensus to continuously increase the penetration of renewable energy, including photovoltaics, wind power, and biomass power generation. With the advantages of distributed generation, fully exploiting the available scattered energy sources and increasing the efficiency of energy use can truly achieve the goal of environmental protection, energy conservation and emission reduction.

The traditional power distribution system is designed to only have the function of distributing the power to the end user. The design is mainly based on the vertical connection scheme. Its characteristics are centralized power generation, decentralized consumption, and limited interconnection capacity between different power grid control areas. In the future, the penetration rate of distributed energy in China will increase substantially. This is an important factor that drives the gradual change of grid control mode from central control to distributed multi-tier control. With the increasing degree of liberalization of the electricity market and the continuous development of photovoltaic technology, the construction of large-scale power plants needs to face enormous economic pressures, which is far less cost-effective than the construction of small-scale distributed photovoltaic power plants.

The quality assurance and safety of distributed photovoltaic energy in the power supply is very attractive to those countries that strongly rely on fossil energy as the power supply and the aging of power transmission infrastructure. As the distributed technology continues to mature, power plants from 1kW to 10MW capacity are connected to the low-voltage power grid, which provides electricity to the grid. Distributed energy needs to be integrated into the grid rather than simply connected to the power grid.

The benefits of integrating distributed PV power plants into grid planning are as follows:

Short-term benefits: Reduce the transportation loss caused by the transmission and distribution of electricity, improve the service quality and sustainability of the grid during peak hours, and reduce greenhouse gas emissions.

Medium- and long-term benefits: Delay the investment in future power grid expansion, and reduce the huge extra power generation equipment that is added to meet the load requirements during the peak period of power usage. Distributed photovoltaic power plants are generally required to be deployed at power consumption points, such as urban areas. At the same time, photovoltaic modules can also be used as tools for public education on the general knowledge of photovoltaics. They can also awaken public awareness of environmental protection and call for everyone to accept and use clean energy.

Distributed energy sources are usually connected to medium-voltage or low-voltage power distribution systems and have a wide and far-reaching impact on distribution systems. From a technical point of view, for the integration of distributed photovoltaic power plants into the grid, the inverter is the key to controlling the mutual influence between the two. In order to better integrate the distributed photovoltaic power plant with the local power grid system and better coordinate the interaction between the distributed photovoltaic power plant and the distribution network, such as controlling the quality of energy and ensuring the safety of the system and the power grid, the core of these problems is solved. It is a photovoltaic inverter.

This article analyzes from two aspects. First, the impact of distributed photovoltaic power plants on distribution networks, the current technical obstacles; Second, the impact of distribution networks on distributed photovoltaic power plants. The key to solving the technical problems caused by the interaction between the two is the inverter. Through the discussion of the interaction, the future of inverter technology innovation and development is outlined.

I. Impact of distributed photovoltaic power plants on distribution networks

The author discusses the impact of distributed PV power plants on distribution networks from the following aspects, including several common problems in the power industry: voltage, current, and power. And how to solve the problem, how does the inverter work?

Inverter control voltage fluctuation

In traditional distribution network system design, electrical energy is delivered at the highest pressure and consumed at low pressure. With the increase of the penetration rate of distributed energy, the power flow will become more complex. Under the influence of distributed photovoltaic power plant operation, low voltage distribution system may cause undesirable over-voltage conditions.

Distribution system staff are obliged to guarantee power quality for the grid customer. It is relatively common for voltage levels to change due to load changes in a distribution network. Technical specifications will give an acceptable over-range of specified values. For example, in China, the voltage change of the municipal low-voltage power grid changes 10% above and below the rated value and is usually considered acceptable. Generally, the power input from a distributed photovoltaic power station is 2 to 5% higher than the national standard compared to the rated voltage of the access point. In general, there is a general rule in the power grid that the power generated by a distributed photovoltaic power plant cannot exceed 33% of the low-voltage network and 50% of the medium-voltage network. The purpose of this design is also to avoid voltage fluctuations in the power grid.

German researcher Rutschmann's research shows that when large-scale distributed photovoltaic power plants are connected to voltage-controlled distribution network lines, the voltage of the end customer will increase due to the presence of reverse power flow. The value of the voltage rise is determined by the ratio of the capacity and load of the distributed photovoltaic power plant. At present, due to the small-scale construction of distributed photovoltaic power plants in China, there is no report on the related voltage increase. Rutschmann's research in Germany shows that these phenomena occur in rural power grids. The reason is that the load on rural users is too small. Therefore, distributed photovoltaic power plants installed in urban areas have very low probability of such problems. At the same time, due to the overvoltage protection function added to the current design of the inverter, once the voltage reaches the upper limit, the inverter automatically switches to the voltage controller, so it is also effective to avoid such accidents.

In general, a reasonable strategy is to strictly control the ratio between the load of the customer terminal and the power of the corresponding distributed photovoltaic power plant to avoid a large asymmetry.

On the other hand, because the output power of a distributed photovoltaic power plant is subject to certain volatility due to the amount of solar radiation, the volatility of the output power will cause fluctuations in the voltage of the distribution network circuit, especially in those circuits where the load is low. on. Through the numerical simulation of the distribution network of residential communities with distributed photovoltaic power stations, it is found that the slowdown of output power changes can effectively reduce the voltage changes. The rate at which the output power of the distributed photovoltaic power plant is controlled is one of the functions of the inverter. That is to say, the voltage fluctuation caused by the distributed photovoltaic power plant to the distribution network can be prevented through the control design of the inverter.

Harmonic and interharmonic currents

In general, the ideal AC power should be a pure sinusoidal waveform, but the power supply waveform is often distorted due to the output impedance and nonlinear load of the power grid system in production and life. China's voltage fundamental frequency is 50Hz. After the distorted AC non-sinusoidal signal is subjected to Fourier transform analysis, its voltage composition can be decomposed into a linear combination of the components of frequency multiplication (100 Hz, 150 Hz,...) In addition to the fundamental frequency (50 Hz). The frequency multiplication component is called harmonic:harmonic. The ratio of the harmonic frequency to the fundamental frequency (n=fn/f1) is called the number of harmonics. Non-integer harmonics sometimes exist in the power grid and are called non-harmonics or interharmonics.

Harmonics are actually a kind of interference. Harmonic currents will produce a large number of times of electric pollution and cause the power grid to be "contaminated." Harmonic harm is very serious. Harmonics reduce the efficiency of the production, transmission, and utilization of electrical energy, causing electrical equipment to overheat, generate vibration and noise, and cause aging of insulation, reduced service life, and even failure or burn. Harmonics can cause partial or parallel resonance or series resonance of the power system, which can amplify the harmonic content and cause the destruction of capacitors and other equipment. Harmonics can also cause relay protection and automatic device malfunctions, causing confusion in the measurement of electrical energy. Harmonics can cause serious interference to communication equipment and electronic equipment outside the power system. The effects and hazards of interharmonics are equivalent to the effects and hazards of integer harmonic voltages.

When many inverters are operated in the same low voltage network, although each individual inverter meets the electrical design specifications, the harmonic currents they operate at the same time may generate harmonic voltages that exceed the specifications. Especially when the impedance and resonant frequency of the power grid change, harmonic currents generated by the multi-inverter harm are most likely to appear. At this time, the processing capability of the current-controlled electronic components will be greatly reduced, while at the same time a large number of non-sinusoidal Waves cannot be fully dealt with.

The characteristics of inverters that may harm harmonic currents in low-voltage power grids are summarized as follows:

In a strong network, the inverter is not the main source of harm to harmonic currents, and the main source is still some non-linear load of the power grid. However, when the grid's impedance and resonance frequency suddenly change, the inverter will cause harmonic current harm to the grid.

In weak networks, the increase in the number of inverters connected to the grid directly increases the probability of generation of harmonic currents.

Direct current from the inverter

For distributed photovoltaic power plants, monitoring of DC currents is essential. The impact of DC on grid equipment is mainly concentrated in distribution transformers, residual current devices (RCD), converters, (active) watt-hour meters, metal structures and cables, the most significant of which was found in residual current devices (RCD). And on the converter, the main manifestations of harmonic distortion, loss, heat and noise.

At present, many inverters use transformers to inherently suppress the occurrence of any direct current. The transformerless inverter has great technical and economic advantages, such as: higher efficiency and less weight, small footprint, low cost, and it has increasingly attracted the attention of the industry. Modern pulse width modulation technology makes it possible to suppress the DC current in the output of the inverter of the five transformers. However, when a positive and negative imbalance exists in the grid voltage waveform, this situation is equivalent to even-order harmonics. Harmonic distortion caused by this imbalance will affect the operation of the pulse width modulation controlled inverter, especially when the inverter is synchronized with the voltage waveform at the zero crossing point of the power grid.

The features of the inverter's direct current hazard are summarized as follows:

Inverters using pulse width modulation technology can reduce the occurrence of DC currents, and do not need to take into account coordinated dynamic behavior and dynamic efficiency.

Single-phase current-mode inverters also do not produce significant DC current components, even when even-order harmonics of the grid voltage are present.

At present, there is no research that shows that the DC power-related disturbances in the distribution network where distributed photovoltaic power plants are installed are caused by the distributed photovoltaic power plants themselves.

Ground fault and leakage current

When the distributed photovoltaic system is exposed to outdoor weather conditions, the current of the photovoltaic system is accidentally connected with the earth and causes insulation failure. This condition is called ground fault. During the 25-year service period of the entire photovoltaic system, this phenomenon is certain. Probability occurs. Even with a good design, ground faults will still occur. The most likely location is in connection boxes, switches, or inverters. The reason may be due to the damage and aging of electronic components or materials. Although in recent years, specialized technical specifications, better design, and better-performing electronic components, these improvement factors have continuously improved the reliability of photovoltaic systems, and at the same time significantly reduced the possibility of ground faults. However, with the advent of BIPV design, a large number of wires in BIPV exist in buildings, which greatly increases the possibility of ground faults. Although leakage protectors can provide maximum protection for photovoltaic systems and workers, this issue is still worth continuing attention and research.

Another consideration is the effect that the capacitor may have on the leakage current. The capacitance here refers to the following: Capacitance in the DC and AC parts of the PV system, the grounding capacitance of the PV module and the bracket system, and the electromagnetic resistance in the inverter. Disturbing filter capacitance. When the AC voltage component plays a role in these capacitors, it is possible to charge the ground. Take an inverter without transformer as an example. When the AC voltage component acts on the capacitor of the filter in the inverter, the capacitor will generate a leakage current, and the leakage current will flow into the ground line. Under normal conditions, the ground line There is no current.

The characteristics of ground fault and leakage current are summarized as follows:

The contact voltage in the photovoltaic system, in particular, the topology of the inverter has the greatest impact on the leakage current. No significant correlation has been found between leakage currents and meteorological conditions such as solar radiation, humidity, atmospheric pressure, and wind speed. In addition, a large number of studies have also shown that the use of aluminum frame components than frameless components, when the reverse contact, the leakage current rate is higher than 50%, while the positive contact is no difference.

When a photovoltaic system has multiple inverters, the potential energy of the ground current will cause a problem area. If the ground wire is destroyed at this moment, it will cause danger in the problem area.

Short circuit capacity of power grid

The short-circuit fault protection of the distribution network is achieved through protection devices such as overcurrent relays and fuses. In general, distributed photovoltaic power plants do not significantly increase the short-circuit failure rate on one side of the distribution network.

The reason is as follows:

1. The short-circuit current of the PV array is self-limited, typically not exceeding 10% to 20% of the rated maximum power current.

2. The undervoltage relay is normally installed in the inverter.

3. Most of the inverters used in distributed power plants are current-controlled. Such inverters are in the state of over-current limit protection when they generate interference from the power grid.

For high-permeability distribution networks, short-circuit faults are more likely to occur when there are certain conditions. For example, short circuit faults are likely to occur at the end of a long distribution line with high impedance, or when the distribution line is overloaded for a long time. Most photovoltaic systems not only fail to detect the occurrence of a fault, but also provide a large part of the short-circuit current while also preventing fault detection. A large number of studies have demonstrated that modern pulse width modulated inverters applied to distributed power supply contribute little to short-circuit fault currents.

The correct strategy is to set up three different levels of short-circuit fault protection at the three different interfaces of the grid, distributed photovoltaic power plants and customers.

Unplanned island phenomenon

When the grid is interrupted, each grid-connected PV system is still operating and connected to the local load in an independent operation. This phenomenon is called islanding. Although current photovoltaic inverters generally adopt the design of predicting islanding phenomenon and cutting off the circuit, it is still impossible to completely avoid the special circumstances in which multiple inverters affect each other in a distributed photovoltaic system in which multiple inverters are connected in parallel. , there is the problem of being unable to detect the island phenomenon.

Power value and load capacity value

The power value of a distributed photovoltaic power plant is determined by the economic value of the production of electricity. It consists of the following components: power generation efficiency; the distance between a given plant location and the load, ie the transportation cost; and the power output during certain periods of time. Quantity (such as peak time); and the balance of output is the match between load and production electricity.

Traditionally, due to inherently uncontrollable solar energy, photovoltaics cannot be used as a stable source of peak-hour power supply in power grid operation management systems. However, when the summer heat wave hits, it is also the arrival of the peak wave of electricity, which means that when the grid enters the most intense moment, precisely when the temperature is the highest, and when the solar radiation is the strongest, the distributed photovoltaic power station can reach it. 60 to 80% of the maximum output power. This point can be used to solve the contradiction between the high demand for local electricity and the incompetence of the entire grid when the peak time arrives.

Another interesting concept is "effective load-carrying capacity (ELCC)", a concept that reflects the ability of a generator to effectively meet an existing load. For a distributed photovoltaic power plant, ELCC represents the percentage of the electrical energy it supplies to the grid as a percentage of the electrical energy needed to load the grid at that time (that is, the value of the grid's load capacity).

The results of a simulation study of distributed photovoltaic power plants based on load data from the United States of America during the period 2002-2003 and based on the amount of radiation generated per hour are assuming that the penetration rate of a distributed photovoltaic power plant is 10-20%. Photovoltaic plants have an average ELCC value between 35 and 55%. From this result, a simple conclusion can be deduced that the construction of a distributed photovoltaic power plant with a basic grid capacity of 10-20% can solve the 35-55% power consumption problem.

Research in New York City in the United States in 2004 showed that at the hottest time of the year in the city, the highest peak of electricity consumption in cities can reach 19 degrees per hour. Simulations of power generation simulations have shown that setting the penetration rate of distributed photovoltaic power plants in New York City to 10% can reduce this figure to 4.5 degrees per hour.

Research shows that the increase in the penetration rate of distributed photovoltaic power plants will greatly alleviate the problem of insufficient local power supply capacity at the peak time.

The study also pointed out that the future trend is to combine distributed photovoltaic power generation with energy management and control, which will make energy consumption more economical and more rational. Examples are as follows: Air-conditioning can use intelligent control of solar radiation, and the PV intelligent temperature control center will regulate the air-conditioning temperature in real time according to the amount of solar radiation and power consumption at that time.

Inverter added value for grid control

With the continuous increase of the penetration rate of distributed photovoltaic power stations, inverters using pulse width modulation technology will be used in large quantities. Not only do they perform the traditional functions of DC to AC, but they also bring a lot of additional benefits to distribution networks.

Reduce voltage harmonics

Can effectively improve the quality of electrical energy. When the inverter is running, it is equivalent to an active filter, which can effectively reduce voltage harmonics in the power grid.

Reactive power control

When the inverter is connected with the load, the inverter not only can supply active power, but also can generate or absorb a part of reactive power. Therefore, the inverter with pulse width adjustment technology used in the distributed photovoltaic power station can compensate the allocated power. The lack or excess of reactive power in the grid has a good effect on voltage regulation.

Power Factor Control and Symmetrical Phase Control

When the grid voltage level reaches a critical high level, not only reactive power control can be performed, active power can also be continuously provided, and the voltage on the transmission line can be further prevented from increasing. In addition, the three-phase inverter can provide unbalanced current to help the grid phase unbalanced voltage return to the balance position.

The stability of the power grid and the prevention of islanding in operation

With the increase of the penetration rate of distributed photovoltaic power stations, distributed photovoltaic energy can be used as an important backup energy supply when the power grid fails or the operating efficiency is declining, and it can effectively control the occurrence of operational islanding.

Having fully developed all the added value of the inverter, it is possible to perform a reassessment of the distributed PV plant's ability to provide all services for the user and the grid, while giving a new cost and benefit model. This is bound to promote the integrated optimization design of grid-connected photovoltaic power plants.

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