The impact of distributed photovoltaic on distribution network and countermeasures

With the widespread access of distributed photovoltaics in recent years, its randomness, volatility, uncontrollability, large spatial distribution differences, and inability to adjust beyond the upper and lower limits of resources have caused major changes in the entire power grid system.

In terms of market investment, a large number of distributed photovoltaics are connected to the distribution network, which has caused the country’s investment in the distribution network and technological innovation to increase simultaneously; in terms of market entities, in addition to the original power grid companies, there are distributed photovoltaic owners (investors), distributed photovoltaic Many new entities such as energy operators, comprehensive energy service providers, load aggregators, and platform service providers have diversified; in terms of business operations, horizontal aspects such as spontaneous self-use, surplus power grid, microgrid, incremental power distribution, and virtual power plant market transactions have emerged. Multi-energy-multi-service-diversified operating model; in terms of user demand, in addition to affordable power supply, green, flexible, personalized, and sustainable have gradually become basic needs; in terms of technical support, distributed energy resources are interconnected and distributed The interaction between photovoltaics and the power grid and the sharing of resources by market entities require technical support such as digitalization, artificial intelligence, edge computing, and blockchain.

These changes are bound to have a profound impact on the operation mode and technical support of future power grids.

Influence 1: Impact of electricity consumption

The output of distributed photovoltaic power generation is affected by uncontrollable factors such as light, wind, temperature and other natural conditions. It is connected to the distribution network at a multi-point, multiple, dispersed, interstitial and random high proportion, causing the power grid to fluctuate and is not easy to absorb. The control and maintenance are complicated, which brings great hidden dangers to the power balance regulation and power grid carrying capacity.

Influence 2: Impact on power quality

Distributed photovoltaic access can be started or stopped at any time and the energy output can be adjusted according to the different needs of users. The uncertainty of access time and output will directly or indirectly cause voltage flicker; at the same time, distributed photovoltaic access is mostly used The electronic inverter is used for control and regulation. Frequent switching and use will generate a large number of harmonic components. The excess harmonics will cause pollution to the distribution network, increase the loss of power equipment, shorten its life, and even directly affect the operation.

Influence 3: Impact on distribution network voltage

The time-varying characteristics of distributed photovoltaic power generation make it easy for changes in reactive power input and load active power to affect system voltage fluctuations and steady-state voltage distribution in the distribution network, which in turn affects the stability of the voltage of each user in the power grid.

Influence 4: Impact of system protection

With the access of distributed photovoltaics, the distribution network gradually forms a huge radiating multi-source power supply network system. The traditional unidirectional relay protection system has its protection problems due to distance changes, over-current changes, fault current capacity and direction changes, etc. The mechanism and fixed value are highly protected and have great uncertainty.

Influence 5: Impact on operation and maintenance security

When distributed photovoltaic generates the island effect, it will threaten the safety of power grid maintenance workers, increase the hazard index of maintenance faults, and seriously affect the reliability of power supply in the distribution network.

Influence 6: Impact of power grid planning

Distributed photovoltaic access adds more uncertain factors to power grid planning - geographical differences directly affect the amount of power generation, the harmonics generated and the difficulty of detecting voltage fluctuations increase the load carrying capacity of the grid. In addition, it also increases the difficulty of grid transformation. and construction costs. Therefore, power grid planning needs to consider the influences of many parties before making optimal choices.

In order to deal with the impact of a large proportion of distributed photovoltaic being connected to the distribution network, government agencies, power grid companies, power companies, scientific research institutes, etc. have brainstormed and proposed relevant solutions from multiple perspectives such as policy, technology, market, platform, and project. Guide and strengthen the upgrading and transformation of the distribution network, and strive to ensure that all distributed photovoltaics are connected.

Response Option 1: Distributed photovoltaic + energy storage

The energy storage device can smooth the output power of the power supply through appropriate charging and discharging, thereby reducing the impact or fluctuation caused by distributed photovoltaic access. Combining distributed photovoltaic and energy storage technology can greatly improve the energy utilization and economy of the power grid system. , to increase the scale of distributed photovoltaic grid connection.

Corespondence Plan 2: Strengthen the sensing and control capabilities of distribution network equipment

Use intelligent grid-connected circuit breakers or communication modules adapted to user grid-connected inverters to achieve distributed photovoltaic monitoring and safe grid-connected management and control, and build a smart interconnection system for power distribution equipment to strengthen power distribution devices and new digital technologies Integration enables panoramic perception of the operating status and environmental information of distribution networks such as distribution stations, distribution lines, distributed photovoltaics, charging stations/piles, etc.

Response 3: Flexible AC and DC hybrid control technology

Apply devices or systems similar to energy routers to aggregate distributed resources in the station area to build microgrids, and achieve operational strategies such as coordination and autonomy within a single microgrid, energy mutual aid between multiple microgrids, and collaborative optimization between microgrids and distribution networks. Realize the coordination between flexible resources such as high-proportion distributed power generation and the power grid to meet regional reliable power supply needs.

Coping plan four: Grid connection security and stability strategy

Establish photovoltaic power generation unit simulation models, site detailed simulation models and parameter libraries, system simulation to restore large-scale photovoltaic off-grid accidents, simulate the impact of photovoltaic grid connection on power quality, and clarify photovoltaic power generation grid connection stability boundary conditions and technical requirements as well as high-level /Grid connection tests such as low voltage ride through and grid adaptability. In addition, the photovoltaic station active support control system should be strengthened to realize the station's rapid response to system voltage and frequency, so as to have rapid ride-through or cut-off capabilities to ensure the stability of the power grid.

Countermeasure 5: Offline and online analysis to achieve schedulability

Conduct offline and online analysis of photovoltaic power generation, use probability to coordinate various uncertain issues caused by the connection of photovoltaic power generation to the grid, monitor and analyze the operational risks of connecting to the grid, and track the change process of the grid operating status online to make reasonable arrangements and optimize the way the grid operates.

Coping plan 6: Numerical simulation and power prediction

Through typical weather identification, artificial intelligence modeling, power prediction models and adaptive switching systems, numerical analysis materials are used to improve the simulation and prediction capabilities of different weather processes, analyze future photovoltaic resource change trends, and establish distributed photovoltaic fluctuation processes and power predictions. correlation to predict photovoltaic power generation.

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