In the context of the carbon peak and neutrality targets, China is accelerating the construction of a new type of power system, and the proportion of new energy is gradually increasing. Distributed PV has been the main form of PV development. In 2022, the domestic newly installed photovoltaic capacity was 87.41GW and distributed photovoltaics once again exceeded the ground centralized power stations, accounting for 58% of the total. The explosive growth of distributed photovoltaics will not only bring new risks to the safe operation of the distribution network, but also pose new challenges to the traditional power grid management model.
I. Many local authorities issued policy documents aiming at rectifying the distributed PV market
Recently, authorities in Hubei, Hunan, Henan and Liaoning provinces issued policy documents marking the beginning of rectifying the distributed PV market. During the rectification period, the filing and grid connection of projects were suspended. According to relevant statistics, more than thirteen provinces or subordinate counties across the country have issued policy documents to strengthen distributed management. These documents are summarized in the table below [1]:
The rectification focuses on four aspects:
(1) Strengthening registration management;
(2) Consumption restrictions on new additions. Sorting out the distributed scale of low-voltage distribution network that can be installed and suspending registration in places with high consumption pressure;
(3) Configuration of energy storage;
(4) Participation in trading. Those of 10kV or more are to participate in scheduling, peaking, and market-oriented trading.
Among the above reasons, the main technical reason is the restriction of new consumption. Under conditions of high penetration, the technical risks of distributed photovoltaics are problems such as power reverse transmission and user overvoltage, which affects consumption.
II. Analysis of power reverse transmission and user overvoltage problems
According to the provisions of GB/T12325-2008 Power Quality Supply Voltage Deviation, the low-voltage distribution network node voltage (single-phase 220V, three-phase 380V) deviation should not be higher than +7% of the nominal voltage, and not lower than -10% of the nominal voltage. The occurrence of overvoltage may cause damage to electrical equipment and systems or affect their normal operation.
Although power reverse transmission and overvoltage appear as two individual problems, in fact, load overvoltage is closely related to the size of the user-side photovoltaic reverse transmission grid power. Theoretical analysis shows that the load voltage is closely related to the transformer outlet voltage (if there is a voltage regulating transformer, it depends on the transformer gear setting), line impedance (depending on the power supply radius), photovoltaic penetration rate (depending on the photovoltaic capacity), and load rate (depending on the user load).
Since the load characteristics of low-voltage users are inconsistent with the photovoltaic power generation characteristics, the peak load hours at night are intertwined with the peak hours of photovoltaic power during the day, resulting in significant voltage changes at each node of the low-voltage power grid. During the day when the photovoltaic power generation is in excess, that is, when the photovoltaic power is greater than the user's load power, the power flow will inevitably be fed back to the upper-level grid, and the load voltage UL>Us will be generated, and even overvoltage will occur. On the other extreme, low voltage may occur during heavy load periods at night.
Figure 1 Analysis of the range of voltage fluctuations in the low-voltage distribution network affected by photovoltaics
Analyzing the calculation example in Figure 1, when the user-side distributed photovoltaic penetration rate reaches 1 (that is, the photovoltaic capacity equals the distribution transformer capacity, the existing limit), and the user is running with light load, the user load overvoltage will reach 460V, exceeding 21% of the nominal voltage of 380V, thus exceeding the existing national standard limit. It can also be seen from the figure that in some scenarios with a large power supply radius (long power supply lines), when the photovoltaic power is not available at night, there is a low voltage problem.
To sum up, the development of distributed photovoltaics will inevitably bring about the phenomenon of power reverse transmission, and the randomness, volatility, and intermittent nature of photovoltaics will further cause voltage over-limit problems in low-voltage distribution networks.
III. Distinguishing between power reverse transmission and overvoltage problems from a scientific perspective
With the development of distributed photovoltaics, power reverse transmission is a normal phenomenon. On September 14, 2021, the National Energy Administration issued a news release that responds to questions on “how to regulate photovoltaic installation when distributed photovoltaic has exceeded the carrying capacity of local power grids”. To quote part of the response: Grid enterprises should fully consider the demand for large-scale access to distributed photovoltaics, strengthen the upgrading of the distribution network, and strive to achieve as much connectivity as possible. For distributed photovoltaic access to the grid, reference can be taken from the Guidelines on Evaluation of the Carrying Capacity of Distributed Power Supply Access to the Grid (DL/T 2041-2019) (the “Guidelines” hereinafter). According to the Guidelines, “……If distributed power supplies cause reverse power transmission to the power grid of 220kV and above, it should be assessed as a red alert. The connection of new distributed power supply projects should be suspended until the carrying capacity of the power grid is effectively improved.” For nearby stations with consumption conditions, grid enterprises can increase capacity or build new distribution transformers. For installed capacity that exceeds the carrying capacity of the local power grid, power grid enterprises can take appropriate counter-power transmission measures to provide grid connection services for distributed power sources.
Figure 2: Screenshot of NEA's response webpage
To summarize the NEA comments, the core points are: 1. Power reverse transmission is allowed, and the calculation range of power reverse transmission is the power grid of 220kV and above; 2. The power grid can take technical measures to improve the carrying capacity.
IV. Technical measures should be taken to improve the carrying capacity of the distribution networks
Improving the carrying capacity of the distribution network to accommodate high penetration of distributed PV access requires a systematic solution that includes at least the following three key factors:
Figure 3 Distributed photovoltaic hierarchical regulation architecture
(1) Establishing a technical specification management system for user-side distributed PV equipment
China's photovoltaic industry has been developing for more than two decades, and China has become the world's manufacturing superpower in terms of production with substantial number of manufacturers of photovoltaic equipment. Although the PV equipment produced by mainstream manufacturers has superior performance, the equipment specifications can be diverse, and the equipment functions and interfaces can be inconsistent. The product catalogue designated by the government departments is designed in a way where each user makes their purchasing decisions independently, and this has resulted in a myriad of varying low-voltage rooftop photovoltaic equipment. Moreover, the current communication network of low-voltage distribution network is not perfect and there are blind spots. Therefore, there is an urgent need for a communication management mechanism with high compatibility, high reliability, and high adaptability to achieve unified specification and access at the data interface level of low-voltage rooftop photovoltaic equipment. On the premise of being measurable and exploitable, for rooftop photovoltaic equipment that is not a grid asset, it is especially necessary to use market mechanisms to realize the “regulation and management” of rooftop photovoltaics by the grid. For photovoltaic power stations connected to 35kV and 6MW or above, the power grid can adopt the direct control and direct regulation mode. However, for rooftop photovoltaics connected to 220V, the power grid is not suitable for direct control and direct regulation mode. To simply cut off the rooftop photovoltaics, grid interaction needs to be achieved through market models of third-party Internet load aggregators such as demand response or virtual power plants.
(2) Establishing cloud-edge collaborative distribution network control system
At present, power grid enterprises have made breakthrough progress in the construction of “observable, measurable, controllable and adjustable” distributed power platforms and distributed photovoltaic cluster control technology. However, as distributed photovoltaics are not power grid assets, and manufacturers have many varying specifications and models, there are still gaps in the unified standardization of communication interfaces for distributed photovoltaics (especially low-voltage rooftop photovoltaics) and adaptive communication access management of photovoltaic equipment. High compatibility, high reliability, and highly adaptive communication interfaces are the basis for realizing distributed photovoltaic management. Furthermore, it is also necessary to develop photovoltaic short-term power forecasting, load forecasting and source-grid-load (storage) coordinated dispatch by taking advantage of artificial intelligence technology.
Insufficient distributed photovoltaic consumption capacity also shows that the coordination degree of distribution network load (storage) needs to be improved. First, we must make full use of the active and reactive power controllability of the photovoltaic inverter itself; secondly, we must also make full use of the potential demand response capabilities of user loads and user-side energy storage to minimise out-of-sync between load peak and photovoltaic power generation peak. Finally, users should be encouraged to build microgrids and micro-networks, develop electricity and hydrogen (ammonia, methanol, etc.) collaborative models, improve the consumption capacity of distributed photovoltaics through multi-energy complementation, and form support for the distribution network.
(3) Developing power quality control devices for distribution networks
Due to the volatility, randomness and intermittent nature of photovoltaic power generation, high-penetration distribution networks have power quality problems such as voltage over-limits (overvoltage and undervoltage), voltage fluctuations, three-phase imbalance, and harmonics. Among them, overvoltage directly affects the safe use of electricity by users, thus posing the most serious technical risk and is the most crucial factor affecting the consumption of distributed photovoltaics.
The distribution network power quality control device options are given in the table [2]. Among them, the on-load voltage regulator transformer is an effective voltage control method that requires the most attention. Internationally, especially in some European countries, tap adjustment is a crucial means of voltage regulation in low-voltage distribution networks [3]. However, distribution networks in China usually do not consider tap adjustment. The main reason is that currently there is load regulation. It is related to the manufacturing level of the voltage transformer. One option is to use a mechanical switch for tap adjustment. Its main disadvantage is that it requires manual operation, is not suitable for frequent operation scenarios, and is prone to arcing during the switching process. The other is to use a thyristor valve group for tap adjustment. The main disadvantage of joint voltage regulation is that the on-state power loss is large, heat dissipation needs to be considered, and there can be false triggers which means low reliability. Therefore, there is a need to further develop on-load voltage regulating transformers based on solid-state taps based on power electronic equipment.
Summarizing the policy documents issued by various places to strengthen the management of distributed photovoltaics, the main technical consideration for limiting new installations is the consumption capacity of distributed photovoltaics. Power grid companies are also required to regularly announce the carrying capacity of the distribution network which becomes the basis for government red line management. By establishing a technical specification management system for user-side distributed photovoltaic equipment and a cloud-edge collaborative distribution network control system, the development of distribution network power quality management devices will significantly improve the distribution network carrying capacity. For example, through the adjustment of the on-load voltage regulating transformer, it is possible to ensure that the voltage does not exceed the limit even if the rooftop photovoltaic installed capacity is twice the capacity of the transformer in the station radius. From this perspective, we can also see the crucial value and significance of developing the technical equipment level of distribution networks.
In the construction of new power systems, improving the carrying capacity of distribution grids and more consumption of distributed power sources are the key tasks of intelligent distribution grids. It is necessary to establish a technical specification management system for user-side distributed photovoltaic equipment, a cloud-edge collaborative distribution network control system, and prioritize the development of power quality management devices.
Reference
[1] Zhihui Photovoltaic WeChat public account: “Yet Another Location Where Distributed Photovoltaic Projects Require Pre-Authorization”, September 21, 2023
[2] Zhao Dongyuan, Wang Xuan: Practical Control Technology of Power Quality[M]. China Electric Power Press, 2016
[3] Long C, Procopiou A T, Ochoa L F, et al. Performance of OLTC-based control strategies for LV networks with photovoltaics [C]//2015 IEEE Power & Energy Society General Meeting.Denver, USA: IEEE,2015:1-5.
Zhao Dongyuan is the director of the Engineering Application Technology Research Office of the Energy Internet Research Institute of Tsinghua University and a professor-level senior engineer. He has long been engaged in research on smart grids, high-power power electronics technology and power quality technology. His current works aim to promote top-level design and demonstration projects of multiple new power systems.
Contact: zhaodongyuan@tsinghua.edu.cn
