Analysis of the situation where the system load exceeds the transformer capacity configuration when the transformer and photovoltaic are used at the same time.
Phenomenon and cause
1. Power fluctuation superposition: The power generation power of the photovoltaic system is affected by factors such as light intensity and weather conditions, and fluctuates significantly. When there is sufficient sunlight during the day, the power generation power may increase significantly in a short period of time; while on cloudy days, cloudy days or in the morning and evening, the power drops sharply. If the system load itself is also in an unstable state at this time, such as the frequent start and stop of large equipment in industrial production, resulting in a large fluctuation in load power, the superposition of the two can easily cause the total system power to exceed the rated capacity of the transformer instantly. For example, in an industrial park equipped with a certain scale of photovoltaic power stations, when clouds suddenly appear in the afternoon to block the sun, the photovoltaic power drops sharply. At the same time, large equipment in several factories in the park starts at the same time, and the system load that was originally close to the upper limit of the transformer capacity is instantly overloaded, causing the transformer temperature to rise rapidly and emit abnormal sounds.
2. Unreasonable planning of photovoltaic installed capacity: When promoting photovoltaic projects, some regions have not fully combined the actual capacity of local transformers and future load growth trends for scientific planning. In order to pursue more photovoltaic power generation benefits, some users or enterprises blindly expand the scale of photovoltaic installations and connect a large number of photovoltaic equipment without in-depth evaluation of the original power supply system. For example, in some old communities, the transformer capacity has not been upgraded for many years. As residents' enthusiasm for photovoltaic power generation increases, they install photovoltaic panels on their roofs, and the total amount of installation far exceeds the transformer's tolerance, resulting in frequent instability in the community power supply, and even frequent tripping during peak power consumption in summer.
3. Insufficient load growth estimation: With economic development and the improvement of people's living standards, various types of electrical equipment are increasing. Whether it is the rise of emerging industries in the industrial field or the popularization of high-power electrical appliances in residents' lives, the demand for electricity continues to rise. If the future load growth estimation is too conservative in the planning stage of the transformer and photovoltaic system, and sufficient capacity margin is not reserved, when the actual load growth rate exceeds expectations, coupled with photovoltaic access, it is very easy to cause the system load to exceed the transformer capacity. For example, in recent years, new stores have been set up in a certain commercial area, and the catering, entertainment and other industries have brought a large amount of new electricity demand. At the same time, photovoltaic systems have been installed on the roofs of some buildings in the area. The capacity of the transformer originally designed can no longer meet the total demand of the existing and new loads and photovoltaic access, and power supply tension often occurs.
Impact
1. Transformer overheating or even damage: When the system load exceeds the transformer capacity, the current of the transformer winding increases. According to Joule's law Q=I2Rt (where Q is heat, I is current, R is resistance, and t is time), the heat generated by the winding increases significantly. Being in this overloaded and heated state for a long time will accelerate the aging of the transformer insulation material and reduce the insulation performance. In severe cases, it may cause a short circuit in the winding, causing damage to the transformer and leading to a large-scale power outage. For example, in a rural distribution network connected to a photovoltaic power station, due to the large number of electrical equipment such as air conditioners turned on during the high temperature period in summer, coupled with the instability of photovoltaic power generation, the transformer was overloaded for a long time, and the insulation material eventually burned out, and the transformer was completely damaged, affecting the normal power supply of many surrounding villages.
2. Power quality degradation: On the one hand, overload operation will reduce the transformer output voltage, resulting in excessive voltage deviation. For some equipment with high requirements for voltage stability, such as precision electronic equipment, industrial automation production lines, etc., low voltage may cause the equipment to fail to work properly or even damage the equipment. On the other hand, the harmonics generated by the photovoltaic system and the load interact when the transformer is overloaded, which may further amplify the harmonic content, affect the power quality of the power grid, and interfere with the normal operation of other electrical equipment, such as causing additional vibration and noise in the motor, reducing the service life of the equipment. For example, in a factory power grid with both photovoltaic access and a large number of industrial equipment, the voltage deviation reached ±10% because the system load exceeded the transformer capacity, causing multiple imported precision processing equipment in the factory to frequently alarm and shut down, and harmonic pollution caused some lighting fixtures to flicker.
3. Reduced power supply reliability: The system load exceeds the transformer capacity configuration, which will increase the risk of power outages. Once the transformer stops operating due to an overload fault, it will directly cause a power outage in the area it supplies power to, affecting residents' lives, industrial production and commercial operations. Even if the transformer is not completely damaged, frequent overload warnings and protection actions will cause intermittent power supply, seriously affecting power supply reliability. For example, in an old neighborhood of a city, due to insufficient transformer capacity and excessive photovoltaic access, there are multiple power outages every week during the peak period of summer electricity consumption, which brings great inconvenience to residents' daily life and also causes economic losses to commercial activities in the neighborhood.
Countermeasures
1. Reasonable planning and capacity expansion: Conduct a comprehensive survey of the existing power grid and load conditions, combine the distribution of photovoltaic resources with future development plans, use big data analysis and load forecasting models to accurately predict the load growth trend. On this basis, scientifically determine the scale of photovoltaic access according to the transformer load rate and remaining capacity. For areas with great load growth potential and rich photovoltaic resources, if the existing transformer capacity cannot meet the demand, the transformer capacity should be expanded and upgraded in time. For example, during the planning stage of a new industrial park, through detailed load research and forecasting, it is expected that the load will increase by 50% in the next 3-5 years. At the same time, considering that a large number of roofs in the park can be used to install photovoltaics, it is finally decided to upgrade the original 1000kVA transformer to 2000kVA, and reasonably plan 500kW photovoltaic access capacity to ensure the stability and sustainability of power supply.
2. Install adjustment equipment: Install a maximum power point tracking (MPPT) device in the photovoltaic system to adjust the working state of the photovoltaic panel in real time so that it always outputs at maximum power and reduces power fluctuations caused by changes in light. At the same time, configure a dynamic reactive power compensation device (SVG) to compensate in real time according to the reactive power demand of the system, stabilize the voltage, improve the power factor, and reduce the load pressure of the transformer. For example, in a rural power grid connected to a 1MW photovoltaic power station, after installing MPPT and SVG devices, the fluctuation range of photovoltaic power was reduced by 30%, and the output voltage deviation of the transformer was controlled within ±5%, which effectively improved the power quality and transformer operating conditions.
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3. Optimize operation management: Establish a smart grid monitoring system to monitor the operating status of transformers, photovoltaic systems and loads in real time, including parameters such as voltage, current, and power. Through data analysis, predict possible overload risks in advance and take timely adjustment measures, such as adjusting the output power of photovoltaic inverters and guiding users to use electricity at off-peak times. For example, a city's smart grid monitoring center uses big data analysis technology to conduct real-time monitoring and analysis of transformer and photovoltaic system operating data throughout the city. When it finds that the transformer load rate in a certain area is close to 80% and has a trend of continuing to rise, it sends peak-shifting electricity consumption reminders to large commercial users in the area through a mobile phone APP, successfully avoiding the occurrence of transformer overload.