Design of AC Electrical System of Photovoltaic Power Station

What is the design of AC electrical system of photovoltaic power station?

The designer of the AC electrical system of a photovoltaic power station must first consider the access system plan, analyze the grid-connected access method and voltage level, select the grid-connected access point, and determine the access plan. Of course, necessary electrical calculations must be performed. For example, according to the recent grid operation data provided by the power supply company, the photovoltaic power station can operate at 90% output, and calculate the load at the maximum output at 12:00 noon (or around 2:00 pm). Perform power flow analysis, estimate voltage fluctuations and deviations, and predict the limits of voltage fluctuations; perform reactive power calculations and propose reactive power compensation configurations; perform short-circuit current calculations and power quality calculations and analysis.

The photovoltaic power station should be connected to the power system according to the power station’s own installed capacity, local power supply network conditions, power quality and other technical requirements to select the appropriate access voltage level.

The actual output power of the solar photovoltaic power generation device changes with the change of the light intensity. When the light intensity is strongest during the day, the output power of the power generation device is the largest, and when there is almost no light at night, the output power is basically zero. Therefore, in addition to equipment failure factors, the output power of the power generation device varies with natural factors such as sunshine, weather, season, and temperature, and the output power is extremely unstable.

The change process of solar illuminance is a gentle and continuous process. For the extreme voltage fluctuation of 10-20MWp photovoltaic power plant from the maximum output of full capacity to zero output, the frequency of this voltage fluctuation is relatively low, and it is expected to be on the order of minutes. Therefore, it is more appropriate to take 2.5% as the limit of voltage fluctuation.

According to the “Technical Regulations of China Grid Corporation of Photovoltaic Power Stations Connecting to the Power Grid” (Q/GDW617-2011): “Large and medium-sized photovoltaic power stations should be equipped with reactive voltage control systems and have reactive power and voltage control capabilities.” Within its reactive power output range, photovoltaic power plants should have the ability to regulate reactive power output according to the voltage level of the grid-connected point and participate in grid voltage regulation. Parameters such as the regulation method, reference voltage, and voltage difference rate should be remotely set by the grid dispatching agency .
Taking the electrical primary part of a 20MWp photovoltaic power station as an example, the following design considerations can be made.

1.1 Power transmission and distribution system

The whole station can be set to three levels of voltage: 0.27kV, 10kV and 110kV.
There are two main transformers and a 110kV step-up station in the station, and the power station is connected to the grid with a 110kV line for transmission. The capacity of the main transformer is 1.25MV·A.

Each solar photovoltaic power generation unit is equipped with a step-up transformer, and the step-up transformer adopts a three-phase 1250kV·A oil-immersed transformer. The photovoltaic module array, DC combiner box, inverter and step-up transformer are arranged on-site in units of 1MW and connected to the 10kV power distribution room via a 10kV cable.
When the photovoltaic power station is connected to the grid, the three-phase voltage unbalance of the grid connection point does not exceed the value specified in the “Power Quality Three-phase Voltage Unbalance” (GB15543-1995). The allowable value of balance is generally 1.3%.

For example, there is no high-power rotating equipment in the project, and the reactive power consumption is very small. An appropriate automatic switching reactive power compensation device can be set up according to the installed capacity to provide reactive power compensation for the step-up transformer and line of the power station.

In order to improve the reliability of the power supply of the power station, the power supply for the power station is connected by a 10kV bus and connected by a 10kV (construction power supply). The two power supplies are mutually backup.

1.2 Selection of main electrical equipment

(1) Step-up transformer

The 10kV step-up transformer uses three-phase oil-immersed distribution transformer, model S11-1250/10, rated capacity 1250kV·A; voltage ratio (10.5±2)×2.5%/0.27kV, wiring group DYN11, short-circuit impedance Ud= 4.5%, if it is a 20MWp photovoltaic power plant, a total of 20 units are used.
The transformer is equipped with temperature control facilities with alarm and trip signals. The trip signal is connected to the 10kV high-voltage switch cabinet and the low-voltage side line switch of the transformer, and the temperature signal is connected to the integrated automation monitoring system.

(2) 10kV power distribution device

For 10kV power distribution device single bus wiring, armored metal-enclosed manual switchgear should be selected, vacuum circuit breakers should be used, and comprehensive protection devices such as step-up transformers and capacitors should be used. It is designed according to the voltage level of 10kV, and the rated breaking current of the vacuum circuit breaker is selected.

(3) Low-voltage power distribution device

Low-voltage switchgear can choose MNS type low-voltage withdrawable switchgear. The incoming line circuit breaker is a frame circuit breaker, equipped with an intelligent release, and the rated breaking current is 50kA.

In the design of large-scale centralized photovoltaic power stations, the primary electrical equipment can be considered as integrated box transformer equipment that integrates DC cabinets, inverters, and step-up transformers.

The integrated integrated box transformer design eliminates the low-voltage compartment, and the low-voltage side of the booster transformer is directly connected to the AC side of the inverter. The user site only needs to consider the connection between the DC side and the high-voltage side of the booster transformer. This choice reduces the footprint of the equipment body. Taking the 1MWp photovoltaic power generation system as an example, the integrated solution covers an area of ​​17.6m2, while the non-integrated solution covers an area of ​​30m2, which can save 40% of the area. In fact, the greater advantage is that for non-integrated solutions, the joint debugging test of the boost converter and the inverter can only be performed on site, while the integrated solution can be completed in advance at the TYCORUN ENERGY ups lithium battery factory, which shortens the on-site debugging work. However, it is emphasized here that the designer or user must select qualified products with good ventilation and cooling performance and convenient operation and maintenance in the later period, otherwise it will have negative effects.

1.3 Total station lighting

Photovoltaic power station lighting is divided into normal lighting and emergency lighting. The lighting power source is taken from the station’s AC power supply, and the emergency lighting fixtures are equipped with batteries. The emergency time should not be less than 30min.

The photovoltaic complex can use energy-saving fluorescent lamps as the light source for normal lighting. The lighting box circuit is separated from the socket circuit, and the socket circuit is equipped with a leakage protector.

1.4 Layout of electrical equipment

A photovoltaic complex needs to be built in the photovoltaic power station, which can be arranged on a single floor; the distribution room, relay room, centralized control room and DC screen, metering screen, UPS screen, integrated automation screen, etc. are arranged separately.
The 10kV power distribution device adopts a complete set of switch cabinets in the user, and the 10kV feeder should use cables.

1.5 Cable laying and cable fire protection

The 10kV power distribution room and relay room of the photovoltaic power station are equipped with cable trenches. In the solar module square array, the bridge trough box is used to lay along the back of the photovoltaic module, and the cable outlet DC combiner box is laid along the cable trench.
The cable channel shall be in accordance with the “Design Regulations for Cable Selection and Laying of Power Plants and Substations” and the “Code for Fire Protection Design of Thermal Power Plants and Substations” to prevent cables from catching fire and extending combustion. In the building, the cables are led to the openings of the electrical cabinets, panels, control panels, and platforms, and the holes in the walls and floors where the cables penetrate, shall be blocked by fire resistance. The branch of the cable channel, the entrance to the power distribution room, and the central control room should be blocked by fire resistance.