In order to facilitate the management of the distributed power system and the safety of the equipment, it is generally realized through an AC power distribution cabinet. The AC power distribution cabinet is an AC power distribution unit used to realize the functions of output, detection, display, and equipment protection of the inverter’s output power. The output interface is provided for the inverter through the AC power distribution cabinet, and the output AC circuit breaker is configured to be directly connected to the grid (or for AC loads). When the photovoltaic power generation system fails and needs to be repaired, it will not affect the safety of the photovoltaic power generation system and the grid (or load), and at the same time ensure the personal safety of maintenance personnel. The main equipment on the AC side includes AC circuit breakers, AC lightning protectors, metering energy meters (with communication interfaces), voltage and current meters, etc. For distributed power grid-connected power generation systems, power quality analyzers are also required.
1. AC circuit breaker
In photovoltaic power generation systems, AC circuit breakers are used to protect the output of the inverter from overcurrent hazards. When the output terminal of the inverter fails, it can quickly cut off the faulty circuit to prevent further expansion of the accident.
The specification of AC circuit breaker needs to be determined according to the rated value of the voltage and current on the output side of the inverter. Generally speaking, the rated voltage of the AC circuit breaker is not less than the rated value of the inverter’s output AC voltage, and the rated current of the overcurrent release is not less than the rated value of the inverter’s output current. In the photovoltaic grid-connected system, an AC grid-connected circuit breaker needs to be installed on the grid-connected side, and the selection of its parameters is the same as that of the AC circuit breaker on the output side of the inverter.
2. Exchange meter
In order to facilitate the observation of the operation of the inverter, an AC voltmeter and an AC ammeter are arranged on the output side of the inverter. The AC voltmeter and AC ammeter are mainly based on the rated working voltage and the maximum AC current output by the inverter to select the meter range, and the accuracy of the meter is selected according to the standard level of the measurement meter. The output voltage and current of the inverter can be displayed in real time through the configured AC voltmeter and AC ammeter.
3. Metering electric energy meter
In the distributed power system, two electric meters need to be installed. One piece is installed at the outlet of the distributed power source, that is, at the outlet of the inverter, and is used to measure the total power generation of the distributed power source. Because the state’s financial subsidies for distributed power sources are settled according to the amount of electricity generated, the electricity price subsidy standard is 0.37 yuan/kW·h (commissioned after January 1, 2018). The other is installed at the grid-connected point and has a two-way metering function (to go online and offline), because the distributed power supply is mainly for “self-generated and self-used, and surplus electricity goes online.” Taking Jiangsu Province as an example, the on-grid electricity is calculated based on the benchmark electricity price for coal-fired units in Jiangsu Province, which is 0.391 yuan/kW·h.
When designing electric energy meters with distributed power sources, on the premise of meeting the requirements of related technologies and standards for electric energy measurement, it should also have the functions of two-way active and four-quadrant reactive power measurement, as well as the functions of local communication and remote communication through the electric energy information collection terminal. Power plant operators can view the operation of distributed power sources on the spot through the electric energy meter, and it is also convenient for remote dispatchers to grasp the operation status of the photovoltaic power generation system in time.
4. Power quality analyzer
In the photovoltaic grid-connected power generation system, the inverter inverts direct current into alternating current, and the inverted alternating current more or less contains some high-order spectral waves. These harmonic components will cause serious harm to the power supply system. Harmonic components will increase the additional losses of power generation, transmission, supply and use equipment, overheat the equipment, reduce the efficiency and utilization of the equipment, affect the operation of relay protection and automatic devices, and interfere with the normal operation of the communication system. Therefore, the power grid has clear requirements for the harmonic components of photovoltaic power generation grid-connected systems. In addition, voltage deviation, voltage imbalance, DC component, voltage fluctuation, and flicker will also occur after inverter. Only when the magnitude of these values meets the relevant standards, the photovoltaic power generation system is allowed to be integrated into the grid.
The power quality analyzer is installed on the output side of the inverter to detect the power quality output by the inverter. Once the inverter output power quality does not meet the technical requirements for grid connection, measures can be taken to remove the photovoltaic power generation system from the grid.
The requirement for a power quality analyzer is to be able to provide various power quality index parameters, and to record various power quality data, and provide detailed information to understand and analyze the status of power quality.
5. AC lightning protector
In the photovoltaic grid-connected system, if the line is struck by lightning, overvoltage will be generated. If the lightning current cannot flow into the ground quickly, the lightning will invade through the cables at the grid connection point, causing damage to power distribution equipment and electrical equipment, causing fire, and even causing serious consequences such as personal injury and death accidents. In order to prevent damage to the equipment due to lightning strikes on the grid-connected side of the photovoltaic power generation system, an AC lightning protection device is installed at the grid-connected point. Once the line is struck by secondary induced lightning or operating overvoltage, the lightning protector will instantaneously discharge the overvoltage to the ground, so as to protect the equipment and personal safety.
The components in the AC power distribution cabinet should be reasonably arranged, the wiring should be neat, and the insulation between the electrical appliances should meet the relevant national standards; the incoming and outgoing wires must be connected to the wiring terminals. There must be isolation protection between the high current terminals, the general voltage terminals and the weak current terminals, the AC power distribution cabinet must have obvious disconnection points for the connected equipment and lines, and the equipment and circuits can be disconnected step by step during maintenance to ensure the safety of maintenance personnel and related equipment.