The large-scale photovoltaic grid-connected power station system consists of solar cell components, brackets, combiner boxes, inverters, step-up transformers, power distribution rooms, lightning protection systems and high-voltage power grids. As shown in Figure 1.
According to the needs of the project, this 100MWp photovoltaic grid-connected power generation system adopts the design scheme of block power generation, one-step boost, and centralized grid-connection. The system is designed as 100 1MWp grid-connected power generation units, equipped with 200 500kW grid-connected inverters. Including isolation transformer, the rated output voltage is three-phase 270V, 50Hz; through 100 high-efficiency 10kV double-split step-up transformers (0.27/0.27/10kV, 1000kVA) T connected to the local 10kV medium-voltage power grid to achieve grid-connected power generation. The principle block diagram of the 1MWp photovoltaic grid-connected system is shown in Figure 2.
1. solar cell module
In this project, a polycrystalline silicon solar cell module with a power of 210Wp is selected for the solar cell module. Its working voltage is about 29.6V, and the open circuit voltage is about 36.5V. According to the maximum power point tracking (MPPT) working voltage of the grid-connected inverter Range (450-820V), each battery string is designed according to 20 battery modules connected in series, each 500kw grid-connected unit needs to be equipped with 120 battery strings, 2400 battery modules, and its power is 504kWp: 1MWp system needs 240 A total of 4,800 battery modules are required for each battery string; the entire 100MWp system requires 24,000 battery strings and a total of 480,000 battery modules. The project site is shown in Figure 3.
2. combiner box
In order to reduce the connection lines between the photovoltaic cell modules and the inverter and facilitate maintenance operations, the DC side adopts a segmented connection and a step-by-step confluence method, that is, the direct current output from the photovoltaic array is converged through the photovoltaic array combiner box. For the overall design scheme, a lightning protection combiner box of model PVS-16M is selected. Each combiner box has 16 channels of DC input, and each channel has current detection. In this way, each 500kW grid-connected unit needs to be equipped with 8 combiner boxes (16 channels of DC input); the entire 100MWp system needs 1600 combiner boxes.
The connection mode of the combiner box is 16 in and 1 out, that is, 16 channels of batteries of the same specification are input in series and then combined to output 11 channels of DC. The combiner box has the following characteristics:
①The protection level is IP65, which is waterproof, ash-proof, rust-proof and sun-proof, which can meet the requirements of outdoor installation and use;
②It can be connected to 16 battery strings at the same time;
③The open-circuit voltage value of each connected battery string can be up to DC1000V
④It has 16 protection and control channels, and the positive and negative poles of each channel are equipped with high-voltage DC fuses (the maximum current is 15A), and the withstand voltage value can reach DC1000V;
⑤The combiner box is equipped with 16 current monitoring devices, which monitor the current of each battery string and upload it to the upper computer monitoring device through the RS485 communication interface;
⑥The output of the DC confluence is equipped with a photovoltaic special lightning protection device between the positive pole and the ground, the negative pole to the ground and the positive pole and the negative pole;
⑦ The output end of the DC combiner is equipped with a segmentable DC circuit breaker. The electrical principle block diagram of this combiner box is shown in Figure 4.
3. Automatic power generation control
Automatic Generation Control (AGC) refers to the use of computer systems, communication networks and controllable equipment, online calculation of control strategies according to real-time operating conditions of the power grid, and automatic closed-loop control of the active power output of power generation equipment.
AGC is a grid-connected power plant generator set within the specified output adjustment range, tracking the instructions issued by power dispatching, and adjusting the power generation output in real time according to a certain adjustment rate to meet the power system frequency and tie line power control requirements. The AGC in the photovoltaic power station receives the load needs from dispatching or the power station and distributes it to the inverter in the power station according to a certain adjustment strategy to realize the optimal distribution and adjustment of the entire photovoltaic power station, and maintain the transmission power of the power station tie line to keep the exchanged electric energy. or close to the specified value.
4. Automatic voltage control
Automatic Voltage Control (AVC) refers to the use of computer systems, communication networks and controllable equipment, online calculation of control strategies according to real-time operating conditions of the power grid, automatic closed-loop control of reactive power and voltage regulation equipment, in order to achieve a reasonable Reactive voltage distribution.
AVC is to ensure that the photovoltaic power station can automatically adjust its reactive power according to the instructions of the power dispatching department, control the voltage of the grid connection point within the normal operating range, and adjust the control accuracy of the speedometer to meet the requirements of power system voltage regulation. The AVC in the photovoltaic power station can receive the bus voltage and total reactive load settings from dispatch, as well as the bus voltage and reactive power settings in the power station, and adjust the grid-connected inverter reactive power and reactive power compensation equipment through certain strategies. The input amount or the transformation ratio of the step-up transformer of the substation is used to adjust the reactive power and voltage of the power station so that the voltage of the grid connection point is within the normal operating range.
The setting of the AGC/AVC function improves the operation quality of the photovoltaic power station grid, and meets the current requirements for the “unattended, less manned” operation mode of the photovoltaic power station.