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65-68 DC bus and DC power distribution design

Author: Source: Datetime: 2016-12-22 17:04:12
4.1.3.2 DC bus and DC power distribution design

Each of the inverters is connected to a number of strings of photovoltaic modules, which are connected to the inverter through a DC combiner box and a DC distribution cabinet.

DC converters to meet the outdoor installation requirements, insulation protection to IP64 or IP65 protection class, while access to more than 8 solar cells in series, each current exceeds 10A, the largest access to the largest PV series of open Voltage up to DC1000V, the fuse voltage is not less than DC1000V. Each group has a diode string of anti-anti-anti-anti-protection function, with a dedicated photovoltaic arrester, positive and negative are equipped with lightning protection function, the use of positive and negative pole respectively in series four-pole circuit breaker to improve the DC voltage value can withstand the DC voltage Not less than DC 1000V.
DC converters are also equipped with surge protection device, with a lightning protection function.

Now, the photovoltaic system customers are no longer use the above general convergence box, but the selection of high-end products, that is intelligent convergence box. This busbar is not only used to connect the PV array and DC power distribution cabinet to the inverter, provide convergence and provide lightning protection and overcurrent protection. In order to improve the reliability and applicability of the system, it also has the function of monitoring the operation of solar panels Voltage, power, mine and DC circuit breaker status, relay contact output and other functions, and with wind speed, temperature, radiation and other sensors, the interface for customers to choose. The device also comes standard with RS485 / RS232 digital communication interface, can be measured and collected data uploaded to the power plant monitoring system, user-friendly and timely and accurate grasp of the status of solar panels, and photovoltaic power generation system has been realized after the operation and maintenance Of the intelligent management.

The overall design of the hardware system based on DSP, which mainly includes convergence module, DSP control and display module, power supply module, detection module, environment monitoring module and communication module. Exchange module to achieve the convergence of multi-channel photovoltaic cells, including the convergence of the main license, circuit breakers and lightning protection devices.



Road detection data real-time monitoring and display. Power supply module to achieve self-powered system. Detection module detects multiple input current and input voltage and the data into the master DSP chip. Environmental monitoring module real-time monitoring of the convergence box temperature and smoke, etc., and the data into the master DSP chip. The communication module realizes the remote communication between the DSP chip and the host computer.
Road detection data real-time monitoring and display
DC lightning protection power distribution cabinet is the convergence box output DC cable access after the second convergence, and then connected to the grid inverter. The distribution cabinet with DC input circuit breaker, anti-diodes, photovoltaic lightning protection, easy operation and maintenance. DC lightning protection power distribution cabinet electrical diagram shown in Figure 4-18 "
Road detection data real-time monitoring and display
In the system design, the use of domestic high-quality and efficient 280Wp polysilicon components, calculate the number of components in series, the component must be based on the operating voltage and inverter DC input voltage range, while the components need to consider the open-circuit voltage temperature coefficient.

From the general grid-connected inverter performance parameters table, the inverter system used in power plant system is generally the highest voltage of 880V, the minimum MPPT voltage of 480V, such as polysilicon module open circuit voltage of 44.8V, peak operating voltage of 35.2V , Component open-circuit voltage temperature coefficient is 0.34% / ° C: The number of series connections is calculated from 17 to 18. In order to ensure the efficiency of power generation and the rational arrangement of the square, using 18 components for a component string. For the parallel design of components, it is based on the peak operating current of the component and the maximum allowable input current of the inverter. The 6-channel sink 1 of the DC combiner box is suitable, that is, 6 strings of components are together. If a 20MWp photovoltaic power plant using 280Wp battery components 73440 block, with 18 string 6 of the components of the series and parallel way, through the rigorous selection of components to ensure that the combination of square loss is less than 3%.

Here draw attention to the reverse current capacity of solar cells (RCA). If a piece of 36 solar battery modules in series. Rated operating current of 5A, the reverse voltage (that is, the diode forward voltage) is 18V, can withstand the reverse current of 20A (need to be measured); The positive open-circuit voltage of the solar module is 21V. Assuming that two modules are connected in parallel, one of them is blocked, and the reverse current injected into the other is only about 2A (required to be measured) under standard daylight. Under unprotected conditions The maximum number of components that can be paralleled is 10 blocks.

Today, more and more PV component suppliers began to mark this parameter in the data list. For small or medium-sized power plants, this parameter has no meaning. Most suppliers believe that IEC 61215 in Section 10.3 of the insulation resistance test is sufficient, but for large-scale photovoltaic power plant is not enough. In other words, for strings that do not have a string fuse or blocking diodes installed, the number of RCAs will determine the number of strings. If the installation string insurance, making the system becomes complicated. The new standard IEC50380 in Section 3.6.2 clearly states that components must be marked reverse current capacity (RCA) values. The higher the value of the reverse current capability, the greater can be the number of parallels in an uninsured string. When each string is obscured, it is not damaged by hot spots.

The strings connected in parallel should be protected, that is, direct current (DC). Note: The technical parameters of the so-called "PV insurance" do not exist yet. The short-circuit current between the strings is a slow-rising short-circuit current. As a result, overheating / annealing / burning of the junction boxes and cables can be caused by a continuous arc. In general, the recommended current values can be found on the data sheet provided by the component supplier, trying to find the recommended type of insurance.

In the case of no insurance, reverse current capability parameters should be provided. For a parallel string, the parallel number of strings should not be skewed by 60% of its reverse current tolerance. 

For example:

The reverse current capability (RCA) of the module = 25A;

Short circuit of the component = 6A;

Number of shunt groups: 4 (= RCA100%>;
The correct number of strings: 3 (= RCA72%>;

Approximately 28% safety factor.



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