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Solution of Centralized Placement of String Inverters

2021-09-26 14:36

String inverters are widely applied in new built ground-mounted power plant projects due to its advantage of Multiple MPPTs, high efficient generation and lower cost. The characteristic and location of the string inverter also affect the generation benefit of the entire on-grid solar power plant. In accordance with the investment cost and generation benefits of large-scale power plant, we developed many solutions regarding the location of the inverters in practice. After comprehensive considering the investment cost, generation benefit and operation conditions, we compared the solution of centralized placement of string inverters on a platform and distributed placement of string inverters.

 

                     

 

Regarding the most common ground-mounted subarray size of 3.15MW, Goodwe combined 35kV pad-mounted substation and solution of centralized placement string inverter on a platform based on its advantage of research and development as well as its industry resource. This solution has the advantage of reducing the cable loss as far as possible, shorter and faster communication routes, easy installation and later period operation compared to the traditional placement solution.

Centralized String inverter soltution

 

Reducing system cable loss

The centralized placement solution has the characteristic of longer DC cables and shorter AC cables usage compared to traditional distributed installation. DC cable has less corona loss and radio interference loss. DC cables also have higher transmission voltage than AC cables, so the DC cable loss is lower than the AC cable loss. As a result, the solution can lower the overall system loss.

Besides, to lower the LCOE generation cost, in ‘Criterion of Solar power generation efficiency NB/T 10394-2020)’ published on 23rd Oct, 2020, it increases the limit of oversizing ratio to 1.8:1 based on ‘Nominal capacity’ calculated from AC side. This regulation direct the way for the development of large scale ground-mounted power plant. With the increase of oversizing ratio, the inverter will maintain nominal output power or 1.1 times nominal output power for 80% of time in a day. Therefore, the centralized structure inverter layout can compensate the loss of power generation in the environment of oversizing and peak shaving. After calculation, the power acquisition rate of the grid connection point is 1% higher than that of the traditional decentralized scheme. This is still the case when aluminum wires are used on the AC side of the distributed scheme, but now more and more aluminum wires are used in ground-mounted power plants.

However, for the early stage cable investment, the cable cost of centralized placement string inverters is a bit higher than that of traditional distributed type. The approximate calculation is shown in the table below,

Distributed Placement Scheme

Power Cable Model

Quantity(m)

UnitCost(Yuan/m

In Total(Ten thousand Yuan)

DC Cable

PV1-F 1X4

25400

3.2

8.128

AC Cable

YJLV 3X120

1710

26.5

4.5315

AC Cable

YJLV 3X150

820

32.8

2.6896

In Total

15.3491

Centralized Placement Scheme

Power Cable Model

Quantity(m)

UnitCost(Yuan/m

In Total (Ten thousand Yuan)

DC Cable

PV1-F 1X4

17020

3.2

5.4464

DC Cable

PV1-F 1X6

36030

4.7

16.9341

AC Cable

YJLV 3X120

140

26.5

0.371

In Total

22.7515

 

However, the power generation income is much higher than the initial equipment investment.

 

Shorter Communication Route and Rapid Fault Location

Due to the centralized installation of inverters, whether RS485 or PLC communication is used, its communication route is very short. The distance between inverter and data collector can generally be less than 20m, which  greatly improved the communication reliability.

Short distance communication makes possibilities for different communication modes of inverter. For example, Ethernet can be used to realize the communication from the inverter to the data logger. Goodwe’s Ethernet field communication rate can reach 100mbit / s. The improvement of communication rate can improve the scanning rate and reliability in the application scenario of IV scanning.

 

Convenient for Operation and Maintenance

Benefits from the platform installation scheme, the inverter manufacturer can provide the prefabrication of the integrated platform in the factory according to the needs of customers, including the installation of inverter, the setting out and wiring from AC cable to the low-voltage side of pad-mounted substation. It greatly reduces the construction period of the project and helps the rapid construction of the power station and get benefits.

For example, a 80 MW ground power station may require nearly 1,000 acres of land. In the general operation and maintenance of the inverters, according to some detection requirements of the operation and maintenance management. It is usually necessary to check the electrical integrity of the structure, rust, dust accumulation and other phenomena. Checking the operation regularly including the tightening of the screw, the insulation of the voltage and current on the DC side, etc is also needed. At the same time, the inverters need to be cleaned and checked monthly to check the operating temperature and the temperature measurement of AC and DC cable heads. We can note that a large part of the equipment data recording work requires on-site measurements. The centralized placement scheme of group-series inverters makes the operation and maintenance of inverters more convenient. Inverter faults are discovered in time to ensure efficient power generation.

 

Summary

Although centralized placement of inverters slightly increases the cost of cable investment, it greatly reduces the cost of construction, operation and maintenance, and the overall electricity cost is lower than the traditional distributed scheme. At the same time, in the operating environment of the ground-mounted power station with high oversizing and high power generation, higher power generation benefits can be obtained. In the future, the ground-mounted power station will be popularized in a larger range.

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