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71-75 AC electrical system design of photovoltaic power plant

Author: Source: Datetime: 2016-12-24 18:14:15


4.1.5 AC electrical system design of photovoltaic power plant

Designers must first consider the access system plan, conduct grid-connected manners and voltage level analysis, select grid-connected access points, and determine access schemes. • Of course, the necessary electrical calculations, such as those based on grid- According to the recent operation data, the PV power plant can run at 90% of the output power, calculate the load at 12:00 noon (or around 2:00 pm), calculate the load, analyze the power flow, estimate the voltage fluctuation and deviation, The reactive power calculation, the proposed reactive power compensation configuration; short-circuit current calculation and power quality calculation and analysis.
PV power plant access to the power system should be installed according to their own capacity of the power station, the local power supply network, power quality and other technical requirements to select the appropriate access voltage level.
Refer to "Jiangsu PV power station access system guidelines (2010 version)", the total capacity of 10 ~ 20MWp photovoltaic power plants, and grid voltage level is generally 20 ~ 110kV, see Table 4-7.

Table 4-7 Jiangsu Electric Power installed on the photovoltaic power station access voltage rating requirements

No.

Total installed capacity G

Access voltage level requirements

1

200kWp≤G

400V

2

200kWp

10kV

3

3kWp

10kV or 20kV

4

10kWp

20kV  or 110kV

5

G>20kWp

110kV

The actual output power of solar photovoltaic power plant changes with the change of light intensity, the strongest daytime light intensity, the largest power plant output, almost no light at night, the output power is basically zero. Therefore, in addition to equipment failure factors, the power output device with the sunshine, weather, season, temperature and other natural factors, the output power is very unstable. The voltage fluctuation limits (d) for different voltage fluctuation frequencies (f) are shown in Table 4-8 according to the requirements of "Voltage fluctuation and flicker of power quality" (GB / T 12326-2008).


Table 4-8 System voltage fluctuation limits

r/(times/h)

D/%

r/(times/h)

D/%

r≤1

3

10≤r≤100

1.5

l

2,5

100

1

The change of solar irradiance is a gentle and continuous process. The voltage fluctuation frequency is extremely low for the extreme voltage fluctuation of 10 ~ 20MWp PV power plant, which drops suddenly to zero output from the full capacity of the full capacity. The voltage fluctuation frequency is expected to be minute level , So the voltage fluctuation limits take 2.5% is more appropriate.
For the reactive power configuration, according to the "State Grid Corporation of photovoltaic power plant access grid technical requirements" (Q / GDW 617-2011) 6.2: " for the green access to public network of large and medium-sized photovoltaic power plant, The power can compensate for the station when the photovoltaic power station  full collection system.All the main transformer inductive reactive power and photovoltaic power station sent out the general line of inductive reactive power and its  configuration of all inductive reactive power capacity to compensate for the photovoltaic power plant to send out half of the line charge (GB / T 19964-2012), section   6. 1.3: "Photovoltaic power plants to take full advantage of grid-connected inverter reactive power capacity and its ability to adjust" (GB / T 19964-2012), "power" and  "photovoltaic power plant access power system technical requirements" When the reactive power capacity of the inverter to meet the needs of the system voltage  adjustment should be installed in the photovoltaic power generation capacity of the appropriate reactive power compensation device, if necessary, the installation of  dynamic reactive power compensation device "; 6.2.2" Through 10 ~ 35Kv voltage grid-connected photovoltaic power station power factor should be able to 0.98 ~  0.98 lag in the range of continuous adjustable.Special demand, with the power grid enterprises in consultation with the relevant requirements, for the access plan to calculate no Work compensation capacity.


According to "State Grid Corporation of photovoltaic power plant access to the technical provisions of the grid" (Q / GDW617-2011) requirements: "Large and medium-sized photovoltaic power plants should be equipped with reactive voltage control system, with reactive power and voltage control. In the reactive power output range, the PV power plant shall have the ability to adjust the reactive power output according to the voltage level of the interconnection point and participate in the adjustment of the  grid voltage. The adjustment method, reference voltage and voltage regulation rate shall be set remotely by the power dispatching agency.


Taking a 20MWp PV power station as an example, the following design considerations can be made.

4.1.5.1 Power Transmission and Distribution System

The total station can be set to three voltage: 0.27kV, 10kV and 110kV.

Integration-solar-generator-variable-room

solar-generator-first-schematic-diagram
(A) - transformation box change room
(B) - sub-schematic

Station has two main transformers and 110kV booster station, power station with a return transmission line 110kV access grid. Main transformer
The capacity is 1.25MV • A.

Each solar photovoltaic power generation unit sets up a step-up transformer, step-up transformer uses three-phase 1250kV • A oil-immersed transformer. The array of PV modules, DC converters, inverters and step-up transformers are arranged in 1MW units and connected to 10kV distribution rooms via 10kV cables.

When the PV power plant is connected to the grid, the unbalance degree of the three-phase voltage of the interconnection point does not exceed the value stipulated in GB 15543-1995, which is connected to the common connection point. Allowed value of the imbalance is generally 1.3%.

Such as engineering without high-power rotation and other equipment, reactive power consumption is very small, according to the installed capacity of the appropriate automatic switching reactive power compensation device for the step-up transformer substation, line, etc. to provide reactive power compensation.

In order to improve the reliability of the power station, the power supply of the power station is connected with 10kV bus, 10kV (construction power source), and the two power sources are reserved for each other.

4.1.5.2 Selection of major electrical equipment

(1> step-up transformer
10kV step-up transformer uses three-phase oil-immersed distribution transformer, type S11-1250 / 10, rated capacity of 1250kV.A; voltage ratio (10.5 ± 2) * 2.5% / 0.27kV, wiring group DYN11, short-circuit impedance Ud = 4.5%, such as 20MWp photovoltaic power plants, a total of 20 units.
Transformer installation with alarm and trip signal temperature control facilities. The trip signal is connected to the 10kV high-voltage switchgear and the low-voltage side line switch of the transformer, and the temperature signal is connected to the integrated automatic monitoring system.

(2) 10kV power distribution device
10kV power distribution unit single bus connection, should be selected armored metal enclosed manual switchgear, vacuum circuit breaker, configure the step-up transformer, capacitor integrated protection device. Designed for 10kV voltage rating and rated breaking current for vacuum circuit breakers


(3) low-voltage power distribution devices
Low-voltage switch cabinet can choose MNS-type low-voltage withdrawable switchgear. The circuit breaker chooses the frame circuit breaker, and the intelligent trip unit is equipped with rated breaking current of 50kA.
In the design of large-scale centralized PV power plant, the main equipment of the primary electrical can be considered to use the integrated cabinet, DC cabinet, inverter and step-up transformer. Figure 4-21 shows the ZGSF11-GN-1000/35 - Integraticm Me Change Rooms and the Me Change A Chart and the technical specifications (Table 4-9).


Table 4-9 - Technical Specifications of the Transformer Box (ZGSF11 to GN-1000/35)

Participation

ZGSF11-GN-1000/35

DC side parameters

 

The largest DC power Zhuang

880V

Starting voltage

520V

Full load MPTT voltage range

500820V( or wider range

Maximum DC power

1100kW

Built-in DC cabinet

2*8Circuit (each branch current can be monitored and can be independently off), with anti-reverse diodes

 Maximum input current

16X250A

 AC side parameters

 

Rated output power

loookV

Rated output voltage

310kV/35kV(optional)

Rated output frequency 50

50HZ

 Power Factor

-0.95ahead~+0.956Hysteresis

system

 

Reserved standard communication cabinet

Width X Depth X Height 600 X 600 X 2000 mm

Through groom

Inverters and transformers provide RS485 interfaces, respectively

Length X width X high

2 X 500kW( 6000mm X 2750mm X 2900mm)    1000kw( 6500mm X 1900mm X 3000mm)

weight

11000kg

other

With low-voltage power distribution control system, station power, cooling systems, lighting, fire and anti-condensation and other functions

Integrated integrated box design, eliminating the low-voltage room, low voltage side of the transformer and the inverter AC side of the direct connection, the user site only need to consider the DC side and step-up transformer to high-voltage side of the connection. Such an option reduces the footprint of the device body. LMWP photovoltaic power generation system is now an example, the integrated program covers 17. 6m2, rather than an integrated program covers an area of 30m2, can save 40% of the area.

 In fact, the greater advantage is that for non-integrated solution, step-up transformer and inverter can only be carried out on-site joint test, and integrated programs can be completed in the wide re-completed, shortening the site commissioning. However, here I emphasize that the designer or user, you must use good ventilation cooling performance, easy operation and maintenance of qualified products, or it will have a negative effect.

4.1.5.3 Total station lighting
Photovoltaic power station lighting is divided into normal lighting and emergency lighting. Lighting power from station power AC power, emergency lighting comes with a battery. Emergency time should not: less than; 30minn.
Energy-saving fluorescent lamps can be used as a normal lighting source. The light box circuit is separated from the socket circuit, and the socket circuit is equipped with a leakage protector.

4.1.5.4 Electrical equipment arrangement
In the photovoltaic power station to be built a photovoltaic integrated building can be a single layout; were arranged power distribution room, relay room, centralized control room and the layout of DC screen, UPS screen measurement screen, integrated automation screen. 10kV power distribution units with indoor complete switchgear, 10kV feeders should be used cables.

4.1.5.5 cable laying and cable fire
The 10kV power distribution room and relay room of the PV power station are equipped with cable trenches, and the solar module array is installed along the back of the PV module. The DC outlet of the cable is laid along the cable trench. The cable channel shall be set up according to the provisions of the Design Rules for Cable Selection and Laying of Power Stations and Substation and the Fire Prevention Code for Thermal Power Plant and Substation Design to prevent the fire from burning. Cable in the  uilding lead to electrical cabinets, discs or control panel, the opening parts of the station, cable through the wall, the floor of the hole, should be implemented fire blocking. Cable channel branch, into the power distribution room, centralized control room entrance should be implemented fire blocking.


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