About the choice of reactive power compensation capacity of photovoltaic power station

With the continuous growth of global energy consumption, as oil and natural gas energy is gradually becoming more stringent, coal energy pollution is intensifying, and electricity supply is increasingly becoming a "card neck" problem, solar energy has attracted more attention. In recent years, due to the continuous introduction of new photovoltaic policies across the country, various types of good news about the photovoltaic power generation industry are increasing. There has been a surge in the installation of photovoltaic power plants in various places, and at the same time, the development of photovoltaic-related industries has also been promoted.

As a new type of energy, photovoltaic power generation is being recognized and accepted by people. In 2013, the rushing of photovoltaic power plants appeared in all parts of the country. With the increasing proportion of photovoltaic power plants connected to the grid, the input and construction cost of reactive power compensation in photovoltaic power plants has become the focus of scholars and designers. Through the calculation of the reactive power loss of large-scale photovoltaic power plants, the paper selects a reasonable compensation capacity and compares it with the traditional reactive power compensation capacity configuration calculation method to obtain the optimal configuration principle of reactive power compensation capacity.

Static var generators (SVG) are increasingly being used as replacements for static var compensators (SVCs) in photovoltaic power plants. This paper selects a 20MW photovoltaic power station in Qinghai as a project case, calculates the reactive loss of the power station, and obtains the capacity configuration of the reactive power compensation device of the power station, and compares it with the capacity configuration of the traditional reactive power compensation device to obtain photovoltaic. Optimal configuration of capacity of power station reactive power compensation device.

1 Photovoltaic power station reactive power loss

The calculation of reactive power loss of photovoltaic power station is mainly the calculation of line reactive loss and the calculation of transformer reactive loss. The required configuration capacity for reactive power compensation can be obtained by calculating the sum of the above losses.

1.1 Line reactive loss calculation

QL=3I2X

Reactive loss due to QL-line reactance, kvar; current at I-line rated power, A

I=P/(√3 U cos?Vertebra)

P-line rated power, kW; U-line rated line voltage kv; cos - power factor; X-line equivalent reactance;

X=xL

X-wire unit length reactance, Ω/km; L-line length, km.

1.2 Calculation of reactive power loss of a single transformer

- transformer reactive loss, kvar; - transformer short-circuit voltage percentage; - transformer no-load current percentage; - load factor; - transformer rated capacity, kvA.

1.3 System reactive power loss

Q=QL+QT

2 Typical photovoltaic power station reactive power loss calculation

The article selects a 20MW photovoltaic power station in Qinghai as a project case. The installed capacity of the power station is 20MW, which is divided into 20 1MW photovoltaic power generation units. 20 power generation units are connected to the first-level confluence through the square-box confluence box, and 20 inverters are used. After the boosting unit is boosted and boosted, the three collector circuits feed the electric energy into the three high-voltage switchgear of the opening and closing station, and finally send it to the public network to realize the grid connection of the power station.

2.1 Calculation of reactive power loss of power station collector line

Since the photovoltaic grid-connected power station is the power supply end, in general, only the active energy is supplied to the power grid, that is, the DC power of the solar photovoltaic array is converted into the same frequency and the same phase of the AC power loss to the power grid, so the photovoltaic policy inner low voltage set The electric cable is not used as the main factor of the reactive power loss of the photovoltaic power station cable. The calculation of the reactive power loss of the photovoltaic power station cable is mainly the reactive power loss of the 35kv collector cable, the reactive power loss of the cable of 200m 35kv, and 6km. Overcurrent loss of overhead lines.

2.1.1 35kv collector lines generate reactive losses

Since the power station uses the YJV22-35kv-3x70 cable as the collector cable, the three loop collector cable usage is shown in Table 1:

Table 1

The current of each collector loop is shown in Table 2:

Table 2

Check the table 3 to get the YJV22-35kv-3x70 cable unit length reactance is x=0.132 Ω/km

The reactive loss of the collector circuit is shown in Table 3:

table 3

2.1.2 Photovoltaic power station to the tower outside the tower cable reactive loss

The 35kv cable is sent to the YJV22-35kv-3x240 cable, and the cable length per unit is x=0.0916 Ω/km.

The reactive power loss of the 35kv transmission line is shown in Table 4:

Table 4

2.1.3 Overcurrent loss of overhead lines

The overhead line of the station uses LGJ-240 steel core aluminum stranded wire, and the steel core aluminum stranded wire has a unit length reactance of x=0.386 Ω/km.

The overhead loss of overhead lines is shown in Table 5:

table 5

In summary, the cable reactive loss is: 20.8+5+625.32=651.12 kvar

2.2 Transformer reactive power loss calculation

The power station has a total of 20 photovoltaic power generation units. The installed capacity of each photovoltaic power generation unit is 1MW. The system diagram of 1MW photovoltaic power generation unit is shown in Figure 1:

As shown in Fig. 1, each photovoltaic power generation unit consists of a number of combiner boxes, which constitutes a level 1 confluence unit, and two sets of DC cabinets form a level 2 confluence unit, consisting of two 0.5MW inverters and one S11-1000kvA, 35/0.3-0.3. Kv constitutes an inverter boosting unit. In the photovoltaic power generation unit, since the inverter itself has the reactive power compensation effect, it is only necessary to calculate the reactive power loss of 20 box type transformers.

Box transformer parameters:

Model: S11-1000kvA, 35/0.3-0.3kv

Capacity: 1000kvA Short circuit impedance: 6.5%

No-load current ratio of transformer: 0.31% Number of transformers: 20

The following calculation method is a method for calculating the reactive loss of a single transformer.

Then, the calculation method for the reactive loss of 20 box-type transformers in the power station is:

The reactive loss generated by the transformer under different load conditions can be obtained, as shown in Table 6:

By calculating the reactive power loss under various load conditions of the transformer, it can be concluded that the reactive power loss consumed by the transformer under minimum load is 62kvar, and the reactive power loss under full load is 1362kvar. According to the statistics of the average power generation load of some photovoltaic power plants built in Qinghai, the average power generation load of the 20MW photovoltaic power station is about 70%, that is, the transformer has a reactive power loss of 699kvar under 70% load, and the system is reactive. Power loss

Q=QL+QT=651.12+699=1350.12kvar

Through the above calculation, a reactive power compensation device with a compensation capacity of 1.4 MVar can be configured for the power station. According to the traditional reactive power compensation capacity algorithm, the reactive power compensation capacity of the 20MW photovoltaic power plant needs to be about 10% of the installed capacity of the power station, that is, 2MVar. It can be seen from the comparison that the reactive power compensation is based on the traditional algorithm. The capacity of the device is calculated. Taking a 20MW power station as an example, it will cause a waste of 700kvar device capacity per month, and the excess compensation capacity accounts for about 35% of the total configured capacity.

3 Compensation for excessive capacity

If the capacity allocation of the reactive power compensation device is too large, it will not only increase the investment cost of the project, but also cause waste of project input, and there may be over-compensation, which causes the voltage at the station to rise, causing overvoltage conditions. In recent years, most of the reactive power compensation devices in photovoltaic power plants are buck-type complete reactive power compensation devices. The physical structure consists of step-down transformers and reactive power compensation complete control cabinets. The capacity and size of step-down transformers are mostly reactive and reactive. The capacity of the compensation device is proportional to the capacity of the compensation device. The larger the installation capacity and size of the transformer, the larger the corresponding transformer foundation. If the capacity of the reactive power compensation device is unreasonably configured, the compensation capacity configuration is too large. It will cause the no-load loss of the transformer of the compensation device and the increase of the cooling loss of the device, which will reduce the efficiency and economic benefit of the equipment.

4 Conclusion

The National Energy Administration’s Notice of the National Energy Administration on the Annual Increase in the Scale of Photovoltaic Power Generation in 2014 is mentioned in 2014. The total domestic new scale is 1400MW. It can be seen that in the next year or even years, photovoltaic power generation It is still an emerging energy industry supported by the state. Although the inverter in the photovoltaic power station has already realized the function of reactive power regulation, the photovoltaic power station needs to have a certain reactive spare capacity to provide the grid with power failure or abnormality. Reactive power support, preventing voltage collapse has become a rigid requirement for PV power plant equipment configuration, reasonably calculating the reactive power loss of photovoltaic power plants, and obtaining a reasonable reactive power compensation capacity configuration can avoid relying on traditional algorithms to configure over-capacity reactive power compensation devices. The investment waste of the photovoltaic power station reactive power compensation device is increased, and the investment return of the photovoltaic power station is increased from the side.