The so-called short-circuit current is an important parameter value in the design of the power system. It refers to the current when the power system is short-circuited in the specified operating mode. It is actually a contradiction value, it not only reflects the closeness and stability of the interconnection of the power system (when the short circuit, any contact line with the focus will contribute short-circuit current to this point), but also reflects the short circuit at this point. The size of the short circuit current. The short circuit capacity is small, the system is unstable, the connection is not strong; the short circuit capacity is large, the short circuit current exceeds the standard, the equipment is expensive, and the control measures are complicated.

At present, the grid connection is getting closer and closer. The short-circuit current exceeds the standard is a very big problem. It is an important constraint for power system analysis and calculation. (Short-circuit current is the basic condition for grid equipment selection: mainly the circuit breaker interrupt current, 220kV current mainstream is 50kA, 500kV mainstream is 63kA, short-circuit current exceeds the interrupted capacity means safety hazard, so the improvement of grid short-circuit current data means unsatisfied The required equipment should be replaced, which not only causes a significant waste of grid investment, but also jeopardizes the safe operation of the grid.

First, the influencing factors of short-circuit current

Effect of generator on short-circuit current

According to relevant calculations and research, 300MW and 600MW units are connected to 220kV system, and the short-circuit currents provided to nearby busbars are 2kA and 4kA respectively; the short-circuit current provided by 600MW units to 500kV systems to nearby busbars is 2kA. The figure below shows the effect of a 1000MW unit on the short-circuit current after it is connected to a 500kV system.

The effect of step-down on short-circuit current

1) Influence of 500kV step-down variation of different short-circuit impedance on 220kV side short-circuit current

If two transformers with a short-circuit impedance of 12% are replaced by a parallel operation with a short-circuit impedance of 15%, the short-circuit current supplied to the 220kV bus will be reduced by 3~5kA; if it is replaced by a short-circuit impedance of 20%, It can be reduced by 7~11kA. The high-impedance transformer of the newly built or expanded 500kV substation can effectively reduce the short-circuit current of 220kV. (The high impedance is that the network loss is large and the operating efficiency drops)

2) Influence of different main transformer configurations (short-circuit impedance is 15%) on 220kV side short-circuit current

When the 500kV side short-circuit current is 60kA, the short-circuit currents supplied to the 220kV side of the three 750MVA, three 1000MVA, and three 1200MVA main transformers are 29kA, 35.8kA, and 40.7KA, respectively, and the four units can reach 36.7kA respectively. 45kA, 50.6kA. Therefore, after a 3~4 main transformer in a 500kV substation, the denominator is generally considered to reduce its short-circuit current of 220kV. (The following table shows the calculation and analysis in the actual project)

Second, measures to reduce short-circuit current

500kV layer short-circuit current solution

1) Substation busbar segmentation operation. Different substation outlets are connected to different busbars to reduce the electrical connection between the lines. At present, such measures are often used to limit the short-circuit current in the actual production process, and the effect is good, but the denominator operation brings about a reduction in power supply reliability, which needs to be weighed. consider. The following is the Jingmen UHV 500kV side denominator operation scheme, the effect is very good, but because of the impact on reliability, and the UHV safety is very sensitive, it has not been implemented.

2) Install a series reactor on the line. For example: the 8Ω series reactor impedance standard value is 0.0032, which is equivalent to the 50km wire type 4&TImes; LGJ-500 line, which lengthens the electrical contact and reduces the short circuit current. This measure is currently implemented in the 500kV Huangdu-泗泾 line in Shanghai. Some important lines in the vicinity of the Three Gorges will also implement such projects. Among them, there is a feasibility study for the person and a 500kV change. I feel that this measure belongs to If the root cause is not cured, it can meet the stage requirements, but there are many problems, such as increasing the network loss, reducing the stability of the system, and the demand for reactive power will increase, especially when considering N-1.

3) 500kV network structure optimization, such measures are not well discussed in depth, depending on the actual situation and analytical calculations.

220kV layer short-circuit current solution

1) Partition slice operation. Fragmentation is the most direct and effective measure to reduce short-circuit current. Taking Beijing Power Grid as an example: the 220kV power grid is divided into several zones with a 220kV busbar of 2~3 500kV substations as the center, forming a double-ring network structure with 220kV busbars of adjacent 500kV substations as the power supply center. The power grids are relatively independent in the normal mode. The 220kV powers of each zone can support each other, meeting the requirements of the 500kV main transformer and 220kV line steady state N-1, N-2. The idea of ​​Shanghai Power Grid is different.

2) Other measures. For example, high-impedance equipment, line adjustment, 220kV denominator operation, etc., are also effective means, but not as good as partitioning and fragmentation, so the 220kV layer partitioning operation and related network analysis and optimization are the fundamental to limit the short-circuit current. The measures are also the engineering basis for the current provincial companies.

Third, foreign measures to limit short-circuit current

Three-phase short-circuit current: Countries with higher short-circuit current levels, such as Germany and France, have adopted a quick de-disclosure in the event of a fault and segmented the busbar to limit the short-circuit current value. Although the busbar disconnection measure is simple and effective, it is generally only used when necessary, because it may reduce the safety margin of the system and limit the flexibility of operation and accident handling. Domestically, it pays more attention to safety, so it is not used much. In fact, it is also a question of thinking. Personally, it is a relatively good measure.

Single-phase short-circuit current: The single-phase grounding short-circuit current is mainly related to the system neutral grounding method and the zero-sequence impedance of the loop. France uses a transformer neutral point via a small reactance grounding method. Germany does not use an autotransformer as a system contact to limit single-phase short-circuit. In some countries, the neutral point of the transformer in the 110kV and above voltage grids is directly grounded, resulting in the single-phase grounding short-circuit current of the system is greater than its three-phase short-circuit current, such as the United Kingdom and Russia. Other countries, such as the United States, operate some of the large-capacity Y/Y/△ (500/230/35kV) autotransformer Δ side openings in the system to increase the zero-sequence impedance of the transformer. However, many countries believe that this is not good for operation. The 500kV side neutral point of the main transformer of the generator transformer group of Gezhouba Dajiang Power Station is designed and installed with small reactance grounding, which not only solves the problem of excessive single-phase grounding short-circuit current, but also solves the problem of large number of hydropower plants, large changes in operation mode, and system grounding. The short-circuit current changes too much, making the grounding protection difficult to set.

Fourth, short-circuit current and district power supply details

A more detailed summary of the computational analysis process.

Model and assumption

For the "independent partition" grid, you can simulate the following simple diagram. Line XL represents the equivalent line from the local power plant to the sub-area 500kV substation; S1..S4 represents the same type or different types of transformers in the 500kV substation.

For the “interconnected partition” grid, you can simulate it with the grid structure shown below. The meaning of the elements in the figure is the same as the above figure.

Assumptions:

1) The main transformer ratio is 525kV/242kV/35kV;

2) According to the current manufacturing capacity of the equipment, the new substation vision short-circuit current level, the 500kV bus line is controlled according to 63kA, and the 220kV bus line is controlled according to 50kA;

3) The maximum capacity of a single transformer in a substation, the number of transformers operating in parallel, shall be such that the short-circuit capacity of the 220kV busbar does not exceed the allowable value;

4) Since the short-circuit current of the 500kV busbar and the 220kV busbar in the parallel operation of the main transformer is only affected by the percentage of the main transformer high and medium voltage short-circuit voltage, only the percentage of the short-circuit voltage of the main transformer high-voltage side is analyzed.

5) Since the resistance in the grid is much smaller than the reactance, the influence of the resistance in the grid and the main transformer is ignored.

6) Since the two-phase grounding short-circuit current level of the 220kV busbar is generally relatively low, the single-phase grounding short-circuit current level can be reduced to be close to the three-phase short-circuit current by adding a small reactance at the main transformer neutral point, so only the following Analyze the three-phase short circuit of the substation bus.

7) The percentage of short-circuit voltage of the high-side side of the 500kV main transformer is within the current applicable range in China, and does not exceed 20%. The short-circuit voltage percentage of the transformer exceeds 20%, which has two drawbacks. The first is the reactive voltage balance and voltage stability problem, followed by the transient stability problem.

Calculation Model of 220kV Bus Short Circuit Current

The short-circuit current of 220kV busbar of 500kV substation mainly consists of two parts: short-circuit current injected into the 220kV busbar by the 500kV system (referred to as short-circuit current component of 500kV) and maximum short-circuit current component of 220kV busbar injected into the substation of 220kV power grid (referred to as "220kV short-circuit current component" ").

In the sub-area grid, under the component parameters shown in Figure 61, the short-circuit current injected by the 500kV system into the 220kV bus B242 through the transformer is:

It can be seen from the above equation that the short-circuit current of the 500kV system injected into the 220kV system by the transformer is related to the short-circuit capacity of the 500kV system, the capacity of the transformer, and the percentage of the short-circuit voltage.

The 220kV short-circuit current component is mainly related to the capacity, access mode and power grid structure of 220kV local power plants.

Analysis of short-circuit current provided by 500kV system

Assuming that the transformers in the substation are of the same type, the following table shows the short-circuit current values ​​of the 220kV busbars injected into the transformers with different capacity configurations when the 500kV system has different injection short-circuit currents (the percentage of short-circuit voltage of the transformers with capacities of 750MVA, 1000MVA, 1500MVA) Take 12%, 16%, 19% respectively)

It can be seen that the larger the transformer capacity, the smaller the short-circuit current is supplied to the 220kV busbar per 100MVA of the variable capacity.

220kV independent partition power configuration and power supply capability

The maximum value of the 220kV short-circuit current component is calculated by limiting the short-circuit current component of 500kV, thereby deriving the power configuration of the 220kV independent partition. The power supply capacity of the 220kV independent zone is equal to the sum of the power supply capacity of the 500kV substation and the power supply capacity of the 220kV local power plant.

1) Short-circuit current supplied by the local power plant to the 220kV bus

In the 220kV independent partition, according to the power plant and boosting parameters shown in Figure 1, the short-circuit current provided by the local power plant to the partition bus B242 is as follows:

Different capacity units are connected to the 500kV substation to contribute to the short-circuit current of the 220kV bus. (When the unit capacity is 600MW, Xd is taken as 0.2, other units Xd" is taken as 0.16, and the percentage of boosting and short-circuit voltage is all selected as 17%, that is, Xd"+Uk=0.33~0.37, take 0.8~0.85, power plant The equivalent 220kV line connecting the 220kV bus of the 500kV substation is 50km long and the wire type is LGJ-2*630.)

(1) The short-circuit current contribution of the power plant to the 500kV 220kV busbar is about (0.6~0.7) kA/100MW.

(2) A 300MVA unit can be connected to the system to provide a short-circuit current component of about 2.1kA; a 400MVA unit can be connected to the system to provide a short-circuit current component of about 2.5kA; a 600MW unit can be connected to a 220kV system. Short-circuit current component of about 3.8kA.

(3) The power plant with a capacity of 300MW or above and a total of 1200MW is connected to the 220kV power grid. The short-circuit current level that may be provided to the 220kV system is about 6.9~7.4kA, and the power plant with a total capacity of 1800MW is connected to the 220kV power grid. The short circuit current level that may be provided is approximately 12.4kA.

(4) The farther the equivalence distance of the local power plant is connected to the 500kV substation, the smaller the short-circuit current is provided, and vice versa.

2) Short-circuit current supplied by the local power plant to the 500kV bus

In the 220kV full division, the short-circuit current supplied by the local power plant to the 500kV bus is as follows:

The contribution of different capacity units to the 500kV busbar short-circuit current is estimated when the 50km line is connected to the 500kV substation.

1) After calculation, the short-circuit current injected into the 500kV side of the 220kV power plant is much smaller than the short-circuit current injected into the 220kV bus, which is about (0.18~0.27) kA/100MW;

2) The power plant with a total capacity of 1200MW and above is connected to the 220kV system, and the short-circuit current level provided to the 500kV bus is between 2 and 3kA. Compared with the 500kV side short-circuit current injected into the 500kV grid, the 220kV power plant unit has little effect on the short-circuit capacity on the 500kV side.

3) Similarly, the farther the equivalent distance of a local power plant is connected to a 500kV substation, the smaller the short-circuit current it provides, and vice versa. If the power plant is larger, the short-circuit current it provides will be larger, and vice versa.

2) Analysis of power supply capability

The power supply capacity of the 220kV independent zone depends on the operation of the main transformer of the 500kV substation in the sub-area (parallel operation or split operation), the short-circuit current level of the 500kV busbar of the 500kV substation, the power plant capacity in the sub-area and the network connection of the sub-area.

When using 750MVA main transformer:

In general, the transformer of the substation considers the overload capacity of 1.3 times. The substation only considers the configuration of the same type of transformer, and the system has the mode to adjust to the transformer without overload during the allowable overload time of the transformer.

When the 500kV transformer of the district grid selects a single capacity of 750MVA, the short-circuit voltage percentage is 12%, and the number of main transformers is 2~4, the power supply capacity of this zone is about 4400~4700MW, and the partition load transfer capacity varies with the number of transformers. The increase is proportional to the increase.

When using 1000MVA main transformer:

Therefore, when the partition 500kV selects the short-circuit voltage percentage as 16% and the single-capacity 1000MVA main transformer:

(1) When the number of main transformers in the substation is 2~4, the power supply capacity of the subzone is about 5000~5400MW, and the capacity of the zone load transfer increases proportionally with the increase of the number of transformers.

(2) From the operational point of view, the partition power supply capacity should not exceed 4000MW, otherwise the short-circuit current provided by the unit will be too large, resulting in the partition of only 500kV main transformers. The 500kV substation 220kV is forced to run the denominator, reducing the power supply. Reliability and operational flexibility.

When using 1500MVA main transformer:

The transformer capacity is 1500MVA, the short-circuit voltage percentage is 19%, and the power supply capacity in the zone varies greatly when the number of main transformers is different. Between 5000 and 6400MW, each additional main transformer in the zone will increase the power supply capacity of 500-800MW; The capacity of the partition load transfer increases in proportion to the increase in the number of transformers. It can be seen that the main transformer of 1500 MVA is suitable for a partition with a large partition load and a small local power plant.

in conclusion:

(1) For passive partitions:

For a zone with a predicted load of 4000 MW or less, a transformer of 750 MVA is preferred, and the percentage of short-circuit voltage is 12% or more;

For a zone with a predicted load value of 4000 to 5000 MW, a transformer of 1000 MVA may be preferred, and the percentage of short-circuit voltage is 16% or more;

(2) For active partitions:

The capacity of the transformer should not be chosen very large, otherwise, the power supply capacity of the partition will not rise and fall;

For a zone with a predicted load of 4000 MW or less, a transformer of 750 MVA is preferred, and the percentage of short-circuit voltage is 12% or more;

For partitions with a predicted load value of 4000 MW or more, a 1000 MVA transformer should be preferred, and the percentage of short-circuit voltage should be 16% or more. Try to control the power supply scale within the partition.

220kV interconnected partition power supply configuration and power supply capacity

For the analysis, the 500kV substation configuration in the sub-area is exactly the same, the distance between stations is 100km; the percentage of short-circuit voltage of 750MVA transformer is 12%, the percentage of short-circuit voltage of 1000MVA transformer is 16%, and the percentage of short-circuit voltage of 1500MVA transformer is 19%; 50km distance access to the interconnection system.

1) Short circuit current analysis

When the interconnection distance is 100km, the short-circuit current values ​​provided by the partitions to the 220kV busbars are mostly around 7kA under different configurations of the 500kV substation main transformer, and the short-circuit current value will increase as the distance between the interconnected zones decreases. The short-circuit current supplied to the 500kV side busbar of the substation is also related to the percentage of the short-circuit voltage of the main transformer. The short-circuit current value is relatively small, and the electromagnetic ring network operation has little effect on the short-circuit capacity on the 500kV side.

Open the tie line between the partitions, which can reduce the short-circuit current of the 220kV busbar of the substation of the interconnected system, and the short-circuit current can be reduced when the transformer operation mode is adjusted from the parallel operation to the split operation. The decoupling operation of the contact line between the interconnected partitions or the main transformer operation is related to reducing the electrical short-circuit current between the 220kV bus and the interconnected partition.

After calculation, four transformers are operated in parallel (capacity is 750~1500MVA). When the interconnection distance is 31~38km, the short-circuit current value provided is equal to the short-circuit current value of the 500kV substation parallel operation.

2) Analysis of access capacity of local power plants

With the increase of short-circuit current injected into the 500kV system, the capacity of the unit that can be connected to the power plant is gradually reduced. With the increase of the main transformer capacity of the interconnected system, the capacity of the unit that can be connected to the power plant is gradually reduced.

From the previous theoretical knowledge, the maximum unit allowed to access the grid is also related to factors such as the percentage of transformer short-circuit voltage and the distance between interconnected zones. The following conclusions can be drawn: When the injection current is constant at 500kV, the percentage of short-circuit voltage of the transformer Increase, the capacity of the power plant can be gradually increased in the accessible place; when the percentage of the transformer short-circuit voltage remains unchanged, the capacity of the unit that can be connected to the power plant increases as the distance between the interconnected zones increases.

It can be seen that when the short-circuit current injected into the 500kV system is 55kA, the capacity of the unit that can be connected to the local power plant is reduced by about 1000MW.

3) Analysis of power supply capability

The main consideration is the study of the power supply capability when the mode is "2 units - 2 units".

Interconnected zones are suitable for passive areas or areas with small power plant capacity. Interconnecting two 500kV supply areas can improve the reliability of their respective power supply and limit the ability to access local power plants, but the 500kV main transformer after interconnection operation The load factor is improved. In summary, it can be seen from Table 614 that when the 500kV system injection short-circuit current is less than 55kA, the power supply capacity of the interconnected partition grid will be greater than the sum of the power supply capabilities of the corresponding two independent grid grids, and greater than 55kA. It's small.

Estimation of grid partition size and number of partitions

The impact of the load value and the number of 500kV substations on the division of 220kV partitions is linked and works together.

(Finish)

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