How to configure the box-type substation?

1. High-voltage side configuration of box substation

(1) Box transformers are divided into terminal type and ring network type according to the primary wiring mode of the high voltage side. No need to consider additional wiring is the terminal box change: if the high-voltage side has one input and multiple output or multiple input and one output (mostly one input, two output or three output, or two input, one output or two output), both It can be regarded as a ring network box change.

(2) The high-voltage side of the European box substation has a circuit breaker configuration. There are also load switch-fuse combination protection. Considering the cost of the box transformer, when the capacity of box transformer does not exceed 800 kV·A and the high-voltage side protection configuration requirements are not high, the latter is often used.

(3) Most of the high-voltage load switches in the European box substation are FLN-12 type sulfur hexafluoride load switches.

(4) The rated current of the protection high-voltage fuse melt is selected by the following formula:

Irr=KIgmax

Where Irr——the rated current of the melt, A;

K-coefficient, usually 1.3:

Igmax——The rated current of the high voltage side of the transformer.

(5) Considering the safety of operation and maintenance, the high-pressure side configuration of the European type box transformer should meet the following requirements.

① There is a grounding device when the load switch is in the off state to ensure the safety of maintenance.

②The load switch and the isolating switch are mutually locked to ensure safe operation.

(6) American box substation is assembled in the same oil-immersed box because the high-voltage load switch-fuse combination electric appliance and transformer are combined into one, including position load switch, backup fuse, plug-in fuse and transformer. The ring network type is realized by changing the internal connection end of the load switch.

2. The configuration of the low-voltage main distribution cabinet of the box substation

The low-voltage main distribution cabinet of box substation generally needs to be equipped with incoming low-voltage disconnecting switch, low-voltage main circuit breaker and low-voltage main metering device. The low-voltage isolating switch is to meet the needs of daily maintenance and must be installed. The low-voltage main circuit breaker is selected according to the transformer capacity and the short-circuit current at the installation location of the circuit breaker.

Transformer low-voltage main circuit breakers generally use frame-type intelligent circuit breakers, and their selection and setting requirements are as follows.

(1) The rated current of the circuit breaker. Generally, 2.0 to 2.5 times the transformer capacity value is sufficient. The corresponding rated breaking current is greater than the short-circuit current value on the low-voltage side of the transformer, and calibration is rarely required.

(2) Long time delay (that is, inverse time limit, overload) overcurrent release setting current Lset, it is required to meet In≤Iset1≤Iz. Among them, In is the rated current of the low-voltage side of the transformer, and Iz is the current carrying capacity of the low-voltage bus. In order to make full use of the capacity of the transformer without affecting its service life, Iset1 should be equal to or close to In.

(3) Short time delay (ie definite time, over current) over current and tripping set current Iset2, it is required to meet 1.2Ifj≤Iset2≤Id/1.3. Among them, Ifj is the short-time load peak current of the transformer, and Id is the single-phase ground fault current of the transformer. This requirement is easy to meet for the transformers connected to D and yn11, but it is not easy to meet the transformers connected to Y and yn0. This is the reason why D and yn11 transformers are often used in box transformers. The setting time of the short-delay overcurrent release of the transformer low-voltage main circuit breaker is generally 0.6 s, which can ensure that the time difference with the next-stage circuit breaker is not less than 0.2 s.

(4) The instantaneous overcurrent release setting current Iset3. Due to the short delay overcurrent protection, in order to ensure better selectivity, the value of Iset3 can be set larger, generally Iset1, which is 12-15 times the value.

3. Reactive power automatic compensation device of box substation

According to the current national standards, the reactive power compensation capacity is generally required to be configured according to 30% of the transformer capacity, which of course meets the requirements under the conditions of high natural power factor. However, with the rapid popularization of inductive household appliances such as air conditioners, washing machines, refrigerators, energy-saving lamps, etc., the configuration of 30% of the transformer capacity sometimes fails to achieve the desired effect. It should be configured according to 40% to 60% of the transformer capacity, and 10 control loops are automatically compensated. Although the cost has increased, the cost of the entire box change is negligible, and the compensation effect is good.

4. The configuration of the low-voltage outlet cabinet

The number of loops of the low-voltage outlet cabinet of the box substation is generally determined according to the load distribution of its power supply area and 1 to 3 loops are properly reserved. The specification and model of the circuit breaker selected for each loop is determined according to its corresponding calculated load of. The selection and setting requirements of the low-voltage circuit breakers of each feeder are similar to those of the main transformer circuit breaker. But the premise is to know the calculated load current of each feeder. It can be reasonably configured.