I. selection of high voltage circuit breaker - type and type

The type and type of high-voltage circuit breaker shall be selected according to the installation location, environment and service conditions of the circuit breaker. Vacuum circuit breaker has been widely used in 35kV and below power system, and it has the trend of replacing oil circuit breaker. SF6 circuit breaker has also been applied in 10-35kV urban and rural power grid construction and transformation. Hygk- 307 circuit breaker analyzer produced by Wuhan Huaying power (www.hy-dl. CN) is used for parameter detection and analysis of high voltage circuit breaker

Most of the operating mechanisms of high-voltage circuit breakers are supplied by the manufacturer. Only some oil less circuit breakers have several types of operating mechanisms such as electromagnetic, spring or hydraulic. The general electromagnetic operating mechanism needs to be equipped with a special DC closing power supply, but its structure is simple and reliable; the spring structure is relatively complex, and the adjustment requirements are high; the processing accuracy of the hydraulic operating mechanism is high. The type of operating mechanism can be selected according to the convenience of installation and commissioning and operation reliability.

II. Selection of high voltage circuit breaker

1. Rated current

The rated current is the effective value of the continuous current of the circuit breaker under the specified service and performance conditions. Rated current shall be selected from R10 series specified in GB / t762. It should be noted that the circuit breaker does not have the specified continuous over-current capacity. When selecting the rated current of the circuit breaker, any load current that may occur in operation shall be taken into account, and they shall not exceed the rated current as long-term effects. The selection of rated current should also take into account the needs of grid development. In DL / t593, although it is specified to increase the temperature rise test current value, the current is increased to 1.1 times of the rated current, it is not allowed to be misunderstood that the operating load current (i.e. rated current) can be continuously increased.

2. Rated voltage

The rated voltage (i.e. maximum voltage) of the high voltage circuit breaker is as follows:

3.6, 7.2, 12 (11.5), 24, 40.5, 72.5126252 (245), 363550kv. The rated voltage of the circuit breaker indicates the highest system voltage it can bear for a long time in operation. The voltage that the circuit breaker bears for a long time in operation shall not exceed its rated value.

The rated voltage of the circuit breaker shall be equal to or greater than the maximum voltage of the system. 

When selecting the rated voltage of the circuit breaker, the relevant provisions in DL / t593 shall also be considered.

3. Rated short-circuit breaking current

3.1 rated short-circuit breaking current

Rated short-circuit braaking current refers to the effective value of AC component in rated short-circuit current.

3.2 rated short-circuit making current

Rated short-circuit making current refers to the highest peak value of rated short-circuit current, which is equal to 2.5 times of rated short-circuit breaking current value, which is calculated according to the time constant τ of DC component attenuation of the system as 45ms.

3.3 rated short-time withstand current and its duration rating

The rated short-time withstand current is equal to the rated short-circuit breaking current, and its duration rating is 4S for 110kV and below, and 2S for 220kV and above.

3.4 rated peak withstand current

The rated peak withstand current is equal to the rated short-circuit making current, that is, the rated short-circuit breaking current multiplied by a factor of 2.5, which is determined by the generator circuit breaker standard (for example, 2.7-2.8).Under the rated voltage, the maximum short-circuit current that the circuit breaker can ensure normal breaking is called rated breaking current I NBR. In the high-voltage circuit breaker, its value should not be less than the cycle component of the actual short-circuit current at the moment of breaking, that is to say, only 20% of the non cycle component is included in the type test of the high-voltage circuit breaker produced in China. Generally, for medium and slow speed circuit breakers, due to the long breaking time (> 0.1s), the short-circuit current aperiodic component decays more, which can meet the requirements of the national standard that the aperiodic component does not exceed 20% of the amplitude of the periodic component. When using the fast protection and high-speed circuit breaker, the breaking time is less than 0.1s. When the short circuit is near the power supply, the aperidic component of the short circuit current may exceed 20% of the periodic component, so it needs to be checked. High voltage circuit breaker

selection dynamic stability verification before the circuit breaker is closed, if there is a short-circuit fault on the line, during the closing process of the circuit breaker, there is a huge short-circuit current passing (pre breakdown) between the dynamic and static contacts when they are not in contact, which is more likely to occur contact fusion welding and suffer damage from electric power.

When the circuit breaker closes the short-circuit current, it will inevitably trip automatically after it is connected. At this time, it is also required to cut off the short-circuit current. Therefore, the rated closing current is one of the important parameters of the circuit breaker. In order to ensure the safety of the circuit breaker when closing the short circuit, the rated closing current incl of the circuit breaker shall not be less than the maximum impact value ICH of the short circuit current.

3.5 rated frequency

The rated frequency of power system in China is 50Hz.

3.6 rated insulation level

When selecting a circuit breaker, it shall be indicated whether it is an indoor circuit breaker or an outdoor circuit breaker.

The insulation of the circuit breaker includes: the insulation of the main circuit to ground; the insulation between phases; the insulation between the break of the circuit breaker; the insulation between the break of circuit breaker that plays the role of liaison (that is, the isolation break that plays the role of isolation).

The insualtion level of the circuit breaker shall be selected from the table specified in DL / t615-2013.

III. selection of high voltage circuit breaker-thermal stability verification

When the circuit is short circuited, the short circuit current will make the conductor temperature rise rapidly. However, after a short circuit, the protection device of the line acts quickly to remove the short circuit fault, so the time for the short circuit current to pass through the conductor is very short, usually not more than 2- 3s. Therefore, in the process of short circuit, the heat dissipation of the

surrounding medium can be ignored, that is to say, in the short circuit time, the conductor and the surrounding medium are adiabatic, and the heat generated by the short circuit current in the conductor is all used to make the conductor temperature rise.

Because the short-circuit current is a variable current and contains non periodic components, it is very difficult to calculate the heat

Q1 generated by the short-circuit in the conductor and the maximum temperature reached. For this reason, a "short-circuit heating pseudo time" Tmax is introduced. It is assumed that the heat generated by the constant short-circuit stable current passing through the conductor in this time is exactly equal to the heat generated by the actual short-circuit current IK passing through the conductor in the actual short-circuit time.