The Functions And Differences Of High-voltage Load Switches, Circuit Breakers, And Isolation Switches

The functions and differences of high-voltage load switches, circuit breakers, and isolation switches

Purpose of high-voltage isolation switch:

The isolation switch is mainly used to reliably isolate the parts that need to be cut off from the live parts in the high-voltage distribution device, in order to ensure the safety of maintenance work.

The contacts of the isolation switch are all exposed in the air and have obvious disconnection points. The isolation switch does not have an arc extinguishing device, so it cannot be used to cut off load current or short-circuit current. Otherwise, under high voltage, the disconnection point will generate a strong arc and it is difficult to extinguish it on its own. It may even cause arcing (phase to ground or phase to phase short circuit), burn equipment, and endanger personal safety, which is a serious accident called "pulling the isolation switch with load". Isolation switches can also be used to switch certain circuits to change the operation mode of the system.

For example:

In a dual bus circuit, an isolation switch can be used to switch the running circuit from one bus to another. At the same time, it can also be used to operate some small current circuits.

(1) After opening, establish a reliable insulation gap and separate the equipment or line that needs maintenance from the power supply with a clear disconnection point to ensure the safety of maintenance personnel and equipment.

(2) Change the wiring according to operational needs.

The switch is composed of a base, rod type post insulator, conductive knife, left and right contacts, and transmission parts, also known as a V-type isolation switch. The isolation switch can be equipped with grounding switch as needed and is widely used in voltage levels of 35-110kV.

The function of high-voltage load switch:

(1) Purpose

High voltage load switch is an electrical appliance with functions between high voltage circuit breaker and high voltage isolation switch. High voltage load switch is often used in series with high voltage fuse; Used to control power transformers. The high-voltage load switch has a simple arc extinguishing device because it can turn on and off a certain amount of load current and overload current. However, it cannot disconnect short-circuit current, so it is usually used in series with high-voltage fuses to provide short-circuit protection.

(2) Characteristics

It can isolate the power supply and has obvious disconnection points, mostly used for fixed high-voltage equipment. There is no arc extinguishing device, which can pass through normal working current and short circuit circuit circuit in the closed state. It is strictly prohibited to connect and disconnect circuits with load, and it is often used in series with high-voltage circuit breakers.

(3) Function

Used for breaking load current and making short-circuit current, used for breaking load current, closed-loop current, no-load transformer and cable charging current, and making short-circuit current. Load switches equipped with grounding switches can withstand short-circuit currents. A load switch fuse combination device with a tripping device and a high breaking capacity fuse is equipped with a phase failure tripping function, which can cut off short-circuit current and serve as a protective device.

(High voltage load switch model)

From the perspective of usage environment, there are indoor and outdoor types;

From the perspective of arc extinguishing forms and extinguishing media, there are oil, compressed air, gas production, vacuum, SF6, etc;

SF6 type load switch:

The load switch using SF6 (Sulfur hexafluoride) gas as insulation and arc extinguishing medium is called SF6 load switch. It can be used to make and break load current and overload current, as well as make and break no-load lines, no-load transformers and capacitor banks. Used in series with fuses, it can break short circuit currents and has strong arc extinguishing ability. It is suitable for large shopping malls, industrial and mining enterprises, urban residential areas, power distribution, switching stations, airports, subways, sports venues, hospitals, small secondary substations, and other places

Advantages of SF6 load switch:

(1) The functions of switch, opening, and grounding are integrated and packaged in an epoxy resin shell filled with SF6 gas, with three positions of interlocking and a compact mechanism.

(2) High safety and reliability.

(3) The relay protection device with no power supply compensates for the lack of protection of the fuse in the overcurrent part of the transformer.

(4) Small size, light weight, easy maintenance, convenient and safe operation.

High voltage circuit breaker

A circuit breaker, also known as a high-voltage switch, can not only cut off or close the no-load current and load current in the high-voltage circuit, but also cut off the overload current and short-circuit current through the relay protection device when a system fault occurs.

(1) The Function and Function of Circuit Breakers

High voltage circuit breakers are one of the most important equipment in the power system, used to connect and disconnect circuits under normal conditions, and switch equipment that can quickly disconnect fault currents in case of faults. Including:

● Cut off or close the no-load current of high-voltage lines;

Cut off or close the load current of high-voltage lines;

Cut off or close the fault current of the high-voltage line.

(2) Classification and characteristics of circuit breakers

High voltage circuit breaker is mainly divided into multi oil circuit breaker, low oil circuit breaker, Sulfur hexafluoride circuit breaker, vacuum circuit breaker, compressed air circuit breaker, etc. Their characteristics are as follows:

1) Multi oil circuit breaker

It is simple to implement and inexpensive, but due to its high oil consumption, large volume, heavy maintenance workload, and susceptibility to explosions and fires.

Multi oil circuit breaker is a high-voltage circuit breaker that uses insulating oil as the arc extinguishing medium and the main insulation medium. Its structure is simple, the process requirements are low, but its volume is large, and it uses more steel and insulating oil.

2) Low oil circuit breaker

It uses less oil and has a small and sturdy fuel tank structure, which has the characteristics of material saving, explosion-proof and fireproof. The use of low oil circuit breakers is safe, greatly simplifying the distribution device, making it small in size and easy to transport.

Low oil circuit breakers have a simple structure, a long history of use, and rich operating experience. Currently, they are still dominant in the power system. Low oil circuit breakers below 20kV produced in China are indoor, while 35kV and above are outdoor.

3) SF6 circuit breaker

Good electrical performance, high fracture voltage. The operation, maintenance and overhaul of the equipment are very convenient, the overhaul cycle is long, the breaking performance is good, and the floor area is small. In particular, the SF6 fully enclosed composite electrical appliances can greatly reduce the floor area of the Electrical substation.

Generally speaking, due to the excellent performance of SF6 insulation medium itself, SF6 circuit breakers have gained numerous excellent characteristics.

High fracture voltage. The 220kV SF6 circuit breaker only requires a single break, so its structure is simple and convenient for manufacturing, installation, debugging, and operation;

● Allow multiple circuit breakers and long maintenance cycles;

● Good breaking performance, SF6 circuit breaker with high breaking current, short arc extinguishing time, no serious current cutoff phenomenon, and no reignition when breaking capacitive current;

Not easily ignited, safer to operate;

● Less land occupation;

The price is high.

4) Vacuum circuit breaker

Vacuum is used as insulation and arc extinguishing medium, with small volume and light weight. However, it has high requirements for arc extinguishing chamber technology and materials, and can be operated continuously multiple times. It has good breaking performance, short action time, simple operation and maintenance, high reliability, and is suitable for multiple operations in distribution systems. It is widely used in 35kV and below systems.

5) Air circuit breaker

Air circuit breakers are also known as compressed air circuit breakers. Use pre stored compressed air to extinguish the arc. Compressed air not only serves as an arc extinguishing and insulation medium, but also as the driving force for transmission.

Air circuit breakers have high short-circuit capacity, fast action time, small size, light weight, and no fire hazard. However, their structure is complex and expensive, requiring the installation of compressed air systems. They are mainly used in systems with high requirements for electrical parameters and circuit breaking time at 110kV and above, as well as in situations where large generator outlets require high rated current and circuit breaking. In China, it is commonly used in large capacity power systems of 110kV and above, and has been basically replaced by SF6 circuit breakers.

6) SF6 fully enclosed switchgear (GIS)

It is to optimize the design of the primary equipment in the substation other than transformers, including circuit breakers, disconnectors, grounding switches, current transformers, Voltage transformer, lightning arresters, buses, incoming and outgoing line bushings or cable terminals according to the system layout and organically combine them into a whole.

Its main advantages are small footprint, less space occupation, high operational reliability, long maintenance cycle, unaffected by external environmental conditions, strong seismic resistance, etc. The main drawback is that it is expensive. Currently, it has been increasingly applied, mainly for 110-500kV systems.

(3) Main parameters of circuit breakers

1) Rated voltage

The rated voltage is the standard voltage for long-term operation of circuit breakers. Due to the allowable fluctuation of the working voltage in the power system, the circuit breaker also specifies the highest working voltage.

Table 1 Rated Voltage and Maximum Operating Voltage of Circuit Breakers

When selecting a circuit breaker, the rated voltage should be selected first.

2) Rated current

Rated current refers to the long-term operating current allowed by a circuit breaker. The rated currents specified in China are 200A, 400A, 630A, 1000A, 1600A, 2000A, 3150A, 4000A, 5000A, 6300A, 8000A, 12500A, 16000A, and 20000A.

3) Rated breaking current IbrN

The maximum circuit current that a circuit breaker can break at rated voltage is called the rated breaking current.

The rated breaking current specified in China is 1.6kA, 3.15kA, 6.3kA, 8kA, 10kA, 12.5kA, 16kA, 20kA, 25kA, 31.5kA, 40kA, 50kA, 63kA, 80kA, and 100kA.

4) Rated breaking capacity SbrN

Rated breaking capacity, used to characterize the breaking capacity of a circuit breaker.

5) Rated making current Imc

The rated making current represents the making capacity of the circuit breaker, and its size is:

The difference between circuit breakers, load switches, and isolation switches

(1) Isolation switch

It can only disconnect and connect the no-load current of the zd system, and serves as a clear disconnection point for the main wiring system, and as a clear disconnection point for the system during maintenance.

(2) Load switch

It is possible to disconnect and connect the normal load current of the system, but it cannot separate the fault current of the version system.

(3) Circuit breaker

In addition to being able to disconnect and connect the normal load current of the system, it can also disconnect and connect the fault and short circuit current of the system

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