The switchgear is not single equipment. This is the name of the entire system which consists of isolators, controllers, meters, relays, circuit breakers, transformers, and all other pieces of equipment required for operating and controlling the system.
For effective and efficient connectivity between generating stations and load centers, switchgear systems are needed. More precisely for the control and protection of a power system proper switchgear system is mandatory.
Selection of Switchgear
The below-mentioned conditions are considered during the selection of which gear system.
- The working voltage,
- The load current,
- The fault current level,
- The atmospheric condition,
- Normal operations and remote operations.
Types of Switchgear
There are mainly two types of which gear available.
- LT Switchgear
- HT Switchgear
Types of Faults occur in an Electrical System
Overcurrent fault: The causes of overcurrent faults are short circuits in the mainline, excessive-high corona occurring in the system, and in some events, the excessive overloading of the system causes an overcurrent fault.
Low voltage fault: Due to a short circuit in the system the voltage level drops. The voltage drop may also occur sometimes due to the failure of alternator fields.
Unbalance fault: Due to short circuits in one or more phases in the system, the line voltages become unbalanced. Because when all three phases are not healthy, the currents flowing through all three phrases are not the same or balanced. This unbalanced condition initiates fault in the system.
Reverse power fault: This type of fault occurs in the interconnected system. If an alternator suddenly fails, it starts behaving as a motor. Therefore it will receive power from the system instead of delivering power to the system. In that case Power Will flow in the opposite direction. This is called reverse power.
Transient overvoltage fault: Due to lightning and sometimes due to switching operation, there may be a transient overvoltage occurring in the system. This transient overvoltage or transient disturbance may lead to a fault in the system resulting in insulation failure.
Causes of Short Circuit Currents
There are several causes of short circuit currents in an electrical system.
- Single-phase to earth short circuit fault.
- Phase to phase short circuit fault.
- Double-phase to earth short circuit fault.
- Triple-phase to earth short circuit fault.
- Short circuit of all phases together.
Different Pieces of Equipment used in a Power System
An isolator is nothing but a simple switch. With the help of isolators, one can isolate a part of the system for maintenance and other purposes. There are several types of isolators available in the market. For example,
- Vertical break type isolator,
- Horizontal break type isolator,
- Vertical pantograph type isolator.
Auto Recloser Scheme
Whenever there is a sudden fault that occurs in the system, the auto recloser judges the type of fault whether it is transient or permanent. After the first tripping of the circuit breaker due to the fault, the auto recloser closes the line again. If the fault persists, the circuit breaker will trip again and tries to reclose with the help of the auto recloser. In this way, the circuit breaker opens and closes 2 to 3 times before it permanently comes to the open condition if the fault is permanent. If the fault is purely transient, the circuit breaker remains in its closed position after it is reclosed by the auto recloser.
Bus Bar System
The bus system facilitates the distribution of the power to different feeders. Also, it facilitates to receive power from different sources. ACSR conductors or aluminum pipes of suitable sizes are used for constructing bus bar systems in a substation.
Functions of Busbars
- It is mainly used for avoiding interruption in the system.
- The system also helps to isolate a feeder or a bay or a small part of the system without interruption of the entire system.
- It also facilitates connecting a number of sources and loads to the switchgear system.
General Types of Busbar System
- Single busbar system,
- Single busbar system with sectionalisation,
- Ring busbar system,
- Double busbar system.
Single Busbar System
In a single busbar system, all the feeders and sources are connected to a single bus. Therefore if any problem arises in the main single bus, the substation will go to a complete shutdown. This is the simplest form of busbar systems but the flexibility of the system is almost zero.
Single Busbar System with Sectionalisation
In a single busbar system with sectionalisation is almost similar to a simple single busbar system. But here one isolator is given in the middle of the busbar. Generally, the feeders are divided into two groups and connected to either side of the section isolator. In the same manner, the sources are also divided into two groups and connected to either side of section isolator. When one section of the bus becomes defective the other section of the bus can be kept alive by keeping the sectionalisation isolator open.
Ring Bus System
The ring bus system is another simple form of busbar. Here each feeder has two circuit breakers one from the left another from the right. Again each of the circuit breakers in the system shares two feeders. In this manner, the entire system forms an interconnected ring. The main advantage of the system is that when one feeder is gone under shutdown there may be no disturbance in the supply of other feeders.
Double Bus System
The double bus system is the most commonly used system in high voltage substations. In this scheme, there are two identical buses located parallel to each other. Each of the feeders can be connected either of the buses with the necessary switching operations. During defect or shutdown of anyone busbar, another can take the entire load of the substation by shifting all the feeders to that healthy bus. This shifting can be done online by proper switching operation therefore there is no power interruption occurred in the system.
A circuit breaker is such a mechanical device that can be switched on and off during normal as well as the faulty conditions of the system. During an abnormal condition when the current crosses its pre-specified limit the circuit breaker trips to isolate the faulty portion of the system. An electrical fuse does the same. But in HV applications we generally do not use fuses because fuses are not as reliable as circuit breakers and also fuses do not have circuit making capability. Instead of using fuses, we use circuit breakers generally in medium voltage and mandatorily in the High Voltage systems.
Types of Circuit Breaker
- Air Blast Circuit Breaker
- Oil Circuit Breaker
- SF6 Circuit Breaker
- Vacuum Circuit Breaker
Air Blast Circuit Breaker
The compressed air in high pressure is used for arc quenching in an air blast circuit breaker. During operation of the breaker a blast of highly compressed air between fixed and moving contacts of the breaker extinguishers the arc.
Oil Circuit Breaker
The oil circuit breakers can be categorized into two different types. These are bulk oil circuit breaker and minimum oil circuit breaker. In bulk oil circuit breaker the oil is utilized as the insulating as well as the arc quenching medium. The entire mechanism along with fixed and moving contacts are entirely immersed in a container filled with insulating oil. Normally the container of the bulk oil circuit breaker is made of iron or steel.
In the minimum oil circuit breaker, the insulation of the circuit breaker is provided with the help of solid porcelain. Only the arcing chamber is filled with oil. Here in the minimum oil circuit breaker, the oil is only used for arc quenching.
SF6 Circuit Breaker
This type of circuit breaker uses SF6 gas as the arc quenching medium. Sf6 gas is a very good absorber of free electrons. Because of this typical property of the gas, it is quite suitable for using as the arc quenching medium in circuit breakers.
Vacuum Circuit Breaker
In a vacuum circuit breaker, there are vacuum vessels or containers in which the current interruption takes place. These vessels are called vacuum interrupter. Each of the vacuum interrupters contains both moving and fixed contacts inside it. These contacts are made of chrome copper. One metallic bellow is provided with the moving contact. It facilitates the movement of the moving contact inside the vacuum interrupter without breaking the vacuum inside the interrupter. Due to the absence of any medium inside the interrupter, the arc initiated due to the metal vapor gets extinguished during the first current zero of the fault current. This type of breaker is fire hazard-free, noise-free and requires very little maintenance.