Switchgear is a piece of equipment that is mainly used to switch, control, and protect electrical components and circuits. Basically, switchgear is a combination of electrical switches, circuit breakers, isolators, relays, and fuses. The arrangement of these devices is used for interrupting current under normal and abnormal operating conditions. All the components of the switchgear are perfectly packed into a properly earthed metal container. Switchgear does not only protect the load from being damaged, but it also maintains the safety of transmission cables and switchgear itself. The switches used in households make use of air as an insulating medium; whereas, the circuit breakers installed to protect the applications working on high magnitude current and voltages use oil, special gases, or vacuum for insulation. Some of the switchgear also use a combination of various types of insulators.
Features of a Switchgear
The four essential features of the switchgear are given below:
Reliability is one of the most important features of switchgear. The safety of the entire power system depends on the switchgear, hence it must be able to cut off the faulty section from the system quickly and flawlessly.
Absolutely Certain Discrimination
Switchgear must be able to discriminate the faulty parts of the system from the healthy parts. In case a section of the system gets ruptured due to power overflow, the switchgear should immediately isolate the faulty section without affecting the operation of the healthy sections of the system. If the switchgear is not able to completely discriminate between healthy and faulty sections of the system, the fault is much likely to spread and damage the whole system.
The fast response of the switchgear is the prime requirement. This is because it is necessary for the switchgear to identify the faulty device and cut it off from the system as soon as possible to avoid the damaging of transformers, generators, and other major pieces of equipment. Slow response switchgear is inefficient in fault detection; therefore is dangerous to use and is not generally preferred.
Provision for Manual Control
Provision for manual control is one of the most important features of switchgear. This is because in case the automatic control of the device fails, the operator should be able to carry out the operation manually.
Components of a Switchgear
Switchgear makes use of two major types of components, namely power conducting components and control system components.
Power Conducting Components
The power conducting components of switchgear include switches, fuses, circuit breakers, insulators, lightning arrestors, etc. These components are mainly used to interrupt the flow of electric power through the circuit.
Control System Components
The control system components of the switchgear include control panels, current or potential transformers, relays, etc. These components are mainly used to monitor, control and protect the system as well as the power conducting components.
Classification of a Switchgear
Depending on the Voltage Level
On the basis of the voltage level, the switchgear can be broadly classified into three categories, namely low voltage switchgear, medium voltage switchgear, and high voltage switchgear.
Low voltage (LV) Switchgear
Low voltage switchgear is used in the power electronic circuits that deal with the voltage level up to 1KV. Some of the most commonly used low voltage devices required to protect the low voltage systems include air circuit breakers, oil circuit breakers, earth leakage circuit breakers, miniature circuit breakers, moulded case circuit breakers, switch fuse units, HRC fuses, etc.
Medium voltage (MV) Switchgear
The electrical systems that work up to 36 kV are known as medium voltage systems. The systems working within this particular level of voltage make use of medium voltage switchgear. Medium voltage switchgear typically includes equipment such as bulk oil circuit breakers, minimum oil circuit breakers, etc. The interruption medium used by medium voltage switchgear may be either oil, vacuum, or other special non-toxic, inert, and insulating gases such as SF6. On the basis of the type of interruption medium used, the medium voltage switchgear can be categorized into air, vacuum, and gas-insulated switchgear. Similarly, on the basis of casing and installation, different types of medium voltage switchgear include metal-enclosed indoor type, metal-enclosed outdoor type, outdoor type medium voltage switchgear without metal casing, etc.
High voltage (HV) Switchgear
The electrical system working at voltages above 36kV make use of high voltage switchgear. The main equipment used in the construction of high voltage switchgear is the high voltage circuit breaker. The interruption medium generally opted by the high voltage switchgear is SF6 gas, vacuum, etc. The devices and systems working at high-level voltages tend to produce high arcing during the switching operation, which is why special care must be taken while designing such a system. The reliability of such devices is required to be extremely high. It must be noted that the switching and tripping operations of the high voltage switchgear are observed very rarely.
Depending on the Voltage to be Handled
On the basis of the voltage to be handled, the switchgear can be broadly classified into two categories, namely outdoor type switchgear and indoor type switchgear.
Outdoor type Switchgear
For voltages above 66kV, outdoor type switchgear is used. This is because, at such high magnitude voltages, the equipment used by the system such as transformers, switches, circuit breakers, etc., require a considerably large surface area and a wide electrical clearance between the conductors. The installation of such a system indoor is not economical at all.
Indoor type Switchgear
For voltages below 66kV, indoor type switchgear is preferred. The construction of indoor type switchgear is generally a metal-clad type. Here, all the pieces of equipment involved in the system are packed together in an earthed metal case. This type of system can be easily installed indoors because it is more compact and economical in nature.
Use of a Switchgear
1. The prime function of switchgear is similar to that of a switch. It is mainly used to carry, make, and break the flow of current between the source and the load.
2. The use of switchgear is important in various power electronics applications as it protects the connected devices from high magnitude current, typically known as the surge current. In absence of the switchgear, the device gets damaged. This causes service intrusion as the system containing the damaged device stops working.
3. Switchgear is primarily employed to safeguard and protect transformers, generators, and various other electrical circuits and devices from short circuits.
4. Switchgear is used to isolate the devices connected at the load side from the main power supply.
5. It is used to protect the transmission lines from being damaged, thereby improving the life span of the cables and minimizing the need for their frequent maintenance.
6. It is used to improve the availability of the system. This is because switchgear allows the system to have more than one source to feed the load.
7. The switchgear allows the user to have full control over the flow of current through the circuit under both normal and abnormal conditions by opening or closing the electrical circuits accordingly.
8. One of the main functions of switchgear is to detect the overload conditions and trip off immediately in case of an excessive flow of power to the load.
9. A switchgear helps in switching both capacitive as well as inductive currents.
10. In the case of a short circuit, switchgear interrupts the flow of current and helps avoid property damage and accidents.
Advantages of a Switchgear
The main advantage of switchgear is that it can be operated manually as well as automatically. This means that under normal conditions the user can access and operate the switchgear manually, while during the abnormal condition it can operate automatically. In case of a mishap, the device tends to detect the fault mechanically. The faulty device is then detached from the system, thereby saving the system from severe damage. Switchgear ensures proper energy utilization and complete safety of the user.