From 3 KV to 36 KV switchgear system is categorized as medium voltage switch gear or MV switc gear in the main system. This switchgear has many types which contain metal-enclosed indoor type switch gear, metal-enclosed outdoor type switch gear, outdoor type switch gear without metal enclosure, etc in the switch gear. The interruption medium of this switch gear may be insulating oil, SF6 gas or vacuum of the breaker. The main requirement of the power network is to interrupt current during faulty conditions irrespective of what type of CB is used in the MV switch gear system n the level method. Although it may be capable of functioning in other conditions also.
Medium voltage switchgear should be capable of,
Normal ON/OFF switching operation.
Short circuit current interruption.
Switching of capacitive currents.
Switching of inductive currents.
Some special applications.
The main focus of circuit breaker design is that all circuit breakers should be capable of interrupting short circuit current with a high degree of reliability and safety. The number of faulty tripping occurred during the total lifespan of a circuit breaker mainly depends upon the location of the system, the quality of the system, and environment conditions. If the number of tripping is much high, the best choice is a vacuum circuit breaker as it may not require any maintenance up to 100 faulty trippings with short circuit current up to 25 KA. Whereas, other circuit breakers require maintenance after 15 to 20 faulty tripping with the same short circuit current of CB. The substations reunited in rural areas are generally of an outdoor type, and most of them are the unattended type. Hence for this type of application maintenance-free outdoor type, medium voltage switchgear is most suitable. The porcelain clad vacuum circuit breaker meets this demand against the conventional indoor kiosks.
Switching of Capacitive Current
The capacitor bank is used in the medium voltage power system to improve the power factor of the system. Unloaded cable and unloaded overhead lines has also capacitive charging current. Re-ionization in the contact gap causes overvoltage in the system. The vacuum circuit breaker meets the requirement.
While switching on a capacitor bank, a high rate of rising of making current will flow through the CB contacts. A circuit breaker with liquid quenching medium and tulip contacts may suffer from contact pin retardation. Vacuum medium voltage switchgear is the most perfect choice for this purpose, as a vacuum circuit breaker has low electric arcing during short pre arcing time.
Switching of Inductive Current
Older VCB had a current chopping level of 20 A when these breakers were used to switch transformers, a special surge protection device was required. Modern VCB has a very low chopping current which is about 2 – 4A. Hence modern vacuum medium voltage switchgear is very much suitable for switching an unloaded transformer of the region. As the modern VCB chops the current at a very low level, there is no need for additional surge protection devices. Hence VCB is suitable for very low inductive load switching. But when the inductive current in the system is low but not very low VCB is the best choice in the method.
An electric arc furnace is required to be switched OFF and ON frequently. The current to be switched may be from 0 to 8 times of the rated current of the furnace. An electric arc furnace is to be switched ON and OFF at its normal rated current up to 2000A, around 100 times per day in the switchgear. A normal, SF6 circuit breaker, air circuit breaker and oil circuit breaker are not at all economical for this frequent operation. A standard vacuum circuit breaker is the most suitable alternative for this frequent high current circuit breaker operation.
Another application of medium voltage switchgear is a single-phase railway track system. The main function of the circuit breaker associated with the railway traction system is to interrupt short circuits, on the overhead catenary system which occurs frequently and it is transient. Hence, a circuit breaker used for this purpose should have, short braking time for small contact gap, short arcing time, quick breaking, and VCB is the best possible solution. Arcing energy is much higher in the single-phase CB than 3 phase CB in the circuit breaker. It is still much lower in a vacuum circuit breaker than that in a conventional circuit breaker. The number of short circuits occurs in the overhead catenary system is much higher than those occurring on the electrical transmission system. Medium voltage switchgear with a vacuum circuit breaker is most suitable for traction application. We can conclude that, in the medium-voltage system where the tripping rate is very high, MV Vacuum Switchgear is the most suitable solution which contains the region.
It is very much required to establish an electrical substation at the load center. Since establishing a substation at the load center is quite economical and profitable in many aspects. As it reduces the length of feeders and due to short length feeders, the quality of voltage regulation improves. But the main obstruction of establishing a substation at the load center is space. Generally, the main load center of any place is situated in a very congested place where sufficient land for establishing conventional electrical substations is very hardly available in the system.
This problem can be solved by using gas-insulated switchgear technology in this system. In this type of switchgear, all the necessary components of switchgear can be assembled in very limited space in the bay. GIS is a kind of metal-enclosed switchgear in the vacuum.
SF6 gas is used as insulation between live parts of the equipment and earthed metal enclosure the bay. This type of switchgear means, and gas-insulated switchgear is available from 12 KV systems to 800 KV system of the switchgear. For establishing electrical substation it contains a very limited place so SF6 insulated electrical switchgear plays a major role in the bay.
Gas-insulated components of the substation are generally,
Electrical bus bars.
Electrical isolators or disconnectors.
Surge arrestors or lightning arresters.
The substation, assembled by gas-insulated switchgear, is popularly known as gas-insulated metal-enclosed substation (GIMES) of the system. GAMES technology is not a very recent invention, it is successfully running for over thirty years.
In gas-insulated medium-voltage switchgear, vacuum technology is used as an interrupting purpose and SF6 gas is used as an insulation material of the region. Although for both interruption and insulation, SF6 gas is used in a much medium voltage GIS system. But for such equipment rated SF6 gas pressures are different for interruption and insulation of the region.
As vacuum technology is not available for high voltage, contain the gas-insulated switchgear system above 72.5 KV, and only it has the SF6 which is used for the interruption medium and insulation in the switchgear.
There are different types of gas-insulated metal-enclosed switchgear available depending upon their constructional feature.
Isolated Phase GIS
In this configuration, each phase of the bay is assembled separately in the system. That is, for each phase, one pole of a circuit breaker, a single-pole of the electrical isolator, one phase assembly of the current transformer is assembled together. This type of GIS requires a larger bay width as compared to other gas-insulated switchgear systems.
In this configuration, all three phases of a circuit breaker, 3 phases of disconnected, and three-phase current transformer are encapsulated in an individual metal enclosure. The arrangement forms a three-phase module for the element in the system. The size of this type of module is one-third of the isolated phase GIS contains the method.
Hybrid GIS System
It is a suitable combination of an isolated phase and three-phase common elements. Here three-phase common bus bar system simplifies the connection from the bus bar contains the range. The isolated phase equipment prevents phase to phase faults of the method. This is an optimum design which has space requirement and maintenance facility of the member.
In this GIS or gas-insulated switchgear system than one functional element is encapsulated in a single metal enclosure. it contains the three-phase circuit breaker, and current transformer, and earth switches, and even other feeder elements are covered together in a single metal capsule of the method.
Highly Integrated System
This design was introduced in the year 2000, where, total substation equipment is encapsulated together in single enclosure housing. This single unit gas insulated substation has gained user appreciation which has a complete solution for an outdoor substation, in a single unit of the range. As such, only equipment (HIS) is a substitute for a total outdoor switchyard.
Highly Integrated System
This design was introduced in the year 2000, where, total substation equipment is encapsulated together in single enclosure housing. This single unit gas insulated substation has gained user appreciation which has a complete solution for an outdoor substation, in a single uniting this part. As such, only equipment (HIS) is a substitute for a total outdoor switchyard.
Design and Construction of Metal Enclosed Switchgear
This type of switchgear is designed and constructed in such a way, that, normal operation, maintenance, an inspection can be carried out very easily and safely. All the components can easily be replaced. Another essential feature of metal-enclosed switchgear is that it should have a proper interlocking arrangement between different components to ensure the safest operation.
To ensure safe and proper sequence of operation, the following provisions are to be provided,
The circuit breaker cannot be removed or drawn out from its compartment unless it is in the OFF condition.
Not only that, but it should also be impossible to isolate the circuit breaker from the bus bar unless it is in OFF condition.
It is impossible to switch ON the circuit breaker in its service position unless the auxiliary circuit is connected with it.
The indoor switchgear housing has three chambers, and namely the circuit breaker chamber (Front), and bus bar chamber (Top), and finally it has current transformer cum cable connection chamber (Rear) of the bay. The metering and really panel consist of a separate low voltage chamber that may or may not be fixed with the main gear housing.
The withdrawable partition comprises a wheel mounted truck with an operating mechanism, interrupters, and isolated units. The spring charging mechanism may be either hand-operated or motor operated by the system. For motor operated spring charging mechanism, a manual charging provision should be provided to charge the spring during failure of spring charging motor or absence of auxiliary supply.
The withdrawal portion with CB generally has three positions i.e. SERVICE, TEST, and ISOLATED position. The positions should be clearly visible from the front of the switchgear.
Three bus bars for three-phase run parallel inside the bus bar chamber in the region. The bus bars are made of either aluminum or copper and cross-section of the bars depends upon the current rating of the system of the method. The bus is supported on epoxy support insulators in this region.
The current transformers and cable termination chamber of earthed metal constriction are designed to be able to mount the CT and have a provision for cable termination.
Gas Insulated Medium Voltage Switchgear
This is a fixed type metal-enclosed design that does not have any withdrawal portion. This design of metal-enclosed switchgear, and mainly has two main compartments and namely CB compartment and bus bar compartment of the system. Here bus bar compartment is provided with the three-position switch marked with SERVICE, ISOLATED, and EARTH. The CB compartment consists of three interrupters. The interrupters are generally of vacuum type but some manufacturers also design gas isolated medium voltage switchgear with SF6 interrupters of the system. In this configuration, all three phases of a circuit breaker, 3 phases of disconnected, and three-phase current transformer are encapsulated in an individual metal enclosure. The arrangement forms a three-phase module for the element in the system. The size of this type of module is one-third of the isolated phase GIS contains the method.
Metal Enclosed Outdoor Type Medium Voltage Switchgear
This is basically similar to metal-enclosed indoor type medium voltage switchgear, except its external housing.