AIM: To study to an introductory level, about switch gears and protective schemes.

SWITCH GEAR

A great demand of electricity is a notable feature of modern civilisation. Most of the

energy is required for lighting, heating, domestic appliances, industrial electrical machinery

and electrical traction. This importance of electric supply has constructed such

circumstances that we must secure the power system from large faults and provide

protection to the machineries and devices used and to ensure maximum continuity of the

power supply. For this purpose, machines such as generators and motors are needed to be

switched on and off many times. Means provided to achieve this are called ‘Switch gears’.

“The appratus used for switching, controlling and protecting electrical circuits and

equipments is known as switchgear.”

The term switchgear, used in association with the electric power system, or grid, refers to the combination of electrical disconnects, fuses and/or circuit breakers used to isolate electrical equipment. Switchgear is used both to de-energize equipment to allow work to be done and to clear faults downstream. Switchgear is a non-count noun, much like the software term “code,” and is never used as “switchgears.”

The very earliest central power stations used simple open knife switches, mounted on insulating panels of marble or asbestos. Power levels and voltages rapidly escalated, making open manually-operated switches too dangerous to use for anything other than isolation of a de-energized circuit. Oil-filled equipment allowed arc energy to be contained and safely controlled. By the early 20th century, a switchgear line-up would be a metal-enclosed structure with electrically-operated switching elements, using oil circuit breakers. Today, oil-filled equipment has largely been replaced by air-blast, vacuum, or SF6 equipment, allowing large currents and power levels to be safely controlled by automatic equipment incorporating digital controls, protection, metering and communications.

SWITCH GEARS AT SUBSTATIONS & HOUSINGS

Typically switchgear in substations is located on both the high voltage and the low voltage side of large power transformers. The switchgear located on the low voltage side of the transformers in distribution type substations, now are typically located in what is called a Power Distribution Centre (PDC). Inside this building are typically smaller, medium-voltage (~15kV) circuit breakers feeding the distribution system. Also contained inside these Power Control Centres are various relays, meters, and other communication equipment allowing for intelligent control of the substation.

For industrial applications, a transformer and switchgear (Load Breaking Switch Fuse Unit) line-up may be combined in housing, called a unitized substation or USS.

Switchgear for low voltages may be entirely enclosed within a building. For transmission levels of voltage (high voltages over 66 kV), often switchgear will be mounted outdoors and insulated by air, though this requires a large amount of space. Gas- [or oil- or vacuum-] insulated switchgear used for transmission-level voltages saves space, although it has a higher equipment cost.

At small substations, switches may be manually operated, but at important switching stations on the transmission network all devices have motor operators to allow for remote control.

TYPES & CLASSIFICATION OF SWITCH GEARS

A piece of switchgear may be a simple open air isolator switch or it may be insulated by some other substance. An effective although more costly form of switchgear is gas insulated switchgear (GIS), where the conductors and contacts are insulated by pressurized sulphur hexafluoride gas (SF6). Other common types are oil [or vacuum] insulated switchgear. A simple fuse used at our home, or a toggle switch is a simple low tension switch gear.

Several different classifications of switchgear can be made:

By the current rating.

By interrupting rating (maximum short circuit current that the device can safely interrupt)

o Circuit breakers can open and close on fault currents o Load-break/Load-make switches can switch normal system load currents o Isolators may only be operated while the circuit is dead, or the load current is

very small.

By voltage class: o Low voltage (less than 1,000 volts AC) o Medium voltage (1,000–35,000 volts AC) o High voltage (more than 35,000 volts AC)

By insulating medium: o Air o Gas (SF6 or mixtures) o Oil o Vacuum

By construction type: o Indoor (further classified by IP (Ingress Protection) class or NEMA enclosure

type) o Outdoor o Industrial o Utility o Marine o Draw-out elements (removable without many tools) o Fixed elements (bolted fasteners) o Live-front o Dead-front o Open o Metal-enclosed o Metal-clad o Metal enclose & Metal clad o Arc-resistant

By interrupting device: o Fuses o Air Blast Circuit Breaker o Minimum Oil Circuit Breaker o Oil Circuit Breaker o Vacuum Circuit Breaker o Gas (SF6) Circuit breaker

By operating method: o Manually-operated o Motor-operated o Solenoid/stored energy operated

By type of current: o Alternating current o Direct current

By application: o Transmission system o Distribution.

ESSENTIAL FEATURES OF SWITCH GEARS

Essential features of switch gears are:

(1) Complete Reliability: With continued trend of interconnection and the increasing capacity of generating stations, the need for a reliable switch gear has become of paramount importance. This is not surprising because switch gear is added to the power system to improve the reliability. When fault occurs on any part of the system, the switch gear must operate to isolate the faulty section from the remainder circuit.

(2) Absolutely Certain Discrimination: When fault occurs on any section of the power system, the switch gear must be able to discriminate between the faulty section and healthy section. It should isolate the faulty section from the system without affecting the healthy part. This will ensure the continuity of supply of power.

(3) Quick Operation: When fault occurs on any section of the power system, the switch gear must operate quickly so that no damage is done to generators, transformers and other equipment by the short circuit currents. If fault is not cleared by the switch gear quickly, it is likely to spread into healthy parts, thus endangering complete shutdown of the system.

(4) Provision for Manual control: A switch gear must have provision for manual control. In case the electrical (or electronic) controls fail, the necessary operation can be carried out through manual control.

(5) Provision for Instruments: There must be provisions for instruments which may be required. These maybe in the form of ammeter or voltmeter on the unit itself, or necessary current and voltage transformers for connecting to the main switch board or a separate instrument panel.

[Photograph of a Substation]

DIFFRENT SWITCH GEARS

One of the basic functions of switchgear is protection, which is interruption of short-circuit and overload fault currents while maintaining service to unaffected circuits. Switchgear also provides isolation of circuits from power supplies. Switchgear is also used to enhance system availability by allowing more than one source to feed a load.

Switch gear covers a wide range of equipment concerned with switching and interrupting currents under both normal and abnormal conditions. It includes switches, fuses, circuit breakers, relays and other equipments. A brief account of these devices is given below.

SWITCHES: A switch is a device which is used to open or close an electrical circuit in a convenient way. It can be used under full-load or no-load conditions but it cannot interrupt the fault currents. When the contacts of a switch are opened, an arc is produced in the air between the contacts. This is particularly true for circuits of high voltage and large current capacity. The switches may be classified into (1) air switches (2) oil switches. The contacts of the former are opened in air and that of the latter are opened in oil.

Oil Switch

i. Air-break switch: It is an air switch and is designed to open a circuit under load. In order to quench the arc that occurs on opening such a switch, special arcing horns are provided. Arcing horns are pieces of metals between which arc is formed during opening operation. As the switch opens, these horns are spread farther and farther apart. Consequently, the arc is lengthened, cooled and interrupted. Air-break switches are generally used outdoor for circuits of medium capacity such as lines supplying an industrial load from a main transmission line or feeder.

ii. Isolator or disconnecting switch: It is essentially a knife switch and is designed to open a circuit under no load. Its main purpose is to isolate one portion of the circuit from the other and is not intended to be opened while current is flowing in the line. Such switches are generally used on both sides of circuit breakers in order that repairs and replacement of circuit breakers can be made without any danger. They should never be opened until the circuit breaker in the same circuit has been opened and should always be closed before the circuit breaker is closed.

iii. Oil switches: As the name implies, the contacts of such switches are opened under oil, usually transformer oil. The effect of oil is to cool and quench the arc that tends to form when the circuit is opened. These switches are used for circuits of high voltage and large current carrying capacities.

FUSES: A fuse is a short piece of wire or thin strip which melts when excessive current flows through it for sufficient time. It is inserted in series with the circuit to be protected. Under normal operating conditions, the fuse element it at a temperature below its melting point. Therefore, it carries the normal load current without overheating. However, when a short circuit or overload occurs, the current through the fuse element increases beyond its rated capacity. This raises the temperature and the fuse element melts (or blows out), disconnecting the circuit protected by it.

[Top: Fuse on line, Right: Close-up photo of fuse, Left: fuse in a circuit.]

CIRCUIT BREAKER: A circuit breaker is equipment which can open or close a circuit under all conditions viz. no load, full load and fault conditions. It is so designed that it can be operated manually (or by remote control) under normal conditions and automatically under fault conditions. For the latter operation, a relay circuit is used with a circuit breaker. The circuit breaker essentially consists of moving and fixed contacts enclosed in strong metal tank and immersed in oil, known as transformer oil.

[Photograph of a large oil circuit breaker, under inspection]

[Fig: Fault clearing process] [Photograph of circuit breakers at a sub-station] Under normal operating conditions, the contacts remain closed and the circuit breaker carries the full-load current continuously. In this condition, the e.m.f. in the secondary winding of current transformer (C.T.) is insufficient to operate the trip coil of the breaker but the contacts can be opened (and hence the circuit can be opened) by manual or remote control. When a fault occurs, the resulting overcurrent in the C.T. primary winding increases the secondary e.m.f. This energises the trip coil of the breaker and moving contacts are pulled down, thus opening the contacts and hence the circuit. The arc produced during the opening operation is quenched by the oil. It is interesting to note that relay performs the function of detecting a fault whereas the circuit breaker does the actual circuit interruption.

RELAYS: A relay is a device which detects the fault and supplies information to the breaker for circuit interruption. It can be divided into three parts viz.

(i) The primary winding of a current transformer (C.T.) which is connected in series with the circuit to be protected. The primary winding often consists of the main conductor itself.

(ii) The second circuit is the secondary winding of C.T. connected to the relay operating coil.

(iii) The third circuit is the tripping circuit which consists of a source of supply, trip coil of circuit breaker and the relay stationary contacts. Under normal load conditions, the e.m.f. of the secondary winding of C.T. is small and the current flowing in the relay operating coil is insufficient to close the relay contacts. This keeps the trip coil of the circuit breaker unenergised. Consequently, the contacts of the circuit breaker remain closed and it carries the normal load current. When a fault occurs, a large current flows through the primary of C.T. This increases the secondary e.m.f. and hence the current through the relay operating coil. The relay contacts are closed and the trip coil of the circuit breaker is energised to open the contacts of the circuit breaker.

[Siemens overload relay family]

POWER SYSTEM PROTECTION

The primary purpose of power system protection is to ensure safe operation of power systems, thus to care for the safety of people, personnel and equipment. Furthermore, the task is to minimize the impact of un- avoidable faults in the system. From an electrical point of view, dangerous situations can occur from overcurrents and overvoltages. For example, an asynchronous coupling of networks results in high currents. Earth faults can cause high touch voltages and therefore endanger people. The general problem is always voltage and/or current out of limit. Hence, the aim is to avoid overcurrents and overvoltages to guarantee secure operation of power systems. For the safety of the components it is also necessary to regard device specific concerns, for example oil temperature in transformers, gas pressure in gas insulated components etc. These points are not directly related to electrical values, but, as mentioned, they always come from or lead to un-allowed high voltages or currents. Another issue is mechanical stress. Whenever power is converted electromechanically, one has to consider not only the electrical but also the mechanical equipment. An example is mechanical resonance of steam turbines due to under frequency.

Nowadays, electromechanical protection devices are replaced by microprocessor based relays with a number of integrated features. Currents and voltages are suitably transformed and isolated from the line quantities by instrument transformers and converted into digital form. These values are inputs for several algorithms which then reach tripping decisions. For the design and coordination of protective relays in a network, some overall rules have become widely accepted:

Discrimination: A protection system should disconnect only the faulted part (or the smallest possible part containing the fault) of the system in order to minimize fault consequences. It is the quality of the protective system to distinguish between normal and abnormal conditions and also its location i.e. within protective zone or elsewhere. Reliability & Stability: A protection system has to care for reliable function of relays in order to improve reliability. Reliable functionalities are planed and referred to as backup protection. Moreover, reliability is reached by combining different protection principles, for example distance and differential protection for transmission lines. Stability of the system is the quality of the system due to which system remains inoperative & stable under certain conditions. Requirement adequateness: There is variety of faults and disturbances that exists in the power system. It is impossible to provide protection against each and every abnormal condition due to economical reasons. But in spite of that system must provide adequate protection. Simplicity & Economy: As a rule, “protection cost should not be more than 5% of the total cost.” Protective system should be as simple as possible so that it is easily operative and maintained.

Zones of protection Fig: One-line diagram of a portion of an electric power system illustrating primary relaying

Figure illustrates primary relaying. The first observation is that circuit breakers are located in the connections to each power element. This provision makes it possible to disconnect only a faulty element. Occasionally, a breaker between two adjacent elements may be omitted, in which event both elements must be disconnected for a failure in either one.

The second observation is that, without at this time knowing how it is accomplished, a separate zone of protection is established around each system element. The significance of this is that any failure occurring within a given zone will cause the tripping (i.e. opening) of all circuit breakers within that zone, and only those breakers. It will become evident that, for failures within the region where two adjacent protective zones overlap, more breakers will be tripped than the minimum necessary to disconnect the faulty element. But, if there were no overlap, a failure in a region between zones would not lie in either zone, and therefore no breakers would be tripped. The overlap is the lesser of the two evils. The extent of the overlap is relatively small, and the probability of failure in this region is low; consequently, the tripping of too many breakers will be quite infrequent. Finally, it will be observed that adjacent protective zones of Fig. overlap around a circuit breaker. This is the

preferred practice because, for failures anywhere except in the overlap region, the minimum number of circuit breakers needs to be tripped. When it becomes desirable for economic or space-saving reasons to overlap on one side of a breaker, as is frequently true in metal-clad switchgear the relaying equipment of the zone that overlaps the breaker must be arranged to trip not only the breakers within its zone but also one or more breakers of the adjacent zone, in order to completely disconnect certain faults. This is illustrated in Fig given below, where it can be seen that, for a short circuit at X, the circuit breakers of zone B, including breaker C, will be tripped; but, since the short circuit is outside zone A, the relaying equipment of zone B must also trip certain breakers in zone A if that is necessary to interrupt the flow of short circuit current from zone A to the fault. This is not a disadvantage for a fault at X, but the same breakers in zone A will be tripped unnecessarily for other faults in zone B to the right of breaker C. Whether this unnecessary tripping is objectionable will depend on the particular application.

Fig.: Overlapping adjacent protective zones on one side of a circuit breaker

References: [1] Art and science of protective relaying, C.R. Mason [2] Principles of power systems, V.K. Mehta & Rohit Mehta [3] Gas Insulated switchgear up to 145kV brochure, LS Industrial systems [4] Protection of power system with distributed generation: State of art, Martin Geidl [5] Evaluation and Development of Transmission Line Fault Locating Techniques, E.O. Schweitzer

Websites: [1] www.google.com [2] www.wikipedia.com [3] www.flickr.com [4] www.lsis.biz