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Advanced Diploma of Electrical Engineering

Module 4: Circuit Breakers and Switchgears

Module 4.3

E-mail: [email protected] Web Site: www.idc-online.com

AUSTRALIA CANADA IRELAND NEW ZEALAND SINGAPORE SOUTH AFRICA UNITED KINGDOM UNITED STATES


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7

Medium Voltage Switchgear

In this chapter, we will learn about medium voltage switchgear of indoor and outdoor types, theirmajor components and safety features

Learning objectives

Indoor and outdoor switchgear

Comparison

Metal enclosed switchgear basics

Major components

Safety features

Protection

Switchgear ratings Typical switchgear example

7.1 Switchgear options

7.1.1 Definition of Switchgear

Assembly which houses the equipment used for isolation, switching or control of electrical circuits

7.1.2 Meaning of medium voltage in this text

All voltages above 1000V AC are classified as High Voltage (HV). Medium voltage (MV) is a sub-class of HV and is generally used for representing Voltages above 1000V and up to and including

36000V.

7.1.3 Varieties of MV switchgear

Medium voltage switchgear is available in the following types of executions based on the location

where the switchgear is installed:

Indoor type

Outdoor type

Whichever method of installation is adopted, the design and construction of the switchgear must

take into account the following: Safety of personnel (those concerned with operation and maintenance and where the

switchgear is placed in areas open to public, any person who can gain access to the

equipment)


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104 Switchgear and Distribution Systems104

Ease of operation

Adequate clearances for maintenance

Access to internal components of the switchgear

Protection against external influences

7.2 Outdoor MV switchgearOutdoor switchgear is mostly of open type construction, with the switchgear and associated

equipment mounted on specially designed structures. The clearances of live parts to ground must

be out of arms-reachof persons standing close to the equipment and adequate to prevent arcing

or electrocution. (This translates to the minimum phase-to-ground clearance for the voltage

concerned + 2.5 meters in the vertical plane). Most utility distribution equipment placed along

public roads is designed using this type of construction. This construction is also adopted in

switchyards where the entire distribution equipment such as busbars, isolators, circuit breakers,

power transformers, current/voltage transformers etc. are placed on structures and interconnected

using bare conductors and connector hardware. A typical MV circuit breaker of outdoor

construction is shown in Figure 7.1.

Figure 7.1An outdoor circuit breaker with its mounting structure

The circuit breaker interrupter is placed within a housing made of porcelain and the operating

mechanism and auxiliary components are within the control unit in a sheet steel enclosure just

below the circuit breaker. This enclosure must be of weatherproof construction and protectedagainst ingress of rain water and dust. The advantages of this construction are:

Easier to locate and repair faults

Lower capital cost

However, there are several drawbacks in this design, these being:

Equipment are exposed to weather

More prone to ingress of water and dust and associated problems

Exposure to lightning strike (can be prevented by proper shielding)

Higher/more frequent need for maintenance

Unauthorized access and danger of electrocution

Must be designed to withstand seismic activity where applicable (because of elevatedmounting)


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Medium Voltage Switchgear105

An alternative to the above method is to place the entire equipment in a weatherproof enclosure and

with some form of access protection such as fencing. In addition, the internal parts of the

switchgear can be accessed only by the use of a special purpose key or handle thus preventing the

chances of unsafe condition even if some unauthorized person were to enter the fenced area. A

typical switchgear cubicle designed for outdoor use is shown in Figure 7.2.

Figure 7.2An outdoor vacuum circuit breaker within a metal enclosure

Note that even though the equipment is within an enclosure, the terminals are accessible so as to

facilitate connection of overhead connections to other outdoor equipment.

This type of construction avoids the drawbacks of the open execution discussed earlier. The

advantages are: Well protected against weather action and dust

Better safety against accidental contact

Not exposed to direct lightning strike because of grounded metal enclosure

Easier access for maintenance

There are however certain drawbacks in this construction too such as:

Higher capital cost

Will require additional protection against unauthorized access (fencing)

Higher degree of protection against dust and water ingress

The advantages however far outweigh the drawbacks and situations where a greater degree of

continuity of power is desired, this option offers a good choice.

7.3 Indoor MV switchgear

The practice of using outdoor switchgear and in particular open type design is generally limited to

utility distribution circuits. In certain specific industrial applications such as EHV switchyards,

open type HV switchgear design is adopted. Also, in large yards such as raw material open storage

facilities where most equipment is of outdoor type MV distribution equipment of outdoor metal

enclosed design is preferred. Otherwise, indoor switchgear is the norm in most cases, since

industrial distribution schemes require absolute continuity of power supply.

Indoor switchgear come in two types: the metal-enclosed and metal clad varieties. Metal clad type

is a subset of metal enclosed type. In both types, external earthed metal enclosure is a must. Inmetal clad type, there must be segregation of specified internal parts by earthed metal barriers (for

example, control compartments, busbar chamber, and cable chambers must have earthed metal


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barriers between them). In the case of metal enclosed design, the barriers may be of insulating

material. For example, IEC standard provides for metal enclosed switchgear to have non-metallic

barriers and such switchgear is referred to as metal enclosed, compartmentalized switchgear.

The important components of metal clad/metal enclosed switchgear are:

Outer enclosure and internal barriers

Power circuit components

Bus bars

Switching element (CB or Isolator) or MV fuses

Cable chamber

CT and PT

Auxiliary devices, power supply and wiring

Earthing arrangement

Figure 7.3 shows an example of metal enclosed type switchgear. The different chambers have

barriers of insulating sheet, thus fulfilling the need for compartmentalization. The entire panel is

provided with sheet steel enclosure which will be connected to the plant earthing system for

protection from environmental influence, access prevention and safety against accidental contact.

Figure 7.3An indoor metal enclosed switchgear panel

Figure 7.4 shows a vacuum circuit breaker panel of metal clad type. The arrangement is of 2-tier

construction. Barriers between circuit breaker and other chambers are made of metal. A step-down

control supply transformer and a potential transformer form a part of the switchgear. Busbars run in

separate chambers and are supported with insulating barriers between each panel.


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Figure 7.4An indoor metal clad vacuum circuit breaker panel

Figures 7.5 and 7.6 show the internals of the panel illustrated earlier in Figure 7.4. The 2-tier

arrangement of circuit breakers is clearly visible in Figure 7.5. The breakers are of horizontal draw

out type and can slide out of the panel on horizontal retractable support rails. The busbar chamber

runs in the central portion of the cubicle and the circuit breakers slide into the module and connect

to the busbars. The lower sliding contact of the top tier and the upper sliding contact of the bottom

tier provide connection to the bus bars. The outgoing cables get connected through the upper

sliding contact of the top tier and the lower sliding contact of the bottom tier. These sliding contacts

are insulated by bushings fixed to the metal barrier between the breaker chamber and the bus/cable

chambers. Current transformers can be seen in Figure 7.6 from the front with the breaker removed.

Figure 7.5An indoor metal clad vacuum circuit breaker panel shown with side covers open

A self-retracting metal shutter provides safety against accidental contact and can be locked in the

closed position (refer to Figure 7.6) when work has to be carried out in the panel. The breaker can


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be racked-in and out of the panel without opening the panel door by a mechanism in the front of the

panel. This mechanism is linked to the shutter opening mechanism and thus provides safety while

opening the panel for removal of the circuit breaker. An earth bus is provided in the panel and earth

connection to the circuit breaker assembly is obtained by means of a scraping earth contact. The

earth bus of all the panels are interconnected and earthed to the facilitys protective earth conductor

system. In many designs, an earth switch is also provided and enables the outgoing cable terminals

to be connected to the ground thus ensuring safety.

Figure 7.6Internal view of vacuum circuit breaker module (with breaker drawn out)

Control schemes on the breaker trolley connect to the panel control circuit by sliding control

contacts. In most panels, there are three distinct positions of the circuit breaker assembly:

SERVICE position where power sliding contacts and control sliding contacts makecontact (between panel and breaker trolley)

Breaker DRAWN OUT position where both sets of contacts do NOT make contact.

TEST position where only control sliding contacts make contact.

Thus, in the TEST position, it is possible to operate the breaker without the power circuit getting

energized.

Several safety interlocks (mechanical in most cases) are also provided to ensure that O&M

personnel are not exposed to unsafe conditions. A few of these are:

Panel door cannot be opened when the breaker is in SERVICE position

The breaker cannot be racked in/out when it is on

If racking out a breaker which is on, it will trip before the power sliding contactsseparate.

If the earth switch is on, then breaker cannot be racked in.

There may be other specific interlocks depending on the constructional features of the panel.


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7.4 MV switchgear panel configurations

Switchgear panels may be of different configurations. Sectionalized panels have busbars arranged

in more than one section and connected through a bus coupler circuit-breaker. Double busbar

configurations are also possible with a breaker circuit capable of being connected to either busbar

through a set of isolators. Ring main units (RMU) are often adopted where ring/mesh type

distribution is used. A typical ring main circuit has two isolators which connect the RMU to the

ring and a circuit breaker for feeding the load. In some cases, the circuit breaker may be substitutedwith a fused isolator with the fuse providing short circuit protection. Some of the RMUs may have

more than one outgoing feeder and some may connect to radial take-off connections and to

diagonal mesh arms and thus incorporate more than two isolators. A typical RMU is shown in

Figure 7.7. Facilities for padlocking are also provided for the purpose of safety during operation.

Figure 7.7

A typical RMU

Some of the RMU designs use SF6 gas as the insulation medium. Normally insulation between live

components and to earth is by air (air-insulated switchgear). Use of SF6 as insulation medium

provides many advantages. These being: Compactness (lower clearances due to better dielectric strength of SF6)

Switchgear sealed hermetically and not affected by external environment

Reliable performance with no possibility of faults due to external objects, vermin etc.

Such a unit incorporating vacuum interrupters is shown in Figure 7.8.


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Figure 7.8SF6 insulated RMU with door open

7.5 MV switchgear auxiliary devices

Commonly used auxiliary devices in MV switchgear include the following:

Auxiliary AC and DC power supply and wiring

Control terminal blocks

Control and protection elements

Measuring elements (CT and PT) secondary circuits

Utility elements (panel lightning, anti-condensation heater and utility socket andpanel ventilation where applicable)

Protection elements

Auxiliary DC supply is for the purpose of control of the switchgear. Protection circuits require DC

battery source or some form of stored energy supply. Trip coils are usually of dc so that failsafe

tripping is possible even though the voltage in the associated power circuit may drop to a very low

value during a short circuit fault. Auxiliary AC supply is normally required for spring charging

motors of circuit breakers and for supply to the panel utilities such as socket outlet, panel lamp,

space heater, etc.

Internal wiring of the panel is done through control wiring ducts and in some of the designs, a

control bus arrangement is provided at the top of the panel to provide through connection. Such

buses are provided for auxiliary dc and ac power supply, PT secondary wiring and other special

purpose connections such as common trip signals. Clearly segregated control terminal blocks are

provided in each panel for intra-panel and external wire connections. The segregation is on the

basis of the control functions for which the terminals are used. The terminals for different purposes

may be sized suitably based on the circuit current likely to be handled. CT wiring terminals are

usually provided with special shorting-type terminals which can be used to short CT terminals

before devices such as meters and relays are disconnected from the CT secondary circuit.

Any switchgear deploys several control elements for its operation. Some of them are:o Command switch/pushbuttons for operation of switchgear

o Other operation selectors (Remote/Local etc.)o Auxiliary contacts of switching deviceo Protection relays/releases


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o Status indicating lampso Annunciation devices (visual, audio)

In some of the installations one or more of these devices can be placed external to the switchgear in

a separate control panel.

Measuring elements in the form of current transformers and potential transformers are provided as

a part of the switchgear panels. Other associated devices include: All secondary wiring from PT/CT

Protection of PT output (breaker/fuse)

PT fuse failure detecting device

Indicating instruments, recorders, transducers, switches for selective measurement (ofa given phase).

Switchgear and the outgoing feeders fed by it require various protection devices for safe

disconnection in the event of short circuits, earth faults and other abnormal operating conditions.

These include:

Relays for sensing faults in individual circuits

Protection for other internal (busbar) faults Inter-trip (sending and receiving ends)

Protection of equipment fed by a circuit (motor or transformer)

We will deal with protection in detail in a later chapter.

7.6 MV switchgear ratings

Manufactured Medium Voltage (MV) switchgear panels are rated according to the following main

specifications:

7.6.1 Nominal voltageThis designed average voltage of a system, e.g. 11kV.The actual voltage will typically fluctuate

between 10.5 and 11.5kV (95 to 105%).

7.6.2 Rated voltage

This is the voltage level at which the equipment will be expected to perform at continuously under

normal operating conditions, e.g. 12 kV for a system with a nominal voltage of 11kV.

The rated voltage will determine the insulation properties of the panel. Each main component,

including individual circuit-breakers, busbars, cable terminations, etc, must be rated to this voltage,

or higher. (Main components refer to those components that form part of the main voltage circuit

of the panel, as opposed to the control circuit.) For example, if a 7.2 kV circuit-breaker is installedin a 12 kV panel, it will mean that the whole panel is rated only 7.2 kV.

The normal practice is to rate switchgear panels 10 % higher than the required nominal voltage,

e.g. 12 kV for a 11 kV system, 36 kV for a 33 kV system, etc.

7.6.3 Power frequency withstand voltage

This is the 50 or 60 Hz voltage that the switchgear can with stand for 1 minute. For 12kV, related

equipment the applied test voltage is 28kV. Switchgear manufacturers use this test to prove the

insulation of their equipment after manufacture in routine testing.

Commissioning engineers also use this test to prove the integrity of the equipment before switchingon the power. This test is also known as a pressure test.


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7.6.4 Impulse voltage

This is the highest peak voltage the equipment will be able to withstand for a very short period of

time, as in the case of a voltage peak associated with lightning, switching or other transients, e.g.

95 kV for 12Kv equipment.

BIL standards are set by IEC 60.

The same principle as above applies, i.e. the impulse voltage rating of the panel is equal to the

rating of the lowest rated main component.

The impulse level that a panel will experience may be controlled by installing surge suppressers,

which will limit the peak voltage to a certain level. This is illustrated in Figure 7.9.

In limiting the peak voltage, e.g. 45 kV, the surge arrestor will conduct a large current, say 10 kA

to earth.

Figure 7.9Impulse voltage

7.6.5 Surge arrestor

Surge arrestors are installed at the transition point between e.g., an overhead line and a transformer,

and as close to the transformer terminals as possible.

Surge arrestors are also installed as motor terminals and on the ends of overhead lines. They are

unusually connected between phases and earth.

Three types of surge arrestors are used:

Rod spark gapped

Multiple gapped arrestors

Zinc (metal) oxide surge arrestors

In applying surge arrestors the voltage rating, lighting density and the surge rating should beconsidered in determining the insulation coordination of the power system.

7.6.6 Full load current

This is the maximum load current that may pass continuously through the switchgear panel.

Contrary to the voltage rating, not every main component needs to have the same current rating.

Every individual circuit breaker or switch will be rated according to the maximum load current that

will pass through it.

The incoming circuit-breaker/disconnector and the main busbars will usually be rated the same.

This value is obtained by adding all the individual feeder ratings, multiplied by a load or diversity

factor. This factor is smaller than one, and is determined by the types of loads connected to thepanel. Individual loads will not run at full capacity simultaneously, hence the load factor. This

calculation can be illustrated as shown in Table 7.1.


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Table 7.1Evaluation of switchgear current rating

Feeder No. Current Rating (A)

1 200

2 100

3 3504 600

5 150

6 250

Total 1650

Load factor 60%

Required Incomer/Busbars 990A

A good estimate of the load factor can be done by looking at the historical values recorded for the

relevant feeders, if available.

Due to economy of scale, MV circuit-breakers/disconnectors are manufactured in a few standardsizes, for example 630 A, 1250 A, 1600 A, 2000 A and 2500 A according to IEC standards.

7.6.7 Fault current

The magnitude of fault current that a switchgear panel must withstand is not determined by the load

connected to it, but by the properties of the supply to it. Usually, a MV panel will be supplied via a

HV/MV transformer(s). This transformer will then determine the magnitude of the fault currents

that may flow through the panel.

Electrical faults usually occur due to breakdown of the insulating media between live conductors or

between a live conductor and earth. This breakdown may be caused by any one or more of several

factors, e.g. mechanical damage, overheating, voltage surges (caused by lightning or switching),ingress of a conducting medium, ionisation of air, deterioration of the insulating media due to an

unfriendly environment or old age, or misuse of equipment.

Faults are classified into two major groups: symmetrical and unbalanced (asymmetrical).

Symmetrical faults involve all three phases and cause severe fault currents and system

disturbances. Unbalanced faults include phase-to-phase, phase-to-ground, and phase-to-phase-to-

ground faults. They are not as severe as symmetrical faults because not all three phases are

involved. The least severe fault condition is a single phase-to-ground fault with the transformer

neutral earthed through a resistor or reactor. However, if not cleared quickly, unbalanced faults will

usually develop into symmetrical faults. These different types of faults are illustrated in Figure

7.10.

In distribution systems with / (HV/mV) connection and the star point solidly earthed the singlephase fault current may be higher than the 3-phase symmetrical fault current due to the zero

sequence reactance of the transformer being lower than the positive sequence reactance.


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Figure 7.10Electrical fault types

Switchgear needs to be rated to withstand and break the worst possible fault current, which is a

solid three-phase short-circuit close to the switchgear. Solid means that there is no arc resistance.

Normally arc resistance will be present, but this value is unpredictable, as it will depend on where

exactly the fault occurs, the actual arcing distance, the properties of the insulating medium at that

exact instance (which will be changing all the time due to the heating effect of the arc), etc.

Arc resistance will decrease the fault current flowing.

Exact calculations of prospective fault currents can be quite complex, and are usually performed

with the aid of computer simulation software (see Chapter 5). However, when a few allowable

assumptions are made, approximate fault currents can be determined quite easily and quickly with

pen and paper (plus calculator, preferably!). These approximate values will be conservative, giving

the worst case, and can therefore be confidently used for the ratings of switchgear panels.

These assumptions are the following:

Assume the fault occurs very close to the switchgear. This means that the cableimpedance between the switchgear and the fault may be ignored.

Ignore any arc resistance

Ignore the cable impedance between the transformer secondary and the switchgear, ifthe transformer is located in the vicinity of the substation. If not, the cable impedance

may reduce the possible fault current quite substantially, and should be included for

economic considerations (a lower rated switchgear panel, at lower cost, may be

installed)

When adding cable impedance, assume the phase angle between the cable impedanceand transformer reactance are zero, hence the values may be added without complex

algebra, and values readily available form cable manufacturers tables may be used

Ignore complex algebra when calculating and using transformer internal impedance

7.6.8 DC offset

The phenomenon called DC Offset should be taken into account when rating circuit breakers.This is illustrated in Figure 7.11. This phenomenon occurs due to the presence of inductive

reactance in the system.


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Figure 7.11Illustration of DC offset

The peak current value will depend on the power factor of the system at the time of the fault. This

maximum practical value of this peak current is 2.55 times the RMS fault current, as determined by

the system impedance. This relationship is illustrated in Figure 7.12.

Figure 7.12Asymmetry factor chart

Although the peak currents associated with the DC Offset is of relative short duration, they must be

taken into consideration when rating switchgear panels, as additional mechanical stresses arecaused by the electromagnetic forces associated with these high peak currents.


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The rate of decay of the DC Offset, as illustrated in Figure 7.12, is depended on the relationship

L/R of the system. The higher the inductance, the slower the rate of decay. Normally, the DC

Offset will decay to zero within the first 3 to 4 cycles. Therefore, at the instant that the circuit

breaker opens, these peaks have all but disappeared, and it will not be a cause for concern.

However, when a circuit breaker is installed close to high inductive sources, like generators and

large induction motors, the current waveform may still be substantially asymmetrical at the instant

of the circuit breaker opening. This may cause the breaker to interrupt a higher current value than itwas rated for, and the DC Offset may cause an extended arcing period within the breaker. The

resultant thermal energy may be higher than the breaker can withstand, with the result that the

breaker may blow up.

Therefore, for these applications, a specialized generator circuit breaker should be installed, which

is designed to withstand these factors.

The switching device shall be capable of performing at least the following operating functions:

Make and break full-load current

Carry the prospective fault current

Make prospective fault currentIf fitted with protection devices, also break prospective fault current

A manually operated switch need not be capable of breaking the prospective fault current.

A switch which will not make prospective fault current may be approved where satisfactory

interlocking is provided.

Standard fault current ratings for switchgear include:

16 kA

20 kA

25 kA 31.5 kA

40 kA

50 kA

60 kA

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