MV Dstribution Substation

8/8/2019 MV Dstribution Substation

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CHAPTER 03

Medium Voltage System Maintenance

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NATIONAL DIPLOMA IN ENGINEERING SCIENCES

INTRODUCTION

The physical structure of most power systems consists of

generation facilities feeding bulk power into a high-voltage bulk

transmission network, that in turn serves any number of

distribution substations. A typical distribution substation will

serve from one to as many as ten feeder circuits. A typical

feeder circuit may serve numerous loads of all types. A light to

medium industrial customer may take service from the distribution

feeder circuit primary, while a large industrial load complex may

take service directly from the bulk transmission system. All

other customers, including residential and commercial, are

typically served from the secondary of distribution transformers

that are in turn connected to a distribution feeder circuit.

Figure 6.1 illustrates a representative portion of a typicalconfiguration.

The Electric Power System is usually divided into three

segments, which are generation, transmission, and distribution.

In a broad definition, the distribution system is that part of

the electric utility system between the bulk power source and the

customers service switches.


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A distribution system includes the following components

Sub-transmission system;

Distribution substations;

Distribution primary feeders;

Distribution transformers;

Secondary circuits;

Some distribution system engineers define the distribution

system as that part of the electric utility system between the

distribution substations and the consumers service entrance.

3.1 PRIMARY SUBUSTATIONS

Following diagram shows a Typical distribution substation

arrangement


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Nature of a typical substation

The voltage of the high side bus can be anywhere from 34.5

kV all the way to 345 kV and beyond.

The average or preferred high side voltage level is

approximately 115 to 138 kV.

The average substation consists of twotransformers with an

impedance of approximately 10 percent (0.1 p.u.).

The low voltage bus in a multiple transformer substation isusually split (contains a normally open breaker or switch)

to alleviate circulating currents as well as reduce the

short circuit current seen by the system.

Normally, Two or more feeders are connected to each bus

through a feeder breaker.

On smaller substations where short circuit levels are lower,

a re-closeris sometimes used instead of abreaker.

Short circuit levels at the terminals of the low voltage bus

are generally kept at 12 000 amperes or less although there

are many systems where much higher levels can be found.

3.1.1 Main Equipmentsinatypical Substation

Power transformers

Circuit breakers

Current transformer

Capacitor voltage transformer (CVT)

EarthingTransformers

Protective Relays


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BREATHER

Breather is mounted on the top of the conservator tank

& it is a small cylindrical unit containing silica gel to

absorb moisture of air entering the conservator tank.

RADIAT

ORThese are fitted for cooling the transformer oil. The

hot oil circulate through these unit where it becomes cool

due to the air touching.

WINDINGS

Transformer windings are wound with rectangular, cable

paper insulated copper wires. There are two windings HV side

and LV side.

ON LOAD TAPCHANGER

Tap changer is used for on load changing of the voltageratio. It is a three-phase unit located in one container,

which is placed in the transformer tank. Each phase

regulating winding is located at the star point of HV side

of transformer. Tap changer is controlled from a motor drive

unit, fixed to the transformer tank. Transformer voltage may

have to be constantly regulated and it is often very

inconvenient to cut off the power supply each time. On load

tap changer solve such problems, and are being used

increasingly as a means of offering better power supply

service as well as for general power receiving purposes.

BUCHHOLZ RELAYThis relay is situated in the pipe connected between

the transformer and the conservator. Relay is a gas actuated

relay which is meant for the protection of oil immersed

transformer from insulation failure, core heating or any

type of internal fault which may cause the heating of coil

beyond the specified temperature due to this faults either

alarm circuit or the trip circuit operate.

PRESSURE RELIEF VALVE

If is the transformer oil vapor pressure increase in

transformer tank a signal is came to relief valve and it is

operated.

TEMPERATUREMETERS

Those meters measure oil temperature and winding

temperature and meters are set to fixed temperature.

ARCHING HORNS

Arching horns are situated top of the transformer it is

protected transformer by lightings.


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Electrical Characteristic parameters of a transformer

Ratedpower (Pn): the conventional apparent-power in kVA on

which other design parameter values and the construction of the

transformer are based. Manufacturing tests and guarantees are

referred to this rating

Frequency: for power distribution systems of the kinddiscussed in this guide, the frequency will be 50 Hz or 60 Hz

Ratedprimaryandsecondary voltages: For a primary winding

capable of operating at more than one voltage level, a kVA rating

corresponding to each level must be given. The secondary rated

voltage is its open circuit value

Rated insulation levels are given by overvoltage-withstand

test values at power frequency, and by high voltage impulse tests

values which simulate lightning discharges. At the voltage levels

discussed in this guide, over voltages caused by MV switching

operations are generally less severe than those due to lightning,

so that no separate tests for switching -surge withstand

capability are madeOff-circuittap-selectorswitch generally allows a choice of

up to 2.5% and 5% level about the rated voltage of the

highest voltage winding. The transformer must be de -energized

before this switch is operated

Windingconfigurations are indicated in diagrammatic form by

standard symbols for star, delta and inter-connected-star

windings; (and combinations of these for special duty, e.g. six-

or twelve-phase rectifier transformers, etc.) and in an IEC-

recommended alphanumeric code. This code is read from left -to-

right, the first letter refers to the highest voltage winding,

the second letter to the next highest, and so on:

Capital letters refer to the highest voltage windingD = delta

Y = star

Z = interconnected-star (or zigzag)

N = neutral connection brought out to a terminal

Lower-case letters are used for tertiary and secondary windingsd= delta

y= star

z = interconnected-star (or zigzag)

n= neutral connection brought out to a terminal

A number from 0 to 11, corresponding to those, on a clock dial

(0 is used instead of 12) follows any pair of letters to

indicate the phase change (if any) which occurs during thetransformation.

A very common winding configuration used for distribution

transformers is that of a Dyn 11 transformer, which has a delta

MV winding with a star-connected secondary winding the neutral

point of which is brought out to a terminal. The phase change

through the transformer is +30 degrees, i.e. phase 1 secondary

voltage is at 11 oclock when phase 1 of the primary voltage is

at 12 oclock.


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Circuit breakers

A circuit breaker is defined as a mechanical switching

device capable of making, carrying and breaking currents under

normal circuit conditions and also making, carrying and breaking

for a specified time, and breaking currents under specified

abnormal circuit conditions such as a short circuit Circuit

breakers are generally classified according to the interrupting

medium used to cool and elongate the electrical arc permitting

interruption. The types are:

Oil circuit breaker

Air blast circuit breaker

Vacuum circuit breaker

SF6 gas circuit breaker

Oil circuit breakers have been widely used in the utility

industry in the past but have been replaced by other breaker

technologies for newer installations. Two designs exist bulk

oil (dead tank designs) and oil minimum breaker technology (live

tank design). Bulk oil circuit breakers were designed as single-

tank or three-tank mechanisms; generally, at higher voltages,

three-tank designs were dominant. Oil circuit breakers were large

and required significant foundations to support the weight and

impact loads occurring during operation. Environmental concerns

forcing the necessity of oil retention systems, maintenance

costs, and the development of the SF6 gas circuit breaker have

led to the gradual replacement of the oil circuit breaker for new

installations.

Oil circuit breaker development has been relatively static for

many years. The design of the interrupter employs the arc causedwhen the contacts are parted and the breaker starts to operate.

The electrical arc generates hydrogen gas due to the

decomposition of the insulating mineral oil. The interrupter is

designed to use the gas as a cooling mechanism to cool the arc

and to use the pressure to elongate the arc through a grid (arc

chutes), allowing extinguishing of the arc when the current

passes through zero.


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Vacuum circuit breakers use an interrupter that is a small

cylinder enclosing the moving contacts under a high vacuum. When

the contacts part, an arc is formed from contact erosion. The arc

products are immediately forced to and deposited on a metallic

shield surrounding the contacts. Without anything to sustain thearc, it is quickly extinguished.

Gas (SF6) circuit breakers generally employ SF6 (sulfur

hexaflouride) as an interrupting and sometimes as an insulating

medium. In single puffer mechanisms, the interrupter is

designed to compress the gas during the opening stroke and usethe compressed gas as a transfer mechanism to cool the arc and to

elongate the arc through a grid (arc chutes), allowing

extinguishing of the arc when the current passes t hrough zero. In

other designs, the arc heats the SF6 gas and the resulting

pressure is used for elongating and interrupting the arc. Some

older two-pressure SF6 breakers employed a pump to provide the

high-pressure SF6 gas for arc interruption.


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SF6 Gas circuit breakers

typically operate at pressures

between six and seven

atmospheres. The dielectric

strength of SF6 gas reducessignificantly at lower

pressures, normally as a result

of lower ambient temperatures.

Monitoring of the density of

the SF6 gas is critical and

some designs will block

operation of the circuit

breaker in the event of low gas

density.

Currenttransformer

A current transformer is a device for

measuring a current flowing through a power system

and inputting the measured current to a protective

relay system. Electrical power distribution

systems may require the use of a variety of

circuit condition monitoring devices to facilitate

the detection and location of system malfunctions.

Current transformers and current sensors are well

known in the field of electronic circuit breakers,providing the general function of powering the

electronics within the circuit breaker trip unit

and sensing the circuit current within the

protected circuit. Ground fault circuit breakers

for alternating current distribution circuits are

commonly used to protect people against dangerous shocks due to

line-to-ground current flow through someone's body. Ground fault

circuit breakers must be able to detect current flow between line

conductors and ground at current levels. Upon detection of such a

ground fault current, the contacts of the circuit breaker are

opened to de energize the circuit. Current transformers are an

integral part of ground fault circuit breakers. Current

transformer assemblies are often positioned between the line side

of a trip unit of a circuit breaker and the load side in order to

monitor the current there between. Current transformers in

electrical substations measure the system currents at

predetermined measuring points of the switchgear with a certain

measurement inaccuracy. The measuring points are typically

located at all incoming and outgoing lines and possibly also


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within the system, e.g. for the busbar protection. The current

measurement signals are used for protective functions, for

monitoring the substation, for calculating performance data for

operating purposes or for consumption billing and for the

representation on a display. The output of the current

transformer provides a representation of the current flowingthrough the assembly that is being monitored. Associated

monitoring and control instrumentation in combination with the

current transformer may provide critical system functions such as

overload protection and power usage monitoring.

Capacitor voltagetransformer (CVT)-

A capacitor voltage transformer (CVT), or

capacitance coupled voltage transformer (CCVT)

is a transformer used in power systems to step

down extra high voltage signals and provide a

low voltage signal, for measurement or to

operate a protective relay. In its most basic

form the device consists of three parts: two

capacitors across which the transmission line

signal is split, an inductive element to tune

the device to the line frequency, and a

transformer to isolate and further step down the

voltage for the instrumentation or protective

relay. The device has at least four terminals: a

terminal for connection to the high voltage signal, a groundterminal, and two secondary terminals which connect to the

instrumentation or protective relay.

CVTs are typically single-phase

devices used for measuring

voltages in excess of one hundred

kilovolts where the use of

voltage transformers would be

uneconomical. In practice,

capacitor C1is often constructed

as a stack of smaller capacitors

connected in series. This provides a large voltage drop across C1

and a relatively small voltage drop across C 2.

The CVT is also useful in communication systems. CVTs in

combination with wave traps are used for filtering high frequency

communication signals from power frequency. This f orms a carrier

communication network throughout the transmission network.


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Volt

e System Maintenane

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Earthing Transformers

In areas where earth point is not

available, a neutral point is created

using an earthing transformer. Earthingtransformer, having the zig-zag

(interstar) winding is used to achieve

the required zero phase impendence

stage which provides the possibility of

neutral earthing condition. In addition

an auxiliary windings can also be

provided to meet the requirement of an

auxiliary power supply.

Earthing transformers are usually oil immersed and may be

installed outdoor. As for connection, the earthing can beconnected directly, through an arc -suppression reactor or through

a neutral earthing reactor or resistor. In cases where a separate

reactor is connected between the transformer neutral and earth,

the reactor and the transformer can be incorporated into the same

tank.

Line Traps

High voltage transmission lines are

also used for transmitting carrier signals

between 30 kHz and 500 kHz for remote

control, voice communication, remote

metering & protection, and so forth, and

are often referred to as Power Line

Carrier (PLC) systems.

Line traps prevent

transmission of these high

frequency signals to unwanted

directions without loss of energy

at power frequency.

Line traps are series-

connected to the transmission

lines, and are designed to

withstand the rated power

frequency current and the short-

circuit current to which the lines

are subjected.


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3.1.2Different MVserviceconnectionsSingle-line service connection

The substation is supplied by a

single circuit tee-off from a MV

distributor (cable or line).In general,

the MV service is connected into a panel

containing a load-break/ isolating

switch-fuse combination and earthing

switches, as shown in Figure B11.

In some countries a pole-mounted

transformer with no MV switchgear or

fuses (at the pole) constitutes the

substation. This type of MV service is

very common in rural areas.

Protection and switching devices are

remote from the transformer, and

generally control a main overhead line,

from which a number of these elementary

service lines are tapped.

Ring-main service connection

Ring-main units (RMU) are normally

connected to form a MV ring main or

interconnector-distributor(2), such that

the RMU busbars carry the full ring-main

or interconnector current (see Fig. B12).

The RMU consists of three units,

integrated to form a single assembly,

2 incoming units, each containing a load break/isolating

switch and a circuit earthing switch

1 outgoing and general protection unit, containing a load -

break switch and MV fuses, or a combined load-break/fuse

switch, or a circuit-breaker and isolating switch, together

with a circuit-earthing switch in each case.

All load-break switches and earthing switches are fully

rated for short-circuit currentmaking duty.This arrangement

provides the user with a two-source supply, thereby reducing

considerably any interruption of service due to system faults or

operations by the supply authority, etc.

The main application for RMUs is in utility supply MV

underground-cable networks in urban areas.


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Parallel feeders service connection

Where a MV supply connection to two

lines or cables originating from the samebusbar of a substation is possible, a

similar MV switchboard to that of a RMU

is commonly used (see Fig. B13).

The main operational difference

between this arrangement and that of a

RMU is that the two incoming panels are

mutually interlocked, such that one

incoming switch only can be closed at a

time, i.e. its closure prevents the

closure of the other.On the loss of power

supply, the closed incoming switch must be opened and the

(formerly open) switch can then be closed. The sequence may be

carried out manually or automatically.

This type of switchboard is used particularly in networks of

medium-load density and in rapidly-expanding urban areas supplied

by MV underground cable systems.

3.1.3Protection of transformer and circuitsGeneral protection

The electrical equipment and circuits in a substation must

be protected in order to avoid or to control damage due to

abnormal currents and/or voltages. All equipment normally used in

power system installations have standardized short-time withstand

ratings for overcurrent and overvoltage. The role of protective

scheme is to ensure that this withstand limits can never be

exceeded. In general, this means that fault conditions must be

cleared as fast as possible without missing to ensure

coordination between protective devices upstream and downstream

the equipement to be protected. This means, when there is a fault

in a network, generally several protective devices see the faultat the same time but only one must act.

These devices may be:

Fuses which clear the faulty circuit directly or together

with a mechanical tripping attachment, which opens an

associated three-phase load-break switch

Relays which act indirectly on the circuit -breaker coil


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Transformer protection

Stresses due to the supply network

Some voltage surges can occur on the network such as:

Atmospheric voltage surgesAtmospheric voltage surges are caused by a stroke of

lightning falling on or near an overhead line.

Operating voltage surges

A sudden change in the established operating conditions

in an electrical network causes transient phenomena to occur.

This is generally a high frequency or damped oscillation

voltage surge wave.

For both voltage surges, the overvoltage protection device

generally used is a varistor (Zinc Oxide).In most cases, voltagesurges protection has no action on switchgear.

Stresses due to the load

Overloading is frequently due to the coincidental demand of

a number of small loads, or to an increase in the apparent power

(kVA) demand of the installation, due to expansion in a factory,

with consequent building extensions, and so on. Load increases

raise the temperature of the wirings and of the insulation

material. As a result, temperature increases involve a reduction

of the equipment working life. Overload protection devices can belocated on primary or secondary side of the transformer.

The protection against overloading of a transformer is now

provided by a digital relay which acts to trip the circuit-

breaker on the secondary side of the transformer. Such relay,

generally called thermal overload relay, art ificially simulates

the temperature, taking into account the time constant of the

transformer. Some of them are able to take into account the

effect of harmonic currents due to non linear loads (rectifiers,

computer equipment, variable speed drives).This type of relay is

also able to predict the time before overload tripping and the

waiting time after tripping. So, this information is very helpful

to control load shedding operation.

In addition, larger oil-immersed transformers frequently

have thermostats with two settings, one for alarm purposes and

the other for tripping.

Dry-type transformers use heat sensors embedded in the

hottest part of the windings insulation for alarm and tripping.


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Internal faults

The protection of transformers by transformer-mounted

devices, against the effects of internal faults, is provided on

transformers which are fitted with air breathing conservatortanks by the classical Buchholz mechanical relay (see Fig. B15).

These relays can detect a slow accumulation of gases which

results from the arcing of incipient faults in the winding

insulation or from the ingress of air due to an oil leak.

This first level of detection generally gives an alarm, but if

the condition deteriorates further, a second level of detection

will trip the upstream circuit-breaker.

An oil-surge detection feature of the Buchholz relay will

trip the upstream circuit breaker instantaneously if a surge of

oil occurs in the pipe connecting the main tank with the

conservator tank.

Such a surge can only occur due to the displacement of oil

caused by a rapidly formed bubble of gas, generated by an arc of

short-circuit current in the oil.

By specially designing the cooling-oil radiator elements to

perform a concerting action, totally filled types o f

transformer as large as 10 MVA are now currently available.

Expansion of the oil is accommodated without an excessive

rise in pressure by the bellows effect of the radiator

elements. A full description of these transformers is given in

Sub-clause 4.4 (see Fig. B16).Evidently the Buchholz devices mentioned above cannot be

applied to this design; a modern counterpart has been developed

however, which measures:

The accumulation of gas

Overpressure

Over temperature

The first two conditions trip the upstream circuit-breaker,

and the third condition trips the downstream circuit-breaker of

the transformer.

Internal phase-to-phase short-circuit

Internal phase-to-phase short-circuits must be detected and

cleared by:

3 fuses on the primary side of the transformer or

An over current relay that trips a circuit-breaker upstream

of the transformer


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Internal phase-to-earth short-circuit

This is the most common type

of internal fault. It must be

detected by an earth fault relay.

Earth fault current can becalculated with the sum of the 3

primary phase currents (if 3

current transformers are used) or

by a specific core current

transformer. If a great

sensitivity is needed, specific

core current transformer will be

preferred. In such a case, a two

current transformers set is sufficient (see Fig. B17).

Discrimination between the protective devices upstream and

downstream of the transformer

The consumer-type substation with LV metering requires

discriminative operation between the MV fuses or MV circuit-

breaker and the LV circuit-breaker or fuses.

The rating of the MV fuses will be chosen according to the

characteristics of the transformer.

The tripping characteristics of the LV circuit-breaker must

be such that, for an overload or short-circuit condition

downstream of its location, the breaker will trip sufficiently

quickly to ensure that the MV fuses or the MV circuit -breaker

will not be adversely affected by the passage of overcurrent

through them.

The tripping performance curves for MV fuses or MV circuit-

breaker and LV circuit breakers are given by graphs of time-to-

operate against current passing through them. Both curves have

the general inverse-time/current form (with an abrupt

discontinuity in the CB curve at the current value above which

instantaneous tripping occurs).

These curves are shown typically

in Figure B18.


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3.2.0 Distribution Line Protection Devices

Overhead distribution lines are generally to faults due to

high winds, lighting, falling tree, birds etc. Most faults are

transient in nature and the system would be ready for operation

again as soon as the fault has been interrupted by the system

protection. The fault current might have been caused by a falling

tree, which falls across the high voltage line .In many tree wil l

fall off the line again after a circuit breaker has de-energized

line. Alighting stroke will cause the same transient fault; a

flash over will cause a short circuit current to flow, but as

soon as this current is interrupted, the system is back to normal

again.

Usually majority of the faults recoded in an overhead line

system is of a transient in nature. This is in contrast to a

cable network distribution system where most faults are of a

permanent nature.

Drop Down Lift Off (DDLO)

A non-current-limiting

device interruption the

current by the melting of a

fuse element and an arc is

drawn inside a gas evolving

type. These may be recharged

with limited cost.

An expulsion fuses in

a holder, arranged in such a

way that the expulsion fuse

tube drops out of the

electrical circuit when the fuse has operated. These are commonly

used in the CEB distribution network mainly for the protection of

distribution transformers and some cases for sectionalizing spur

MV lines.


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Air break Switch

A switch device, which is

normally, only used as a

disconnection, i.e. only

operated in a de energized

system. However a very

limiting making and breaking

and breaking performance.

Contact velocity at making is

operator dependent; an arcing

horn may give a high arcing contact velocities at oppugn

sufficient for the interruption of load transformers.

The switch can in most cases be equipped with a load current

interrupting device. Still, the switch has only very limited

making performance .In the CE, the A

S were installed in area

boundaries, interconnection points and on long spur line etc. to

facilitate isolation of section for fault location, maintenance

and repair works.

Load Break Switch

The so-called general purpose

switch is according to standards

defined as follows; mechanical

switching device capable of making,

carrying the breaking currents under

normal. A circuit condition, which may

include specified operating overload

conditions, such as those of a short

circuit .It may also be capable of as those of a short circuit.

It may be capable to making but not breaking of short circuit

current. The load break switch contains some special arts .One of

the interrupter head. It reduces their formed When the switch is

operate.


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Auto Recloser

Auto reclosers are self contaminated devises that make and

break the distribution system under normal at and fault

conditions. A basic feature of a

recloser is to reclose immediately

once the circuit under which it

served breaks due to temporary

fault. Recloser will lock out its

operation whenever it senses a

permanent fault clears before lock

out, recloser will reset for another

cycle of operation.

Before CEB has introduced auto

reclosers to the distribution system, only

DDLOs are provided as the protective

devices.

But this needs some one to operate the

DDLO in order to isolate the line from the

power supply. Therefore by introducing auto

reclosers to distribution system, the speed

of fault clearing has improved and hence

which promotes the stability of the power

system. Because of these reasons the concepts

of auto reclosers entered as a time and money saving method [the

interruption period becomes less].

The minimum requirement for installing an auto recloser is

100 km 1 MVA. The reclosers are sensitive for over current, and

in modern types sensitive earth faults too.

There are three types of auto reclosers

available in medium voltage system in CEB.

SF6 gas auto recloser

Oil auto recloser

Vacuum auto recloser


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The following CT ratios are available in CEB 33KV CT ratio

300/100/1, 11KV CT ratio 300/100/1.The main advantage of

Programmable type Auto Recloser is;

This type is programmable and therefore different settings

can be given

Availability of memory facility, data for later viewing can

be stored. This will includes the no of tripping occurred,

the no of occurred as over current and earth fault or

sensitive earth fault and the percentage of these f ailures .

The only thing is to maintain the pressure of the gas in

side at specified region given in the name plate.

The tripping period (dead time) for this type when

connecting in the distribution system are selected as follows;

1st tripping operation 0.25 sec.

2nd

tripping operation 0.50 sec

3rd tripping operation 1.00 sec

4th

tripping operation trip /lockout

MV Dstribution Substation

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