Protection against lightning and switchingovervoltage…MV surge arresters

TT33

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Why the transformer needs to be protected

During operation, an electrical distribution network undergoesexternal stress such as overvoltage.This overvoltage often damages the electrical equipment when theamplitude exceeds the insulation level for which the equipmentwas sized.

The transformer has the same insulation level as the otherequipment but may undergo a higher level of stress when anovervoltage occurs. Indeed, it acts as a high input impedance inpulsed operating conditions. It therefore provides the best point ofwave reflection and is consequently one of the most exposed tiveparts on the network.

This is why it is necessary to protect the transformer from theoutside environment and limit overvoltage to an acceptable level,i.e. by ensuring there is a margin in relation to the device’sinsulation level. The best way of doing this is to install surgearresters close to the transformer’s connection points.

overvoltagefactor

protectionlevels

transformerwithstand level

Taken from Schneider ElectricTechnical booklet N°151

types ofovervoltage

2 to 4

≤ √3

1

withstandat 2.5 Un

surge arrester’s

range of effect

llightning impulsewithstand

Un

due to switching

due to power frequency

- atmospheric

- due to interruption*10 to 15

* multiple arcing in specific operating conditions

Overvoltage withstand of the transformer and the surge arrester

Why overvoltage causes a problem

An overvoltage is a voltage whose value is abnormally high inrelation to the network operating voltage. It may be caused byeither:- atmospheric conditions, in which case it is external to the

network,- switching operations or resonance phenomena, in which case

the cause is said to be internal,- or the untimely opening of the circuit-breaker during

transformer magnetisation.

Atmospheric overvoltageThis is caused by atmospheric discharge (from lightning), and maybe generated :

- either directly, when lightning strikes the transformer’s feed lineconductors,

- or indirectly, when lightning strikes the ground or metallicstructures near the line, thus generating induced current and anincrease in the ground potential.

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Switching overvoltageThis is caused by maintenance work on the networks during whichcircuit-breakers or fuses interrupt inductive or capacitive loads.

Overvoltage caused by untimely opening on energizingThe circuit-breaker may open just after energizing if the protectiondevices have not been set correctly, or if the user does not havesufficient knowledge of the making currents or has forgotten toimplement the “H2 check”. This untimely opening duringtransformer magnetisation causes significant overvoltage.

The ideal solution to the problem is the surgearresterThe surge arrester is a static device designed to limit the amplitudeof overvoltage, which may be generated at a given point on thenetwork.

In =10 kA

Voltage at surgearrester terminals

Current viasurge arrester

Ures =75 kV

Uc =15 kV

OperatingVoltage

Dynamic behaviour of a surge arrester

The overvoltage is limited by running off the current into the earthvia the surge arrester.The surge arrester is permanently connected to the network, closeto the transformer’s terminals. When an overvoltage wave ispropagated over the network and becomes greater than the surgearrester’s limitation voltage, the resistance of the surge arresterbecomes temporarily very weak, allowing the current to be run offto earth and hence limiting the voltage at the transformer’sterminals. It spontaneously recovers its insulation quality once thevoltage has reached its normal value again..This solution offers a genuine advantage in terms of networkoperation since it does not involve interrupting the supply.

Voltage

Time

Without a surge arrester

With a surge arrester

Behaviour of surge arrester in overvoltage conditions

Choice of surge arrester, and its purpose

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To justify the use of a surge arrester, it is necessary to assess therisk of lightning striking using various parameters such as :- the lightning impact level Nk, which is the number of days ayear thunder has been detected in a given place,- the type of network,- the topology of the place.

To define a surge arrester it is necessary to know the network’scharacteristics, and notably the neutral point connectionconditioning the choice of constant operating voltage.The transformer manufacturer is usually unaware of the networkparameters; furthermore, probability calculations differ according tothe type of network and the country.

The main electrical sizing criteria are:• the maximum constant voltage Umcov, dependent on therated voltage Un and neutral point connection,• the rated current (e.g. 10 kA),• the residual voltage at In (8/20 µs wave),• creepage distance.

Rated voltagetransformer Un

Info onneutral pointconnection

If no info, takeUc=Umax =1.1 x UnNo

Yes

Uc = 1,4*Umax / 1,732

Umax = 1.1 x Un

Uc = Umax

Impedance neutralwith Z < 40 Ω

Uc = 1,5*Umax / 1,732

Choice of the surge arrester

Grounded neutralImpedance neutralwith Z > 40 Ω

or isolated power

Selected surge arrester

Simplified diagram of the surge arrester selection procedure

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ImplementationIf the distance between the surge arrester and the transformer tobe protected is significant, the wave will be reflected at theterminals of this last one, whose impedance is high, causing stresswell above the surge arrester’s level of protection. In this situation,the surge arrester no longer ensures its protective role.It is consequently imperative to :- install the surge arrester in the immediate vicinity of thetransformer (less than 10 metres if possible), and even directly onthis one.- connect the earth of the surge arrester directly to the mass ofthe transformer ; in this way the transformer will undergo theovervoltage limited by the surge arrester.

Remember that the impedance of an earth cable is 1µH/m, whichmeans that if a lightning impulse of 1 kA/µs with a rising front is runoff to earth, the voltage drop is 1 kV/m.

The discharge counterAs the surge arrester has a limited service life, it should bechanged when the number of discharges exceeds 500.The number of discharges can be tracked using a dischargecounter, automatically installed with the surge arrester’s earthconnection.

Conclusions and recommendations

It is essential to install phase-earth surge arresters in the followingcases:

When the lightning impact level Nk is greater than 25, the riskof direct or induced atmospheric overvoltage is directly proportionalto Nk (e.g. Indonesia Nk = 180 ).

During the occasional switching (less than 10 operations ayear) of a transformer with a weak load, or during a magnetisationperiod.

We strongly recommend that phase-earth surge arresters beinstalled in the following case:

When the substation is supplied by a network includingoverhead parts, then a cable which is longer than 20m (case of anoverhead-underground network).

What you should remember:

Surge arresters should beinstalled if:

- Nk > à 25

- switching transformers withweak inductive load

- overhead-underground supply

Surge arrester ideal installation