■ Merlin Gerin ■ Modicon ■ Square D ■ Telemecanique

Can you explain theadvantages of thevarious groundingsystems?If not, this text willserve a purpose…

KEEP IN MIND

We find five major grounding systems in MV installations:■ ungrounded neutral■ resistance grounding■ reactance grounding■ Petersen coil grounding■ direct grounding

Grounding systems

Each type of grounding system has its advantages and itsdrawbacks. The choice depends on the usage constraints of the network, on safety rules and on local habits.

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We consider the network power to be infinite. The mesh voltages are thenimposed and will not vary, even in the event of a ground fault.

We shall show the network in the following diagram form:

In normal balanced operation

U = nominal operating mesh voltageC1 = C2 = C3 = total capacity per phase

Before the fault:

Whatever the grounding mode, in balanced operation:■ point N has the same potential as the ground.■ the sum of the phase voltages is zero.■ the ground current Io is zero.

V1 + V2 + V3 = 0

Five methods are used to fix the potential of theneutral of Medium Voltage networks with respectto the ground.They are differentiated by the kind (capacity,resistance, inductance) and the value (zero to infinity) of the grounding link impedance Zn.

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C3C2C1

U3

U2

U1

Ph 3

Ph 2

Ph 1

V3V2V1

N

Zn Io = 0?

V3

V1

V2

0 to ∞

C3C2C1

U3

U2

U1

Ph 3

Ph 2

Ph 1

V3V2V1

N

Zn

Vo

Io

If

Ic

In the case of a phase/ground fault

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U: nominal operating mesh voltage.V: phase voltage before the fault.C1 = C2 = C3 = total capacity per phase.

During the fault, the faulty phase modifies the potential of the ground.In our example:

Three zero sequence appear.

They are in phase:

V0

V1 = 0; V2 = V3 = U

V3

V1 = 0

V2

V0 V0 V0

V1 + V2 + V3 = 0 + V2 cos 30° + V3 cos 30°

= 3 V 32

+ 3 V 32

= 0 + U cos 30° + U cos 30°

= 3 V = 3V0

V0 = V

Whatever the grounding mode, in the case of a full phase/ground fault.■ the mesh voltages do not change.■ one phase voltage is cancelled out and the two others take the value ofthe mesh voltages.■ the module of the sum of the phase voltages is equal to three times themodule of the phase voltage in balanced operation.

THE UNGROUNDEDNEUTRAL

Case of the ungrounded neutral Zn = ∞The grounding mode is said to be ungrounded neutral when there is nointentional grounding in the neutral.

The ungrounded neutral network is a solution frequently used for industrialnetworks (≤ 15 kV) requiring continuity service.

It is rarely used in transmission line networks due to the overvoltages ofwhich it is the centre (e.g. lightning stroke on the line).

An ungrounded neutral network implies:■ use of superinsulated equipment■ continuous monitoring of insulation and the needfor rapid location of the fault.

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C3C2C1

U3

U2

U1

Ph 3

Ph 2

Ph 1

V3V2V1

N

Ic2

If = Ic

Ic3

U: phase-to-phase voltage.V: balanced phase voltage.The capacity C1 is short-circuited.The mesh voltages do not change, but the ground is at the potential of thefaulty phase.

The fault current: Ic = Ic2 + Ic3

V3

V2

Un

Ic3 Ic2

Ic

V1

U: phase-to-phase voltage.

Ic = Ic2 + Ic3

⇒ Ic = C2 ω U2 32

+ C3 ω U3 32

Ic = C2 ω U 3 = If

Ic = Ic2 cos 30° + Ic3 cos 30°and I = C ω U and C1 = C2 = C3

L1

L2

L3

l 1

l 2

l 3

N

P1 S1

P2 S2

U/ 3

P1

P2

S1

S2

U/ 3

P1

P2

Re

LωRc?

THE UNGROUNDEDNEUTRAL (cont’d)

The fault current is limited to capacitive currents, which technicallyauthorizes continuity of service. This means that the equipment must beable to operate with the network mesh voltage.

☞ Remark: the capacitive current depends on the number of km of cablesused for it to flow. It must be ensured that it is not a risk for persons andequipment.

☞ Remark: the phase voltage transformers complying with the IEC standardcan run for 8 hours with 1.9 V.

In normal operation, the o vervolta ges do not flo w off to the gr ound,the equipment must also be superinsulated.

Insulation monitoring is compulsor y and, should a fault occur, qualified maintenance staff must take rapid action.

Fault detection m ust be rapid, either by: ■ zero sequence voltage relays.■ or a continuous insulation monitor.

Refer to the protection device guide appendix: binder C - chapter 2-3 a)2.7 pages A3 to A7.

Case of voltage transformers,star cabled, P2 grounded(refer to binder C, chapter 2-3 a)1.1.1).

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where Rc is the equivalent impedance of the load P.Re is the internal equivalent resistance of the VT.Lω is the internal equivalent inductance.

The VT precision power commonly used is 50 to 100 VA.

The equivalent resistance brought to the primary where U: primary mesh voltage

P: precision power.

Rc = V2

P = U2

3P

The equivalent diagram of a VT is shown below:

load P

THE UNGROUNDEDNEUTRAL (cont’d)

☞ Remark 1: with respect to alternating current, Lω represents severalmegohms. The equivalent impedance of the VT is close to Rc, i.e. a fewtens of kiloohms to a few hundreds of kiloohms according to the voltage.

In alternating current, the contribution of the VT's on a grounding fault willbe of the order of a few tens of milliamps in a 10 kV network. No groundingsystem can be disturbed by the contribution of such a current level.

☞ Remark 2: with respect to direct current Lω = 0The equivalent impedance of VT is then close to Re, i.e. a few hundreds of ohms.

If an insulation monitor injects direct current, the monitor would concludethere was a fault if the P2’s were directly grounded. This is why in this casethe P2’s must be grounded via a capacitor and a CARDEW in parallel inorder to prevent the direct current to loop round on itself by the VT’s.

By opening eac h supplied cir cuit, star ting with the least critical: the fault will be located when opening of the circuit concerned causes it to clear.

By selective detection: the protection devices will be said to be selectiveif the only protection device to react is the one immediatly upstream fromthe fault. Only the receiver concerned is disconnected.

This process is only suitable when the capacitive currents are strongenough to be detected.

A fault on an ungrounded neutral network can belocated by opening each circuit, starting with theleast critical, or by selective detection.

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THE UNGROUNDEDNEUTRAL (cont’d)

Grounding systems

Ic4

Ir4

Ic1

Ir1

Ic3

Ir3

Ic2

Ir2Ic1+

Ic3+

Ic4

If = Ic1+Ic2+Ic3+Ic4

Ir1 > 1.3 Ic1

Ir1 < 0.3 (Ic2+Ic3+Ic4)

Ir2 > 1.3 Ic2

Ir2 < 0.3 (Ic1+Ic3+Ic4) Ir3 > 1.3 Ic3

Ir3 < 0.3 (Ic1+Ic2+Ic4)

Ir4 > 1.3 Ic4

Ir4 < 0.3 (Ic1+Ic2+Ic3)

Ic4

Ir4

Ic1

Ir1

Ic3

Ir3

Ic2

Ir2Ic1+

Ic3+

Ic4

If = Ic1+Ic2+Ic3+Ic4

Ir1 < 0.3 (Ic2+Ic3+Ic4)Ir2 < 0.3 (Ic1+Ic3+Ic4)

Ir3 < 0.3 (Ic1+Ic2+Ic4)Ir4 < 0.3 (Ic1+Ic2+Ic3)

Selective fault detection is possible if thecapacitive currents are sufficient. Note that all the feeders see a faulty current:

Selective faulty detection is possible if thecapacitive currents are sufficient.Note that all the feeders see a faulty current:

Conditions for achieving discrimination

Note that the currentdirection is not the samein the faulty as in thesound feeders.

Conditions for achieving discrimination

Since the incoming cables are generally short, we shall not take theircapacitive current into account.

■ by directional gr ound rela ys (67N)(refer to binder C, chapter 2-3 a)2.7, page A8)For this type of relay, the zero sequence voltage must be measured.

When designing equipment intended for an ungrounded neutral network,we must limit the risk of ferromagnetic resonance by appropriately loadingthe VT’s (refer to binder C, chapter 2-3 a)1.1.1).

For further information on the causes of ferromagnetic resonance, consulttechnical booklet no. 31.

Two methods can be considered: ■ by zero sequence rela ys (50G - 51G), the threshold Ir of which is set at a value > 1.3 Ico. (refer binder C, chapter 2-3 a)2.7 page A15)Ir: setting currentIco is the contribution in capacitive current of the protected feeder.

With relays Max. In 50G - 51G

With directional ground relays

detectiondirection

RESISTANCE GROUNDING Zn is a resistance of varying value which both limits the ground fault currentand ensures a good flow-off of the overvoltages (limited current > twice thecapacitive current of the network:

This type of grounding is normally used for MV public and industrialdistribution networks.

Resistance grounding limits the fault current,dampens overvoltages and simplifies selectivefault detection.

Tripping must occur at the first fault to limit damage.The resistance must be calculated to dissipate theheating due to the fault current flow.

IL ≥ 2 x Ic

Grounding systems

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V: phase voltage before faultU: mesh voltage

IL

If

V3

V2

V1

Ic3 Ic2

Ic

V0 V0 V0

If = Ic + IL

Ic = Ic2 + Ic3

IL = VoRn

and Vo = V = U3

therefore IL = URn 3

C3C2C1

U3

U2

U1

Ph 3

Ph 2

Ph 1

V3V2V1

N

?

Vo

IL

If

Ic2

Ic3

ILRn

If2 = IL2 + Ic2

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RESISTANCE GROUNDING(cont’d)

Grounding systems

Ic4Ic1 Ic3Ic2

Ic1+

Ic3+

Ic4+IL

If = Ic1+Ic2+Ic3+Ic4+IL

IL

In>

3000/5

51N

IL = 50 AThe relay does not allowcorrect setting:

5 to 20% In = 0.05 x 3000to 0.2 x 3000

= 150 to 600 A

It is insensitive to thefault current relay setting range

5 to 20% In

Ic4Ic1 Ic3Ic2

Ic1+

Ic3+

Ic4+IL

If = Ic1+Ic2+Ic3+Ic4+IL

IL

In>50/5

51G

IL = 50 ASupply the relay by a toroideither at the incomer or atthe transformer neutral.

relay setting range5 to 20% In i.e.:

2.5 to 10 primary Amps

Let us check that the use of a zero sequence toroid is appropriate.

Refer to binder C, chapter 2-3 a) 2.7 appendix pages A12 to A14.

In medium voltage, the fault current is generally limited between 10 and100 A. For protection devices of rotating machines, it is often less than 20 A. This current causes the resistance to heat up, and this heat must be dissipated. The faulty line must be quickly de-energized.Selective fault detection may be performed using simple zero sequenceovercurrent relays, supplied by toroid, or by the sum of the CT currents of each phase.

Application e xample: choosing the sensor according to the relay.The relay chosen has a setting range of 5 to 20% In. Is supply by the sum of the 3 CT’s suitable in this case?

RESISTANCE GROUNDING(cont’d)

☞ Remark: if the fault current is limited by the resistance to a value lessthan 30 A, the voltage transformers will have to be loaded as in the ungroundedneutral mode.

If we have to determine the grounding resistance Rn and the correspondinglimited current (IL), we shall apply the following rules:

■ The current IL must be greater than or equal to twice the capacitivecurrent of the network in the event of a ground fault IL ≥ 2 Ic (anticipate thepossibility of a network extension. Ic could then increase).

■ The current IL must be less than the maxim um overcurrent that the cablescreens could withstand.

■ The relation 5 A ≤ IL ≤ 20 A should preferably be respected in a currentcomprising MV motors.

■ To ensure proper protection at receiver level, the threshold settings Irshould not exceed 0.2 IL : Ir ≤ 0.2 IL■ The relation Ir ≥ 1.3 Ic must be respected in order to obtain discriminationas regards sound link protection devices.Ic is the capacitive current of the protected link in the event of a phase-ground fault.

■ If the ground current is measured by 3 CT’s of rating In, Ir ≥ 0.06 In mustbe obtained.

■ The withstand of resistance Rn must allow the flow of the current IL for themaximum ground fault removal time, or, vice versa, removal of the groundfault must be sufficiently fast so as not to damage the resistance Rn.

IL = URn 3

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REACTANCE GROUNDING The resistance is replaced by a reactance.

For networks with voltages greater than 40 kV, reactance is preferred toresistance since heating is less and construction easier.

Reactance grounding limits the fault current,allows selective fault detection and reduces heatdissipation.On the other hand, we find again the overvoltageproblems and the need for tripping at the first fault.

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IL

IfV3

V2

V1

Ic3 Ic2

Ic

V0 V0 V0

The currents Ic and IL are in opposition of phases.

Since the resistance of the reactance is high, it will be difficult for any lineovervoltages to flow off.

C3C2C1

U3

U2

U1

Ph 3

Ph 2

Ph 1

V3V2V1

N

?

Vo

IL

If

Ic2

Ic3

ILZn

Ic = Ic2 + Ic3

Zn ≈ Lω

IL = V0Zn

= UZn 3

If = Ic + IL

If = UCω 32

+ UCω 32

- U3 Lω

and Ic2 = Ic3 = U C ω

therefore:

If = Ic2 cos 30° + Ic3 cos 30° - V0Lω

If = U 3 C ω - 13 Lω

REACTANCE GROUNDING(cont’d)

The current flowing through the reactance is of a few hundreds of ampsand considerably greater than the capacitive current (a few hundreds ofmilliamps to a few amps).Selective fault detection will be performed simply.

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Ic4Ic1 Ic3Ic2

Ic1+

Ic3+

Ic4+IL

If = Ic1+Ic2+Ic3+Ic4+IL

IL

A

Ic2Ic1

If = Ic1+Ic2+Ig1+Ig2

IfA

Ig1 Ig2

A simple zero sequence relay (51N or 51G) is sufficient to detect a fault in A.

In general: IL = a few hundreds of amps.The capacitive currents represent a fewamps or a few hundreds of milliamps.Ic1 + Ic2 + Ic3 + Ic4 is negligible faced with IL

REACTANCE GROUNDING(cont’d)

Case of two transformer incomers in parallel with grounded neutral

☞ Remark 1: a fault in A is seen by the sensors of both incomers.

☞ Remark 2: the current direction is not the same in both circuits.

To preserve a sound incomer, ground directionals (67N) or earth leakagezone protection devices (87B) could be used.

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Ic2Ic1

If = Ic1+Ic2+IL1+IL2

IL1+Ic1+Ic2A

IL1

IL1

IL2

PETERSEN COILGROUNDING

This system also known as the arc suppression coil is based on the factthat the fault current is the sum of the inductive current IL limited by thereactance and of the capacitive current of the network.These currents are in opposition of phases and cancel each other out: 3 L C ω2 = 1

Since the fault current is theoretically zero, the fault may persist longerwithout damage.

This grounding mode is used above all in Eastern Europe. The fault isdetected at coil level.

☞ Remark: the coil is never perfectly tuned. If a fault occurs, there is stilla current at fault level.

Petersen coil grounding ensures continuity of serviceprovided certain conditions are met.On the other hand, we find again the overvoltageproblems and selective detection is trickier than inthe ungrounded neutral mode.

Grounding systems

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C3C2C1

U3

U2

U1

Ph 3

Ph 2

Ph 1

V3V2V1

N

?

Vo

IL

If

Ic2

Ic3

ILLω

Ic = Ic2 + Ic3

IL = VoZn

= UZn 3

if Ic = IL ⇒ If = 0

ILV3

V2

V1

Ic3 Ic2

Ic

V0 V0 V0

If = 0

V1 + V2 + V3 = 3Vo

DIRECT GROUNDING With the neutral directly grounded, the fault current between phase and groundhas practically the same value as the three-phase short-circuit current.

Although this type of network is seldom used in Europe, it is, however, widespread on North American distribution networks.

Direct grounding reduces overvoltage risks andsimplifies selective detection.On the other hand, tripping is compulsory at thefirst fault, the fault current is very strong andpersons are at risk from contact voltages.

Grounding systems

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Ic4Ic1 Ic3Ic2

Ic1+

Ic3+

Ic4+IL

If = Ic1+Ic2+Ic3+Ic4+IL

IL IL = a few kiloamps.The capacitive currents represent a fewamps or a few hundreds of milliamps.Ic1 + Ic2 + Ic3 + Ic4 is negligible faced with IL.