Neutral Grounding

Presented By Ashvin G. PatelAsst. Prof. (E.E.)

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Introduction

• Solidly grounded i.e. the neutral is connected directly to the ground without any intentional impedance between the neutral and the ground.

• Generally the neutral of the generator is connected through resistance to limit the stator short circuit current and also for stability reasons.

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Advantages of Neutral Grounding:

1) Voltages of the phases are limited to phase to ground voltages.

2) The high voltages due to arcing grounds or transient line to ground faults are eliminated.

3) Sensitive protective relays against line to ground faults can be used.

4) The over-voltages due to lightning are discharged to ground, other wise there will be positive reflection at the isolated neutral of the system.

The following are the advantages of operating with isolated neutral:

1) It is possible to maintain the supply with a fault on one line.2) Interference with communication lines is reduced because of

the absence of zero sequence currents.

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Effectively Grounded System:

AIEE Standard No. 32, May 1947, defines the effective grounding as:

A system or a portion of a system can be said to be effectively grounded when for all points on the system or specified portion thereof the ratio of zero sequence reactance to positive sequence reactance (X0/X1) is not greater than 3 and the ratio of zero sequence resistance to positive sequence reactance is not greater than 1 for any condition of operation and for any amount of generator capacity. The effective grounded systems are less expensive than any

other type of grounding for all operating voltages because for such a system the maximum line to ground voltage during a fault does not exceed 80% of the line voltage whereas for all other groundings the voltage or the healthy phases rises to about 100% line-to-line voltage.

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Ungrounded System: Under balanced conditions the potential of the neutral is held at

ground due to the presence of the shunt capacitance of the system (Fig. 1 (a)). Under balanced condition the vector diagram is given in Fig. 1 (b).

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Cont…

A charging current of three times the per phase charging current flows. The voltage of the healthy phases rise to √3Vph.

The presence of inductance and capacitance in the system leads to what’s known as Arcing grounds and the voltage of the system may rise to dangerously high values as explained.

These voltages can be eliminated by connecting an inductance of suitable value between the natural and the ground.

If the value of the inductive reactance is such that the fault current IL balances exactly the charging current, then the grounding is known as resonant grounding or ground fault neutralizer or Peterson coil.

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Resonant Grounding: The value of inductance such that 3IC=IL so that theoretically there

is no current in the fault or it is so small that the arc will not maintain itself and the fault is extinguished.

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Cont…

If C is the capacitance to ground of each phase, then the charging current will IC=VphωC.

If L is the inductance to be connected between the neutral and the ground, then IL=

For balance condition IL=

L=1/(3ω2C) Resonant grounding will reduce the line interruption due to

transient line to grounding faults which will not be possible with other forms of grounding.

Ground fault neutralizers should not be used where(i) Fully graded insulation transformers are used as the neutrals of

such transformers arc not sufficiently well insulated.(ii) Auto-transformers having a ratio greater than 0.95 to 1 are

used.

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ωLVph

ωLV

3Vph

phωC

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Cont… The coils of the ground fault neutralizes are ten-minutes time-

rated on system where permanent ground faults can be located and removed by ground relays or other suitable means. Otherwise, continuous time-rated neutralizes are used on all other systems. However, if for any reason more current flows through the fault neutralizer a circuit breaker closes after a certain time-lag and the earth-fault current flows through the parallel circuit by-passing the arc Suppression coil (Fig. 3).

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Methods of Neutral Grounding:

(i) Solid grounding(ii) Resistance grounding(iii) Reactance grounding(iv) Voltage transformer grounding(v) Zig-zag transformer grounding.

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Solid Grounding/Effective Grounding: Consider L-G fault on phase c. The neutral and terminal c are at

earth potential. The reversed vector is shown at VC. The voltage of the healthy

phases remains unchanged i.e. phase to ground voltages and the currents are as shown in Fig 4 (b).

The charging current will be fully eliminated. Since in this system of grounding the voltage of the healthy phases in case of a line to ground fault does not exceed 80% of the L-L and is much less as compared to other forms of grounding.

On system 115 kV and above additional savings are possible because of the transformers with the insulation graded towards the neutral are less costly.

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Cont…

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Resistance Grounding: The value of the resistance commonly used is quite high (in order

to limit power loss in resistor during L-G fault) as compared with the system reactance. With the increase in operating voltage the value of resistance required for grounding also increases so as to limit (25% of full load current) the short circuit current during line to ground faults. Resistance grounding is normally used where the charging current is small i.e. for low voltage short length over-head lines.

Resistance grounding reduces the arcing ground hazards and it permits ready relaying of ground faults. In certain situations, resistance grounding has helped in improving the stability of the system during ground fault by replacing the power dropped, as a result of low voltage, with an approximately equal power loss in the resistor, thus reducing the advance in phase of the generators.

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Cont…

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Reactance grounded system: A reactance grounded system is one in which the neutral is

grounded through impedance which is highly reactive. In fact whether a system is solidly grounded or reactance grounded depends upon the ratio of .

For reactance grounded system > 3.0 For solid grounded system < 3.0 When a neutral is solidly grounded, but if >3, the system is

presumed to be reactance grounded rather than solidly grounded Reactance grounding lies between effective grounding and resonant grounding.

The value of reactance required is to keep currents within safe limits. This method of grounding may be used for grounding the neutral of synchronous motors and capacitors and also for circuits having large charging currents.

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1

0

XX

1

0

XX

1

0

XX

1

0

XX

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Earthing transformers: If a neutral point is required which otherwise is not available (e.g.

delta connection, bus bar points etc.) a zig-zag transformer is used. These transformers do not have secondary winding.

Each limb of the transformer has two identical Winding would differentially such that under normal conditions the total flux in each limb is negligibly, small and therefore, the transformer draws very little magnetizing current.

The grounding transformers are of short time rating usually 10 seconds to 1 minute. Therefore, the sizes of such transformers are small as compared to the power transformers of the same ratings.

If a zigzag transformer is not available, a star delta transformer can be used without loading the delta side as shown in fig 7.

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Cont…

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Generator Neutral Breaker: When a line to ground fault occurs the generator armature and

field circuit breakers arc tripped and the input to the prime mover is shut off. With these operations the current through the fault does not necessarily stop immediately because a certain time is required for the generator field flux to decay. The fault current can be reduced to a very low value immediately after the fault, if a generator neutral breaker is employed and it is also tripped simultaneously along with the field and armature breakers. In case the Value of the neutral impedance is very high and the fault current is limited, there is no need for a neutral breaker.

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Grounding Practice:1) One grounding is normally provided at each voltage level.

Between generation and distribution, there are various voltage levels; it is desirable to have ground available attach voltage level.

2) The generators are normally provided with resistance grounding and synchronous motors or synchronous capacitors are provided with reactance grounding.

3) Where several generators are connected to a common neutral bus, the bus is connected to ground through a single grounding device. Disconnect switches can be used to ground the desired generators to the neutral bus.

4) Where several generators are operating in parallel, only one generator neutral is grounded. This is done to avoid the interference of zero sequence currents. Normally two grounds are available in a station but only one is used at a time. The other is used when the first generator is out of service.

5) For low voltages up to 600 volts and for high voltages above 33 kV solid grounding is used whereas for medium voltages between 3.3 kV and 33 kV resistance or reactance grounding is used. 19

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