EARTHING

EARTHING

          INTRODUCTION

Earthing means a connection to the general mass of earth.  The use of earthing is so widespread in an electric system that at practically every point in the system, from the generating system to the consumers’ equipment, earth connections are made.

Earthing is divided into two main categories:

• Neutral Earthing
• General Earthing

          OBJECTIVE OF EARTHING

          Neutral Earthing

This is the earthing of the star or neutral point of power system lines and apparatus.

The objective of neutral earthing are:

a)    To reduce the voltage stress due to switching and lightning surges and to discharge safely into the ground over voltages occurring in the system.

b)   To permit the use of graded insulation in H.V. and E.H.V systems with consequent reduction in weight, size and cost.

c)    To control the fault currents to satisfactory values.

d)   To ensure the operation of ground or earth fault relays.

                  General Earthing

This is a term applied to all earthing of metal parts of lines and apparatus used in electrical systems and equipment used in the utilisation of electrical energy other than neutral earthing.

The objects of general earthing are:

a)    To provide protection to plant and personnel due to accidental grounding of equipment.

b)   To cordon off the zone of dead line working to make it safe during working to prevent electrostatic and electromagnetic induction and also accidental contact from other energized lines and apparatus.

Examples of general earthing are the earthing of the frames of generators, rotors, motors, tanks of transformers, circuit breakers, body of domestic apparatus, lines, electric stoves, electric irons etc.

3.0     NEUTRAL EARTHING

The various methods of neutral earthing are:

a)    Solid Earthing or Effectively Grounded Earthing

b)   Resistance Earthing

c)    Reactance Earthing

d)   Arc suppression coil earthing.

However before discussing the effects, the merits and demerits of the above methods, an isolated Neutral system is considered.

3.1     ISOLATED NEUTRAL SYSTEM

Each line conductor has a capacitance to the earth and the magnitude of this capacitance is the same in a perfectly transposed three phase line.  With balanced voltages applied to such a line, the capacitance currents will be equal in magnitude as shown above.  Assume an earth fault in conductor B.  Hence no capacity current flows between the phase B and earth.

 But the voltage across the other two phases rises to phase to phase voltage, as shown.

The fault phase B supplied the currents ICGR and ICGY.  These being capacitive

Currents, no current flows when the line capacitance is charged.  Hence, an arcing takes place at the faulted point.  During this period, the line capacitance discharges and capacitive current once again flows.  This repetitive cycle of charging and discharging causes intermittent arcing at the point of fault and also gives rise to abnormal voltages across the healthy phases due to the capacitance effect.  In practice, voltages of 3 to 4 times the system phase voltage may occur thereby causing damage to the system insulation.  Hence isolated neutral system is not being practiced.

                                             Solid Earthing

In solid earthing a direct metallic connection is made between the system neutral and the ground.  The ground electrode resistance will be very small usually less than one ohm.

          The Main Advantages are:

a)    There is no abnormal voltage rise on the other healthy phases.

b)   Permits the use of discriminative protective gear.

c)    No voltage stress on the system insulation.

d)   Efficient and correct operation of Earth fault Relays is ensured.

e)    Additional savings are possible in power transformers of 132KV and above with the use of graded insulation.

f)     No arcing grounds.

3.22   Disadvantages are:

a)    On overhead transmission lines, a majority of the faults are to the ground.  Thus, the number of severe shocks to the system is relatively much greater than with resistance or reactance grounding.

b)   The ground fault current is generally lower than the three-phase current.  But near generating stations, it may be relatively higher and may exceed the three phase short circuit currents.  In such cases circuit breakers with higher rupturing capacity are required.

c)    The increased ground fault currents affect neighboring telecommunication circuits.

Most of the adverse effects have been overcome nowadays by the use of high rupturing capacity, high speed circuit breaker and fast acting protective relays.  Hence in the world over, it is the practice to adopt solid earthing for the neutrals of power systems.

3.3     Resistance Earthing

This is one form of impedance earthing and introduced when it becomes necessary to limit the earth fault current.  The resistance used may be a solid metallic resistor or a liquid resistor or a metallic resistor immersed in a liquid like transformer oil.

         The main advantages are:

1)   Permits the use of discriminative gear.

2)   Effects of arcing grounds are avoided with suitable low ohmic resistance.

3)   Ground fault currents are reduced, thus obviating the harmful effects of the large currents associated with solid earthing.

4)   Interference with adjoining communication circuits is avoided.

The disadvantages are:

1)   System neutral will almost invariably be fully displaced in the case of a ground fault, thereby necessitating the use of 100% lightning Arresters at an increase in cost.

2)   Cost of transformers will increase because graded insulation cannot be used.

Resistance earthing, if at all used, is limited to system voltages of 33KV and below and when the total system capacity does not exceed 5000 KVA.

3.4     Reactance Earthing

This is another form of impedance earthing also called `Peferson Coil Earthing' after the name of the inventor.

This is a logical development of reactance earthing and is based on a value of reactance in the system neutral such that the reactance current due to the coil exactly neutralises the network capacitance current at the fault.  The resultant capacity current is theoretically nil and in any case inadequate to maintain the arc.  Hence the name `arc suppression coil'.

Parallel Operation of Transformers

          The following conditions must be strictly observed in order that 3-phase transformers may operate in parallel.

(a)  The secondaries must have the same phase sequence or the same phase rotation.

(b)  All corresponding secondary line voltages must be in phase.

(c)  The same inherent phase angle difference between primary and secondary terminals.

(d)  Same polarity.

(e)  The secondaries must give the same magnitude of line voltages.

In addition, it is desirable that:

(f)   The impedances of each transformer, referred to its own rating should be the same, i.e. each transformer should have the same percentage or per unit resistance and reactance.

         If conditions (a) to (e) are not complied with, the secondaries will simply short-circuit one another and no output will be possible.

If condition (f) is not complied with, the transformers will not share the total load in proportion to their ratings and one transformer will become over-loaded before the total output reaches the sum of the individual ratings. It is difficult to ensure that transformers in parallel have identical per unit impedance and this affects the load sharing.

          Three Winding Power Transformers

          An example of an EHV substation having three different voltages is a 330KV substation with voltages at 330KV, 132KV and 11KV.

          A comparison is now made as whether to have two winding transformers of 330/132KV and 132/11KV or three winding transformers of 330/132/11KV in an EHV substation with three voltages.  The 11KV load in such a substation is to meet the local loads around the substation and also for the requirements of the station auxiliary supplies.  This load may be around 10 to 15MVA.

The two schemes are shown by single line diagrams as follows: 

Comparing scheme (B) with scheme (A) we have the following merits and demerits

       Merits

(a)  The number of transformers, circuit breakers, CT’s, isolators and control panels is reduced to a minimum.  There is therefore a considerable saving in the cost of equipment required.

(b)  There is considerable saving in the cost of civil engineering and structural works because of the fewer equipment.

(c)  The layout is simple and occupies less space because of the fewer equipment

and operation is also simple.

(d)  There is saving in energy because of the reduced transformation losses.

(e)  Besides, it is inevitable to provide a third winding in a star-star connected power transformer.  This third winding in such transformers is also called a `Stabilizing Winding' or ‘Tertiary Winding’.  This winding is connected in a closed delta to provide a circulating path for the third harmonic voltages and zero sequence currents or ground fault currents.

It is pertinent to note here that a star-star connection is almost always resorted to in the case of EHV transformers of 132KV and above such as in 330/132KV transformers.  The reason being that the cost of such a transformer is cheaper because the windings need be insulated for only 1/Ö3 times of the line voltage instead of for the full line voltage of Ö3 times the star voltage with a delta winding. Such a closed delta winding can be made use of for the third voltage, without the necessity of having a separate transformer.

Demerits

(a)  The main disadvantage is the increased fault level at 11KV because the voltage is directly transformed from 330 to 11KV.  Hence 11KV switchgear of adequately higher rupturing capacity will have to be installed.

The cost of such switchgear may be much more than that of such switchgear installed in the secondary of a 132/11KV transformer.

(b)  Since the third winding is a closed delta, an artificial neutral has to be necessarily

created by the use of earthing transformers.  This is a disadvantage as it adds to the initial cost.

THREE-PHASE UNIT VERSUS SINGLE-PHASE UNITS

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Three-phase unit versus single-phase units:

Since the transmission system is 3-phase, transformers may be built as 3-phase single units or as three single-phase units into delta and star combinations or groups.

Advantages of 3 phase units

• They occupy less space
• No extra support equipment is required to form a 3-phase Delta or Star

connection.

• They are cheaper
• They can be transported from factory as a compact unit, erected and

commissioned at site quickly

• Compact on-load tap changing (OLTC) gear can be provided as a built in unit.

Disadvantages of 3 phase units

·         Problem of transportation in case of large capacity units weighing more than 100 tons.

·         Takes time in assembling, erecting and commissioning if parts are dismantled and sent to site.

·         The cost of one spare 3-phase transformer is more.

·         Change of connections from star to delta or vice-versa cannot be done.

·         If reconditioning is undertaken then the complete unit has to be taken out of service and this becomes a problem if no spare capacity is available.

          Advantages of Single-Phase Units

• The cost of a spare transformer is the cost of a single-phase unit, which is comparatively very much less than the cost of a complete spare 3-phase unit.
• They can be transported to site as completely assembled units and commissioned quickly.
• Reconditioning can be undertaken on individual units with a minimum outage time.
• It is possible to obtain different possible pairs of connections between the primary and secondary.

          Disadvantages of Single-Phase Units

• They occupy more space
• They require additional support structure to form 3-phase connections.
• Expenditure on civil engineering works is more
• The problem of providing on-load tap changing gear and even if provided the cost of providing tap changing gear on each unit works out costlier by at least 50% when compared to a compact unit in a 3-phase transformer.

          Considering all the above, there is little argument in favour of the adoption of single-phase units as compared to 3-phase units.  Single-phase units are the only choice where 3-phase units cannot be transported because of their weight and dimensions and also if there are no facilities at site for the assembly, preparation and commissioning of the 3-phase unit

          Power Transformers

These are transformers of high rating of generally not less than 5MVA and 33KV and the rating also increases with the voltage rating. They may be of the step-up type installed at generating stations or of the step-down type installed at substations. They have a high utilisation factor, which means that they are arranged to work at a constant load equal to their rating.  Hence their maximum efficiency is designed to be at or near full load.  Such power transformers installed in substations are provided with OLTC gear to regulate the voltage to be within permissible limits during peak load and off peak load hours.

However, generator step-up power transformers are provided with only off circuit taps.

          Distribution Transformers

These are transformers installed in H.V. distribution feeders to meet consumer voltage requirements.  These are generally rated at 11KV and have a rating not exceeding 1000KVA.  These transformers are characterised by an intermittent variable load, which is usually considerably less than the full load rating.  They are therefore designed to have their maximum efficiency at between half and three quarter of full load.  These transformers are not provided with any OLTC gear but with only off circuit taps.

        Auto Transformers

An Auto Transformer is a transformer with a common winding for both primary and secondary. They are used in place of two winding power transformers where the ratio of transformation does not exceed 2 as they are cheaper than two winding transformers such as in a 132KV/66KV system or 66KV/33KV system.