VECTOR GROUP TEST OF X-MER

Vector Group Test of Transformer

The vector group of transformer is an essential property for successful parallel operation of transformers. Hence every electrical power transformer must undergo through vector group test of transformer at factory site for ensuring the customer specified vector group of transformer.

The phase sequence or the order in which the phases reach their maximum positive voltages, must be identical for two paralleled transformers. Otherwise, during the cycle, each pair of phases will be short circuited.

We know that, the primary and secondary coils on any one limb have induced emfs that are in time - phase. Let's consider two transformers of same number primary turns and the primary windings are connected in star. The secondary number of turns per phase in both transformers are also same. But the first transformer has star connected secondary and other transformer has delta connected secondary. If same voltages are applied in primary of both transformers, the secondary induced emf in each phase will be in same time-phase with that of respective primary phase, as because the the primary and secondary coils of same phase are wound on the same limb in the core of transformer. In first transformer, as the secondary is star connected, the secondary line voltage is √3 times of induced voltage per secondary phase coil. But in case of second transformer, where secondary is delta connected, the line voltage is equal to induced voltage per secondary phase coil. If we go through the vector diagram of secondary line voltages of both transformer, we will easily find that there will be a clear 30^o angular difference between the line voltages of these transformers. Now, if we try to run these transformers in parallel then there will be a circulating current flows between the transformers as because there is a phase angle difference between their secondary line voltages. This phase difference can not be compensated. Thus two sets of connections giving secondary voltages with a phase displacement can not be intended for parallel operation of transformers.The several secondary connections are available in respect of various primary three phase connection in a the three phase transformer. So for same primary applied three phase voltage there may be different three phase secondary voltages with various magnitudes and phases for different internal connection of the transformer. Let's have a discussion in detail by example for better understanding.

The following table gives the connections for which from the view point of phase sequence and angular divergences, transformer can be operated parallel. According to their vector relation, all three phase transformers are divided into different vector group of transformer. All electrical power transformers of a particular vector group can easily be operated in parallel if they fulfill other condition for parallel operation of transformers.

GROUP	Connection	Connection
0 (0o)	Yy0	Dd0
1 ( 30o)	Yd1	Dy1
6 ( 180o)	Yy6	Dd6
11 ( - 30o)	Yd11	Dy11

Procedure of vector group test of transformer

Let’s have a YNd₁₁ transformer.

1) Connect neutral point of star connected winding with earth.

2) Join 1U of HV and 2W of LV together.

3) Apply 415V, three phase supply to HV terminals.

4) Measure voltages between terminals 2U – 1N, 2V – 1N, 2W – 1N, that means voltages between each LV terminal and HV neutral.

5) Also measure voltages between terminals 2V – 1V, 2W – 1W and 2V – 1W.

For YNd₁₁ transformer, we will find,
2U – 1N > 2V – 1N > 2W – 1N
2V – 1W > 2V – 1V or 2W – 1W

The vector group test of transformer for other group can also be done in similar way.

EARTH FAULT PROTECTION

Restricted Earth Fault Protection

Restricted Earth Fault Protection of Transformer

An external fault in the star side will result in current flowing in the line current transformer of the affected phase and at the same time a balancing current flows in the neutral current transformer, hence the resultant electric current in the relay is therefore zero. So this REF relay will not be actuated for external earth fault. But during internal fault the neutral current transformer only carries the unbalance fault current and operation of Restricted Earth Fault Relay takes place. This scheme of restricted earth fault protection is very sensitive for internal earth fault of electrical power transformer. The protection scheme is comparatively cheaper than differential protection scheme

Restricted earth fault protection is provided in electrical power transformer for sensing internal earth fault of the transformer. In this scheme the CT secondary of each phase of electrical power transformer are connected together as shown in the figure. Then common terminals are connected to the secondary of a Neutral Current Transformer or NCT. The CT or Current Transformer connected to the neutral of power transformer is called Neutral Current Transformer or Neutral CT or simply NCT. Whenever there is an unbalancing in between three phases of the power transformer, a resultant unbalance current flow through the close path connected to the common terminals of the CT secondaries. An unbalance current will also flow through the neutral of power transformer and hence there will be a secondary current in Neutral CT because of this unbalance neutral current. In Restricted Earth Fault scheme the common terminals of phase CTs are connected to the secondary of Neutral CT in such a manner that secondary unbalance current of phase CTs and the secondary current of Neutral CT will oppose each other. If these both electric currents are equal in amplitude there will not be any resultant current circulate through the said close path. The Restricted Earth Fault Relay is connected in this close path. Hence the relay will not response even there is an unbalancing in phase current of the power transformer.

ALL ABOUT THE CURRENT TRANSFORMER

Definition of Instrument Transformer

Instrument transformers means current transformer & voltage transformer are used in electrical power system for stepping down currents and voltages of the system for metering and protection purpose. Actually relays and meters used for protection and metering, are not designed for high currents and voltages.

High currents or voltages of electrical power system can not be directly fed to relays and meters. Current transformer steps down rated system current to 1 Amp or 5 Amp similarly voltage transformer steps down system voltages to 110V. The relays and meters are generally designed for 1 Amp, 5 Amp and 110V.

Definition of current transformer(CT)

A current transformer (CT) is an instrument transformer in which the secondary current is substantially proportional to primary current and differs in phase from it by ideally zero degree.

CT Accuracy Class or Current Transformer Class

A CT is similar to a electrical power transformer to some extent, but there are some difference in construction and operation principle. For metering and indication purpose, accuracy of ratio, between primary and secondary currents are essential within normal working range. Normally accuracy of current transformer required upto 125% of rated current; as because allowable system current must be below 125% of rated current. Rather it is desirable the CT core to be saturated after this limit since the unnecessary electrical stresses due to system over current can be prevented from the metering instrument connected to the secondary of the current transformer as secondary current does not go above a desired limit even primary current of the CT rises to a very high value than its ratings. So accuracy within working range is main criteria of a current transformer used for metering purpose. The degree of accuracy of a Metering CT is expressed by CT Accuracy Class or simply Current Transformer Class or CT Class.

But in the case of protection, the CT may not have the accuracy level as good as metering CT although it is desired not to be saturated during high fault current passes through primary. So core of protection CT is so designed that it would not be saturated for long range of currents. If saturation of the core comes at lower level of primary current the proper reflection of primary current will not come to secondary, hence relays connected to the secondary may not function properly and protection system losses its reliability.

Suppose you have one CT with current ratio 400/1A and its protection core is situated at 500A. If the primary current of the CT becomes 1000A the secondary current will still be 1.25A as because the secondary current will not increase after 1.25A because of saturation. If actuating current of the relay connected the secondary circuit of the CT is 1.5A, it will not be operated at all even fault level of the power circuit is 1000A.

The degree of accuracy of a Protection CT may not be as fine as Metering CT but it is also expressed byCT Accuracy Class or simply Current Transformer Class or CT Class as in the case of Metering Current Transformer but in little bit different manner.

Theory of Current Transformer or CT

A current transformer functions with the same basic working principle of electrical power transformer, as we discussed earlier, but here is some difference. If aelectrical power transformer or other general purpose transformer, primary current varies with load or secondary current. In case of current transformer, primary current is the system current and this primary current or system current transforms to the CT secondary, hence secondary current or burden current depends upon primary current of the current transformer.

Are you confused? OK let us clear you.

In a power transformer, if load is disconnected, there will be only magnetizing current flows in the primary. The primary of the power transformer takes current from the source proportional to the load connected with secondary . But in case of Current transformer, the primary is connected in series with power line. So current through its primary is nothing but the current flows through that power line. The primary current of the CT, hence does not depend upon whether the load or burden is connected to the secondary or not or what is the impedance value of burden. Generally current transformer has very few turns in primary where as secondary turns is large in number. Say N_p is number of turns in CT primary and I_p is the current through primary. Hence the primary AT is equal to N_pI_p AT.

If number of turns in secondary and secondary current in that CT are N_s and I_s respectively then Secondary AT is equal to N_sI_s AT.

In an ideal CT the primary AT is exactly is equal in magnitude to secondary AT.

So from the above statement it is clear that if a CT has one turn in primary and 400 turns in secondary winding, if it has 400 A current in primary then it will have 1A in secondary burden.
Thus the turn ratio of the CT is 400/1A

Error in Current Transformer or CT

But in an actual current transformer, errors with which we are connected can best be considered through a study of phasor diagram for a CT,

I_s - Secondary Current
E_s - Secondary induced emf
I_p - primary Current
E_p - primary induced emf
K_T - turns ratio = numbers of secondary turns/number of primary turns
I_o - Excitation Current
I_m - magnetizing component of I_o
I_w - core loss component of I_o
Φ_m - main flux.

Let us take flux as reference. EMF E_s and E_p lags behind the flux by 90^o. The magnitude of the passers E_s and E_p are proportional to secondary and primary turns. The excitation current I_o which is made up of two components I_m and I_w. The secondary current I_o lags behind the secondary induced emf E_s by an angle Φ _s. The secondary current is now transferred to the primary side by reversing I_s and multiplied by the turns ratio K_T. The total current flows through the primary I_p is then vector sum of K_T I_s and I_o.

The Current Error or Ratio Error in Current Transformer or CT

From above passer diagram it is clear that primary current I_p is not exactly equal to the secondary current multiplied by turns ratio, i.e. K_TI_s. This difference is due to the primary current is contributed by the core excitation current. The error in current transformer introduced due to this difference is called current error of CT or Current error of current transformer or some times Ratio Error in Current Transformer.

Hence, the percentage current error =	|Ip| − |KT.Is|	X 100 %
	Ip

Phase Error or Phase Angle Error in Current Transformer

For a ideal current transformer the angle between the primary and reversed secondary current vector is zero. But for an actual current transformer there is always a difference in phase between two due to the fact that primary current has to supply the component of the exiting current. The angle between the above two phases in termed as Phase Angle Error in Current Transformer or CT. Here in the phasor diagram it is β the phase angle error is usually expressed in minutes.

Cause of error in current transformer

The total primary current is not actually transformed in CT. One part of the primary current is consumed for core excitation and remaining is actually transformers with turns ratio of CT so there is error in current transformer means there are both Ratio Error in Current Transformer as well as a Phase Angle Error in Current Transformer.

How to reduce error in current transformer

It is desirable to reduce these errors, for better performance. For achieving minimum error in current transformer, one can follow the following,

1) Using a core of high permeability and low hysteresis loss magnetic materials.
2) Keeping the rated burden to the nearer value of the actual burden.
3) Ensuring minimum length of flux path and increasing cross – sectional area of the core, minimizing joint of the core.
4) Lowering the secondary internal impedance.

Substation Grounding / Earthing

Substation Grounding/Earthing

The sole purpose of substation grounding/earthing is to protect the equipment from surges and lightning strikes and to protect the operating persons in the substation. The substation earthing system is necessary for connecting neutral points of transformers and generators to ground and also for connecting the non current carrying metal parts such as structures, overhead shielding wires, tanks, frames, etc to earth. Earthing ofsurge arresters is through the earthing system. The function of substation earthing system is to provide a grounding mat below the earth surface in and around the substation which will have uniformly zero potential with respect to ground and lower earth resistance to ensure that

• To provide discharge path for lightning over voltages coming via rod-gaps, surge arresters, and shielding wires etc. .
• To ensure safety of the operating staff by limiting voltage gradient at ground level in the substation
• To provide low resistance path to the earthing switch earthed terminals, so as to discharge the trapped charge (Due to charging currents even the line is dead still charge remains which causes dangerous shocks) to earth prior to maintenance and repairs.

Earth Resistance

Earth Resistance is the resistance offered by the earth electrode to the flow of current in to the ground. To provide a sufficiently low resistance path to the earth to minimize the rise in earth potential with respect to a remote earth fault. Persons touching any of the non current carrying grounded parts shall not receive a dangerous shock during an earth fault. Each structure, transformer tank, body of equipment, etc, should be connected to earthing mat by their own earth connection.

Generally lower earth resistance is preferable but for certain applications following earth resistance are satisfactory

Large Power Station s– 0.5 Ohm

Major Power Stations - 1.0 Ohm

Small Substation – 2.0 Ohm

In all Other Cases – 8.0 Ohm

Step Potential and Touch Potential

Grounding system in a electrical system is designed to achieve low earth resistance and also to achieve safe ‘Step Potential ‘and ‘Touch Potential’.

Step Potential:

Step potential is the potential difference between the feet of a person standing on the floor of the substation, with 0.5 m spacing between the feet (one step), through the flow of earth fault current through the grounding system.

Touch Potential:

Touch potential is a potential difference between the fingers of raised hand touching the faulted structure and the feet of the person standing on the substation floor. The person should not get a shock even if the grounded structure is carrying fault current, i.e, The Touch Potential should be very small.

Step Potential and Touch Potential

Types of Grounding:

Un earthed Systems:

It is used no more. The neutral is not connected to the earth, also called as insulated neutral system.

Solid grounding or effective grounding:

The neutral is directly connected to the earth without any impedance between neutral and ground.

Resistance grounding:

Resistance is connected between the neutral and the ground.

Reactance grounding:

Reactance is connected between the neutral and ground.

Resonant Grounding:

An adjustable reactor of correctly selected value to compensate the capacitive earth current is connected between the neutral and the earth. The coil is called Arc Suppression Coil or Earth Fault Neutralizer.

For Studying more about Different Types Of Groundings Please see Power System Earthing

Different Grounding Equipment in Electrical Substation

• Earthing Electrodes
• Earthing Mat
• Risers
• Overhead shielding wire (Earthed)

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Different Equipments and Ground Connections

Apparatus	Parts to be Earthed	Method Of Connection
Power Transformer	Transformer tank	Connect the earthing bolt on transformer tank to station earth. Connect the neutral to earthing system
High Voltage Circuit Breakers	Operating mechanism, frame	Connect the earthing bolt on the frame and the operating mechanism of Circuit Breaker to earthing system
Surge Arrester	Lower Earth Point	To be directly connected to the earth mat
Support of bushing insulators, lightning arresters, fuse, etc..	Device Flange or Base Plate	Connect the earthing bolt of the device to the station earthing system
Potential Transformer	Potential transformer tank, LV neutral.	Connect the transformer earthing bolt to earthing system Connect LV neutral of phase lead to case with flexible copper conductor
Isolator	Isolator frame, operating mechanism, bedplate	Weld the isolator base frame, connects it to the bolt on operating mechanism base plate and station earth.
Current Transformer	Secondary winding and metal case	Connect secondary winding to earthing bolt on transformer case with a flexible copper conductor.

Power System Earthing

It is necessary to earth a power system at a suitable point by a suitable method as it offers many advantages as

1. It provide Safety to the electrical equipments against over-current
2. It provides better safety
3. It reduces the maintenance expenditure
4. It improves the service reliabilty
5. It provides improved lightning protection

Solid Earthing

When the neutral of the power transformer and generator is directly connected to the earth, then the system is said to be solidly earthed. The solidly earthing does not make a zero impedance circuit as generator or transformer would have its own reactance in series with the neutral circuit. The direct earthing of a generator without external impedance causes earth fault current from the generator to exceed the maximum 3-phase fault current if the impedance of the generator is too low. This results in Stator winding damage as the short circuit current during fault will exceed the short circuit rating of the winding for which it was designed. For this system of earthing, it is necessary that the earth fault current shall be in the range of 25% to 100% of the 3-phase fault current to prevent the development of high transit over voltages.

Resistance Earthing

In resistance earthing the neutral of the generator or transformer is connected to the earth trough a resistance in series.

Advantage Of Resistance Earthing are:

1. It reduces the line voltage drop caused when earth fault occurs
2. It reduces electric shock hazards to the persons, caused by stray earth fault currents in the return path
3. It reduces the mechanical stresses in the circuit carrying fault current
4. It reduces the effect of burning of faulted electrical equipment

The magnitude of the resistance to be used should be such that it should limit the earth fault current to a value which will reduce minimum damage at the point of the fault.

Type Of Earth Resistance Methods

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Reactance Earthing:

In reactance earthing a reactor is connected in between the neutral of the machine and earth. A low reactance is connected in series with the neutral of the machine to limit the earth fault current through the generator. This current should not be greater than the 3-phase fault current of the generator. The earth fault current of the earthed system should not be less than 25% of the 3-phase fault current in order to minimize the transient voltages

MAIN COMPONENTS OF SUBSTATION

Substation Components

Electric Substations are the part of the power system and used for transferring power from generating points to load centers. Some of the important components of substation are

Busbars:

Various incoming and outgoing circuits are connected to busbars. Busbars receive power from incoming circuits and deliver power to outgoing circuits.

Bus Bars

Surge arrestors or Lightning arrester:

Surge Arresters or Lightning Arresters discharge the over voltage surges to earth and protect the equipment insulation from switching surges and lightning surges. Surge arresters are generally connected between phase conductor and ground. In a Substation surge arrester is located at the starting of the substation as seen from incoming transmission lines and is the first equipment of the substation. Surge arresters are also provided near the transformer terminals phase to ground. Two type of surge arresters are available 1) Gapped Arresters 2) Gapless Zinc – Oxide arresters.

Lightning Arrester or Surge Arreseter

Isolators or Disconnecting Switches:

Isolators are provided for isolation from live parts for the purpose of maintenance. Isolators are located at either side of the circuit breaker. Isolators are operated under no load. Isolator does not have any rating for current breaking or current making. Isolators are interlocked with circuit breakers

Types of Isolators are

1. Central rotating, horizontal swing
2. Centre-Break
3. Vertical swing
4. Pantograph type

Isolators

Earth Switch:

Earth Switch is used to discharge the voltage on the circuit to the earth for safety. Earth switch is mounted on the frame of the isolators. Earth Switch is located for each incomer transmission line  and each side of the busbar section

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Current Transformer:

Current transformers are used for Stepping down current for measurement, protection and control. Current transformers are of two types

1. Protective CT 
2. Measuring CT

Current Transformer

Voltage Transformer:

Voltage transformers are used to step down the voltage for measurement, protection and control. Voltage transformers are of two types.

1. Electro magnetic type
2. Capacitive VT located on the feeder side of the Circuit Breaker.

Voltage Transformer

Circuit Breaker:

Circuit Breaker is used for Switching during normal and abnormal operating conditions. It is used to interrupt the short circuit currents. It is used to interrupt short circuit currents. Circuit Breaker operations include.

1. Closing
2. Opening
3. Auto – reclosing

Circuit Breaker is located near every switching point and also located at the both ends of every protection zone.

SF6 Circuit Breaker

Power Transformers:

Power Transformers are used to step up or step – down a.c. voltages and to transfer electrical power from one voltage level to another. Tap changers are used for voltage control.

Power Transformer

Shunt Reactors:

Shunt Reactors are used for long EHV transmission lines to control voltage during low – load period. Shunt reactors is also used to compensate shunt capacitance of transmission line during low load periods.  Usually Shunt reactors are unswitched.

Shunt Reactors Bank

Shunt Capacitance:

   Shunt capacitors are used for compensating reactive power of lagging power factor. Shunt Capacitors are used for improving the power factor. It is also used for voltage control during heavy lagging power factor loads. Shunt Capacitors are located at the receiving stations and distribution substations. Shunt Capacitors are switched in during heavy loads and switched – off during low loads.

Shunt Capacitor Bank

Series Capacitor:

Series Capacitors are used for some long EHV a.c lines to improve power transferability. Capacitors are located at the sending end / receiving end of the lines. Series Capacitors are provided with by – pass circuit breaker and protective spark – gaps.

Series Capacitors

Series Reactors

Series reactors are used to limit short – circuit current and to limit current surges associated with fluctuating loads. Series reactors are located at the strategic locations such that the fault levels are reduced.

Series Reactors

Lightning Protection:

Lightning protection is used to protect substation equipment from direct lightning strokes. Lightning Masts are located at the outdoor yard. Overhead Shielding wires are used to cover entire outdoor yard.

Lightning Masts

Isolated Phase Bus System:

Isolated Phase Bus System provides connection between Generator and the main Transformer. It carries very high currents.

Isolated Phase Bus Duct

Neutral Grounding Equipment:

Neutral Grounding Equipment are Resistors and reactors. They are used to limit the short circuit current during ground fault. They are connected between neutral point and ground.

• For Studying more on Power System Earthing Click Here

neutral Grounding resistor

Line Trap:

Line Trap consists of Inductive coil usually connected in the outdoor yard incoming line. Line traps are usually mounted above Capacitor Voltage Transformer (CVT) or on separate structure.

Wave Trap or Line Trap

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Insulators:

Used for Insulation purpose. Different types of insulators are porcelain, Glass, Epoxy.

Porcelain Insulators

Power Cables:

Power Cables are used to carry the power. They are single core and three core. Types of power cables are PVC insulated, XLPE insulated.

XLPE insulated Power Cable

Control Cables:

Control Cables are for protection, control and measurement etc.. They are of low voltage and PVC insulated. Control Cables are Multi core and Shielded.

XLPE insulated Control Cables

Station Earthing System:

Station Earthing System includes Earth Mat and Earth electrodes placed below ground level. These Earth Mat and Earth electrode is connected to the equipment structures, neutral points for the purpose of Equipment earthing  and neutral point earthing.

Function earthing system is to provide low resistance earthing for

1. Discharging currents from the surge arresters, overhead shielding, earthing switches
2. For equipment body earthing
3. For safe touch potential and step potential in substation.

Earthing Mat

Metering, Control and Relay panels:

To house various measuring Instruments, control Instruments, Protective relays. They are located in air-conditioned building. Control Cables are laid between Switchyard equipment and these panels.

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Metering and Control Panel