PROTECTION OF TRANSFORMER                        CHAPTER - 5

Introduction

The transformer is heart of power system. Power transformer is a major equipment in power system. It requires highly reliable protection devices. The protective scheme depends on size of transformer. The ratings of transformers used in transmission and distribution systems range from few kVA to several hundred MVA. For medium capacity transformers over current relays are used. For large transformers differential protection scheme is used.

Abnormalities and faults

The faults occurring in power transformer are earth fault, phase to phase faults, inter turn faults, over heating because of overloading, core heating. The most common type of faults are, earth faults, inter turn faults.

Through fault: It is a fault which lies outside the protective zone of the transformer but fed through the transformer.

Following Table shows, abnormal conditions; the protection necessary for power transformer.

Abnormal Condition/Fault	Protection Employed
Over load	I)              Thermal overload relays II)             Temperature relays.
Earth faults	i)              Differential protection. ii)             Earth fault relay
Through faults	i)              Graded time lag over current relay ii)             HRC fuses
Incipient faults below oil level resulting into  decomposition of oil.	i)              Buchholz relay i.e. gas actuated relay ii)             Pressure relief valve
Large internal faults: Phase to phase, phase to ground, below oil level.	Buchholz relay and circuit breaker.
Faults in tap changer.	i)              Percentage differential protection ii)             High speed over current relay.
High voltage surges due to lightning & switching	I)              Horn gaps. II)             Lightning arrestor.

For other transformers the protections provided are as per following table:

Abnormal Condition/Fault	Protection Employed
Small distribution transformer	HV fuses for earth fault protection and phase fault protection
Transformers in important location rating 500 kVA above.	i)              Restricted earth fault protection. ii)             Over current protection. iii)            Buchholz relay.

While selecting protection scheme for transformer the following information is necessary.

i)              kVA rating.

ii)             Whether it is indoor or outdoor.

iii)            Voltage ratio.

iv)           Connections of winding.

v)            Percentage reactance.

vi)           Neutral point earthing, value of resistance.

vii)          Conservator is provided or not.

viii)         Value of system earthing resistance.

ix)           Details of connection of CTs and relays.

x)            Fault level at transformer terminals.

xi)           Network diagram showing position of transformer.

Differential protection for transformer

The differential protection is based on current circulation principle. It is also known as Merz-Price protection. CTs will be used with pilot wire and relay coils. The CTs will be connected in primary side and secondary side of the transformer to be protected. The system is fundamentally same as that for alternator. However, the scheme used for alternator can not be directly used for transformer protection. This is because of following reasons:

1.    One group of CTs is to be connected in primary side of transformer and second group of CTs is to be connected in secondary side. The currents of primary side and secondary of transformer are different. But for differential protection, under normal working condition, the CTs should send same amount of current in the pilot wire so the CT ratio of respective side is selected such that they send same current in pilot wire so the CT ratio of respective side Ct ratios are different but pilot wire currents are the same under normal working condition.

2.    The primary side and secondary side currents of transformer have phase difference. This is because the transformer may be delta-star or star-delta connected. So CTs used for protection have to be connected is a particular way.

Following table shows the connection to be made for the CTs used in differential protection of power transformer.

Power Transformer Connections		CT Connections
Primary	Secondary	Primary	Secondary
Star with neutral earthed	Delta	Delta	Star
Delta	Delta	Star	Star
Star	Star with neutral earthed	Delta	Delta
Delta	Star with neutral earthed	Star	Delta

This, we can conclude that CTs on star side of transformer should be connected in delta and CTs on delta side should be connected in star.

Differential protection scheme Merz-Price protection scheme for delta-delta power transformer.

The scheme for protection of delta/delta power transformer is an shown in Fig. This protection scheme gives protection against phase to ground and phase to phase fault. The CTs on primary and secondary side are connected in star. All the CTs are identical.

During normal working conditions the CT secondary currents are equal and no current flows through the relay coil.

When line to ground fault or phase to phase fault occurs, the currents through the CT secondary’s are no longer the same. A difference current is produced which flows through pilot wires and relay coils. The respective relay coil is thus energized, it sends trip signal to circuit breakers on primary and secondary side. The CBs then operate and transformer is disconnected.

Differential protection scheme/Merz-price protection scheme for delta-star power transformer:

The CTs on delta side of power transformer are connected in star and CTs on star side of transformer are connected in delta as shown in figure.

The working of this scheme is exactly the same as explained for delta-delta power transformer. In the similar way diagram can be drawn for Star-delta transformer. The working of that scheme will be similar.

SOLVED EXAMPLES

Example: A 3-phase, 33/6.6 kV, star delta transformer is protected by Merz-Price circulating current system. If the CTs on the low voltage side have a ratio of 300/5. Determine the ratio of CTs on the high voltage side.

Solution: Suppose current flowing in secondary side of transformer is 300 Amp. This current is primary current for the CTs connected to delta side of transformer. The secondary current of the CT will be 5 Amp. Since ratio is 300/5.

(Note: For both side CTs the primary side is single turn i.e. bar type or line conductor itself acting as primary winding.)

For differential protection scheme, when operating conditions are normal, the CT secondary currents flowing through pilot wire from both group have to be equal. Refer Figure we have drawn it for the CTs only. The CT1, CT2, CT3 are in primary side of transformer and these are connected in delta and CT1, CT2, CT3 are in secondary side of transformer and these are connected in star.

The pilot wire current = CT secondary current (connected a delta side of transformer)

                                   =  5 Amp.

The CTs connected in delta should also send 5 Amp. In the pilot wire i.e.  same current.

So current induced in the CTs secondary should be 5/√3 Amp.(For delta circuit, line current is √3 times phase current.

Thus the CTs connected in delta have a current of 5/√3 Amp in their secondary.

(Note: For both side CTs the primary is bar type i.e. single turn or the line conductor)

Primary apparent power         = Secondary apparent power

                             √3V₁I₁         = √3V2I2

           √3 x (33 x 1000) x I₁    = √3 x (6.6 x 1000) x 300

We have already assumed that secondary carries 300 Amp.

                                        I₁      = 60 Amp

This is primary current of main transformer and it is primary current of CT on HV side.

So Primary current of HV side CT = 60 Amp

Secondary current of HV side CT = 5/√3 Amp.

                                                Ratio = 60: 5/√3 or 60/(5/√3)

Example 2: A 3-phase Transformer of 220/10000 line voltage is connected in star/delta. The protective transformers on 220 V side have a current ratio 600/5. What should be the CT ration on 11000 V side?

Solution: The connection diagram will be similar to example A.

Suppose the line current on 220 V side is 600 Amp. i.e. CT primary current (I₁) is 600 Amp. The CT secondary i.e. phase current in delta connected CTs current will be = 5 Amp…. (Therefore the ratio is 600/5). The current coming out in pilot wires (i.e. line current) from delta connected CTs is 5√3 … (for delta connection IL - √3I_ph).

Under normal condition, same current is supposed to flow in pilot wires from 11000 V side CTs. Therefore, secondary current of star connected CTs is 5/√3 Amp.

Primary apparent power = Secondary apparent power

                        √3V₁I₁  =          √3V₂I₂

           √3 x 220 x 600 =          √3 x 11000 x I₂

                            I₂      =          12 Amp.

This is line current of secondary of main transformer which is primary current for star connected CTS.

                       Primary current         12

So CT ratio = ---------------------- =     ----- or 12: 5√3

                     Secondary Current     5√3

Difficulties in differential protection scheme:

1.    CT ratio error during short circuit : This is because of inherent characteristics of individual CT.

2.    The difference in pilot wire lengths causes current difference.

3.    During short circuit condition, the CT core may saturate due to high flux density.

Percentage differential protection: To increase reliability of differential protection percentage differential protection or biased differential protection is used.

Fig. Shows, arrangement of biased differential protection for delta – Star power transformer.

The percentage differential relay has an operating coil and restraining coil in each phase. The operating coil is connected between mid-point to restraining coils and neutral pilot wire.

Under normal working conditions, the currents in pilot wires fed from CTs are equal and no differential current flows through operating coil. When fault occurs in the transformer winding, the balance is disturbed and differential current flows through operating coil and parts of restraining coil. Trip signal is thus produced and CB is operated.

OVER CURRENT PROTECTION

                                   

When cost of differential relying is not justified, over current protection is provided. Following relays are used for over current protection. Instantaneous over current relay, inverse time relay, directional over current relay, static over current relay, thermal relays.

HRC fuses, shows dropout fuses are also used up to 11 kV.

The three over current relays R₁,R₂, R₃ are connected in CTs secondary circuit.

When fault occurs in the protected zone, the CT secondary current increases and it operates the circuit breaker.

This scheme is used for short circuit faults, phase faults, earth faults.

EARTH FAULT PROTECTION

When the fault current flows through earth return path, the fault is called earth fault. The earth faults area frequent. So earth fault protection is important.

Residual earth fault relay: When earth fault does not take place, the sum of three secondary currents of CT is zero.  i.e. I_R +I_Y + I_B =0.

The sum of these currents is called residual current. When earth fault develops, the sum of three currents is not zero, the residual current flows through earth fault relay.

This protection is also called unrestricted earth fault protection. In another scheme, the relay is connected in neutral of the transformer.

The neutral acts as CT primary. When fault occurs, the fault current flows through this earth connection and secondary current inc4reases causing earth fault relay operation. The amount of neutral current flowing does depend on type of earthing system provided.

This is also called unrestricted earth fault protection.

                                               

Earth fault Protection with Core Balance Current Transformer (CBCT)/zero Sequence CT: A core balance CT is used in this scheme. The core of CBCT has large cross-sectional area so that it does not saturate when fault current produces high flux density in it.

The core embraces the three conductors R, Y, B. Under normal condition, the components of fluxes produced by three currents are balanced and CT secondary current is negligible and relay does not operate.

When earth fault develops the balance is disturbed and sufficient current is induced in CT secondary thus relay is operated.

RESTRICTED EARTH FAULT PROTECTION

The previous schemes for earth fault come under unrestricted category i.e. the scheme may operate for a fault beyond the transformer winding.

If a fault(F1) occurs, which is beyond transformer it causes current I1 and I2 flow through CT secondary’s and resultant current through R (relay) is negligible and it does not operate.

If fault F2 occurs, current I2 flows and I1 is negligible so I2 flows through R and relay is operated.Thus, this scheme can distinguish between the faults that are away from transformer winding and within transformer winding.

                                               

INTER TURN FAULT PROTECTION

The normal differential protection scheme can protect the alternator against phase to ground or phase to phase faults. It cannot protect the alternator from inter-turn faults. This is because current entering and leaving the alternator winding remains the same. Even though some turns of any phase are shorted. So CT secondary currents do not change and operating coil does not carry any current.

The inter-turn faults later on develop earth fault, so inter-turn fault must be detected at an early stage.

This scheme is used for multi-turn alternators. For multi-turn alternator, the winding of each phase is divided into two equal sections (S₁ and S₂). Two identical CTs are connected in respective windings. The CT secondary's are connected in phase opposition and a relay R is connected in CT secondary loop.

The functioning of this scheme is similar to differential protection principle.

Under normal operating conditions, the currents through S₁ and S₂ are same, so CT secondary’s carry same current. And no current flows through relay R.

If the adjacent turns of S₁ are shorted, currents shared by S₁ and S₂ differs hence there is differ3ence in CT secondary currents. The difference current flows through the loop and relay now carries current and produces trip signal for CB.

THERMAL OVERHEATING PROTECTION OF LARGE TRANSFORMER

Temperature sensors are kept in the vicinity of transformer winding. Thermocouples, resistance temperature detectors can be used. These are arranged in the bridge circuit, when temperature rises above safe value, the bridge balance is disturbed and voltage is available across two points of that bridge. This voltage signal can be used to operate the alarm or to operate the circuit breaker.

Some typical settings for oil temperature are as follows:

            Switch ON fans          :60⁰C

            Alarm ON                    : 95⁰C

            CB trip                         : 120⁰C

Oil Thermometer: It is commonly used for oil filled transformer. It is equipped with alarm contacts to give indication of excessive increase in temperature. Accordingly signal can be processed to make the fans ON.

However, thermometer is not a reliable fault detecting device because it does not detect winding temperature correctly. This is because the transformer oil has much longer time constant than windings themselves i.e. winding temperature heats up the oil, oil temperature increases and this oil temperature is measured by thermometer.

So by the time thermometer measures the oil temperature, actual winding temperature is much higher.

BUCHHOLZ RELAY

It is gas actuated relay. It is connected in the pipe installed between transformer tank and conservator.

                                               

Construction: It consists of a domed vessel. There are two mercury switches. The upper mercury switch is attached to a float. This float is hinged. The lower mercury switch is attached to the flap and the mechanism in hinged.

Alarm circuit contacts are brought to upper mercury switch and trip circuit contacts are brought to lower mercury switch.

The upper mercury switch operates during incipient faults and lower mercury switch operates in case of severe faults.

                                               

Working: Under normal working conditions, the position of mercury switches. It is seen that the alarm circuit contacts and trip circuit contacts are not bridged by liquid mercury in the switch. In case of incipient faults, the heating oil occurs, the oil decomposes and gases are formed. The majority portion of gases is hydrogen (70%). This gas is accumulated at the topside near domb. Due to gas pressure, the float comes down and mercury level shifts closing the alarm circuit contacts. Thus alarm circuit is complete and alarm sounds.

In case of severe faults, gas formation is rapid and large amount of gas rushes from transformer to Buchholz relay. During its travel, the gas produces sufficient pressure on the flap, the flap turns in anticlockwise direction, the mercury switch also tilts and mercury level inside the mercury switch shifts, closing the trip circuit contact. Thus, trip signal is produced causing operation of circuit breaker.

The Buchholz relay is used for transformer having rating greater than 750 kVA because of economy reasons.

Advantages:

1.    Simple.

2.    Fault detection is faster.

3.    Operation is reliable.

Disadvantages;

1.    Only applicable for oil immersed transformer.

2.    Only the faults below oil level can be detected.

EXERCISE

1.    Write a short not on abnormalities and faults in transformer.

2.    What are the difficulties in differential protection scheme used for transformer?

3.    How  CTs used in differential protection of 3-phase transformer are to be connected?

4.    How over current protection is provided in case of transformer? Explain any one scheme.

5.    Explain earth fault protection using CBCT.

6.    Explain restricted earth fault protection scheme with a neat diagram.

7.    Write a short note on thermal overheating protection for transformer.

8.    Explain any one scheme for inter turn fault protection for transformer.

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