TRANSFORMER FAULTS & PROTECTIONS
For reliable operation, power system should be robust and follow 3S characteristics like: sensitivity, selectivity and speed of the protection system. Protection system should have approach for 360° coverage and beyond for uninterrupted power supply.
- Rajesh Chourishi
A transformer is a critical and expensive link of any power transmission network. Sometimesa minor fault in ‘Powertransmissionsystem elements’may create a major damage. Thus,its consumers may face an extended outage.Since repairing and replacement of faulty elements like transformer consumes substantialtime, we may haveto install astandby transformer in the sub-stations. This is a redundancy strategy, but theprotectionsystem of any transformer isprovided to avoid any damage to thetransformer and to maintain uninterrupted power supply.
Typical Phenomena in Transformers
Magnetizing inrush Current and Over-excitation are the commonphenomena in transformers.
Magnetizing inrush currents
The magnetizing inrush current is developed in the transformer when a transformer is switched ONfrom either side.It is also evident when thesupplyvoltage is restoredafter clearing a line fault.
The magnetizing inrush current has adominant component of 2nd harmonic whichcan be used to diﬀerentiate between inrush currentand a faultcurrent.
Over-excitation or Over-fluxing
Generator transformerunits may be overexcited due to accelerationor deceleration of the turbine during sudden loading or load throw. Transformer over excitation can be a result of overvoltage and low system frequency. An overexcited transformer is not atransformer fault. It is an abnormal networkcondition.Excitation current has adominant component of 5th harmonic current whichcan be used to diﬀerentiate between excitation current and a faultcurrent.
Fault Scenarios and Solutions
Some of the fault scenarios along with their solutions are briefed below. Ignorance of their symptoms may resultin major faults.
It may occurbecause of overloading or failure of forcedcooling units. During adaily load cycle, the temporary loading mayexceed the rated power of the transformer. Since heating time constant of the oil (in response to overloading of the transformer) is very high, transformer temperature cannot reach up to maximum allowable limit instantaneously. However, ifsuch condition is sustained for a long time, then it maycause insulation damage. According toTransformer standards, short time overloading up to 1.5 times the rated loadcan be allowed. Therefore, overcurrent relays cannot be usedfor overload monitoring. Over current relay have to be setabove predicted short time overload but under the damage curve of the transformer.
Underperforming cooling system
Failure of cooling systems may heat oil, deterioratinginsulationwhich reduces life. Also, excessive temperature decomposes oil into gases. Suchprolonged condition may damage the transformer. Therefore it should be kept under close supervision along with an alarm to initiateappropriate actionbefore the transformer becomes overheated.
Poor Quality of Oil and leakage
Thereliability of transformer depends on the dielectric strength of the oil. The dielectric strength of the oil is the mostimportant property of the oil. Insulation will be deteriorated if thedielectric strength of oil isreducedby thepresence of moisture andother impurities.The oil leakages and level must be monitored regularly. If theoil level is low, a breakdown in insulation may occur. To avoid such situationSilica gel breather and an oil levelmonitor is installed with transformer oilconservator.
Performance of Silica Gel Breather
The silica gel type breather is used to supply dryair to the oil conservator. The silica gel has the capability to absorb moisture up to 20% of its weight. Change of color from blue to pale pink gives anindication of its saturation, and it is timeto replace.
Deterioration of insulation
With time and temperature, transformer insulation deteriorates. It is not possible to accurately predict the length of life under widely varying service conditions. However, the part of the winding, operating at the higher temperature will deteriorate fastwhen compared to another part of winding.,Thus either by improving the transformer cooling or by reducing the load, fast deterioration of insulation can be avoided.
Failure of Insulation
Insulation failure is also an alarming situation. It may occur due to the following reasons.
Poor quality of oil
Aging effect on insulation
Over current due to external faults
Corona discharges in the insulation
Formation of Gas
To avoid theformation of gasses; overheating of thetransformer due to overloading, over fluxing, and flow of heavy fault current should be avoided.
Pressurized gas in the tap-changer compartment gives an indication of overheating or breakdown/fault inside the transformer. Circuit breakersare tripped,using a pressure relay,before thedevelopment ofexcessive pressure.
Venting of Pressurized Gas
Venting of over pressurized gasesindicates a flashover or a short-circuit in an oil-filledtransformer. With the help of a pressure relief valve, overpressure can be released.The valve closes automatically when the pressure is restored to normalcy.
Oil level Indicator
Too low or highlevel of oil in oil conservator indicates some abnormal condition with the transformer. Oil level indicator shows the oil level and has contacts for minimum and maximum oil level alarm.
Classification-Faults and Protections
Faults in the power system cannot be prevented, but with the help of power system automation and fast protective relayoperations, major damage in the power system elements can be limited.
In a span of its life, thetransformer has to withstand variousInternal faults, External faults along with different type of abnormal operating conditions. These can be classified as shown below.
Faults in the transformer’s elements are considered as internal fault while those happen outside the transformer are regarded as external faults. Some of the faults appear in the transformer due to anabnormal condition of the network.
Figure 1:-Classification of Faults in Transformer
Transformers internal faults
From all thefaults described above, only some important type of the faults and their protectionare explainedbelow.
It is a flashover between the conductors of thesame phase.It occurs due to thedevelopment of mechanical forces andlocal overheatingbetween the conductors during theflow of high fault current, which damages the insulation between the conductors. It createsarcingand decomposes oil into gases.This fault would be detected by “Rate-of-rise of pressure Relay” and “Gas Detector Relay.”
It is a fault between two different phases. Since It does not have any relation with theground, therefore its magnitude is very high, and it is controlled only by source impedance and leakage impedance of the transformer.
Earth faults in a delta-connected winding
The magnitude of the earth-fault current depends on the earth resistance, fault impedance and the location of the fault within the delta winding. If afault occurs at the mid-point of the delta winding, fault impedance would be maximum and fault current would be equal or less than rated current. If a fault occurs close to one end of the delta winding, fault current would be approximately equal to phase-to-ground fault current.
Figure 2:- Earth current according to position of the faults in a delta-connected winding
Earth faults in Star secondary winding.
Since, reactance decreases towards the neutral, therefore fault current closer to neutral will be more as compared to the fault at end-point of the winding. In the following figure, avariation of fault current with respect to neutral is shown.
Figure 3:-:- Earth current according to theposition of afault in star winding.
Earth fault in impedance earthed star secondary winding.
In this situation, the fault current is controlled by the grounding impedance and winding impedance connected between grounding impedance and the position of thefault. The primary current is approximately proportional to the square of the fraction winding short-circuited by fault position and neutral of the winding. In the following figure, avariation of fault current with respect to neutral is shown.
Figure 4:-EarthFault Current in Impedance earthed Star secondary winding
Figure 5:-EarthFault Current in Solidly earthed Stars secondary winding
Protection system for internal faults
The differential protection system is applied to protect the transformers from internal faults.
Transformer differential protection system
Principle and characteristics of differential protection system for the transformersare described below.
A transformer differential protection relay compares the incoming current to the protected zone with the outgoing currentfrom the protected zone and senses the flow of unbalanced currents. The protective zone of a differential relayincludes power transformer, buses, and cables between the currenttransformers of both sides. Therefore,adifferential relay has a largeprotective zone. In the absence of a fault in the protected zone, this unbalance tends to be small or zero. For the operation of the differential current relay, thedifferential current through the relay should be more than the setting of the relay.
To provide differential protection for all three-phase power transformer variants, it is necessary to provide following types of compensation.
Current Magnitude Compensation
Phase Angle Shift
Zero Sequence Current Compensation
In theoperational condition of a transformer, thedifferential relay must be ableto handle, Magnetizing inrushcurrent,Internal faults, External faults and Over-flux or Over-excitation of the transformer.
In the following cases of Differential Protection; apart from common characteristics only important characteristics are described.
Differential Protection of two-winding standard power transformers
i) Star-star power transformer: -
In this configuration phase between current and voltage is 0° or 180°. Current Compensation is also needed. Both CTs have to be connectedin“Delta.”
Figure 6:-Differential Protection for Star-star standard Transformer
i) For Delta-star power transformer
Delta star connection in power transformer introduces ±30° phase shift in balanced voltage and current. Therefore, to diminish this phase shift, CTs on theDeltatransformer winding are connected in Star,and the CT on the Star transformer winding areconnected in Delta, as shown in the fig given below.Advanced relays have logics for internal Delta Star compensation.
Figure 7:-Differential Protection for Delta-star standard Transformer
Differential protection for autotransformers
In thecaseof Auto-transformers CTs secondary’s connections for earth fault protection alone, and combined phase and earth fault protection are different.
For theprotection of auto-transformer alsocurrent compensation is required.If tertiary winding of Auto-transformeris not loaded and it isused as a delta-connected equalizer windingthus, it is protected as a two-winding power transformer. In some cases, tertiary delta winding is used to provide reactive power compensation. Thus, it is protected as thethree-winding power transformer.
Figure 8:-YNd5 connected Auto-transformer with loaded tertiary winding
Figure 9:- YNd5 connected Auto-transformer with unloaded tertiary winding
Protection of 3-winding power transformers
The protection of three winding transformersis complicated by the fact that line CTs for each winding are usually based on different MVA levels and will not themselves achieve balance under through current conditions. To achieve current balancing,interposing CTs would be required.The relay Unit used in this scheme have three restraining coils and one operating coil.
Figure 10:-ThreeWinding TransformerFigure 11:-ThreeWindingTransformer(One power source)(Three power source)
Differential Protection for Special Converter Transformers
Numerical differential protection relay canprotect standard three-phase power transformers without any external interposing CTs.However, in thecase of special converter transformer Numerical relay will be unable to set additional, non-standard angle of phase shift, caused by special winding connections. It would be able to compensate phase shift nearest standard transformer vector group only.Consequently, a permanent false differential current would be usedin circulation.
For a 15°phase shift,a false differential current up to 26% of the ‘through-load current’will be in circulation. Therefore, a minimum pickup for differential protection has to set at least twice of this value. Thus the relay for low-level internal fault will not besensitive.
Transformer:External faults and protections
Transformer faults(External)compriseof phase faults and earth faults.
Phase fault protections
Some popular protection configurations with relays are briefed below.
With significant difference between the fault levels, overcurrent relay is not suitable for backup protection. In this condition ‘Under impedancerelay’ can be used. It can operate for theminimum short-circuit level.
Distance relays can be used as aback-up for the transformer differential relayinstead of non-directionalunder-impedance relays
Time-overcurrent relay is applied to the feeder of all power transformers. It provides backupto all the differential protection and other protective relays on the load- side of the transformer. In the absence of differential protection,it actsas a primary overcurrent protectionrelay.Timeovercurrent relays with an instantaneous element for high-fault currentsare used in each phase.
Earth fault protections
Power transformers with impedance or solidly earthedneutral can be equipped with different types of earth faultrelays to protect the earthed winding. Some popular protection configurations with relays are briefed below.
Low-impedance residual overcurrentrelay
Thisrelay can beconnected to acurrent transformer connected either in phase for residual connection or in neutral earth fault. The relay can operate for ground faults as well as for magnetizing inrushcurrent if it contains a zero sequence component. Therefore, therelay must have a delaymore than the duration of the inrush current.
High-impedance restricted relay
This relayprovides a sensitive high-speedrestraint protection. The relay is used insolidly grounded and impedance grounded networks. Its setting forfault current should be above the sensitivity of the relay.
Low-impedance restricted relay
Sometimes it is possible to use a low impedance current relay as a grounddifferential relay instead of a high impedance relay. The current transformer in theneutral can then have a different ratio of thephase CTs since an auxiliary CT can be usedfor ratio correction.If a phase CT gets saturated during a short-circuit, the relay may operate. Thereforethe relayshould be released by a residualvoltage relay.
Residual voltage relay
A residual voltage relay connected to potentialtransformers connected (in broken delta) measures the neutral displacement for anyground fault. In solidly or effectively grounded networks, it can be used as abackup ground fault relay. It can be set to operate in case the grounding of thenetwork is lost and ‘residual current relays’fails to operate.
Harmonic restraint overcurrent relay
This relay provided with a second-harmonic restraint. Since It is stable for the inrush current, therefore, its time setting is independent of the duration of the inrush current.
In this system, theTank is connected to ground through a current transformer. An instantaneous overcurrent relay is connected to the current transformer. The relay operates for ground faults inside the tankand flashovers on the bushings.
Flashover and earth fault protectionsfor low voltage Rectiformeror frequencyconverters.
To protect damage due to flashover between theprimary and secondary winding of the transformer. Aninstantaneous voltage relay is to be connected to a voltage transformer.Itis already connected between the neutral point of the low voltage system and ground.If a ground-fault occurs on thedcsystem with Rectifiers or frequency converters connected to the low voltagesystem. The voltage transformer will get saturated by dc and get damaged. Therefore, voltage transformer is not allowed with rectifiers/ converter. In such cases, the voltage transformer mustbe replaced by aresistor to limitthe voltage. If a ground fault occurs inthe low voltage network, itwill bedetected by a miniature circuit breaker (MCB)in the relay circuit
Bird’s eye view-Faults and applicable Protections
Different type of transformersfaults and applicableprotection are depicted below.
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