Presentation on Power Transformer & Distribution Transformer Protection

Presented byCh. Alamgir Hossain Deputy General Manager System Protection & Metering Circle PGCB,Dhaka

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Protection of Transformer Requirements of Transformer Protection-To protect the equipment form external and internal faults Type of Transformer Faults: A: Internal Faults 1.Earth Fault 2.Phase to Phase Fault 3.Inter-turn Fault 4.Core Fault 5.Tank Fault- ex. Loss of Oil 6.Slug Formation 2

Protection of Transformer: cont. External Faults: 1. Over Loading – Causes I2R losses 2. System Faults- Causes Mechanical stress to transformer 3. Over Voltage- Transient Over Voltage- LAs are provided - System O/V- Causes Over Flexing which increases Iran losses and damage insulation.

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Protection of Transformer: cont.

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Protection of Transformer: cont. Inrush Current ¾ A transformer steel core's retains a static magnetic field when power is removed. ¾ When power is then reapplied, the residual field will cause a high inrush current until the effect of the remnant magnetism is reduced, usually after a few cycles of the applied alternating current. ¾ Transformer protection devices must be selected to allow this harmless inrush to . ¾ When a transformer is energized, the magnetizing inrush currents are estimated as multiples up to 08 to 10 times of the transformer's rated current. ‰ Factors that affect the Inrush Current ¾The source impedance ¾The size of the transformer ¾The point of wave when the switch closes ¾The magnetic properties of the core ¾The remanence magnetism of the core 5

Protection of Transformer: cont. ‰ Protections provided by external protective relays: ƒTransformer Differential Protection (87T) ƒRestricted Earth Fault (REF) Protection (87N) ƒTime Delayed Over Current & Earth Fault Protection (51/51N) ƒInstantaneous Over Current & Earth Fault Protection (50/50N) ƒThermal Overload Protection (49) ƒOver Fluxing Protection (24) ‰ Mechanical/Self Protection (Provided within the transformer ): ƒMain Tank Buchholz Protection ƒOLTC Buchholz Protection ƒPressure Relief Device (PRD) Protection ƒWinding Temperature Protection ƒOil Temperature Protection ƒOil Level Alarm ƒMain Tank Buchholz Alarm 6

Protection of Transformer: cont. Conventional Protection Scheme of a Two Winding Transformer

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Protection of Transformer: cont. Conventional Protection Scheme of a Two Winding Transformer Including REF

87N

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Protection of Transformer: cont. Conventional Protection Scheme of a Three Winding Transformer

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Protection of Transformer: cont. Conventional Protection Scheme of an Auto-Transformer

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Protection of Transformer: cont. Grouping of protection: As transformer protections are not duplicated. So that protection functions are divided in two groups to obtain some redundancy. 1. Group A Protection 2. Group B protection The group A and group B protection are connected to separate DC source. All the Group-A and Group-B protection functions energize separate lockout relays (86-1 & 86-2) respectively to trip the circuit breaker during fault.

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Protection of Transformer: cont. Generally Group-A protection consists of¾Differential Protection (87T) ¾Time Delayed Over Current & Earth Fault Protection (51/51N)_HV ¾Instantaneous Over Current & Earth Fault Protection (50/50N)_HV ¾Thermal Over Load Protection (49) ¾Main Tank Buchholz Relay Protection

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Protection of Transformer: cont. Generally Group-B protection consists of¾Restricted Earth Fault Protection (87N) ¾Time Delayed Over Current & Earth Fault Protection (51/51N)_LV ¾Instantaneous Over Current & Earth Fault Protection (50/50N)_LV ¾OLTC Buchholz Relay Protection ¾Pressure Relief Device (PRD) Protection ¾Winding Temperature Protection ¾Oil Temperature Protection 13

Protection of Transformer: cont. Transformer Differential Protection (87T) Basics: ƒTransformer differential protection is a unit scheme that compares the current on the primary side of a transformer with that of the secondary side. ƒIf any difference in HV & LV currents exists (beyond the setting value) it is assumed that the transformer has developed a fault and the relay instantaneously trips the relevant circuit breakers. ƒThe principle of operation is made possible by virtue of the fact that large transformers are very efficient and hence under normal operation power-in equals power-out. ƒTransformer differential protection detects faults within the differential protected zone, including inter-turn short circuits. Fig: Zone of Differential Protection 14

Protection of Transformer: cont. Transformer Differential Protection (87T) Principle of Operation: The operating principle employed by transformer differential protection is the MerzPrice circulating current system as shown in the figure. Under normal conditions I1and I2 are made equal and opposite such that the resultant current through the relay is zero. An internal fault produces an unbalance or 'spill' current that is detected by the relay, leading to operation to isolate the fault.

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Protection of Transformer: cont. Transformer Differential Protection (87T) ‰ Transformer Differential 10 Protection (87T) Relay Types: ¾ High Impedance Type ¾ Low Impedance Type ƒ In a low impedance protection scheme, the differential protection can 5 have the protection characteristic set typically with a two slope restraint characteristic. ƒ 87T relays are generally of 1 I d min low impedance type. 1

Differential current |I1+I2 |

< 25 I Unrestrained

High

Trips Blocks 5

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Irestrain (|I1 |+ |I2 |)/2 16

Protection of Transformer: cont. Transformer Differential Protection (87T)

Transformer Differential Protection With Matching CT

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Protection of Transformer: cont. Transformer Differential Protection (87T)

Factors to be considered in selecting 87T relay: ¾ Extremely stable under through fault conditions and magnetic inrush. ¾ Very fast to operate for an internal fault ¾ Able to restrain second harmonics and block fifth harmonics.

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Protection of Transformer: cont. Restricted Earth Fault Protection (87N)

Basics: ¾ Conventional earth fault protection using over-current elements fails to provide adequate protection for transformer. ¾ Restricted earth fault, or zero-sequence differential protection is implemented in transformer star winding. ¾ It’s a unit protection and operation of relay is instantaneous. ¾ It offers a significant improvement in sensitivity over traditional differential protection ¾ It does not respond to load current. 19

Protection of Transformer: cont. Restricted Earth Fault Protection (87N)

Basics: ¾ Ground current in the transformer neutral is used as a reference and is compared to zero-sequence current at the terminals to determine if a fault is internal to the transformer. ¾ The relay is operative for faults within the region between current transformers, that is, for faults on the star winding in question. ¾ The relay will remain stable for all faults outside this zone. ¾ 87N relays are also available high impedance and low impedance type. ¾ Generally 87N relays are of High Impedance type.

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Protection of Transformer: cont. Restricted Earth Fault Protection (87N) Basic 87N Scheme:

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Protection of Transformer: cont. Restricted Earth Fault Protection (87N)

(a)

(b)

Fig. Restricted earth fault protection : (a) neutral earthed within the protected zone (b) neutral not earthed within the protected zone

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Protection of Transformer: cont. Time Delayed/Instantaneous Over Current & Earth Fault Protection (50/50N & 51/51N)

¾Used on all feeding circuits of transformer ¾Provide back up for internal faults ¾Also provide back up for system faults ¾Instantaneous high element ¾51/51N can be definite time or inverse time to achieve proper relay co-ordination from upstream to downstream

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Protection of Transformer: cont. Thermal Overload Protection (49)

¾The transformer winding hot-spot temperature is another quantity that should be used for protection of transformers. ¾Protection based on winding hot-spot temperature can potentially prevent short circuits and catastrophic transformer failure, as excessive winding hot-spot temperatures cause degradation and eventual failure of the winding insulation. ¾During over load conditions excessive load current through the transformer causes over heating of the winding and insulating oil. 24

Protection of Transformer: cont. Thermal Overload Protection (49) • To prevent damage of the winding insulation thermal overload protection is used. • It is basically a over current protection implemented in both windings of the transformer • Tripping is time delayed. First alarm is generated then finally tripping occurs.

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Protection of Transformer: cont. Thermal Overload Protection (49) Rise of temperature

Trip

Alarm

τ

Time 26

Protection of Transformer: cont. Over Fluxing Protection (24)

Transformer over-fluxing can be a result of • Overvoltage • Low system frequency ¾A transformer is designed to operate at or below a maximum magnetic flux density in the transformer core. ¾Above that design limit the eddy currents in the core and nearby conductive components cause overheating which within a very short time may cause severe damage. ¾The magnetic flux in the core is proportional to the voltage applied to the winding divided by the impedance of the winding. 27

Protection of Transformer: cont. Over Fluxing Protection (24) • The flux in the core increases with either increasing voltage or decreasing frequencyEMF = E = 4.44fΦmN =>E/f = 4.44 ΦmN • During startup or shutdown of generator-connected transformers, or following a load rejection, the transformer may experience an excessive ratio of volts to hertz, that is, become overexcited. • Overexcited transformers become overheated and damaged • Over fluxing protection is specially required for Generator Transformers

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Protection of Transformer: cont. Mechanical Protection ¾There are several mechanical protection relays installed on transformers . ¾Transformer mechanical protection relays operate by sensing operational parameters like oil pressure, oil level, gas evolved, oil & winding temperature. ¾Generally these relays are built in features of power and distribution transformers having capacity more than 10MVA.

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Protection of Transformer: cont. Mechanical Protection: cont. Buchholz Relays (63):

¾Buchholz relay is a mechanical protection device for monitoring the gas and oil movements in oil immersed transformers. ¾It is used on practically all power transformers with the exception of small distribution transformers. ¾In addition with the main tank Buchholz relay another Buchholz relay is mounted on OLTC (On Load Tap Changer). ¾Location of main tank Buchholz relay is given as follows:

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Protection of Transformer: cont. Buchholz Relay: contd. Fig. Buchholz Relay

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Protection of Transformer: cont. Buchholz Relay: contd. ¾The internal mechanism of a Buchholz relay mainly comprises two floats. ¾During normal operation, the relay is completely filled with oil keeping the floats in their top limit or reset position. ¾The mechanisms in the relays respond to: ƒ Slight faults causing a slow evolution of gas in the transformer (e.g. overheating). ƒSerious faults creating an immediate surge of oil (e.g. shortcircuits etc.). ƒUnattended Oil leakage may lead to operation of the Buchholz relay. 32

Protection of Transformer: cont. Buchholz Relay: contd. Schematic Diagram of a conventional Buchholz Relay Arrangement

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Protection of Transformer: cont. Buchholz Relay: contd.

Schematic Diagram of a modern Buchholz Relay (Oil Surge) Arrangement

An oil-surge detection feature of the Buchholz relay will trip the upstream circuit-breaker “instantaneously” if a surge of oil occurs in the pipe connecting the main tank with the conservator tank. Such a surge can only occur due to the displacement of oil caused by a rapidly formed bubble of gas, generated by an arc of short-circuit current in the oil.

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Protection of Transformer: cont. Buchholz Relay: contd. Schematic Diagram of a Buchholz Relay Mounting Arrangement

Θ = 3-5°

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Protection of Transformer: cont. Buchholz Relay: contd. Buchholz Relay Operation: ¾When a slight fault occurs in the transformer, the small bubbles of gas which upwards towards the oil conservator tank are trapped in the relay housing this causing its oil level to fall. ¾ As a result, the upper float drops and activates the external alarm switch. ¾ If gas continues to be generated then the second float operates the second switch that is normally used to isolate (trip) the transformer. ¾If an arc forms, gas accumulation is rapid, and oil flows rapidly into the conservator which is called oil surge. ¾This flow of oil operates a switch attached to a vane located in the path of the moving oil. ¾This switch normally will operate a circuit breaker to isolate the apparatus before the fault causes additional damage 36

Protection of Transformer: cont. Pressure Relief Device (PRD) Basics: ¾Pressure Relief Device is a safety element of the transformer employs to prevent heavy damages of the tank in the case of sudden rise of the internal pressure. ¾These device has been designed in order to remove the excess pressure in a very short time as soon as the pressure in the tank rises above predetermined safe limit ¾PRD operates and allows the pressure to drip instantaneously and avoids damage to transformer body. ¾During internal faults of a power transformer, there will be an increase of temperature associated with formation of gases, impurities in oil and thus increase in pressure. ¾This pressure is sufficient to damage the transformer. ¾The pressure relief device is applied to prevent the transformer from this danger 37

Protection of Transformer: cont. Pressure Relief Device (PRD): cont. Fig. Pressure Relief Device

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Protection of Transformer: cont. Pressure Relief Device (PRD): cont. ¾The pressure relief device consists of a spring which normally is uncompressed and when the pressure increased in the transformer, the spring get compressed and give a path of gases to go out of the transformer. ¾Compressing the spring will close an electrical , and this will give trip to circuit breakers associated with alarm. ¾Following Figure (shows the pressure relief device in the normal condition (before the fault occurrence):

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Protection of Transformer: cont. Pressure Relief Device (PRD): cont. Following figure shows the fault condition at which the compressed gases get age to let the gases out from the transformer.

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Protection of Transformer: cont. Winding & Oil Temperature Protection Winding Temperature Indicator ¾By making a "Thermal Image" of the winding the Winding Temperature Indicator, simulates the winding temperature. ¾The temperature of the winding depends on the transformer load (i.e. the current through the winding) and the temperature of the cooling medium (the oil). ¾Temperature is measured with a bulb in a pocket. ¾It has a specially designed heating element, to measure the transformer load. ¾This heating element is a thermal model of the winding. ¾The heating element is connected to the current transformer (CT) via a Matching Resistance or a Matching Unit, to allow setting the correct winding temperature gradient. 41

Protection of Transformer: cont. Winding Temperature Protection: cont. ¾Generally winding temperature indicator consists of four s which are normally open and closes in series according to pre set closing value (temperature). ¾These s can be assigned as follows:1.The first is used for automatic operation of first fan group. 2.The second is used for automatic operation of second fan group, this value is higher than the first setting. 3.If the cooling fans are not sufficient to retain the transformer temperature to its normal value, the third is applied to feed alarm circuit. 4.As a last step, the fourth is applied for tripping to prevent the transformer from high temperatures. Normally it trips the load side breaker, (i.e. the secondary side CB) 42

Protection of Transformer: cont. Oil Temperature Protection Oil Temperature Indicator: ¾Oil temperature indicator is similar to winding temperature indicator except that it depends only on the temperature transferred by the bulb (no current transformer is used). ¾This consists only of two s. ¾These s are similar to the third and the fourth s of the winding temperature indicator but with preset values less than winding temperature indicator by approximately 5-10 degrees. 43

Protection of Transformer: cont. Transformer Dehydrating Breather Transformer Dehydrating Breather:

¾A transformer breather is an accessory of an oil filled type transformer which is attached to the oil conservator tank. ¾When the insulating oil of the transformer gets heated up, it expands and goes back to the conservator tank and subsequently pushes the dry air out of the conservator tank through the breather 44

Protection of Transformer: cont. Transformer Dehydrating Breather

• It is filled with some desiccating agent, e.g. silica gel. • When the oil cools down, it retracts and sucks fresh air from the atmosphere through the breather • the silica gel dries up the moisture content of the air that goes back in to the conservator tank. • If the silica gel looses its moisture absorbing capability then the oil in the conservator gets contaminated and eventually losses the insulating property.

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Protection of Transformer: cont. Oil Level Indicator ¾Oil level indicators with magnetic t are usually used on transformers' conservators. ¾It is mounted on the body of the conservator. ¾Its function is to give a visual alarm of the oil level contained in the conservator. ¾Furthermore they are provided with micro switches to signal the alarm in case the oil level reaches its minimum and/or maximum.

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Thanks to All

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