Transformers protection, an introduction

Protection & Coordination Session 11: Transformer protection

Transformer Protection Typical Functions: Differential (87T), Lockout auxiliary (86), Overcurrent and short circuit (50/51), Ground fault (50G). Additional Functions: Directional overcurrent 67, Voltage and Power metering V & S

Transformer Protection Components of electrical power distribution systems: The utility portion up to the service entrance in a plant will, typically, have the following major elements: -Overhead high voltage transmission line (eg. 115kV or 230kV) -High voltage disconnect switches (eg. 115kV or 230kV) -Lightning arresters (station, distribution and maybe riser pole types) -Capacitive voltage transformers -Power transformers (eg. 230kV/27.6kV, 100MVA) -M.V. circuit breakers (main, tie and feeders) -Instrument transformers -Relay/metering panels -Overhead distribution lines and poles -Fuses, insulators & switches -Underground cables, terminations & splices -Distribution transformers (single and three phase) -Reclosers and/or sectionalizers -System control & data acquisition equipment

Transformer Protection From the service entrance boards downstream to the different loads and load centers, the major elements ar e: -M.V. service entrance board or gear complete with breakers or switches/fuses combination -Main distribution transformers (dry or liquid filled eg. 27.6kV/600V) -600V distribution board complete with molded case breakers/switches and/or power breakers -Secondary distribution transformers (600/120/208V) -600V motor control centers -M.V. motor starters complete with motor protective relays -Distribution/lighting panels complete with molded case circuit breakers or fuses -Interconnecting wires/cables and loads (motors, lighting, appliances, office equipment, heating equipment and others)

Transformer Protection Fuses for distribution systems transformers: in distribution systems, three-phase transformers and three phase banks (i.e. 3 single phase connected to provide a delta or a Y 3-phase configuration) are common. In general, the protection of the power transformers is provided through the use of protective relays (o/c or differential and over current ground) and gas relays. The distribution transformers are protected by fuses (current limiting and expulsion types). The distribution transformers are either overhead (pole mounted) transformers or installed in above or below grade vaults or pad mounted. The connection and protection to each type differ significantly. Pad mounts can be classified into radial feed and loop feed. Pole mounted transformers: They have ahead of them current limiting fuses and distribution cutouts with fuse links with speed T or K as defined in ANSI C37.100 other speeds are also available to achieve proper co-ordination between the fuses and upstream/downstream protective devices. There is another type of pole-mounted transformers, which is the completely self-protected one (CSP). Primary fuses and lightning arresters are included with the transformer, thus there is no need to any external protective device except for a current limiting fuse.

Transformer Protection Pad mounted transformers: They will have load or fault sensing (expulsion) type fuses that are accessible from outside the transformer to remove and replace and in series with these fuses are current limiting back-up fuses under the oil and is inaccessible without de-energizing the transformers and removing the transformers from the site and probably breaking the welds of the cover. The partial range current limiting (C.L.) fuse operates without discharging flame, gases or other by-products of expulsive action. This combination of series fuses provides the current-time characteristics of a coordinated full range current limiting fuse (C.L.). C.L. fuse is selected to operate only on internal failure of the transformer (permanent shorts and faults). Vault mounted transformers: A series of current limiting and expulsion type (with power fuses or fuse links) fuses are mounted on the pole or the wall of the vault and are considered the primary protection. Medium voltage fuses (2.4 to 72kV) can be classified according to the following; they are either distribution fuse cutouts or power fuses. The power fuses can further be classified into expulsion type and current limiting. Distribution fuse cutouts were developed for use in overhead distribution circuits (a connection to distribution transformers, supplying residential areas or small commercial/industrial plants). Pole mounted capacitor banks, used for voltage regulation or power factor correction, can be protected by such fuses. A distribution fuse cutout consists of a special insulating support and fuse holder.

Transformer Protection Protection of the transformer in a transformer station: The power transformers can have any of the following connections: Y-delta, delta-YG, Y-D-YG, delta-YG-YG, Y-ZG, Y-ZG-ZG. In order to connect windings of the same transformer in parallel or to connect single phase transformers to form polyphase ones, it is necessary to do the connection taking into account the polarity marks on the transformers. Polarity markings designate the relative instantaneous directions of current in the transformer leads. Transformers, single phase or polyphase have all their leads marked with a standard system of lettering designating the transformer polarity. Faults in transformers: The protection to power transformers (example of rating 75/100/125MVA, 230kV/27.6kV) will be against internal faults and is provided through the use of gas relays (in the transformer tank and tap changer enclosure) and one differential relay (3-phase type). Another differential relay of a different type, redundant (for dependability reasons) may, also, be used. Other faults that may occur and do not warrant a trip but rather an alarm or the operation of a bank of fans are: overload and overheating of the transformer. For over-voltage protection, horn gap protectors, lightning arresters and transformer rod gaps may be used. Overexciting of the transformer may occur and an over-voltage relay may be used to indicate or alarm, rather than to trip. Other devices that are found on transformers in order to indicate or protect against a certain abnormality are: thermometers (foor oil temperature), winding temperature indicating devices (thermometers) and pressure relief devices.

Transformer Protection Thermometers: They indicate top liquid temperature (ambient plus temperature rise of transformer). Thermometers may have a bi-metal spiral in the metal housing, temperature-indicating pointer & drag pointer (resettable). The liquid tight well for the sensing element allows the removal of the thermometer with no further steps. Thermometers may have up to three micro switches, the first for the fans (close @ 70 C, open @ 65 C), the second for the alarm (close @ 85, open @ 80) and the third for the trip (close @ 100, open @ 95). The winding temperature thermometers can be classified into direct or C.T. type. The first type has a capillary tube equipped with porcelain insulator that isolates the sensing bulb from the thermometer. The bulb is usually placed in direct contact with the low voltage bus. The second type consists of thermometer bulb, a resistor or thermocouple inserted in resistance heating element that is energized from a current transformer. The heating element and temperature sensitive device (thermocouple or resistor) are mounted in a dry well on the tank wall; the whole assembly is then immersed in the top liquid. Pressure relied devices: Pressure relief devices are used in sealed transformers. The common types are the diaphragm and the mechanical automatic reseal. The diaphragm is designed to rupture before damaging pressure can build up. After operation, the diaphragm has to be replaced. The automatic reseal type will maintain its seal until the threshold pressure (eg. 8psi + 1) is reached, at that point the valve snaps open, the full operation takes about 2 milliseconds. The device automatically re-closes and reseals when the internal pressure drops to 4 psi approximately. Remote and local indicators to show that the device had operated are, usually, available but not necessarily as a standard part of the pressure relief valve (on the transformer).

Transformer Protection Restricted ground (earth) faults: They may occur when the transformer neutral is grounded through high impedance, special protection is applied. Restricted earth fault protection is applied on transformers in order to detect ground-faults on a given winding more sensitively than overall transformer differential protection is able to do. Restricted earth fault protection – as well as transformer differential protection – is based on the principle of comparison of measured variables by comparing the residual current of the phase current transformers of the given winding with the current of the associated grounded star point. Since residual current may occur due to transient saturation during high throughfault currents restraining is required for restricted earth fault protection. For this purpose two different measuring principles are available: • Biased restricted earth fault protection • High impedance restricted earth fault protection The protection for this application is provided through the use of 4 current transformers (sensors), one in each phase and the fourth in the neutral circuit (between the neutral and the first terminal of the grounding resistor). The over-current relay connected in a differential protection configuration, is connected across the C.T. in the neutral circuit as well as across the other three current transformers (connected in parallel). For external faults the secondary currents will circulate in the current transformers windings and not through the operating coil (circuit) of the relay; while in case of internal faults it will flow through the relay (this approach is also used in generator protection under similar conditions).

Transformer Protection Differential protection: The differential protection is one that operates when the vectorial difference of two or more electrical quantities of the same type exceeds a per-determined value. What makes a protection differential is the way the circuit is connected. The most extensively used current differential relay is the percentage. Such relay has operating and restraint coils or circuits. There are two types of percentage differential relays: the fixed and the variable. The fixed has the following property: the ratio of the differential operating current (the difference of the two secondary currents of the current transformers on the high and low voltage power transformer windings divided by 2) to the average restraining current (the sum of the two secondary currents of the current transformers on the high and low voltage power transformer windings divided by 2) is a fixed value.

Transformer Protection Chracteristics of variable percentage differential relays: the slope of the operating current to the restraint current increases with higher through currents. The current in the operating coil tends to operate the relay; the current in the restraint coil tends to prevent this operation. This design is necessary to restrain the relay from operation for any faults outside the protected zone, for any unbalanced conditions or for the discrepancies in the characteristics of the current transformers, thus, avoiding nuisance tripping. The choice of the percent slope value is function of three factors. The unbalance in the secondary outputs of the differential zone C.T. may be caused by the tap changer of the power transformer. Secondly, there will be an error current due to the mismatch of the C.T. taps; it is based on the average of the restraint currents. Finally, the ratio errors of the current transformers themselves may affect the unbalance of the secondary outputs. All these factors have to be taken into consideration to get the maximum possible percent unbalance. Transformer overexcitation: Transformers (power & distribution) draw a steady state magnetizing current under normal operation (exciting current). This current can vary between 1 and 5% (depending on the design and type of steel used in the core) of the rated full load current of the transformer. Subjecting transformers to over-voltage increases the exciting current significantly and due to the non-linear magnetizing characteristics of the core, harmonics (third and fifth) will be present. A pick-up setting above the steady state exciting current would seem good enough, however, the amount of unbalance and the limited restraint at high emergency through current loads, may dictate a higher

Transformer Protection setting. A dedicated over-excitation protection is provided, when it is desired to protect against over-voltage (for short times). Differential relays can have a fifth harmonic filter to restrain the relay from operation (when overexcited). This will prevent the relay (instantaneous) operation, due to over-voltage. The percentage of the fifth harmonic (due to over-voltage conditions) is approximately 35% of the fundamental. The third harmonic is also present when over-exciting the transformer. Transformer inrush currents: When transformers are energized, a large inrush transient current will flow through the primary of the transformer (and not seen by the secondary windings). Thus, the operating coil of the differential relay will receive currents with high peak values, leading to greater tendency for the relay to operate. The magnitude and wave shape of the inrush current is function of the magnitude (point) of the supply voltage, at which the transformer is energized, the residual flux and its relationship in polarity and magnitude with respect to the instantaneous value of the steady state flux (corresponding to the particular initial energization point) and the ratio of saturation flux density to the operating flux density at rated voltage. The duration of the inrush is affected by the size of the transformer bank and the resistance in the power system from the source to the transformer. The magnitude will be affected by the size of the power system, type of steel used in the transformer core and its saturation density and the residual flux level. For three phase transformers, the electrical connections of the windings and the magnetic coupling between the phases, affect the inrush. The inrush current is rich in second harmonic component; it varies from 20% to 65%. There are

Transformer Protection another two types of inrush currents that occur with transformers, other than the initial, the first is the recovery that is the inrush that occurs after a fault external to the bank has been cleared and the voltage rises from the fault condition to the normal level. The second is the sympathetic inrush, which is the inrush to an already energized transformer while another one in parallel with the first is being energized. Differential relays may have a second harmonic restraint element to prevent the relay operation during transformer energization. Characteristics of current transformers: - Correct secondary currents of the current transformers to reflect the different voltages (and consequently the full load and fault currents) of the secondary of the power transformer to its primary. - If there is a phase shift angle between the secondary and primary, the C.T. connection on each side should compensate for such difference (eg. On the same transformer, for a delta or a zig-zag connected power transformer windings, the current transformers have to be connected in wye; on the other hand for a wye connected power transformer windings, the current transformers are to be connected in delta). - The secondary currents through the differential relay should not cause the relay to operate under external (through) faults or maximum emergency load and should operate the relay for internal faults (should be sufficiently higher than the restraint pick-up level). The C.T. ratio is chosen so that it will give a secondary current close to but less than the nominal rated current of the relay at maximum load condition. Two winding percentage differential relays can be used for three winding transformers, provided the current transformers on the secondary sides of the transformer are connected in parallel. Its advantage is its cost saving.

Transformer Protection In a network where more than one station is fed from the same transmission line through power transformers, a fault in the protected zone of the transformer should trip the breaker or breakers (main) on the low voltage side, remote tripping of the breakers upstream of this transformer (at terminal stations) and all low voltage side breakers (main) of transformers connected to the same line. The load break switch on the high voltage side, connected to the transformer high voltage bushing, is also tripped open. When the switch opens, the remote trip signal will be terminated (because of the disconnect switch interlock used in the remote trip circuit), the terminal breakers and l.v. side breakers connected to the other than the faulty transformer are re-closed. Gas relays: For gas protection, Buchholz (pressure type) relays connected between the tank and the conservator (in the piping) are used. One of the 2 elements of this relay is a gas-collecting chamber. After a certain amount of gas is collected, a contact is closed to sound an alarm. The second element contains a vane that is operated by the rush of oil through the piping. The first is used to protect against slow breakdown of insulation while the second against severe faults occurring inside the transformer.

Transformer Protection An example showing typical calculations & verifications: Provide the differential and gas accumulation/sudden release protection to a 100 MVA power transformer, 220/25KV with +/-16% tap changer. Assume that the available relays have a pick-up setting between 20-50% of relay rating with an adjustable slope of 20-50% and another with fixed slope and restrained pick-up between 20 and 50% and unrestrained pick-up of 8, 13, 20 x relay nominal current. The pressure gas relays have two settings, for the trip 5.2 - 17.2 KPa and for the alarm 200-400 CC. The tap changer gas pressure trip can be set between 35-390 KPa. I excitation = 5% of full load primary, C.T. error = 2.5%, relay rating = 5A, C.T. primary current = 1.5 x full load current of transformer. I secondary = 262 x 220/25 = 2300 A Using a 1200/800/200:5A C.T. on the primary side (the C.Ts are delta connected), 3500 : 5A C.T. on secondary side (the C.Ts are wye connected) of the power transformer. Turns ratio of C.T. on primary winding = 800/5 = 160, on secondary = 3500/5 = 700. Relay current due to primary C.T. at f.l. = (262/160) 1.732 = 2.8 amp, relay current due to secondary C.T. at f.l. = 2300/700 = 3.28 amp, relay current ratio = 3.28/2.8 = 1.17. Mismatch at midpoint changer and full load = 17%. At 220 + 16% = 255KV tap, maintaining secondary voltage at 25KV, primary full load current = 100 (1000)/255 (1.732) = 226 amp, relay current = 226/160(1.732) = 2.45amp.,voltage of 25 KV on the secondary while primary = 220 - 16 % = 185 KV primary full load current = 100(1000)/185(1.732) = 312 amp., relay current = 312/160 (1.732) = 3.4 A

Transformer Protection Mismatch for + 16% = 3.28/2.45, mismatch for -16% = 3.28/3.4 which are 34 % and -4 %, respectively. The maximum mismatch = 34 %, add 6 % as safety margin. Thus the slope adjustment = 40 % (range is 20 to 50 %). The pick-up level under full load current = inaccuracies + exciting current + allowance for the limited restraint at emergency load through currents = 2.5(5/100) + (1.732)(5(262)/160)(100) + (3.28-2.45) = .125 + .14 + .83 = 1.1 amp, the pick-up setting = 40%(5) = 2 A (range 20 to 50 %). The unrestrained instantaneous triping current = 13 x 5 = 65 amp. secondary relay current. VERIFICATIONS: Assuming a 100 MVAbase and 220/25 KVbase, Ibase (@ primary side) = 100/(1.732)(220) = 262 amp., Ibase (@ secondary side) = 2300 amp. Assuming an infinite source, 11% impedance transformer, a 3-phase fault on the secondary of the transformer beyond the differential protection zone will produce 9 p.u. fault current (1/.11), Iprimary = 2358 amp., Isec. = 20700 amp. The current from the primary side into the relay = (1.732)2358/160 = 25.5 amp., from the secondary side = 20700//700 = 29.6 amp. A mismatch of 29.6/25.5 = 16 %. Assuming the tap changer to be at 220KV + 16% = 255 KV and the impedance = 14%, the short circuit fault current of a 3-phase fault = 1/.14 = 7.16 p.u., full load primary current = 232 amp., SCC on the primary side = 1661 A, SCC on the secondary side = 16468 A, relay current from primary side = 1661(1.732)/160 = 18 A, relay current from the secondary side = 16468/700 = 23.5, the mismatch = 30%.

Transformer Protection Assuming the tap changer at 220 KV - 16% = 185 KV and the impedance = 8%, a 3-phase fault current = 1/.08 = 12.5 p.u., primary current = 312 A, SCC on primary side = 3900 A, SCC on secondary side = 28750 A, relay current from prim. side = 42.2 A, relay current from sec. side = 41.1 A, mismatch = 3% GAS RELAYS: Main tank alarm = 200 cc, main tank trip = 17 KPa above static head at relay level, tap changer trip = 100 KPa.

Transformers protection, an introduction

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Transformers protection, an introduction