Industrial Power Transformers -- Operation and maintenance [6b]

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<<cont. from part 1

Circulating current protection

FIG. 92 shows an explanatory diagram illustrating the principle of the circulating current system. Current transformers (which have similar characteristics and ratios) are connected on both sides of the machine winding and a relay is connected across the pilot wires between the two current transformers.

FIG. 92 Explanatory diagram to illustrate principle of circulating current protection: (a) healthy condition or external fault; (b) internal fault condition and (c) illustrating equipotential points

Under healthy or through-fault conditions, the current distribution is as shown in Fig. 92(a), no current flowing in the relay winding. Should a fault occur as shown in Fig. 92(b), the conditions of balance are upset and current flows in the relay winding to cause operation. It will be noted that in FIG. 92(b) the fault is shown at a point between the two current transformers (the location of these determine the extent of the protected zone). If the fault had occurred beyond, say, the right-hand current transformer, then operation would not occur as the fault current would then flow through both current transformers thus maintaining the balance, as shown in FIG. 92(a).

In order that the symmetry of the burden on the current transformers shall not be upset and thus cause an out-of-balance current to pass through the relay, causing operation when not intended, it is essential that the relay be connected to the pilot wires at points of equipotential. This is illustrated in FIG. 92(c), such equipotential points being those as a and b, a1 and b1 , etc. In practice it is rarely possible to connect the relay to the actual physical mid-point in the run of the pilots and it is usual to make the connection to convenient points at the switchgear and to insert balancing resistances in the shorter length of pilot wire. The resistances should be adjustable so that accurate balance can be obtained when testing before commissioning the plant. Some complications arise when circulating current protection is applied to a power transformer because a phase shift may be introduced which can vary with different primary/secondary connections and there will be a magnitude difference between the load current entering the primary and that leaving the secondary.

Correction for a phase shift is made by connecting the current transformers on one side of the power transformer in such a way that the resultant currents fed into the pilot cables are displaced in phase from the individual phase cur rents by an angle equal to the phase shift between the primary and secondary currents of the power transformer. This phase displacement of the current transformer secondary currents must also be in the same direction as that between the primary and secondary main currents.

The most familiar form of power transformer connection is that of delta/ star, the phase shift between the primary and secondary sides being 30º. This is compensated by connecting the current transformers associated with the delta winding in star and those associated with the star winding in delta. In order that the secondary currents from the two groups of current transformers may have the same magnitude, the secondary ratings must differ, those of the star-connected current transformers being 5 A and those of the delta-connected group being 2.89 A, that is 5/_3.

If the power transformer is connected delta/delta, there is no phase shift between primary and secondary line currents. Similarly, there is no phase shift in the case of star/star-connected power transformers, but phase correction is applied at both sets of current transformers, the reason being that only by this means can the protective system be stable under external ground fault conditions. Thus, both sets of current transformers will be delta connected so that the secondary currents in the pilots from each set will be displaced in phase by 30º from the line currents but both will coincide, a necessary requirement of circulating current protection. It is obvious that similarity in phase could be achieved if both sets of current transformers are connected in star, but it can be shown that, in this case, the protective system would be stable on through faults between phases but not for ground faults. This is demonstrated numerically in FIG. 93, noting that in FIG. 93(a) the secondary currents entering and leaving the pilots are not the same at both ends and therefore do not sum up to zero at the relays, whereas in FIG. 93(b) the reverse is true and no current appears in the relay coils. The 2:1:1 current distribution shown in FIG. 93 on the ungrounded side of the transformer pertains only to such a transformer with a closed-delta tertiary winding. This winding is not shown in the diagram. Its function is to provide a short-circuit path for the flow of harmonic components in the magnetizing current. The distribution applies also when the core is a three-phase type as opposed to shell type.

FIG. 93 Showing stable and unstable conditions on through ground faults, with circulating current protection applied to a star/star transformer, due to methods of connecting current transformers.

The switching in of a power transformer causes a transient surge of magnetizing current to flow in the primary winding, a current which has no balancing counterpart in the secondary circuit. Because of this a 'spill' current will appear in the relay windings for the duration of the surge and will, if of sufficient magnitude, lead to isolation of the circuit. This unwanted operation can be avoided by adding time delay to the protection but, as the in-rush current persists for some cycles, such delay may render protection ineffective under true fault conditions. A better solution may lie in the use of harmonic restraint, and relays of this type are shown in Figs 97-99.

FIG. 94 is a demonstration diagram of connections of a three-phase, delta/star-connected transformer equipped with circulating current protection and shows the distribution of the short-circuit fault currents arising from a winding fault to ground on the star-connected winding, when the neutral point of the latter is solidly grounded. The current phasor diagrams drawn for a one-to one ratio, corresponding to the conditions of FIG. 94, are given in FIG. 95 in which the phasors have the following significance.

FIG. 94 Circulating current protection for a three-phase delta/ star-connected transformer, showing operation under internal ground fault conditions

FIG. 95 Current phasor diagrams corresponding to the conditions of FIG. 94

FIG. 95(a): IA; IB; IC are the normal balanced load currents in the primary delta-connected power transformer windings.

IR; IY; IB are the normal balanced load currents in the primary main lines.

IAF is the short-circuit fault current in the power transformer primary winding A2A1 and in the line R corresponding to the fault current Iaf, set up in the short-circuited portion of the power transformer secondary winding over a2a1.

Its magnitude is such that the ampere-turns given by IAF multiplied by the total number of turns in the primary winding A2A1 equal the ampere-turns given by the fault current Iaf in the short-circuited portion of the secondary winding a2a1 multiplied by the number of secondary turns short circuited.

The phase angle fp of IAF with respect to the normal voltage across A2A1 is given by the expression cos_1 (Rp/Zp) where Rp is the resistance of the primary winding A2A1 plus the resistance of the short-circuited portion of the secondary winding a2a1 and Zp is the impedance of the short-circuited portion of the secondary windings a2a1 with respect to the whole of the primary windings A2A1, all quantities being referred to the primary side. _IAF is the short-circuit fault current in the line B, and is IAF in the line R, but flowing in the reverse direction to IAF with respect to the line R.

IA _ IAF is the total current in the winding A2A1, that is the phasor sum of the load current and the fault current in the winding.

IR _ IAF is the total current in the line R, that is the phasor sum of the load current and the fault current in the line.

IB _ IAF is the total current in the main line B, that is the phasor sum of the load current and the fault current in the line.

FIG. 95(b): Ir; Iy; Ib, are the normal balanced load currents in the secondary star-connected power transformer windings and in the secondary main lines.

Iaf is the short-circuit fault current in that part of the power transformer secondary winding a2a1 between the grounded neutral and the winding ground fault.

Its magnitude and phase angle fs, with respect to the normal voltage across the winding a2a1, are determined by the impedance of the short-circuited portion of the secondary winding a2a1 with respect to the whole of the primary winding A2A1, and by the resistance Raf of the short-circuited portion of a2a1. The magnitude of Iaf is given by the expression Vaf/Zaf, where Vaf is the normal voltage across the short-circuited portion of the winding a2a1 and Zaf is the impedance referred to earlier in terms of the secondary side of the transformer. The phase angle fs, with respect to the normal voltage across a2a1, is cos_1 (Raf/Zaf).

FIG. 95(c): iR; iY; iB are the normal balanced currents in the star-connected secondary windings of the current transformers and in the lines connected thereto on the primary side of the power transformer. They are the currents due to the normal balanced load currents in the primary power lines R, Y, B.

iAF is the fault current in the current transformer secondary winding over MALA and in the line V connected to it, and corresponds to the current IAF in the primary power line R.

_iAF is the fault current transformer secondary winding MCLC and in the line X connected to it, and corresponds to the current _IAF in the primary power line B.

iR _ iAF is the total current in the current transformer secondary winding MALA and in the line V connected to it, that is the phasor sum of the currents due to the load current and the fault current in the primary power line R.

iB _ iAF is the load current in the current transformer secondary winding MCLC and in the line X connected to it, that is the phasor sum of the current due to the load current and the fault current in the primary power line B.

The relative angular displacements between the currents of FIG. 95(c) are the same as those of FIG. 95(a).

FIG. 95(d) ia; ib; ic, are the normal balanced currents in the delta-connected secondary windings of the current transformers on the secondary side of the power transformer. They are the currents due to the normal balanced load currents in the secondary power lines r, y, b.

iv; iw; ix are the normal balanced currents in the lines to the delta-connected secondary windings of the current transformers on the secondary side of the power transformer. They are the line currents corresponding to the currents in the current transformer secondary windings which are due to the normal balanced load currents in the secondary power lines r, y, b.

This diagram bears no fault current phasors, showing that no fault currents flow through the current transformers on the secondary side of the power transformers.

The currents which flow through the protective relays are thus the fault currents iAF and _iAF of FIG. 95(c), the magnitudes of which depend, for a given power transformer, upon the amount of the power transformer winding short circuited and its position with respect to the whole winding on the other side of the power transformer.

So far no mention has been made of the problem which arises when a power transformer is provided with facilities for tap changing. It has been noted that for stability under healthy or though-fault conditions, identical outputs from each group of current transformers are an essential feature of circulating current protection. It is clearly impossible for the current transformers to be matched at all tap positions unless these (the CTs) are also correspondingly tapped. This solution is generally impracticable if only because of the nature of the task of changing current transformer tappings each time a tap change is made on the power transformer. The latter function is often automatic so that it would then be necessary to make the tap changes on the current transformers automatic and simultaneous. Because of this and the normal inequalities which occur between current transformers, many schemes for the protection of transformers have been devised in which steps have been taken to eliminate the difficulties and some of these schemes will be noted later. Tap changing and current transformer inequalities can be largely avoided by using a circulating current scheme which employs a biased differential relay, indicated typically in FIG. 99.

In each pole of this relay, there are, in addition to the operating coil, two bias or restraining windings. Under through-fault conditions, when operation is not required, no current should flow through the operating coil but, because of imperfect matching of the current transformers, and the effects due to tap changing, some spill current may flow in the operating coil. This, however, will not cause operation unless the ratio of operating to bias current for which the relay is set is exceeded and the restraint or bias which is applied automatically increases as the through-fault current increases, thus enabling sensitive settings to be obtained with a high degree of stability.

To understand the operation of the bias coils, consider the protective system firstly under through-fault conditions (i.e. a fault outside the protected zone), and then under internal fault conditions:

(a) Through-fault conditions: if a three-phase short circuit occurred on the feeder side of the system beyond the circuit breaker the current circulating in the pilot wires would pass through the whole of the relay bias coils, and any out-of-balance current which might occur due to discrepancies in the ratios of the protective current transformers would flow through the relay operating coil. Under these conditions the biasing torque predominates, so preventing relay operation.

(b) Internal fault conditions: imagine now a three-phase fault at the power transformer terminals on the star-connected side and that the power flow is as shown in FIG. 96. Fault current flows through the three current transformers designated A on the delta-connected side of the power transformer but not through the set B on the star side. Therefore, the current transformer secondary currents circulate via the pilot wires, through one-half of the bias coils and the operating coils back to the current transformer neutral connection. Under these conditions the relay operating torque pre dominates. The protective system operates correctly when the transformer is fed from either or both directions and for all types of faults.

FIG. 96 Biased differential protection applied to a delta/star connected three-phase transformer

C.B. circuit breaker; C.T. protective current transformer;

T.C. trip coil; C.B.A. circuit breaker auxiliary switch;

B.C. bias coil; O.C. operating coil;

P.F. & L. protector fuse and link; B. battery.

High-speed protection of power transformers by biased differential harmonic restraint

For many years the GEC Type DMH relay has provided differential protection for two-winding or three-winding power transformers with a high degree of stability against through-faults and is immune to the heavy magnetizing cur rent in-rush that flows when a transformer is first energized. The relay is avail able in two forms:

(a) for use with line current transformers with ratios matched to the load cur rent to give zero differential current under healthy conditions;

(b) with tapped interposing transformers for use with standard line current transformers of any ratio.

In this relay the preponderance of second harmonic appearing in the in-rush current is detected and is used to restrain its action, thus discriminating between a fault and the normal magnetizing current in-rush. The relay employs rectifier bridge comparators in each phase which feed their outputs through transistor amplifiers to sensitive polarized relays, resulting in:

(i) an operating current which is a function of the differential current;

(ii) a restraining current, the value of which depends on the second harmonic of the differential current;

(iii) a bias current which is a function of the through-current and stabilizes the relay against heavy through-faults.

The relay is provided with an instantaneous overcurrent unit in each phase to protect against faults heavy enough to saturate the line current transformers, under which conditions the harmonics generated would tend to restrain the main unit. These overcurrent units have a fixed setting of 8 times the current transformer secondary rating and are fed from saturable current transformers to prevent operation on peak in-rush current which may momentarily exceed this value.

The operation of the main unit is briefly as follows:

Under through-current conditions, current is passed by the two restraint rectifier bridges through the polarized relay in the non-operating direction.

In conditions of internal fault there will be a difference between primary and secondary current, and the difference flows in the operating circuit so that the operating rectifier passes a current to the polarized relay in the operative direction. Operation depends on the relative magnitude of the total restraint and differential currents, and the ratio of these currents to cause operation is controlled by a shunt resistor across the restraint rectifiers. Under magnetizing in-rush conditions, the second-harmonic component is extracted by the tuned circuit and the current is passed to the relay in the non-operating condition.

In addition to the second-harmonic component, the in-rush current contains a third-harmonic component, its proportion being large but less than the second. No restraint against the third harmonic is provided as there would be danger that the relay might be delayed in operating under heavy internal fault conditions, due to the current transformer saturation producing third harmonics in the secondary waveform.

Figures 97 and 98 show typical application diagrams for three-phase two-winding, and three-phase three-winding transformers.

FIG. 97 Typical application of GEC Type DMH biased differential harmonic restraint relay for a three-phase, two-winding transformer (GEC Measurements)

Duo-Bias differential transformer protection

Another development, basically of the conventional current-balance scheme already discussed but using a special relay compensated to override the complications associated with transformer protection, is that by Reyrolle Protection. This is shown in FIG. 99. It is a diagram of their 'Duo-Bias' relay scheme applied to a single phase, and functioning under various conditions as follows:

Under load or through-fault conditions, the current transformer secondary currents circulate through the primary winding of the bias transformer, the rectified output of which is applied to a bias winding on a transductor via a shunt resistor. Out-of-balance current flows from the center tap on the primary winding of the bias transformer, energizing the transductor input winding and the harmonic-bias unit.

FIG. 98 Typical application of GEC Type DMH biased differential harmonic restraint relay for a three-phase, three-winding transformer (GEC Measurements)

The input and output windings of the transductor are inductively linked but there is no inductive linking between these and the bias windings. So long as the transformer being protected is sound the transductor bias winding is energized by full-wave rectified current which is proportional to the load or through-fault current, and this bias current saturates the transductor.

Out-of-balance currents in the transductor input winding, produced by power transformer tap changing or current transformer mismatch, superimpose an alternating m.m.f. on the DC bias m.m.f., as shown in FIG. 100 but the resulting change in working flux density is small and the output to the relay negligible.

The tappings on the shunt resistor are used for adjusting the relationship between the bias transformer primary current and the input to the transductor bias winding. This resistor also serves to suppress the ripple in the bias m.m.f. due to ripple in the bias current, because it provides a low impedance non-inductive shunt path across the highly inductive bias winding for the AC content of the bias current.

If the power transformer develops a fault, the operating m.m.f. produced by the secondary fault current in the transductor input winding exceeds the bias m.m.f., resulting in a large change in working flux density which produces a correspondingly large voltage across the relay winding, and the resultant cur rent operates the relay. Operation of the relay cannot occur unless the operating m.m.f. exceeds the bias m.m.f., and as the m.m.f. is proportional to the load or through-fault current, the required operating m.m.f. (and hence the operating current) is also proportional to the load or through-fault current.

The harmonic-bias unit shown in FIG. 99 is a simple tuned circuit which responds to the second-harmonic component of the magnetizing current.

When magnetizing in-rush current flows through the relay operating circuit the rectified output of the harmonic-bias unit is injected into the transductor bias winding and restrains the relay.

FIG. 99 Duo-Bias transformer protection, single-phase diagram (NEl-Reyrolle Protection)

FIG. 100 Fluxes due to operating and biasing ampere-turns (NEl-Reyrolle Protection)

Transformer differential relays generally have a basic setting which is the fault current required to operate them with no through-current in the differential system and internal fault current fed from only one set of current transformers. In the case of the Duo-Bias relay, this is 20 percent of the relay rating. The actual value of the fault current at which the differential relay will operate is thus the basic setting value under no-load conditions but when load current is flowing the setting will be higher, depending upon the amount of load and the bias setting in use. With an internal ground fault in which the cur rent is limited by a neutral-grounding resistor, the load current might well be little affected by the fault and, therefore, when considering such a condition, the effect of load current on the setting should be taken into account.

FIG. 101 Duo-Bias protection for a two-winding transformer (Reyrolle Protection)

FIG. 101 shows a diagram for a three-phase assembly of Duo-Bias relays applied to the protection of a two-winding transformer. When applied to a three-winding transformer, the relay is identical except for a change of tapping on the primary winding of the bias transformer. Further details of this type of protection are given in The J & P Switchgear Book (Butterworths).

Opposed-voltage protection

The essential difference between this and the circulating current scheme is that under normal conditions no current circulates in the pilot wires, the e.m.f.s generated at either end of the pilots being balanced against each other. This is basically the well-known 'opposed-voltage' scheme, a typical arrangement of which is shown in FIG. 102. This particular scheme is known as 'Translay' and was developed originally by Metropolitan-Vickers Electrical Co. Ltd (subsequently GEC Measurements).

The two diagrams illustrate the operation of the protection for through-fault conditions, and for internal fault conditions. This scheme is also more fully described in The J & P Switchgear Book, which refers particularly to feeder protection, but in general applies as well to transformers.

Overcurrent and ground leakage protection

As indicated earlier, it is not always economical to fit circulating current protection for the smaller sizes of power transformers up to, say, 1000 kVA (and in some cases larger than this). Adequate protection can be provided by means of simple overcurrent and ground fault relays, the latter preferably of the restricted form on the LV side.

A typical diagram is shown in FIG. 103 where it will be seen that the HV side comprises three overcurrent and one ground leakage relays, while the LV arrangement is similar with the addition of a neutral current transformer if the power transformer neutral is grounded. With this type of protection no balancing of current transformers on the primary and secondary sides of the power transformer is necessary, and hence similar characteristics and definite ratios are unnecessary. Further, the ground leakage relays are instantaneous in operation, and ground fault settings as low as 20 percent can usually be obtained without difficulty. Line to line faults are dealt with by the overcurrent relays, which operate with a time lag and are graded with the overcurrent relays on other parts of the system.

For ungrounded windings (delta or star) the apparatus would consist of a three-pole overcurrent relay of the inverse, definite minimum, time lag type and a single pole instantaneous ground leakage relay with or without series resistor depending on the type of relay. This is shown at the left-hand side of FIG. 103 and by the full lines at the right-hand side: this is the overcurrent and plain ground leakage system of protection.

FIG. 102 Translay protection applied to a transformer feeder (GEC Measurements)

FIG. 103 Overcurrent and unrestricted ground fault protection of a three-phase delta/star-connected transformer.

If the power transformer neutral point is grounded, as shown dotted at the right-hand side of FIG. 103, an additional current transformer is provided in the neutral connection with its secondary winding in parallel with the three line current transformers; this protection is known as the overcurrent and restricted ground leakage system. With an external ground fault (say to the right of the current transformers on the star-connected side of the power transformer), current flows in one of the line current transformers and in the neutral current transformer and the polarities are so arranged that current circulates between the two secondaries. The ground leakage relay is thus connected across equipotential points; no current flows in it, and it does not operate. With an internal ground fault, fault current flows either in the neutral current transformer only, or in opposition in the line and neutral current transformers; the relay is then energized and operates.

To balance the line and neutral current transformers with external ground faults, a dummy balancing impedance equal to the impedance of one of the overcurrent elements is connected in series with the neutral current transformer as shown in FIG. 103 so that the burdens of the line and neutral cur rent transformers are equalized. FIG. 104 shows in diagrammatic form the current distribution for restricted ground fault protection for faults inside and external to the protected zone.

Dealing next with the question of protection against interturn faults within the transformer, it has already been stated that such faults are more likely to occur in the HV windings and therefore it is only necessary as a rule to install protective gear on the HV side. When, however, the LV side of the transformer is designed for a voltage which is higher than normal, the degree of susceptibility of the windings to interturn insulation failure is comparable to that of HV windings, bearing in mind, of course, the influence of the type of circuit, that is overhead lines, underground cables, or merely short connecting leads, to which the windings are connected.

FIG. 104 Diagrams showing restricted ground fault protection for transformers

Restricted ground fault protection: high-impedance principle

The current balance scheme will only protect a transformer against ground faults within the area between the current transformers, hence the title 'restricted ground fault protection.' The major difficulty experienced with the scheme is that of retaining stability on through-faults when unequal saturation of the cur rent transformers occurs during the first few cycles after the fault zero.

This is overcome by using a high-impedance relay, which has a high-value stabilizing resistor in its circuit, such as the Reyrolle Protection type 4B3 relay illustrated in FIG. 105. The relay element is AC energized via a full-wave rectifier in series with the setting resistors R1 to R7. The non-linear resistors M1 and M2 limit the peak output voltages of the current transformers and protect both the relay components and the current transformers. The capacitor C together with the resistors R1 to R7 form a low-pass filter which ensures that the primary fault setting of the scheme at harmonic frequencies will be greater than the setting at the fundamental frequency.

FIG. 105 Circuit diagram of high-impedance relay Reyrolle Protection Type 4B3 relay (NEl-Reyrolle Protection)

FIG. 106 shows the diagrammatic representation of a high-impedance restricted ground fault current balance scheme used with a three-phase, two winding transformer. The performance of the relays can be calculated with certainty for both stability and fault setting, and the voltage setting adjusted by means of the links across the resistors which are marked in volts on the face of the relay.

FIG. 106 High impedance restricted ground fault protection scheme applied to a three-phase, two-winding transformer (Reyrolle Protection)

The stability of the scheme depends on the voltage setting being greater than the maximum voltage which can appear across the relay under a given through-fault condition. Assuming the worst case condition that one CT is fully saturated, making its excitation impedance negligible, then the maximum voltage Vmax is given by V N RR max () __ 1 CT L

where I is the maximum steady-state through-fault current

N is the current transformer turns ratio

RCT is the current transformer secondary winding resistance

RL is the pilot loop resistance.

The fault setting is calculated in the usual manner taking the excitation cur rents of the current transformers in parallel with the relay:

Primary fault setting _ N(I0 _ I1 _ I2 _ I3) where N is the current transformer turns ratio

I0 is the relay operating current

I1; I2; I3 are the current transformer excitation currents.

This scheme is unaffected by load, external fault and magnetizing in-rush cur rents. It will protect a winding which has a solidly grounded neutral but not if it is grounded through a resistance.

Replacement of electromagnetic relays

As explained earlier in this section, the types of electromagnetic relays described have been the basic means of providing protection for electrical plant and equipment for more than 60 years. With the electronic revolution of the 1980s these are gradually being replaced by more sophisticated types of relays, initially utilizing transistor circuitry, a few small microprocessors and more recently these have totally changed to microprocessor technology. These modern devices continue to perform the same tasks, taking signals from current and voltage transformers in the circuits being protected, but these signals, instead of causing a disc to rotate or an armature to be attracted, are processed by amplifiers, comparators or digital processors in order to produce the necessary trip signal to the controlling circuit breaker. The principles of protection remain unchanged, but the following description of modern biased differential protection relays gives an indication of the effects which recent developments have had on the equipment involved.

FIG. 107 Modern Static differential protection relay (GEC Measurements)

The present-day GEC Measurements equivalent of the DMH relay is the MBCH shown in FIG. 107. This is from their Midos range and was introduced in the mid-1980s. FIG. 108 shows the functional block diagram, from which it will be seen that the philosophy of operation is basically similar to that of the DMH. The outputs from each bias restraint transformer T3 to T5, proportional to the primary line currents, are rectified and summed to produce a bias restraint voltage. Any resulting difference current is circulated through transformers T1 and T2. The output from T1 is rectified and combined with the bias voltage to produce a signal which is applied to the amplitude comparator. The comparator output is in the form of pulses which vary in width depending on the amplitude of the combined bias and difference voltages. Where the measurements of the interval between these pulses indicate less than a preset time, an internal fault is indicated and a trip signal is initiated after a short delay, the magnitude of which is set by the bias. If, during this delay, the instantaneous value of differential current falls below the threshold and remains below for longer than a further preset time, as it would during transformer magnetizing in-rush conditions, the trip timer is reset and operation of the relay blocked.

FIG. 108 Block diagram: biased differential protection relay type MBCH 13 with three biased inputs (GEC Measurements)

An unrestrained high-set circuit, which monitors the differential current, will override the amplitude comparator circuit and operate the relay output element when the difference current is above the high-set setting.

Even under normal operating conditions, unbalanced currents, spill current, may appear. The magnitude of the spill current depends largely on the effect of tap changing. During through-faults the level of spill current will rise as function of the fault current level. In order to avoid unwanted operation due to spill current and yet maintain high sensitivity for internal faults, when the difference current may be relatively small, the variable percentage bias restraint characteristic shown in FIG. 109 is used. The setting Is is defined as the mini mum current, fed into one of the bias inputs and the differential circuit to cause operation. This is adjustable between 10 and 50 percent of rated current.

FIG. 109 Typical percentage bias characteristic (GEC Measurements)

The initial bias slope is 20 percent from zero to rated current. This ensures sensitivity to faults while allowing a 15 percent current transformer ratio mis match when the power transformer is at the limit of its tapping range, plus 5 percent for CT ratio error. Above rated current, extra errors may be gradually introduced as a result of CT saturation. The bias slope is therefore increased to 80 percent to compensate for this.

At the inception of a through-fault the bias is increased to more than 100 percent. It then falls exponentially to the steady-state characteristic shown in FIG. 109. The transient bias, matches the transient differential currents that result from CT saturation during through-faults, so ensuring stability. However, during internal faults this transient bias is suppressed to ensure that no additional delay in operation is caused.

The most significant change in operating philosophy made possible by the use of more elaborate electronic circuitry is the method of providing restraint during magnetizing in-rush conditions. The relay makes use of the fact that the magnetizing in-rush current waveform is characterized by a period during each cycle when little or no current flows, as shown in FIG. 110. By measuring this characteristic zero period, the relay is able to determine whether the difference current is due to magnetizing in-rush current or to genuine fault current and thereby inhibit operation only during the in-rush condition. This technique enables operating times to be speeded up even during periods of significant line CT saturation.

FIG. 110 Typical magnetizing in-rush waveforms (GEC Measurements)

The relay can also discriminate against increases in magnetizing current which can occur under conditions of sudden loss of load from the system.

Such sudden loss of load may cause a 10-20 percent increase in voltage at the input terminals of the transformer until such time as tapchangers or other system voltage control equipment is able to respond. This might briefly lead the transformer into saturation with a resultant large increase in exciting cur rent which will be seen only by the input line CTs. However, exciting currents resulting from saturation have a waveshape, as shown in FIG. 111, which also has a period during each cycle for which the current remains close to or at zero. By detecting this in a similar manner to that used to identify magnetizing in-rush current, the relay is able to remain inoperative to this over excitation current. It should be noted that where large and potentially damaging over excitation currents can occur, for example following tripping of the EHV side of a large generator transformer while it remains connected to the generator on the LV side, separate overfluxing protection should be installed. This will be discussed further in Section 7.1.

FIG. 111 Magnetizing current with transformer overfluxed (GEC Measurements)

The relay also incorporates an unrestrained instantaneous high-set feature to provide very fast clearance of heavy internal faults. This instantaneous feature has an auto-ranging setting, normally low at normal load throughput, but rising to a higher value under heavy through-fault conditions. This will not trip on magnetizing in-rush current provided the first peak of this does not exceed 12 times the rated r.m.s current.

FIG. 112 shows a typical application using three MBCH 12 relays to protect a delta/star-connected transformer and using an additional restricted ground fault relay connected into the differential circuitry, in association with a current transformer connected into the transformer neutral. Supplementing the differential protection by a restricted ground fault relay in this way can be beneficial, especially when the transformer neutral is grounded via a current limiting resistor which limits ground fault current to a maximum of about normal full-load current.

FIG. 112 Typical connection diagram for MBCH 12 relays protecting a DY 1 transformer with integral restricted ground fault relay (GEC Measurements)

The GEC Measurements' logical successor to the Midos range of protection relays is their 'K Range' introduced in the mid-1990s. These are truly microprocessor based which makes possible the facility for 'communication' between the relay and computer-based SCADA systems concerned with plant monitoring, thus greatly reducing the extent of operator involvement. For transformer-biased differential protection the appropriate relay is the KBCH. Settings can be input into the K Range relays by means of a keypad on the relay face and these are displayed by a liquid crystal display. Where the relay power supply is non-secure, relays are available which derive their energy from the fault current to provide for circuit breaker tripping. The greatly increased amount of information which can be output from the relay if used in conjunction with a central data logging computer includes, for example, a post-incident log which can be of great assistance in fault investigation and diagnosis.

The gas- and oil-actuated relay

The gas- and oil-actuated (Buchholz) relay has been used extensively in the UK for disconnecting a transformer from the supply upon the occurrence of an interturn fault or any other internal failure which generates gases in sufficient quantities to operate the device and to actuate the controlling circuit breaker.

The modern transformer is a very reliable piece of electrical equipment and however infrequent breakdowns may be, they must be guarded against and all possible steps taken to maintain continuity of supply. Any means of indicating the development of a fault within the transformer, particularly in the incipient stages, may avoid major breakdowns and sudden failure of the power supply.

The gas-operated relay is designed for this particular duty and depends for its operation on the fact that most internal faults within the transformer generate gases. The service record over many years shows clearly that the relay is extremely sensitive in operation and that it is possible to detect faults in their incipient stages, thus minimizing damage and saving valuable time in effecting the necessary repairs. The gas-operated relay can only be fitted to transformers having conservator vessels, and is installed in the pipeline between the transformer and its conservator tank. The relay comprises an oil-tight container fitted with two internal elements which operate mercury switches connected to external alarm and tripping circuits. Normally, the device is full of oil and the elements, due to their buoyancy, rotate on their supports until they engage their respective stops. An incipient fault within the transformer generates small bubbles of gas which, in passing upwards towards the conservator, become trapped in the housing of the relay, thereby causing the oil level to fall. The upper element rotates as the oil level within the relay falls, and when sufficient oil has been displaced the mercury switch contacts close, thus completing the external alarm circuit.

In the event of a serious fault within the transformer, the gas generation is more violent and the oil displaced by the gas bubbles flows through the connecting pipe to the conservator. This abnormal flow of oil causes the lower element to be deflected, thus actuating the contacts of the second mercury switch and completing the tripping circuit of the transformer circuit breaker, so disconnecting the transformer from the supply.

Gas within the device can be collected from a small valve at the top of the relay for analysis and from the results obtained an approximate diagnosis of the trouble may be formed. Some of the faults against which the relay will give protection are:

(1) core-bolt insulation failure (2) short-circuited core laminations (3) bad electrical contacts (4) local overheating (5) loss of oil due to leakage (6)ingress of air into the oil system These would normally initiate an audible or visible alarm via the upper element, while the following more serious faults would trip the transformer from the supply:

(a) short circuit between phases (b) winding ground fault (c) winding short circuit (d) puncture of bushings Typical values of the oil velocity required to operate the lower element under oil surge conditions and the volume of gas required to operate the upper alarm element are given in Table 6.

Table 6

A view of a dismantled double-element relay is shown in FIG. 113 and the recommended arrangement for mounting the relay is shown in FIG. 114. It is essential when designing the transformer tank that all gas rising from the transformer shall pass into the relay pipe and not collect in stray pockets, for other wise an accumulation of gas would delay the operation of the alarm float. For testing purposes, a test valve is provided on the relay for connection to a source of air supply. A suitable testing equipment comprises a small air vessel with a pressure gauge and a suitable length of rubber tubing. The air chamber is filled to a pressure of approximately 42 g/mm^2. Slow release of the air to the relay operates the upper float while quick release causes the tripping float to operate.

When transformers are to be installed in countries subject to groundquake tremors, mining blasting effects or traction applications, a relay having magnetically operated reed switches instead of mercury type should be specified.

Interturn failures All types of coils are liable to interturn insulation failure, and the order of susceptibility may be given as crossover, continuous-disc and spiral coils. A purely interturn fault is distinguished by localized burning of the conductors of the coil affected, and often by extensive charring of the interturn insulation of the coil; distortion of the conductors is not a feature of a true interturn insulation fault. Severe coil distortion is direct and positive evidence of an external short circuit across the whole or a major portion of the winding.

It is generally the case that an initial interturn insulation failure does not draw sufficient current from the line to operate an ordinary overload circuit breaker or even more sensitive balanced protective gear. The transformer will, in fact, only be disconnected from the line automatically when the fault has extended to such a degree as to embrace a considerable portion of the affected winding. This may take one of the forms shown in FIG. 115 in which the fault is confined strictly to the winding in FIG. 115(a), while in FIG. 115(b) it burns through to ground in the incipient stage of the failure.

FIG. 113 Gas- and oil-actuated relay dismantled to show the position of the elements and test jet (Weir Electrical Instruments)

FIG. 114 Arrangement for mounting the gas- and oil-actuated relay

FIG. 115 Interturn insulation failures: (a) winding insulated from ground and (b) winding neutral point grounded

If the fault occurs on the primary winding the short-circuited turn acts as an autotransformer load on the winding, and the reactance is that between the short-circuited turns and the whole of the affected phase winding. If the fault takes place on the secondary winding the short-circuited turns act as an ordinary double winding load, and the reactance is that between the short-circuited turns and the whole of the corresponding primary phase winding.

The following example gives an idea of the relative order of magnitudes of the quantities involved.

Tests were carried out on a typical step-down 250 kVA, 50 Hz, three-phase, core-type transformer. The design data were as follows: HV phase voltage, 2800 V; LV phase voltage, 237 V; volts per turn, 7.38; turns per HV phase winding, 380; turns per LV phase winding, 32; normal impedance, 3.25 percent; normal reactance, 3.08 percent; axial length of each HV and LV phase winding, 16.4 in. The HV winding on each phase consisted of a total of 380 turns and tapping points were obtained at 28 intervals of 16 turns and two intervals of 12 turns. Both ends of each tapping point were brought out for testing so that they could be short circuited. Impedance tests were made, first short circuiting one tapping section only at a time, starting at the top and working down the core limb, taking each consecutive interval in turn and, subsequently, short circuiting different series and parallel groups of tappings up to eight in number, at various positions throughout the entire length of the limb.

This made it possible to plot impedances, primary line currents and currents in short-circuited winding sections against the relative position of the short circuited turns in the complete winding and the number of winding sections short circuited. Tests were also made, applying voltage to the HV winding and to the LV winding, to simulate the conditions of a fault on the primary or on the secondary winding. In all cases the current in the short circuit was the normal full-load current of the HV winding, namely, 29.8 A.

Figures 116- 118 give some of the results of this particular series of tests. They are fairly self-explanatory and show how the position and number of the short-circuited turns affect the primary current drawn from the line. The illustrations apply to the case where the fault occurs on one phase of the primary windings, which, for this series of tests, were star connected.

FIG. 116 Curves showing the relationship between primary line current and average position of short-circuited turns in a typical three-phase, core-type transformer

FIG. 117 Curves showing the relationship between the percentage impedance and average position of short-circuited turns in a typical three-phase, core-type transformer

FIG. 118 Curves showing the relationship between the current in the short-circuited turns and the average position of the short circuited turns in a typical three-phase, core-type transformer

It will be seen from these curves that when relatively few turns are short circuited, on the one hand extremely large currents flow in the short-circuited turns, while relatively small currents are drawn from the primary lines, and at first glance these appear to be opposing facts. They are easily reconciled, how ever, when it is pointed out that the high currents in the few short-circuited turns are due to the low impedances between those turns and the primary winding, while the smallness of the current drawn from the primary lines is due to the high ratio of total primary turns to short-circuited turns. As the number of turns short-circuited increases, the impedance increases (up to a point) and the current in the short-circuit decreases, while the ratio of turns cited above decreases and more current is drawn from the primary lines.

It will be noted that, bearing in mind the numbers of turns short circuited, the impedances shown by FIG. 117 really are very high relative to the nor mal transformer impedance, and this is accounted for by the relatively high reactance produced by the dissymmetry between the primary winding and the short-circuited turns.

It is to be borne in mind that the minimum number of turns short circuited in these tests was 16, and that they were all in series. In the usual interturn fault, first one turn, then a second turn and so on are short circuited in parallel, in which case impedances are lower than those shown in FIG. 117, short circuit currents are higher, and primary line currents are lower. The usual result of this is thus: severe local burning out of the faulty turns, small primary line currents, but no untoward distortion of the windings.

The following major conclusions may be drawn from the data obtained from the tests, bearing in mind that the maximum portion of winding short circuited was limited to one-third of the total winding on one limb.

For a given number of turns short circuited the impedance is a minimum when the axial center of the turns coincides with the axial center of the winding, and the line current is then a maximum for that number of turns; the variation of impedance throughout the length of the winding increases with the number of turns short circuited. Impedances increase with the number of turns short circuited, and the increases are greatest when the short-circuited turns are at the ends of the winding. For a given number of turns short circuited, the current in the short-circuited turns is highest when the axial center of the turns coincides with the axial center of the winding; the short-circuit current decreases with increasing number of turns. The primary line current increases with an increase in the number of turns short circuited as for a given increase of the latter the impedance increase is proportionately less, so that the resulting ampere-turns in the short circuit are greater. The turns in the whole winding are constant, and therefore the line current increases proportionately to the short-circuit ampere-turns.

The characteristics disclosed by the curves apply generally to single-phase and polyphase transformers however the windings may be connected and for faults on the primary or secondary winding. Currents and impedances are of the same order of magnitude for similar interturn faults on either winding of a given transformer.

Line currents and phase voltages become unbalanced to a degree depending upon the extent of the winding fault and the transformer connections.

The curves illustrate clearly the reason why an initial breakdown of inter turn insulation, involving a few turns only, fails to operate automatic protective gear, and they demonstrate that the supply can be interrupted only when sufficient turns are embraced by the fault to provide sufficient primary current to operate the protective equipment.

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