118 NATIONAL POWER SYSTEMS CONFERENCE, NPSC 2002
Transformer Differential Relay – The Utility
Perspective
Sanjoy Mukherjee and Rajarsi Ray
The power transformer is one of the most important carry the inrush current without blowing;only in the event of
links in a power transmission system.It also possesses the an internal fault did the fuse blow and permit the relay to
greatest range of characteristics and certain special features operate.
which make complete protection difficult.A review of the
conditions must always be done before detailed application of
protection is considered.The protection engineer has to
believe in the fact that if a suitable protection cannot be
designed for an electrical system, it is better not to have it.
A differential system can be arranged to cover the Kick-
R
complete transformer;this is possible because of the high fuse
efficiency of transformer operation and the close equivalence
of ampere-turns developed on the primary and secondary Fig1. Connection of kickfuse across relay coil.
windings.The basic considerations for application of The drawback of this technology was the formation
differential protection to a three phase transformer are of high resistance at the fuse base thereby causing
transformer ratio, connection, tap changers, magnetising maltrippings during switching.
inrush and harmonics.Whereas the first three considerations This technology was soon followed in the seventies
can be easily taken care of,the main challenge is posed by by induction pattern relays of the I.D.M.T. type which
inrush current and presence of harmonics. provided suitable time delay during switching
In general the transformer primary currents do not conditions.Two induction electromagnets operated on a
equal their secondary currents and the secondary connections single disc to produce opposite torques.A small time delay in
do not correspond to those of the primary.In order that the operation was produced by an appropriate movement of the
current flowing through the relay operating coil should nearly disc in combination with the braking action on the disc by a
equal zero during normal operating conditions and when permanent magnet.One such I.D.M.T. relay even employed
external short circuits appear, it is necessary to do everything CTs at two different stations with bottom coil of relays
to have the secondary currents of the current transformers on connected via pilot wires thus giving differential protection
the transformer primary and secondary sides of equal order to the feeder as well as the transformer.The prime
and coinciding in phase.This is acheived by accordingly disadvantage of using this low-set relay was the low speed
selecting the CT ratios,having the method of connection of operation under fault conditions.
the CTs made in conformance with the vector group The need for quicker fault clearance time gradually
connection of the three phase power transformer and by the developed relays with immunity to magnetising inrush
use of additional current balancing units in the scheme. currents.The current curve during the magnetising inrush
Current balancing in differential protection circuits contains pronounced harmonics, whereas that due to an
were initially done in the sixties with the aid of intermediate
autotransformers connected in parallel with the operating Main CT Main CT
coil.The ratio of the autotransformers were calculated from
the ratio of actual star and delta side currents.However, the
whole idea of development of transformer differential
protection hovers around the idea of tackling the magnetising BIAS
inrush phenomenon and the restraint feature provision to
various harmonics.
The magnetising inrush produces current input to the
energised winding which has no equivalent on the other sides
of the transformer.The whole of the inrush current CT1
appears,therefore, as unbalance and superficially is not
distinguishable from internal fault current.Since the
phenomenon is transient, stability can be maintained by
Filter
providing a small time delay.The scheme during the late
sixties employed an instantaneous relay shunted by a fuse,
CT2 OP
better known as the kickfuse.The fuse was chosen so as to
Rajarsi Ray and
Sanjoy Mukherjee, CESC limited, 4,Sashi Sekhar Bose Row,Kolkata.
Fig 2.General internal arrangement.
INDIAN INSTITUTE OF TECHNOLOGY, KHARAGPUR 721302, DECEMBER 27-29, 2002 119
internal fault condition,is sinusoidal.Advantage was taken of The use of traditional second harmonic restraint to
this fact in the design of the relay,which was restrained by all block the relay during switching conditions resulted in a
the harmonic frequencies but operated under fault conditions, significant slowing of the relay operation during heavy
when fundamental frequency was predominant.This design internal faults due to the presence of second harmonics as a
was structured sometime in the eighties. result of saturation of the line CTs.To overcome this, one
manufacturer came up with a new waveform recognition
In Fig.2 CT1 is the auxiliary CT for bias coils while technique during the mid nineties for detecting magnetising
CT2 is the same for operating coil.Note that bias coil is in inrush.The inrush current waveform is characterised by a
parallel to the harmonic filter circuit. period of each cycle, simultaneously in all the three phases,
An acceptor circuit,tuned to the fundamental where its magnitude is very small (nearly zero).By measuring
frequency was connected in series with the relay operating the time of this period of low current, an inrush condition was
coil, the voltage developed across this tuned circuit being identified.
connected through a resistor to energise the relay restraining
coil.For fundamental frequencies, the tuned circuit, having
low impedance, effectively shorted the bias coil while for
harmonic currents, the tuned circuit, having high impedance, A
passed a high portion of the operating coil current through the
bias coil.The ratio of turns of operating and bias coils ensured
adequate restraint during switching surges.These generation B
of relays had settable bias settings and a fixed Second
Harmonic restraint setting.This core design was employed by
many manufacturers, who switched technolgies from a C
mixture of electromechanical and electronic to a fully static
one.Over-excitation, resulting from over-voltages due to
sudden tripping of major loads or underfrequencies caused
heavy magnetising currents and often such relays have ZERO CROSSINGS
proved to cause inadvertent trippings. Fig3. Waveform recognition technique.
Necessity being the mother of invention, the early From our utility point of view, on different
nineties saw a reputed manufacturer come up with the occassions, power transformers located at unmanned
solution of detecting over-excitation by measuring fifth distributing stations, needed to be switched ON load
harmonic component of differential current.A fixed following underfrequency trippings.On several occassions, it
percentage of fifth harmonic restraint was introduced as an was observed that these relays had caused unwanted trippings
added feature to the relay. during switching on load.Load currents, being superimposed
Another interesting feature, which tookover the with the inrush waveforms, caused the zero crossings to shift,
construction of trip characteristics of differential relays thereby causing unnecessary trippings.This proved to be the
during this period was the method employed by disadvantage of this make of relays.
manufacturers to take into account the saturation of current Another feature of transformer differential relays
transformers.A study of the issue of CT saturation, which came hand-in-hand with the various developments was
particularly at higher end of through faults, showed that CT the high-set feature.If current exceeds the highset value, the
transient errors increase somewhat linearly with through relay does not wait to be restrained by the second or fifth
currents.Thus, if a restraining characteristics were provided harmonic content in the differential current.This element is
having a slope increasing at higher through currents, it would necessary to ensure fast operation for internal fault, even with
stabilize the differential relay even at the face of CT saturated CTs.
saturation.Hence it became necessary to have characteristics For several decades, the power system protection
for the relays as shown in Fig. 3. relay had experienced many important changes and arrived to
employing fully numerical technology by the mid-
nineties.With the fast development in the area of digital
technology, it is now possible to provide powerful protection
algorithms within the cost effective hardware modules for
dedicated differential protection applications.The features and
advantages provided by these devices operating on numerical
technology have proved beneficial in quicker isolation of
faults resulting in stability of power system under abnormal
Idiff
operating conditions.
The fundamental principle of operation in any
numerical relay is the input of real time analog signals
1 p.u. followed by galvanically isolated analog to digital convertor
circuits and further processing and
Ibias measurements thereon.The advantages of numerical
Fig3. Differential relay Trip characteristics technology are manifold.The physical requirements of having
120 NATIONAL POWER SYSTEMS CONFERENCE, NPSC 2002
separate interposing current transformers were eliminated by waveforms, readily available with the communication
the introduction of software ICTs.Trippings related to software of the numerical relay. The oscillographic traces and
accessories of transformers such as Buccholz, PRV etc., were Fourier analysis suggested that 2nd harmonic content was
also included into the numerical relay thus eliminating the existent for a considerable amount of time in the differential
requirement for auxilliary relays.Another added advantage of current (due to the discharging current of long cable through
employing the numerical technology was the incorporation of the solidlly earhed transformer HV winding). The matter was
disturbance recording feature into the relay itself.Fault referred to the manufacturer who then provided with a newer
waveforms alongwith trip details with date and time version of the relay.
stampings have proved beneficial in analysis of disturbance FOURIER ANALYSIS RESULTS:
for future corrective actions.It is not that a numerical WDG. FC no.1 FC no.2 FC no.3
transformer differential relay never operates undesirably, but IL1 100.00 40.331 33.048
the information stored in the relay is so very helpful that the IL2 100.00 63.079 45.249
exact cause of its operation can be determined and prompt IL3 100.00 16.917 31.177
corrective action can be taken which is not always true with The new relay had additional features of “Time for
other forms of protective gears.This is the true silver lining to cross blocking” and “Choice of a further ‘nth.’ harmonic
the numerical technology.From the utility’s perspective, we restraint”. Cross blocking feature enables harmonic
have experienced a couple of occassions in which the stabilization of all phases even if the 2nd harmonic component
numerical transformer differential relays have operated in the current exceeds the permissible set value in one of the
undesirably.The feedback when given to the manufacturers, phases. From a close study of the nature of the differential
helped in upgradation of versions leading to advancements. current, a time delay of 100msec was given to the cross-
Case-1: blocking feature. With the new relay and settings, the
One 132/33 kV, 50 MVA power transformer, controlled problem seemed to be non-existent until it tripped once again
from remote by a circuit breaker through a 25 km long 132 during the de-energisation of the circuit from HV side.
kV cable occasionally tripped through the operation of a
static transformer differential relay at the time of de-
energisation from the HV side. Several investigations
revealed no possible cause for such undesirable operations.
Subsequently, the discrete component type static relay was
replaced with a numerical transformer differential relay with
conventional settings, which also yielded same results. The
operation of numerical relay gave the opportunity of studying
the waveforms recorded during tripping.
Fig.5 Waveforms on tripping with the new version relay.
FOURIER ANALYSIS RESULTS:
WDG. FC no.1 FC no.2 FC no.3
IL1 100.00 15.346 58.691
IL2 100.00 130.33 80.672
IL3 100.00 18.489 40.326
Waveform studies and Fourier Analysis suggested a
considerable amount of 3rd harmonic content in the current at
the time of deenergisation even after 100msec. (Refer Fig.5)
With conventional settings, only 2nd and 5th harmonic
restraint features were given to the relay to cater for inrush
stabilization and overfluxing. Now, with the available
Fig.4. Waveforms on tripping at the instant of de-energisation waveform analysis, it was decided that a non-conventional 3rd
from remote end. harmonic restraint feature might be adopted in place of the 5th
harmonic restraint. The overfluxing protection was switched
The discharging currents of the long length of the cable over to a separate relay. The flexibility in the settings option
through the transformer neutral were clearly visible.(Refer of ‘Choice of a further nth harmonic restraint’ and its
Fig.4) Subsequently, Fourier analysis was done on the
INDIAN INSTITUTE OF TECHNOLOGY, KHARAGPUR 721302, DECEMBER 27-29, 2002 121
‘Percentage content in differential current’ in addition to the
2nd harmonic restraint feature facilitated the prompt action.
Fig.6 Waveforms obtained from new relay with 3rd.
Harmonic restraint features.
FOURIER ANALYSIS RESULTS:
WDG. FC no.1 FC no.2 FC no.3
IL1 100.00 23.834 53.874
IL2 100.00 146.17 143.64
IL3 100.00 58.383 42.170
With the modified settings, laboratory tests were
conducted feeding the relay with the waveform obtained
during the prior tripping of the relay. Satisfied with the
stability of the relay, it was put into circuit and till date no
inadvertent trippings have been observed during the circuit
deenergisation procedure. Fig. 6 shows the waveform
recordings and Fourier analysis results with forced triggering
of the new relay with modified settings during de-
energisation of the transformer from remote end. These
confirmed the stability of the relay even with considerable
amount of 3rd harmonic in the current. Fig.7 Waveforms with current and voltage signal spikes
Case-2: causing highset element operation of transformer differential
On different isolated occasions, 16 MVA, 33/11.5 – relay.
6.25 kV, power transformers located at our unmanned
distributing stations, tripped through the operation of a
particular make of numerical transformer differential relay
during switching on ‘on load’ (both 11.5 kV and 6.25 kV
sides) from far end manned substations. The fault
annunciation stored in the relays indicated the operation of
highset element of the differential relays. There were no
faults, either in the transformers or in any outgoing feeders.
The following waveforms were retrieved from the relay after
tripping.
It was evident from the waveforms (Fig.7) that there
were spikes in the current waveforms that apparently caused
operation of highset element of the relays. At one of the
stations, a numerical transformer differential relay of
different make was installed to operate in tandem off the
same CTs. On a particular switching, mal-tripping occurred Fig 8. Current waveforms from original relay.
and waveforms from both the numerical relays were
downloaded and compared (Fig.8 and 9). Spikes were once
again observed in the originally commissioned relay and they
were absent in the repeater relay.
122 NATIONAL POWER SYSTEMS CONFERENCE, NPSC 2002
Frequency tracking is used for sampling technique in the
relay. Relay measures the power system frequency and in
accordance to that determines the sampling frequency. For
example, if the measured frequency is 50 Hz., the samples for
gap detection and disturbance recorders will be executed each
20/40 = 0.5msec. (Since
relays were sampling at 40 samples/cycle). If, however the
measured frequency is 51 Hz., the samples will be executed
19.61/40 =0.49msec. This is to secure an exact number of
samples required for protection.
Upon getting the feedback, the manufacturer
recently has come up with an upgraded version of the relay
with modified software, fixing the upper limit of frequency
tracking to 65 Hz., thus avoiding the relay response to high
frequency spikes. Minor hardware modifications with revised
design PCBs were also incorporated. With the new version of
relays, which have been commissioned, it seems that the
problem has been attended to.
Modern day trends have gone ahead in using PLCs in
the structure of differential relays alongwith programmable
sequence logic editors in the software thus greatly reducing
inter-relay wiring and auxilliary relay requirement.Perhaps
this is not the end.Present day and future requirements would
yield more sophisticated version of relays which would
probably operate on common software platform and have
features of load pattern study, etcetra..With time, the
protection engineer will harness technology to its utmost so
as to ease his own job of providing protection to the electrical
equipment.
Fig 9.HV-phase currents from repeater relay.
The waveforms as downloaded from both the relays
(Fig. 8 and 9) were referred to the manufacturer for their
views and suggestions. We expressed our grave concern to
the manufacturer regarding the above and were apprehensive
about the efficacy of their relays and its suitability in our
system where remote closing of transformers with load is
quite common.
According to the product support engineers, although
the relays were designed for a maximum working frequency
of 60 Hz., there were no limitations for frequency tracking.
Thus the relays tripped on high frequency signals, when the
frequency (of spikes) were much above the designed
frequency of 60 Hz. Since there were no upper limit to the
frequency tracking, all the signals were sampled by the
relays, which in turn led to the drift in sampling rate thus
causing maloperations.
