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CH APTE R 7 Proving Test of Duo bias Transformer
Protection
INTRODUCTION
The use of unit systems of protection is now almost
standard practice in all important electrical power sys-
tems. M ost unit systems are of the balanced class and
are based on the assumption that under through-fault
conditions the currents entering and leaving the pro-
tected zone are equal to one another, or bear some fixed
relationship to one another. In applying unit protection
to power-transformers two special problems arise, nam-
ely, the unbalancing effect of tappings on the trans-
former w indings w hich cause the relationship between
the magnitudes of the input and output currents to vary,
and the magnetizing inrush current which occurs when
switching on a transformer with its output side open-
circuited or very lightly loaded. The first problem is
usually dealt with by employing bias or restraint on the
relays so that the current required to operate the protec-
tion increases roughly in proportion to the straight-
through fault-current. The second problem presents
much greater difficulty. It can be dealt with by introduc-
ing time lags, as in conventional systems such as that
using our type-TJG relay, or by methods which in some
way or other differentiate between normal internal-fault
currents and magnetizing inrush currents in such a way
that the protection operates for the former but not for
the latter. O ne difference between these two currents,
which m ay be used for the purpose mentioned, lies in
their wave-forms, fault-currents being nearly sinusoidal,
whereas magnetizing currents contain appreciable sec-
ond harmonic. The duo-bias system of transformer pro-
tection derives its magnetizing stability b y taking this
into account.
GENERAL PRINCIPLES OF DUO-BIAS
TRANSFORMER PROTECTION
The principles of duo-bias protection are now fairly well
known. It is a Merz-Price system with biasing to take
care of tap-changing, and harmonic restraint to coun-
teract the effect of magnetizing inrush currents. A
schematic diagram for one phase of a three-phase trans-
former is shown in fig. 1. It differs from most other
differential systems of transformer protection in that the
relay is fed from the secondary winding of a transductor,
the primary winding of which is connected across the
pilots in the usual way . Biasing is obtained by d.c. excita-
tion of the transductor via a separate d.c. control-
winding which is fed from an auxiliary transformer in
series with the pilots. On internal faults the transductor
acts more or less as a transformer but on external faults
the saturation of the transductor core by the d.c.
control-winding prevents the unbalance current present
in the pilots from being transferred to the relay. W ith
magnetizing inrush currents the harmonic-restraint cir-
cuit has appreciable second-harmonic output. This is
RELAY
FIG. 1. DUO BIAS DIFFERENTIAL PROTECTION
WITH TRANSDUCTOR RELAY
rectified and fed into the d.c. control-winding on the
transductor thus biasing the protection in the same way
as does straight-through fault-current.
Fig. 2 shows the interconnections between the relays
in the protection of a three-phase transformer. It should
be noted that the outputs of the three filter-units are
paralleled and fed through the transductor bias-
windings of all three phases connected in series, thus
ensuring adequate restraint in all relays under condi-
tions of magnetizing inrush.
TESTS ON DUO-BIAS PROTECTION
Research and exhaustive testing are continually finding
new ways of improving the performance of protective
systems generally. The Reyrolle Research and Certifica-
tion Laboratories are very fully equipped for work of
this kind and the majority of system conditions can
readily be simulated. For transformer protection, how-
ever, a fundamen tal difficulty exists in connection with
the production of magnetizing inrush currents which
represent service conditions sufficiently accurately. It is
therefore desirable to supplement laboratory tests on
transform er differential-pro tection with tests on site. It
is not often that facilities for site tests are available but
through the courtesy of the Central Electricity Author-
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CVRRENT-TRANSFORrlERS
ON PRIMARY
POWER TRANSFORMER
CRRENT.TRANSFORMERS
ON SECONDARY
TRANSDUCTORS
SECOND HARMONIC
FILTERS
FIG. 2.
PROTECTION 0~ A 3-~~4sE TRANSFORMER
RELAY
ity, Eastern Division, it was possible to test the duo-bias
system thoroughly at their Rayleigh Transform ing Sta-
tion recently and these tests, together with comprehen-
sive laboratory tests, have fully proved the performance
of the system. Before dealing with the site tests we give a
brief outline of the laboratory tests.
Secondary injection Tests
A large num ber of tests were mad e using secondary-
injection circuits, and these provided data on the trans-
ductor and filter characteristics, and the ratings of com-
ponents.
A detailed investigation of the percentage-bias
characteristic showed that the overall relay performance
was almost unaffected by phase variations between the
bias and the operating inputs to the transductor, and that
the settings were similar irrespective of whether the
inputs were switched or slowly increased. Further tests
were made to determine the effects of harmonic content
and frequency variations.
The operating-time of the protection at three times
the setting of the relays was shown to be approximately
60ms. with no through-load current, and 85ms. with
full-load current. Furthermore, it was proved that the
asymmetry of the fault-current made little difference to
the operating-time, the actual times varying by only 5ms.
between fully symmetrical and fully asymm etrical condi-
tions.
The detailed data obtained by means of low-current
testing techniques were confirmed by tests using primary
injection as described below.
Primary-injection
Tests
In order to simulate site conditions as closely as poss-
ible, a number of laboratory tests were made using cir-
cuits incorporating 500-kVA and 2500-kVA power-
transformers.
For the majority of the tests the transformer had a
3-phase rating of 500-kV A, 660 /48 volts, with delta/star
windings. On the H.V. (delta) side three 25/l current-
transformers (i.e. 200/l using 8 primary turns) were
connected in star, and on the L.V. (star) side three 600/l
(i.e. 347 /0.58 ) current-transformers were connected in
delta. With these ratios the steady-state unbalance cur-
rent was negligible. To simulate power-transformer
ratio-changes (due to tap-changing) of plus and minus
12& , the number of primary turns used on the H.V.
current-transformers was altered from 8 to 9 and 7
respectively.
Fig. 4 shows the magnetization curves of the current-
transformers used in these tests, but other current-
transformer designs have also been tested.
Fault-settings without through-load were measured in
terms of the H.V. current, and were less than 36 for all
phase-to-phase and phase-to-earth faults, the variation
in the settings obtained for the six fault-conditions being
less than 5 . These figures applied for faults on both the
H.V. side and on the L.V. side of the power-transformer.
The effect on the fault-settings of 100 three-phase
load (using the circuit shown in fig. 3) is shown in fig. 5,
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FIG. 3. TEST-CIRCUIT LOAD
AND
FAULT
CONDITIONS.
from which it is apparent that the phase-angle between
fault and load is unimportant in deciding the sensitivity.
Tests also proved correct operation with high values of
fault-current and current-transformer burden (such that
the current-transformers saturated). The operating-time
I
I
+_
100
CURRENT IA ,I I TIMES AVERAGE,
FIG. 4.
MAGNETIZATION
CURVES
OF
THE
CURRENT-
TRANSFORMERS USED IN PRIMARY-INJECTION TESTS.
of the protection is shown in fig. 6.
The stability of the protection under through-fault
conditions, the fault being applied on the secondary
(L.V.) side after the transformer had been energized,
was proved under normal and maximum tap-change
conditions with H.V. current-transformer burdens of up
to 8 ohms. Fig. 7 shows a typical record of the relay-
operating current and L.V. primary current, the latter
corresponding to approximately 15 times the current-
transformer rated-current. The record shows the safety
margin a t an extreme tap-change position, and illus-
trates clearly that the relay output, resulting from the
magnetizing current of the power-transformer prior to
closure onto the fault on the L.V. side, is low relative to
the relay operating-level. Tests were also made to
demonstrate the stability when the H.V. side was ener-
gized with an external fault already applied on the L.V.
side, and to prove that repeated fault-current asymm etry
did not prejudice the stability of the protection.
Tests to prove the performance under conditions of
magnetizing inrush current were made in the laboratory
on the 500-kV A transformer and also on the 2,500-k VA
6.6-kV 3-phase transformer. For the former, the ratio of
the H.V. current-transformers was 200/l, and peak cur-
rents of up to 6,l times the current-transformer rating
were obtained, the time-constant of the magnetizing
inrush current-decrement being 35m s (X/R = 11).
These tests were made with repeated point-on-wave
switching, and the protection remained stable through-
out, oscillograph records showing that the value of the
transient relay-operating current never exceeded half
that required for operation. A further test was made
using current-transformers of ratio 25/ 1, when the pro-
tection remained stable with a peak magn etizing-current
equivalent to approximately 30 times the current-
transformer rating.
Similar tests were made on the 2,500-k VA trans-
former using 100 /l and 200 /l current-transformers of
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27w
FIG. 5. RED-PHASE-TO-EARTH FAULT-SE-ITING WITH 100
PER CENT ~ PHASE
LOAD.
differing designs. Peak surges of up to 14 times the
current-transformer rating and time-constants of
105ms. were obtained on these tests.
Site Tests
The characteristics of duo-bias protection concerned
with fault-settings and stability under through-fault con-
ditions are independ ent of source-impeda nce and trans-
former size. Stability under conditions of magnetizing
inrush current is howev er depend ent upon both the
magnitu de and the time-constant of the inrush current.
The laboratory tests demonstrated the stability of the
protection with heavy inrush currents but the time-
constants of these inrush currents w ere much shorter
OPERATING-CURRENT IN TERtlS OF tl LTlPLES OF FAULT-SETTING
FIG. 6 OPERATING-TIME OF DUO-BIAS PROTECTION.
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FIG. 7. RELAY-OPERATING CURRENT AND PRIMARY CURRENT UNDER THROUGH-FAULT CONDITIONS.
than those usually associated with large power-
transformers. The site tests at Rayleigh were made,
therefore, to prove stability with an inrush cu rrent of
long time-constant.
The tests were made on 3O-MVA and 6Q-MVA
132/2 2-kV transformers (see Table 1 opposite) using
the current-transformers available on site. The
magnetization-curves of these current-transformers are
shown in fig. 9. It should be noted that these current-
transformers have a much higher knee-point than those
which would normally be supplied for duo-bias protec-
tion. The use of these current-transformers does not,
however, ease the test-condition, since here we are con-
cerned with the output of a particular current-
transformer (which will be higher the better the
current-transformer) and not with the balancing of the
outputs of current-transformers.
Across the output of each power-transformer was
permanently connected a 150-kV A aux iliary trans-
former, the secondary winding of which was open-
circuited. The magnetizing-current of this transformer
would produce very little bias, and did not therefore
affect the validity of the tests.
Throug hout the tests Dud e11 oscillograph records
were taken of the primary-current and relay-current in
each phase, a nd the harmonic-bias current was recorded
FIG. 8. CURRENTS DURING MAGNETIZING SURGE
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Table l Data of Rayleigh Transformers
Reference
T3 T2B
Rating
I
I
1 30 MVA: 160 MVA:
ON/OFB-cooled ON/OFB-cooled
(15 MVA (30 MVA
ON-rating)
ON-rating)
Connection
Voltage
Star-Delta
132133 kV
Star-Delta
132133 kV
1 Impedance
1
10.3
1
12.4
Ratio of
associated
H.V. current-
transformers
15010.5
25010.5
on a moving-film cathode-ray oscillograph. Fig. 8 is a
typical record and shows that the relay-current is well
within the operating-level of the relay.
Whereas the laboratory tests were made with control
of asymmetry, thus permitting testing always under the
most severe conditions of primary-currents, such control
was not possible on site, and a large n umber of switching
operations were necessary. A total of 69 switching oper-
ations were made during these tests.
In many tests the harmonic bias was deliberately
reduced below its normal level by altering the primary
turns on the harmonic-bias reactor, the bias produced
being in direct ratio to the number of primary turns.
Although the harmonic bias was reduced to 4 of its
normal value protection still remained stable.
Some of the more significant results are given overleaf
in Tables 3 and 4.
Examination of the results given above (and of the
oscillograms taken) show that:
(a) Th e greater the inrush current the greater the
harmonic bias produced.
(b) The greater the harmonic bias the less the relay
current for corresponding inrush currents.
(c) T he continuation of the asymm etrical wave due
to the longer time-constant did not produce any
adverse effect on the stability of the protection.
CONCLUSION
From the laboratory and site tests described it can be
concluded that:
1)
2)
Duo-bias protection is stable with through-fault
currents of at least fifteen times the rated current
of the current-transformers with magnetizing
inrush surges having maximum peak values
exceeding any likely to be found in practice, and
also that it is stable with magnetizing surges hav-
ing time-constants of at least 6 seconds.
The fault-settings of the protection are less than
40 per cent of the current-transformer rating with
t
FIG. 9. MAGNETIZATION CURVES OFTHE CURRENT
TRANSFORMERSUSED IN SITE TESTS.
no through-load, and less than 6 0 per cent of the
current-transformer rating with 100 per cent
three-phase through-load. The phase-angle bet-
ween the load-currents and the fault-currents is
unimportant.
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Transformer No:-
Steady-state Magnetization-current
Time-constant
Normal lead-burden
Current-transformers-
Ratio
Secondary turns
D.C. resistance
Excitation curve
Red and blue phases-l 1 A (approx.;
Yellow phase 6 A (approx.,
2 sets (approx.)
4.6 ohms/phase
3.4 A (approx.)
6 sets (approx.)
6 ohms/phase
150/75/0*5 (used as 150/0.5)
295 of 19 s.w.g.
5 ohms
Fig. 9
Table &Results of Tests on Transformer T3
Table 2-Site-testing Data
T3 T2B
25OlO.5
495 of 19 s.w.g.
3.5 ohms
Fig. 8
Nominal turns on
harmonic-bias
Table AResults of Tests on Transformer T2B
Nominal turns on Lead-
harmonic-bias burden
reactor (per cent) (ohms/phase)
Peak primary
Relay-current Harmonic-
current
( of operating-
bias
(A)
current) current
(m-Q
Red
Yellow Blue Red Yellow Blue
I
100
I
4.6
1
490 330
1
220
1
25
1
30
1
18
1
35
1
100
4.6 230 320
140
21
18 34 28
57 4.6 570
410 230 31 38 32 29
57 6.6 340 180 160 30 25 28 9
33 6.6 570 320 220 36 56 No record 8
33
6.6 110 120
170 29 33
27
Very small
(3) The operating-time of the protection is less than
100 milliseconds at 3 times the setting under all
conditions of load and fault-current asymmetry,
and is less than 65 milliseconds at 3 times the
setting for internal faults with no through-load.
(4) The correct performance of the system is unaf-
fected by the presence of harmonics higher than
the second, and by departures from the nominal
frequency greatly exceeding anything likely to
occur in practice.
These additional tests and appreciable operating
experience with duo-bias protection have provided val-
uable confirmation that this system of transformer pro-
tection is basically sound in principle, and that it can be
applied with confidence to the largest and most impor-
tant transformers in service.
127
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