proving test

7/23/2019 Proving Test

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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.

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