Ziegler Transformer Diff

81 Basic Physics

8 '1'1. ansformer Differential Protection

R

,, I

Tnnsfonners are importsrii system components available in many different construclions. The range of HV transfonners reaches from small distribution transformers (from lookVA) up to large transformers having several hundred MVA. Apartfrom the

large number of simple two and Ihiee-winding transformers, a range of complex constructions in the form of multi-winding and regulating transfonners also exist, Differential protection provides fast and selective short-circuitproiection on its own, or as a supplement to Buchholz (gas pressure) protection, It is usually applied on transformers above approx. I MVA. On larger units above approx. 5 MVA it is standard. The transformer differential protection contains a number of supplementary functions (adaption to transfonnation ratio and vector group, stabilisation against in-rush and overexcitation) and therefore requires some fundamental consideration forthe coringuration and setting calculation,

T

X

T

^.

I=I'

I-w =I -w

I~'j' 2"2

'T'Xoj"o2

I '2'

RT =Rj+R2'

Figure 8.2 Simplified transformer equivalent circuit

I according to the magnetismg curve is required. In the electrical equivalent circuit, this excitation requirement corresponds to the main reactance X . The leakage flux @ I and @, 2 are only linked to theirrespective own windings and make up the leakage rearlances X, I and X, 2. Rj and Rz are the respective winding resistances. All currents and impedances are referred to the primary side,

X = Un!ICOrresponds to the slope of the magnetismg curve. During load and particuIarly in the event of short-circuits, the operating point is below the knee-point in the steep portion of the curve. The magnetismg current at nominal voltage only amounts to approx. 0.2% IN, i. e. in the non-saturated segment of the curve* X is approx. 500 times larger than the nominal impedance of the transformer and approx. 5000 times greater than the leakage reactances, During load and short-circuit conditions* a simplified equivalent circuit may therefore be used for the calculations (Fig. 8.2). The series reactance XT corresponds to the short-circuit voltage in %, relative to the nominal impedance of the transformer:

IfX-T t', XT' ~" .XTN T~ 100 2

_UN UN

TN~ I^"N 'N

8.1 Basic Physics To better undersumd the protection response during short-circuits and switching opera tions, the physical principles of the transforrner are initially covered in detail. t8-11

(82)

The series resistance corresponds to the ohmic short-circuit voltage in %* and is also based on the nontinalimpedance. For calculation onhe short-circuit current, the resistance may be neglected;it must only be considered when calculating the DC time conslant

EQ"tv@fom, cine"it of a Iran. $fon, ,er

Table 8.1 lists typical transformer data. t8-21

The primary and secondary winding are linked via a magnetic core by means of the main flux @ (Fig. 8.1). To obtain the flux, the magnetismg curreni(excitation current) W

11

12 \

^.

; I'2 by

'02

Equivalent electromagnetic circuit Figure 8.1 Equivalent circuit of a transformer

X

R

Of

u, I ;

146

When energising a transfonner, one-sided overexcitation results, due to reinanance causing large magnetising current flow (in-rush current).

W

^

In-rush 18-3 to 8-71

^

,, I

I

R'

o2

The flux does notretum to zero when the transfonnerisswitchedoff, but remains arthe

^.

I, , I

X

2

Equivalent electric circuit

I u, '

reinanance point Ok. ,, which may be above 80% of the nominal induction. When the transfonneris re-energised, the flux increase starts at this point. Depending on theenergising instant on the sinusoidal voltage (point on wave), an off-set course of the flux can result. For the large flux values in the saturation range, a very large magnetismg current is required, and cyclic current peaks will result. The curve form corresponds with the sinusoidal half-waves of a simple half-waye rectified AC currentthat decays with a very large time constant(Fig. 8.3).

8 Transformer Differential Protection

8.1 Basic Physics

Table 8.1 Typical hallsfomier data Rating

Tnnsfonnation ratio

MVA

(kV/ICV)

600

4001230

Rush

Short-circuit voltage "x-T in %

Open circuit current %I

0.25

19

IN 10

Rated power in MVA 0.5. .,. 1.0 1.0 10 >, O

I,

6

300

2301/10

24

0.1

40

110110

17

0.1

16

30/10

8.0

0.2

6.3

30/10

7.5

0.2

Time constant in seconds

0.16. ... 0.2 0.2 .....,. 2

I 2 .... 720

4 2

5 10

50 too

Rated transformer power in MVA 0.63

1010.4

4.0

500 ^

0.15

Figure 8.4 Typical rush cument of a star delta transformer

The rush current is particularly large when cores of cold rolled steel with a nominal ^.

induction (1.6 to 1.8 Tesla) are operated close to the saturation induction (approx. 2 Tesla).

IA

On a three-phase transformer, a three-phase rush current will result, which depends on

IB

^

the vectorgroup and the method of star-point earthing on the transformer. [8-3 and 8-7]

+ I,

+.

In general, two phases willsaturate and draw large magnetismg currents. On star delta transfonners, these currents are coupled to the non-saturated phase via the delta wind~

I

ing. This causes the typical rush currents as shown in Fig. 8.4.

OScillogram:

The rush currents in the three phases can be calculated from the required magnetisation of the two saturated core-limbsA and C with the given equations, The current on phase B thereby corresponds to the current in the delta winding ID. Please refer to the liters-

IA

ture for the theoretical analysis [8-7]. The shown OScillogram of an in-rush occurrence

I

confirms the calculated curves. IC

Amplitude and time constant depend on the transformer size. (Fig. 8.5). It must be noted that a similar rush current also arises when a close-in externalshort-

circuitis switched off and the transformer is re-magnetised by the recovery of the volt age. It however is substantially smaller than the in-rush following energising of a switched-off transfonner,

JVV1'!11UVU^ Innsh -

Setting the slope of the first brunch to 25% and the second to 50% is appropriate for nomial applications. Stability in the event of large differential currents that arise in the event of CT saturation is ensured by the integrated saturation detector with temporary blocking of the trip output (Fig. 8.12).

Incorrect operation due to in-rush currents ordue to over-excitation is prevented by the blocking functions described above. During internal faults, with severe CT saturation, harmonics which can cause delay of the trip output may also be present. In this case,

A'sec

_ 0.2A IA 25,000A IA 5A 44,000A

IC,

18-sec.

_ 1,000 43,949

C-sec.

A-sec.

the high set stage IDjFp>> will however respond.

B-sec. ,

IC-,, C!

That means an adaptation to the rating 1050MVA. Measuring algorithm

The adapted measured values are subjected to numeric evaluation according to the differentialprotection measuring principle.

The pick-up characteristic has three stages which is typical for numerical protection. Compared to generator protection, the basic pick-up threshold IDlFF>, and the slope of the first branch must have less sensitive settings as the magnetismg currents of the transfomier* and ratio errors due to tap changers result in false differential currents.

Adaptation of the transfomiation ratio in accordance with the tap changer tap position would in theory be possible, is however not implemented in practice due to the increased complexity.

A cornmon setting value is 20% IN fortransfonners withouttap changer and up to 30% on transformers with tap changers having ^22% tap changer range.

High-current stage

The short-circuit current flowing through a transfonner is limited by the short-circuit reactance to I, - (1001, ,, %). In the event of external faults, the differential current can therefore also notbe greater than this, even under the most severe nori-syrnmetricalsaturalion of the CTS.

If the impping currents are greater than this maximum current, then tripping without any stabilising may be camed out without delay. The transfonner differential protection 7UT6 has a corresponding nori-stabilised high-

current stage. It ensures fast tripping also in those cases when during internal faults with large DC off-set short-circuit current and severe CT saturation, a temporary pickup of the rush-blocking takes place. As extreme saturation is riot to be expected with the limited short-circuit currents flowing through the transformer, IDFF>> may also be sello a value below the current corresponding to the short-circuit reactance.

The high-current stage responds to the fundamental wave of the short-circuit current which meanstha. the DC component and the high order harmonics of the rush currents

8 Transfom, er Differential Protection

8.2 Numerical Measured Value Processing

^re elmtinated. Including a securlty margin of 20%, a setting of approximately 60% of

while the current alitie in-feed side is inversely proportional to the square of the num-

IRUsh 1/2 may be applied as the maximum value of the fundamental wave in the rush

ber of short-circuited turns,

currentis only approximately 50% of the rush current peak, The factory pre-setting is 7-5'IN, Tmf, . It should be appropriate for most applications.

The fault currents may in this case easily be calculated: h.U

In case of internal transfomier faults near the terntinals, high fault currents may lead to

IF' '

fast CT saturation, This is always the case when smalltransfonners are connected to a system with high short-circuit power.

Therefore, a fast momentary value processing high-set element is additionally provided. It operates when two sannles exceed twice the IDjFF>> setting value, This ensures ultra high speed tripping before saturation occurs, The measuring principle was dis-

and

h'U2N 2 I '2N'a IK" "'~"'~

The primary short-circuit currentis very small for faults close to the star-point, so that the differential protection will only pick up for earth-faults closer to the transformer terntinals.

cussed in section 4.2.2.

The following is an example of this:

Earthf""!, dtff'eyen!iaip, otec, ion

Ex@inPIe 8-3. . Currents during tram. ^fomiere@rthf""!ts

During an earth-faulton an earthed transformer winding, short-circuitcurrents that can

Given:

cause severe damage winnow. On the in-feed side the corresponding currents may be

The reactance is inversely proportional to the square of the short-circuited turns, while the inducing voltage decreases linearly, Accordingly the parabolic curve results,

Transformer 20 MVA, U, N = 132 kV, U2N = 13.8 kV Circuit according to Fig, 8.14. CT on the 132 kV side: 10011 A

relatively smallifthe short-circuit currentis only linked by a few turns of the second ary winding. On solidly earthed winding star-points the ratio is extreme if the fault is only a few turns away from the star-point(Fig, 8.13).

(8.8)

" U, ./; '75'UN i^;

Earth currentlimited by R to 2000A Faultloca, ion 20% from the star-point. Wanted:

Will the differential protection detect the fault(setting 25% IN)?

Solution: The primary short-circuitcurrent(=tripping current)is:

whereby the winding resistance has a limiting action close to the star-point,

I = ^--:----2000A = 23A

Fig. 8.13 also applies for inter-turn short-circuit whereby the current then does riot return via earth bun flows via the short-circuit bridge without earth connection. It must be noted that the fault current IF in this case is not detected by a relay connected at the star-point earth.

With a setting of 25% I, , which is equivalentt0 25 A the differential protoction would only just failto pick up. The protection coverage during earth-faults on the 13.8 kV winding therefore is almost 80%,

If the star-point is eached via an impedance, the conditions shown in Fig. 8.14 apply. The secondary current is linearly proportional to the number of short-circuited turns,

In the example at hand, it is therefore advisable roconnect a CT with earth currentrelay

(pick-up threshold 200A) in the earth connection of the star-point, to increase themrige of protection coverage to 90%. This protection however requires a large time delay setting as the earth curTent relay in the system must trip faster to maintain selectivity.

IF 10 per Unit

U,

h. Un

UR

I 6

h. U

% 100

IP I

I

Infeed

IF

I

Infoed

4

side ^.

8

I I 2

,

IK

^.

IK

I

Max

side

IF RE

co .. .

I

IF

I I

, I

O 20 40 60 80 100

Short-circLiiled winding part h in %

'18/1re 8.13 Earth-fault on a musformer winding with sond earthing t8-131

O~

20 40 60

80 100

Short. circuitsd winding part h in %

Figure 8.14 Earth-fault on a transformer winding with resistive earthing 18-131

8.2 Numerical Measured Value Processing

8 Transfonner Differential Protection

Below, the earth current differential protection is described. It facilitates non-delayed tripping for this fault condition.

In this case, a stabilising current is additionally effective: (8-9)

I, . 11; -I, **I_I!; ,I, **I Restric, ed earlhf""!IP, Diectio"

The tripping condition in this range is as follows:

The earth current differential protection (restricted earth-fault protection) is an ideal supplement of the phase fault protection, in particular on transfonner windings with star-point earthing via an impedance (earth currentlimiter). Thereby the pick-up sensitivity during earth-faults is improved. t8-141

therefore:

The protection principle makes use of a comparison of the star-point current 1, * with the summated current of the feeder^* (Fig. 8.15). To improve the pick-up sensitivity while maintaining good selectivity, the so-called product relays were applied with conventional technology, These polarise the tripping current with the current of the star-point so that the protection obtains it's highest sensinvity (maximum torque) when the tripping current (differential current) and the starpoint current are in the same direction (have the same polarity).

(8-10)

II;I^lad+k, ,I,

(8-11)

1';I^I, **+k, -( I; _!,** I_11; ,^* I, The following equation can be derived: I* ^^

set

(8-12)

,,,-*,-IF;:^-F;:I

The numerical protection 7UT6 uses this principle with numerical technology.

During external earth-faults, 1, * and If* are in the same direction (have opposite

The following relationships apply:

signs)* so that only stabilising current flows and 00 differential current results.

During internal faults 1, * and 1, ** flow into the protected object and therefore have

I* 10 ,, "N

the same sign (opposite direction).

'0 ' IR+Is+IT ' 3'10 Two angle ranges must be distinguished for the tripping ctiterion:

In Fig. 8.16 the pick-up characteristic is shown. The right hand side of the diagram designates the range of internal faults. Here, the protection has high sensitivity. The

"ideal"internal faultis located at @( 16'1 If*) = 0' on the right hand side, outside the

The basic tripping range is defined by the approximate phase coincidence of 1, * and 1, ** : -90' ^ @(!j*/If*) S +90'. In this range, 00 stabilising applies, The inp-

diagram, while the "ideal" external fault appears on the left hand side at 180',

ping current corresponds to the star-point current 10 so that the protection has a consinntly low pick-up threshold: 10 ' 10 > I, et

The extended impping range extends from 90'(k, ^ co) up to 130'(k, = I). The larger the k, -factor setting is, the more the tripping range will be restricted.

The extended tripping range is defined by the angle difference > 90', which, during internal faults, only occurs in the presence of CT saturation:

+90' ^ @( If /If*) ^+270 '~ IA

^. ^ ^

I*

I 00

I. 4

120

2.0

1.0

^

,. I. .

100

10 > 4 3

\

90

\ \

^ ,

1800

Restricted earth-fault

protection

.. ..

10 ZIPh^ I '0 IN

..

..

External fault

Figure 8.15 ,-

130

2

r ------ - - -J

A1E

Q Lint

1.0

4.0

,- IC

^" I

1, I may also be applied in the transfonner sun-point. This would however have to be time graded with the system earth-fault relays to ensure selective impping.

52

7SJ600 r

I 51N

51BF

50 51 I - - .

7UT6,3 F~ ~~~~~I

I 87 TL .-

A Load 52

87 TH

7VH600

49 I

-----,

52

3Y Y

r ~~

Yrn I 51

Bu

50 51

59 I 50

50 BF

7RW600 .--7SJ600

51 N .

Figure 8.27 Auto-transformer protection 172

----

7SJ600

For very large transformer banks, forexample a system coupler, aredundant protection with high speed tripping should be provided. In central Europe the differential pro^C lion is usually duplicated and connected to separate CT cores. Anglo-Saxon protection usually applies a normal differential protection 87TL together with a Hl protection 87TH. In this case the relay 87TL is connected to the transfonner bushing CTS while the relay 87TH is connected to CTS at the switch gear (Fig, 8.28). In particularwhen connecting in I I^ CB applications, the HIProtection has advantages because the numerous CTS can simply be connected in parallel. With normal low

8.5 Application Examples for Transformer Protection

8 Transfonner Differential Pronection

impedance protection, the connection to CTS of I I^ CB switch gearmust be applied to a 4 terniinaldifferentialproiection (7UT635)(compare previous examples). A distance protection (21) is applied to both the high and low voltage side. The fast

tripping stage is set to Teach about 80% into the transfonnerthereby providing fast protestion in this range. Iris also possible to apply a zone that reaches through the trans-

fomer in combination with a directional comparison logic to obtain 100% protection coverage. In the latter case, the redundant differential protection may be omitted.

For the remaining protection functions the comments made in the previous example apply also here.

The primary differential protection 87TP is the protection for the auto transfonner which also covers the primary winding of the exciting transfonner. The numerical relay 7UT613 is well suited for this purpose. Alternatively, a high impedance protection

could also be applied for this purpose* as shown in Fig, 8.28. The phase regulating transfomier connection makes a star delta conversion necessary for the secondary dif-

forential protection 87TS (illustrated by a triangle in the figure). The 7UT613 is also suitable for this purpose.

The analysis of current distribution and measured value adaptation goes beyond the scope of this book. mis covered in detailin the referred publication and may be used as an illustrative example.

Protection of regulari"g tram. $10n"ers

The regulating transformer adds a longitudinal or quadrilateral variable voltage to the feeder voltage. With the longitudinal voltage* the voltage magnitude and reactive power flow is controlled while the quadrilateral voltage regulates the active (real) power now. A combined regulating transfonnerconiains both longitudinal and quadtilateralregulation.

Additional current relays for earth-faults and back-up protection must be applied as shown in Fig. 8.29. Alternatively a distance protection may also be applied as back-up protection. Protection of generatortr",,,$former Minirs

The unit protection encompasses a number of protection components. 17-11 Only the

The complete regulating unit consists of a phase regulator and an exciting transformer

differential protection is elucidated on here,

which can be housed in a single tank or in separate tanks,

The common configurations are shown in Fig. 8.30.

Depending on the construction, the excitation transformer can be connected in star or

The necessary ratio and vector group correction is represented in the diagram by interposing CTS. In the case of numerical relays this adaption is done by numerical coinpu-

delta configuration.

The protection configuration for these complex transformers demands a detailed analysis of the current distribution for load and short-circuit conditions. Fig, 8.29 shows an example which was described in detailin a publication. t8-171

tation in the relay. This was explained in detailin section 8.2.

On small generators, the differential protection covers both the generator and trans former (overall differential protection).

Phase regulator I~~~~~~~~'~~-----,

A

A

A I

A1

50/50N

Exciting

5,151N

Itransforrner

I B"

,

87 TP

50/50N

-,

r~~~~

5,151N

A

A

87 TS

A

51N 50N 51N ^ ^ ^

Figure 8.29 Protection of a phase shifting regulating transformer Figure 8.30 Various differential protection schemes for generator units

8 Transformer Differential Protection

8.5 Application Examples for Transformer Protection

from the generator it is 10.1N. 0, ,., at XII"= 10%; together therefore approximately ^

17.1N_G. ,. u, i, , if the rating of the generator and unit transformer are assumed to be approximately equal. Ifthe rating of the auxiliary transformer is approximately 10% of the unit rating, the resulting short-circuit current is therefore 170.1N. A, ,.. T, ,,, f, and fur

*

01^

thennore this current has an extremely long DC time constant of up to several looms (refer to table 7-1 of section 7.1). The ideal but expensive solution arthis point is a Iinearcore with the same data as the set of CTS at the generatorterininals, The alternative connection to a set of CTS at the auxiliaries side (dashed line alternative in Fig. 8.31) avoids this large over dimensioning, however at the cost of selectivity.

,^IQ ^

. ... 8 ^.

A ... ..

:52: ..

.,

The CT cores that are connected to the differential protection of the auxiliary transformer on the generatorside must be dimensioned such that the protection trips without

A

*)

G

87G

delay during internal faults. This remains a very stringent requirement even if, in the

,' I^ 87r

case of numerical relays (7UT612), a saturation free conversion time of only less than

a quarter cycle is required (refer to the following example), Stability for fault currents flowing through the transformer i. e. for faults in the auxiliaries, on the other hand present no problems,

_ _ I

52

Example 8-4: Dimensioning of the CTS at the @,, tilia, y tm"^former

Auxiliaries Figure831

Differential protection

*) same ratio as generator CTS

Given:

Generatorunit230 MVA (fN = SOHz)

Generator: 200MVA, 10.5 kV, XI, " = 15%, R, = 0.63-10'' Ohm

for a generator unit

Unittransfonner: 230 MVA, 110110.5 kV, "x. T = 13.2%, "R. T = 0.14% Auxiliary transfonner: 25 MVA, 10,515 kV* "x. AunT ' 14%, MR. A",. T ' 0.64%

On larger units, a dedicated differential protection is provided for the generator, on the

one hand for the higher sensitivity (10-15% instead of 25-30% IN), and on the other hand for the selective indication of generator faults. The variants by and c) are available for this purpose. The German Powersystem Relaying Coinimitee recoinments variant b),

On a unit with generator breaker, the variant d) may be applied, because the unittrans-

former which also provides the auxiliary supply may be started and operated sepa-

Task:

Calculate the dimensions for the CTS of the differential protection across the auxiliary transfomier

Solution: The following ctiteria apply to the differential protection 7UT612 with regard to the CT dimensions:

Fasttripping during internal faults: saturation free conversion ^ 4ms Stability during an external fault: over dimensioning factor KTF ^ 1.2

rarely.

The complete arrangement of the differential protection on a large unit is shown in Fig. 8.31. The alternatives for the CT connections are shown with dashed lines. If the

differential protection of the unit transfonner is connected to the CT core in front (at

.

the generator side) of the auxiliary supply transfonner (standard connection according 10 the Gemian Power System Relaying Coinimttee), then the CT primary current should be matched to the generator transfonner. In any eventit should not be smaller by more than factor 4, as an interposing CT would be required in this case (refer to example 8-2).

, ,

^ ^ ^

^

^

Based on the rated current of the auxiliary transformer, the CT must be dimensioned for a large overcurrent factor if the protection of the unittransfonner must remain stable in the event of a short-circuit at the auxiliary transformer ternxinals on the generator

side. The current ilowing from the unit transfomier is 6.7. IN. T, ,, r. , arux. T ' 15%, and

, ,

o

C^ . .

@

e^

^ ,

Figure 8.32 Circuit diagram for example 8-4

8 Tnnsformer Differential Protection

8.5 Application Examples for Tnnsformer Protection

Formtemalfaults the ctitical case is a short-circuit at the terniinals on the

The corresponding equivalent time constant is calculated with the equa-

generatorside (F1):

lion (523) in section 5.7:

Rated current of the generator:

IN-c ,

SN tMVAj. 10

T=

IF-G ' 'c 'IF-T ' TT _ 81- 0.42 + 105 - 0.30

200 - 10

= 0.35 s

81 + 105

IF-G ' IF-T 11 ICJ\

^^^

Short-circuit current from the generator:

The over dimensioning factor for 4ms saturation free conversion is obtained with the equations 5-21 and 5-22, in section 5.7, The value is extracted from the corresponding diagram in Fig. 5.14: KTF = 0.75

I, _G ' '~~ ' ~" ' '11