DDT

liED Measurements I Type DDT The type DDT relay is a medium or high bias-slope differential unit designed for protection of twowinding power transformers over about 10 MVA rating against internal phase and earth faults. Basically the relay is an induction disc unit with a pair of bias or restraint coils (in addition to an operating coil) to prevent operation by external faults. Types ODT12 and DDT32 are single and triple pole versions respectively.

OPERATION The relay is connected in a Merz-Price configuration to corresponding matched current transformers On either side of the protected transformer. The C.T. secondaries provide a through current in the relay restraint coils which produce a continuous torque on the disc in the contact opening direction. The differential of the C.T. secondary currents flows in the relay operating coil. Under normal load conditions the C.T. secondary currents are equal and no current flows in the operating coil. If these currents become unequal due to a fault in the transformers. the resulting differential current energises the operating coil which produces a torque on the disc in the contact closing direction. The contacts close when the ratio of the differential current to the through current exceeds the slope of the relay operating characteristic determined by the turns ratio of the operating and restraint coils. The bias slope is chosen so that the relay is insensitive to unbalanced external lead burdens which normally give a lower ratio of differential current to through current than an internal fault. In addition a fairly high bias slope is required to prevent maloperation by C.T. differential currents arising from:

(a) tap changing on transformers giving mismatch of the CT:s.

(b) different C.T. ratios and hence saturation levels giving differential currents under through fault conditions.

(c) magnetising inrush giving secondary currents in one set of C,T:s only.

To prevent maloperation by magnetising inrush the relay function is delayed by a selected time until the initial current peaks have decayed to a

Type DDT12 relay

tolerable level (determined by the percentage bias). The mmimum operating current of the relay is determined by the tension of the disc control spring and can be adjusted by rotating a knurled moulded disc against a graduated scale.

CURRENT SETTING 40-100% (adjustable) of 0'5. 10 or 50 amps (C.T. secondary) 50 or 60 cis

PERCENTAGE BIAS 20%,30% or 40% (selected by taps) The percentaged bias is defined at the minimum

. spill current current settmg (40"";) as ............................_- X 100

through current

OPERATING TIME 010 second to 025 second (adjustable) at 5 times current setting (see characteristic)

,

-IOpettlting cGi1 Burden (C.T,) at current setting

Current setting 50 c!s 60 c!s

;

30% -I 0 35VA' 04 VAl 40% 06 VA 07 VA "~i(;4"~,(}5VA~i , 100% 37 VA 4,5 VA

Operating Coil Impedance At the rated current, the operating coil impedance does not exceed 0'182 ohms and is 008 ohms at satu ration.

Current Transformer Knee-point Voltage The knee point is defined as the point on the magnetisation curve at which a 10% increase in excitation voltage produces a 50% increase in excitation current. The minimum knee- point voltage is calculated as follows:

V,= 21, (R,+ R, R,) (star connected CT.'s) or V, ~ (21 y3) I, [R, + 3(H, + R,)l

(delta connected C.T:s) where [t = Maximum fault current

(CT. secondary amps) H, = C.T. secondary resistance (ohms) R, Lead resistance between C,T.'s and

relay (ohms) H, = Impedance of one half of relay bias

winding (ohms) bias winding burden (VA)

A (rated currentj'-"

AUXILIARY UNITS AND OPERATION INDICATORS An auxiliary attracted armature unit with a hand reset operation indicator for either shunt ( seal in) or series seal in is fitted as standard.

Standard Coil Ratings Voltage operated (shunt) auxiliary units are available with nominal ratings of 30, 110,125 or 220 volts d.c. Current operated (series) auxiliary units:

BURDENS

Bias coil 1--,

I BUIden (C T ) 011 rated current

20% I 02 VA I 03 VA

r/I nLTlum oporatmg currem I" amps (two l8Ps)

,

0'5 seco:,d c.1front latmg to amps

) Col: Il~SiSlilnCQ in ohrrs

0,1 and 0-3 0,2 and 2,0 06 am: 24

18 !l:"!d 22 22 fL,d 92 92 and 188

9,2 and 21 60 and 0"125 029 and C 031

Other coil ratings can be supplied for both types of auxiliary unit.

Contacts Two pairs of electrically separate normally open self or hand reset contacts are fitted and

will make and carry 7500 VA for 3s with maxima af 30 A and 660 V a.C. or d.c.

CASES The relays afe supplied in drawout cases available for either flush or projecting mounting and finished in phenolic black. Standard relays are finished to BS.2011 :20/40/4 and are suitable for normal tropical use; relays for use in exceptionally severe environments can be finished to BS.2011 :20/50/56 at extra cost.

The drawout case offers many advantages including ease of maintenance and testing, and is fitted with contacts which short circuit the associated current transformers on withdrawal of the unit. A filter is filled to equalise the pressu re inside and outside the case without admitting dust.

MQ1(:ml.l11 Ovorall Dimonslons

Relay Case HOlght - ! W;dth i Dopth * in'> I mrr I ins : mm i ins rum

DDT12 (Singlo Pole, I 1D 9';' : 233 , , 6ft I i 170 I 71 '.7

32 3D (VerL) 201 524 i 6't I 170! 71 197

?cle) 3D (Horil,) 91 235 i 171 ' 454 I 11 1 197 i

"Add 3 ins (76 mM) for maximUM length of 2 SA termin

,

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, I ! T I ;1000 I

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90 o

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~ 100 0; 70

~60

a 500

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~

~ 40 ,. i'i 30

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to

o , 4 6 8 Hi 12 14 16 18 THROUGH CURRENT (TIMES FUll LOAD)

Ope/aling characteristics (30% bias tap)

T 1.00

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0'

0

0

0

0 I I

01--+ I

200

0 V V 2

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4 6 8 10 12 THROUGH CURRENT (lIMES fULL LOAD)

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16 isj4

Operating characteristics (40% bias tap)

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55 "

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'00 38

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340

fS 320 8 300

~ 280 ~ 260-"0'E ;::: 220 ?E 200 : 180 OPERATING ClJlI:ur !AMM) ~ 160

14 Differential circuit impedance characteristic .20 .00

0

0

2 4 6 to 12 f6 18" FAULT CURRENT (TIMES FULL LOAD)

Time current characteristics at 30% bias, time multip!;er setting 1'0

INFORMATION REQUIRED WITH ORDER Relay type Trip circuit current (series seal in) Current transformer secondary rating Trip circuit voltage (shunt reinforcing) Frequency Operation indicator inscription if required Trip circuit (series seal in or shunt Auxiliary contacts (hand or self reset)

reinforcing) Case finish and mode of mounting

TRANSFORMER

CONFIGURATION

AA

BB

CC

T TING

~ A PHASE a PHASE '11

F OPERATING COIL~ NOTEBIAS ALL THREE ELEMENTS ARE

AUX

COilS'--< THE SAME AND ONL Y ONE

UNIT IS SHOWN. ALARM AND TRIP CIRCUITS ARE PARALLELED"

L

.........

.J ! L

---- .......,Jll ! L i

~...

ALARM TRIP

Dy 11

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r

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r I 1.1 "'"' ..........

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r-- r--'> Q,tJ t 1~ ,~i-:~ tB ;q,= ~ ~7 ~ TO ~ !J ~ ~~ ~ ~~ ~~ ~ ~ ~a ~9 I!5 6 II 11 13 14 IS 16 17

-r I

r"

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BIAS alAS OP OP

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BIAS

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C SHOR

ACTSCONT

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'......J DISC UNIT

TYPJCJI appiicalion and internal circuit diagram of type DDT32 relay wilh series seal in

EARTHING ARRANGEMENTS Although not included in the diagram, it is assumed

that secondary C.T. andior V.T. circuits will be

earthed as necessary in compliance with standard

safety requirements and determined by the switch

gear contractor or user. If in doubt, please consult

GEC Measurementsfor advice.

Our policY is ono 0: continuous product development and the r:ght is reserved to SUGply equipmof't which rT'