Protection Application Handbook (англ. яз.)

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VOLTAGE DROP IN SECONDARY CABLING.

Voltage Transformers

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VOLTAGE DROP IN SECONDARY CABLING.

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INTRODUCTION

CT Requirements

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1. INTRODUCTION

Many types of protection relays are used in a power system. Dif-

ferential protection relays, distance protection relays, overcurrent

protection relays of varying types etc. will all have different re-

quirements on the current transformer cores depending on the

design of the individual protection relay.

Instantaneous and delayed protection relays will also have differ-

ent requirements. A DC component of the fault current need often

to be considered for the instantaneous types of protection relays.

Modern solid-state protection relays, of static or numerical types,

provides a much lower burden on the current transformer cores

and often also have lower requirements on the core output as

they normally are designed to operate with saturated CT cores

which was not the case for the old types of electro-mechanical re-

lays

Hereafter follows a brieﬁng of requirements that modern ABB sol-

id state protection relays will set on current transformer cores. For

special types we further refer to manuals for the respective relay

and to tests of behavior at saturated current transformers per-

formed with the different products.

To explain the general expressions used we will show a normal

current transformer circuit.

Figure 1. The current transformer principle circuit with magnetizing reac-

tance and secondary resistance.

OTHER

PHASES

OVERCURRENT PROTECTION RELAYS

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For phase to phase faults the loop-resistance will include the ca-

ble resistance “

” plus the resistance of Phase measuring relays

“R

+ R

”.

For phase to earth fault loops the resistance will include two

times the cable resistance “

” plus the total resistance of

Phase and Neutral measuring relays “

+ R

”.

Core voltage output can either be the knee-point voltage, if

known, or the secondary voltage output “

” calculated with help

of 5P or 10P data, ALF and the secondary resistance according

to:

= ALF x I

+ R

)

where

“

” is the secondary limiting emf

“I

” is the CT rated secondary current

“R

” is the CT secondary resistance at 75°C and

“R

” is the CT rated burden, calculated as resistance.

Normally, modern relays provides a pure resistive burden.

Of course values given in ANSI, or other standards, can be used

in similar ways to calculate the cores secondary output and the

achieved value can be used instead. The differences in the volt-

age output at selected deﬁnition of core is of size some 10-20%

and is not of importance for the calculation.

2. OVERCURRENT PROTECTION RELAYS

Overcurrent relays are used both as short-circuit and earth-fault

protection. They can be instantaneous or delayed, deﬁnite- or in-

verse time. Current transformer cores must give sufﬁcient output

to ensure correct operation.

OVERCURRENT PROTECTION RELAYS

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The required CT core output is given below:

INSTANTANEOUS PROTECTION Core shall not saturate for an AC

current, smaller than

“2xI

set

”. The DC component doesn’t need to

be considered as ABB overcurrent relays are designed with short

impulse limit time to secure operation also when only a very short

current pulse is achieved due to heavy saturated current trans-

former cores. Due to high setting of instantaneous stages the re-

quirement often will be quite high.

INVERSE TIME DELAYED OVERCURRENT PROTECTION RELAYS

Core may not saturate for an AC current less than “20xI

set

”.

The 20 times factor is required to ensure that the inverse time

characteristic will be correct and no extra delay will be introduced

in some relay (due to saturation in a CT core). Such a delay

would mean a lack of selectivity. If required, the factor 20 can be

changed to “the Maximum fault current of interest for selectivity

divided by the Set current”.

DEFINITE-TIME DELAYED OVERCURRENT PROTECTION RELAYS

Core may not saturate for a current “I” less than “2xI

set

” to secure

operation.

The current output is calculated:

Both phase and earth-faults values should be checked when

earth fault currents are high. Normally however the short circuit

protection will give the highest requirement.

Overcurrent relays have moderate requirement on accuracy. 5P

or 10P class according can be used without problems. If a low ac-

curacy class is used this should be considered when selecting

the setting. Especially the margin should be increased for earth

faults when the summation of the three phases is done as the

measuring error is increased when several current transformer

cores are involved.

Loop

--------------=

IMPEDANCE AND DISTANCE PROTECTION RELAYS

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3. IMPEDANCE AND DISTANCE PROTECTION

RELAYS

Distance protection relays

Distance protection is an instantaneous impedance measuring

protection for MV and HV power lines. The general requirement

on current transformer cores which can be used for ABB Dis-

tance protection relays, of modern static and numerical type, is

that the core may not saturate within 50 ms for a fault at the end

of zone 1. Differences of acceptable saturation time between dif-

ferent relay types exists but a use of 50 ms is generally suitable.

For detailed information about the requirement we refer to the

manual of each relay. Saturation is allowed for internal faults as

the relays are designed to operate with saturated CT core without

any delay in operation. Saturation due to the DC component must

be considered due to the instantaneous operation of the protec-

tion.

The empiric formula for a CT free from saturation is:

= I

(X/R+l)(R

) where

“I

” is the current through the own line for a fault at set reach of

zone 1 and

“I

” is calculated as:

where

is the total impedance for a fault at zone 1 reach.

X/R is the ratio of X/R up to the zone 1 reach.

give the secondary resistance of the current transformer

core and the connected burden to the current transformer termi-

nal.

To calculate the required output voltage for a saturated free volt-

age the following formula can be used. The secondary time con-

stant is then considered to be high and the inﬂuence neglected:

-------------------=

w1 e

0.05

------------

–







1+×







+()=

DIFFERENTIAL PROTECTION RELAYS

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where

is the primary time constant.

4. DIFFERENTIAL PROTECTION RELAYS

There are many types of Differential protection relays used for

many different applications. Here follows a summary of the most

commonly used differential relays from ABB.

High impedance protection relays.

CT cores used together with high impedance protection must all

have identical turn ratio. T

urn correction is not allowed. Normally

separate cores must be provided for this kind of protection on all

involved current transformers. However sometimes high imped-

ance type of relays at Restricted earth fault protection application

can be used on the same core together with the Transformer dif-

ferential protection if interposing CT:s, or insulated input trans-

formers, are provided.

All cores must have a saturation voltage

sat

>2U

to ensure operation at internal faults.

Relay operating voltage U

is calculated as:

> I

smax

+ R

Loop

) where

smax

” is the max secondary through fault current and

Loop

” is the max loop resistance seen from connection point.

Two-way cable resistance must normally be used but for applica-

tions as pure phase fault protection single way cable resistance

can be used.

Transformer differential protection.

Transformer differential protection relays are percentage restraint

differential protection relays. The operation level is increased at

through faults to ensure stability even with the tap changer in an

end position and with differences between high and low voltage

CT cores. Generally the CT cores should not saturate for any

DIFFERENTIAL PROTECTION RELAYS

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through fault but the percentage stabilization and an internal sta-

bilization for current transformer saturation means that the re-

quirement can be limited.

Following formula should be used:

E2 ≥ K

x I

smax

+ R

Loop

) where

is the transient overdimensioning factor

smax

is the max secondary through fault current and

Loop

is the max loop resistance seen from connection point.

should be selected dependent of the type of Protection relay

supplied and the application.

= 2 should be used for RADSB with interposing CT’s.

= 3 should be used for RADSB without interposing CT’s or

RET 521 DIfferential protection function.

= 4 should be used for SPAD 346 or RET 316.

For one- and a half, Ring busbar or two breaker arrangements

separate stabilizing inputs shall always be used for CT's where

through fault currents can occur.

The modern relays are designed to operate correctly for heavy in-

ternal faults also with saturated CT's so the through fault condi-

tion to ensure that stability is achieved for outer faults will be

dimensioning for the involved cores. It is advisable to use as sim-

ilar saturation level (in current) for the involved current transform-

ers.

Accuracy class 5P according to IEC185, or similar accuracy

class in other standards, should preferable be used.

Pilot wire differential relay RADHL

RADHL operates with a circulating current in the pilot wires. Cur-

rent transformer cores must be provided with the same ratio at

the two terminals but don’t need to be of the same type. The CT

accuracy requirements are based on the most severe external

fault under symmetrical current conditions. Under these condi-

tions and with the CT burden composed of the CT secondary and

lead resistances, plus an allowance of 5 VA for the largest single

DIFFERENTIAL PROTECTION RELAYS

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phase burden of the RADHL summation CT, the CT shouldn’t ex-

ceed 10% accuracy

This gives an expression

where “

” is the maximum secondary AC fault current, “E

” is the

CT:s secondary voltage with 10P (or 5P) accuracy and

“E

” is the

voltage across the regulating diodes reﬂected to the primary side

of summation CT.

For 1A RADHL at ph-ef faults,

“E

” is 0.7 V and for 1A RADHL at

ph-ph faults

“U

” is 3.9 V

For 5A RADH, at ph-ph faults

“E

” is 0.15 V and for 5A RADHL at

ph-ef faults

“U

” is 0.78 V

“R

” is the cable resistance (one way for ph-ph and two way for

ph-ef faults),

“R

” is the CT:s secondary winding resistance and

“R

” is the summation CT resistance, reﬂected to the primary

side of the summation current transformer, 0.2 Ω.

When so selected no saturation due to DC component in asym-

metrical fault currents will cause maloperation. Neither will CT:s

that saturates during an internal fault, due to AC or DC, prevent

operation. General prudence suggests a limitation of the maxi-

mum fault currents to 100 times nominal current or 250 A sec-

ondary which ever is the smallest.

When current transformers of similar characteristic are provided

at both ends of the line the through faults will always saturate the

current transformers equal at both ends and smaller cores can

then be used.

The formula:

E2 ≥ 20 x I

+ R

+ 5/I

)

–

sct

--------------------------------------=

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should then be fulﬁlled, where:

is the load of other equipment connected to the same core.

is cable resistance (one way for ph-ph and two way for ph-ef

faults. If earth fault current is low only one way is sufﬁcient).

Busbar differential protection type RADSS or REB 103 and

pilot wire differential protection RADSL

RADSS/L and REB 103 are moderate impedance restraint pro-

tection relays which due to extreme speed and restraining char-

acteristic is independent of CT saturation for both internal and

external faults.

To secure operation at internal faults the CT secondary limiting

emf

“E

” or the CT knee-point voltage is:

where

“I

” is the RADSS/L, REB103 operating current

“R

” is the total differential circuit resistance

“28 Ω” is the secondary winding resistance of the aux CT in the

differential circuit, referred to the primary side and

“n

= 20V” is the forward voltage drop at the full wave rectiﬁer

in the aux CT at the differential circuit secondary side.

Loop resistance

The permissible loop resistance for secure through fault stability

seen from the relay is:

where

“S” is the setting of the slope

“R

” is the total differential circuit resistance and

where

“n

” is the turns ratio of the aux CT in the differential circuit and is

10,

28+()n

+=≥

S1S–()⁄×=

× R

d11

×()++=