Protection Application Handbook (англ. яз.)

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POLE DISCORDANCE RELAYS

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Pole discordance relay “PD” is necessary to prevent service with

only one or two phases of a circuit breaker closed. This will cause

an unsymmetry that can lead to damage of other apparatus in the

network. Unsymmetrical conditions can only be accepted during

a single pole Auto reclosing cycle.

When single pole Auto reclosing is used a blocking of the PD is

necessary during the single phase dead time, see ﬁgure 9.

The timing sequence at closing and opening of breakers and the

timing of the operation at a pole discrepancy tripping must be

checked carefully. It is e. g. often necessary to operate the ﬂag

relay ﬁrst and the tripping from this relay to achieve correct indi-

cation at a trip as the main contacts will rather fast take away the

actuating quantity when the remaining poles are opened.

Tripping from PD should be routed to other trip coil than the one

used by manual opening. This is due to the fact that at manual

opening there could be an incipient fault in the circuit. This incip-

ient fault could result in that only one or two phases operate and

the MCB for the circuit is tripped. PD would thus not be able to

operate if connected to the same coil as manual opening.

Operation of PD shall in addition to tripping the own circuit break-

er start Circuit Breaker failure relay, lock-out the own circuit

breaker and give an alarm.

A normal time delay for a PD relay should be approximately

150-200 ms when blocking during single pole Auto reclose is per-

formed. If not the necessary time is about 1,2 sec.

Note

: Some customers do not rely only on the auxiliary contacts

and want to detect Pole discordance also by measuring the resid-

ual current through the breaker at breaker for some second at

opening and closing.

We have however good experience of the contact based principle

and recommend only this simple principle.

Residual current measurement at closing and opening is not a re-

liable function either as there is no load current when a line is

closed from one end only and on the other hand normal residual

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currents can occur if the closing is onto a fault or e. g. energizes

a transformer at the far end of the system.

Conclusion is to use a simple contact based Pole Discordance

function which is simple and reliable. The residual current based

principle is more complicated and can give difﬁculties in setting

of sensitivity etc.

INTRODUCTION

Earth Fault Protection

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1. INTRODUCTION

The fault statistic shows that earth faults are the dominating fault

type and therefor the earth fault protection is of main importance

in a network.

The type of earth fault protection used is dependent of the sys-

tem earthing principle used. In the following the earth fault pro-

tection for solidly- (effectively), reactance-, high resistive- and

resonance earthed systems are covered.

2. EFFECTIVELY EARTHED SYSTEMS

In the effectively earthed systems all transformers are normally

connected to earth and will thus feed earth fault current to the

fault. The contribution from all earthing locations gives special re-

quirements for the protection system.

2.1 FAULT RESISTANCE AND FAULT

CURRENT LEVELS

In order to calculate fault currents in an effectively earthed sys-

tem we must use the representation with symmetrical compo-

nents.

The symmetrical component scheme for a 132 kV system with a

fault according to Figure 1 is shown in Figure 2.

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Figure 1. An earth fault in a direct, effectively earthed system.

Figure 2. The symmetrical components are used to calculate the “I

” current

for the fault in ﬁgure 1, “3I

” is the total fault current.

The distribution of fault currents, from the different system earth-

ing points, can be derived from the distribution in the zero se-

quence network (see ﬁgure 2). By inserting varying fault

resistances one can get the fault current level.

Positive sequence

Negative sequence

Zero sequence

Uh/√3

I01

I02

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The fault resistance “R

”, consists of the arc resistance and the

tower foot resistance. The arc resistance is calculated by the for-

mula:

arc

= 28700a / I

1.4

(according to Warrington)

where “a”, is the arc length in meter, normally the insulator length,

and “I

” is the fault current in “A”.

A calculation will show that values will differs from below 1 Ω for

heavy faults, up to 50-400 Ω for high resistive earth faults.

The tower foot resistance depends on the earthing effectiveness

of the towers, whether top lines are used etc. For the tower foot

resistance values from below 10 Ω up to 50 Ω have been docu-

mented.

2.2 NEUTRAL POINT VOLTAGES

The occurring neutral point voltage, at different locations, can be

seen in ﬁgure 2. The designate “U

”, represents the neutral point

voltage (3U

= U

). It’s to be noted that “U

” is generated by the

earth fault current “I

” through the zero sequence source. This im-

plies that the angle between “U

” and “I

” is always equal to the

zero sequence source angle, independent of the fault resistance

and the angle between the faulty phase voltage and the line cur-

rent in the faulty phase.

It must also be noted that “U

” will be very low when sensitive

earth fault relays are used in a strong network with low zero se-

quence source impedances.

As an example we can use the 132kV network according to ﬁgure

1 and 2. With an “I

” setting of 120A, the “I

” is 40A and with a

zero sequence source impedance of say 20 Ω, the zero se-

quence voltage component “U

” will be 40x10 = 400V and “3U

”

will then be 1200V. This will, with an open delta winding with 110V

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secondary, mean a percentage voltage of 1.6%, i.e. the polariz-

ing sensitivity of directional earth fault relays must be high.

In an open delta secondary circuit there is a voltage also during

normal service due to unbalances in the network. The voltage is

mainly of third harmonic and of size 0,2-0,5% with conventional

VT:s and 1-3% together with CVT:s.

This means that the sensitive directional earth fault protection

must be provided with a third harmonic ﬁlter when used together

with CVT:s. The ﬁltering must be quite heavy to ensure correct di-

rectional measuring for 1% fundamental content also with third

harmonic contents of say 3%.

2.3 RESTRICTED EARTH FAULT PROTECTION

(REF).

For solidly earthed systems a restricted earth fault protection is

often provided as a complement to the normal transformer differ-

ential relay. The advantage with the restricted earth fault relays is

their high sensitivity. Sensitivities of 2-8% can be achieved. The

level is dependent of the current transformers magnetizing cur-

rents whereas the normal differential relay will have sensitivities

of 20-40%.

Restricted earth fault relays are also very quick due to the simple

measuring principle and the measurement of one winding only.

The differential relay requires percentage through fault and sec-

ond harmonic inrush stabilization which always will limit the min-

imum operating time.

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The connection of a restricted earth fault relay is shown in Figure

3. It is connected across each transformer winding in the ﬁgure.

Figure 3. A restricted earth fault relay for an YNdyn transformer.

It is quite common to connect the Restricted earth fault relay in

the same current circuit as the transformer differential relay. This

will due to the differences in measuring principle limit the differ-

ential relays possibility to detect earth faults. Such faults are de-

tected by the REF. The mixed connection is shown in the low

voltage winding of the transformer, see ﬁgure 3.

To Diff

protection

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The common principle for Restricted earth fault relays is the high

impedance principle, see ﬁgure 4.

Figure 4. The high impedance principle.

The relay provides a high impedance to the current. The current

will, for through loads and through faults, circulate in the current

transformer circuits, not go through the relay.

For a through fault one current transformer might saturate when

the other still will feed current. For such a case a voltage can be

achieved across the relay. The calculations are made with the

worst situations in mind and an operating voltage “U

” is calcu-

lated:

where

“I

Fmax

” is the maximum through fault current at the secondary

side,

“R

” is the current transformer secondary resistance and

“R

” is the loop resistance of the circuit.

e1 e2

Rct RL RL Rct

Xm2

Xm1

Secondary current and voltage with

no voltage limiter.

Z=>0

at sat.

Fmax

+()≥

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The maximum operating voltage have to be calculated (both neu-

tral loop and phase loop must be checked) and the relay set high-

er than the highest achieved value.

For an internal fault the circulation is not possible and due to the

high impedance the current transformers will immediately satu-

rate and a rms voltage with the size of current transformer satu-

ration voltage will be achieved across the relay. Due to the fast

saturation very high top voltages can be achieved. To prevent the

risk of ﬂashover in the circuit, a voltage limiter must be included.

The voltage limiter can be either of type surge arrester or voltage

dependent resistor.

The relay sensitivity is decided by the total current in the circuit

according to the formula:

where

“n” is the CT ratio, “I

” is the current through the relay, “I

res

”

is the current through the voltage limiter and “∑I

mag

” is the sum of

the magnetizing currents from all CT’s in the circuit (normally 4).

It should be remembered that the vectorial sum of the currents

must be used. The current measurement have to be DC insensi-

tive to allow a use of AC components of the fault current in the

calculations.

2.4 LOGARITMIC INVERSE RELAY

Detection of earth fault and back-up tripping with maintained se-

lectivity in a solidly (effectively) earthed system is rather compli-

cated due to the infeed of fault current from different direction

concerning all faults. A special inverse characteristic with a log-

aritmic curve has been developed to be suitable for these appli-

cations. The principle for earth fault relays in a effectively earthed

system is shown in ﬁgure 5 and the logaritmic inverse character-

istic is shown in ﬁgure 6. The inverse characteristic is selected so

res

ΣI

mag

++()≥

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that if the current of the largest infeed is less than 80% of the

faulty objects current selectivity is achieved.

Figure 5. Earth fault protection in a solidly earthed system.

Figure 6. The logaritmic inverse characteristic. A fault current of the biggest

infeed less than 0,8 times the current of the faulty object, gives a selective

tripping.

I I