06_Overcurrent and Impedance Protection

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Overcurrent and Impedance Protection

Power Automation 2

Power Transmission and Distribution

Power AutomationProgress. It‘s that simple.

Presenter: Dr. Hans-Joachim Herrmann PTD PA13Phone +49 911 433 8266E-Mail: [email protected]

Generator ProtectionOvercurrent and Impedance

Protection

Power Automation 3



Summary

The short circuit is a heavy fault for a generator. In that case the short-circuit protection is a standard protection for a generator. The electromotive force (e.m.f.) supplies at the moment of a short circuit the following current :

with: VN - nominal voltageXd“ - subtransient reactance

This subtransient short-circuit current is reduced with the typical time constants of a generator. The type of the excitation has an important influence on the magnitude of the short circuit current. In the case of a short circuit near the terminals the voltage is low and the voltage regulator starts to increase the excitation. A higher current is the result.Dimensioning the co-ordination of the voltage regulator and excitation system (including excitation winding) based on a short circuit current for a time of 8-10s is appr. 1,8 of the nominal generator current.The settings of an overcurrent time relay is appr. 1,3 - 1,4 x IN . If the short- circuit current can be lower after a time an undervoltage seal-in must be activated.The short-circuit current depends on the fault type.The ratio is ISC(1) : ISC(2) : ISC(3) = 5:3:2. In the case of a no load field voltage and a full load field voltage the range of the short-circuit current is appr. 1:2,5.For a turbo generator the following steady state short-circuit currents are typically:

three-phase fault: ISC ??(0,8 ... 2)INphase-to-phase fault: ISC ??(1,2 ... 3)INsingle-phase fault: ISC ??(2 ... 5)IN(grounding of the generator star point)

If the generator is grounded with a low ohmic resistor the short-circuitcurrent is limited to the nominal current.

Part 1: - Overcurrent Protection- Impedance Protection

Part2: - Differential Protection

"31,1

d

NK(3) X?

??

VI

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Short Circuit Faults

A) Internal Faults1) Isolated star point

(or high ohmic)

L1

L2

L3

3phase short circuit

Phase to phase short circuitInterturn fault

2) Low ohmic star point(or solid)

Single phase short circuit(Resistor limits the fault current)

Faults from 1) are also additional possible to ground

B) External Faults

Transformer

Grid

Generator is the e.m.f. for the short circuit currents and becomes to stress? Back-up protection

Power Automation 5



Typical Short Circuit Curves

''k

2 ??

k2 ??

? ? d'T't-

-2 ekk

?? ''' ??

? ? dTt-

-2 ekk

'' ?? ??

Tgt-

2 ek ?? ''?Combinations:

Driving Voltages

Actual reactances

Time Constants

V??p < V?p < Vp

X??d < X?d < Xd

T??d < T?d < Td

? ? ? ? ? ? ? ? ? ?????

?

?

????

?

?

???????? ??????? sin Tt

-e-tsin -tsin T

t-

e--tsin Tt

-e2i gddk

kkkkkk''''''''' IIIIII

Short circuit current:

Subtransient part transient part Steady stateshort circuit current

DC - current

I??SC= Subtransient short-circuit currentI?SC = Transient short-circuit currentISC = Steady state short-circuit current

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Example of a Short-Circuit Curve

Data: SN = 200 MVA (Synchronous generator)

xd = 1,78 x´d = 0,24 x´´d = 0,16TG = 0,097s T´d = 1,125s T´´d = 0,05s

Rotor (e.m.f) voltages

Vp Vp´ Vp´´

Turbo generator 2,25 1,11 1,07

Salient pole generator 1,8 1,2 1,13

Without Dc offset (? = 0°)

Short circuit currents

NNd

K

NNd

KNNd

K

6,7 X

1,07

4,6 X

1,11 1,26

X2,25

III

IIIIII

????

?

???

????

´´

´. VN. VN

Fully asymmetrical DC component (? = 90°)

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Short-Circuit Current within the Generator(Three-phase fault)

R, Xd

?

?

Ik

0,5 1

?Driving voltage (e.m.f.) increases linear

? Synchronous direct-axis reactance increasessquare with the number of windings ( ? 2 ·Xd)

? Short-circuit current on the terminals is lower than within

? Short circuit near the star point has small short-circuit currents

Power Automation 8



Overcurrent Time Protection

I >>, t

I >, t

I >, t

I >, t

G

I > - Generator

I >>

I > - Network ? t

l

t

? t:grading time(approx. 0,3 s)

? Protection is connected on the star-point-CT (proof against internal faults - at open circuit breaker too)

? Pick-up value I>; Ipick-up ? (1,2.....1,4) IN, G

? Co-ordination of grading time with the network protection

? I>>-stage: responsible for faults near or within the generator with a short tripping time (0,1 s) ? Busbar connection: reverse interlocking is recommended for radial network? Unit transformer connection: Ipick-up > 1,5 I ??SC(3, G) t ? 0,1 s

Power Automation 9



Overcurrent Time Protection with Undervoltage Seal-in

Logic:

G

I>, tU< Pick-up

T-Seal-in

QSR

I>

V<&

? 1

>1

? The excitation cannot maintain the short-circuit current, because the excitation voltage is to low.

? At long fault durations the short-circuit current can be close or lower than the nominal current ISC< INSettings: V< ? (0,75....0,8) VN

V: positive sequenceT- seal-in > T-I>

Attention: If the generator breaker is open blocking of the undervoltage seal-in at units with generator and network circuit breakers.

Advantage of Seal-in: No Overfunction at VT Failures

Power Automation 10



Inverse Time Overcurrent Protection (51)

In ANSI or English speaking regions are very often used the inverse time overcurrent protection. The trip time depends on the amount of current. In the applications different types of characteristics are available.

G

I

t

Grading time

Example: Very Inverse (ANSI)

D 0.0982 1 - I

I

3.922 t 2

P

?

????

?

?

????

?

?

?

????

??

?IP - Pick-up current(General pickup is fix at 1,1 IP)D - Time multiplier (0,5s to 15 s)

The same characteristic is used

Power Automation 11



Inverse Time Overcurrent protection influenced by voltage (51 V)

The voltage dependency of the short circuit current is considered

Voltage controlled:If there is an undervoltage pick-up, the inverse time characteristic will be released with a low pick-up value (IPICK-UP < ING)(Used at generator-busbar schemes)

Voltage restraint:The pick-up threshold of inverse time overcurrent protection depends on voltage level. Low voltage reduces the pick-up threshold by a factor (see figure). There is a linear dependency of the generator voltage.(Used at generator-transformer schemes)

Note:To avoid an overfunction in the case of an fuse failure (voltage circuit interruption), there is recommended a blocking via an external m.c.b. or the internal fuse - failure -monitor function.

0,25

0,25

1,0

1,0

Factor

V/VNG

Power Automation 12



Impedance Protection

G

Z<

XG

Z<

XT

Z1

0,1s

tNetz +? tI>

t

T1 0,7X Z ?

? Correct measuring of the transformer reactance? Measuring of a fictitious generator reactance

(short circuit at the generator terminals: V ? 0, I =ISC,G ? Z ? 0)

? Settings value of the protection is the unit transformer reactance(Requirement: Z1 ? 0,7 · XT)

Power Automation 13



Impedance Measuring over the Unit Transformer (Part 1)

Additional Infeed

Z< Z<ZT ZT

IK1IK2

IKZL

Three-phase fault

? Correct measurement of the impedance

? Setting problems at short lines because:

? Through the additional infeed theimpedance is measured always tolarge

? Underreach of the protection (Trip in a longer time)

LTr

2T Z Ratio

1 Z ??

LTr

2TMeas Z Ratio

1 Z Z ??? L

Tr2TMeas Z

Ratio1

Z Z 1K1

K2 ????

????

????

II

RatioTr

RatioTr : Transformer Ratio

Power Automation 14



Impedance Measuring over the Unit Transformer (Part 2)

L1

L2

L3

L1

L2

L3

Phase-to-phase fault Single-phase fault

Pick-up of the protectionin three phases

Accurate measurement:

In the other impedance loops the impedance is measuredtoo large (lower current).

Pick-up of the protection in two phases

Impedance is measured too large for the zero sequence impedance(There is no zero in short circuit current on the generator side.)

L2

L2

maxph,

E-ph.Meas

V

V Z

II?? ? ?0L0TLT

ph.-ph.

ph.-ph.Meas Z Z

21

Z Z V

Z ?????I

Power Automation 15



Characteristics of the Impedance Protection

T1 0,7X Z ?

X

R

X2

X1B

X1

R1 R1B R2

No direction element is necessary because the star point CT is used.

? At open generator circuit breaker the stage Z1B can be actived

? Co-ordination of zone Z2 with the networkprotection (Measuring errors cause an underreach)

t

Z1 Z1BZ2

t(Z1) t(Z1B)

t(Z2)

t(I>)

Power Automation 16



Scheme with an Impedance or Distance Protection

ZG: Generator ZT: Transformer ZL: LineTransformer LineLine

ZG ZT

ZL

ZT (backward)

I > (ZG)

ZG

2. Stage (ZT) ZT

ZL

t

Z

G

? Fast back-up protection for the whole system.

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06_Overcurrent and Impedance Protection