05 underexcitation protection

7/23/2019 05 Underexcitation Protection

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UnderexcitationProtection


2/28

Power Automation 2

Power Transmission and Distribution

Power AutomationProgress. Itsthatsimple.

Presenter: Dr. Hans-Joachim Herrmann

PTD PA13

Phone +49 911 433 8266E-Mail: [email protected]

Generator Protection

Underexcitation Protection

(Loss of Field protection)


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Power Automation 3



Reasons for Underexcitation

? Failure of the excitation device

? short circuit in the excitation circuit

? interruption in the excitation circuit

? Maloperation of the automatic voltageregulator

? Incorrect handling of the voltage regulator(generator, transformer)

? Generator running with capacitive load

Countermeasure:

Underexcitation Protectionexcitationdevice

GS

3~

ZLoad

Note: This protection is also called

Loss of Field Protection


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Power Automation 4



Consequences of Excitation Failures

Influence Quantities:? type of construction of the generator

? design of the excitation

? grid conditions

? magnitude of delivered active power

?

type of the voltage and power regulator

Consequences:

? rotor acceleration

? local overheating in the rotor and stator

? over-voltages in the rotor

? mechanical shocks onto the foundation? grid starts oscillating


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Power Automation 5



Relation of Current and Voltage in a SynchronousGenerator

machine equation: VP = V - IjX

cylindrical-rotor machine: X~Xd; VP = V- jXd I(turbo generator)

salient-pole machine:

(hydro generator)

X:=Xq; Xd

exact: VP = V - j(XdId + XqIq)

reduced: VP ~ V - jXqI

ZLVVP

X I

Simplified equivalent circuit:

Vector diagram:

V

Iexc

I

Im

Re

? = rotor angle? = load angle

?

VpI jX

?

V / jX


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Power Automation 6



Possible Design of the Generator Capability Diagram

Definition:

+P

(W)

+Q

(Var)

under excited

over excited

+P

(W)

+Q

(Var)

over

excitedunder

excited

Operating

area

Operating

area

Static stabilitylimit

Static

stabilitylimit

Preferred design


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Power Automation 7



Capability Curve of a Turbo Generator

type of generator: TLRI 108/46

nominal apparent power SN = 200 MVAnominal voltage VN = 15.750 kVnominal current IN = 7.331 kAnominal frequency fN = 50.0 Hzpower factor cos ? N = 0.8cold-air temperature Tx = 40.00 C

MVAr 140 120 MVAr100 80 60 40 20 0 20 40 60 80 100 120 140 160 180

underexcited overexcited

Q

P

MW

220

200

180

160

140

120

100

80

60

400,2

0,4

0,6

0,7

0,

8

0,85

0,

9

0,95

0,97

5

0,

975

0,95

0,

9

0,2

0,4

0,6

0,7

0,8

0,85

cosphi

cosp

hi


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Power Automation 8



Load Diagram of a Synchronous Machine (Cylindrical-rotor Machine)

dynamicstabilitylimit

steadystatestabilitylimit

theoreticallylimit

turbine limit

stator limit

rotor limit

P

overexcitedunderexcited

Q

SN

VP If

? N

? N

N

d

2

N ;'

SX

V?? d

2

N

X

V

Xd: synchronous reactanceXd: transient reactance

The generator capability curve describes

the stability limits of the generator


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Power Automation 9



Per Unit Capability Diagram of a Synchronous Generatorin the Case of Undervoltage (V = 0.9 VN)

In the case of an under-voltage the generator capability curvemoves to right and reduces the stability limits of the generator

1/xd

0.81/xd

0.851 V=1; I=1;

V=0.9; I=1.11

Stabilitylimit

Q [p.u]

P [p.u]

overexcitedunderexcited


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Power Automation 10



Conclusions for the Protection Design

A good underexcitation protection should consider both facts(1 and 2)

1. The generator capability curve describes in theunderexcitation region the stability limit of a generator

2. In the case of an undervoltage the stability becomesmuch more critical (moves to active power axis)

The transformation of the generator diagram into the

admittance diagram is the solution, because:

? its direct proportional to the per unit generator diagram

(only the reactive axis must be multiplied by -1)? the settings can be easy read out from the generator diagram

? it considers right the undervoltage behaviour

A

B


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Power Automation 11



Definitions for Converting the GeneratorDiagram into the Admittance Diagram

QjPSIVS ???? ?

BjGYV

IY ???

Transformation:

2222

*

V

Qj-

V

P

V

Qj-P

V

S

VV

VIY ???

?

??

?

?

2

2

V

Q-B

V

PG

?

?

Q

P+

+-

B

G+

+ -

Note: In the per unit calculation is VN = 1

Complex Power: Admittance:

G: Conductance

B: Susceptance

In the per unit

representation

the diagrams

are the same,

only there is a

phase shift in

the reactive

part of 180


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Power Automation 12



a) Excitation Current (IEXC

)

- stabile for over-excitation

- insecure for under-excitation (IEXC can be smaller than IEXC, N)

b) Direct Measuring of the Rotor Angle (? )

-steady stability limit depends on?

or 2?

- transversal reactance cannot be neglected with small excitationof

turbogenerators (Xq is also depending from 2 ? )

c) Reactive power I-QI>, Impedance I-ZI

? Admittance calculation guaranteesa right behaviour, if the voltages

decreases

? 3 independent characteristics and 3

timer

? characteristic 1,2 is adaptated onthe steady state curve;

? additional inquiry of the field voltage

(release a short trip time)

? characteristic 3 is adaptated on the

dynamic stability limit curve

? blocking of the protection at V


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Power Automation 14


Power AutomationProgress. Itsthatsimple. Combination of

? stator criterion (straight line characteristics)

? rotor criterion (DC undervoltage in the excitation circuit)

Case no. 1: only rotor criterion fulfilled:

no alarm, no trip

Case no. 2: only stator criterion fulfilled ( char. 1,2):only alarm, eventually long-time delayed trip (e.g. 10s)

Case no. 3: rotor and stator criterion fulfilled (char. 1,2):

alarm and short-time delayed trip (e.g.


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Power Automation 15



Conversion of the Reactive Power into 1/xd

Example: Generator capability diagram (figure 9)

Generator: SN =200 MVA CT, VT: knV =VN =15,75 kV

Stability limit: Q =90 MVAr knl = 8000A/1A

1. Calculation the longitudinal reactance :

2. Conversion into secondary values :

3. Setting value for Char. 1:

?????

?

?

?

?

?

?

?

2,76Q

3Q3 N

2

2

N

d U

U

X 2,223N

Ndd ??? UXx I

2,38kk

WpN,GN,

WpN,GN,

d

nUGN,Nsek

nlNsekGN,

ddsek ?????? U

Ux

U

Uxx

I

I

I

I

?? 0,421

dsekx ??? 80, 10,42

d1x

1?

3

100V/3

16kV

2,22Nd ?? Qx S

or


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Power Automation 16



Measuring Algorithm

1) Filtering of the input values

2) Calculation of the positive sequence values

3) Calculation of the complex power

4) Calculation of the admittance values

GS3~

iL1,2,3 uL1,2,37UM6

Li

Lv

,,, L3L2L1 III

,,, L3L2L1 VVV

1I

1V

,j iLrLL III ??

,j iLrLL VVV ??

1I

1V

*IUS ? QjPS ??

S 21U

SY ? BjG

1j

1 ????

XRY

symmetr.

comp.

fourierfilter

(50Hz)


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Power Automation 17



Generator diagram is transferred in the impedance plan (e.g. X=V2/Q).

(Stability limit is represented as a circular arc.)

? characteristic: Offset-MHO

? tripping zone inside the circle

? characteristic 1, tdelay ? 0...0.3 s (for high

load generator and field failure)

? characteristic 2, tdelay ? 0.5 - 3 s (for lowload generator, section field voltage failure)

Underexcitation Protection with Criterion ImpedanceI-ZI

05 underexcitation protection

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