Capability Curve-Based Generator Protection Minimizes...

© 2019 SEL

Capability Curve-Based Generator Protection Minimizes Generator Stressand Maintains Power System Stability

Matchyaraju Alla, Armando Guzmán,Dale Finney, and Normann Fischer

Schweitzer Engineering Laboratories, Inc.

Generator Capability Curve

0

50

100

150

200

Q (M

VA

R)

-150

-100

-50

P(MW)

15010050

Lagging PF

Leading PF

0.95

0.90

1

2

3

1. I2R rotor heating

2. I2R stator heating

3. End-iron heating for a round rotor or stability for a salient-pole generator

Fringe Flux Increases During LOF Conditions

End-Core Heating Limit VariesWith the Terminal Voltage

0 0.2 0.4 0.6 0.8 1P (pu)

-0.6

-0.4

-0.2

0

VT = 1.05 pu

VT = 1.0 pu

Q (p

u)

( ) =

21 T

d

k • VCenter P,Q 0,X

= 2 T

d

k • VRadiusX

Steady-State Stability Limit (SSSL)

SSSL is

• valid when the automatic voltage regulator (AVR) is in manualmode

• not valid during transients dueto changes in Xd

Loss of SSSL rarely occurswith modern AVRs

( )I SE

EQ

E VP sinX

= δ

Rotor Position (deg)

PM

P

δ δ 0

Rea

l Pow

er (W

)

SSSL Varies With Terminal Voltage and System Impedance

Q

0P

Weak System

Strong System

Reduced VT

= −

2T

S d

V 1 1Center 0,2 X X

= +

2T

d S

V 1 1Radius2 X X

=2

RATED T

d

MVA • VIntersectionX

Underexcitation Limiter

0

50

100

150

200

Q (M

VA

R)

-150

-100

-50

P(MW)

15010050

H2 Pressure (kPA) Lagging PF

Leading PF

0.95

20610335 0.90

UEL

• The UEL acts to increase the VT setpoint to prevent generator underexcitation

• The UEL should coordinate with SSSL and end-core heating

• The UEL can change based on VT

K, where K = 0, 1, or 2

Coolant pressure/temperature

LOF Element – Impedance PlaneX

Xd

Scheme 1

RXʹd / 2

Z11 pu

Xd

Z2

X

Scheme 1

HeavyLoad

RXʹd / 2

XʹqZ11 pu

Xd

Xʹd

Z2

X

Xd

Scheme 1

Xq

HeavyLoad

RXʹd / 2

LightLoad

Z11 pu

Xd

Z2

LOF Protection – Impedance Scheme 1

0 0.2 0.4 0.6 0.8

–0.6

–0.4

–0.2

0

0.2

0.4

0.6

MW

Capability Curve

MV

AR

UEL

Zone 2 Zone 1

LOF Element – Impedance PlaneX

Xd

Xʹq

Xq

Xʹd / 2XS

DirectionalElement

HeavyLoad

R

LightLoad

Scheme 2

Z1Xʹd

Z2

1.1Xd + XS

1.1Xd – X'd/2

LOF Protection – Impedance Scheme 2

0 0.2 0.4 0.6 0.8

–0.6

–0.4

–0.2

0

0.2

0.4

0.6

MW

Capability Curve

MV

AR

UEL

Zone 1

Zone 2

LOF Protection in the Admittance Plane

• The admittance plane preserves the shape of GCC

• GCC varies with voltage

• SSSL is fixed

B (pu)

0.8

0.6

0.4

0.2

0

SSSLUEL

GCC

G (p

u)

0.4 0 -0.4 -0.81.2 0.8

Z2Z1

• Zone 1 operates quickly for severe events

• Zone 2 coordinates with the UEL

• Zone 3 provides an alarm for operating point encroachment on SSSL

• Zone 4 provides an alarm for generator operation near GCC limits

GCC-Based LOF Protection and MonitoringQ

P

Lagg

ing

Pow

er F

acto

rLe

adin

g P

ower

Fac

tor

Zone 2

Zone 4

Zone 1

Zone 3

Zone 1 Operates for Severe LOF Conditions

• Operates for LOF at heavy loading conditions

• Is set in the power plane but operates in the admittance plane

• Can be set similarly to traditional approaches

Lagg

ing

Pow

er F

acto

rLe

adin

gP

ower

Fac

tor

Heavy Load

Zone 1

P

GCC

Q

Zone 2 Coordinates With UEL

• UEL can have a curved ormulti-segmented characteristic

• Zone 2 element matches the shape of the UEL and varies with VT

K , where K = 0, 1, or 2• Zone 2 element includes

an accelerated path for undervoltage conditions

UEL

Lagg

ing

Pow

er F

acto

rLe

adin

gP

ower

Fac

tor

P

GCC

Q

Light Load

Zone 2

Zone 3 Is Dedicated to Coordinate With SSSL

• Implements the SSSL equation by using only Xd and XS

• Varies with VT2

• Alarms when thePQ locus crosses the SSSL

• Trips when the PQ locus crosses the SSSL AND the AVR is in manual mode OR VT collapses

− = − −

*2 2T T

puS d

j3• V j3• VZ3 Re S • SX X

Zone 4 Monitors GCC Limits

• Is set based on GCC• Adapts to changes in

cooling capabilities• Alarms for any segment

violation

Q

Lead

ing

Po

wer

Fac

tor

Minimum CoolingMaximum Cooling

Lagg

ing

Po

wer

Fac

tor

3

2

1

P

UEL With Static Characteristic in the Power PlaneK = 0 at VT = 1.00 pu

0 5 10 15 20 25 30 35 40 45

-30

-25

-20

-15

-10

0

P (MW)

Q (M

VA

R)

UEL

GCC

-5


Zone 1 Per Impedance Scheme 2

Zone 1 Settings

Value (pu)

40P1P 0.6 pu

Tilt –5 deg

0 5 10 15 20 25 30 35 40 45

-30

-25

-20

-15

-10

0

P (MW)

Q (M

VA

R)

UEL

Zone 1 (VT = 1.0 pu)

GCC

-5


Coordination of UEL and Zone 2 Characteristics

UEL Settings in AVR [P, Q] (Primary)

Zone 2 Settings Value (Primary)

[40, –12.6] [UELP1, UELQ1] [40, –12.6][20, –18] [UELP2, UELQ2] [20, –18][0, –19.8] [UELP3, UELQ3] [0, –19.8]

Margin 10%Characteristic LinearVoltage Dependency (k) 0

0 5 10 15 20 25 30 35 40 45

-30

-25

-20

-15

-10

0

P (MW)

Q (M

VA

R)

Zone 2

UEL


GCC

-5


SSSL Characteristic

Zone 3 Settings

Value (pu)

Xd 1.8

XS 0.2

Voltage Acceleration

0.8

0 5 10 15 20 25 30 35 40 45

-30

-25

-20

-15

-10

0

P (MW)

Q (M

VA

R)

Zone 2

UEL



GCC

-5


0 5 10 15 20 25 30 35 40 45

-30

-25

-20

-15

-10

0

P (MW)

Q (M

VA

R)

Zone 2Zone 4



GCC

-5

UEL


• UEL, Zone 2, and Zone 4 are static

• Zone 1 and Zone 3 change in direct proportion to VT

2

0 5 10 15 20 25 30 35 40 45

-30

-25

-20

-15

-10

0

P (MW)

Q (M

VA

R)

Zone 2Zone 3

UEL

GCC

-5

Zone 1


Zone 3 moves closerto UEL

0 5 10 15 20 25 30 35 40 45

-30

-25

-20

-15

-10

0

P (MW)

Q (M

VA

R)

Zone 2Zone 3

UEL

GCC

-5

Zone 1


• LOF conditions for weak systems can lead to pole slipping

• Stable power swings can enter Zone 3 and Zone 1

0 5 10 15 20 25 30 35 40 45

-30

-25

-20

-15

-10

0

P (MW)

Q (M

VA

R)

Zone 3

Zone 1

GCC

-5

Zone 2UEL

UEL With Dynamic CharacteristicK = 1 at VT = 1.00 pu

0 5 10 15 20 25 30 35 40 45

-30

-25

-20

-15

-10

-5

0

P (MW)

Q (M

VA

R)

UEL

Zone 1


Coordination of UEL and Zone 2 Characteristics

UEL Settings in AVR [P, Q] (Primary)

Zone 2 Settings Value (Primary)

[40, –12.6] [UELP1, UELQ1] [40, –12.6][20, –18] [UELP2, UELQ2] [20, –18][0, –19.8] [UELP3, UELQ3] [0, –19.8]

Margin 10%Characteristic QuadraticVoltage Dependency (k)

1

0 5 10 15 20 25 30 35 40 45

-30

-25

-20

-15

-10

-5

0

P (MW)

Q (M

VA

R)

Zone 2

UEL

Zone 1


0 5 10 15 20 25 30 35 40 45

-30

-25

-20

-15

-10

-5

0

P (MW)

Q (M

VA

R)

Zone 3

UEL

Zone 1

Zone 2


0 5 10 15 20 25 30 35 40 45

-30

-25

-20

-15

-10

-5

0

P (MW)

Q (M

VA

R)

Zone 4 Zone 2

UEL

Zone 1

Zone 3


• UEL, Zone 2, and Zone 4 vary in direct proportion to VT

• Zone 1 and Zone 3 vary in direct proportion to VT

2

0 5 10 15 20 25 30 35 40 45

-30

-25

-20

-15

-10

-5

0

P (MW)

Q (M

VA

R)

UEL

Zone 2

Zone 3 Zone 1


• Zone 3 encroachesinto GCC

• UEL can protect generator operationfrom reaching SSSL

0 5 10 15 20 25 30 35 40 45

-30

-25

-20

-15

-10

-5

0

P (MW)

Q (M

VA

R)

UEL Zone 2

Zone 3 Zone 1

Case 1: Lack of CoordinationBetween Zone 2 and UEL

Imag

inar

y (s

econ

dary

ohm

s)–20 –10 0 10 20 30

–40

–30

–20

–10

0

10

20

30

Real (secondary ohms)

Capability Curve

Zone 2

UEL

0 20 40 60 80 100

–60

–40

–20

0

20

40

60

80

MW

Capability Curve

Zone 2

MVA

R

UEL

Apparent Impedance Entered Zone 2, Causing Undesired Operation

–60

–40

–20

0

20

40

60

80

MW

MVA

R Operating Point

0 20 40 60 80 100

–40

–30

–20

–10

0

10

20

30


Imag

inar

y (s

econ

dary

ohm

s)

Operating Point

–20 –10 0 10 20 30

After Improving Coordination,Apparent Impedance Is Outside Zone 2

–60

–40

–20

0

20

40

60

80

MW

MVA

R Operating Point

0 20 40 60 80 100

–40

–30

–20

–10

0

10

20

30


Imag

inar

y (s

econ

dary

ohm

s)–20 –10 0 10 20 30

Operating Point

Seconds

-1

0-0.5

Q (p

u)0.5 1 1.5 2 2.5 3 3.5 4 4.5

20

0.51.5

1

0P (p

u)

0.5 1 1.5 2 2.5 3 3.5 4 4.5

0.5

1.51

I T (p

u)

0.5 1 1.5 2 2.5 3 3.5 4 4.5

0.5

1

0.5

VT

(pu)

1 1.5 2 2.5 3 3.5 4 4.5

(a)

(b)

(c)

(d)

Case 2: Loss of Stability When AVRWas in Manual Mode During Black Start

P (M

W)

Steady State PE

PM

δ (degrees)

Generator Lost Stability When Mechanical Power Was Increased Beyond SSSL

q-axis

d-axis

NRotor

S Rotor

SStator

The Relay Operated Whenthe Apparent Impedance Entered Zone 2

-2 -1.5 -1 -0.5 0 0.5 1 1.5 2R (pu)

-2

-1.5

-1

-0.5

0

0.5

1

X (p

u)

Zone 1

Zone 2

Directional Element

SSSL

Xd

X'd2

0 10 20 30 40 50 60P (MW)

-30

-25

-20

-15

-10

-5

0

Q (M

VA

R)

Zone 3 Would Have Operated Faster ThanTraditional Protection

Conclusions• LOF scheme is tailored to GCC and improves

generator protection and monitoring• LOF scheme includes four zones, each with a

specific purpose• Scheme is easy to set Enter UEL and GCC characteristics directly to configure

the scheme

Enter XS and Xd to replicate SSSL

Conclusions• A graphical view of the elements in the PQ and

impedance planes minimizes setting errors• Achieve improved coordination with UEL The K setting allows the relay to exactly match the

dynamics of the UEL

The characteristic can expand and contract accordingto a measurement of cooling capability

Improved coordination can maximize utilization of the generator

Questions?

Capability Curve-Based Generator Protection Minimizes...

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