NERC PRC-026 Evaluation

Dr. Juergen Holbach Quanta Technology

Raleigh, NC

Overview

Ø Basics of Power Swing Ø  Impact on Distance Protection Relays Ø NERC PRC026

Simple two machine model

Vs Va Vb Vr

I*Xs1 I*Xs2I*Xl

ϑIa

Real power in relation to voltage angle

ϑ

P

Ps=Pr

Pmax

90 180

stableworkingarea

)sin(21

ϑSlS

RS

XXXVVP++

⋅=

Impedances in relation to the voltage angle

I*Zs2

Vs

Va Vb

Vr

I*Zs1 I*Zl

ϑ=60

Ia

Z loadϑ=60

R

X

Impedance seen by the relay inthe electr. center

Impedanceseen by the relay on Bus Blooking into the motor

Impedances in relation to the voltage angle

ϑ=120

VaIa

VrVs Vb

I*Zs2I*Zs1 I*Zl

Z loadϑ=60

R

X

Z loadϑ=120

Impedances in relation to the voltage angle

I*Zs2

VsVb

Vr

I*Zs1 I*Zl

ϑ=180

Ia

Va=0

Z loadϑ=60

R

X

Z loadϑ=120

Z loadϑ=180

Impedances trajectory for a system with losses

X

Impedancetrajectoryseenbytherelayintheelectr.center

R

Z loadϑ=60

Z loadϑ=120

Z loadϑ=180

ImpedancetrajectoryseenbytherelayonBusBlook ingintothemotor

Impedances trajectory with unequal machine voltages

VsVa

Vr

ϑ=0Ia

X

RZ loadϑ=60

Z loadϑ=120

Z loadϑ=180

ϕ

Z loadϑ=0

Power Swing Trajectories fro different conditions

B

A

0.2

0.3

0.4

0.5

0.6 0.65 0.7 0.75 0.8

0.85

0.9

0.95

1.05

1.1

1.15

1.21.251.31.41.51.61.7

∅=20°

∅=25°30°40° 35°45°50°

1.71.61.5

1.41.31.251.2

∅=340°

1.15

1.1

1.05

∅=335°

0.95

0.9

0.85

0.80.75

0.7 0.65 0.6

∅=0°

1.81.92.0

2.5

5.060°

75°90°120°150°180°21

0°240°270°285°∅= 300°

B

A

E

E

B

A

E

E

B

A

E

E

NERC PRC 026 requirement

Ø  Criterion A: Ø  An impedance-based relay used for tripping is expected to not trip for a stable power swing

when the relay characteristic is completely contained within the unstable power swing region. The unstable power swing region is formed by the union of three shapes in the impedance (RX) plane (see Figure 3‑10); (1) a lower loss-of-synchronism circle based on a ratio of the sending-end to receiving-end voltages of 0.7; (2) an upper loss-of-synchronism circle based on a ratio of the sending-end to receiving-end voltages of 1.43; (3) a lens that connects the endpoints of the total system impedance (with the parallel transfer impedance removed) bound by varying the sending-end and receiving-end voltages from 0.0 to 1.0 per unit, while maintaining a constant system separation angle across the total system impedance where:

Ø  1. The system separation angle is at least 120 degrees or an angle less than 120 degrees where a documented transient stability analysis demonstrates that the expected maximum stable separation angle is less than 120 degrees.

Ø  2. All generation is in service and all transmission BES elements are in their normal operating state when calculating the system impedance.

Ø  3. Saturated (transient or sub-transient) reactance is used for all machines.

Impedances trajectories

B

A

0.2

0.3

0.4

0.5

0.6 0.65 0.7 0.75 0.8

0.85

0.9

0.95

1.05

1.1

1.15

1.21.251.31.41.51.61.7

∅=20°

∅=25°30°40° 35°45°50°

1.71.61.5

1.41.31.251.2

∅=340°

1.15

1.1

1.05

∅=335°

0.95

0.9

0.85

0.80.75

0.7 0.65 0.6

∅=0°

1.81.92.0

2.5

5.060°

75°90°120°150°180°21

0°240°270°285°∅= 300°

B

A

E

E

B

A

E

E

B

A

E

E

PRC-026-1 Attachment B- Criterion A

PRC-026: Power System Model Simplification

PRC-026: Lens Point 1

Evaluating Limitation

PRC026 Limit

PRC026 Limit-Transformed

Summary

Ø  With NERC PRC026, Protection engineers are required to check that stable Power Swings impedances will not enter any fast tripping Distance Protection Zone

Ø  Evaluation process requires l  Building Equivalent Network Model around the line

under investigation l  Calculation of limiting Stable Power Swing impedances l  Check if impedance enter fast tripping Distance Zone