ECE422_8-VoltageStability

8/18/2019 ECE422_8-VoltageStability

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Spring 2015

Instructor: Kai

Sun

ECE 422

Power System Operations & Planning

8 ‐ Voltage Stability


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Voltage Stability

• Voltage stability is concerned with the ability of a power system to maintain

acceptable voltages at all buses in the system under normal conditions and

after being subjected to a disturbance.

• A system

enters

a state

of

voltage

instability

(or

voltage

collapse)

when

a

disturbance, e.g. increase in load demand, or change in system condition

causes a progressive and uncontrollable decline in voltage

• The main factor causing instability is the inability of the power system to

meet the

demand

for

reactive

power

• Voltage stability problems normally occur in heavily stressed systems.

• References:

– Chapter 14

of Kundur’s book

– “Survey of the voltage collapse phenomenon”, NERC Report, Aug. 1991

– EPRI Tutorial’s Chapter 6

– Carson W. Taylor, “Power System Voltage Stability” McGraw Hil, 1994

– “Voltage Stability Assessment: Concepts, Practices and Tools”, IEEE whitepaper, Aug. 2002


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Voltage Stability vs. Rotor Angle stability

• Voltage

stability

is

basically

load

stability,

and

rotor

angle

stability

is

basically

generator stability

• For rotor angle stability, we are often concerned with integrating remote power

plants to a large system over long transmission lines.

• Voltage stability is concerned with load areas and load characteristics. In a large

interconnected system,

voltage

collapse

of

a load

area

is

possible

without

loss

of

synchronism of any generators.

• Transient voltage stability is usually closely associated with transient rotor angle

stability. If voltage collapse at a point in a transmission system remote from loads, it

is, in nature, an angle instability problem.

Typical region with voltage stability concerns


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A simple radial system

where

Z LD decreases (assume constant Z LN )

S

LN LD

E I

Z Z

2 2( cos cos ) ( sin sin )

S

LN LD LN LD

E I

Z Z Z Z

1 S

LN

E I

Z F

2

1 2 cos( ) LD LD

LN LN

Z Z F

Z Z

• How does V R change as P R increases?

R LDV Z I cos R RP V I

1 LD R LD S

LN

Z V Z I E Z F

2

cos cosS LD R R

LN

E Z P V I

F Z


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• Voltage stability depends on the

load characteristics

– Under normal

conditions,

to

increase power PR, a load control

strategy usually decreases Z LD

– However,

when

Z LD


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Constant P

P R=P

Constant Z

P R=aV R2

• Voltage collapse happens when the system passes the critical point

(also called the “nose point” or “knee point”).

Z LD decreases (assume constant Z LN )

P‐V Curve


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Normalized P‐V curves ( varies)

• Normally, only the operating points above the critical points represent

satisfactory operating conditions

A sudden reduction in (increase in QR)


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IEEE 39‐bus test system

P Aera 1 - V530Uniformly scale up the area load with

constant


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Influence of Generation Characteristics

• Generator AVRs provide the primary source

of voltage support to maintain constant

terminal voltages under normal conditions.

• During conditions of low/high voltages, the Q

output of

a generator

may

reach

its

limit.

Consequently, the terminal voltage is not

longer maintained constant

• Then, with constant field current, the point of

constant voltage is now E q behind the

synchronous reactance

X S X d .

That

increases

the network reactance significantly to further

aggravating the voltage collapse condition

• It is important to maintain the voltage control

capability of generators

• The degree

of

voltage

stability

cannot

be

judged based only on how close the bus

voltage is to the normal voltage level

Voltage collapse happens when the Q limit is reached


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12

3

1

2

3

1GW generation

tripped by SPS

4

4

Faulty zone 3

relay

5

5

6 8

67

7

8

Loss of key

hydro units

Tripped by

Zone 3 relay

9

9

10

Tree

contact

and

relay

mis-opt.

Example of Voltage Collapse -

July 2nd, 1996 Western

Cascading Event


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•On July 3rd, 1996, i.e. the following day,

– A similar chain of events happened to cause voltages in Boise area to

decline.

– Different from the previous day, Idaho Power Company system

operators noted the declining voltages and immediately took the only

option available: shedding of Boise area load

– Then, the

system

returned

to

normal

within

1 hour

•Lessons learned:

– The July

2nd and

3rd events

in

Boise,

Idaho

area

emphasize

the

need

for

effective and sufficient, rapidly responsive dynamic Mvar reserve.

– The July 3rd events illustrate the importance of system operators’

situational awareness and rapid responses.


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Prevention of Voltage Instability

• Application of

var compensating

devices

– Ensure adequate stability margin (in MW or Mvar)

– Selection of sizes, ratings and locations of the devices (especially for dynamic

reactive reserves, e.g. synchronous condensers and SVCs)

– Design

criteria

based

on

maximum

allowable

post‐

contingency

voltage

drop

are

often NOT satisfactory from voltage stability viewpoint

– Important to recognize voltage control areas and weak boundaries.

• Control of transformer tap changers

– Can be controlled either locally or centrally

– Where tap changing is detrimental, tap changing should be blocked when the

source‐side voltage sags and unblocked when voltage recovers

• Control of network voltage and generator reactive output

– Improvement on AVRs

– Add secondary

coordinated

outer

‐loop

voltage

control

(e.g.

the

hierarchical,

automatic two/three‐layer voltage control)


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Prevention of Voltage Instability (cont’d)

• Coordination of protections/controls

– Adequate coordination ensured based on dynamic simulation studies

– Tripping of equipment to protect from overloaded conditions should be the last

resort. The overloaded conditions could be relieved by adequate control measures

before isolating

the

equipment.

• Under‐voltage load shedding (UVLS)

– For unplanned or extreme situations; analogous to UFLS

– Provide a low‐cost means of preventing widespread system collapse

– Particularly attractive

if

conditions

leading

to

voltage

instability

are

of

low

probability but consequences are high

– Characteristics and locations of the loads to be shed are more important for voltage

problems than for frequency problems

– Should be designed to distinguish between faults, transient voltage dips, and low

voltage conditions leading

to

voltage

collapse


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E S =1.0pu, Z LN = j0.5pu, cos=0.97.• Sketch the P R-V R curve at bus R and find values

of P R and V R at the critical point

• If the real power load is represented by a ZIPload mode P R=P0(V R

2+0.9V R+4.0) , where P0

varies depending on the load level. Estimate theminimum |V R| before voltage collapse.

2

1 2 cos( )

2 2cos( ) 2.486

LD LD

LN LN

Z Z F

Z Z

10.634 pu LD R S

LN

Z V E

Z F

2

cos 0.780 puS LD R

LN

E Z P

F Z

Example: P‐V Curve vs. ZIP Load Model

Documents

ECE422_8-VoltageStability