transmission planning technical guide appendix e · pdf filetransmission planning technical...

Transmission Planning Technical Guide

Appendix E

Dynamic Stability Simulation

Voltage Sag Guideline

© ISO New England Inc. System Planning December 6, 2013

Table of Content

VOLTAGE SAG PARAMETERS ........................................................................................................................................ 1

VOLTAGE SAG INTRODUCTION .................................................................................................................................... 2

THE CAUSE OF VOLTAGE SAGS .................................................................................................................................... 2

VOLTAGE SAG EFFECTS .................................................................................................................................................. 3

POWER QUALITY ................................................................................................................................................................. 3

LOSS OF LOAD ..................................................................................................................................................................... 3

EMERGENCY AND STANDBY POWER .................................................................................................................................... 4

MODELING LIMITATIONS ..................................................................................................................................................... 4

VOLTAGE SAG MITIGATION OPTIONS ....................................................................................................................... 4

APPLICABLE STANDARDS ............................................................................................................................................ 5

RECOMMENDATION ......................................................................................................................................................... 5

APPENDIX A ......................................................................................................................................................................... 6

THE CBEMA AND ITIC CURVES [REFERENCE 3] ..................................................................................................... 6

FIGURE 2 – CBEMA CURVE ................................................................................................................................................ 6

FIGURE 3 – ITIC CURVE ...................................................................................................................................................... 6

COMPOSITE CURVE [REFERENCE 10] (IEEE P1564_99_01.DOC) ........................................................................... 7

FIGURE 4 – COMPOSITE CURVE FROM IEEE P1564 ............................................................................................................. 7

EVOLUTION OF POWER ACCEPTABILITY CURVES ................................................................................................ 8

TABLE 1 – LISTING OF ALTERNATIVE POWER ACCEPTABILITY CURVES FROM [3] ............................................................... 8

SYSTEM VOLTAGE SAG SIMULATIONS ...................................................................................................................... 9

FIGURE 5 – SIMULATED 115 KV VOLTAGE SAG OF SECTION 392 ......................................................................................... 9

REFERENCES ..................................................................................................................................................................... 10

Appendix E-Voltage Sag Guideline

1

VOLTAGE SAG PARAMETERS

The minimum post-fault positive sequence voltage sag must remain above 70% of nominal voltage and

must not exceed 250 milliseconds below 80% of nominal voltage within 10 seconds following a fault.

These limits are supported by the typical sag tolerances shown in Figures C.5 to C.10 in IEEE Standard

1346-1998.

100

90

80

0

Maximum duration

of 3-Ph (positive sequence)

sag below 80%

Minimum

post-fault sag

Fault sag

Post Transient

Voltage

10 s

70

%

No

min

al V

olta

ge

Time

Figure 1– Transient Voltage Sag Parameters


2

Voltage Sag Introduction

The intent of this guideline is to avoid uncontrolled significant load shedding that may lead to

unintended system performance, such as widespread system collapse. This guide is not intended

as a standard of utility supply to individual customers, as a standard of power quality, nor used

for transmission or distribution protection design.

The voltage sag resulting from a system short circuit or fault depends on the location of the fault

in relation to the measured voltage, and may vary from zero to a few percent of normal. The

duration of the sag is determined by the fault clearing time and ranges from as low as 3 cycles on

345kV systems to one or more seconds on 34.5 kV sub-transmission systems. Following the fault

clearing, the voltage passes through a transient recovery period before settling to the post-fault

value. During this oscillatory transient period, additional voltage dips typically occur

immediately after the voltage attempts to return to the pre-fault level. The starting voltage, sag,

and duration of these post-fault transient under-voltages are a measure of the system strength.

The performance indicators for prompt restoration of voltage are the object of this guide.

This guide:

provides background information on the cause and effects of voltage sags associated

with transmission faults

discusses relevant IEEE Recommended Practices and Guides and technical papers

which discuss voltage sags and their effect on transmission system performance or

utilization equipment

(IEEE has developed Recommended Practices to guide equipment manufacturers and

customers regarding end-use equipment sensitivity to voltage transients and the

application of power-conditioning equipment. Note that there are no utility

transmission design or operating voltage sag standards.)

defines guideline criteria for the post-fault sag magnitude and duration.

This Voltage Sag Guideline applies only to the transmission system and therefore cannot be

assumed to represent the voltage at the points of utilization. The voltage sag during the fault is

not covered by this guide.

The Cause of Voltage Sags

During the fault period, the active power transferred from the generators to the system is

reduced, causing the generators’ internal angles to advance. When the fault is cleared, the

generators have to supply the pre-fault active load again and their internal angle moves toward to

their pre-fault value. This slowing of the local generators draws inrush decelerating power from

the remote generators and, coupled with motors’ demand for accelerating power (the motors

have slowed down during the lower fault voltage), causes a new voltage sag on the system. This

second sag is then followed by an oscillatory transition to the post-fault steady-state voltage, as

the machine prime mover power is again in balance with the electric load.


3

VOLTAGE SAG EFFECTS

Power Quality

The voltage sags caused by high voltage transmission faults are classified as “Instantaneous”

sags in IEEE (Institute of Electrical and Electronics Engineers) Standard 1159-1995 Table 2 (0.4

cycles to 30 cycles, 0.1 to 0.9 per-unit voltage) and the post-fault voltage swings are categorized

as “Momentary” (30 cycles to 3 seconds, 0.1 to 0.9 per-unit voltage). The voltage dips affect

sensitive loads such as computer, computer-based equipment, power conversion, etc. The voltage

sags also contribute to the deterioration of the power quality, which the utilities strive to maintain

for their customers. An IEEE T&D Working Group on Distribution Voltage Quality (WG

15.06.05) has commissioned a Task Force on Voltage Sag Indices (P1564 and CIGRE WG 36-

07) to draft a document to survey and propose methods to measure, characterize and index

voltage sag disturbances. Among some of the indices is a set of indices, similar to the popular

SAIFI (System Average Interruption Frequency Index) defined in IEEE Standard 1366-1999.

The proposed index (EPRI-Electrotek Indices, § 7.3 [Reference 10]) is SARFIx (System Average

RMS (Variation) Frequency Index of magnitude less than threshold x), along with subsets:

SIARFIx (System Instantaneous Average RMS (Variation) Frequency Index of magnitude

less than threshold x),

SMARFIx (System Momentary Average RMS (Variation) Frequency Index of magnitude

less than threshold x)

STARFIx (System Temporary Average RMS (Variation) Frequency Index of magnitude less

than threshold x).

The thresholds and time durations of these indices are apparently chosen to correspond to

standard reference equipment susceptibility curves such as the CBEMA curve (Computer

Business Equipment Manufacturers Association) and the more recent ITIC (Information

Technology Industry Council, the CBEMA replacement) Voltage-versus-Time curves, appearing

in Figures 2 & 3. These curves are included in this report for convenience. It is of interest to note

that the expected sag in these CBEMA and ITIC curves, during the “Momentary” period (from

30 cycles to 3 seconds) identified in IEEE Standard 1159-1995, is not lower than 80%.

Loss of Load

Voltage sags affect all types of customers, depending on the sensitivity of their equipment and

the extent of their power conditioning. IEEE Standard 493-1997 (“Gold Book”) [Reference 12]

reports that voltages to 85-90% of nominal as short as 16 milliseconds have triggered immediate

outages of critical industrial processes. In the same Standard 493-1997, Table 9-12 accounts for

sags ranging from 90% to 70% with durations up to 1250 milliseconds to include the effect of

motor starting.

Domestic and commercial electronic (computer) loads are more likely to ride through a sag if the

magnitude and duration are within the ITIC curve. Central air conditioners’ internal protection

may operate or their motors may stall and contribute to a protracted recovery [Reference 11].

Florida Power & Light had hundreds of megawatts of load lost during transients due to this

phenomenon. Industrial loads are the most vulnerable to severe voltage sags. IEEE Standard

141-1993 (“Red Book”) [Reference 7] Figure 3-9 compares probable sag magnitudes to a 80%

baseline and advises designers to consider both the sag voltage magnitude and duration when


4

specifying equipment performance capability during voltage sags. If a motor control contactor is

unable to ride through a voltage sag, the motor and associated process is interrupted.

Emergency and Standby Power

The Western Electricity Systems Coordinating Council’s (WECC), formally the Western

Systems Coordinating Council (WSCC), voltage sag criteria are based in part on a need to

maintain a margin for nuclear unit auxiliary undervoltage protection and load transfer

[Reference 1. A more general application is found in the setting guidelines of load-transfer

devices in IEEE Standard 446-1987 (“Orange Book”) [Reference 9]. In Section 4.3.6 of this

Standard, typical transfer threshold settings of 75% to 95% of pickup are given, with pickup

settings raging from 85% to 98% of nominal. Time delays are on the order of 1 second. This

means that voltages below 80% ( the limit suggested on the ITIC curve) are likely to initiate

automatic load transfers.

Modeling Limitations

In order to determine if a voltage disturbance for a system transmission fault falls below a level

and duration set in transient voltage criteria, the power system model should represent the

dynamic response of load as accurately as possible. Steady state and dynamic power studies in

New England have relied on static load models (constant admittance and current). An analysis

done by Florida Power & Light [Reference 11] shows that the voltage recovery is worse for a

simulation with a mix of static and motor models [Reference 10, Figure 4], than with the static

load model only. This was confirmed in a feasibility study conducted by American

Superconductor for CMP in September 2000: the post-fault voltage sag was about 0.08 per-unit

lower when the industrial, commercial and residential load models were modified to include

80%, 60% and 40%, respectively, of high and low inertia motor load.

Voltage Sag Mitigation Options

Voltage sags in power systems are unavoidable. The system can be designed and operated to

minimize severe voltage sags. High speed fault clearing, special protection systems, field

forcing, transmission reinforcements, and transmission interface transfer limits can be considered

by generation and transmission owners as options to improve voltage sag performance.

Customers can apply power-conditioning technologies such as Uninterruptible Power Supplies

(UPS), and Distributed-Superconducting Magnetic Energy Storage System (D-SMES) to

sensitive loads. IEEE Standard 1346-1998 [Reference 13] lists voltage sags as the greatest

financial risk due to lack of compatibility of electric supply systems with electronic process

equipment, and offers a method to evaluate the financial impact of incompatibility as well

suggestions of financial analysis of alternatives to improve compatibility.


5

APPLICABLE STANDARDS

IEEE Std 1159-1995 “IEEE Recommended Practice for Monitoring Electric Power Quality”

IEEE Std 1250-1995 “Guide for Service to Equipment Sensitive to Momentary Voltage

Disturbances”

IEEE Std 141-1993 “IEEE Recommended Practice for Electric Power Distribution for Industrial

Plants” (“Red Book”)

IEEE Std 493-1997 “IEEE Recommended Practice for the Design of Reliable Industrial and

Commercial Power Systems” (“Gold Book”)

IEEE Std 1346-1998 “IEEE Recommended Practice for Evaluating Electric Power System

Compatibility with Electronic Process Equipment”

P1564, Task Force on Voltage Sag Indices (proposed standard, in progress)

RECOMMENDATION

The IEEE standards leave the sag and duration up to the specific user and application (Section

9.7 in IEEE Standard 493-1997 [Reference 12] states that “utilization equipment response to sags

must be known from manufacturer specifications or from performance test data. Both supply

characteristics and equipment response data sets are required…”). In this context, Planning and

Operating Engineers are faced with judging between the probability of undesirable load loss and

the imposition of stricter design criteria and operating limits. Based on the references, the

industry has accepted 80% as a typical sag magnitude for momentary voltage sags at the point of

utilization. It should be noted that the IEEE remains divided on the adoption of standard sag

limits for application to utility transmission or distribution systems, and no IEEE standards for

this exist today.


6

APPENDIX A

THE CBEMA and ITIC CURVES [Reference 3]

The well known CBEMA and ITIC power acceptability curves are not the direct objective of this

voltage sag guideline and were not developed as a guide for utility supply. Rather, these curves

were intended to be used as a guide for equipment manufacturers and utility customers in making

decisions for power conditioning equipment for sensitive loads. The curves are reproduced here

for reference and comparison to this voltage sag guide only.

Note that the voltage scale shows percent deviation from pre-sag operating voltage, not percent

of nominal voltage.

0.0001 0.001 0.01 0.1 1 10 100 1000

-100

-50

0

50

100

150

200

250

TIME IN SECONDS

PE

RC

EN

T C

HA

NG

E I

N B

US

VO

LTA

GE

8.33

ms

OVERVOLTAGE CONDITIONS

UNDERVOLTAGE CONDITIONS

0.5

CY

CL

E

RATED

VOLTAGE

ACCEPTABLE

POWER

Figure 2 – CBEMA Curve

0.0001 0.001 0.01 0.1 1 10 100 1000

-100

-50

0

50

100

150

200

250

TIME IN SECONDS

PER

CEN

T C

HAN

GE

IN B

US

VOLT

AGE

8.33

ms

OVERVOLTAGE CONDITIONS

UNDERVOLTAGE CONDITIONS

0.5

CYC

LE

RATED

VOLTAGE

ACCEPTABLE

POWER

10%+--

Figure 3 – ITIC Curve


7

Composite Curve [Reference 10] (IEEE P1564_99_01.doc)

This curve was reproduced from a draft document posted on the IEEE P1564 Working Group

site, and may be adopted into the proposed Standard for classifying voltage sags.

This curve applies to equipment at the point of utilization.

Figure 4 – Composite Curve From IEEE P1564

5 00 %

2 00 %

S A R F I (E U E )

1 40 %

1 20 %

1 10 %

1 00 %

1 m s 3 m s 0 .5 ~ 2 0 m s 0 .5 se c 3 s e c 1 0 s e c 6 0 se c

9 0%

8 0%

7 0%

5 0%

1 0%

0 %

0 0 .5 ~ 2 0 m s 30 ~ 3 s e c 1 m in

V o l ta g e M a g n i tu d e v s . E v e n t D u ra t io n

N o In s ta n ta n e o u s

I n te rr u p ti o n s

1 0 % is 1 1 5 9

T h r e sh o ld L e v e l fo r

In te r r u p tio n s

1 1 5 9 M o m e n ta r y

In te r ru p tio n s

(E U E S M A R F I)

1 1 5 9 T e m p o r a r y

In te r ru p ti o n s

( E U E S TA R FI )

In s ta n ta n e o u s

S a g s /S w e ll s (S IA R F I)

M o m e n ta r y

S a g s /S w e l ls

( S M A R FI )

Te m p o ra r y

S a g s/S w e ll s ( S T A R F I)

T r a ns ien ts

1 µ s

1 6 0 µ s * A g r a p h i ca l c o m b in a t io n fr o m t h e fo ll o w in g re s o u r ce d o c u m e n t s:

ITIC : In fo r m a tio n Te c h n o lo g y In d u st ry C o u n c il (w a s C B E M A ) 1 9 9 6 V o lta g e Q u a l ity G u id e l in e s ;

IE E E 1 1 5 9 - 1 9 9 5 "C a te g o r ie s" T a b le fo r P o w e r S y ste m E M P h e n o m e n a ;

"E U E " i s a n a b b re v ia tio n f o r a p a p e r p re s e n te d a t th e IE E E P E S S u m m e r M e e tin g i n B e rl in , G e r m a n y,

t it l e d In d i ce s fo r A s se ss in g U ti li ty D i st ri b u tio n S y ste m R M S V a ri a tio n P e r fo r m a n ce . A u th o r s w e r e

Du g a n -E le c tr o te k, W a c la w ia k - U n ite d I llu m in a ti n g C o ., a n d S u n d a r a m o f E P RI.

5 0 % is ty p ic a l m o to r

co n ta cto r b r e a k p o in t.


8

Evolution of Power Acceptability Curves

The following table lists the chronology of significant power acceptability curves. According to

[Reference 3] the CBEMA curve was redesigned in 1996 and renamed for its supporting

organization.

Curve Year Application Source

FIPS Power

Acceptability Curve1978

Automatic Data

Processing EquipmentU.S. Federal Government

CBEMA Curve 1978Computer Business

Equipment

Computer Business

Equipment Manufacturers

Association

ITIC Curve 1996Information Technology

Equipment

Information Technology

Industry Council

Failure Rate Curves

for Industrial Loads1972 Industrial Loads IEEE Standard 493

AC Line Voltage

Tolerences1974 Mainframe Computers IEEE Standard 446

IEEE Emerald Book 1992Sensitive Electronic

EquipmentIEEE Standard 1100

Table 1 – Listing of Alternative Power Acceptability Curves from [3]


9

SYSTEM VOLTAGE SAG SIMULATIONS

The following plots were generated from a G.E. PSLF simulation for the loss of Section 392

(similar to the fault which occurred on June 1, 2001 at Maine Yankee Substation). The two traces

on each plot show the difference between a slow (0.6 second) and fast (0.2 second) operation of

the overcurrent Special Protection Systems at Maxcy’s and Bucksport.

Figure 5 – Simulated 115 kV Voltage Sag of Section 392


10

REFERENCES

1-Western Systems Coordinating Council “Supporting Document For Reliability Criteria For

Transmission System Planning” – August 1994.

2- Daniel Sabin, Electrotek “Indices Used to Assess RMS Voltage Variations”- July 2000.

3-R.S.Thallam & G.T.Heydt “Power Acceptability and Voltage Sag Indices in the Three Phase

Sense”.

4- Daniel Sabin, T.E Grebe & A.Sundaram “RMS Voltage Variation Statistical Analysis for a

Survey of Distribution System Power Quality Performance”.

5-IEEE Std 1159-1995 “IEEE Recommended Practice for Monitoring Electric Power Quality”.

6-IEEE Std 1250-1995 “IEEE Guide for Service to Equipment Sensitive to Momentary Voltage

Disturbances”.

7-IEEE Std 141-1993 “IEEE Recommended Practice for Electric Power Distribution for

Industrial Plants” (IEEE Red Book).

8-IEEE Std 242-1986 “IEEE Recommended Practice for Protection and Coordination of

Industrial and Commercial Power Systems” (IEEE Buff Book).

9-IEEE Std 446-1987 “IEEE Recommended Practice for Emergency and Standby Power

Systems for Industrial and Commercial Applications” (IEEE Orange Book).

10-IEEE P1564 Task Force on Voltage Sag Indices (Working Group on Distribution Voltage

Quality) “Voltage Sag Indices-Draft 1.2” – December 2000.

11- John W. Shaffer “Air Conditioner Response to Transmission Faults” – IEEE Transactions

on Power Systems – May 1997.

12-IEEE Std 493-1997 “IEEE Recommended Practice for the Design of Reliable Industrial and

Commercial Power Systems” (IEEE Gold Book).

13-IEEE Std 1346-1998 “IEEE Recommended Practice for Evaluating Electric Power System

Compatibility with Electronic Process Equipment”.

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