Var-501-wecc-3.1

VAR-501-WECC-3.1 – Power System Stabilizer

A. Introduction
1. Title:

Power System Stabilizer (PSS)

2. Number: VAR-501-WECC-3.1
3. Purpose: To ensure the Western Interconnection is operated in a coordinated manner
under normal and abnormal conditions by establishing the performance criteria for
WECC power system stabilizers.
4. Applicability:
4.1 Generator Operator
4.2 Generator Owner
5. Facilities: This standard applies to synchronous generators, connected to the Bulk
Electric System, that meet the definition of Commercial Operation.
6. Effective Date: The first day of the first quarter following regulatory approval, except
for Requirement R3.
For units placed in first-time service after regulatory approval, Requirement R3 is
effective the first day of the first quarter following final regulatory approval.
For units placed in service prior to final regulatory approval, Requirement R3 is effective
the first day of the first quarter that is five years after regulatory approval.

B. Requirements and Measures
R1. Each Generator Owner shall provide to its Transmission Operator, the Generator
Owner’s written Operating Procedure or other document(s) describing those known
circumstances during which the Generator Owner’s PSS will not be providing an active
signal to the Automatic Voltage Regulator (AVR), within 180 days of any of the
following events: [Violation Risk Factor: Low] [Time Horizon: Planning Horizon]
• The effective date of this standard;
• The PSS’s Commercial Operation date; or
• Any changes to the PSS operating specifications.
M1. Each Generator Owner will have documented evidence that it provided to its
Transmission Operator, within the time allotted as described in the procedures
required under Requirement R1, written Operating Procedures or other document(s)
describing those known circumstances during which the Generator Owner’s PSS will
not be providing an active signal to the AVR.
For auditing purposes, because Requirement R1 conditions are intended to be
unchanged unless the Transmission Operator is otherwise notified, the Generator
Owner only needs to provide the documentation to the Transmission Operator one
time, or whenever the operating specifications change.

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VAR-501-WECC-3.1 – Power System Stabilizer

For auditing purposes, if a PSS is in service but is not providing an active signal to the
AVR as described in Requirement R1, the disabled period does not count against the
Requirement R2 mandate to be in service except as otherwise allowed.
R2. Each Generator Operator shall have its PSS in service while synchronized, except
during any of the following: [Violation Risk Factor: Medium] [Time Horizon: Operating
Assessment]
•

Component failure

•

Testing of a Bulk Electric System Element affecting or affected by the PSS

•

Maintenance

•

As agreed upon by the Generator Operator and the Transmission Operator

A PSS that is out of service for less than 30 minutes does not create a violation
of this Requirement, regardless of cause.
M2. Each Generator Operator will have documentation of each claimed exception
specified in Requirement R2. Documentation may include, but is not limited to:
•

A written explanation covering the bulleted exception that describes the
circumstances of the exception as allowed in Requirement R2.

•

Documented evidence that the Generator Operator and the Transmission
Operator agreed the PSS would not be operating during a specified set of
circumstances, where the exception is claimed under the last bullet of
Requirement R2.

For auditing purposes, the presumption is that the PSS was in service unless otherwise
exempted in Requirement R2. Evidence need only be provided to prove the
circumstances during which the PSS was not in service for periods in excess of 30
minutes.
R3. Each Generator Owner shall tune its PSS to meet the following inter-area mode
criteria, except as specified in Requirement R3, Part 3.5 below: [Violation Risk Factor:
Medium] [Time Horizon: Operating Assessment]
3.1. PSS shall be set to provide the measured, simulated, or calculated compensated
Vt/Vref frequency response of the excitation system and synchronous machine
such that the phase angle will not exceed ± 30 degrees through the frequency
range from 0.2 Hertz to the lesser of 1.0 Hertz or the highest frequency at which
the phase of the Vt/Vref frequency response does not exceed 90 degrees.
3.2. PSS output limits shall be set to provide at least ±5% of the synchronous
machine’s nominal terminal voltage.
3.3. PSS gain shall be set to between 1/3 and 1/2 of maximum practical gain.
3.4. PSS washout time constant shall be no greater than 30 seconds.
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VAR-501-WECC-3.1 – Power System Stabilizer

3.5. Units that have an excitation system or PSS that is incapable of meeting the
tuning requirements of Requirement R3 are exempt from Requirement R3 until
the voltage regulator is either replaced or retrofitted such that the PSS becomes
capable of meeting the tuning requirements.
M3. Each Generator Owner will have documented evidence that its PSS was tuned to
meet the specifications of Requirement R3.
If the exception under Requirement R3, Part 3.5, is claimed, the Generator Owner will
have documented evidence describing: 1) the conditions that render the PSS incapable
of meeting the tuning requirements, and 2) the date the voltage regulator was last
replaced or retrofitted.
R4. Each Generator Owner shall install and complete start-up testing of a PSS on its
generator within 180 days of either of the following events: [Violation Risk Factor:
Medium] [Time Horizon: Operational Assessment]
•

The Generator Owner connects a generator to the BES, after achieving
Commercial Operation, and after the Effective Date of this standard.

•

The Generator Owner replaces the voltage regulator on its existing excitation
system, after achieving Commercial Operation for its generator that is
connected to the BES, and after the Effective Date of this standard.

M4. Each Generator Owner will have evidence that it installed and completed start-up
testing of a PSS on its generator within 180 days of either of the conditions described
in Requirement R4, and when those conditions occur after the Effective Date of this
standard.
For auditing purposes of Requirement R4, bullet one only applies to equipment on its
initial (first energization) connection to the BES.
R5. Each Generator Owner shall repair or replace a PSS within 24 months of that PSS
becoming incapable of meeting the tuning specifications stated in Requirement R3.
[Violation Risk Factor: Medium] [Time Horizon: Operational Assessment]
M5. Each Generator Owner will have evidence that it repaired or replaced its PSS within
24 months of that PSS becoming incapable of meeting the tuning specifications of
Requirement R3. Evidence may include, but is not limited to, documentation of the
date the PSS became incapable of meeting the Requirement R3 tuning specifications,
and the date the PSS was returned to service, demonstrating that the span of time
between the two events was less than 24 months.

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VAR-501-WECC-3.1 – Power System Stabilizer

C. Compliance
1. Compliance Monitoring Process
1.1 Compliance Enforcement Authority
NERC or the Regional Entity, or any entity as otherwise designated by an
Applicable Governmental Authority, in their respective roles of monitoring and/or
enforcing compliance with mandatory and enforceable Reliability Standards in
their respective jurisdictions.
1.2 Compliance Monitoring and Assessment Processes
•

Compliance Audits

•

Self-Certifications

•

Spot Checking

•

Compliance Investigations

•

Self-Reporting

•

Complaints

1.3 Evidence Retention
The following evidence retention periods identify the period of time an entity is
required to retain specific evidence to demonstrate compliance. For instances
where the evidence retention period specified below is shorter than the time since
the last audit, the Compliance Enforcement Authority may ask an entity to provide
other evidence to show that it was compliant for the full time period since the last
audit.
Each Generator Operator shall keep evidence for all Requirements of the
document for a period of three years plus calendar current.
1.4 Additional Compliance Information
None

D. Regional Differences
None

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VAR-501-WECC-3.1 – Power System Stabilizer

Table of Compliance Elements
R

Time
Horizon

VRF

Violation Severity Levels
Lower VSL

Moderate VSL

High VSL

Severe VSL

R1

Planning
Horizon

Low

NA

NA

NA

The Generator Owner
failed to provide its PSS
operating specifications
to the Transmission
Operator as required in
Requirement R1.

R2

Operations
Assessment

Medium

Each Generator
Operator not having
its PSS in service while
synchronized in
accordance with
Requirement R2, for
more than 30 minutes
but less than 60
minutes.

Each Generator
Operator not having its
PSS in service while
synchronized in
accordance with
Requirement R2, for
more than 60 minutes
but less than 120
minutes.

Each Generator
Operator not having
its PSS in service while
synchronized in
accordance with
Requirement R2, for
more than 120
minutes but less than
180 minutes.

Each Generator
Operator not having
its PSS in service while
synchronized in
accordance with
Requirement R2, for
more than 180
minutes.

R3

Operations
Assessment

Medium

The Generator
Owner’s PSS failed to
meet any of the
required
performances in
Requirement R3, two
times or fewer during
the audit period.

The Generator Owner’s
PSS failed to meet any
of the required
performances in
Requirement R3, three
times during the audit
period.

The Generator
Owner’s PSS failed to
meet any of the
required performances
in Requirement R3,
four times during the
audit period.

The Generator
Owner’s PSS failed to
meet any of the
required performances
in Requirement R3,
five times or more
during the audit
period.

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VAR-501-WECC-3.1 – Power System Stabilizer

R

Time
Horizon

VRF

Violation Severity Levels

R4

Operational
Assessment

Medium

NA

NA

NA

The Generator Owner
failed to install on its
generator a PSS, as
required in
Requirement R4.

R5

Operational
Assessment

Medium

NA

NA

NA

The Generator Owner
failed to repair or
replace a nonoperational PSS as
required in
Requirement R5.

Lower VSL

Moderate VSL

High VSL

Severe VSL

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VAR-501-WECC-3.1 – Power System Stabilizer

Version History
Version

Date

1

April 16, 2008

1

October 28, 2008

Action
Permanent Replacement
Standard for VAR-STD-002b-1
Adopted by NERC Board of
Trustees

1

April 21, 2011

FERC Order issued approving
VAR501-WECC-1 (FERC approval
effective June 27, 2011;
Effective Date July 1, 2011)

2

November 13, 2014

Adopted by NERC Board of
Trustees

2

March 3, 2015

FERC letter order approved
VAR-501-WECC-2

3

February 9, 2017

Adopted by NERC Board of
Trustees

3

April 28, 2017

FERC letter order approved
VAR-501-WECC-3

August 10, 2017

Adopted by the NERC Board of
Trustees

3.1

Change Tracking

Errata

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VAR-501-WECC-3.1 – Power System Stabilizer

Guideline and Technical Basis
PSS systems are used to minimize real power oscillations by rapidly adjusting the field of the
generator to dampen the low-frequency oscillations.
It is necessary for large numbers of PSS devices to be in operation in the Western
Interconnection to provide the required system damping while still allowing for some of these
units to be out of service whenever necessary.
Mandate to Install a PSS
Nothing in this Regional Reliability Standard (RSS) should be construed to require installation of
a PSS solely because a PSS is not currently installed as of the Effective Date of this RRS. Rather,
installation is only mandated on the occurrence of either of the triggering events described in
Requirement R4, Bullet 1 or Bullet 2, after the Effective Date of the RRS.
It should be noted that a PSS is neither Transmission nor generation.
Requirement R1
Requirement R1 addresses normal operating conditions.
Requirement R1 recognizes that PSS systems have varying states, such as on, off, active, and
non-active. As long as the PSS is operating in accordance with the documentation provided to
the Transmission Operator, this is not considered a status change for purposes of this standard.
This Requirement eliminates the requirement to count hours as required in the previous
version of this standard while also allowing the Generator Owner to create a unit-specific
operating plan.
The intent of Requirement R1 is to provide the Transmission Operator, the PSS operating zone
in which the PSS is “active” providing damping to the power system. Some PSS may be
programmed to become “active” at a specified megawatt loading level and above while others
may be programmed to be “active” in a particular band of megawatt loading levels and are
“non-active” only when passing through the “rough zone” or some other band. A “rough zone”
is a megawatt loading band in which the generator-turbine system could contribute to system
instability.
Requirement R2
This Requirement only applies when the PSS is out of service for a period greater than 30
minutes.
Unlike Requirement R1, Requirement R2 addresses exceptions to normal operation.
The intent of Requirement R2 is to remove the previous requirement to log hours for PSS in
service. In this standard’s previous version, the logged hours were totaled quarterly to meet the
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VAR-501-WECC-3.1 – Power System Stabilizer

98% in-service requirement. Instead of documenting the number of hours excluded, this
Requirement simplifies the process by allowing the Generator Operator to communicate to the
Transmission Operator the circumstances that render the PSS unavailable to the Transmission
Operator (such as component failure, maintenance, and testing).
Requirement R3
Nothing in this RSS should be construed to mandate the design criteria for the equipment used
to produce the tuning output of the PSS. Rather, Requirement R3 is intended to address the
design criteria for the tuning output of the PSS.
Unlike the language in Requirement R5 that looks backward to address units that were once
operating but are no longer capable of operating, Requirement R3 looks forward, requiring that
units be tuned to the specified parameters.
The PSS transfer function should compensate the phase characteristics of the generator,
exciter, and power (GEP) system transfer function so the compensated transfer function
((PSS(s) * GEP(s)) has a phase characteristic of ± 30 degrees in the frequency range.
The GEP(s) transfer function is a theoretical transfer function and its phase characteristic
cannot be directly measured during field tests (only via simulation). Thus, the Requirement
recognizes the practical approach of measuring the frequency response between voltage
reference set point and terminal voltage (Et/Vref) and using the phase characteristic of such
frequency response as being the phase characteristic of GEP(s). The phase characteristic of
Et/Vref is a better approximation to the phase characteristic of GEP(s) when the frequency
response Et/Vref is obtained with the generator synchronized to the grid at its minimum stable
power output.
In an effort to allow for reasonable wash-out time constants, the Requirement specifies 0.2 Hz
as the applicable threshold. The 0.2 Hz threshold more closely aligns with the observed
oscillation frequencies.
A properly tuned PSS should provide positive damping to the local mode of oscillation, which
typically has a frequency higher than 1.0 Hz.
This Requirement modifies the requirement associated with the adjustment of the PSS gain.
The standard no longer defines the PSS gain in terms of gain margin but instead requires the
final PSS gain to be between 1/3 (10 dB) and 1/2 (6 dB) of the maximum practical gain that
could be achieved during PSS commissioning. The maximum practical gain might be associated
with the excessive noise or raised higher-frequency oscillations in the closed loop response
(exciter mode) or any other form if there is inadequate closed-loop performance, as
determined during PSS commissioning. It is now part of Measure M3 to show the field test
results that led to the determination of the maximum practical gain.

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VAR-501-WECC-3.1 – Power System Stabilizer

Requirement R4
Requirement R4 requires a Generator Owner to install a PSS on new applicable units or when
excitation systems are replaced or retrofitted on existing applicable units. This Requirement
applies to new excitation systems and not to existing systems that do not have PSS. The
Requirement also allows a reasonable amount of time for the commissioning of new PSS.
Requirement R5
Unlike the language in Requirement R3 that looks forward to ensure that a unit is tuned,
Requirement R5 looks backward. Specifically, the language in Requirement R5, “becoming
incapable,” indicates the unit was previously capable of meeting the tuning requirements in
Requirement R3, but is no longer capable. Restated, Requirement R5 addresses units that were
previously working but are now no longer working.
The intent of Requirement R5 is to remove the “tiered” approach to PSS repair/replacement
following a failure. A simple, streamlined approach to allow the Generator Owner sufficient
time to repair or replace a broken PSS has been written. Consideration has been given for the
need to procure parts or new equipment, schedule an equipment/unit outage, and install and
test the repaired or replaced PSS. It is recognized that in some instances, it may require
(1) replacement of an AVR, and (2) the existence of a PSS, or both the AVR and the PSS may
need to be replaced to achieve a functioning system.
The 24-month time frame is sufficient to return a functional, operating PSS to service.

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VAR-501-WECC-3.1 – Power System Stabilizer

* FOR INFORMATIONAL PURPOSES ONLY *
Enforcement Dates: Standard VAR-501-WECC-3 — Power System Stabilizer
United States
Standard

Requirement

Enforcement Date

VAR-501-WECC-3

TBD

TBD

Inactive Date

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