NERC Petition for approval of PRC-025-1

UNITED STATES OF AMERICA
BEFORE THE
FEDERAL ENERGY REGULATORY COMMISSION

North American Electric Reliability
Corporation

)
)

Docket No. _______

PETITION OF THE
NORTH AMERICAN ELECTRIC RELIABILITY CORPORATION
FOR APPROVAL OF PROPOSED RELIABILITY STANDARD PRC-025-1
(GENERATOR RELAY LOADABILITY)

Gerald W. Cauley
President and Chief Executive Officer
North American Electric Reliability
Corporation
3353 Peachtree Road, N.E.
Suite 600, North Tower
Atlanta, GA 30326
(404) 446-2560
(404) 446-2595 – facsimile

Charles A. Berardesco
Senior Vice President and General Counsel
Holly A. Hawkins
Assistant General Counsel
William H. Edwards
Counsel
Brady A. Walker
Associate Counsel
North American Electric Reliability
Corporation
1325 G Street, N.W., Suite 600
Washington, D.C. 20005
(202) 400-3000
(202) 644-8099 – facsimile
charlie.berardesco@nerc.net
holly.hawkins@nerc.net
william.edwards@nerc.net
brady.walker@nerc.net
Counsel for the North American Electric
Reliability Corporation
September 30, 2013

TABLE OF CONTENTS
I.

EXECUTIVE SUMMARY .................................................................................................... 4

II.

NOTICES AND COMMUNICATIONS ................................................................................ 4

III. BACKGROUND .................................................................................................................... 5
A.

Regulatory Framework ..................................................................................................... 5

B.

NERC Reliability Standards Development Procedure ..................................................... 6

C.

History of Project 2010-13 Relay Loadability ................................................................. 7

IV. JUSTIFICATION FOR APPROVAL..................................................................................... 9
A.
Reliability Benefits and Technical Explanation of Proposed Reliability Standard PRC025-1 9

V.

B.

Commission Directives Addressed ................................................................................ 12

C.

Requirement in Proposed Reliability Standard PRC-025-1 ........................................... 17

D.

Enforceability of PRC-025-1 ......................................................................................... 22
CONCLUSION ..................................................................................................................... 22

Exhibit A

Proposed Reliability Standard PRC-025-1 (Generator Relay Loadability)

Exhibit B

Implementation Plan

Exhibit C

Order No. 672 Criteria

Exhibit D

Consideration of Issues and Directives

Exhibit E

Analysis of Violation Risk Factor and Violation Security Level

Exhibit F

Summary of Development History and Complete Record of Development

Exhibit G

Standard Drafting Team Roster for Project 2010-13.2 Phase 2 Relay Loadability:
Generation

i

UNITED STATES OF AMERICA
BEFORE THE
FEDERAL ENERGY REGULATORY COMMISSION

North American Electric Reliability
Corporation

)
)

Docket No. _______

PETITION OF THE
NORTH AMERICAN ELECTRIC RELIABILITY CORPORATION
FOR APPROVAL OF PROPOSED RELIABILITY STANDARD PRC-025-1
(GENERATOR RELAY LOADABILITY)
Pursuant to Section 215(d)(1) of the Federal Power Act (“FPA”)1 and Section 39.52 of the
Federal Energy Regulatory Commission’s (“FERC” or “Commission”) regulations, the North
American Electric Reliability Corporation (“NERC”)3 hereby submits proposed Reliability
Standard PRC-025-1 – Generator Relay Loadability for Commission approval. NERC requests
that the Commission approve proposed Reliability Standard PRC-025-1 (Exhibit A) and find
that the proposed Reliability Standard is just, reasonable, not unduly discriminatory or
preferential, and in the public interest.4 NERC also requests approval of the associated
implementation plan (Exhibit B) and Violation Risk Factor (“VRF”) and Violation Severity
Level (“VSL”) (included in Exhibit A and explained in Exhibit E). Proposed PRC-025-1 was
developed to respond to Commission directives in Order No. 7335 to address generator protective
relay loadability.

1

16 U.S.C. § 824o (2006).
18 C.F.R. § 39.5 (2013).
3
The Commission certified NERC as the electric reliability organization (“ERO”) in accordance with
Section 215 of the FPA on July 20, 2006. N. Am. Elec. Reliability Corp., 116 FERC ¶ 61,062 (2006).
4
Unless otherwise designated, all capitalized terms shall have the meaning set forth in the Glossary of Terms
Used in NERC Reliability Standards, available at http://www.nerc.com/files/Glossary_of_Terms.pdf
5
Transmission Relay Loadability Standard, Order No. 733, 130 FERC ¶ 61,221, at P 104-08 (2010), order
on reh’g and clarification, Order No. 733-A, 134 FERC ¶ 61,127, order on reh’g and clarification, Order No. 733B, 136 FERC ¶ 61,185 (2011).
2

2

NERC submits proposed Reliability Standard PRC-025-1 to meet the Commission’s
September 30, 2013 deadline6 to develop a new generator relay loadability Reliability Standard;
however, NERC requests the Commission delay its approval of proposed Reliability Standard
PRC-025-1 until proposed Reliability Standard PRC-023-3 – Transmission Relay Loadability is
submitted to the Commission as a supplement to this petition. Proposed PRC-023-3 will be
presented to the Board for approval in November 2013 and filed with the Commission by the end
of the year. During the development of proposed Reliability Standard PRC-025-1, clarifying
changes to PRC-023-2 were identified by the standard drafting team as necessary to establish a
bright-line between the applicability of load-responsive protective relays in the transmission and
generator relay loadability Reliability Standards. As a result, a supplemental Standard
Authorization Request was approved by the Standards Committee at its January 16-17, 2013
meeting to authorize the standard drafting team to make the corresponding changes. NERC
requests the Commission take concurrent action on the proposed Reliability Standards PRC-0251 and PRC-023-3 to preserve consistency between proposed Reliability Standards PRC-025 and
PRC-023.
As required by Section 39.5(a)7 of the Commission’s regulations, this petition presents
the technical basis and purpose of proposed Reliability Standard PRC-025-1, a summary of the
development history (Exhibit F), and a demonstration that the proposed Reliability Standard
meets the criteria identified by the Commission in Order No. 6728 (Exhibit C). Proposed

6

NERC was granted a one-year extension of time until September 30, 2013 to develop a new generator relay
loadability standard. See Notice of Extension of Time, Docket No. RM08-13-001 (issued Feb. 15, 2012).
7
18 C.F.R. § 39.5(a) (2013).
8
The Commission specified in Order No. 672 certain general factors it would consider when assessing
whether a particular Reliability Standard is just and reasonable. See Rules Concerning Certification of the Electric
Reliability Organization; and Procedures for the Establishment, Approval, and Enforcement of Electric Reliability
Standards, Order No. 672, FERC Stats. & Regs. ¶ 31,204, at P 262, 321-37, order on reh’g, Order No. 672-A,
FERC Stats. & Regs. ¶ 31,212 (2006).

3

Reliability Standard PRC-025-1 was approved by the NERC Board of Trustees on August 15,
2013.9
I.

EXECUTIVE SUMMARY
Proposed Reliability Standard PRC-025-1 addresses generator Facilities protective relay

loadability. The proposed Reliability Standard is designed to prevent generator tripping when
conditions do not pose a direct risk to the generator and associated equipment and will reduce the
risk of unnecessary generator tripping—events that increase the severity of disturbances.
Proposed PRC-025-1 requires Generator Owners, Transmission Owners, and Distribution
Providers to apply an appropriate setting for load-responsive relays based on calculations or
simulations for conditions established in Attachment 1 of the proposed Reliability Standard.
The Attachment 1 criteria are representative of the short-term conditions during which
generation Facilities have, in the past, disconnected when otherwise capable of providing
Reactive Power resources. By minimizing these risks, proposed Reliability Standard PRC-025-1
serves the important reliability goal of limiting the risk for severe power system disturbances.
II.

NOTICES AND COMMUNICATIONS

Notices and communications with respect to this filing may be addressed to the
following:10

9

See NERC Board of Trustees Agenda Item 7b, available at
http://www.nerc.com/gov/bot/BOT%20May%209%202013%20%20Boston%20MA/7b-Board%20Writeup%20Phase%202%20Relay%20Loadability%20Generation%20-%20PRC-025-1.pdf.
10
Persons to be included on the Commission’s service list are identified by an asterisk. NERC respectfully
requests a waiver of Rule 203 of the Commission’s regulations, 18 C.F.R. § 385.203 (2013), to allow the inclusion
of more than two persons on the service list in this proceeding.

4

Charles A. Berardesco*
Senior Vice President and General Counsel
Holly A. Hawkins*
Assistant General Counsel
William H. Edwards*
Counsel
Brady A. Walker*
Associate Counsel
North American Electric Reliability
Corporation
1325 G Street, N.W., Suite 600
Washington, D.C. 20005
(202) 400-3000
(202) 644-8099 – facsimile
charlie.berardesco@nerc.net
holly.hawkins@nerc.net
william.edwards@nerc.net
brady.walker@nerc.net
III.

Mark G. Lauby*
Vice President and Director of Standards
Howard Gugel*
Director of Standards Development
North American Electric Reliability
Corporation
3353 Peachtree Road, N.E.
Suite 600, North Tower
Atlanta, GA 30326
(404) 446-2560
(404) 446-2595 – facsimile
mark.lauby@nerc.net
howard.gugel@nerc.net

BACKGROUND
A.

Regulatory Framework

By enacting the Energy Policy Act of 2005,11 Congress entrusted the Commission with
the duties of approving and enforcing rules to ensure the reliability of the Nation’s Bulk-Power
System, and with the duties of certifying an ERO that would be charged with developing and
enforcing mandatory Reliability Standards, subject to Commission approval. Section 215(b)(1)12
of the FPA states that all users, owners, and operators of the Bulk-Power System in the United
States will be subject to Commission-approved Reliability Standards. Section 215(d)(5)13 of the
FPA authorizes the Commission to order the ERO to submit a new or modified Reliability
Standard. Section 39.5(a)14 of the Commission’s regulations requires the ERO to file with the
Commission for its approval each Reliability Standard that the ERO proposes should become

11
12
13
14

16 U.S.C. § 824o (2006).
Id. § 824(b)(1).
Id. § 824o(d)(5).
18 C.F.R. § 39.5(a) (2013).

5

mandatory and enforceable in the United States, and each modification to a Reliability Standard
that the ERO proposes should be made effective.
The Commission has the regulatory responsibility to approve Reliability Standards that
protect the reliability of the Bulk-Power System and to ensure that such Reliability Standards are
just, reasonable, not unduly discriminatory or preferential, and in the public interest. Pursuant to
Section 215(d)(2) of the FPA15 and Section 39.5(c)16 of the Commission’s regulations, the
Commission will give due weight to the technical expertise of the ERO with respect to the
content of a Reliability Standard.
B.

NERC Reliability Standards Development Procedure

The proposed Reliability Standard was developed in an open and fair manner and in
accordance with the Commission-approved Reliability Standard development process.17 NERC
develops Reliability Standards in accordance with Section 300 (Reliability Standards
Development) of its Rules of Procedure and the NERC Standard Processes Manual.18 In its ERO
Certification Order, the Commission found that NERC’s proposed rules provide for reasonable
notice and opportunity for public comment, due process, openness, and a balance of interests in
developing Reliability Standards and thus satisfies certain of the criteria for approving Reliability
Standards. The development process is open to any person or entity with a legitimate interest in
the reliability of the Bulk-Power System. NERC considers the comments of all stakeholders, and
15

16 U.S.C. § 824o(d)(2).
18 C.F.R. § 39.5(c)(1).
17
Order No. 672 at P 334 (“Further, in considering whether a proposed Reliability Standard meets the legal
standard of review, we will entertain comments about whether the ERO implemented its Commission-approved
Reliability Standard development process for the development of the particular proposed Reliability Standard in a
proper manner, especially whether the process was open and fair. However, we caution that we will not be
sympathetic to arguments by interested parties that choose, for whatever reason, not to participate in the ERO’s
Reliability Standard development process if it is conducted in good faith in accordance with the procedures
approved by FERC.”).
18
The NERC Rules of Procedure are available at http://www.nerc.com/AboutNERC/Pages/Rules-ofProcedure.aspx. The NERC Standard Processes Manual is available at
http://www.nerc.com/comm/SC/Documents/Appendix_3A_StandardsProcessesManual.pdf.
16

6

a vote of stakeholders and the NERC Board of Trustees is required to approve a Reliability
Standard before the Reliability Standard is submitted to the Commission for approval.
C.

History of Project 2010-13 Relay Loadability
a)

PRC-023-1 — Transmission Relay Loadability

NERC developed Reliability Standard PRC-023-1 to address key August 14, 2003
blackout19 recommendations regarding relay loadability issues. Relay loadability issues were
found to have played a pivotal role in accelerating and spreading the early part of the cascading
outage in Ohio and Michigan during the blackout. Relay loadability refers to the ability of
protective relays to restrain operation for load conditions. As protective relays can respond only
to measured voltage and current, they must be set such that they will detect the faults for which
they must operate while avoiding unnecessary operation under non-fault load conditions.
The currently-effective PRC-023-1 Reliability Standard required certain Transmission
Owners, Generator Owners and Distribution Providers to set protective relays to maintain
reliable protection for all fault conditions while meeting specified criteria to ensure settings do
not contribute to cascading outages.
Reliability Standard PRC-023-1 specifically addresses Recommendation 8A20 approved
by the NERC Board of Trustees in February 2004, and the U.S.-Canada Power System Outage

19

U.S.-Canada Power System Outage Task Force, Final Report on the August 14, 2003 Blackout in the
United States and Canada: Causes and Recommendations, at 80 (2004) (“2003 Blackout Report”).
20
NERC, August 14, 2003 Blackout: NERC Actions to Prevent and Mitigate the Impacts of Future Cascading
Blackouts, at 13 (Feb. 10, 2004). Recommendation 8a of the NERC Blackout Report provides:
All transmission owners shall, no later than September 30, 2004, evaluate the zone 3
relay settings on all transmission lines operating at 230 kV and above for the purpose
of verifying that each zone 3 relay is not set to trip on load under extreme emergency
conditions. In each case that a zone 3 relay is set so as to trip on load under extreme
conditions, the transmission operator shall reset, upgrade, replace, or otherwise
mitigate the overreach of those relays as soon as possible and on a priority basis, but
no later than December 31, 2005. Upon completing analysis of its application of
zone 3 relays, each transmission owner may no later than December 31, 2004,
submit justification to NERC for applying zone 3 relays outside of these

7

Task Force’s Recommendation 21A,21 “Make More Effective and Wider Use of System
Protection Measures,” as included in the 2003 Blackout Report.
The Commission issued a Notice of Proposed Rulemaking (“NOPR”) proposing to
approve Reliability Standard PRC-023-1 on May 21, 2009.22 The Commission approved
Reliability Standard PRC-023-1 in Order No. 733. Also in the Final Rule, the Commission
directed NERC to: (1) make certain modifications to the approved Reliability Standard PRC023-1; (2) submit a timeline for NERC’s development of a new Reliability Standard to address
generator protective relay loadability; and (3) develop a new Reliability Standard addressing the
issue of protective relay operation due to power swings.23
b)

Project 2010-13

To respond to the directives in Order No. 733, NERC proposed to address the
Commission’s directives in three phases in Project 2010-13. Phase I focused on making specific
modifications24 to Reliability Standard PRC-023-1 identified in Order No. 733. Phase I was
completed and the revised Reliability Standard PRC-023-225 became mandatory on July 1, 2012.
Phase II has focused on developing a new Reliability Standard to address generator relay
loadability as proposed in this petition. Phase III will focus on developing requirements that
address relay operations due to power swings. Phase III is currently under development and is
tentatively scheduled to be completed by December 2014. The NERC Planning Committee, on

21
22
23
24
25

recommended parameters. The Planning Committee shall review such exceptions to
ensure they do not increase the risk of widening a cascading failure of the power
system.
2003 Blackout Report at 156-59.
Transmission Relay Loadability Standard, Notice of Proposed Rulemaking, 127 FERC ¶ 61,175 (2009).
Order No. 733 at P 104-P 108.
Id. at P 47.
Transmission Relay Loadability Standard, Order No. 759, 138 FERC ¶ 61,197 (2012) (“Order No. 759”).

8

August 19, 2013, approved a System Protection and Control Subcommittee report26, developed
with support from the System Analysis and Modeling Subcommittee, intended to inform the
development process.
IV.

JUSTIFICATION FOR APPROVAL
As discussed in detail in Exhibit C, proposed Reliability Standard PRC-025-1 satisfies

the Commission’s criteria in Order No. 672 and is just, reasonable, not unduly discriminatory or
preferential, and in the public interest. The reliability benefits of proposed Reliability Standard
PRC-025-1 are discussed below along with an explanation of how the proposed Reliability
Standard satisfies the Commission’s directives related to generator relay loadability in Order
733. Also included is a detailed explanation of the content of Reliability Standard PRC-025-1
and associated changes in proposed Reliability Standard PRC-023-3, which is currently in formal
development.
A.
Reliability Benefits and Technical Explanation of Proposed Reliability
Standard PRC-025-1
Analyses of power system disturbances over the last twenty-five years have found
generators to have tripped unnecessarily—an occurrence that has the potential to extend the
scope and duration of a disturbance. During the recovery phase of a disturbance, the disturbance
may exhibit a “voltage disturbance” behavior pattern, wherein system voltage is widely
depressed. In order to support the system during this phase of a disturbance, proposed
Reliability Standard PRC-025-1 establishes criteria for setting load-responsive relays such that
individual generators may provide Reactive Power within their dynamic capability during
transient time periods. Premature or unnecessary tripping of generators during this period can

26

Protection System Response to Power Swings, NERC System Protection and Control Subcommittee,
.approved by the NERC Planning Committee on August 19, 2013.

9

increase the severity of the voltage disturbance making it essential to assure this dynamic
capability is available to support system recovery.
Proposed Reliability Standard PRC-025-1 establishes a risk-based Requirement in which
the Generator Owner, Transmission Owner, or Distribution Provider that applies load-responsive
relays must identify the type of protective relay and its application, and apply an appropriate
setting based on its calculations or simulations of conditions established in Attachment 1 to
proposed Reliability Standard PRC-025-1.
NERC’s proposed Reliability Standard PRC-025-1 addresses the issue of generator relay
loadability by establishing a new Reliability Standard for load-responsive protective relays
applied on generating Facilities for the conditions, namely depressed voltages, observed during
the August 2003 blackout. Proposed Reliability Standard PRC-025-1 includes criteria for loadresponsive protective relays on generator step-up (“GSU”) transformers and on unit auxiliary
transformers (“UAT”) that supply station service power to support the on-line operation of
generating units or generating plants. These transformers are referred to as station power, UATs,
or station service transformer(s) and are used to provide overall auxiliary power to the generator
station when the generator is running. Loss of these transformers will result in the removal of
the generator from service.
The Guidelines and Technical Justification can be found in the Application Guidelines
section the proposed Reliability Standard (Exhibit A). The document provides analysis of
protective functions and generator performance addressed within this Reliability Standard. The
relay setting criteria are based on the system conditions observed during the August 2003
Blackout. The criteria for relays applied on synchronous generators, GSU transformers, and
Elements that connect the GSU transformer(s) to the Transmission system are based on the

10

response of the synchronous generator to depressed Transmission System voltage. Under this
condition the generator will respond by increasing its Reactive Power output to support its
terminal voltage – a response known as field-forcing. The criteria for relays applied on these
Elements are similar because relays applied on each of these Elements are challenged by the
loadability condition resulting from the increased generator output. An allowance is made for
relays applied on the transmission side of the GSU transformer to account for Reactive Power
losses in the transformer.
The criteria for relays applied on asynchronous generators, their GSU transformers, and
Elements that connect the GSU transformer(s) to the Transmission system are based on the
response of the asynchronous generator to depressed Transmission System voltage.
Asynchronous generators do not have excitation systems and will not respond to a disturbance
with the same magnitude of apparent power that a synchronous generator will respond.
However, asynchronous generators will support the system during a disturbance and the criteria
account for the generator response and any static or dynamic Reactive Power devices that
contribute to the power flow. The criteria for relays applied on these Elements are the same
because relays applied on each of these Elements are challenged by the loadability condition
resulting from the increased generator output. An allowance is not made for relays applied on
the transmission side of the GSU transformer because the Reactive Power losses are not
significant for asynchronous generators.
The criteria for relays applied on UAT transformers are based on the increased current
requirements of station service load during a depressed voltage condition. In this the case the
current is based on the generator terminal voltage associated with a depressed system voltage. A
conservative allowance is provided to avoid complex calculations for this load condition. As an

11

alternative, entities may base the setting on actual current measured when the generator is
operating at its maximum gross output.
Generator Owners, Transmission Owners, and Distribution Providers may at times find
the relay setting criteria are in conflict with their desired protection goals. In such cases, it is
suggested that entities consider the requirement within this Reliability Standard and its desired
protection goals, and perform modifications to its protective relays or protection philosophies as
necessary to achieve both.
B.

Commission Directives Addressed

Proposed Reliability Standard PRC-025-1 addresses and meets the Commission’s
directives in Order No. 733 related to generator relay loadability as outlined below.
a)

Proposed Reliability Standard PRC-025-1 is aligned with the
Requirements and expected outcome of PRC-023-1

The Commission declined to adopt its NOPR proposal to require the previously approved
Reliability Standard PRC-023-1 to address issues of generator step-up and auxiliary transformer
loadability.27 The Commission stated that “it does not matter if generator step-up and auxiliary
transformer loadability is addressed in a separate Reliability Standard, so long as the ERO
addresses the issue in a timely manner and in a way that is coordinated with the Requirements
and expected outcome of PRC-023-1.”28 In Order No. 733, the Commission also stated:
We also expect that the ERO will develop the Reliability Standard
addressing generator relay loadability as a new Standard, with its
own individual timeline, and not as a revision to an existing
Standard. While we agree that PRC-001-1 requires, among other
things, the coordination of generator and transmission protection
systems, we think that generator relay loadability, like transmission
relay loadability, should be addressed in its own Reliability

27
28

Id. at P 104.
Id.

12

Standard if it is not to be addressed with transmission relay
loadability.29
During the development of proposed Reliability Standard PRC-025-1, the standard
drafting team and industry stakeholders identified potential compliance overlap and reliability
gaps between Reliability Standard PRC-023-2 and proposed Reliability Standard PRC-025-1.
Reliability Standard PRC-023-2 and proposed Reliability Standard PRC-025-1 overlap with
regard to the application of load-responsive protective relays on transmission lines that connect
the generating plant or generating units to the Transmission System. Proposed Reliability
Standard PRC-025-1 introduced criteria for relays applied at the terminals of these lines. At the
same time Requirement R1, Criterion 6 of Reliability Standard PRC-023-2 requires entities to
“set transmission line relays applied on transmission lines connected to generation stations
remote to load so they do not operate at or below 230% of aggregated generation nameplate
capability.” The compliance overlap would result in a finding of a non-compliance with both
Reliability Standards—generation and transmission—unless revisions are made to avoid
overlap—two sets of Requirements applying to the same relay—between the two Reliability
Standards.
Coordinating changes to Reliability Standard PRC-023-2 are necessary to properly align
proposed Reliability Standard PRC-025-1 with Reliability Standard PRC-023-2. First,
Requirement R1, Criterion 6 of PRC-023-2 was removed and the applicability section of PRC023-2 was revised to exclude “Elements that connect the GSU transformer(s) to the
Transmission system that are used exclusively to export energy directly from a Bulk Electric
System generating unit or generating plant.” These changes avoid overlap with the requirements
in PRC-025-1 that apply to these Facilities.
29

Id. at P 106.

13

Second, PRC-025-1 was developed to include relay loadability requirements for all load
responsive protective relays applied at the terminals of generators and GSU transformers.
Corresponding modifications are being developed to revise the applicability of PRC-023-3 and to
remove section 2.4 of Attachment A to avoid overlap or gaps between the two proposed
Reliability Standards. The applicability sections for the two proposed Reliability Standards are
based on the location where the relays are applied and are independent of the intended protection
function.
The proposed applicability for PRC-025-1 includes that all relays that may operate in
response to increased generator output during stressed system conditions, assuring that these
relays are addressed in one Reliability Standard. The loadability challenge presented to relays
applied at the terminals of generators and GSU transformers is different than the transmission
relay loadability conditions observed during the August 2003 Blackout. The transmission relay
loadability requirements are based on assuring relays do not operate unnecessarily to trip
transmission elements during conditions of depressed transmission system voltage (0.85 per unit)
and high transmission power factor angle (30 degrees). Since the objective is to keep
transmission Elements in service when the Elements are not at risk of thermal overloading, the
setting requirements are based on the Facility Rating. Similarly, the generator relay loadability
requirements ensure relays do not operate unnecessarily to trip generating units during depressed
Transmission System voltage. However, the voltage and power factor conditions that challenge
relays applied at the terminals of generators and GSU transformers are significantly different.
The voltage and power factor angle will be higher, particularly for synchronous generators, due
to the increased Reactive Power output from the generator to support voltage at its terminals.
For example, generator terminal voltage may approach 0.95 per unit during depressed

14

transmission system voltage and the power factor angle may approach 60 degrees. The generator
relay loadability requirements, therefore, are based on the generator capability rather than the
Facility Rating of the generator or GSU transformer.
The protective relays applied at the terminals of generators and GSU transformers will be
challenged by the increased generator output during stressed system conditions regardless of the
intended protection function; e.g., whether they are applied to protect the generator or GSU
transformer, or to provide backup protection for the Transmission System. Thus, to prevent
unnecessary tripping of the generator, the relay loadability requirements for these relays must be
independent of the intended protection function.
The applicability requirements in PRC-025-1 and corresponding applicability proposed in
PRC-023-3 address the Commission’s concern that all generator and GSU transformer loadresponsive protective relays are subject to appropriate requirements in a Reliability Standard.
Basing applicability on the physical location where the relay is applied provides the following
advantages:
(i)

Facilitates establishing generator relay loadability requirements based on the
physics associated with increased generator output during stressed system
conditions.

(ii)

Avoids ambiguity whether the intended protection function is for the generating
unit or the Transmission System. For example, a relay may be applied at the
terminals of a generator to provide backup protection for the GSU transformer,
but because the relay setting must “over-reach” the GSU transformer terminals
the relay inherently provides backup protection for the high-voltage bus and
close-in portions of transmission lines.

15

(iii)

Provides clear division of applicability between the generator and transmission
relay loadability Reliability Standards based on the physical location, independent
of the entity that owns the relay.
b)

Proposed Reliability Standard PRC-025-1 is Timely

In Order No. 733, the Commission directed “the ERO to submit to the Commission an
updated and specific timeline to explain when it expects to develop and submit this proposed
Standard.” Further, the Commission stated it “will not hesitate to direct the development of a
new Reliability Standard if the ERO fails to propose a Standard in a timely manner.”30 NERC
submitted a specific timeline to the Commission. The Commission granted a one-year extension
of time to develop Reliability Standard for generator relay loadability on February 15, 2012,
allowing NERC until September 30, 2013 to complete the Reliability Standard pursuant to Order
No. 733.31 With this petition, NERC has timely submitted the proposed Reliability Standard.
c)

Consideration of a Generic Rating Percentage for Generator Step-up
Transformers

In Order No. 733, the Commission encouraged NERC to “consider whether a generic
rating percentage can be established for generator step-up transformers and, if so, determine that
percentage.”32 Proposed Reliability Standard PRC-025-1 establishes a Requirement that each
Generator Owner, Transmission Owner, and Distribution Provider to apply settings on its loadresponsive protective relays for GSU transformers.
For relays applied on the generator side of GSU transformers connected to synchronous
generator units, the proposed Reliability Standard establishes settings based on 100 percent of
the generator unit’s maximum gross Real Power capability in megawatts (MW), as reported to
30

Id. at P 103.
See Transmission Relay Loadability Reliability Standard, Notice of Extension of Time Docket, No. RM0813-001 (Feb. 15, 2012).
32
Order No. 733 at P 108.
31

16

the Transmission Planner, and Reactive Power capability, in megavoltampere-reactive (Mvar),
equal to 150 percent of the MW value derived from the generator nameplate megavoltampere
(MVA) rating at rated power factor. A similar generic criterion is established for relays applied
at the high-side of GSU transformers connected to synchronous generators,
For relays applied at the generator side of GSU transformers connected to asynchronous
generator units, the proposed Reliability Standard establishes settings based on 130 percent of
the generator unit’s aggregate installed maximum rated MVA output (including the Mvar output
of any static or dynamic reactive power devices) of the aggregated generators at rated power
factor. Asynchronous generator criteria also include inverter-based installations. A similar
generic criterion is established for relays applied at the high-side of GSU transformers connected
to asynchronous generators,
While these generic criteria achieve the goal of simplifying the calculations necessary to
establish relay settings, these generic criteria are conservative to assure they provide adequate
relay loadability for all applications. In some cases these generic criteria may be overly
conservative due to limitations of the generating unit. To address such cases, proposed
Reliability Standard PRC-025-1 provides multiple options for most applications, allowing
entities to use simpler calculations yielding more restrictive settings, more complex calculations
yielding less restrictive settings, or based on the modeled output of the generating plant or
generating unit.
C.

Requirement in Proposed Reliability Standard PRC-025-1

Proposed Reliability Standard PRC-025-1 establishes one Requirement for relay settings
on each load-responsive protective relay. The Requirement is as follows:33

33

A full technical justification is included in Exhibit A.

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R1. Each Generator Owner, Transmission Owner, and Distribution
Provider shall apply settings that are in accordance with PRC-0251 – Attachment 1: Relay Settings, on each load-responsive
protective relay while maintaining reliable fault protection.
[Violation Risk Factor: High][Time Horizon: Long-Term
Planning].
Requirement R1 is risk-based. For ease of use, a table providing the relevant criteria was
developed including application, relay type, voltage to consider, and the pickup setting. The
criteria table is listed in Attachment 1 of proposed Reliability Standard PRC-025-1. Based on
the criteria table, an entity must set its load-responsive relay to the appropriate setting based on
the entity’s calculation or simulation for the specified conditions. Each responsible entity must
be aware of each protective relay subject to the proposed Reliability Standard and set the relay
using an appropriate option established in the criteria table. The proposed Reliability Standard
furthers reliability by establishing setting criteria to prevent operation for short-term conditions
during which generation Facilities are capable of providing the system with increased Reactive
Power. It is under these circumstances that generation Facilities have historically been
disconnected. In previous disturbance events, the disconnecting of generation Facilities has
increased the severity of the event.
The basis for the proposed Reliability Standard’s loadability criteria for relays applied at
the terminals of synchronous generators or low-side of the GSU transformer are the dynamic
generating unit loading values observed during the August 2003 blackout, other subsequent
system events, and simulations of generating unit response to similar system conditions. The
Reactive Power output observed during field-forcing in these events and simulations approaches
a value equal to 150 percent of the Real Power (MW) capability of the generating unit when the
generator is operating at its Real Power capability. In the System Protection and Control

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Subcommittee technical reference document34, two operating conditions were examined based
on these events and simulations: (1) when the unit is operating at rated Real Power in MW with a
level of Reactive Power output in Mvar which is equivalent to 150 percent times the rated MW
value (representing some level of field-forcing); and (2) when the unit is operating at its declared
low active Real Power operating limit (e.g., 40 percent of rated Real Power, with a level of
Reactive Power output in Mvar which is equivalent to 175 percent times the rated MW value
(representing some additional level of field-forcing)).
Both conditions above are evaluated with the GSU transformer high-side voltage at 0.85
per unit. These load operating points are believed to be conservatively high levels of Reactive
Power out of the generator with a 0.85 per unit high-side voltage which is based on these
observations. However, the drafting team evaluated the benefit of defining two operating points
and determined, for the purposes of this proposed Reliability Standard, that the second load point
(40 percent) offered no additional benefit and only increased the complexity for an entity to
determine how to comply with the Reliability Standard. Given the conservative nature of the
criteria, which may not be achievable by all generating units, an alternate method is provided to
determine the Reactive Power output by simulation. In addition, to account for Reactive Power
losses in the GSU transformer, a reduced level of output of 120 percent times the rated MW
value is provided for relays applied at the high-side of the GSU transformer and on Elements that
connect a GSU transformer to the Transmission system and are used exclusively to export energy
directly from a Bulk Electric System generating unit or generating plant.

34

Technical Reference Document: Power Plant and Transmission System Protection Coordination – Revision
1, NERC System Protection and Control Subcommittee (Jul. 2010), available at
http://www.nerc.com/docs/pc/spctf/Gen%20Prot%20Coord%20Rev1%20Final%2007-30-2010.pdf. The technical
reference document was approved by the NERC Planning Committee on July 30, 2010.

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The basis for the proposed Reliability Standard’s loadability criteria for relays applied at
the terminals of asynchronous generators or low-side of the GSU transformer is the expected
dynamic generating unit loading for the same system conditions used for synchronous
generators. Asynchronous generators do not have excitation systems and will not respond to a
disturbance with the same magnitude of apparent power that a synchronous generator will
respond. However, asynchronous generators will support the system during a disturbance.
The generator output used to determine settings is derived from 130 percent of the
maximum aggregate nameplate MVA output at rated power factor including the Mvar output of
any static or dynamic Reactive Power devices. This is determined by summing the total MW and
Mvar capability of the generation equipment behind the relay and any static or dynamic Reactive
Power devices that contribute to the power flow through the relay. This calculation
approximates the stressed system conditions.
Asynchronous generators do not produce as much Reactive Power as synchronous
generators; the voltage drop due to Reactive Power flow through the GSU transformer is not as
significant. Therefore, the generator bus voltage can be conservatively estimated by reflecting
the high-side nominal voltage to the generator-side based on the GSU transformer’s turns ratio
and the setting basis is the same for relays on either side of the GSU transformer, and for relays
applied on Elements that connect a GSU transformer to the Transmission system and are used
exclusively to export energy directly from a Bulk Electric System generating unit or generating
plant.
The basis for the proposed Reliability Standard’s loadability criteria for relays applied on
UATs is based on the expected performance of station service load during depressed system
voltage. The performance of the UAT loads during stressed system conditions is very difficult to

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determine. Rather than requiring responsible entities to determine the response of UAT loads to
depressed voltage, the technical experts writing the proposed Reliability Standard elected to
increase the margin to 150 percent from that used elsewhere in this proposed Reliability Standard
(e.g., 115 percent) and use a generator bus voltage of 1.0 per unit. A minimum pickup current
based on 150 percent of maximum transformer nameplate MVA rating at 1.0 per unit generator
bus voltage will provide adequate transformer protection based on IEEE C37.9135 at full load
conditions while providing sufficient relay loadability to prevent a trip of the UAT, and
subsequent unit trip, due to increased UAT load current during stressed system voltage
conditions.
Because of the various design and loading characteristics of UATs, two options are
provided to accommodate an entity’s protection philosophy while preventing the UAT
transformer phase time overcurrent relays from operating during the dynamic conditions
anticipated by this proposed Reliability Standard. These options are based on the transformer
bus voltage corresponding to 1.0 per unit nominal voltage on the high-side winding of the UAT.
For the first option, the overcurrent element shall be set greater than 150 percent of the
calculated current derived from the UAT maximum nameplate MVA rating. This is a simple
calculation that approximates the stressed system conditions. For the second option, the
overcurrent element shall be set greater than 150 percent of the UAT measured current at the
generator maximum gross MW capability reported to the Transmission Planner. This allows for a
reduced setting pickup compared to the first option. This is a more involved calculation that
approximates the stressed system conditions by allowing the entity to consider the actual load

35

IEEE Guide for Protecting Power Transformers, IEEE Std C37.91-2008 (Revision of IEEE Std C37.912000) (2008).

21

placed on the UAT based on the generator’s maximum gross MW capability reported to the
Transmission Planner.
D.

Enforceability of PRC-025-1

The proposed Reliability Standard PRC-025-1 contains a Measure that supports the
Requirement by clearly identifying acceptable evidence of compliance and how the Requirement
will be enforced. The Implementation Plan also discusses the documentation necessary to
comply with the proposed Reliability Standard. The VSL provides further guidance on the way
that NERC will enforce the Requirements of the proposed Reliability Standard. The VRF and
VSL for the proposed Reliability Standard comport with NERC and Commission guidelines
related to their assignment. The VSL has been developed based on the situations an auditor may
encounter during a compliance audit. For a detailed review of the VRF, VSL, and the analysis of
how the VRF and VSL were developed using these guidelines, see Exhibit E.
V.

CONCLUSION
For the reasons set forth above, NERC respectfully requests that the Commission:
•

approve the proposed Reliability Standard, the VRF and VSL (explained in Exhibit E),
and other associated elements included in Exhibit A; and

•

approve the implementation plan included in Exhibit B.
Respectfully submitted,
/s/ Brady A. Walker

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Charles A. Berardesco
Senior Vice President and General Counsel
Holly A. Hawkins
Assistant General Counsel
William H. Edwards
Counsel
Brady A. Walker
Associate Counsel
North American Electric Reliability
Corporation
1325 G Street, N.W., Suite 600
Washington, D.C. 20005
(202) 400-3000
(202) 644-8099 – facsimile
charlie.berardesco@nerc.net
holly.hawkins@nerc.net
william.edwards@nerc.net
brady.walker@nerc.net
Counsel for the North American Electric
Reliability Corporation
Date: September 30, 2013

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