Reliability Standard PRC-010-2

PRC-010-2 – Under Voltage Load Shedding
A. Introduction
1.

Title:

2.
3.

Number: PRC-010-2
Purpose: To establish an integrated and coordinated approach to the design,
evaluation, and reliable operation of Undervoltage Load Shedding Programs (UVLS
Programs).

4.

Applicability:

Undervoltage Load Shedding

4.1. Functional Entities:
4.1.1 Planning Coordinator.
4.1.2 Transmission Planner.
4.1.3 Undervoltage load shedding (UVLS) entities – Distribution Providers and
Transmission Owners responsible for the ownership, operation, or
control of UVLS equipment as required by the UVLS Program established
by the Transmission Planner or Planning Coordinator.
5.

Effective Date: See Project 2008-02.2 Implementation Plan.

B. Requirements and Measures
R1.

Each Planning Coordinator or Transmission Planner that is developing a UVLS Program
shall evaluate its effectiveness and subsequently provide the UVLS Program’s
specifications and implementation schedule to the UVLS entities responsible for
implementing the UVLS Program. The evaluation shall include, but is not limited to,
studies and analyses that show: [Violation Risk Factor: High] [Time Horizon: Long‐term
Planning]
1.1. The implementation of the UVLS Program resolves the identified
undervoltage issues that led to its development and design.
1.2. The UVLS Program is integrated through coordination with generator voltage
ride‐through capabilities and other protection and control systems, including,
but not limited to, transmission line protection, autoreclosing, Remedial Action
Schemes, and other undervoltage‐based load shedding programs.

M1. Acceptable evidence may include, but is not limited to, date‐stamped studies and
analyses, reports, or other documentation detailing the effectiveness of the UVLS
Program, and date‐stamped communications showing that the UVLS Program
specifications and implementation schedule were provided to UVLS entities.
R2.

Each UVLS entity shall adhere to the UVLS Program specifications and implementation
schedule determined by its Planning Coordinator or Transmission Planner associated
with UVLS Program development per Requirement R1 or with any Corrective Action
Plans per Requirement R5. [Violation Risk Factor: High] [Time Horizon: Long‐term
Planning]

Page 1 of 29

PRC-010-2 – Under Voltage Load Shedding
M2. Acceptable evidence must include date‐stamped documentation on the completion of
actions and may include, but is not limited to, identifying the equipment armed with
UVLS relays, the UVLS relay settings, associated Load summaries, work management
program records, work orders, and maintenance records.
R3.

Each Planning Coordinator or Transmission Planner shall perform a comprehensive
assessment to evaluate the effectiveness of each of its UVLS Programs at least once
every 60 calendar months. Each assessment shall include, but is not limited to, studies
and analyses that evaluate whether: [Violation Risk Factor: Medium] [Time Horizon:
Long‐term Planning]
3.1. The UVLS Program resolves the identified undervoltage issues for which the
UVLS Program is designed.
3.2. The UVLS Program is integrated through coordination with generator voltage
ride‐through capabilities and other protection and control systems, including,
but not limited to, transmission line protection, autoreclosing, Remedial Action
Schemes, and other undervoltage‐based load shedding programs.

M3. Acceptable evidence may include, but is not limited to, date‐stamped reports or other
documentation detailing the assessment of the UVLS Program.
R4.

Each Planning Coordinator or Transmission Planner shall, within 12 calendar months
of an event that resulted in a voltage excursion for which its UVLS Program was
designed to operate, perform an assessment to evaluate: [Violation Risk Factor:
Medium] [Time Horizon: Operations Planning]
4.1. Whether its UVLS Program resolved the undervoltage issues associated with the
event, and
4.2. The performance (i.e., operation and non-operation) of the UVLS Program
equipment.

M4. Acceptable evidence may include, but is not limited to, date‐stamped event data,
event analysis reports, or other documentation detailing the assessment of the UVLS
Program and associated equipment.
R5.

Each Planning Coordinator or Transmission Planner that identifies deficiencies during
an assessment performed in either Requirement R3 or R4 shall develop a Corrective
Action Plan to address the deficiencies and subsequently provide the Corrective
Action Plan, including an implementation schedule, to UVLS entities within three
calendar months of completing the assessment. [Violation Risk Factor: Medium] [Time
Horizon: Operations Planning]

M5. Acceptable evidence must include a date‐stamped Corrective Action Plan that
addresses identified deficiencies and may also include date‐stamped reports or other
documentation supporting the Corrective Action Plan. Evidence should also include
date‐stamped communications showing that the Corrective Action Plan and an
associated implementation schedule were provided to UVLS entities.

Page 2 of 29

PRC-010-2 – Under Voltage Load Shedding
R6.

Each Planning Coordinator that has a UVLS Program in its area shall update a database
containing data necessary to model the UVLS Program(s) in its area for use in event
analyses and assessments of the UVLS Program at least once each calendar year.
[Violation Risk Factor: Lower] [Time Horizon: Operations Planning]

M6. Acceptable evidence may include, but is not limited to, date‐stamped spreadsheets,
database reports, or other documentation demonstrating a UVLS Program database
was updated.
R7.

Each UVLS entity shall provide data to its Planning Coordinator according to the
format and schedule specified by the Planning Coordinator to support maintenance of
a UVLS Program database. [Violation Risk Factor: Lower] [Time Horizon: Operations
Planning]

M7. Acceptable evidence may include, but is not limited to, date‐stamped emails, letters,
or other documentation demonstrating data was provided to the Planning
Coordinator as specified.
R8.

Each Planning Coordinator that has a UVLS Program in its area shall provide its UVLS
Program database to other Planning Coordinators and Transmission Planners within
its Interconnection, and other functional entities with a reliability need, within 30
calendar days of a written request. [Violation Risk Factor: Lower] [Time Horizon:
Operations Planning]

M8. Acceptable evidence may include, but is not limited to, date‐stamped emails, letters,
or other documentation demonstrating that the UVLS Program database was
provided within 30 calendar days of receipt of a written request.
C. Compliance
1.

Compliance Monitoring Process
1.1. Compliance Enforcement Authority
As defined in the NERC Rules of Procedure, “Compliance Enforcement Authority”
means NERC or the Regional Entity in their respective roles of monitoring and
enforcing compliance with the NERC Reliability Standards.
1.2. 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.
The Planning Coordinator, Transmission Planner, Distribution Provider, and
Transmission Owner shall keep data or evidence to show compliance as identified

Page 3 of 29

PRC-010-2 – Under Voltage Load Shedding
below unless directed by its Compliance Enforcement Authority to retain specific
evidence for a longer period of time as part of an investigation.
The applicable entity shall retain documentation as evidence for six calendar
years.
If an applicable entity is found non‐compliant, it shall keep information related to
the non‐compliance until mitigation is complete and approved, or for the time
specified above, whichever is longer.
The Compliance Enforcement Authority shall keep the last audit records and all
requested and submitted subsequent audit records.
1.3. Compliance Monitoring and Assessment Processes
“Compliance Monitoring and Assessment Processes” refers to the identification of
the processes that will be used to evaluate data or information for the purpose of
assessing performance or outcomes with the associated reliability standard.
1.4. Additional Compliance Information
None.

Page 4 of 29

PRC-010-2 – Under Voltage Load Shedding

Table of Compliance Elements
R#

Time
Horizon
Long‐term
Planning

R1

Violation Severity Levels

VRF
Lower VSL

Moderate VSL

High VSL

Severe VSL

N/A

The applicable entity
that developed the
UVLS Program failed to
evaluate the program’s
effectiveness and
subsequently provide
the UVLS Program’s
specifications and
implementation
schedule to UVLS
entities in accordance
with Requirement R1,
including the items
specified in Parts 1.1
and 1.2.

High

N/A

N/A

Page 5 of 29

PRC-010-2 – Under Voltage Load Shedding

R#

Time
Horizon
Long‐term
Planning

Violation Severity Levels
VRF
Lower VSL

Moderate VSL

High

R2

N/A

N/A

High VSL

Severe VSL

The applicable entity
failed to adhere to the
UVLS Program
specifications in
accordance with
Requirement R2.

The applicable entity
failed to adhere to the
UVLS Program
specifications and
implementation
schedule in accordance
with Requirement R2.

OR
The applicable entity
failed to adhere to the
implementation
schedule in accordance
with Requirement R2.

Long‐term
Planning

R3

Medium

N/A

N/A

N/A

The applicable entity
failed to perform an
assessment at least
once during the 60
calendar months in
accordance with
Requirement R3,
including the items
specified in Parts 3.1
and 3.2.

Page 6 of 29

PRC-010-2 – Under Voltage Load Shedding

R#

Time
Horizon
Operations
Planning

R4

Violation Severity Levels
VRF
Medium

Lower VSL

Moderate VSL

High VSL

Severe VSL

The applicable
entity performed an
assessment in
accordance with
Requirement R4
within a time period
greater than 12
calendar months
but less than or
equal to 13 calendar
months after an
applicable event.

The applicable entity
performed an
assessment in
accordance with
Requirement R4
within a time period
greater than 13
calendar months but
less than or equal to
14 calendar months
after an applicable
event.

The applicable entity
performed an
assessment in
accordance with
Requirement R4 within
a time period greater
than 14 calendar
months but less than or
equal to 15 calendar
months after an
applicable event.

The applicable entity
performed an
assessment in
accordance with
Requirement R4 within
a time period greater
than 15 calendar
months after an
applicable event.
OR
The applicable entity
failed to perform an
assessment in
accordance with
Requirement R4.

Page 7 of 29

PRC-010-2 – Under Voltage Load Shedding

R#

Time
Horizon
Operations
Planning

R5

Violation Severity Levels
VRF
Medium

Lower VSL

Moderate VSL

High VSL

Severe VSL

The applicable
entity developed a
Corrective Action
Plan and provided it
to UVLS entities in
accordance with
Requirement R5 but
was late by less than
or equal to 15
calendar days.

The applicable entity
developed a
Corrective Action
Plan and provided it
to UVLS entities in
accordance with
Requirement R5 but
was late by more
than 15 calendar
days but less than or
equal to 30 calendar
days.

The applicable entity
developed a Corrective
Action Plan and
provided it to UVLS
entities in accordance
with Requirement R5
but was late by more
than 30 calendar days
but less than or equal
to 45 calendar days.

The applicable entity
developed a Corrective
Action Plan and
provided it to UVLS
entities in accordance
with Requirement R5
but was late by more
than 45 calendar days.
OR
The responsible entity
failed to develop a
Corrective Action Plan
or provide it to UVLS
entities in accordance
with Requirement R5.

Page 8 of 29

PRC-010-2 – Under Voltage Load Shedding

R#

Time
Horizon
Operations
Planning

R6

Violation Severity Levels
VRF
Lower

Lower VSL

Moderate VSL

High VSL

The applicable
entity updated the
database in
accordance with
Requirement R6 but
was late by less than
or equal to 30
calendar days.

The applicable entity
updated the
database in
accordance with
Requirement R6 but
was late by more
than 30 calendar
days but less than or
equal to 60 calendar
days.

The applicable entity
updated the database
in accordance with
Requirement R6 but
was late by more than
60 calendar days but
less than or equal to 90
calendar days.

Severe VSL
The applicable entity
updated the database
in accordance with
Requirement R6 but
was late by more than
90 calendar days.
OR
The applicable entity
failed to update the
database in accordance
with Requirement R6.

Page 9 of 29

PRC-010-2 – Under Voltage Load Shedding

R#

Time
Horizon
Operations
Planning

R7

Violation Severity Levels
VRF
Lower

Lower VSL

Moderate VSL

High VSL

Severe VSL

The applicable
entity provided data
in accordance with
Requirement R7 but
was late by less than
or equal to 30
calendar days per
the specified
schedule.

The applicable entity
provided data in
accordance with
Requirement R7 but
was late by more
than 30 calendar
days but less than or
equal to 60 calendar
days per the
specified schedule.

The applicable entity
provided data in
accordance with
Requirement R7 but
was late by more than
60 calendar days but
less than or equal to 90
calendar days per the
specified schedule.

The applicable entity
provided data in
accordance with
Requirement R7 but
was late by more than
90 calendar days per
the specified schedule.

OR
The applicable
entity provided data
in accordance with
Requirement R7 but
the data was not
provided according
to the specified
format.

OR
The applicable entity
failed to provide data
in accordance with
Requirement R7.

Page 10 of 29

PRC-010-2 – Under Voltage Load Shedding

R#

Time
Horizon
Operations
Planning

R8

Violation Severity Levels
VRF
Lower

Lower VSL

Moderate VSL

High VSL

The applicable
entity provided its
UVLS Program
database in
accordance with
Requirement R8 but
was late by less than
or equal to 15
calendar days.

The applicable entity
provided its UVLS
Program database in
accordance with
Requirement R8 but
was late by more
than 15 calendar
days but less than or
equal to 30 calendar
days.

The applicable entity
provided its UVLS
Program database in
accordance with
Requirement R8 but
was late by more than
30 calendar days but
less than or equal to 45
calendar days.

Severe VSL
The applicable entity
provided its UVLS
Program database in
accordance with
Requirement R8 but
was late by more than
45 calendar days.
OR
The applicable entity
failed to provide its
UVLS Program
database in accordance
with Requirement R8.

Page 11 of 29

PRC-010-2 – Under Voltage Load Shedding
D. Regional Variances
None.
E. Interpretations
None.
F. Associated Documents
None.

Version History
Version

Date

0

February 8, 2005

Adopted by NERC
Board of Trustees

0

April 1, 2005

Effective Date

0

Adopted by NERC
February 7, 2013
Board of Trustees

R2 and associated elements for
retirement as part of the Paragraph 81
project (Project 2013-02) pending
applicable regulatory approval.

November 13,
2014

Adopted by NERC
Board of Trustees

Revisions made under Project 2008-02:
Undervoltage Load Shedding (UVLS) &
Underfrequency Load Shedding (UFLS)
to address directive issued in FERC
Order No. 763.

May 7, 2015

Adopted by NERC
Board of Trustees

Revisions made under Project 200802.2: Undervoltage Load Shedding
(UVLS): Misoperation to include UVLS
equipment.

November 19,
2015

FERC Letter Order
issued approving
PRC-010-2. Docket
RD15-5-000

1

2

2

Action

Change Tracking

Page 12 of 29

PRC-010-2 – Under Voltage Load Shedding

Guidelines and Technical Basis
Introduction
The standard drafting team provides the following discussion to support the approach to the
standard. The information is meant to enhance the understanding of the reliability needs and
deliverable expectations of each requirement, supported as necessary by technical principles and
industry experience.
Guidelines for UVLS Program Definition
The definition for the term, “Undervoltage Load Shedding Program” or “UVLS Program” includes
automatic load shedding programs that utilize only voltage inputs at locations where action is
taken to shed load. As such, the failure of a single component is unlikely to affect the reliable
operation of the program.
The UVLS Program definition excludes centrally controlled undervoltage‐based load shedding,
which utilizes inputs from multiple locations and may also utilize inputs other than voltages (such
as generator reactive reserves, facility loadings, equipment statuses, etc.). The design and
characteristics of a centrally controlled undervoltage‐based load shedding system are the same
as that of a Remedial Action Scheme (RAS), wherein load shedding is the remedial action.
Therefore, just like for a RAS, the failure of a single component can compromise the reliable
operation of centrally controlled undervoltage‐based load shedding.
To ensure that the applicability of the standard includes only those undervoltage‐based load
shedding systems whose performance has an impact on system reliability, a UVLS Program must
mitigate risk of one or more of the following: voltage instability, voltage collapse, or Cascading
impacting the Bulk Electric System (BES). An example of a program that would not fall under this
category is undervoltage‐based load shedding installed to mitigate damage to equipment or local
loads that are directly affected by the low voltage event.
Figure 1 below is an example of a BES subsystem for which a UVLS system could be used as a
solution to mitigate various issues following the loss of the 345 kV double circuit line between
buses A and B. If the consequence of this Contingency does not impact the BES by leading to
voltage instability, voltage collapse, or Cascading, a UVLS system (installed at either, or both, bus
B and D) used to mitigate this Contingency would not fall under the definition of a UVLS Program.
However, if this same UVLS system is used to mitigate an Adverse Reliability Impact outside this
contained area, it would be classified as a wide‐area undervoltage problem and would fall under
the definition of UVLS Program.

Page 13 of 29

PRC-010-2 – Under Voltage Load Shedding

Figure 1: UVLS Subsystem

Guidelines for Requirements
Table 1 provides a high-level overview of the requirements contained in the standard.
Table 1: High-Level Requirement Overview
Requirement

R1
R2

Entity

PC or TP

Evaluate
Program
Effectiveness

Adhere to
Program
Specifications
and Schedule

Perform
Program
Assessment
(Periodic or
Performance)

UVLS entity

X

PC or TP

X

X

R4

PC or TP

X

X

R5

PC or TP

R7
R8

Update
and/or
Share
Program
Data

X

R3

R6

Develop a
CAP to
Address
Program
Deficiencies

PC
UVLS entity
PC

X
X
X
X

Guidelines for Requirement R1
A UVLS Program may be developed and implemented to either serve as a safety net system
protection measure against unforeseen extreme Contingencies or to achieve specific system
Page 14 of 29

PRC-010-2 – Under Voltage Load Shedding
performance for known transmission Contingencies for which dropping of load is allowed under
Transmission Planning (TPL) Reliability Standards. Regardless of the purpose, it is important that
the UVLS Program being implemented is effective in terms that it mitigates undervoltage
conditions impacting the Bulk Electric System (BES), leading to voltage instability, voltage
collapse, or Cascading. Consideration should be given to voltage set points and time delays, rate
of voltage decay or recovery, power flow levels, etc. when designing a UVLS Program.
For the UVLS Program to be effective in achieving its goal, it is also necessary that the UVLS
Program is coordinated with generator voltage ride‐through capabilities and other protection
and control systems that may have an impact on the performance of the UVLS Program. Some of
these protection and control systems may include, but are not limited to, transmission line
protection, RAS, other undervoltage‐based load shedding programs, autoreclosing, and controls
of shunt capacitors, reactors, and static voltampere-reactive systems (SVSs).
For example, if the purpose of a UVLS Program is to mitigate fault‐induced delayed voltage
recovery (FIDVR) events in a large load center that also includes local generation, it is important
that such a UVLS Program is coordinated with local generators’ voltage ride‐through capabilities.
Generators in the vicinity of a load center are critical to providing dynamic voltage support to the
system during FIDVR events. To maximize the benefit of on-line generation, the best practice may
be to shed load prior to generation trip. However, occasionally, it may be best to let generation
trip prior to load shed. Therefore, the impact of generation tripping should be considered while
designing a UVLS Program.
Another example that can be highlighted is the coordination of a UVLS Program with automatic
shunt reactor tripping devices if there are any on the system. Most likely, any shunt reactors on
the system will trip off automatically after some time delay during low voltage conditions. In such
cases, shunt reactors should be tripped before the load is shed to preserve the system. This may
require coordination of time delays associated with the UVLS Program with shunt reactor tripping
devices.
The examples given above demonstrate that, for a UVLS Program to be effective, proper
consideration should be given to coordination of a UVLS Program with generator ride‐through
capabilities and other protection and control systems.
Guidelines for Requirement R2
Once a Planning Coordinator (PC) or Transmission Planner (TP) has identified a need for a UVLS
Program, the Planning Coordinator or Transmission Planner will develop a program that includes
specifications and an implementation schedule, which are then provided to UVLS entities per
Requirement R1. Specifications may include voltage set points, time delays, amount of load to be
shed, and the location at which load needs to be shed. If UVLS entities do not implement the
UVLS Program according to the specifications and schedule provided, the UVLS Program may not
be effective and may not achieve its intended goal. The UVLS entity must document that all
necessary actions were completed to implement the UVLS Program.

Page 15 of 29

PRC-010-2 – Under Voltage Load Shedding
Similarly, when a Corrective Action Plan (CAP) to address UVLS Program deficiencies is developed
by the Planning Coordinator or Transmission Planner and provided to UVLS entities per
Requirement R5, UVLS entities must comply with the CAP and its associated implementation
schedule to ensure that the UVLS Program is effective. The UVLS entity is required to complete
the actions specified in the CAP, document the plan implementation, and retain the appropriate
evidence to demonstrate implementation and completion.
Deferrals or other relevant changes to the UVLS Program specifications or CAP need to be
documented so that the record includes not only what was planned, but what was implemented.
Depending on the planning and documentation format used by the responsible entity, evidence
of a successful execution could consist of signed‐off work orders, printouts from work
management systems, spreadsheets of planned versus completed work, timesheets, work
inspection reports, paid invoices, photographs, walk‐through reports, or other evidence.
For example, documentation of a CAP provides an auditable progress and completion
confirmation for the identified UVLS Program deficiency:
CAP Example 1 ‐ Corrective actions for a quick triggering problem; preemptive actions for
similar installations:
The PC or TP obtains fault records from a UVLS entity that participates in its UVLS Program
that indicate a group of UVLS relays triggered at the appropriate undervoltage level but
with shorter delays than expected. The PC or TP directed the UVLS entity to schedule on‐
site inspections within three weeks. The results of the inspection confirmed that the
delay-time programmed on the relays was 60 cycles instead of 90 cycles. The PC or TP
then directed the UVLS entity to correct to a 90‐cycle time delay setting of the UVLS relays
identified to have shorter time delay settings within eight weeks.
Applicability to other UVLS relays: The PC or TP then developed a schedule with the UVLS
entity to verify and adjust all remaining UVLS relays time delay settings within a one‐year
period.
The PC or TP verified completion of verification and adjustment of the time delay settings
for all of the UVLS entity’s equipment that participates in the PC or TP UVLS Program
CAP Example 2 ‐ Corrective actions for a firmware problem; preemptive actions for similar
installations:
The PC or TP obtains fault records on 6/4/2014 from a UVLS entity that participates in its
UVLS Program. The UVLS entity also provided the fault records to the manufacturer, who
responded on 6/11/2014 that the Misoperation 1 of the UVLS relay was caused by a bug
in version 2 firmware, and recommended installing version 3 firmware. The PC or TP
approved the UVLS entity’s plan to schedule Version 3 firmware installation on
6/12/2014.
Misoperation of Protection Systems reporting was initiated by the NERC Board of Trustees adopted NERC Rules of Procedure,
Section 1600, Request for Data or Information. Refer to: Request for Data of Information, Protection System Misoperation Data
Collection, August 14, 2014. http://www.nerc.com/pa/RAPA/ProctectionSystem
Misoperations/PRC-004-3%20Section%201600%20Data%20Request_20140729.pdf.
1

Page 16 of 29

PRC-010-2 – Under Voltage Load Shedding
Applicability to other UVLS relays: The PC or TP then developed a schedule with the UVLS
entity to install firmware version 3 at all of the UVLS entity’s UVLS relays that are
determined to be programmed with version 2 firmware. The completion date was
scheduled no-later-than 12/31/2014.
The firmware replacements were completed on 12/4/2014.
Guidelines for Requirement R3
In addition to the initial studies required to develop a UVLS Program, periodic comprehensive
assessments (detailed analyses) are required to ensure its continued effectiveness. This
assessment is required to be completed at least once every 60 calendar months to capture the
accumulated effects of minor changes to the system that have occurred since the last assessment
was completed. However, at any point in time, a Planning Coordinator or Transmission Planner
may also determine that a material change 2 to system topology or operating conditions affects
the performance of the UVLS Program and therefore necessitates the same comprehensive
assessment. Regardless of the trigger, each assessment should include an evaluation of each
UVLS Program to ensure the continued integration through coordination.
This comprehensive assessment complements the TPL‐001‐4 annual assessment requirement to
evaluate the impact of protection systems. The 60-month period is the same time frame used in
TPL‐001‐4 and in PRC‐006‐1.
As specified in Requirement R3, a comprehensive assessment must be performed at least once
every 60 calendar months. If a Planning Coordinator or Transmission Planner conducts a
comprehensive assessment sooner for the reasons discussed above, the 60‐month time period
would restart upon completion of this assessment.
Guidelines for Requirement R4
After a voltage excursion event, the goal of the assessment required in Requirement R4 is to
evaluate: (1) whether the UVLS Program resolved the undervoltage issues, and (2) the
performance of the UVLS Program equipment. The assessment should include event data
analysis, such as the relevant sequence of events leading to the undervoltage conditions (e.g.,
Contingencies, operation of protection systems, and RAS) and field measurements useful to
analyzing the behavior of the system. A comprehensive description of the UVLS Program
operation should be presented, including conditions of the trigger (e.g., voltage levels, time
delays) and amount of load shed for each affected substation. Assessment of the event is
performed to evaluate the level of performance of the program for the event of interest and to
identify deficiencies to be included in a CAP per Requirement R5. Misoperation of UVLS
equipment is addressed as a deficiency. Reporting of UVLS equipment Misoperations are
It is understood that the term material change is not transportable on a continent‐wide basis. This determination must be
made by the Planning Coordinator or Transmission Planner and should be accompanied by documentation to support the
technical rationale for determining material changes.

2

Page 17 of 29

PRC-010-2 – Under Voltage Load Shedding
addressed by the NERC Request for Data and Information, Protection System Misoperation Data
Collection. 3
The studies and analyses showing the effectiveness of the UVLS Program can be similar to what
is required in Requirements R1 and R3, but should include a clear link between the evaluation of
effectiveness (in studies using simulations) and the analysis of the event (with measurements
and event data) that actually occurred. For example, differences between the expected and
actual system behavior for the event of interest should be discussed and modeling assumptions
should be evaluated. Important discrepancies between the simulations and the actual event
should be investigated.
Considering the importance of an event that involves the operation of a UVLS Program, the 12‐
calendar‐month period provides adequate time to analyze the event and perform an assessment
while identifying deficiencies within a reasonable time. This time period is also required in PRC‐
006‐1.
Guidelines for Requirement R5
Requirement R5 promotes the prudent correction of an identified problem during the
assessment of a UVLS Program. Per Requirements R3 and R4, an assessment of an active UVLS
Program is triggered:
•

Within 12 calendar months of an event that resulted in a voltage excursion for which
the program was designed to operate

•

At least once every 60 calendar months. The default time frame of 60 calendar months
or less between assessments has the intention to assure that the cumulative changes to
the network and operating condition affecting the UVLS Program are evaluated

Since every UVLS is unique, if material changes are made to system topology or operating
conditions, the Planning Coordinator or Transmission Planner will decide the degree to which the
change in topology or operating condition becomes a material change sufficient to trigger an
assessment of the existing UVLS Program.
A CAP is a list of actions and an associated timetable for implementation to remedy a specific
problem. It is a proven tool for resolving operational problems. Per Requirement R5, the Planning
Coordinator or Transmission Planner is required to develop a CAP and provide it to UVLS entities
to accomplish the purpose of this requirement, which is to prevent future deficiencies in the UVLS
Program, thereby minimizing risk to the system. Determining the cause of the deficiency is
essential in developing an effective CAP to avoid future re‐occurrence of the same problem. A
CAP can be revised if additional causes are found.
Based on industry experience and operational coordination timeframes, three calendar months
from the date an assessment is completed is a reasonable time frame for development of a CAP,
including time to consider alternative solutions and coordination of resources. The “within three

3

Id.

Page 18 of 29

PRC-010-2 – Under Voltage Load Shedding
calendar months” time frame is solely to develop a CAP, including its implementation schedule,
and provide it to UVLS entities. It does not include the time needed for its implementation by
UVLS entities. This implementation time frame is dictated within the CAP’s associated timetable
for implementation, and the execution of the CAP according to its schedule is required in
Requirement R2.
Guidelines for Requirements R6–R8
An accurate UVLS Program database is necessary for the Planning Coordinator or Transmission
Planner to perform system reliability assessment studies and event analysis studies. Without
accurate data, there is a possibility that annual reliability assessment studies that are performed
by the Planning Coordinator or Transmission Planner can lead to erroneous results and therefore
impact reliability. Also, without the accurate data, it is very difficult for the Planning Coordinator
or Transmission Planner to duplicate a UVLS event and determine the root cause of the problem.
To support a UVLS Program database, it is necessary for each UVLS entity to provide accurate
data to its Planning Coordinator. Each UVLS entity will provide the data according to the specified
format and schedule provided by the Planning Coordinator. This is required in order for the
Planning Coordinator to maintain and support a comprehensive UVLS Program database. By
having a comprehensive database, the Planning Coordinator can embark on a reliability
assessment or event analysis/benchmarking studies, identify the issues with the UVLS Program,
and develop Corrective Action Plans.
The UVLS Program database may include, but is not limited to the following:
•

Owner and operator of the UVLS Program

•

Size and location of customer load, or percent of connected load, to be interrupted

•

Corresponding voltage set points and clearing times

•

Time delay from initiation to trip signal

•

Breaker operating times

•

Any other schemes that are part of or impact the UVLS Programs, such as related
generation protection, islanding schemes, automatic load restoration schemes,
underfrequency load shedding (UFLS), and RAS

Additionally, the UVLS Program database is required to be updated annually (once every calendar
year) by the Planning Coordinator. The intent here is for UVLS entities to review the data annually
and provide changes to the Planning Coordinators so that Planning Coordinators can keep the
databases current and accurate for performing event analysis and other assessments.
Finally, a Planning Coordinator is required to provide information to other Planning Coordinators
and Transmission Planners within its Interconnection, and other functional entities with a
reliability need, within 30 calendar days of receipt of a written request. Thirty calendar days was
selected as the time frame as it is considered to be reasonable and well‐ accepted by the industry.
Also, this requirement of sharing the database with applicable functional entities supports the

Page 19 of 29

PRC-010-2 – Under Voltage Load Shedding
directive provided by FERC that requires an integrated and coordinated approach to UVLS
programs (Paragraph 1509 of FERC Order No. 693).

Frequently Asked Questions
To succinctly address common comment themes that require drafting team response on Project
2008-02 UVLS (proposed PRC-010-1), the drafting team provides the following discussion in the
construct of an FAQ format.
Introduction
This Frequently Asked Questions (FAQ) document was created during the development of PRC010-1 (Undervoltage Load Shedding) 4, 5 to succinctly address common comment themes with
respect to the approach and intent of the Project 2008-02 Undervoltage Load Shedding (UVLS) 6
standard drafting team (“drafting team”). This FAQ document is the outcome of comments
received during comment periods and multiple outreach sessions with industry. All comments
submitted by industry during comment periods may be reviewed on the project page.
Subsequent to the adoption of PRC-010-1, the UVLS drafting team made minor revisions to the
standard address the UVLS Misoperation identification and correction. 7 This FAQ document was
amended to reflect up the approach and intent of the drafting team during the development of
PRC-010-2 concerning Misoperation of UVLS equipment.
Purpose of Standard Revision
1) What is the basis for a revision of the existing UVLS standards?

The initial input into a revision of the existing UVLS standards is FERC Order No. 693, 8 Paragraph
1509, which directed the ERO to develop a modification of PRC-010-0 that “requires that an
integrated and coordinated approach be included in all protection systems on the Bulk-Power
System, including generators and transmission lines, generators’ low voltage ride through
capabilities, and UFLS and UVLS programs.” In addition, The Final Report on the August 14, 2003
Blackout in the United States and Canada: Causes and Recommendations 9 (“August 14 Blackout
Report”) showed that proper coordination would have mitigated effects if UVLS was used as a
tool.

(http://www.nerc.com/_layouts/PrintStandard.aspx?standardnumber=PRC-010-1&title=Undervoltage%20Load%20Shedding).
Adopted by the NERC Board of Trustees on November 14, 2014.
6 (http://www.nerc.com/pa/Stand/Pages/Project-2008-02-Undervoltage-Load-Shedding.aspx).
7 Refer to Project 2010-05.1, which developed PRC-004-3 (Protection System Misoperation Identification and Correction)
concurrently with the development of PRC-010-1. (http://www.nerc.com/pa/Stand/Pages/Project2010-05_Protection_System_
Misoperations.aspx).
8 (http://www.nerc.com/docs/docs/ferc/order_693.pdf).
9 (http://energy.gov/sites/prod/files/oeprod/DocumentsandMedia/BlackoutFinal-Web.pdf).
4
5

Page 20 of 29

PRC-010-2 – Under Voltage Load Shedding
Additional inputs included 1) recommendations from the NERC System Protection and Control
Subcommittee (SPCS) in its December 2010 Technical Review of UVLS-Related Standards 10 to
combine the four existing UVLS standards, revise the applicability to entities responsible for UVLS
program design, implementation, and coordination, specifically include a requirement for
assessment of coordination between UVLS programs and all other protection systems, and
differentiate post-event validation of UVLS program design from verifying correct operation of
UVLS equipment; 2) the existing UVLS standards were not in the current results-based format; 3)
the preceding revision of the underfrequency load shedding (UFLS) standards had similar types
of requirements and had been completed under the construct of a consolidation; and 4) the
Independent Expert Review Panel recommendations, which included an evaluation of the
existing standards’ applicability and level of specificity.
The drafting team agrees that a lack of coordination among protection systems is a key risk to
reliability. As part of the revision to address this, the drafting team also agreed that an evaluation
and consolidation of the existing UVLS standards was necessary to meet current Reliability
Standard development initiatives and to provide clear, comprehensive requirements to address
the application and coordination of UVLS.
2) UVLS programs are not mandatory—is compliance for an optional tool necessary?

The drafting team asserts that a key takeaway from the August 14 Blackout Report is that
coordination of UVLS with other protection systems could have mitigated the effects if UVLS was
used as a tool. Although the use of UVLS is not mandatory, if it is determined that this system
preservation measure is necessary to support reliability and a UVLS program is installed, the
program needs to be properly coordinated, implemented, and assessed due to the inherent
associated reliability risks. As such, there needs to be a level of performance required to properly
protect system reliability. Of note, PRC-010-1 and PRC-010-2 apply to the defined term “UVLS
Program,” which limits the standard’s applicability to only those undervoltage-based load
shedding programs whose performance has an impact on system reliability. 11
Coordination with Project 2009-03 Emergency Operations
3) EOP-003-2 has potential redundant requirements with proposed PRC-010-1—how
is this being addressed?

As part of its five-year review, Project 2009-03 – Emergency Operations (EOP) identified EOP003-2 (Load Shedding Plans), 12 Requirements R2, R4, and R7 as being more properly covered by
Project 2008-02 – UVLS. Both projects were strategically coordinated to move in lockstep from a
timing perspective to address these requirements. Project 2009-03 – EOP proposed to revise and

(http://www.nerc.com/docs/pc/spctf/PRC-010_022%20Report_Approved_20101208.pdf).
The term “UVLS Program” used herein was adopted by the NERC Board of Trustees on November 14, 2014.
12 (http://www.nerc.com/_layouts/PrintStandard.aspx?standardnumber=EOP-003-2&title=Load%20Shedding%20Plans).
10
11

Page 21 of 29

PRC-010-2 – Under Voltage Load Shedding
consolidate EOP-001-2.1b (Emergency Operations Planning), 13 EOP-002-3 (Capacity and Energy
Emergencies), 14 and EOP-003-2 to create EOP-011-1, will retire the noted EOP-003-2
requirements (among other revisions), and the Project 2008-02 – UVLS Mapping Document will
show how PRC-010-1 encompasses the retired content accordingly. Slated to have aligning
effective dates, both EOP-011-1 (Emergency Operations) 15 and PRC-010-1 will be posted and
balloted separately but concurrently, so that industry stakeholders will be able to clearly evaluate
the transition. Please see the posted Project 2008-02 UVLS Project Coordination Plan for more
information.
“UVLS Program” Definition
4) Why is the introduction of the new defined term “UVLS Program” necessary?

The drafting team found it necessary to introduce the term “UVLS Program” for inclusion in the
Glossary of Terms Used in NERC Reliability Standards 16 (“NERC Glossary”) because different types
of UVLS systems need to be treated appropriately with respect to reliability requirements.
Therefore, the term establishes which UVLS systems PRC-010-1 will apply to an: “automatic load
shedding program consisting of distributed relays and controls used to mitigate undervoltage
conditions impacting the Bulk Electric System (BES), leading to voltage instability, voltage
collapse, or Cascading. Centrally controlled undervoltage‐based load shedding is not included.”
The definition excludes locally-applied relays that are designed to protect a contained area or, in
other words, are not designed to mitigate wide-area voltage collapse. This exclusion is not explicit
in these terms in the enforceable language of the definition since the meaning and measurement
of “local” or “wide-area” varies greatly on a continent-wide basis and could potentially be
interpreted differently by auditors and the applicable functional entities. Therefore, the
definition as written is meant to provide flexibility for the Planning Coordinator or Transmission
Planner to determine if a UVLS system falls under the defined term with respect to its impact on
the reliability of the BES (voltage instability, voltage collapse, or Cascading). To further support
the intended exclusion, further discussion and an example are provided on in the PRC-010-1 and
PRC-010-2 Guidelines and Technical Basis section under the heading “Guidelines for UVLS
Program Definition.”
The definition does explicitly note that the term excludes centrally controlled undervoltagebased load shedding. This type of load shedding is excluded because the drafting team asserts
that the design and characteristics of centrally controlled undervoltage-based load shedding are
commensurate with those of a Special Protection System (SPS) or Remedial Action Scheme (RAS)
and should therefore be subject to SPS or RAS-related Reliability Standards. See PRC-010-1 and

(http://www.nerc.com/_layouts/PrintStandard.aspx?standardnumber=EOP-001-2.1b&title=Emergency%20Operations
%20Planning).
14 (http://www.nerc.com/_layouts/PrintStandard.aspx?standardnumber=EOP-002-3&title=Capacity%20and%20Energy%20
Emergencies).
15 (http://www.nerc.com/_layouts/PrintStandard.aspx?standardnumber=EOP-011-1&title=Emergency%20Operations).
16 (http://www.nerc.com/pa/Stand/Glossary%20of%20Terms/Glossary_of_Terms.pdf).
13

Page 22 of 29

PRC-010-2 – Under Voltage Load Shedding
PRC-010-2 Guidelines and Technical Basis section under the heading “Guidelines for UVLS
Program Definition” for further discussion.
5) If the definition excludes certain types of UVLS, does this preclude an “integrated”
approach (FERC Order No. 693, Paragraph 1509)?

The defined term “UVLS Program” clarifies which UVLS systems are subject to the requirements
in PRC-010-1 and PRC-010-2. The resulting exclusions from these versions of the standard do not
preclude an “integrated” approach because the standard requires that an entity coordinate with
all other protection and control systems as necessary, which may include other types of UVLS
(i.e., locally-applied UVLS relays and centrally controlled undervoltage-based load shedding).
6) Where will centrally controlled undervoltage-based load shedding be covered?

As explained immediately above, the Requirements of PRC-010-1 and PRC-010-2 are applicable
to the proposed NERC Glossary term “UVLS Program,” which excludes centrally controlled
undervoltage-based load shedding because its design and characteristics are commensurate with
those of an SPS or RAS. However, the NERC Glossary during the development of PRC-010-1
definition of “Special Protection System” excluded UVLS. Therefore, the work under Project
2010-05.2 – Special Protection Systems (Phase 2 of Protection Systems) combined the NERC
Glossary definition of “Special Protection System” into the single term “Remedial Action
Scheme.” 17 The definition revisions specifically excluded UVLS Programs, therefore including
centrally controlled undervoltage-based shedding.
Consequently, the introduction of the term “UVLS Program” and the conforming revision to the
term “Remedial Action Scheme” explicitly clarifies that RAS-related standards are applicable to
centrally controlled undervoltage-based load shedding. The implementation plan for the revised
definition of “Remedial Action Scheme” will address entities that will have newly identified RAS
resulting from the application of the defined term.
Similar to the coordination effort with Project 2009-03 – EOP explained above, Project 2008-02
– UVLS and Project 2010-05.2 – SPS were coordinated to ensure that the effective dates of the
adopted definitions of “Remedial Action Scheme” and “UVLS Program,” the PRC-010-1 and PRC010-1 Reliability Standards, and all associated retirements align.
7) Is the term “UVLS Program” inclusive of a collection of independent UVLS relays?

No; multiple independent relays do not constitute a program. While the definition stipulates that
a UVLS Program consists of distributed relays and controls, the definition specifies that it must
be “[a]n automatic load shedding program, consisting of distributed relays and controls, used to
mitigate undervoltage conditions impacting the Bulk Electric System(BES), leading to voltage

17

Adopted by the NERC Board of Trustees on November 14, 2014.

Page 23 of 29

PRC-010-2 – Under Voltage Load Shedding
instability, voltage collapse, or Cascading. Centrally controlled undervoltage‐based load shedding
is not included.”
Applicability
8) What is meant by the phrase “Planning Coordinator or Transmission Planner”?

The PRC-010-1 and PRC-010-2 Reliability Standards are applicable to both the Planning
Coordinator and Transmission Planner because either may be responsible for designing and
coordinating the program based on agreements, memorandums of understanding, or tariffs. The
phrase “Planning Coordinator or Transmission Planner” provides the flexibility for applicability to
the entity that will perform the action. The expectation is not that both parties will perform the
action, but rather that the Planning Coordinator and Transmission Planner will engage in
discussion to determine the appropriate responsible entity. In addition, the requirements
containing this phrase have specific language to qualify the responsible entity. For example,
Requirement R1 states: “Each Planning Coordinator or Transmission Planner that is developing a
UVLS Program shall . . .” This language provides clarity that the applicable entity would be the
one that is developing the program.
9) Why is the Transmission Operator not included?

While the Transmission Operator may be involved with UVLS Program activities, the drafting
team did not identify any required performance for the Transmission Operator that was
necessary to capture within PRC-010-1 and PRC-010-2, since the Transmission Operator does not
have the resources necessary to implement program specifications. If responsibilities are
delegated to the Transmission Operator by the Transmission Owner, the Transmission Owner is
still the accountable party.
To the extent that the Transmission Operator is required to have knowledge of system relays and
protection systems, the drafting team notes that this requirement is covered under PRC-001-1.1
(System Protection Coordination), 18 Requirement R1. It is also noted that manual load shedding,
for which the Transmission Operator is responsible, is not in the purview of PRC-010-1 and PRC010-2, as it is covered under current EOP-003-2 and will subsequently be covered by proposed
EOP-011-1 (see Project 2009-03 – Emergency Operations).
10) What about UVLS schemes owned by Transmission Owners, Distribution
Providers, or Transmission Operators that are not required by the planner?

The PRC-010-1 and PRC-010-2 Reliability Standards are applicable to the term “UVLS Program.”
The drafting team notes that, by its defining attributes, a UVLS Program would be required and
developed by a Planning Coordinator or Transmission Planner. The nature of a UVLS scheme
developed or required by a Distribution Provider, Transmission Operator, or Transmission Owner
http://www.nerc.com/_layouts/PrintStandard.aspx?standardnumber=PRC-001-1.1&title=System%20Protection%20
Coordination.
18

Page 24 of 29

PRC-010-2 – Under Voltage Load Shedding
would not meet the attributes of the defined term and would therefore not have the design and
characteristics necessary to be subject to the requirements of PRC-010-1 and PRC-010-2.
Requirements R1, R3, R4, and R5
11) What is required to evaluate the coordination referenced in Requirement R1, part
1.2?

Requirement R1 requires each Planning Coordinator or Transmission Planner that develops a
UVLS Program to evaluate the program’s viability and effectiveness prior to implementation. This
evaluation should include studies and analyses used when developing the program that show
implementation of the program resolves the identified undervoltage issues that led to its design.
These studies and analyses should also show that the UVLS Program is integrated through
coordination with generator voltage ride-through capabilities and other protection and control
systems. As such, the requirement is meant to provide flexibility for an entity to make the proper
determinations, including the considerations for coordination, with respect to program
effectiveness based on system characteristics. For further guidance on and examples of
coordination considerations, please see the portion of the Guidelines and Technical Basis section
under the Requirement R1 heading.
12) Requirements R1, R3, and R4 seem to all require evaluations of program
effectiveness—how are they different?

Requirements R1, R3, and R4 do require evaluations of program effectiveness, but they are each
at distinct points in time.
Requirement R1 requires evaluation of program effectiveness (by way of the qualifying parts) at
the onset of program development, or during the initial planning stage, prior to implementation.
Requirement R3 requires the same objectives of an evaluation of effectiveness, but at the point
of a mandatory periodic review (at least once every 60 calendar months). Requirement R4
addresses the performance of a UVLS Program after an event (for applicable voltage excursion)
to evaluate whether the UVLS Program resolved the undervoltage issues associated with the
event.
It is noted that, because of the separate activities of each requirement, UVLS Program
deficiencies found as a result of the assessments performed in Requirement R3 or R4 would not
be violations of Requirement R1.
13) Requirement R4 would require the Planning Coordinator or Transmission Planner
to review all voltage excursions—isn’t this unduly burdensome?

While Requirement R4 essentially requires the Planning Coordinator or Transmission Planner to
review all voltage excursions to see if they fall below the initializing set points of the UVLS
Program, the drafting team contends that it will be clearly evident if voltage falls below the UVLS

Page 25 of 29

PRC-010-2 – Under Voltage Load Shedding
threshold because either a) UVLS devices will operate; or b) the system will experience the
adverse conditions the UVLS Program was installed to mitigate.
In addition, the drafting team acknowledges that the Planning Coordinator or Transmission
Planner may not have the ability to know when voltage excursions are occurring since they are
not operating entities. However, a process for the Transmission Operator, Transmission Owner,
or Distribution Provider to notify the Transmission Planner or Planning Coordinator of such
voltage excursion events is consistent with standard utility practice.
14) PRC-022-1 required the analysis of UVLS Misoperations. How is this addressed in
PRC-010-1?

One of the recommendations in the SPCS report was to clearly differentiate between the postevent process of validating the effectiveness of the UVLS program design, its coordination with
other protection and control systems, and the potential need to modify the program design
(activities addressed in PRC-010-1) and the process of verifying correct operation of UVLS
equipment. Because PRC-010-1 was not specific concerning the Misoperation of UVLS
equipment, the drafting team made a subsequent revision creating PRC-010-2. Version two (PRC010-2) now requires that the assessment according to Requirement R4 include the performance
(i.e., operation or non-operation) of the UVLS Program equipment.
Relative to the assessment, Requirement R5 requires that a Corrective Action Plan be developed
to address any identified deficiencies. This structure ensures that UVLS Program equipment is
assessed to identify any Misoperation which could affect BES reliability. Although, the UVLS
drafting team maintained during development of PRC-010-1 that verifying correct operation of
UVLS equipment should be addressed in PRC-004, the drafting team included UVLS that is
intended to trip one or more BES Elements in the proposed PRC-004-5.
Requirements R6, R7, and R8
15) Do Requirements R6, R7, and R8 overlap with the requirements of MOD-032-1?

While both MOD-032-1 (Data for Power System Modeling and Analysis) 19 and Requirements R6,
R7, and R8 of PRC-010-1 and PRC-010-2 address data requirements, MOD-032-1 establishes
overarching modeling data requirements with respect to consistency in format and reporting
procedures, whereas the PRC-010-1 and PRC-010-2 requirements address the need to maintain
and share data and databases for the purposes of studies for use in event analyses for UVLS
Programs specifically. While Reliability Standards in general may have overlap in this manner, the
activities in these requirements remain distinctly different.

(http://www.nerc.com/_layouts/PrintStandard.aspx?standardnumber=MOD-032-1&title=Data%20for%20Power%20System
%20Modeling%20and%20Analysis).
19

Page 26 of 29

PRC-010-2 – Under Voltage Load Shedding
16) Requirements R6, R7, and R8 appear to be administrative — doesn’t this conflict
with Paragraph 81 criteria? 20

Proper maintenance and timely sharing of UVLS Program data as required by Requirements R6,
R7, and R8 is necessary to inform the Planning Coordinator or Transmission Planner’s studies and
analyses. While administrative tasks are required, the tasks have a core reliability-based need.
In addition, Requirements R6, R7, and R8 were written to emulate FERC-approved PRC-006-2
(Automatic Underfrequency Load Shedding) 21, 22 data requirements. While some of these
analogous requirements in PRC-006-2 are listed as candidates for Phase 2 of the Paragraph 81
project, they are not yet approved as meeting the criteria; furthermore, the Independent Expert
Review Panel has recommended that these Paragraph 81 candidates not be included for deletion,
citing that “there should be a clear expectation for Planning Coordinators to share data necessary
to determine their UFLS program parameters.”
Rationale

During development of this standard, text boxes were embedded within the standard to explain
the rationale for various parts of the standard. Upon BOT approval, the text from the rationale
text boxes was moved to this section.
Rationale for Applicability
This standard is applicable to Planning Coordinators and Transmission Planners that have or are
developing a UVLS Program, and to Distribution Providers and Transmission Owners responsible
for the ownership, operation, or control of UVLS equipment as required by the UVLS Program
established by the Transmission Planner or Planning Coordinator. These Distribution Providers
and Transmission Owners are referred to as UVLS entities for the purpose of this standard.
The applicability includes both the Planning Coordinator and Transmission Planner because
either may be responsible for designing and coordinating the program based on agreements,
memorandums of understanding, or tariffs.
The phrase “Planning Coordinator or Transmission Planner” provides the latitude for applicability
to the entity that will perform the action. The expectation is not that both parties will perform
the action, but rather that the Planning Coordinator and Transmission Planner will engage in
discussion to determine the appropriate responsible entity.
Rationale for R1
In Paragraph 1509 from Order No. 693, FERC directed NERC to require an integrated and
coordinated approach to all protection systems. The drafting team agrees that a lack of
coordination among protection systems is a key risk to reliability, and that each Planning
Refer to Standards Independent Expert Review Project (IERP). (http://www.nerc.com/pa/Stand/Standard%20
Development%20Plan/Standards_Independent_Experts_Review_Project_Report-SOTC_and_Board.pdf).
21 (http://www.nerc.com/_layouts/PrintStandard.aspx?standardnumber=PRC-006-2&title=Automatic%20Underfrequency
%20Load%20Shedding).
22 Adopted by the NERC Board of Trustees on November 14, 2014.
20

Page 27 of 29

PRC-010-2 – Under Voltage Load Shedding
Coordinator or Transmission Planner that develops a UVLS Program should evaluate the
program’s viability and effectiveness prior to implementation. This evaluation should include
studies and analyses used when developing the program that show implementation of the
program resolves the identified undervoltage conditions that led to its design. These studies and
analyses should also show that the UVLS Program is integrated through coordination with
generator voltage ride‐through capabilities and other protection and control systems. Though
presented as separate items, the drafting team recognizes that the studies that show
coordination considerations and that the program addresses undervoltage issues may be
interrelated and presented as one comprehensive analysis.
In addition, Requirement R1 also requires the Planning Coordinator or Transmission Planner to
provide the UVLS Program’s specifications and implementation schedule to applicable UVLS
entities to implement the program. It is noted that studies to evaluate the effectiveness of the
program should be completed prior to providing the specifications and schedule.
Rationale for R2
UVLS entities must implement a UVLS Program or address any necessary corrective actions for a
UVLS Program according to the specifications and schedule provided by the Planning Coordinator
or Transmission Planner. If UVLS entities do not implement the UVLS Program according to the
specifications and schedule provided, the UVLS Program may not be effective and may not
achieve its intended goal.
Rationale for R3
A periodic comprehensive assessment (detailed analysis) should be conducted to identify and
catalogue the accumulated effects of minor changes to the system that have occurred since the
last assessment was completed, and should include an evaluation of each UVLS Program to
ensure the continued integration through coordination. This comprehensive assessment
supplements the NERC Reliability Standard TPL‐001‐4 annual assessment requirement to
evaluate the impact of protection systems.
Based on the drafting team’s knowledge and experience, and in keeping with time frames
contained in similar requirements from other PRC Reliability Standards, 60 calendar months was
determined to be the maximum amount of time allowable between assessments. Assessments
will be performed sooner than the end of the 60‐calendar month period if the Planning
Coordinator or Transmission Planner determines that there are material changes to system
topology or operating conditions that affect the performance of a UVLS Program. Note that the
60‐calendar‐month time frame would reset after each assessment.
Rationale for R4
A UVLS Program not functioning as expected during a voltage excursion event for which the UVLS
Program was designed to operate presents a critical risk to system reliability. Therefore, a timely
assessment to evaluate (1) whether the UVLS Program resolved the undervoltage issues and (2)
the performance of the UVLS Program equipment associated with the applicable event is
essential. The 12 calendar months (from the date of the event) provides adequate time to
coordinate with other Planning Coordinators, Transmission Planners, Transmission Operators,

Page 28 of 29

PRC-010-2 – Under Voltage Load Shedding
and UVLS entities, simulate pre‐ and post‐event conditions, and complete the performance
assessment.
Rationale for R5
If program deficiencies are identified during an assessment performed in either Requirement R3
or R4, the Planning Coordinator or Transmission Planner must develop a Corrective Action Plan
(CAP) to address the deficiencies. Based on the drafting team’s knowledge and experience with
UVLS studies, three calendar months was determined to provide a judicious balance between the
reliability need to address deficiencies expeditiously and the time needed to consider potential
solutions, coordinate resources, develop a CAP and implementation schedule, and provide the
CAP and schedule to UVLS entities.
It is noted that the three‐month time frame is only to develop the CAP and provide it to UVLS
entities and does not encompass the time UVLS entities have to implement the CAP. Requirement
R2 requires UVLS entities to execute the CAP according to the schedule provided by the Planning
Coordinator or Transmission Planner.
Rationale for R6
Having accurate and current data is required for the Planning Coordinator to perform
undervoltage studies and for use in event analyses. Requirement R6 supports this reliability need
by requiring the Planning Coordinator to update its UVLS Program database at least once each
calendar year.
Rationale for R7
Having accurate and current data is required for the Planning Coordinator to perform
undervoltage studies and for use in event analyses. Requirement R7 supports this reliability need
by requiring the UVLS entity to provide UVLS Program data in accordance with specified
parameters.
Rationale for R8
Requirement R8 supports the integrated and coordinated approach to UVLS programs directed
by Paragraph 1509 of Order No. 693 by requiring that UVLS Program data be shared with
neighboring Planning Coordinators and Transmission Planners within a reasonable time period.
Requests for the database should also be fulfilled for those functional entities that have a
reliability need for the data (such as the Transmission Operators that develop System Operating
Limits and Reliability Coordinators that develop Interconnection Reliability Operating Limits).

Page 29 of 29