PRC-006-SERC-02 Reliability Standard

SERC UFLS Standard: PRC-006-SERC-02

Effective Date
Effective for SERC Region applicable Registered Entities on the first day of the first
calendar quarter after approved by FERC.

Introduction

1.

Title: Automatic Underfrequency Load Shedding Requirements

2.

Number: PRC-006-SERC–02

3.

Purpose: To establish consistent and coordinated requirements for the design,
implementation, and analysis of automatic underfrequency load shedding (UFLS)
programs among all SERC applicable entities.
Applicability:
4.1 Planning Coordinators
4.2 UFLS entities shall mean all entities that are responsible for the ownership,
operation, or control of UFLS equipment as required by the UFLS program
established by the Planning Coordinators. Such entities may include one or more
of the following:
4.2.1 Transmission Owners
4.2.2 Distribution Providers
4.3 Generator Owners

4.

5.

Background
The SERC UFLS Standard: PRC-006-SERC-01 (“SERC UFLS Standard”) was developed to
provide regional UFLS requirements to entities in SERC. UFLS requirements have been
in place at a continent-wide level and within SERC for many years prior to
implementation of federally mandated reliability compliance standards in 2007.
When reliability standards were implemented in 2007, the Federal Energy Regulatory
Commission (“FERC”), which is the government body with regulatory responsibility for
electric reliability, issued FERC Order 693, recognizing 83 NERC Reliability Standards as
enforceable by FERC and applicable to users, owners, and operators of the bulk power
system (BPS). FERC did not approve the NERC UFLS standard, PRC-006-0 in Order 693.
FERC’s reason for not approving PRC-006-0 was that it recognized PRC-006-0 as a “fill-in
the blank standard,” and regional procedures associated with the standard were not
submitted along with the standard. FERC’s ruling in Order 693 required Regional Entities
to provide the regional requirements necessary for completing the UFLS
standard.
In 2008, SERC commenced work on PRC-006-SERC-01. NERC also began work on
revising PRC-006-0 at a continent-wide level. The SERC standard has been developed to
be consistent with the NERC UFLS standard. PRC-006-SERC-02 was developed per
periodic review of the standard.
PRC-006-1 clearly defines the roles and responsibilities of parties to whom the standard
applies. The standard identifies the Planning Coordinator (“PC”) as the entity
responsible for developing UFLS schemes within their PC area. The regional standard
adds specificity not contained in the NERC standard for development and
implementation of a UFLS scheme in the SERC Region that effectively mitigates the
consequences of an underfrequency event.
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SERC UFLS Standard: PRC-006-SERC-02

Requirements and Measures

R1. Each Planning Coordinator shall include its SERC subregion as an identified island in the
criteria (required by the NERC PRC standard on UFLS) for selecting portions of the BPS
that may form islands. [Violation Risk Factor: Medium] [Time Horizon: Long-term
Planning]
1.1

A Planning Coordinator may adjust island boundaries to differ from subregional
boundaries where necessary for the sole purpose of producing a contiguous
subregional island more suitable for simulation.

M1. Each Planning Coordinator shall have evidence such as a methodology,
procedure, report, or other documentation indicating that its criteria included
selection of its SERC subregion(s) as an island per Requirement R1.
R2. Each Planning Coordinator shall select or develop an automatic UFLS scheme (percent
of load to be shed, frequency set points, and time delays) for implementation by UFLS
entities within its area that meets the following minimum requirements: [Violation Risk
Factor: Medium] [Time Horizon: Long-term Planning ]
2.1. Have the capability of shedding at least 30 percent of the Peak Demand (MW)
served from the Planning Coordinator’s transmission system. The Peak
Demand may be either summer or winter as determined by the Planning
Coordinator.
2.2. Shed load with a minimum of three frequency set points.
2.3. The highest frequency set point for relays used to arrest frequency decline shall
be no lower than 59.3 Hz and not higher than 59.5 Hz.
2.3.1 This does not apply to UFLS relays with time delay of one second or longer
and a higher frequency setpoint applied to prevent the frequency from
stalling at less than 60 Hz when recovering from an underfrequency event.
2.4. The lowest frequency set point shall be no lower than 58.4 Hz.
2.5. The difference between frequency set points shall be at least 0.2 Hz but no
greater than 0.5 Hz.
2.6. Time delay (from frequency reaching the set point to the trip signal) shall be at
least six cycles.
M2. Each Planning Coordinator shall have evidence such as reports or other
documentation that the UFLS scheme for its area meets the design requirements
specified in Requirement R2.
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SERC UFLS Standard: PRC-006-SERC-02

R3. Each Planning Coordinator, when performing design assessments specified in the NERC
PRC standard on UFLS, shall conduct simulations of its UFLS scheme for an imbalance
between load and generation of 13%, 22%, and 25% for all identified island(s) where
such imbalance equals [(load minus actual generation output) / load]. [Violation Risk
Factor: High] [Time Horizon: Long-term Planning]
M3. Each Planning Coordinator shall have evidence such as reports or other
documentation that it performed the simulations of its UFLS scheme as required
in Requirement R3.
R4. Each UFLS entity that has a total load of 100 MW or greater in a Planning Coordinator
area in the SERC Region shall implement the UFLS scheme developed by their Planning
Coordinator. UFLS entities may implement the UFLS scheme developed by the Planning
Coordinator by coordinating with other UFLS entities. The UFLS scheme shall meet the
following requirements on May 1 of each calendar year. [Violation Risk Factor:
Medium] [Time Horizon: Operations Planning]
4.1. The percent of load shedding to be implemented shall be based on the actual or
estimated substation or feeder demand (including losses) of the UFLS entities at
the time coincident with the previous year’s actual Peak Demand in the season
specified by the Planning Coordinator in R2.
4. 2. The amount of load in each load shedding step shall be within -1.0 and +3.0 of
the percentage specified by the Planning Coordinator (for example, if the
specified percentage step load shed is 12%, the allowable range is 11 to 15%).
4. 3. The amount of total UFLS load of all steps combined shall be within -1.0 and +5.0
of the percentage specified by the Planning Coordinator for the total UFLS load in
the UFLS scheme.
M4. Each UFLS entity that has a total load of 100 MW or greater in a Planning
Coordinator area in the SERC Region shall have evidence such as reports or other
documentation demonstrating that its implementation of the UFLS scheme on
May 1 of each calendar year meets the requirements of Requirement R4
(including all the data elements in Parts 4.1, 4.2, and 4.3) unless scheme changes
per Requirement R6 are in process.
R5. Each UFLS entity that has a total load less than 100 MW in a Planning Coordinator area
in the SERC Region shall implement the UFLS scheme developed by their Planning
Coordinator, but shall not be required to have more than one UFLS step. UFLS entities
may implement the UFLS scheme developed by the Planning Coordinator by
coordinating with other UFLS entities. The UFLS scheme shall meet the following
requirements on May 1 of each calendar year. [Violation Risk Factor: Medium] [Time
Horizon: Operations Planning].
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SERC UFLS Standard: PRC-006-SERC-02

5.1. The percent of load shedding to be implemented shall be based on the actual or
estimated substation or feeder demand (including losses) of the UFLS entities at
the time coincident with the previous year actual Peak Demand in the season
specified by the Planning Coordinator in R2..
5.2. The amount of total UFLS load shall be within ± 5.0 of the percentage specified by
the Planning Coordinator for the total UFLS load in the UFLS scheme.
M5. Each UFLS entity that has a total load less than 100 MW in a Planning Coordinator
area in the SERC Region shall have evidence such as reports or other
documentation demonstrating that its implementation of the UFLS scheme on
May 1 of each calendar year meets the requirements of Requirement R5
(including all the data elements in Parts 5.1and 5.2) unless scheme changes per
Requirement R6 are in process.
R6.

Each UFLS entity shall implement changes to the UFLS scheme which involve frequency
settings, relay time delays, changes to the percentage of load in the scheme, or changes
to the peak season selected in R2.1 within 18 months of notification by the Planning
Coordinator. [Violation Risk Factor: High] [Time Horizon: Long-term Planning]
M6. Each UFLS entity shall have evidence such as reports or other documentation
demonstrating that it has made the appropriate scheme changes within 18
months per Requirement R6. Such evidence is only required if the Planning
Coordinator makes changes to the UFLS scheme as specified in Requirement R6.

R7. Each Planning Coordinator shall provide the following information to SERC according to
the schedule specified by SERC. [Violation Risk Factor: Lower] [Time Horizon: Longterm Planning]
7.1. Underfrequency trip set points (Hz)
7.2. Total clearing time associated with each set point (sec). This includes the time
from when frequency reaches the set point and ends when the breaker opens.
7.3. Amount of previous year actual or estimated load associated with each set point,
both in percent and in MW. The percentage and the Load demand (MW) shall be
based on the time coincident with the previous year actual Peak Demand.
M7. Each Planning Coordinator shall have evidence such as reports or other
documentation that data specified in Requirement R7 was provided to SERC in
accordance with the schedule.

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SERC UFLS Standard: PRC-006-SERC-02

R8. Each Generator Owner shall provide the following information within 30 days of a
request by SERC to facilitate post-event analysis of frequency disturbances. [Violation
Risk Factor: Lower] [Time Horizon: Long-term Planning]
8.1. Generator protection automatic underfrequency and overfrequency trip set
points (Hz).
8.2. Total clearing time associated with each set point (sec). This is defined as the
time that begins when frequency reaches the set point and ends when the
breaker opens. If inverse time underfrequency relays are used, provide the total
clearing time at 59.0, 58.5, 58.0, and 57.0 Hz.
8.3. Maximum generator net MW that could be tripped automatically due to an
underfrequency or overfrequency condition.
M8. Each Generator Owner shall have evidence such as reports or other
documentation that data specified in Requirement R8 was provided to SERC as
requested.

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SERC UFLS Standard: PRC-006-SERC-02

Compliance

Compliance enforcement authority
SERC Reliability Corporation
Compliance monitoring and assessment process
• Compliance Audit
•

Self-Certification

•

Spot Checking

•

Compliance Violation Investigation

•

Self-Reporting

•

Complaint

Evidence retention
Each Planning Coordinator, UFLS Entity and Generator Owner shall keep data or
evidence to show compliance as identified below unless directed by SERC to
retain specific evidence for a longer period of time as part of an investigation.
Each Planning Coordinator, UFLS Entity and Generator Owner shall retain the
current evidence of each Requirement and Measure as well as any evidence
necessary to show compliance since the last compliance audit.
If a Planning Coordinator, UFLS Entity or Generator Owner is found noncompliant, it shall keep information related to the non-compliance until found
compliant or for the retention period specified above, whichever is longer.
The compliance enforcement authority shall keep the last audit records and all
requested and submitted subsequent audit records.

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SERC UFLS Standard: PRC-006-SERC-02

Time Horizons, Violation Risk Factors, and Violation Severity Levels

Table 1
R#

Time
Horizon

VRF

R1

Long-term
Planning

Medium

R2

Long-term
Planning

Medium

R3

Long-term
Planning

High

R4

Operations
Planning

Medium

Violation Severity Level
Lower

Moderate

High

Severe

N/A

N/A

N/A

The Planning
Coordinator did not have
evidence that its criteria
included selection of its
SERC subregion(s) as an
island, with or without
adjusted boundaries.

The Planning
Coordinator's scheme
did not meet one of the
UFLS system design
requirements identified
in 2.2 through 2.6

The Planning
Coordinator's scheme
did not meet two of the
UFLS system design
requirements identified
in 2.2 through 2.6.

The Planning
Coordinator's scheme
did not meet three of
the UFLS system design
requirements identified
in 2.2 through 2.6.

The Planning
Coordinator's scheme
did not meet 2.1

N/A

The Planning
Coordinator failed to
conduct one of the
required simulations of
its UFLS scheme.

N/A

The Planning
Coordinator failed to
conduct two of the
required simulations of
its UFLS scheme.

The UFLS entity’s
implemented UFLS
scheme had one load
shedding step outside
the range specified in 4.

The UFLS entity’s
implemented UFLS
scheme had two load
shedding steps outside
the range specified in 4.

The UFLS entity’s
implemented UFLS
scheme had three or
more load shedding
steps outside the range

The UFLS entity’s
implemented UFLS
scheme had three or
more load shedding
steps outside the range

OR
Four or more of the UFLS
system design
requirements identified
in 2.2 through 2.6.

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SERC UFLS Standard: PRC-006-SERC-02

Table 1
R#

Time
Horizon

R5

Operations
Planning

R6

Long-term
Planning

VRF

Medium

High

Violation Severity Level
Lower

Moderate

High

Severe

2.

2.

specified in 4.2.

specified in 4.2.

OR

AND

The UFLS entity's
implemented UFLS
scheme had a total load
outside the range
specified in 4.3.

The UFLS entity's
implemented UFLS
scheme had a total load
outside the range
specified in 4.3.

N/A

N/A

N/A

The UFLS entity's
implemented UFLS
scheme had a total load
outside the range
specified in 5.2.

The UFLS entity
implemented required
scheme changes but
made them 1 to 30 days
after the scheduled
date.

The UFLS entity
implemented required
scheme changes but
made them 31 to 40
days after the scheduled
date.

The UFLS entity
implemented required
scheme changes but
made them 41 to 50
days after the scheduled
date.

The UFLS entity
implemented required
scheme changes but
made them more than
50 days after the
scheduled date
OR
The UFLS entity failed to
implement the required
scheme changes.

R7

Long-term
Planning

Lower

The Planning
Coordinator provided
the data required in R7
to SERC 1 to 10 days

The Planning
Coordinator provided
the data required in R7
to SERC 11 to 20 days

The Planning
Coordinator provided
the data required in R7
to SERC 21 to 30 days

The Planning
Coordinator provided
the data required in R7
to SERC more than 30
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SERC UFLS Standard: PRC-006-SERC-02

Table 1
R#

R8

Time
Horizon

Long-term
Planning

VRF

Lower

Violation Severity Level
Lower

Moderate

High

Severe

after the scheduled
submittal date.

after the scheduled
submittal date.

after the scheduled
submittal date.

days after the scheduled
submittal date.

OR

OR

OR

The Planning
Coordinator did not
provide to SERC one
piece of information
listed in R7.
The Generator Owner
provided the data
required in R8 to SERC
11 to 20 days after the
requested submittal
date.

The Planning
Coordinator did not
provide to SERC two
pieces of information
listed in R7.
The Generator Owner
provided the data
required in R8 to SERC
21 to 30 days after the
requested submittal
date.

The Planning
Coordinator did not
provide to SERC any of
the information listed in
R7.
The Generator Owner
provided the data
required in R8 to SERC
more than 30 days after
the requested submittal
date.

OR

OR

OR

The Generator Owner
did not provide to SERC
one piece of information
listed in R8.

The Generator Owner
did not provide to SERC
two pieces of
information listed in R8.

The Generator Owner
did not provide to SERC
any of the information
listed in R8.

The Generator Owner
provided the data
required in R8 to SERC 1
to 10 days after the
requested submittal
date.

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SERC UFLS Standard: PRC-006-SERC-02

Regional Variances

None
Interpretations

None
Guideline and Technical Basis

1. Existing UFLS schemes
Each Planning Coordinator should consider the existing UFLS programs which are in place
and should consider input from the UFLS entities in developing the UFLS scheme.
2. Basis for SERC standard requirements
SERC Standard PRC-006-SERC-02 is not a stand-alone standard, but was written to be
followed in conjunction with NERC Standard PRC-006-1. The primary focus of SERC Standard
PRC-006-SERC-02 was to provide region-specific requirements for the implementation of the
higher tier NERC standard requirements with the goals of a) adding clarity and b) providing
for consistency and a coordinated UFLS scheme for the SERC Region as a whole.
Generally speaking, requirements already in the NERC standard were not repeated in the
SERC standard. Therefore, both the NERC and SERC standards must be followed to ensure
full compliance.
3. Basis for applying a percentage load shedding value to Forecast Load versus Actual Load
The Planning Coordinator will develop a UFLS scheme to meet the performance
requirements of NERC Standard PRC-006-2 Requirement R3 and SERC Standard PRC-006SERC-02 Requirement R2. This development will result in certain percentages of load for
each UFLS entity in the Planning Coordinator’s area for which automatic under frequency
load shedding must be implemented. The Planning Coordinator develops these percentages
based on forecast peak load demand. However, the UFLS entity implements these
percentages based on the previous year’s actual peak demand. Applying the same
percentage to these different base values was intentional to ensure that both the Planning
Coordinator and UFLS entities had a clear, measurable value to use in performing their
respective roles in meeting the standard. Planning Coordinators typically use forecast
demands in their work. Whereas the previous year’s actual (or estimated) demand is
typically more available to UFLS entities. Additionally, the use of percentages based on
t h e s e different base values tends to minimize the error due to the time lag between
design and actual field implementation. Since a percentage is provided by the Planning
Coordinator to the UFLS entities, any differences between the design values (i.e., forecast
load) and the implemented values (i.e., previous year’s actual) would naturally tend to
match up reasonably well. For example, if the total planning area load in MW for which
UFLS was installed during the time of implementation was slightly higher or lower than the
MW value used in the design by the Planning Coordinator, multiplying by the specified
percentage would result in an implemented load shedding scheme that also had a
reasonably similar higher or lower MW value.
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SERC UFLS Standard: PRC-006-SERC-02

4. Basis for May 1 and 18 month time frames
Each UFLS entity must annually review that the amount of UFLS load shedding implemented
is within a certain tolerance as specified by SERC Standard PRC-006-SERC-02 Requirement
R 4 or Requirement R5 by May 1 of the current year. May 1 was chosen to allow sufficient
time after the previous year’s peak occurred to make adjustments in the field to the
implementation if necessary to meet the tolerances specified in Requirement R4 or
Requirement R5. Therefore, the May 1 date applies only to implementation of the existing
percentages of load shedding specified by the Planning Coordinator. On the other hand, the
18-month time frame specified in PRC-006-SERC-02 Requirement R6 is intended to allow
sufficient budgeting, procurement, and installation time for additional equipment, or for
significant setting changes to existing equipment necessary to meet a revised load shedding
scheme design that has been specified by the Planning Coordinator. During this 18-month
transition period, the May 1 measurement of R4 or Requirement R5 would not apply.
5. Basis for smaller entity threshold of 100 MW
Most distribution substations have transformers rated in the range of 10 to 40 MVA. Usually
most transformers would serve 1 to 4 feeders and each feeder will normally carry between
8 and 10 MVA. In general, assuming that each feeder would carry 10 MW, an entity with a
load slightly greater than 100 MW would have at least 10 feeders available. For a program
with three 10 % steps, only 3 feeders would be required to have under frequency load shed
capabilities. The 100 MW threshold seems to provide adequate flexibility for implementing
load shedding in three steps for entities slightly greater than 100 MW.
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 each of the
rationale text boxes was moved to this section.
Rationale for R1:
Studying the Region as an island is required by the NERC standard. Most regions have only one
or a few different UFLS schemes. Where there is more than one scheme, studying this island
demonstrates that the schemes are coordinated and performing adequately. Because there
are so many different UFLS schemes in SERC (18 different schemes were represented in the
2007 SERC UFLS study), the SDT believes that applying the schemes to each subregion as an
island is a necessary additional test of the coordination of the various UFLS schemes. Without
this additional test, a poorly performing scheme may be masked by the large number of good
performing schemes in the Region. A subregion island study, which would have a smaller
number of schemes, would be more likely to uncover the poorly performing scheme and
therefore get it fixed. This approach will result in a much better overall performance of the
UFLS programs in SERC. The SDT recognized that there may be simulation problems due to
opening the ties to utilities outside the subregion. Therefore, the subregion island boundaries
are allowed to be adjusted to produce an island more suitable for simulation.
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SERC UFLS Standard: PRC-006-SERC-02

(Note: The SERC Subregions are identified in paragraph 4.2 of the SERC Reliability Corporation
Bylaws: “The Region is currently geographically divided into five subregions that are identified
as Southeastern, Central, VACAR, Delta, and Gateway.”)
Rationale for R2:
These requirements for the UFLS schemes in SERC have been in place for many years (except
2.6). The SDT believes that these requirements are still needed to ensure consistency for the
various schemes which are used in SERC. Part 2.6 is designed to prevent spurious operations
due to transient frequency swings.
Rationale for R3:
R4 of the NERC standard PRC-006-1 requires the PC to conduct assessments of UFLS schemes
through dynamic simulations to verify that they meet performance requirements for
generation/load imbalances of up to 25%. This requirement defines specific imbalances that are
to be studied within SERC. The 13% and 22% levels were determined from simulations of the
worst case frequency overshoot for the UFLS schemes in SERC.
Rationale for R4:
The SDT believes it is necessary to put a requirement on how well the UFLS scheme is
implemented. This requirement specifies how close the actual load shedding amounts must be
to the percentage of load called for in the scheme. A 4 percentage point range is allowed for
each individual step, but the allowed range for all steps combined is 6 percentage points.
Rationale for R5:
The SDT believes it is necessary to put a requirement on how well the UFLS scheme is
implemented. This requirement specifies how close the actual load shedding amounts must be
to the percentage of load called for in the scheme. The SDT recognizes that UFLS entities with a
load of less than 100 MW may have difficulty in implementing more than one UFLS step and in
meeting a tight tolerance. The basis of the 100 MW comes from typical feeder load dropped by
UFLS relays, and the use of a 100 MW threshold in other regional UFLS standards.
Rationale for R6:
The SDT believes it is necessary to put a requirement on how quickly changes to the scheme
should be implemented. This requirement specifies that changes must be implemented within
18 months of notification by the PC. The 18 month interval was chosen to give a reasonable
amount of time for making changes in the field. All of the SERC Region has existing UFLS
schemes which, based on periodic simulations, have provided reliable protection for years.
Events which result in islanding and an activation of the UFLS schemes are extremely rare in
SERC. Therefore, the SDT does not believe that changes to an existing UFLS scheme will be
needed in less than 18 months. However, if a PC determines there is a need for changing the UFLS
scheme faster than 18 months, then the PC may require the implementation to be done sooner as
allowed by NERC Reliability Standard PRC-006-1.

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SERC UFLS Standard: PRC-006-SERC-02

Rationale for R7:
The NERC standard requires that a UFLS database be maintained by the Planning Coordinator.
This requirement specifies what data must be reported to SERC. A SERC UFLS database is
needed to facilitate data sharing across the SERC Region, with other regions, and with NERC.
Rationale for R8:
The SDT believes that generator over and under frequency tripping data is needed to
supplement the UFLS data provided by the Planning Coordinator for post-event analysis of
frequency disturbances. This requirement states what data must be reported to SERC by the
Generator Owners.
Since the inverse time curve cannot easily be placed into the SERC database, four clearing times
based on data from the curve are requested. These clearing times are intended to cover a
range of frequencies needed for event replication as well as provide information about
generators that trip at a higher frequency than is allowed by the NERC standard.
Version History

Version Date

Action

1

September 19,
2011

SERC Board Approved

1

November 3, 2011

Adopted by NERC Board of Trustees

1

December 20, 2012 FERC Order issued approving PRC-006-SERC–01

1

March 11, 2013

Modified the Rationale and changed the VRF for
Requirement R6 from “Medium” to “High” per a
compliance filing (Filed on 3/11/13)

2

June 28, 2017

SERC Board Approved

2

August 10, 2017

Adopted by NERC Board of Trustees

2

October 16, 2017

FERC Order issued approving PRC-006-SERC-02

Change Tracking

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