PRC-006-NPCC-1 Reliability Standard

Standard PRC-006-NPCC-1 Automatic Underfrequency Load Shedding
A. Introduction
1.

Title:

Automatic Underfrequency Load Shedding

2.

Number:

PRC-006-NPCC-1

3.

Purpose: To provide a regional reliability standard that ensures the development of
an effective automatic underfrequency load shedding (UFLS) program in order to
preserve the security and integrity of the bulk power system during declining system
frequency events in coordination with the NERC UFLS reliability standard
characteristics.

4.

Applicability:
4.1. Generator Owner
4.2. Planning Coordinator
4.3. Distribution Provider
4.4. Transmission Owner

5.

Effective Date: For the Eastern Interconnection & Québec Interconnection portions
of NPCC excluding the Independent Electricity System Operator (IESO) Planning
Coordinator area of NPCC in Ontario, Canada:
The effective date for Requirements R1, R2, R3, R4, R5, R6, and R7 is the first
day of the first calendar quarter following applicable regulatory approval but no
earlier than January 1, 2016 The effective date for Requirements R8 through R23
is the first day of the first calendar quarter two years following applicable
governmental and regulatory approval.
For the Independent Electricity System Operator (IESO) Planning Coordinator’s area
of NPCC in Ontario, Canada:
All requirements are effective the first day of the first calendar quarter following
applicable governmental and regulatory approval but no earlier than April 1,
2017.

B. Requirements

R1 Each Planning Coordinator shall establish requirements for entities aggregating their
UFLS programs for each anticipated island and requirements for compensatory load
shedding based on islanding criteria (required by the NERC PRC Standard on UFLS).
[Violation Risk Factor: Medium] [Time Horizon: Long Term Planning]

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Standard PRC-006-NPCC-1 Automatic Underfrequency Load Shedding

R2

Each Planning Coordinator shall, within 30 days of completion of its system studies
required by the NERC PRC Standard on UFLS, identify to the Regional Entity the
generation facilities within its Planning Coordinator Area necessary to support the
UFLS program performance characteristics. [Violation Risk Factor: Medium] [Time
Horizon: Long Term Planning]

R3

Each Planning Coordinator shall provide to the Transmission Owner, Distribution
Provider, and Generator Owner within 30 days upon written request the requirements
for entities aggregating the UFLS programs and requirements for compensatory load
shedding program derived from each Planning Coordinator’s system studies as
determined by Requirement R1. [Violation Risk Factor: Low] [Time Horizon: Long
Term Planning]

R4

Each Distribution Provider and Transmission Owner in the Eastern Interconnection
portion of NPCC shall implement an automatic UFLS program reflecting normal
operating conditions excluding outages for its Facilities based on frequency thresholds,
total nominal operating time and amounts specified in Attachment C, Tables 1 through
3, or shall collectively implement by mutual agreement with one or more Distribution
Providers and Transmission Owners within the same island identified in Requirement
R1 and acting as a single entity, provide an aggregated automatic UFLS program that
sheds their coincident peak aggregated net Load, based on frequency thresholds, total
nominal operating time and amounts specified in Attachment C, Tables 1 through 3.
[Violation Risk Factor: High] [Time Horizon: Long Term Planning]

R5

Each Distribution Provider or Transmission Owner that must arm its load to trip on
underfrequency in order to meet its requirements as specified and by doing so exceeds
the tolerances and/or deviates from the number of stages and frequency set points of
the UFLS program as specified in the tables contained in Requirement R4 above, as
applicable depending on its total peak net Load shall: [Violation Risk Factor: High]
[Time Horizon: Long Term Planning]
5.1

Inform its Planning Coordinator of the need to exceed the stated tolerances
or the number of stages as shown in UFLS Attachment C, Table 1 if
applicable and

5.2

Provide its Planning Coordinator with a technical study that demonstrates
that the Distribution Providers or Transmission Owners specific deviations

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Standard PRC-006-NPCC-1 Automatic Underfrequency Load Shedding

from the requirements of UFLS Attachment C, Table 1 will not have a
significant adverse impact on the bulk power system.
5.3

Inform its Planning Coordinator of the need to exceed the stated tolerances
of UFLS Attachment C, Table 2 or Table 3, and in the case of Attachment
C, Table 2 only, the need to deviate from providing two stages of UFLS, if
applicable, and

5.4

Provide its Planning Coordinator with an analysis demonstrating that no
alternative load shedding solution is available that would allow the
Distribution Provider or Transmission Owner to comply with UFLS
Attachment C Table 2 or Attachment C Table 3.

R6 Each Distribution Provider and Transmission Owner in the Québec Interconnection
portion of NPCC shall implement an automatic UFLS program for its Facilities based
on the frequency thresholds, slopes, total nominal operating time and amounts
specified in Attachment C, Table 4 or shall collectively implement by mutual
agreement with one or more Distribution Providers and Transmission Owners within
the same island, identified in Requirement R1, an aggregated automatic UFLS program
that sheds Load based on the frequency thresholds, slopes, total nominal operating
time and amounts specified in Attachment C, Table 4. [Violation Risk Factor: High]
[Time Horizon: Long Term Planning]
R7

Each Distribution Provider and Transmission Owner shall set each underfrequency
relay that is part of its region’s UFLS program with the following minimum time
delay:
7.1

Eastern Interconnection – 100 ms

7.2

Québec Interconnection – 200 ms

[Violation Risk Factor: High] [Time Horizon: Long Term Planning]
R8 Each Planning Coordinator shall develop and review once per calendar year settings for
inhibit thresholds (such as but not limited to voltage, current and time) to be utilized
within its region's UFLS program. [Violation Risk Factor: Medium] [Time Horizon:
Long Term Planning]

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Standard PRC-006-NPCC-1 Automatic Underfrequency Load Shedding

R9

Each Planning Coordinator shall provide each Transmission Owner and Distribution
Provider within its Planning Coordinator area the applicable inhibit thresholds within
30 days of the initial determination of those inhibit thresholds and within 30 days of
any changes to those thresholds. [Violation Risk Factor: Medium] [Time Horizon:
Operations Planning]

R10 Each Distribution Provider and Transmission Owner shall implement the inhibit
threshold settings based on the notification provided by the Planning Coordinator in
accordance with Requirement R9. [Violation Risk Factor: High] [Time Horizon:
Operations Planning]
R11 Each Distribution Provider and Transmission Owner shall develop and submit an
implementation plan within 90 days of the request from the Planning Coordinator for
approval by the Planning Coordinator in accordance with R9. [Violation Risk Factor:
Lower] [Time Horizon: Operations Planning]

R12 Each Transmission Owner and Distribution Provider shall annually provide
documentation, with no more than 15 months between updates, to its Planning
Coordinator of the actual net Load that would have been shed by the UFLS relays at
each UFLS stage coincident with their integrated hourly peak net Load during the
previous year, as determined by measuring actual metered Load through the switches
that would be opened by the UFLS relays. [Violation Risk Factor: Lower] [Time
Horizon: Long Term Planning]
R13 Each Generator Owner shall set each generator underfrequency trip relay, if so
equipped, below the appropriate generator underfrequency trip protection settings
threshold curve in Figure 1, except as otherwise exempted in Requirements R16 and
R19. [Violation Risk Factor: High] [Time Horizon: Long Term Planning]

R14 Each Generator Owner shall transmit the generator underfrequency trip setting and
time delay to its Planning Coordinator within 45 days of the Planning Coordinator’s
request. [Violation Risk Factor: High] [Time Horizon: Operations Planning]

R15 Each Generator Owner with a new generating unit, scheduled to be in service on or
after the effective date of this Standard, or an existing generator increasing its net

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Standard PRC-006-NPCC-1 Automatic Underfrequency Load Shedding

capability by greater than 10% shall: [Violation Risk Factor: High] [Time Horizon:
Long Term Planning]

15.1 Design measures to prevent the generating unit from tripping directly or
indirectly for underfrequency conditions above the appropriate generator
tripping threshold curve in Figure 1.
15.2 Design auxiliary system(s) or devices used for the control and protection of
auxiliary system(s), necessary for the generating unit operation such that
they will not trip the generating unit during underfrequency conditions
above the appropriate generator underfrequency trip protection settings
threshold curve in Figure 1.
R16 Each Generator Owner of existing non-nuclear units in service prior to the effective
date of this standard that have underfrequency protections set to trip above the
appropriate curve in Figure 1 shall: [Violation Risk Factor: High] [Time Horizon: Long
Term Planning]
16.1 Set the underfrequency protection to operate at the lowest frequency
allowed by the plant design and licensing limitations.
16.2 Transmit the existing underfrequency settings and any changes to the
underfrequency settings along with the technical basis for the settings to the
Planning Coordinator.
16.3 Have compensatory load shedding, as provided by a Distribution Provider
or Transmission Owner that is adequate to compensate for the loss of their
generator due to early tripping.
R17 Each Planning Coordinator in Ontario, Quebec and the Maritime provinces shall apply
the criteria described in Attachment A to determine the compensatory load shedding
that is required in Requirement R16.3 for generating units in its respective NPCC area.
[Violation Risk Factor: High] [Time Horizon: Long Term Planning]
R18 Each Generator Owner, Distribution Provider or Transmission Owner within the
Planning Coordinator area of ISO-NE or the New York ISO shall apply the criteria
described in Attachment B to determine the compensatory load shedding that is
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Standard PRC-006-NPCC-1 Automatic Underfrequency Load Shedding

required in Requirement R16.3 for generating units in its respective NPCC area.
[Violation Risk Factor: High] [Time Horizon: Long Term Planning]
R19 Each Generator Owner of existing nuclear generating plants with units that have
underfrequency relay threshold settings above the Eastern Interconnection generator
tripping curve in Figure 1, based on their licensing design basis, shall: [Violation Risk
Factor: High] [Time Horizon: Long Term Planning]
19.1

Set the underfrequency protection to operate at as low a frequency as
possible in accordance with the plant design and licensing limitations but
not greater than 57.8Hz.

19.2

Set the frequency trip setting upper tolerance to no greater than + 0.1 Hz.

19.3

Transmit the initial frequency trip setting and any changes to the setting
and the technical basis for the settings to the Planning Coordinator.

R20 The Planning Coordinator shall update its UFLS program database as specified by the
NERC PRC Standard on UFLS. This database shall include the following
information: [Violation Risk Factor: Lower] [Time Horizon: Operations Planning]
20.1

For each UFLS relay, including those used for compensatory load
shedding, the amount and location of load shed at peak, the corresponding
frequency threshold and time delay settings.

20.2

The buses at which the Load is modeled in the NPCC library power flow
case.

20.3

A list of all generating units that may be tripped for underfrequency
conditions above the appropriate generator underfrequency trip protection
settings threshold curve in Figure 1, including the frequency trip threshold
and time delay for each protection system.

20.4

The location and amount of additional elements to be switched for voltage
control that are coordinated with UFLS program tripping.

20.5

A list of all UFLS relay inhibit functions along with the corresponding
settings and locations of these relays.

R21 Each Planning Coordinator shall notify each Distribution Provider, Transmission
Owner, and Generator Owner within its Planning Coordinator area of changes to load

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Standard PRC-006-NPCC-1 Automatic Underfrequency Load Shedding

distribution needed to satisfy UFLS program performance characteristics as specified
by the NERC PRC Standard on UFLS.[Violation Risk Factor: High] [Time Horizon:
Long Term Planning]
R22 Each Distribution Provider, Transmission Owner and Generator Owner shall
implement the load distribution changes based on the notification provided by the
Planning Coordinator in accordance with Requirement R21. [Violation Risk Factor:
High] [Time Horizon: Long Term Planning]
R23 Each Distribution Provider, Transmission Owner and Generator Owner shall develop
and submit an implementation plan within 90 days of the request from the Planning
Coordinator for approval by the Planning Coordinator in accordance with Requirement
R21. [Violation Risk Factor: Lower] [Time Horizon: Operations Planning]

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Standard PRC-006-NPCC-1 Automatic Underfrequency Load Shedding

Figure 1
Thresholds for Setting Underfrequency Trip Protection for Generators

Frequency (Hz)
60

59.5

59

58.5

58

57.5

57

56.5

56
Eastern Interconnection Generator Tripping
55.5

Quebec Interconnection Generator Tripping

0.1

1

10

100

1000

55
10000

Time (sec)

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Standard PRC-006-NPCC-1 Automatic Underfrequency Load Shedding
C. Measures

M1

Each Planning Coordinator shall have evidence such as reports, system studies and/or
real time power flow data captured from actual system events and other dated
documentation that demonstrates it meets Requirement R1.

M2. Each Planning Coordinator shall have evidence such as dated documentation that

demonstrates that it meets requirement R2.
M3 Each Planning Coordinator shall have evidence such as dated documentation that
demonstrates that it meets Requirement R3.
M4 Each Distribution Provider and Transmission Owner in the Eastern Interconnection
portion of NPCC shall have evidence such as documentation or reports containing the
location and amount of load to be tripped, and the corresponding frequency thresholds,
on those circuits included in its UFLS program to achieve the individual and
cumulative percentages identified in Requirement R4. (Attachment C Tables 1-3).
M5 Each Distribution Provider or Transmission Owner shall have evidence such as reports,
analysis, system studies and dated documentation that demonstrates that it meets
Requirement R5.
M6 Each Distribution Provider and Transmission Owner in the Québec Interconnection
shall have evidence such as documentation or reports containing the location and
amount of load to be tripped and the corresponding frequency thresholds on those
circuits included in its UFLS program to achieve the load values identified in Table 4
of Requirement R6. (Attachment C Table 4).
M7 Each Distribution Provider and Transmission Owner shall have evidence such as
documentation or reports that their underfrequency relays have been set with the
minimum time delay, in accordance with Requirement R7.
M8 Each Planning Coordinator shall have evidence such as reports, system studies or
analysis that demonstrates that it meets Requirement R8.

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Standard PRC-006-NPCC-1 Automatic Underfrequency Load Shedding

M9 Each Planning Coordinator shall provide evidence such as letters, emails, or other
dated documentation that demonstrates that it meets Requirement R9.
M10 Each Distribution Provider and Transmission Owner shall provide evidence such as
test reports, data sheets or other documentation that demonstrates that it meets
Requirement R10.
M11 Each Distribution Provider and Transmission Owner shall provide evidence such as
letters, emails or other dated documentation that demonstrates that it meets
Requirement R11.
M12 Each Distribution Provider and Transmission Owner shall provide evidence such as
reports, spreadsheets or other dated documentation submitted to its Planning
Coordinator that indicates the frequency set point, the net amount of load shed and the
percentage of its peak load at each stage of its UFLS program coincident with the
integrated hourly peak of the previous year that demonstrates that it meets Requirement
R12.
M13 Each Generator Owner shall provide evidence such as reports, data sheets,
spreadsheets or other documentation that demonstrates that it meets Requirement R13.
M14 Each Generator Owner shall provide evidence such as emails, letters or other dated
documentation that demonstrates that it meets Requirement R14.
M15 Each Generator Owner shall provide evidence such as reports, data sheets,
specifications, memorandum or other documentation that demonstrates that it meets
Requirement R15.
M16 Each Generator Owner with existing non-nuclear units in service prior to the effective
date of this Standard which have underfrequency tripping that is not compliant with
Requirement R13 shall provide evidence such as reports, spreadsheets, memorandum
or dated documentation demonstrating that it meets Requirement R16.
M17 Each Planning Coordinator in Ontario, Quebec and the Maritime provinces shall
provide evidence such as emails, memorandum or other documentation that

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Standard PRC-006-NPCC-1 Automatic Underfrequency Load Shedding

demonstrates that it followed the methodology described in Attachment A and meets
Requirement R17.
M18 Each Generator Owner, Distribution Provider or Transmission Owner within the
Planning Coordinator area of ISO-NE or the New York ISO shall provide evidence
such as emails, memorandum, or other documentation that demonstrates that it
followed the methodology described in Attachment B and meets Requirement R18.
M19 Each Generator Owner of nuclear units that have been specifically identified by NPCC
as having generator trip settings above the generator trip curve in Figure 1 shall
provide evidence such as letters, reports and dated documentation that demonstrates
that it meets Requirement R19.

M20 Each Planning Coordinator shall provide evidence such as spreadsheets, system
studies, or other documentation that demonstrates that it meets the requirements of
Requirement R20.
M21 Each Planning Coordinator shall provide evidence such as emails, memorandum or
other dated documentation that it meets Requirement R21.
M22 Each Distribution Provider, Transmission Owner and Generator Owner shall provide
evidence such as reports, spreadsheets or other documentation that demonstrates that it
meets Requirement R22.
M23 Each Distribution Provider, Transmission Owner and Generator Owner shall provide
evidence such as letters, emails or other dated documentation that demonstrates it
meets Requirement 23.
D. Compliance
1.

Compliance Monitoring Process
1.1. Compliance Enforcement Authority

NPCC Compliance Committee
1.2. Compliance Monitoring Period and Reset Time Frame

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Standard PRC-006-NPCC-1 Automatic Underfrequency Load Shedding

Not Applicable
1.3. Data Retention

The Distribution Provider and Transmission Owner shall keep evidences for three
calendar years for Measures 4, 5, 6,7,10, 11, and 12.
The Planning Coordinator shall keep evidence for three calendar years for
Measures 1, 2, 3, 8, 9, 20, and 21.
The Planning Coordinator in Ontario, Quebec, and the Maritime Provinces shall
keep evidence for three calendar years for Measure 17.
The Distribution Provider, Transmission Owner, and Generator Owner shall keep
evidences for three calendar years for Measures 18, 22, and 23.
The Generator Owner shall keep evidence for three calendar years for Measures
13, 14, 15, 16, and 19.
1.4. Compliance Monitoring and Assessment Processes

Self -Certifications.
Spot Checking.
Compliance Audits.
Self- Reporting.
Compliance Violation Investigations.
Complaints.
1.5. Additional Compliance Information

None.

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Standard PRC-006-NPCC-1 Automatic Underfrequency Load Shedding
2.

Violation Severity Levels

Requirement
R1

Lower VSL
N/A

Moderate VSL
N/A

High VSL

Severe VSL

Planning Coordinator did not
establish requirements for entities
aggregating their UFLS programs.

Planning Coordinator did not
establish requirements for entities
aggregating their UFLS programs
and did not establish requirements
for compensatory load shedding.

or
Did not establish requirements for
compensatory load shedding.

R2

The Planning Coordinator
identified the generation
facilities within its Planning
Coordinator Area necessary to
support the UFLS program, but
did so more than 30 days but less
than 41 days after completion of
the system studies.

The Planning Coordinator
identified the generation
facilities within its Planning
Coordinator Area necessary to
support the UFLS program, but
did so more than 40 days but less
than 51 days after completion of
the system studies.

The Planning Coordinator
identified the generation facilities
within its Planning Coordinator
Area necessary to support the
UFLS program, but did so more
than 50 days but less than 61 days
after completion of the system
studies.

The Planning Coordinator
identified the generation facilities
within its Planning Coordinator
Area necessary to support the
UFLS program, but did so more
than 60 days after completion of
the system studies.
or
The Planning Coordinator did not
identify the generation facilities
within its Planning Coordinator
Area necessary to support the
UFLS program.

R3

The Planning Coordinator
provided the requested
information, but did so more than
30 days but less than 41 days to
the requesting entity.

The Planning Coordinator
provided the requested
information, but did so more
than 40 days but less than 51
days to the requesting entity.

The Planning Coordinator
provided the requested
information, but did so more than
50 days but less than 61 days to the
requesting entity.

The Planning Coordinator
provided the requested
information, but did so more than
60 days after the request.
or
The Planning Coordinator failed
to provide the requested

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Standard PRC-006-NPCC-1 Automatic Underfrequency Load Shedding
information.

R4

N/A

N/A

N/A

The Distribution Provider or
Transmission Owner failed to
implement an automatic UFLS
program reflecting normal
operating conditions excluding
outages, for its Facilities or
collectively implemented by
mutual agreement with one or
more Distribution Providers and
Transmission Owners within the
same island identified in
Requirement R1, an aggregated
automatic UFLS program that
sheds Load based on frequency
thresholds, total nominal
operating time, and amounts
specified in the appropriate
included tables.

R5

N/A

The Distribution Provider or
Transmission Owner armed its
load to trip on underfrequency in
order to meet its minimum
obligations and by doing so
exceeded the tolerances and/or
deviated from the number of
stages and frequency set points
of the UFLS program as
specified in the tables contained
in Attachment C, as applicable
depending on their total peak net
Load, but did not inform the
Planning Coordinator of the
need to exceed the stated

The Distribution Provider or
Transmission Owner armed its
load to trip on underfrequency in
order to meet its minimum
obligations and by doing so
exceeded the tolerances and/or
deviated from the number of stages
and frequency set points of the
UFLS program as specified in the
tables contained in Attachment C,
as applicable depending on their
total peak net Load, but did not
provide the Planning Coordinator
with an analysis demonstrating that
no alternative load shedding

The Distribution Provider or
Transmission Owner did not arm
its load to trip on
underfrequency in order to meet
its minimum obligations and in
doing so exceeded the tolerances
and/or deviated from the number
of stages and frequency set
points of the UFLS program as
specified in the tables contained
in Attachment C, as applicable
depending on their total peak net
Load.

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Standard PRC-006-NPCC-1 Automatic Underfrequency Load Shedding
tolerances of UFLS Table 2 or
Table 3, and in the case of Table
2 only, the need to deviate from
providing two stages of UFLS.

solution is available that would
allow the Distribution Provider or
Transmission Owner to comply
with the appropriate table.

R6

N/A

N/A

N/A

The Distribution Provider or
Transmission Owner in the
Québec Interconnection portion
of NPCC did not implement an
automatic UFLS program for its
Facilities based on the
frequency thresholds, slopes,
total nominal operating time and
amounts specified in Attachment
C, Table 4 or did not collectively
implement by mutual agreement
with one or more Distribution
Providers and Transmission
Owners within the same island,
identified in Requirement R1, an
aggregated automatic UFLS
program that sheds Load based
on the frequency thresholds,
slopes, total nominal operating
time and amounts specified in
Attachment C, Table 4.

R7

N/A

N/A

N/A

The Distribution Provider or
Transmission Owner failed to set

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Standard PRC-006-NPCC-1 Automatic Underfrequency Load Shedding
an underfrequency relay that is
part of its region’s UFLS
program as specified in
Requirement R7.
R8

N/A

N/A

The Planning Coordinator
developed inhibit thresholds as
specified in Requirement R8 but
did not perform the review once
per calendar year.

The Planning Coordinator did
not develop inhibit thresholds as
specified in Requirement R8.

R9

The Planning Coordinator
provided to a Transmission
Owner or Distribution Provider
within its Planning Coordinator
area the applicable inhibit
thresholds more than 30 days but
less than 41 days of the initial
determination or any subsequent
change to the inhibit thresholds.

The Planning Coordinator
provided to a Transmission
Owner or Distribution Provider
within its Planning Coordinator
area the applicable inhibit
thresholds more than 40 days but
less than 51 days of the initial
determination or any subsequent
change to the inhibit thresholds.

The Planning Coordinator
provided to a Transmission Owner
or Distribution Provider within its
Planning Coordinator area the
applicable inhibit thresholds more
than 50 days but less than 61 days
of the initial determination or any
subsequent change to the inhibit
thresholds.

The Planning Coordinator
provided to a Transmission
Owner or Distribution Provider
within its Planning Coordinator
area the applicable inhibit
thresholds more than 60 days
after the initial determination or
any subsequent change to the
inhibit thresholds.
or
The Planning Coordinator did
not provide to a Transmission
Owner or Distribution Provider
within its Planning Coordinator
area the applicable inhibit
thresholds.

R10

N/A

N/A

N/A

The Distribution Provider or
Transmission Owner did not
implement the inhibit threshold
based on the notification
provided by the Planning
Coordinator in accordance with

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Standard PRC-006-NPCC-1 Automatic Underfrequency Load Shedding
Requirement R9.

R11

The Distribution Provider or
Transmission Owner developed
and submitted its implementation
plan more than 90 days but less
than 101 days after the request
from the Planning Coordinator.

The Distribution Provider or
Transmission Owner developed
and submitted its
implementation plan more than
100 days but less than 111 days
after the request from the
Planning Coordinator.

The Distribution Provider or
Transmission Owner developed
and submitted its implementation
plan more than 110 days but less
than 121 days after the request
from the Planning Coordinator.

The Distribution Provider or
Transmission Owner developed
and submitted its
implementation plan more than
120 days after the request from
the Planning Coordinator.
or
The Distribution Provider or
Transmission Owner did not
develop its implementation plan.

R12

N/A

N/A

N/A

The Transmission Owner or
Distribution Provider did not
provide documentation to its
Planning Coordinator of actual
net load data or updates to the
data that would be shed by the
UFLS relays, as determined by
measuring actual metered load
through the switches that would
be opened by the UFLS relays,
that were armed to shed at each
UFLS stage coincident with their
integrated hourly peak during
the previous year.

R13

N/A

N/A

N/A

The Generator Owner did not set
each generator underfrequency
trip relay, if so equipped, below
the appropriate generator
underfrequency trip protection
settings threshold curve in

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Standard PRC-006-NPCC-1 Automatic Underfrequency Load Shedding
Figure 1, except as otherwise
exempted.

R14

The Generator Owner transmitted
the generator underfrequency trip
setting and time delay to its
Planning Coordinator more than
45 days and less than 56 days of
the Planning Coordinator’s
request.

The Generator Owner
transmitted the generator
underfrequency trip setting and
time delay to its Planning
Coordinator more than 55 days
and less than 66 days of the
Planning Coordinator’s request.

The Generator Owner transmitted
the generator underfrequency trip
setting and time delay to its
Planning Coordinator more than 65
days and less than 76 days of the
Planning Coordinator’s request.

The Generator Owner
transmitted the generator
underfrequency trip setting and
time delay to its Planning
Coordinator more than 75days
after the Planning
Coordinator’s request.
or

The Generator Owner did not
transmit the generator
underfrequency trip setting and
time delay to its Planning
Coordinator.
R15

N/A

N/A

The Generator Owner did not
fulfill the obligation of
Requirement R15; Part 15.1 OR
did not fulfill the obligation of
Requirement R15, Part 15.2.

The Generator Owner did not
fulfill the obligation of
Requirement R15, Part 15.1 and
did not fulfill the obligation of
Requirement R15, Part 15.2.

R16

N/A

The Generator Owner did not
fulfill the obligation of
Requirement R16, Part 16.2.

The Generator Owner did not
fulfill the obligation of
Requirement R16; Part 16.1 OR
did not fulfill the obligation of

The Generator Owner did not
fulfill the obligation of
Requirement R16, Part 16.1 and
did not fulfill the obligation of

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Standard PRC-006-NPCC-1 Automatic Underfrequency Load Shedding
Requirement R16, Part 16.3.

Requirement R16, Part 16.3.

R17

N/A

N/A

N/A

The Planning Coordinator did
not apply the methodology
described in Attachment A to
determine the compensatory load
shedding that is required.

R18

N/A

N/A

N/A

The Generator Owner,
Distribution Provider, or
Transmission Owner did not
apply the methodology described
in Attachment B to determine
the compensatory load shedding
that is required.

R19

N/A

The Generator Owner did not
fulfill the obligation of
Requirement R19, Part 19.3.

The Generator Owner did not
fulfill the obligation of
Requirement R19; Part 19.1 OR
did not fulfill the obligation of
Requirement R19, Part 19.2.

The Generator Owner did not
fulfill the obligation of
Requirement R19, Part 19.1 and
did not fulfill the obligation of
Requirement R19, Part 19.2.

R20

The Planning Coordinator did not
have data in its database for one
of the parameters listed in
Requirement 20, Parts 20.1
through 20.5.

The Planning Coordinator did
not have data in its database for
two of the parameters listed in
Requirement 20, Parts 20.1
through 20.5.

The Planning Coordinator did not
have data in its database for three
of the parameters listed in
Requirement 20, Parts 20.1
through 20.5.

The Planning Coordinator did
not have data in its database for
four or more of the parameters
listed in Requirement 20, Parts
20.1 through 20.5.

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Standard PRC-006-NPCC-1 Automatic Underfrequency Load Shedding

R21

N/A

N/A

N/A

The Planning Coordinator did
not notify a Distribution
Provider, Transmission Owner,
or Generator Owner within its
Planning Coordinator area of
changes to load distribution
needed to satisfy UFLS program
requirements.

R22

N/A

N/A

N/A

The Distribution Provider,
Transmission Owner, or
Generator Owner did not
implement the load distribution
changes based on the
notification provided by the
Planning Coordinator.

R23

The Distribution Provider.
Transmission Owner or Generator
Owner developed and submitted
its implementation plan more than
90 days but less than 101 days
after the request from the
Planning Coordinator.

The Distribution Provider.
Transmission Owner or
Generator Owner developed and
submitted its implementation
plan more than 100 days but less
than 111 days after the request
from the Planning Coordinator.

The Distribution Provider.
Transmission Owner or Generator
Owner developed and submitted its
implementation plan more than
110 days but less than 121 days
after the request from the Planning
Coordinator.

The Distribution Provider.
Transmission Owner or
Generator Owner developed and
submitted its implementation
plan more than 120 days after
the request from the Planning
Coordinator.
or
The Distribution Provider.
Transmission Owner or
Generator Owner did not
develop its implementation plan.

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Standard PRC-006-NPCC-1 Automatic Underfrequency Load Shedding

Version History

Version

Date

Action

1

November 20,
2011

Region BOD Approval

1

February 9,
2012

Adopted by Board of Trustees

Change Tracking

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Standard PRC-006-NPCC-1 Automatic Underfrequency Load Shedding

PRC-006-NPCC-1 Attachment A
Compensatory Load Shedding Criteria for Ontario, Quebec, and the Maritime Provinces:
The Planning Coordinator in Ontario, Quebec and the Maritime provinces is responsible for
establishing the compensatory load shedding requirements for all existing non-nuclear units in its
NPCC area with underfrequency protections set to trip above the appropriate curve in Figure 1.
In addition, it is the Planning Coordinator’s responsibility to communicate these requirements to
the appropriate Distribution Provider or Transmission Owner and to ensure that adequate
compensatory load shedding is provided in all islands identified in Requirement R1 in which the
unit may operate.
The methodology below provides a set of criteria for the Planning Coordinator to follow for
determining compensatory load shedding requirements:
1. The Planning Coordinator shall identify, compile and maintain an updated list of all
existing non-nuclear generating units in service prior to the effective date of this standard
that have underfrequency protections set to trip above the appropriate curve in Figure 1.
The list shall include the following information for each unit:
1.1 Generator name and generating capacity
1.2 Underfrequency protection trip settings, including frequency trip set points and
time delays
1.3 Physical and electrical location of the unit
1.4 All islands within which the unit may operate, as identified in Requirement R1
2. For each generating unit identified in (1) above, the Planning Coordinator shall establish
the requirements for compensatory load shedding based on criteria outlined below:
2.1 Arrange for a Distribution Provider or Transmission Owner that owns UFLS
relays within the island(s) identified by the Planning Coordinator in Requirement
R1 within which the generator may operate to provide compensatory load
shedding.
2.2 The compensatory load shedding that is provided by the Distribution Provider or
Transmission Owner shall be in addition to the amount that the Distribution
Provider or Transmission Owner is required to shed as specified in Requirement
R4..
2.3 The compensatory load shedding shall be provided at the UFLS program stage (or
threshold stage for Quebec) with a frequency threshold setting that corresponds to
the highest frequency at which the subject generator will trip above the
appropriate curve in Figure 1 during an underfrequency event. If the highest
frequency at which the subject generator will trip above the appropriate curve in
Figure 1 does not correspond to a specific UFLS program stage threshold setting,
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Standard PRC-006-NPCC-1 Automatic Underfrequency Load Shedding

the compensatory load shedding shall be provided at the UFLS program stage
with a frequency threshold setting that is higher than the highest frequency at
which the subject generator will trip above the appropriate curve in Figure 1.
2.4 The amount of compensatory load shedding shall be equivalent (±5%) to the
average net generator megawatt output for the prior two calendar years, as
specified by the Planning Coordinator, plus expected station loads to be
transferred to the system upon loss of the facility. The net generation output
should only include those hours when the unit was a net generator to the electric
system.
In the specific instance of a generating unit that has been interconnected to the
electric system for less than two calendar years, the amount of compensatory load
shedding shall be equivalent (±5%) to the maximum claimed seasonal capability
of the generator over two calendar years, plus expected station loads to be
transferred to the system upon loss of the facility.

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Standard PRC-006-NPCC-1 Automatic Underfrequency Load Shedding

PRC-006-NPCC-1 Attachment B
Compensatory Load Shedding Criteria for ISO-NE and NYISO:
The Generator Owner in the New England states or New York State are responsible for
establishing a compensatory load shedding program for all existing non-nuclear units with
underfrequency protection set to trip above the appropriate curve in Figure 1 of this standard.
The Generator Owner shall follow the methodology below to determine compensatory load
shedding requirements:
1. The Generator Owner shall identify and compile a list of all existing non-nuclear
generating units in service prior to the effective date of this standard that has
underfrequency protection set to trip above the appropriate curve in Figure 1. The list
shall include the following information associated with each unit:
1.1 Generator name and generating capacity
1.2 Underfrequency protection trip settings, including frequency trip set points and
time delays
1.3 Physical and electrical location of the unit
1.4 Smallest island within which the unit may operate as identified by the Planning
Coordinator in Requirement R1 of this Standard.
2. For each generating unit identified in (1) above, the Generator Owner shall establish the
requirements for compensatory load shedding based on criteria outlined below:
2.1 In cases where a Distribution Provider or Transmission Owner has coordinated
protection settings with the Generator Owner to cause the generator to trip above
the appropriate curve in Figure 1, the Distribution Provider or Transmission
Owner is responsible to provide the appropriate amount of compensatory load to
be shed within the smallest island identified by the Planning Coordinator in
Requirement R1 of this standard.
2.2 In cases where a Generator Owner has a generator that cannot physically meet the
set points defined by the appropriate curve in Figure 1, the Generator Owner shall
arrange for a Distribution Provider or Transmission Owner to provide the
appropriate amount of compensatory load to be shed within the smallest island
identified by the Planning Coordinator in Requirement R1 of this standard.
2.3 The compensatory load shedding that is provided by the Distribution Provider or
Transmission Owner shall be in addition to the amount that the Distribution
Provider or Transmission Owner is required to shed as specified in Requirement
R4.

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Standard PRC-006-NPCC-1 Automatic Underfrequency Load Shedding

2.4 The compensatory load shedding shall be provided at the UFLS program stage
with the frequency threshold setting at or closest to but above the frequency at
which the subject generator will trip.
2.5 The amount of compensatory load shedding shall be equivalent (±5%) to the

average net generator megawatt output for the prior two calendar years, as
specified by the Planning Coordinator, plus expected station loads to be
transferred to the system upon loss of the facility. The net generation output
should only include those hours when the unit was a net generator to the electric
system.
In the specific instance of a generating unit that has been interconnected to the
electric system for less than two calendar years, the amount of compensatory load
shedding shall be equivalent (±5%) to the maximum claimed seasonal capability
of the generator over two calendar years, plus expected station loads to be
transferred to the system upon loss of the facility.

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Standard PRC-006-NPCC-1 Automatic Underfrequency Load Shedding

PRC-006-NPCC-1 Attachment C
UFLS Table 1: Eastern Interconnection
Distribution Providers and Transmission Owners with 100 MW or more of peak net Load shall
implement a UFLS program with the following attributes:
Frequency
Threshold
(Hz)

Total Nominal
Operating
1
Time (s)

Load Shed at Stage as
% of TO or DP
Load

Cumulative Load Shed as % of
TO or DP Load

59.5

0.30

6.5 – 7.5

6.5 – 7.5

59.3

0.30

6.5 – 7.5

13.5 – 14.5

59.1

0.30

6.5 – 7.5

20.5 – 21.5

58.9

0.30

6.5 – 7.5

27.5 – 28.5

59.5

10.0

2–3

29.5

–
31.5

UFLS Table 2: Eastern Interconnection
Distribution Providers and Transmission Owners with 50 MW or more and less than 100 MW
of peak net Load shall implement a UFLS program with the following attributes:
UFLS Stage

Frequency
Threshold (Hz)

Total Nominal
1
Operating Time(s)

Load Shed at
Stage as % of TO
or DP Load

Cumulative Load
Shed as % of TO
or DP Load

1

59.5

0.30

14-25

14-25

2

59.1

0.30

14-25

28-50

1. The total nominal operating time includes the underfrequency relay operating time plus any interposing
auxiliary relay operating times, communication times, and the rated breaker interrupting time. The
underfrequency relay operating time is measured from the time when frequency passes through the frequency
threshold setpoint, using a test rate of frequency decay of 0.2 Hz per second. If the relay operating time is
dependent on the rate of frequency decay, the underfrequency relay operating time and any subsequent testing of
the UFLS relays shall utilize a test rate of linear frequency decay of 0.2 Hz per second.

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Standard PRC-006-NPCC-1 Automatic Underfrequency Load Shedding

UFLS Table 3: Eastern Interconnection
Distribution Providers and Transmission Owners with 25 MW or more and less than 50 MW of
peak net Load shall implement a UFLS program with the following attributes:
UFLS Stage

Frequency
Threshold (Hz)

Total Nominal
Operating Time
1
(s)

Load Shed at
Stage as % of TO
or DP Load

Cumulative Load
Shed as % of TO
or DP Load

1

59.5

0.30

28-50

28-50

1. The total nominal operating time includes the underfrequency relay operating time plus any interposing
auxiliary relay operating times, communication times, and the rated breaker interrupting time. The
underfrequency relay operating time is measured from the time when frequency passes through the frequency
threshold setpoint, using a test rate of frequency decay of 0.2 Hz per second. If the relay operating time is
dependent on the rate of frequency decay, the underfrequency relay operating time and any subsequent testing of
the UFLS relays shall utilize a test rate of linear frequency decay of 0.2 Hz per second.

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Standard PRC-006-NPCC-1 Automatic Underfrequency Load Shedding

UFLS Table 4: Quebec Interconnection

MW
at peak
Rate

Frequency
(Hz)

(*Load must
be fixed at all
times when
above 60% of
peak load..)

Mvar
at peak

Total
Nominal
Operating
2
Time (s)

Threshold Stage 1

–––

58.5

1000*

1000

0.30

Threshold Stage 2

–––

58.0

800*

800

0.30

Threshold Stage 3

–––

57.5

800

800

0.30

Threshold Stage 4

–––

57.0

800

800

0.30

–––

59.0

500

500

20.0

Slope Stage 1

-0.3 Hz/s

58.5

400

400

0.30

Slope Stage 2

-0.4 Hz/s

59.8

800*

800

0.30

Slope Stage 3

-0.6 Hz/s

59.8

800*

800

0.30

Slope Stage 4

-0.9 Hz/s

59.8

800

800

0.30

Threshold Stage 5
(anti-stall)

2. The total nominal operating time includes the underfrequency relay operating time plus any interposing
auxiliary relay operating times, communications time, and the rated breaker interrupting time. The
underfrequency relay operating time shall be measured from the time when the frequency passes through the
frequency threshold set point.

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