IEC TS 62786-41:2023

Distributed energy resources connection with the grid - Part 41: Requirements for frequency measurement used to control distributed energy resources (DER) and loads

IEC TS 62786-41:2023 (E), which is a Technical Specification, defines minimum requirements for frequency and rate of change of frequency measurements used to control distributed energy resources (DER) and loads connected to electrical power networks.
This document specifies the characteristics of frequency and rate of change of frequency measurements to evaluate their performances. It describes the main use cases of frequency and rate of change of frequency measurements, with associated level of performances. It describes the principle of functional tests to evaluate the specified characteristics and defines the influencing factors that affect these performances, under steady state or dynamic conditions.
This document defines the functional requirements applicable to frequency and rate of change of frequency measurements which can be inside or outside the DER or loads. In the case of DER, this document provides requirements additional to those which are defined in the other parts of IEC 62786 or standards produced by the relevant IEC technical committees (e.g. TC 82 for photovoltaic systems, TC 88 for wind systems, TC 120 for electrical energy storage systems (EES)).
This document is applicable to DER and loads regardless of the voltage level of the point of connection to the grid.
This document does not specify hardware, software or a method for frequency or rate of change of frequency measurement. It does not specify tests linked to environmental conditions associated with hardware devices (climatic, electromagnetic disturbances above 3 kHz, mechanical stress, etc.).
Frequency and rate of change of frequency measurements associated with time stamping are not in the scope of this document. These measurements are already covered by IEC 60255 118 1.
Frequency and rate of change of frequency measurements associated with protection functions or protection relays are not in the scope of this document. These requirements are already covered by IEC 60255-181.

General Information

• Status: Published
• Publication Date: 16-Jul-2023
• ICS: 91.140.50 - Electricity supply systems
• Technical Committee: TC 8 - System aspects of electrical energy supply
• Drafting Committee: JWG 12 - TC 8/JWG 12
• Current Stage: PPUB - Publication issued
• Start Date: 17-Jul-2023
• Completion Date: 17-Jul-2023

Standards Content (Sample)

IEC TS 62786-41 ®
Edition 1.0 2023-07
TECHNICAL
SPECIFICATION
colour
inside
Distributed energy resources connection with the grid –
Part 41: Requirements for frequency measurement used to control distributed
energy resources (DER) and loads

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IEC TS 62786-41 ®
Edition 1.0 2023-07
TECHNICAL
SPECIFICATION
colour
inside
Distributed energy resources connection with the grid –

Part 41: Requirements for frequency measurement used to control distributed

energy resources (DER) and loads

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 91.140.50  ISBN 978-2-8322-7225-1

– 2 – IEC TS 62786-41:2023 © IEC 2023
CONTENTS
FOREWORD . 7
1 Scope . 9
2 Normative references . 9
3 Terms and definitions . 10
4 Performance description . 14
4.1 General . 14
4.2 Input energizing quantities . 14
4.3 Delay time . 15
4.3.1 Description . 15
4.3.2 Reporting of delay time declaration . 15
4.4 Effective resolution and accuracy . 15
4.4.1 Description . 15
4.4.2 Effective measurement resolution . 16
4.4.3 Reporting of the frequency and ROCOF accuracy . 16
4.5 Measuring range, operating range, and rejection of interfering signals . 16
4.6 Timing characteristics . 19
4.6.1 Reporting rate. 19
4.6.2 Settling time . 19
5 Summary of typical performances associated with different use cases . 20
6 Description of functional test principles . 22
6.1 General . 22
6.2 Test reference conditions . 24
6.3 Verification of delay time for frequency and ROCOF measurement . 24
6.3.1 Test description . 24
6.3.2 Example determination of delay time . 25
6.4 Verification of effective resolution for frequency and ROCOF measurement . 27
6.4.1 Test description . 27
6.4.2 Example determination of effective resolution . 29
6.5 Verification of measurement and operating ranges . 29
6.5.1 Verification of measurement and operating ranges under steady state
conditions . 29
6.5.2 Measuring and operating ranges under dynamic conditions . 31
6.5.3 Verification of rejection of interfering interharmonics. 33
6.5.4 Verification of rejection of harmonics . 35
6.6 Verification of settling time . 38
6.6.1 Test description . 38
6.6.2 Verification of settling time for frequency measurement . 39
6.6.3 Example of verification of frequency settling time . 39
6.6.4 Verification of settling time for ROCOF measurement . 40
6.6.5 Example of verification of ROCOF settling time . 40
6.7 Type test report . 41
Annex A (informative) Measurement classes . 43
Annex B (informative) Description of frequency or ROCOF measurement use cases . 44
B.1 Use case "PLL in photovoltaic power generating systems" . 44
B.1.1 Technical background of the use case . 44
B.1.2 Resulting requirements for measurement . 45

B.2 Use case "Primary reserve" . 46
B.2.1 Technical background of the use case . 46
B.2.2 Resulting requirements for measurement . 46
B.2.3 Example of "frequency-watt" function in photovoltaic power generating
systems . 47
B.3 Use case "Secondary reserve – frequency measurement used for centralized
control" . 48
B.3.1 Technical background of the use case . 48
B.3.2 Resulting requirements for measurement . 48
B.4 Use case "Fast frequency-active power proportional controller with dead
band" . 49
B.4.1 Technical background of the use case . 49
B.4.2 Resulting requirements for measurement . 50
B.5 Use case "Fast frequency response" . 51
B.5.1 Technical background of the use case . 51
B.5.2 Resulting requirements for measurement . 51
B.6 Use case "Synthetic inertia" . 51
B.6.1 Technical background of the use case . 51
B.6.2 Resulting requirements for measurement . 52
B.7 Use case "Passive anti-islanding detection" . 52
B.7.1 Technical background of the use case . 52
B.7.2 Resulting requirements for measurement . 53
B.8 Use case "Active anti-islanding detection" . 54
B.8.1 Technical background of the use case . 54
B.8.2 Resulting requirements for measurement . 54
B.9 Use case "ROCOF measurement used for centralized control" . 55
B.9.1 Technical background of the use case . 55
B.9.2 Resulting requirements for measurement . 55
B.10 Use case "Load control with active power management" . 55
B.10.1 Technical background of the use case . 55
B.10.2 Resulting requirements for measurement . 55
B.11 Use case "Self-dispatchable loads" (microgrid applications) . 56
B.11.1 Technical background of the use case . 56
B.11.2 Resulting requirements for measurement . 56
B.12 Use case "Under-frequency load shedding" (UFLS) . 57
B.12.1 Technical background of the use case . 57
B.12.2 Resulting requirements for measurement . 57
Annex C (informative) Summary of requirements expressed in standards and grid
codes related to frequency and ROCOF measurements . 58
Annex D (informative) Maximum ROCOF to be considered on power systems in case
of incidents . 65
D.1 General . 65
D.2 UK . 65
D.3 European continent . 65
D.4 Islands . 65
Annex E (informative) Frequency and rotating vectors . 66
Annex F (informative) Synthetizing input signals with sudden frequency change

without discontinuity in voltage waveform. 68
Annex G (informative) Step test equivalent time sampling technique . 71
G.1 Overview. 71

– 4 – IEC TS 62786-41:2023 © IEC 2023
G.2 Equivalent time sampling . 72
G.3 Determination of settling time using instrument errors . 73
Annex H (informative) Voltage and phase angle changes during transmission line
faults related to the type of transformer connection . 75
H.1 Overview. 75
H.2 Power line short circuit fault and protection . 75
H.3 Voltage magnitude and phase angle change at line fault . 77
H.3.1 General . 77
H.3.2 Balanced-three-phase short circuit fault . 77
H.3.3 Line-to-line short circuit fault . 77
H.4 Conclusion . 79
Annex I (informative) Influencing factors and functional tests . 80
I.1 Influencing factors . 80
I.2 Functional tests . 80
I.2.1 General . 80
I.2.2 Phase step change . 80
I.2.3 Magnitude step change . 82
I.2.4 Combined magnitude and phase step change . 85
I.2.5 Voltage magnitude drop and restoration . 89
I.2.6 Noise . 95
I.2.7 Unbalanced input signal magnitude . 97
I.2.8 Linear ramp of frequency . 99
Bibliography . 104

Figure 1 – Measuring range and operating range without interfering signals . 17
Figure 2 – Measuring range and operating range in the presence of interfering signals . 17
Figure 3 – Settling time description with input signal added . 20
Figure 4 – Example of frequency delay time validation: measurement of delay time for
a power frequency of 50 Hz . 26
Figure 5 – Example of cross-correlations of the normalized frequencies and ROCOF . 26
Figure 6 – Example of frequency modulation used to determine frequency effective
resolution . 29
Figure 7 – Example of frequency modulation used to determine ROCOF effective

resolution . 29
Figure 8 – Example of verification of measurement bandwidth under steady state
conditions . 31
Figure 9 – Example of verification of measuring and operating ranges under dynamic
conditions . 33
Figure 10 – Example of verification of rejection of interfering interharmonics . 34
Figure 11 – Waveforms with superimposed harmonics . 36
Figure 12 – Three-phase harmonic test signals, 0° and 180° harmonic phases . 37
Figure 13 – Example of verification of rejection of harmonics . 38
Figure 14 – Example of verification of frequency settling time using positive 1 Hz step

in frequency . 39
Figure 15 – Example of verification of frequency settling time using negative 1 Hz step
in input frequency . 40
Figure 16 – Example of verification of ROCOF settling time using positive 1 Hz/s step
in ROCOF . 41

Figure 17 – Example of verification of ROCOF settling time using negative 1 Hz/s step
in ROCOF . 41
Figure B.1 – Example of a system diagram of a PV system with a three-phase DC to

AC converter . 44
Figure B.2 – Example of system diagram of a three-phase PV system for voltage
control . 45
Figure B.3 – Example of system diagram of PV system with frequency-watt function . 47
Figure B.4 – Application example of frequency-watt function for PV systems . 48
Figure B.5 – Example of fast frequency-active power proportional controller with dead
band (LFSM-O and LFSM-U characteristics from European Grid Code) . 50
Figure E.1 – Phasor representation of a power system signal, which has amplitude (a),

angle (Φ) and angular velocity (ω) . 66
Figure F.1 – Example of voltage waveform without discontinuity at t = 0,02 s . 69
o
Figure F.2 – Example of voltage waveform with discontinuity at t = 0,02 s. 70
o
Figure G.1 – Example of reports during step response . 71
Figure G.2 – Example of reports during step response with higher resolution . 72
Figure G.3 – Example of reports during step response with higher resolution . 73
Figure H.1 – Voltage phase change by transmission line short circuit fault . 76
Figure H.2 – Transmission line protection sequence and line voltage, frequency
change . 76
Figure H.3 – Voltage and phase angle change at three-phase short circuit . 77
Figure H.4 – Relationship of voltage phase angle between Y-connection side and
Δ-connection side . 78
Figure H.5 – Voltage magnitude and phase angle change at two-phase short circuit
fault 78
Figure I.1 – Frequency error response to +0,3 radian phase step followed by −0,3
radian step . 82
Figure I.2 – ROCOF error response to +0,3 radian phase step followed by −0,3 radian
step 82
Figure I.3 – Frequency error response to magnitude step changes . 84
Figure I.4 – ROCOF error response to steps in magnitude . 85
Figure I.5 – Voltage vectors for test case a) . 86
Figure I.6 – Voltage vectors for test case b) . 87
Figure I.7 – Frequency error responses to combined phase and magnitude steps . 88
Figure I.8 – ROCOF error responses to combined phase and magnitude steps . 89
Figure I.9 – Representation of the input energizing quantity (voltage, RMS) injection . 91
Figure I.10 – Frequency response to voltage drop and restoration . 92
Figure I.11 – ROCOF response to voltage drop and restoration . 94
Figure I.12 – Frequency error absolute values from noise test scenarios a) and b) . 96
Figure I.13 – ROCOF error absolute values from noise test scenarios a) and b) . 97
Figure I.14 – Frequency absolute error due to unbalanced input signal magnitude . 98
Figure I.15 – ROCOF absolute error due to unbalanced input signal magnitude . 99
Figure I.16 – Frequency ramp test scenarios . 100
Figure I.17 – Absolute frequency error during linear ramp of frequency test scenarios . 102
Figure I.18 – Absolute ROCOF error during linear ramp of frequency test scenarios . 103

– 6 – IEC TS 62786-41:2023 © IEC 2023
Table 1 – Performance characteristics presented in Clause 4 . 14
Table 2 – Example of delay time . 15
Table 3 – Example of measurement resolution and maximum absolute error for
frequency and ROCOF measurements . 16
Table 4 – Example of measuring range and operating range for frequency and ROCOF

measurements (taken from an actual instrument) . 18
Table 5 – Example of reporting of settling time and reporting rate . 19
Table 6 – List of use cases and associated requirements . 21
Table 7 – Input signal harmonic magnitudes . 35
Table A.1 – Measurement classes for frequency measurements . 43
Table A.2 – Measurement classes for ROCOF measurements . 43
Table B.1 – Typical requirements for frequency measurement of PLL in PV systems . 46
Table B.2 – Typical requirements for frequency measurement – use case "Primary
reserve" . 46
Table B.3 – Example of requirements of frequency-Watt function of PV systems. 48
Table B.4 – Typical requirements for use case "Secondary reserve – frequency
measurement used for centralized control" . 49
Table B.5 – Typical requirements for frequency measurement – use case "Fast
frequency-active power proportional controller with dead band" . 51
Table B.6 – Typical requirements for frequency measurement – use case "Fast
frequency response" . 51
Table B.7 – Typcial requirements for ROCOF measurement – use case "Synthetic
inertia" . 52
Table B.8 – Set of typical requirements for frequency measurement – use case
"Passive anti-islanding detection" . 53
Table B.9 – Typical requirements for ROCOF measurement – use case "Passive anti-
islanding detection" . 53
Table B.10 – Typical requirements for frequency measurement – use case "Active anti-
islanding detection" . 54
Table B.11 – Typical requirements for ROCOF measurement – use case "ROCOF
measurement used for centralized control" . 55
Table B.12 – Typical requirements for frequency measurement – use case "Load
control with active power management" . 55
Table B.13 – Typical requirements for frequency measurement – use case "Self-
dispatchable loads" . 56
Table B.14 – Typical requirements for frequency measurement – use case "Under-
frequency load shedding" . 57
Table B.15 – Typical requirements for ROCOF measurement – use case "Under-
frequency load shedding" . 57
Table C.1 – Requirements expressed in standards and grid codes related to frequency
and ROCOF measurements . 59
Table I.1 – Influencing factors of frequency and ROCOF measurements . 80
Table I.2 – Test case a) for combined magnitude and phase step change . 86
Table I.3 – Test case b) for combined magnitude and phase step change . 86
Table I.4 – Magnitudes and phase angles for three phase voltages . 97

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
DISTRIBUTED ENERGY RESOURCES CONNECTION WITH THE GRID –

Part 41: Requirements for frequency measurement used to control
distributed energy resources (DER) and loads

FOREWORD
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IEC TS 62786-41 has been prepared by IEC technical committee 8: System aspects of electrical
energy supply. It is a Technical Specification.
The text of this Technical Specification is based on the following documents:
Draft Report on voting
8/1649/DTS 8/1661/RVDTS
Full information on the voting for its approval can be found in the report on voting indicated in
the above table.
The language used for the development of this Technical Specification is English.

– 8 – IEC TS 62786-41:2023 © IEC 2023
This document was drafted in accordance with ISO/IEC Directives, Part 2, and developed in
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at www.iec.ch/members_experts/refdocs. The main document types developed by IEC are
described in greater detail at www.iec.ch/publications.
It has been developed as part of measurement series together with IEC TS 62786-42 on voltage
measurement.
A list of all parts in the IEC 62786 series, published under the general title Distributed energy
resources connection with the grid, can be found on the IEC website.
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under webstore.iec.ch in the data related to the
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• reconfirmed,
• withdrawn,
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DISTRIBUTED ENERGY RESOURCES CONNECTION WITH THE GRID –

Part 41: Requirements for frequency measurement used to control
distributed energy resources (DER) and loads

1 Scope
This part of IEC 62786, which is a Technical Specification, defines minimum requirements for
frequency and rate of change of frequency measurements used to control distributed energy
resources (DER) and loads connected to electrical power networks.
This document specifies the characteristics of frequency and rate of change of frequency
measurements to evaluate their performances. It describes the main use cases of frequency
and rate of change of frequency measurements, with associated level of performances.
It describes the principle of functional tests to evaluate the specified characteristics and defines
the influencing factors that affect these performances, under steady state or dynamic conditions.
This document defines the functional requirements applicable to frequency and rate of change
of frequency measurements which can be inside or outside the DER or loads. In the case of
DER, this document provides requirements additional to those which are defined in the other
parts of IEC 62786 or standards produced by the relevant IEC technical committees (e.g. TC 82
for photovoltaic systems, TC 88 for wind systems, TC 120 for electrical energy storage systems
(EES)).
This document is applicable to DER and loads regardless of the voltage level of the point of
connection to the grid.
This document does not specify hardware, software or a method for frequency or rate of change
of frequency measurement. It does not specify tests linked to environmental conditions
associated with hardware devices (climatic, electromagnetic disturbances above 3 kHz,
mechanical stress, etc.).
Frequency and rate of change of frequency measurements associated with time stamping are
not in the scope of this document. These measurements are already covered by
IEC 60255-118-1 [1] .
Frequency and rate of change of frequency measurements associated with protection functions
or protection relays are not in the scope of this document. These requirements are already
covered by IEC 60255-181 [2].
NOTE As defined in the first paragraph, this document is focused on frequency and rate of change of frequency
measurements used to control DER and loads. But the technical requirements defined in this document, with the list
of declared characteristics and their associated functional tests, can also be applicable for other uses where "fast"
frequency and ROCOF measurement is required (small or large generators of power substations connected to
transmission or distribution grids, power meter devices, power quality instruments, etc.).
2 Normative references
There are no normative references in this document.
___________
Numbers in square brackets refer to the Bibliography.

– 10 – IEC TS 62786-41:2023 © IEC 2023
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminological databases for use in standardization at the following
addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
3.1
instrument
device or measurement function which performs frequency or
ROCOF measurement
Note 1 to entry: As the frequency or ROCOF measuring functions can be performed inside different types of devices
or systems (control system of distributed energy resources, power system loads, protection relays, metering devices,
etc.), the generic term "instrument" is used in this document to designate frequency or ROCOF measuring function
which must be characterized and tested.
3.2
rotating vector
representation of a sinusoidal function where a polar vector rotates at an angular velocity which
can be a non-constant function of time and is expressed in radians per second
Note 1 to entry: The radius of the rotating vector can also be a non-constant function of time.
Note 2 to entry: Rotating vectors can represent periodic or non-periodic sinusoids.
Note 3 to entry: Power system signals can be represented by a combination of signals, each represented by one
rotating vector, each with various angular velocities and various radii. Each of these rotating vectors represents one
component of the power system signal (see Annex E).
Note 4 to entry: The noise component of a power system signal is not represented by a rotating vector. Noise is
represented as a time series.
3.3
phase
angle of a rotating vector
Note 1 to entry: When the rotating vector is described in polar notation, the phase is the angle; when described in
complex notation, the phase is the argument.
3.4
instantaneous phase
phase of a rotating vector at a specific moment of time
Note 1 to entry: Any point along a sinusoidal periodic function can be represented by a complex number. The
instantaneous phase is the argument of that complex number.
3.5
frequency
rate of change of phase of a rotating vector
Note 1 to entry: If the period is a span of time, the unit of frequency is hertz (Hz) in cycles per second.
Note 2 to entry: Frequency can be a non-constant function of time.
3.6
power frequency
values of frequency used in the electricity supply systems

[SOURCE: IEC 60050-601:1985, 601-01-05, modified – In the definition, "conventionally" has
been deleted.]
3.7
instantaneous frequency
rate of change of instantaneous phase
Note 1 to entry: Typical frequency reporting instruments report instantaneous frequency and can report the
changing frequency within one period of the input energizing quantity.
3.8
measured frequency
estimated frequency provided by an instrument
3.9
fundamental component
rotating vector of interest for a waveform that is a sum of rotating vectors
Note 1 to entry: Generally, the fundamental component is the rotating vector with the greatest magnitude;
sometimes it is the component with the lowest frequency (often called the first harmonic). However, neither is always
the case, for example in AC current waveforms or in oscillating signals with sub-harmonics.
3.10
fundamental frequency
frequency of the fundamental component
Note 1 to entry: In AC electrical power systems, the fundamental frequency is to be maintained within relevant
statutory deviation from the nominal frequency.
Note 2 to entry: In three-phase systems, measuring fundamental frequency for all three phases can yield slightly
different measurements due to interference. Frequency can be obtained by a transformation of the individual phases
such as by averaging the three frequency measurements or by calculating the rate change of instantaneous phase
of the positive sequence since positive sequence cancels common-mode interference.
[SOURCE: IEC 60050-103:2009, 103-07-21, modified – In the definition, "of a periodic quantity"
has been deleted. Notes 1 and 2 to entry have been added.]
3.11
nominal frequency
nominal value of power frequency
Note 1 to entry: In conventional power systems, nominal frequency is normally 50 Hz or 60 Hz.
3.12
rate of change of frequency
ROCOF
first time derivative of instantaneous frequency or second time derivative of instantaneous
phase
3.13
harmonic component
rotating vector whose frequency is an integer multiple of the fundamental frequency for a signal
that is the sum of rotating vectors
Note 1 to entry: The fundamental component is the first harmonic component.
3.14
interharmonic component
rotating vector whose frequency is not an integer multiple of the fundamental frequency for a
signal that is a sum of rotating vectors

– 12 – IEC TS 62786-41:2023 © IEC 2023
3.15
sub-harmonic component
interharmonic component having harmonic order lower than one
[SOURCE: IEC 60050-103:2009, 103-07-29]
3.16
settling time
for a step response the duration of the time interval between the instant of the step change of
an input variable and the instant, when the difference between the step response and their
steady-state value remains smaller than the transient value tolerance
SEE: Figure 3
[SOURCE: IEC 60050-351:2013, 351-45-37, modified – Figure 5 inside the terminology entry
has been replaced by Figure 3.]
3.17
fast frequency response
FFR
fast active power response to frequency variations that uses a droop control
3.18
droop control
control loop to control dispatchable generators or loads to ensure
that the active power generation or consumption is a proportional function of the measured
power frequency deviation
Note 1 to entry: The proportionality factor is an inverse of the frequency droop.
3.19
frequency droop
ratio of the per-unit changes in frequency (∆f)/f to the per-unit change in power (∆P)/P
n n
σ = (∆f/f ) / (∆P/P ),
n n
where f is the nominal frequency and P is the DER or load rated power
n n
Note 1 to entry: The frequency droop is a f(P) function, whereas often used characteristic curve is P(f).
Note 2 to entry: The same principle can be applied for a voltage droop.
Note 3 to entry: The frequency gradient of a characteristic curve, which describes the power response to frequency,
is the active power change per frequency change. In a 50 Hz system, a droop of σ % can be transformed into a
gradient g % (in P /Hz) by the formula g = 200/σ; in a 60 Hz system g = 166,7/σ.
n
3.20
synthetic inertia
capability of a grid connected converter to emulate the effect of
inertia of a synchronous generator to a prescribed level of performance
Note 1 to entry: A static converter can provide
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