IEC TS 62898-3-3:2023

Microgrids - Part 3-3: Technical requirements - Self-regulation of dispatchable loads

IEC TS 62898-3-3:2023 deals with frequency and voltage stabilization of AC microgrids by dispatchable loads, which react autonomously on variations of frequency and voltage with a change in active power consumption. Both 50 Hz and 60 Hz electric power systems are covered. This document gives requirements to emulate the self-regulation effect of loads including synthetic inertia.
The loads recommended for this approach are noncritical loads, this means their power modulation will not significantly affect the user as some kind of energy storage is involved which effectively decouples end energy use from the electricity supply by the electric network. The self-regulation of loads is beneficial both in island mode and grid-connected mode. This document gives the details of the self-regulation behaviour but does not stipulate which loads shall participate in this approach as an optional function.
This document covers both continuously controllable loads with droop control and ON/OFF switchable loads with staged settings. The scope of this document is limited to loads connected to the voltage level up to 35 kV. Reactive power for voltage stabilization and DC microgrids are excluded in this document.

General Information

• Status: Published
• Publication Date: 15-Aug-2023
• ICS: 29.240.01 - Power transmission and distribution networks in general
• Technical Committee: SC 8B - Decentralized electrical energy systems
• Drafting Committee: JWG 1 - TC 8/SC 8B/JWG 1
• Current Stage: PPUB - Publication issued
• Start Date: 16-Aug-2023
• Completion Date: 17-Jul-2023

Overview

IEC TS 62898-3-3:2023 is a Technical Specification from the IEC that defines technical requirements for the self-regulation of dispatchable loads in AC microgrids. The document focuses on autonomous load behaviour that supports frequency stabilization and voltage stabilization by changing active power consumption (including emulation of synthetic inertia). It applies to both 50 Hz and 60 Hz systems and to loads connected up to 35 kV. Reactive power provision and DC microgrids are explicitly excluded.

Key topics and technical requirements

• Self-regulation behaviour: Requirements for loads that autonomously adjust active power in response to frequency and/or voltage deviations.
• Load types covered: Continuously controllable loads using droop control and ON/OFF switchable (staged) loads with hysteresis.
• Synthetic inertia: Guidance to emulate inertia-like response from dispatchable loads to support transient stability.
• Operational parameters: Definitions and requirements for dead band, measurement accuracy and resolution, step response objectives, damping and performance/time quality levels.
• Control strategies: Hybrid control approaches that combine voltage and frequency response, prioritization and desynchronization strategies for staged loads.
• Testing and validation: Procedures and criteria for frequency and voltage response tests, and test environment requirements.
• Scope limits: Applies to noncritical loads that can be modulated without significant user impact (often enabled by local energy storage), and only for AC networks up to 35 kV.

Practical applications and users

Who uses IEC TS 62898-3-3:2023:

• Microgrid designers and system integrators implementing autonomous stabilizing functions.
• Utility operators and distribution engineers seeking additional local frequency/voltage support.
• Manufacturers of smart loads, load controllers and inverter/EMS firmware that implement droop or staged load response.
• DER aggregators and energy storage integrators coordinating load-based ancillary services.
• Researchers and consultants developing synthetic inertia and local stability solutions.

Practical benefits:

• Improved local frequency and voltage stability in both islanded and grid-connected microgrids.
• Enhanced resilience by leveraging dispatchable noncritical loads to provide fast active-power support.
• Option to reduce reliance on conventional spinning reserves or centralized controls.

Related standards

• Part of the IEC 62898 microgrids family - consult other parts of the series for complementary system-level and component requirements.
• Normative references listed in the specification should be checked for measurement, testing and terminology consistency.

Keywords: IEC TS 62898-3-3:2023, microgrids, self-regulation, dispatchable loads, frequency stabilization, voltage stabilization, synthetic inertia, droop control, islanded microgrid, grid-connected.

Standards Content (Sample)

IEC TS 62898-3-3 ®
Edition 1.0 2023-08
TECHNICAL
SPECIFICATION
colour
inside
Microgrids –
Part 3-3: Technical requirements – Self-regulation of dispatchable loads

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IEC TS 62898-3-3 ®
Edition 1.0 2023-08
TECHNICAL
SPECIFICATION
colour
inside
Microgrids –
Part 3-3: Technical requirements – Self-regulation of dispatchable loads

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 29.240.01  ISBN 978-2-8322-7336-4

– 2 – IEC TS 62898-3-3:2023 © IEC 2023
CONTENTS
FOREWORD . 4
INTRODUCTION . 6
1 Scope . 7
2 Normative references . 7
3 Terms, definitions, abbreviated terms and symbols . 7
3.1 Terms and definitions . 8
3.2 Abbreviated terms and symbols . 16
3.2.1 Abbreviated terms . 16
3.2.2 Symbols . 16
4 Requirements on self-regulation . 17
4.1 General . 17
4.1.1 Operational ranges . 17
4.1.2 Continuous and discrete control . 17
4.1.3 Dead band . 18
4.1.4 Accuracy and resolution . 18
4.1.5 Step response objective . 19
4.1.6 Damping . 20
4.2 Frequency stabilization . 20
4.2.1 General . 20
4.2.2 Continuously controllable loads . 21
4.2.3 Switchable loads. 23
4.2.4 Recommended default values . 24
4.3 Voltage stabilization . 24
4.3.1 General . 24
4.3.2 Continuously controllable loads . 25
4.3.3 Switchable loads. 26
4.3.4 Recommended default values . 27
4.4 Hybrid controls for both voltage and frequency . 28
5 Testing . 28
5.1 General . 28
5.2 Test for frequency response of self-regulated loads . 30
5.2.1 Purpose . 30
5.2.2 Procedure . 30
5.2.3 Criteria . 30
5.2.4 Comments . 30
5.3 Test for voltage response of self-regulated loads . 30
5.3.1 Purpose . 30
5.3.2 Procedure . 30
5.3.3 Criteria . 31
5.3.4 Comments . 31
Annex A (informative) Background information about the self-regulation effect . 32
Annex B (informative) Choice of coefficients k and k . 34
f U
B.1 General . 34
B.2 Expression of coefficient k for self-regulation of frequency . 34
f
B.3 Example of frequency settings in an isolated microgrid . 35
B.4 Example of frequency settings in a large interconnected network . 36

B.5 Expression of coefficient k for self-regulation of voltage. 37
U
B.6 Example of voltage settings in an isolated microgrid . 38
Annex C (informative) Prioritization of loads . 40
Annex D (informative) Damping measure in electric power systems . 44
Annex E (informative) Formula development on the relation of power and torque . 46
Annex F (informative) Examples for desynchronisation strategies . 47
F.1 General . 47
F.2 Heterogeneous load types . 47
F.3 Fuzzy or randomized control logic . 47
F.4 Emulating continuously controllable loads . 47
Bibliography . 48

Figure 1 – Hysteresis curve of a switchable load . 10
Figure 2 – Typical step response of a system . 12
Figure 3 – Example of P(f) self-regulation before and after activating the dead band . 18
Figure 4 – Bode diagram of a typical differential loop . 21
Figure 5 – Time domain response of first order low-pass filter . 22
Figure 6 – Functional diagram of a combined frequency control function for
continuously controllable dispatchable loads . 22
Figure 7 – Example of a hysteresis controller to control the temperature of a freezer in
response to variations in grid frequency . 23
Figure 8 – Functional diagram of a combined voltage control function for continuously
controllable dispatchable loads . 26
Figure 9 – Schematic diagram for the test environment of a self-regulated load . 28
Figure A.1 – Frequency development after a disturbance . 32
Figure A.2 – Particle model of switchable loads . 33
Figure B.1 – Example of P(f) self-regulation in an isolated microgrid . 36
Figure B.2 – Example of P(f) self-regulation in a large interconnected network . 37
Figure B.3 – Example of P(U) self-regulation in an isolated microgrid . 39
Figure C.1 – Frequency distribution of the power frequency of a 50 Hz network . 40
Figure C.2 – Four different droop curves according to prioritization . 41
Figure C.3 – Schematic representation of voltage probability distribution . 42
Figure D.1 – Typical location for desired eigenvalues . 44

Table 1 – Declared frequency measurement accuracy levels . 18
Table 2 – Declared voltage measurement accuracy levels . 19
Table 3 – Time quality levels . 19
Table 4 – Performance quality levels . 20
Table B.1 – Relationship between k and droop for self-regulation of frequency . 35
f
Table B.2 – Relationship between k and droop for self-regulation of voltage . 38
U
Table C.1 – Frequency domain (example for 50 Hz systems) . 41
Table C.2 – Frequency domain (example for 60 Hz systems) . 41
Table C.3 – Voltage domain (example) . 43

– 4 – IEC TS 62898-3-3:2023 © IEC 2023
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
MICROGRIDS –
Part 3-3: Technical requirements –
Self-regulation of dispatchable loads

FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
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9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of patent
rights. IEC shall not be held responsible for identifying any or all such patent rights.
IEC TS 62898-3-3 has been prepared by subcommittee SC 8B: Decentralized electrical energy
systems, of IEC technical committee TC 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
8B/155/DTS 8B/172/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.

This document was drafted in accordance with ISO/IEC Directives, Part 2, and developed in
accordance with ISO/IEC Directives, Part 1 and ISO/IEC Directives, IEC Supplement, available
at www.iec.ch/members_experts/refdocs. The main document types developed by IEC are
described in greater detail at www.iec.ch/standardsdev/publications.
A list of all parts in the IEC 62898 series, published under the general title Microgrids, 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
specific document. At this date, the document will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
IMPORTANT – The "colour inside" logo on the cover page of this document indicates that it
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contents. Users should therefore print this document using a colour printer.

– 6 – IEC TS 62898-3-3:2023 © IEC 2023
INTRODUCTION
Self-regulation of loads is a phenomenon known very well to transmission system operators,
see Annex A. This effect historically emerged from the dynamic behaviour of electric motors
that were used to directly power mechanical drivetrains, for example for pumps or air blowers.
The higher the rotational speed of the drive, the more active power is used and vice versa. This
effect automatically contributes to frequency stabilization without a supervisory control.
There is also a self-regulation effect on the voltage due to resistive loads. At higher voltages,
the current through a resistive load increases and therefore the active power consumption
increases as well. This increased current also flows through the impedance of the upstream
supply network, resulting in a voltage reduction at the load’s point of connection and vice versa.
This effect helps to stabilise the voltage and is also used indirectly with power system stabilisers
(PSS). Modulated system voltage at transmission level is translated to corresponding changes
of active power consumption of loads at distribution level which dampen low frequency power
oscillations.
This document intends to emulate the above explained beneficial behaviours with dispatchable
loads, which do not affect the functionality with regard to the end user, and to make this effect
available for frequency and voltage stabilization in microgrids. Dispatchable loads can modify
the active power consumption while maintaining their functionality by keeping system
parameters within acceptable ranges. This is usually achieved by the use of an internal energy
storage, for example thermal energy storage in refrigerators, freezers, air conditioners, water
heaters, or electrical energy storage units such as batteries. As the loads respond to the
frequency and voltage they experience, no communication channels or complex control systems
are necessary to include small loads in the common task of keeping the electric system stable.

MICROGRIDS –
Part 3-3: Technical requirements –
Self-regulation of dispatchable loads

1 Scope
This part of IEC 62898 deals with frequency and voltage stabilization of AC microgrids by
dispatchable loads, which react autonomously on variations of frequency and voltage with a
change in active power consumption. Both 50 Hz and 60 Hz electric power systems are covered.
This document gives requirements to emulate the self-regulation effect of loads including
synthetic inertia.
The loads recommended for this approach are noncritical loads, this means their power
modulation will not significantly affect the user as some kind of energy storage is involved which
effectively decouples end energy use from the electricity supply by the electric network. The
self-regulation of loads is beneficial both in island mode and grid-connected mode. This
document gives the details of the self-regulation behaviour but does not stipulate which loads
shall participate in this approach as an optional function.
This document covers both continuously controllable loads with droop control and
ON/OFF-switchable loads with staged settings. The scope of this document is limited to loads
connected to the voltage level up to 35 kV. Reactive power for voltage stabilization and DC
microgrids are excluded in this document.
NOTE 1 If agreed between system operator and grid user, the self-regulating principles outlined in this document
can also be applied to loads in other electricity networks, see IEC/ISO Directives, Part 1:2023, C.4.3.2, Example 1.
NOTE 2 According to 3.1.7, critical loads with an electrical energy storage system such as an uninterruptable power
supply are considered as noncritical and therefore dispatchable.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies.
For undated references, the latest edition of the referenced document (including any
amendments) applies.
ISO/IEC 17025, General requirements for the competence of testing and calibration laboratories
3 Terms, definitions, abbreviated terms and symbols
For the purposes of this document, the following terms, definitions and abbreviated apply.
ISO and IEC maintain terminology databases for use in standardization at the following
addresses:
• IEC Electropedia: available at https://www.electropedia.org/
• ISO Online browsing platform: available at https://www.iso.org/obp

– 8 – IEC TS 62898-3-3:2023 © IEC 2023
3.1 Terms and definitions
3.1.1
accuracy
quality which characterizes the ability of a measuring instrument
to provide an indicated value close to a true value of the measurand
Note 1 to entry: This term is used in the "true value" approach. An updated term using the "uncertainty" approach
is in preparation for edition 2 of this document.
Note 2 to entry: Accuracy is all the better when the indicated value is closer to the corresponding true value.
[SOURCE: IEC 60050-311:2001, 311-06-08, modified – Note 1 to entry has been expanded.]
3.1.2
closed-loop control
process whereby one variable quantity, namely the controlled variable is continuously or
sequentially measured, compared with another variable quantity, namely the reference variable,
and influenced in such a manner as to adjust to the reference variable
Note 1 to entry: Characteristic for closed-loop control is the closed action in which the controlled variable
continuously or sequentially influences itself in the action path of the closed loop.
[SOURCE: IEC 60050-351:2013, 351-47-01, modified – Note 2 to entry has been deleted.]
3.1.3
control loop
set of elements or systems incorporated in the closed action of a closed-loop control
[SOURCE: IEC 60050-351:2013, 351-47-11, modified – Note 1 to entry has been deleted.]
3.1.4
damping coefficient
δ
-δt
positive quantity δ in the expression A e f(x) of an exponentially damped oscillation, where f(x)
is a periodic function
[SOURCE: IEC 60050-103:2009, 103-05-24]
3.1.5
damping ratio
for a linear time-invariant system described by the second order differential equation
ddxx
+ 20⋅ϑ⋅ω ⋅ +ω ⋅x=
dt
dt
the value of the coefficient ϑ,
where
t is the time;
x is a state variable of the system;
ω is the characteristic angular frequency of the system
1−ϑ
Note 1 to entry: When ϑ<1, ω = ω ∙ is the eigen angular frequency of the system.
d 0
[SOURCE: IEC 60050-351:2013, 351-45-19, modified – Note 2 to entry has been deleted.]

3.1.6
dead band
dead zone
finite range of values of the input variable within which a variation of the input variable does not
produce any measurable change in the output variable
Note 1 to entry: When this type of characteristic is intentional, it is sometimes called neutral zone.
[SOURCE: IEC 60050-351:2013, 351-45-15, modified – Note 2 to entry has been deleted.]
3.1.7
dispatchable load
noncritical load
load for which the active power consumption can be modified while maintaining the functionality
of that load within an acceptable range of parameters
Note 1 to entry: Maintaining the load’s functionality is often achieved by use of an internal energy storage.
Note 2 to entry: The use of dispatchability depends on an agreement between grid user and grid operator.
Note 3 to entry: The feature of dispatchability can be made accessible either by self-regulation or remote control.
Note 4 to entry: The reference point for the conformity assessment is the terminal of the load.
3.1.8
droop control
control loop to control dispatchable loads in such a way that the active
power consumption is a function of system frequency, voltage, or both
3.1.9
(electric) island
part of an electric power system that is electrically disconnected from the remainder of the
interconnected electric power system but remains energized from the local electric power
sources
Note 1 to entry: An electric island can be either the result of the action of automatic protections or the result of a
deliberate action.
Note 2 to entry: An electric island can be stable or unstable.
Note 3 to entry: Electric islands can be nested.
[SOURCE: IEC 60050-692:2017, 692-02-11, modified – Note 3 to entry has been added.]
3.1.10
fault ride through
FRT
ability of a load to stay connected during specified faults in the electric power system
3.1.11
(frequency) droop
ratio of the per-unit changes in frequency (Δf)/f (where f is the nominal frequency) to the per-
n n
unit change in power (ΔP)/P (where P is the reference active power):
ref ref
σ = (Δf/f ) / (ΔP/P )
n ref
Note 1 to entry: Frequency droop is f-by-P, whereas the often used characteristic curve is P(f).
Note 2 to entry: The reference active power P is either the nominal active power or the present active power.
ref
Note 3 to entry: The same principle can be applied for a voltage droop.

– 10 – IEC TS 62898-3-3:2023 © IEC 2023
Note 4 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
[SOURCE: IEC 60050-603:1986, 603-04-08, modified – The notes have been added, the
nominal power has been replaced with reference power, and the specific use has
been deleted in the term.]
3.1.12
frequency response
for a linear time-invariant system with a sinusoidal input variable in steady state of the output
variable the ratio of the phasor of the output variable to the phasor of the corresponding input
variable, represented as a function of the angular frequency ω
Note 1 to entry: The frequency response coincides with the transfer function taken on the imaginary axis of the
complex plane.
[SOURCE: IEC 60050-351:2013, 351-45-41, modified – Figure 9, Figure 10 and Note 2 to entry
have been deleted.]
3.1.13
functional diagram
symbolic representation of the actions in a system by functional blocks, summing points and
branching points linked by action lines
Note 1 to entry: The action lines do not necessarily represent physical connections, like electrical wires.
Note 2 to entry: Functional blocks, action lines, summing points, and branching points are elements of the functional
diagram.
[SOURCE: IEC 60050-351:2013, 351-44-01, modified – Figure 1, Figure 2 and Note 3 to entry
have been deleted.]
3.1.14
hysteresis
phenomenon represented by a characteristic curve which has a branch, called ascending
branch, for increasing values of the input variable, and a different branch, called descending
branch, for decreasing values of the input variable
SEE: Figure 1.
Figure 1 – Hysteresis curve of a switchable load

[SOURCE: IEC 60050-351:2013, 351-45-16, modified – The figure has been added and Note 1
to entry has been deleted.]
3.1.15
hysteresis control
two-state control
control scheme where a device is switched ON when an input variable crosses a threshold value
in a given direction, and is switched OFF when the input variable crosses another threshold
value in the opposite direction
3.1.16
(hysteresis) width
difference of the input variable between the ON and OFF switching
states
3.1.17
immunity
ability of a device, equipment or system to perform without degradation in
the presence of a voltage or frequency disturbance
3.1.18
inertia,
property of a rotating rigid body according to which it maintains
its angular velocity in an inertial frame in the absence of an external torque
[SOURCE: IEC 60050-113:2011, 113-03-02, modified –The definition has been modified for the
purpose of rotating reference system.]
3.1.19
inertia constant,
H
ratio of rotational energy stored at nominal frequency and the nominal power
H = E / P
max n
Note 1 to entry: The inertia constant H is half the mechanical starting time T .
m
3.1.20
low-pass filter
filter having a single pass band below a cut-off frequency and a stop band for higher frequencies
[SOURCE: IEC 60050-561:2014, 561-02-26, modified – Note 1 to entry has been deleted.]
3.1.21
measurand
particular quantity subject to measurement
[SOURCE: IEC 60050-311:2001, 311-01-03]
3.1.22
mechanical starting time,
T
m
time of a rotating mass from standstill to nominal frequency while being accelerated with
nominal torque
Note 1 to entry: The mechanical starting time T is twice the inertia constant H.
m
– 12 – IEC TS 62898-3-3:2023 © IEC 2023
3.1.23
microgrid
group of interconnected loads and distributed energy resources
with defined electrical boundaries forming a local electric power system at distribution voltage
levels, that acts as a single controllable entity and is able to operate in island mode
Note 1 to entry: This definition covers both (utility) distribution microgrids and (customer owned) facility microgrids.
[SOURCE: IEC 60050-617:2017, 617-04-22, modified – Reworded to avoid redundancy.]
3.1.24
overshoot
v
m
for a step response of a transfer element the maximum transient deviation from the final
steady-state value of the output variable, mostly used in the form of overshoot ratio
SEE Figure 2 (v )
m
u Input variable
U Initial value of the input variable
U Step height of the input variable
s
v Output variable
V , V Steady-state value before and after application of the step
0 ∞
v Overshoot (maximum transient deviation from the final steady-state value)
m
2·Δv Specified tolerance limit
s
T Step response time
sr
T Settling time
s
Dead time
T
t
Figure 2 – Typical step response of a system
[SOURCE: IEC 60050-351:2013, 351-45-38, modified – The second part of the definition has
been simplified and adapted to the purpose of this document.]

3.1.25
overshoot ratio
ratio between the overshoot and the difference of steady-state values before and after the
application of the step
v / (V – V )
m ꝏ 0
3.1.26
over-voltage ride through
OVRT
ability of a load to stay connected during a limited duration rise of system voltage
3.1.27
power system stability
capability of a power system to regain a steady state, characterized by the synchronous
operation of the generators after a disturbance due, for example, to variation of power or
impedance
[SOURCE: IEC 60050-603:1986, 603-03-01]
3.1.28
primary control
control of generators or loads by their individual controllers which ensures
that the active power flow is a function of the power frequency or network voltage
3.1.29
rebound effect
aggregated increase or decrease of power consumption after synchronised demand response
deactivation
Note 1 to entry: The risk of oscillating rebound effects is small when there is a high diversity in the time constants
of the relevant dispatchable loads, which define the need to switch from on to off and vice versa.
3.1.30
resolution
smallest change in the measurand, or quantity supplied, which causes a perceptible change in
the indication
[SOURCE: IEC 60050-311:2001, 311-03-10]
3.1.31
rate of change of frequency
ROCOF
first-order derivative in time of the frequency
df/dt
3.1.32
secondary control
coordinated control of the active power supplied to the network
by particular generators
Note 1 to entry: The secondary control usually has an integrative component to ensure steady state accuracy of
the controlled variable.
[SOURCE: IEC 60050-603:1986, 603-04-05, modified – Note 1 to entry has been added.]

– 14 – IEC TS 62898-3-3:2023 © IEC 2023
3.1.33
self-regulation
inherent feature of loads to react under certain conditions autonomously on
variations of frequency or voltage or both with a change in the power exchange with the electric
power network
Note 1 to entry: In this document active power is considered only, Q(U) is not covered.
3.1.34
simulated electric power system interface
grid simulator
SEPSI
assembly of test equipment with variable voltage and variable frequency output emulating a
power system at the point of connection
Note 1 to entry: Normally, a SEPSI is voltage source converter with AC/DC/AC structure.
3.1.35
state of energy
SOE
ratio between the currently stored energy from an energy storage at specified operational
conditions and the maximum storable energy when fully charged, typically expressed as a
percentage
Note 1 to entry: Stored energy is the energy which is physically contained in an energy storage. The stored energy
is independent of the charge or discharge power.
[SOURCE: IEC 62933-1:2018, 3.2.4, modified – In the term, “charge” has been changed to
“energy”, the definition now comprises all kinds of energy storage and Note 1 to entry has been
added.]
3.1.36
step response
time response of a linear time-invariant system, which initially is in steady state U , V ,
0 0
produced by application of a step function Δu (t) = K·ε(t) to one of the input variables, where
ε ε
Δv (t) = v(t) – V and Δu (t) = u(t) – U
ε 0 ε 0
Note 1 to entry: The step response of a linear time-invariant system is proportional to the time integral of its impulse
response.
[SOURCE: IEC 60050-351:2013, 351-45-27, modified – Figure 5 and Note 2 to entry have been
deleted.]
3.1.37
step response 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 output variable reaches for the first time a specified
percentage of the difference between the final and the initial steady-state value
SEE Figure 2 (T ).
sr
Note 1 to entry: In the case of PT1-behaviour of a first order low-pass filter with a transfer function K / (1+T ), the
s
output variable reaches 0,95 of the difference between the final and the initial steady-state value after 3 τ. Therefore,
the bandwidth of ± 5 % is an often-used bandwidth around the targeted value.
[SOURCE: IEC 60050-351:2013, 351-45-36, modified – The original note has been deleted and
Note 1 to entry has been added.]

3.1.38
synchronisation
aggregated response of different dispatchable loads within a
synchronous area by simultaneously switching on or off so that the adjustments of their active
power consumption coincide with each other
Note 1 to entry: Synchronisation of dispatchable loads can contribute to frequency instability.
Note 2 to entry: Desynchronisation of dispatchable loads refers to the practice of avoiding synchronisation in the
electric power system.
3.1.39
synthetic inertia
capability of a grid connected converter to emulate the effect of
inertia of a synchronous generator to a prescribed level of performance
3.1.40
time response
variation in time of an output variable, produced by a specified variation of one of the input
variables, under specified operating conditions
[SOURCE: IEC 60050-351:2013, 351-45-09, modified – Note 1 to entry has been deleted.]
3.1.41
torque
component of a moment of force M along a given axis passing through the origin point, thus
T = M⋅e, where e is the unit vector of the axis
Note 1 to entry: Torque is the twisting moment of force with respect to the longitudinal axis of a beam or shaft.
[SOURCE: IEC 60050-113:2011, 113-03-26, modified – The note to entry was reformatted.]
3.1.42
transfer function
for a linear time-invariant system the ratio of the Laplace transform of an output variable to the
Laplace transform of the corresponding input variable, with all initial values equal to zero
[SOURCE: IEC 60050-351:2013, 351-45-39, modified – Note 1 to entry has been deleted.]
3.1.43
uncertainty
parameter, associated with the result of a measurement, that
characterizes the dispersion of the values that could reasonably be attributed to the measurand
Note 1 to entry: This term is used in the "uncertainty" approach.
Note 2 to entry: The parameter can be, for example, a standard deviation (or a given multiple of it), or a half-width
of an interval having a stated level of confidence. Various ways of obtaining uncertainty are defined in ISO/IEC Guide
98.3, Guide to the expression of uncertainty in measurement (GUM).
Note 3 to entry: Uncertainty of measurement comprises, in general, many components. Some of these components
can be evaluated from the statistical distribution of the results of a series of measurements and can be characterized
by experimental standard deviations. The other components, which can also be characterized by standard deviations,
are evaluated from the assumed probability distributions based on experience or other information.
[SOURCE: IEC 60050-311:2001, 311-01-02, modified – The notes are reformatted, and GUM
in Note 2 to entry is written in full.]
3.1.44
under-voltage ride through
UVRT
ability of a load to stay connected during a voltage dip

– 16 – IEC TS 62898-3-3:2023 © IEC 2023
Note 1 to entry: In some documents the term “low voltage ride through (LVRT)”, is used for the same capability.
Note 2 to entry: As a synonym for voltage dip, voltage sag is also used.
3.1.45
voltage dip
voltage sag
sudden voltage reduction at a point in an electric power system, followed by voltage recovery
after a short time interval, from a few periods of the sinusoidal wave of the voltage to a few
seconds
[SOURCE: IEC 60050-614:2016, 614-01-08]
3.1.46
voltage droop
ratio of the per-unit change in voltage (ΔU)/U (where U is the nominal voltage) to the per-unit
n n
change in active power or reactive power
Note 1 to entry: The active power voltage droop is U(P), whereas the often-used characteristic curve is P(U).
Note 2 to entry: The reactive power voltage droop is U(Q), whereas the often-used characteristic curve is Q(U).
3.2 Abbreviated terms and symbols
3.2.1 Abbreviated terms
BIPM Bureau International des Poids et Mesures
EES Electrical energy storage
EMS  Energy management systems
ES Energy storage
EUT Equipment under test
FRT  Fault ride through
GUM Guide to the expression of uncertainty in measurement
OVRT Over-voltage ride through
RMS Root mean square
ROCOF Rate of change of frequency
SEPSI Simulated electric power system interface
SOE State of energy
THD Total harmonic distortion
UVRT Under-voltage ride through
3.2.2 Symbols
f frequency
H inertia constant
P active power
T torque
T mechanical starting time
m
T measurement time window
w
T settling time
s
T step response time
sr
T dead time
t
τ time constant
U voltage
U initial value of the input variable
U step height of the input variable
s
u input variable
v output variable
V steady-state value before application of the step
V steady-state value after application of the step
∞
v overshoot (maximum transient deviation from the final steady-state value)
m
2·∆v specified tolerance limit
s
4 Requirements on self-regulation
4.1 General
4.1.1 Operational ranges
The purpose of 4.1.1 is to define immunity ranges in which the dispatchable loads shall continue
to function as designed to support microgrid stability, notwithstanding intentional disconnecting
according to 4.2.3 and 4.3.3 (switchable loads).
NOTE 1 IEC TS 62749 indicates fluctuations of frequency and voltage which can be expected during normal
operation of power systems. During exceptional conditions, wider frequency tolerances can be applied temporarily
in order to maintain the continuity of electricity supply.
a) Frequency ranges for frequency stabilization:
1) ±5 % during continuous operation,
2) ±15 % during short events up to 5 min;
NOTE 2 In dynamic response, power swings are different in electrical islands as well as in weakly connected grids
compared to the core of a synchronous zone.
b) Voltage ranges for voltage stabilization:
1) +10 % or −15 % during continuous operation,
2) +20 % or −30 % during short events only up to 5 s.
NOTE 3 The wide shall-not-trip area even for a longer time is explained by FRT requirements including UVRT at
70 % U and OVRT at 120 % U for several seconds.
n n
The values in items a) and b) listed above are recommended as default values. Individual
microgrid designs may require different values.
NOTE 4 Within the operational ranges, the control strategies for different loads can be differentiated according to
their priorities, see Annex C.
4.1.2 Continuous and discrete control
In the field of digital controllers, the term "continuous" is defined by the sampling frequency,
the duration of the control cycle, and other dynamic parameters. In this document, the
differentiation between discrete versus continuous refers to frequency and voltage
measurement at input level.
In this document the differentiation between continuously controllable versus switchable loads
concerns mai
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