IEC TR 63401-1:2022

IEC TR 63401-1 ®
Edition 1.0 2022-11
TECHNICAL
REPORT
colour
inside
Dynamic characteristics of inverter-based resources in bulk power systems –
Part 1: Interconnecting inverter-based resources to low short circuit ratio AC
networks
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IEC TR 63401-1 ®
Edition 1.0 2022-11
TECHNICAL
REPORT
colour
inside
Dynamic characteristics of inverter-based resources in bulk power systems –

Part 1: Interconnecting inverter-based resources to low short circuit ratio AC

networks
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 29.020 ISBN 978-2-8322-6143-9

– 2 – IEC TR 63401-1:2022  IEC 2022
CONTENTS
FOREWORD . 6
INTRODUCTION . 8
1 Scope . 9
2 Normative references . 10
3 Terms and definitions . 10
4 Characteristics of low short circuit ratio AC networks . 12
4.1 Definition of low short circuit ratio . 12
4.1.1 General . 12
4.1.2 Low SCR in IEEE Std 1204-1997 . 13
4.1.3 Low SCR in CIGRE B4.62 TB671 . 13
4.2 Stability issues posed by inverter-based resources . 15
4.2.1 General . 15
4.2.2 Static voltage control . 16
4.2.3 Fault ride-through . 16
4.2.4 Multi-frequency oscillation . 16
4.3 Summary . 17
5 Identification of low short circuit ratio AC networks . 17
5.1 Problem statement . 17
5.2 Short circuit ratio for a single-connected REPP system . 18
5.2.1 SCR calculation with fault current . 18
5.2.2 SCR calculation with equivalent circuit . 19
5.3 Short circuit ratio for multi grid-connected WPP system . 26
5.3.1 General . 26
5.3.2 Modal decoupling method . 27
5.3.3 Circuit aggregation method . 38
5.4 Summary . 45
6 Steady state voltage stability issue for low short circuit ratio AC networks . 47
6.1 Problem statements . 47
6.2 Steady state stability analysis method . 47
6.2.1 P-V curve . 47
6.2.2 Q-V curve . 48
6.2.3 Voltage sensitivity analysis . 49
6.2.4 Relation to short circuit ratio . 54
6.3 Control strategy for inverter-based resource . 56
6.3.1 Active power and reactive power control . 56
6.3.2 Voltage control . 58
6.4 Case study . 59
6.4.1 Steady state voltage stability problem – China . 59
6.4.2 Low SCR interconnection experience – Vestas . 62
6.5 Summary . 63
7 Transient issue for low short circuit ratio AC networks . 64
7.1 Problem statement . 64
7.2 Transient characteristic modelling and analysis . 65
7.2.1 Transient stability analysis tools and limitations . 65
7.2.2 Electromagnetic transient (EMT) type models . 66
7.2.3 Transient stability analysis model requirements . 67

7.3 Fault ride-through protection and control issue. 67
7.3.1 General . 67
7.3.2 Hardware protection of inverter-based resource during fault . 68
7.3.3 Unbalancing-voltage ride-through issue . 71
7.3.4 Overvoltage ride-through control strategy . 73
7.3.5 Multiple fault ride-through . 75
7.3.6 Under and over -voltage ride-through in time sequence . 78
7.3.7 Active/reactive current support of inverter-based resource during fault . 79
7.4 Operating experiences . 80
7.4.1 Operating experience – China . 80
7.4.2 Operating experience . 81
7.5 Summary . 83
8 Oscillatory instability issue for low short circuit ratio AC networks. 83
8.1 Problem statement . 83
8.2 Modelling and stability analysis . 86
8.2.1 Analysis and modelling of the inverter in the time-domain . 86
8.2.2 Analysis and modelling of the inverter in the frequency-domain . 86
8.3 Mitigation of the oscillation issues by active damping control . 93
8.4 Cases study based on the benchmark model . 94
8.5 Summary . 99
9 Conclusions . 100
Bibliography . 101

Figure 1 – Measured voltage and current curves of sub-synchronous oscillation . 15
Figure 2 – Schematic diagram of a WPP with no static or dynamic reactive support . 19
Figure 3 – Equivalent circuit representation of the WPP shown in Figure 2 . 20
Figure 4 – A typical SIPES . 24
Figure 5 – Changes of system eigenvalues, and the weakest system eigenvalue’s
damping ratio with SCR in a SIPES. 24
Figure 6 – Schematic diagram of a WPP with static reactive support plant (capacitor
banks) . 25
Figure 7 – Equivalent circuit representation of the WPP shown in Figure 6 . 25
Figure 8 – Schematic diagram of a WPP with dynamic reactive support plant
(synchronous condensers) . 26
Figure 9 – Equivalent circuit representation of the WPP shown in Figure 8 . 26
Figure 10 – Mechanism illustration of decoupling a MIPES into a set of equivalent
SIPESs . 28
Figure 11 – A typical MIPES . 29
Figure 12 – A test wind farm system that contains nine wind turbines . 33
Figure 13 – One-line diagram of 5-infeed PES . 35
Figure 14 – Eigenvalue comparison of 5-infeed PES and its 5 equivalent SIPESs . 36
Figure 15 – The 9-converter heterogeneous system with a IEEE 39-bus network
topology . 37
Figure 16 – The dominant eigenvalues and the damping ratios . 38
Figure 17 – Nearby WPP connected to the same region in a power system. 39
Figure 18 – Equivalent representation of multiple windfarms connecting to a power
system with its Z matrix . 40

– 4 – IEC TR 63401-1:2022  IEC 2022
Figure 19 – Equivalent circuit representation of two WPPs connected to the same
connection point-configuration 2 . 41
Figure 20 – Four WPPs integrated into the system with weak connections . 42
Figure 21 – Multiple WPPs connecting to the same HV bus or HV buses in close
proximity . 43
Figure 22 – Equivalent circuit representation of WPPs connecting to the same HV bus . 43
Figure 23 – Approximate equivalent representation assumed for CSCR method . 43
Figure 24 – System topology . 47
Figure 25 – Typical P-V curve . 47
Figure 26 – System topology . 48
Figure 27 – Typical Q-V curve . 48
Figure 28 – Simplified equivalent circuit of large-scale wind power integration system . 49
Figure 29 – Voltage sensitivity at PCC of large-scale wind power integration system . 51
Figure 30 – Single generator connected to an infinite bus via grid impedance . 52
Figure 31 – P-V curves for a typical generator in a weak grid . 53
Figure 32 – Power limit curve of DFIG . 58
Figure 33 – Voltage control block diagram of the doubly-fed wind turbine . 59
Figure 34 – Network structure of Baicheng grid . 59
Figure 35 – Short circuit capacity of Baicheng network . 60
Figure 36 – P-V curves and V-Q curves . 61
Figure 37 – Reactive power of the wind farm and voltage level at the PCC . 62
Figure 38 – Schematic representation of the study system . 63
Figure 39 – Fault characteristics . 64
Figure 40 – Comparison of VER fault response between transient stability and EMT
models . 66
Figure 41 – Doubly-fed wind turbine rotor-side crowbar protection circuit topology . 69
Figure 42 – Schematic diagram of positive and negative sequence current control of
DFIG converter under grid unbalanced fault . 72
Figure 43 – Comparative analysis of simulation results . 73
Figure 44 – Overvoltage ride-through control flow diagram . 74
Figure 45 – Multiple fault conditions . 75
Figure 46 – Pitch angle control strategy . 76
Figure 47 – Typical characteristics of P and P under multiple fault ride-through . 77
m e
Figure 48 – Characteristics of P and P under multiple fault ride-through . 78
m e
Figure 49 – Under/overvoltage ride-through curve . 78
Figure 50 – Circuit diagram in Jiuquan . 80
Figure 51 – Analysis of wind power disconnection incident . 81
Figure 52 – Demonstration of voltage regulation performance during variable power
output conditions . 82
Figure 53 – Configuration of a system of multiple grid-tied VSIs . 85
Figure 54 – Control schematic diagram and structure of inverter . 87
Figure 55 – Frequency coupling in different frequency range . 90
Figure 56 – Negative resistor caused by PLL . 91
Figure 57 – Negative resistor caused by DVC . 92

Figure 58 – Equivalent circuits of the LC-filter considering the virtual resistor . 93
Figure 59 – Active damping control methods . 94
Figure 60 – Impact of virtual resistance control on the stability . 95
Figure 61 – Impact of line length on the stability . 97
Figure 62 – Impact of PLL on the stability . 98
Figure 63 – Impact of current control loop on the stability . 99

Table 1 – Rated capacity of PEDs in 5-infeed PES in p.u. . 35
Table 2 – Network parameters of 5-infeed PES in p.u. . 36
Table 3 – Relationship between equivalent SIPESs and eigenvalues of Y in 5-infeed PES 36
eq
Table 4 – Control parameters of converters . 37
Table 5 – Wind capacity and SCR values assuming no interaction . 42
Table 6 – The definition of different MISCRs . 45
Table 7 – Comparison of SCR methods . 46
Table 8 – Wind farm’s maximum power under different conditions . 61
Table 9 – New oscillation issues of power systems in the world . 83
Table 10 – Detailed influence frequency ranges of every loop . 89
Table 11 – Approximate distribution of high frequency negative damping range . 93
Table 12 – Typical cases of weak grid parameters . 96

– 6 – IEC TR 63401-1:2022  IEC 2022
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
DYNAMIC CHARACTERISTICS OF INVERTER-BASED
RESOURCES IN BULK POWER SYSTEMS –

Part 1: Interconnecting inverter-based resources
to low short circuit ratio AC networks

FOREWORD
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rights. IEC shall not be held responsible for identifying any or all such patent rights.
IEC TR 63401-1 has been prepared by subcommittee SC 8A: Grid integration of renewable
energy generation, of IEC technical committee TC 8: Systems aspects of electrical energy
supply. It is a Technical Report.
The text of this Technical Report is based on the following documents:
Draft TR Report on voting
8A/109/DTR 8A/113/RVDTR
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 Report 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 63401 series, published under the general title Dynamic
characteristics of inverter-based resources in bulk power systems, 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
contains colours which are considered to be useful for the correct understanding of its
contents. Users should therefore print this document using a colour printer.

– 8 – IEC TR 63401-1:2022  IEC 2022
INTRODUCTION
As the penetration of inverter-based energy generating resources increases, huge challenges
to all sections of the power system including planning, operation, control, etc. have been
created. The impact on the power grid extends from local to the whole power system. New
technical solutions are needed to address the different challenges. The solutions will include
the new technologies, methods and practices, to provide more flexibility and improve the
efficiency of power systems, constantly balancing generation and load.
The purpose of this document (TR) is to specifically focus on information collection from
regulatory agencies, including specifying low short circuit ratio AC networks and the challenges
they pose for inverter-based resources, and methods, indexes, and characteristics of low short
circuit ratio AC networks. This TR addresses renewable energy (RE) integration in low short
circuit ratio AC networks, mainly focusing on the technology development trends, best practices
of RE grid integration, and future standardization activities.
The aim of this TR is to create a strategic, technically oriented and referenced document, which
presents the core and key issues of interconnecting inverter-based resources to low short circuit
ratio AC networks. Renewable energy station developers and owners, transmission systems
operators need to have a common understanding of the key issues based on practices and
challenges between inverter-based resources and AC networks.

DYNAMIC CHARACTERISTICS OF INVERTER-BASED
RESOURCES IN BULK POWER SYSTEMS –

Part 1: Interconnecting inverter-based resources
to low short circuit ratio AC networks

1 Scope
As the use of inverter-based RE power generation resources increases, the use of low short
circuit ratio AC networks is becoming more common. Considering the advantages of short circuit
ratio in stability analysis, the low short circuit ratio is an important indication for describing weak
AC networks. This document focuses on technologies and standardization aspects of
interconnecting inverter-based resources to low short circuit ratio AC networks. A clear
definition of low short circuit ratio AC networks with or without a high proportion of inverter-
based resources and the calculation method is described. The adaptability of traditional
modelling and analytical method for low short circuit ratio AC networks are discussed. Some
new characteristics and challenges will be re-examined, and some adapted control strategies
will be studied. This document covers the following major aspects.
In terms of defining a weak AC network, for example the (X/R) ratio, voltage sensitivity, system
inertia and the short circuit ratio (SCR) are important characteristics. The definition of low short
TM 1
circuit ratio AC networks in IEEE Std 1204 -1997 [1] and in CIGRE B4.62 TB671 [2] is used.
Some stability challenges for inverter-based resources in a low short circuit ratio AC network
(SCR AC) will be analyzed. There are stability challenges in a low short circuit ratio (SCR) AC
network, typically complex static voltage control, risk of failure in fault ride-through situations,
strong control interactions and instability.
In terms of identification of low short circuit ratio (SCR) AC networks, some short circuit ratio -
like index for various applications is introduced. A wind power plant (WPP) is a power station
consisting of a batch of wind turbines or groups of wind turbines, collection lines, main step-up
transformers and other equipment. For a single grid-connected WPP system, a fault current
based calculation method and an equivalent circuit based calculation method are introduced to
make an SCR calculation possible for any given WPP and network topology. For multi grid-
connected WPP systems, eigenvalue decomposition based generalized short circuit ratio
(gSCR) is then proposed and compared against other approaches referred to as equivalent
short circuit ratio (ESCR), composite short circuit ratio (CSCR), and weighted short circuit ratio
(WSCR).
In terms of large scale inverter-based resources integration, the steady-state stability analysis
methods, including the P-V curve, Q-V curve, and voltage sensitivity analysis, are illustrated.
The conventional control strategies of the renewable energy sources are explained. An adaptive
controller designed for the photovoltaic (PV) panels, which can maximize the power output
capability of PV stations under weak-grid conditions, is presented. Finally, the steady-state
voltage stability problem in China that happened recently is illustrated.
___________
Numbers in square brackets refer to the Bibliography.

– 10 – IEC TR 63401-1:2022  IEC 2022
In terms of the transient state stability issue for low short circuit ratio AC networks after large
scale inverter-based resources integration, related issues and phenomena that occur need to
be discussed. Undervoltage ride-through (UVRT), overvoltage ride-through (OVRT) and
multiple fault ride-through occur easily in a low SCR AC network, which bring risk of failure to
fault ride-through. Electromagnetic transient simulations to supplement positive sequence
root-mean-square (RMS) simulations are described, and shortfalls of the RMS models and how
to identify them in simulations are considered.
In terms of the oscillatory stability issue for low short circuit ratio AC networks after large scale
inverter-based resources integration, the impedance-based method is used to analyze the
system stability. For the inverter modelling, three typical inverter models are established,
including: a) only considering the current controller (CC); b) considering CC and phase-locked
loop (PLL); c) considering CC, PLL and voltage controller (VC). Relying on the impedance
analysis method, the effect of PLL, CC, number of inverters, SCR of AC grid is discussed.
Finally, the additional active damping control method is proposed for suppressing the oscillation
phenomenon.
This document discusses the challenges of connecting inverter-based resources to low short
circuit ratio AC networks, key technical issues and emerging technologies. There are the
steady-state stability issue, transient state stability issue, and oscillatory stability issue, which
are the most distinct differences compared to inverter-based resources or traditional generators,
and accordingly brings new challenges to operation, control, protection, etc. Therefore,
technical solutions are needed. The potential solutions will include new technologies, methods
and practices, in order to provide more flexibility and improve the efficiency of power systems.
It is expected that this document can also provide guidance for further standardization on
relevant issues.
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.
IEC 62934, Grid integration of renewable energy generation – Terms and definitions
3 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC 62934 and the
following 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
renewable energy
RE
primary energy, the source of which is constantly replenished and will not become depleted
Note 1 to entry: Examples of renewable energy are: wind, solar, geothermal, hydropower, etc.
Note 2 to entry: Fossil fuels are non-renewable.
[SOURCE: IEC 60050-617:2009, 617-04-11]

3.2
inverter
electric energy converter that changes direct electric current to single-phase or polyphase
alternating currents
[SOURCE: IEC 60050-151:2001, 151-13-46]
3.3
point of common coupling
PCC
point in an electric power system, electrically nearest to a particular load, at which other loads
are, or may be, connected
Note 1 to entry: These loads can be either devices, equipment or systems, or distinct customer's installations
[SOURCE: IEC 60050-614:2016, 614-01-12]
3.4
short circuit current of renewable energy power plant
current that a renewable energy power plant delivers to the point of connection resulting from
a short circuit in the external electric power system
3.5
short circuit ratio
SCR
ratio of the three-phase short circuit power at PCC to the nominal active power of a renewable
energy power plant or generating unit
Note 1 to entry: SCR is a common analytical indicator used in the industry to quantify system strength.
Note 2 to entry: There is no industry consensus on the exact definition and methodology for calculating the SCR,
particularly for applications with several adjacent renewable energy power plants, or for a renewable energy power
plant adjacent to HVDC terminals, see CIGRE B4.62 TB 671 [2].
3.6
voltage dip
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]
3.7
fault ride-through
FRT
ability of a generating unit or power plant to stay connected during specified faults in the electric
power system
3.8
electrical simulation model
set of mathematical formulae or logical functions used in time or frequency domain digital
simulations which describe the dynamic characteristics of a facility or certain equipment
3.9
electromechanical simulation
RMS simulation
dynamic simulation method based on the RMS model, which usually focuses on the
electromechanical processes of the electric power system under disturbance, and where the
typical observation time interval is from several seconds to tens of seconds after disturbance

– 12 – IEC TR 63401-1:2022  IEC 2022
3.10
electromagnetic transient simulation
EMT simulation
dynamic simulation method to model the electromagnetic transient behaviour of an electric
power system, where instantaneous values are used in the process, and the typical observation
time interval is from several microseconds to several seconds after a disturbance
3.11
steady state stability
ability of generators in a system to keep operating synchronously and transit to a new stable
operating state or recover to the original stable operating state under a small power system
disturbance
3.12
transient stability
ability of generators in a system to keep running synchronously and transit to a new stable
operating state or recover to the original stable operating state under a large power system
disturbance
3.13
sub-synchronous oscillation
electrical oscillation occurring in an electric power system at a frequency smaller than the
nominal system frequency and generally sustained for a minute or more
3.14
sub-synchronous resonance
resonance between adjacent equipment in an electric power system, generating oscillations at
a frequency smaller than the nominal system frequency and generally sustained for a minute or
more
3.15
low-frequency oscillation
electrical oscillation occurring in an electric power system at a frequency usually between 0,1
Hz and 3 Hz
Note 1 to entry: According to an extensive survey of IEEE technical literature, the range 0,1 Hz to 3 Hz covers the
majority of low-frequency oscillation events.
4 Characteristics of low short circuit ratio AC networks
4.1 Definition of low short circuit ratio
4.1.1 General
The strength of a power system is a metric used to describe the ability of a power system to
maintain the core characteristics through which it interacts with a connection, namely voltage
and frequency, as steadily as possible, under all operating conditions. The strength or
weakness of a power system is a relative concept and needs to be addressed both in terms of
the system characteristics at a given connection point as well as the size of renewable energy
to be connected to the connection point.
As indications of system strength, the (X/R) ratio of the system impedance seen from the
connection point and the concept of available fault level have also been used. The ability to
stably transfer power over a weak transmission system, from a renewable energy station
connecting point to stronger parts of a network (where generally the load is) has been quantified
by using the sensitivity of the connection point's voltage to the active and reactive power outputs
of the renewable energy station. The maximum stable power transfer capability has been
derived, providing an insight for renewable energy station designers of the potential issues to
be anticipated when power transfer reaches the maximum transfer limits.

Short circuit ratio (SCR), which is a commonly used metric for quantifying the relative power
system impedance seen from a connection point, is an important indication of the strength of
AC networks. The SCR seen by a generator strongly influences its ability to operate
satisfactorily both in steady state and after system disturbances. While this is a very powerful
and simple concept, extending its use to describe the shared impedance seen by multiple
renewable energy power plants (REPPs) connecting to the same part of a network, electrically
close to each other, or close to other power electronic plants such as HVDC converters, has
not been unified across the industry.
4.1.2 Low SCR in IEEE Std 1204-1997
SCR is a widely used index for assessment of the strength of the connection point for HVDC
links, and in particular for the line current commutated (LCC) and capacitor commutated
converter (CCC) technologies. For an HVDC link it is defined as the ratio of fault level at the
connection point to the nominal output active power of the link. It is commonly used at the
planning stage to give an idea of the likely issues caused by integration of the HVDC link into
the network. The SCR seen by a generator strongly influences its ability to operate satisfactorily
both in steady state and following system disturbances. While this is a very powerful and simple
concept, extending its use to describe the shared impedance seen by multiple REPPs
connecting to the same part of a network, electrically close to each other, or close to other
power electronic plant such as HVDC converters, has not been unified across the industry.
The SCR can be often obtained from the following formula:
S
SCR=
(1)
P
N
where
S is the AC system three-phase symmetrical short circuit level in megavolt-amperes (MVA)
at the convertor terminal AC bus with 1,0 p.u. AC terminal voltage;
P is the rated AC power in megawatts (MW).
N
Based on that definition and on typical inverter characteristics (such as the value of the
convertor transformer reactance), the following SCR values can be used to classify an AC/DC
system:
• a high SCR AC/DC system is categorized by a SCR value greater than 3;
• a low SCR AC/DC system is categorized by a SCR value between 2 and 3;
• a very low SCR AC/DC system is categorized by a SCR value lower than 2.
4.1.3 Low SCR in CIGRE B4.62 TB671
Compared to an HVDC link, a WPP has a significantly more complex topology due to the use
of several wind turbines (which may or may not be identical), and the common use of static
reactive support plants such as capacitor banks and dynamic reactive support plants such as
STATCOMs and synchronous condensers. Additionally, it sometimes happens that several
REPPs are located adjacent to each other, or even sometimes connected to the same
connection point. This makes it imperative to evaluate the impact of all connected REPPs. The
two aspects addressed above are difficult to account for using the conventional calculation
method for the SCR. To make it applicable to any given REPP and network topology, an
equivalent circuit based calculation method (ESCR) is proposed as elaborated below for various
configurations. For multi REPP applications results obtained from this approach will then be
compared against two other approaches referred to as composite short circuit ratio (CSCR) and
weighted short circuit ratio (WSCR).
1) Composite short circuit ratio (CSCR)

– 14 – IEC TR 63401-1:2022  IEC 2022
Index based on short circuit ratio, which calculates an aggregate SCR for multiple renewable
energy power plants by creating a common bus and tying all renewable energy power plants of
interest together at that common bus.
The CSCR can be obtained from the following formula:
S
kv
CSCR=
N
(2)
P
∑
ni
i=1
where
S is the short circuit power at the virtual common bus without current contribution from the
kv
renewable energy power plants;
P is the nominal power of renewable energy power plant i;
ni
N is the number of the renewable energy power plants to be considered.
Composite short circuit ratio is used to estimate the equivalent system impedance seen by
multiple renewable energy power plants.
2) Weighted short circuit ratio (WSCR)
Another appropriate index for the calculation of impact of adjacent REPPs is the weighted short
circuit ratio (WSCR), defined by:
N
SP×
∑ SCMVAiiRMW
(3)
i=1
WSCR=
()P
RMWi
where
S is the short circuit capacity at bus i before the connection of REPPi;
SCMVAi
P is the MW rating of renewable energy station i to be connected;
RMWi
N is the number of REPPs fully interacting with each other;
i is the renewable energy station index.
The proposed WSCR calc
...