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\begin{document}

\title{Study on Re-Energization Capability of a Hybrid Windfarm under a
Microgrid-based Restoration Strategy}



\author[1]{Duncan  Maina}%
\author[2]{Nirmal-Kumar Nair}%
\affil[1]{The University of Auckland}%
\affil[2]{University of Auckland}%


\vspace{-1em}



  
  \date{May 26, 2024}


\begingroup
\let\center\flushleft
\let\endcenter\endflushleft
\maketitle
\endgroup





\selectlanguage{english}
\begin{abstract}
Under a microgrid based (bottom-up) restoration strategy, considering a
disaster related outage, the local generation is required to energize
and supply the unaffected part of the network. Considering 100\%
renewable generation, only hydro based generation systems, if well
equipped, can be able to blackstart and re-energize the network. Wind
Energy Conversion Systems (WECSs) have been restricted to the latter
stages of restoration due to their source intermittency and
non-dispatchability and in order for them to participate in the initial
restoration stages, voltage and frequency support auxiliary devices are
required. This paper investigates the capability of a hybrid windfarm to
participate in the initial stages of restoration similar to a
conventional blackstart unit: Re-energization of the various network
components and pick up of load. The auxiliary equipment in this case is
a dump load to absorb excess power produced by the windfarm and a
synchronous condenser to provide a stable voltage reference required for
normal operation of the WECS. Firstly, the nominal range of operation is
determined with an additional pitch control mode linking the dump load
power to individual WECSs introduced for frequency control. Flicker and
harmonics are investigated in determination of the hosting capacity.
Studies are undertaken to investigate energization of transformers,
underground cables, overhead lines and other non-blackstart units
(including other WECSs). Consideration is given to Type 1 and Type 3
WECSs to investigate the capability of a non-inverter and inverter based
WECS. MATLAB/Simulink has been used as the simulation platform due to
its modelling flexibility.%
\end{abstract}%



\sloppy


Study on Re-Energization Capability of a Hybrid Windfarm under a
Microgrid-based Restoration Strategy

Duncan Kaniaru Maina\textsuperscript{1}, Nirmal-Kumar C Nair
\textsuperscript{1}

\textsuperscript{1} Department of Electrical, Computer and Software
Engineering, University of Auckland, Private Bag 92019, Auckland 1142,
Auckland, New Zealand.

Correspondence

Duncan Kaniaru Maina, Email: dmai810@aucklanduni.ac.nz

\textbf{Summary:} Under a microgrid based (bottom-up) restoration
strategy, considering a disaster related outage, the local generation is
required to energize and supply the unaffected part of the network.
Considering 100\% renewable generation, only hydro based generation
systems, if well equipped, can be able to blackstart and re-energize the
network. Wind Energy Conversion Systems (WECSs) have been restricted to
the latter stages of restoration due to their source intermittency and
non-dispatchability and in order for them to participate in the initial
restoration stages, voltage and frequency support auxiliary devices are
required. This paper investigates the capability of a hybrid windfarm to
participate in the initial stages of restoration similar to a
conventional blackstart unit: Re-energization of the various network
components and pick up of load. The auxiliary equipment in this case is
a dump load to absorb excess power produced by the windfarm and a
synchronous condenser to provide a stable voltage reference required for
normal operation of the WECS. Firstly, the nominal range of operation is
determined with an additional pitch control mode linking the dump load
power to individual WECSs introduced for frequency control. Flicker and
harmonics are investigated in determination of the hosting capacity.
Studies are undertaken to investigate energization of transformers,
underground cables, overhead lines and other non-blackstart units
(including other WECSs). Consideration is given to Type 1 and Type 3
WECSs to investigate the capability of a non-inverter and inverter based
WECS. MATLAB/Simulink has been used as the simulation platform due to
its modelling flexibility.

\textbf{Keywords:} DFIG, Network Re-energization, Range of Operation,
SCIG, Hybrid Mode.

1. Introduction

There are 2 common approaches to restoration after a blackout: Bottom-up
and top-down {[}1{]}. Bottom-up approach, also referred to as microgrid
based restoration, relies on local generation with blackstart capability
to re-energize the network while top-down approach relies on
interconnecting lines from blackout-unaffected regions to re-energize
the network. Commonly used blackstart generators in the case of
microgrid formation are hydro and diesel based. Thermal based
generation, if equipped with load rejection capabilities {[}2{]}, can be
able to re-energize the network but it depends on how long the generator
has been on no-load. Activities related to network re-energization
include component (transformer, overhead lines, underground cables)
re-energization, cranking up of non-blackstart units (motor
energization) and load pickup {[}3{]}.

For the last 2 decades there has been a continuous increase in wind and
solar based generation, and this is in aiming for 100\% renewable energy
generation by 2050 {[}4{]}. At the same time, the globe is also seeing
an increase in disasters causing damage to the electricity and
interdependent infrastructure. With disaster related outages, the
trajectory of the disaster remains unknown until it happens and if a
100\% renewable generation is assumed, there is need to investigate the
role of non-hydro based renewable energy sources if they have been left
unscathed. Currently, wind and solar based generation systems are
utilized only after the core grid is stable and only to assist in
picking up load. Focus of this study will be on wind-based generation
systems/wind energy conversion systems (WECSs).

Research on the role of WECSs during restoration has been reviewed in
{[}5{]}, together with other new power system technologies. Focus on
research has been on the optimal use of windfarms during the load pickup
stage {[}6-9{]}. Use of WECSs in earlier stages using support of either
energy storage elements, voltage support devices or Voltage Source
Converter based High Voltage Direct Current (VSC-HVDC) has been proposed
in {[}10-17{]}. {[}18{]} provides a detailed analysis by identification
of limitations of the use of Type 3 WECSs in restoration from a voltage
and frequency standpoint, and the control methodology required to enable
it to blackstart. No research, as per knowledge of the author, has been
done on investigation of hybrid windfarms in re-energizing the network
and this will be the focus of this study.

A comprehensive review of different standalone WECSs has been provided
in {[}19{]}. Gearless-drive PMSG and geared-drive SCIG have been deemed
as the best schemes in terms of performance. In order to ensure a
successful standalone WECS scheme, there must be a stable voltage
reference and an energy storage element. The term `standalone' is used
when the auxiliary elements are located within the WECS. In this study,
the auxiliary equipment (synchronous condenser and dump load) are
located at the windfarm collector point thus the term `hybrid' is used.
A synchronous condenser (SC) is used to provide a stable voltage
reference and required reactive power while a dump load is used to
absorb the excess energy produced by the windfarm. Consideration is only
given to Type 1 and Type 3 WECSs as they depict a non-inverter and
inverter-based technology respectively. In order to focus on the
windfarm response, the instances of re-energization and load pickup are
assumed to occur when there is sufficient power from the WECSs.

The remainder of this paper is organized as follows: Section 2 describes
the test system and component models used. Section 3 firstly discusses
the nominal range of operation whilst looking at the different power
quality issues afterwhich the results of the different network
re-energization related activities are discussed. A brief conclusion is
provided in section 4.

2. Test System Description

The system under study is as shown in Fig. 1. It is a subsection of a
windfarm in which there are multiple strings each of 10 WECSs. The
capacity of the sub-section of the windfarm operational (WECSs
connected) is assumed to be approximately 9MW and as previously
mentioned, the WECSs can either be of Type 1 or Type 3. A comparison
will be provided between the two. This layout is applicable to an
offshore windfarm and for it to be applicable for an onshore windfarm
the HV submarine cable is substituted by an overhead line.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image1/image1}

\end{center}
\end{figure}

\textbf{Figure 1} . Test System

2.1 Type 1 WECS

Type 1 WECS is based on the squirrel cage induction generator (SCIG). In
this study, each SCIG is rated at 2.3MW and in order to achieve an
approximately 9MW capacity windfarm, 4 WECSs are considered operational.
Fig. 2 shows the general schematic of Type 1 WECS. A simple 4 staged
resistive starter {[}20{]} has been used to limit the inrush currents
associated during their synchronization to the system. Parameters of the
SCIG are obtained in {[}21{]}.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image2/image2}

\end{center}
\end{figure}

\textbf{Figure 2} . SCIG based WECS Schematic.

2.2. Type 3 WECS

Type 3 WECS is based on the doubly fed induction generator (DFIG). In
this study, each DFIG is rated at 1.5MW and in order to achieve a 9MW
capacity windfarm, 6 WECSs are considered operational. Fig. 3 shows the
general schematic. Field orientation schemes have been used for both the
rotor side controller (RSC) and grid side controller (GSC) with both
active and reactive power control achieved through the RSC using stator
voltage orientation scheme. DC link voltage control, on the other hand,
is achieved through the GSC using grid voltage orientation scheme.
Parameters of the DFIG are obtained from {[}22{]}.

\textbf{Hosted file}

\verb`image3.emf`  available at \url{https://authorea.com/users/393037/articles/506769-study-on-re-energization-capability-of-a-hybrid-windfarm-under-a-microgrid-based-restoration-strategy}

\textbf{Figure 3} . DFIG based WECS Schematic.

2.3. Synchronous Condenser

An 8 MVA synchronous condenser (SC) has been used to provide a voltage
reference for stable operation of the windfarm. It also has an added
advantage of increasing system inertia, short circuit current, short
term overload and low voltage ride through (LVRT) capabilities. In terms
of modelling, the SC model is simply a synchronous machine with no
mechanical input and a static excitation system {[}23{]}. Excitation
limiters have also been included in the excitation system to obtain a
near accurate response. Excitation limiters included and modelled are
overexcitation limiter (OEL) {[}24{]}, volt/hertz limiter (V/Hz)
{[}24{]} and underexcitation limiter (UEL) {[}25{]}.

2.4. Dump Load

A dump load has been used to absorb the excess energy produced by the
windfarm. A model similar to {[}26{]} has been used in this study. 8
three phase resistors, in series and controlled by ideal GTO switches,
are varied in steps of 17.5 KW from 0 to 4.4625 MW in steps of 17.5 KW.
They are connected in series and controlled by ideal GTO switches.
System frequency has been used as the input to control the switches that
in turn control the resistor banks.

2.5. Transformer

Different sizes of the 33/132 kV collector point step-up transformer
have been considered in the study to investigate the transformer
energization capabilities of the hybrid windfarm. Fig. 4 shows the high
frequency model of up to 100kHz proposed by {[}27{]} suitable for
transformer energization studies. Description and derivation of the
parameters have been discussed in detail in {[}28{]}. The model includes
saturation effect with hysteresis, short circuit impedance incorporating
skin effect, eddy current losses and coupling capacitances.

\textbf{Hosted file}

\verb`image4.emf`  available at \url{https://authorea.com/users/393037/articles/506769-study-on-re-energization-capability-of-a-hybrid-windfarm-under-a-microgrid-based-restoration-strategy}

\textbf{Figure 4.} Single phase Transformer Equivalent Circuit.

2.6. Transmission Line

Different lengths of overhead lines and cables have been used to
investigate the impact of transmission line (overhead lines and
underground cables) energization. In terms of overhead lines, the
lengths that have been undertaken are 50, 100, 150 and 200km lines. For
underground cables, the lengths that have been undertaken are 10, 20, 30
and 40km. Fig. 5 shows the 132kV transmission line parameters used in
this study.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image5/image5}

\end{center}
\end{figure}

\textbf{Figure 5.} Overhead line and Underground cable parameters.

For simplicity and without much loss in accuracy, multiple PI sections
model have been used have been to represent both transmission lines and
cables. Derivation of the underground cable PI section has been done as
{[}29{]}. As a rule of thumb, each individual PI section should not be
longer than (1) {[}27{]}.

\begin{equation}
\text{\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ }L_{\max}=\frac{v}{5\times f_{\max}}\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ (1)\nonumber \\
 \end{equation}

\emph{Where fmax represents the maximum frequency under study, l
represents the length of the cable and v represents the cable velocity.}

2.7. Motors

Motor energization is an important consideration associated with either
cranking of thermal based generation or cold load pick up to show the
effects of inrush currents which are about 6 times the full load rated
current. A 4th order induction motor model {[}30{]} has been used to
represent the motors.

3. Results and discussion

All across the study, voltage and frequency measurements are taken at
the onshore substation at 33 kV level. Field current and excitation
limiter outputs measurements are related to the SC. Also Type 1 WECSs
will be referred to as SCIG based WECSs while Type 3 WECSs will be
referred to as DFIG based WECSs.

3.1. Range of Operation

Pickup and drop-off of pure resistive and inductive load was undertaken
to determine the normal operating range of the windfarm. This was done
to define the limitations on the hybrid windfarm under different
scenarios.

3.1.1. Active Power

The initial connected load was taken as 4MW. At the 40th second, 3.5 MW
of load was dropped, followed by a gradual increase in windspeed from
7m/s to 9m/s at the 60th second. The aim of this was to investigate the
worst-case scenario if there is excess generation at low loading. At the
75th second, 1.5 MW of load is picked up followed by generation decrease
caused by wind gradually decreasing from 9m/s to 7m/s, starting from the
90th second. The aim of this was to investigate the response on impact
of sequential load increase and reduction in generation. Fig. 6 shows
the response.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image6/image6}

\end{center}
\end{figure}

\textbf{Figure 6} . Response of both SCIG and DFIG based WECSs on the
described scenario.

It can be seen that at excess generation conditions and maximum dump
load absorption capacity, the system would collapse on overfrequency for
the SCIG based system and overvoltage for the DFIG based system. The
DFIG based system tried to regulate its power output through the pitch
control system but it was not enough to avoid the overfrequency which
happened at a later stage as compared to the SCIG based system. The
simulation was discontinued at the 80th second due to the collapse thus
other events are not shown. An additional simple pitch regulation scheme
dependent on the dump load power is proposed. It is implemented using a
PI controller obtaining its input from dump load power and if it reaches
80\% of its capacity, the pitch angle is increased to de-load the WECS.
Fig. 7 shows the improved network response.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image7/image7}

\end{center}
\end{figure}

\textbf{Figure 7.} Response of both SCIG and DFIG based WECSs on the
described scenario.

It is seen that there is significant improvement in the voltage and
frequency response thus coordination is required between the dump load
and the WECSs. The DFIG based system performed better in its frequency
response due to presence of MPPT that prevents short term variability
affecting its operation. The response from the SCIG based system is slow
and the additional pitch regulation is not enough during the described
scenario thus depending on the overfrequency protection settings, the
hybrid windfarm operation would collapse. The dump load power is kept
within its limits as compared to the previous scenario. The pitch angle
increased as compared to the previous scenario to de-load the WECS in
trying to keep the frequency within acceptable limits.

3.1.2. Reactive Power

A similar exercise as above was conducted to assess the response of the
network on pick up of theoretically near-pure inductive load (for
example unloaded transformer and motor). In the case of SCIG based
system, the reactive power source in the system is the SC and available
reactive compensation in the form of capacitor banks while for the DFIG
based system, the reactive power source is still the SC and DFIG itself
which has an reactive power operational range of \selectlanguage{ngerman}±30\% of its rated
capacity. The system capability to pick up reactive load, considering
both SCIG and DFIG based systems, is dependent on the network's level of
excitation at the time of load pickup. Fig. 8 shows the response on the
SCIG based system on pickup of different sizes of inductive load at
different levels of excitation.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image8/image8}

\end{center}
\end{figure}

\textbf{Figure 8.} SCIG based system response on pure inductive load
pickup with 90\% level of excitation.

There is significant rise in the field current as expected and above the
1.05pu OEL activation limit for inductive loads of 1MVAR and above. This
prompts the OEL to reduce the AVR reference voltage in turn reducing the
terminal voltage of the SC and in turn of the system. The allowable
steady state minimum voltage is 0.9pu which is when the ride through
strategies that are meant for short term voltage sags are activated. The
level of excitation can be lowered to 50\% by adjusting the SC AVR or by
energization of more capacitor banks. Although lowering of the AVR
reference would lead to lowering of the excitation level, the system
voltage would also decrease, and this is not desirable. It is thus
preferred to energize a capacitor bank as it would lower the excitation
level and at the same time increase the voltage. In this case, a 1.4
MVAr capacitor bank was energized (not shown but energized at
10\textsuperscript{th} second). This drove the SC to operate in
underexcitation mode, so as to pick up more reactive load. Fig. 9 show
the results obtained:\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image9/image9}

\end{center}
\end{figure}

\textbf{Figure 9.} SCIG based system response on pure reactive load
pickup with 90\% level of excitation.

Significant improvements are observed and now 2 and 1MVAr inductive
loads can be picked up with the SC in underexcited mode. The OEL acted
swiftly on picking up of the 4MVAr load. This led to significant drop in
voltage which would lead to system collapse. The action on the pickup of
the 2MVAr load was minor and the voltage still remained within
acceptable technical limit. It is thus recommended for the system to
have 2MVAr as the limit for the SCIG based hybrid system.

A similar exercise is conducted for the DFIG based hybrid system. Fig.
10 shows the results obtained at 100\% excitation level. As observed,
the pickup of even the smallest inductive load block prompted the OEL to
reduce the voltage but not to low levels as compared to the SCIG based
system. This is due to rotor control with the q component of rotor
current re-adjusting to maintain the voltage to within acceptable
limits. Also, the inductive load is picked up by both the DFIG and SC.
On lowering the level of excitation to 50\%, by adjusting the DFIG
reference reactive power so as to drive the SC in underexcitation mode,
Fig. 11 shows the results obtained. Significant improvements are
observed and now for 2, 1 and 0.5MVAr blocks, the voltages do not drop
below 1pu. The OEL still acted significantly on picking up of the 4MVAr
load. It is thus recommended for the system to have 2MVAr, similar to
SCIG based system, as the limit for the DFIG based system.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image10/image10}

\end{center}
\end{figure}

\textbf{Figure 10.} DFIG based system response on pure reactive load
pickup at 100\% level of excitation.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image11/image11}

\end{center}
\end{figure}

\textbf{Figure 11.} DFIG based system response on pure reactive load
pickup at 50\% level of excitation.

3.1.3. Power Quality Assessment

The 2 common issues related to power quality are flicker and harmonics.
Flicker is defined as `an impression of unsteadiness of visual sensation
induced by a light stimulus whose luminance or spectral distribution
fluctuates with time' {[}31{]}. Harmonics, on the other hand, is defined
as `sinusoidal voltages or currents having frequencies that are integer
multiple of the frequency at which the supply system is designed to
operate' {[}31{]}.

Flicker assessment methodology has been described in IEC 61000-4-15
{[}32{]}. Two common metrics are used: Short term flicker taken over a
maximum 10-minute period and long-term flicker taken as a cubic mean of
the short-term flicker values over a 2-hour period. These metrics are
normalized to between 0 and 1 where 1 means `\emph{voltage fluctuations
sufficient to cause noticeable flicker to 50\% of a sample observing
group} ' {[}32{]}. Flicker was investigated by assuming a \selectlanguage{ngerman}±2m/s wind
speed range within a period of 5 seconds. Fig. 12 shows the network
response.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image12/image12}

\end{center}
\end{figure}

\textbf{Figure 12} . Response of SCIG and DFIG based system to rapid
wind speed changes.

From the voltage and frequency observations, it is observed that the
DFIG based system responded better to rapid wind fluctuations compared
to the SCIG based system of which its frequency goes above the
acceptable technical limit of 50.5Hz. The pitch regulation of the SCIG
is much slower as compared to the DFIG based system. The DFIG has an
additional control system in form of the MPPT that enables it to track
and optimize the available windspeed with the turbine characteristics,
thus aiding in pitch control. With regards to the flicker measurement,
the SCIG based system operated beyond the acceptable technical limit of
1 while the DFIG remained below 1 during the 5-second period. Although
this study does not cover flicker mitigation, an active voltage
controller, for example a STATCOM, is required to counter the flicker
especially for the SCIG based system.

Harmonic assessment and acceptable harmonic limits have been formalised
by IEEE standard 519 {[}33{]}. The measurement window is recommended to
be over a 10 cycle period. The technical limits for a voltage signal at
33kV are 5\% for THD and 3\% for any single harmonic. Table 1 below
shows the THD for both SCIG and DFIG based systems for a period of 4
seconds.

\textbf{Table 1.} THD for both SCIG and DFIG based Hybrid System.\selectlanguage{english}
\begin{longtable}[]{@{}lllllll@{}}
\toprule
\textbf{Scenario} \textbf{Time} & \textbf{4 SCIGs} \textbf{SC-8MVA} &
\textbf{5 SCIGs} \textbf{SC-8MVA} & \textbf{6 SCIGs} \textbf{SC-10MVA} &
\textbf{6 DFIGs} \textbf{SC-8MVA} & \textbf{7 DFIGs} \textbf{SC-8MVA} &
\textbf{8 DFIGs} \textbf{SC-10MVA}\tabularnewline
\midrule
\endhead
\textbf{30.0} & 0.09 & 0.20 & 0.54 & 1.20 & 1.25 & 1.07\tabularnewline
\textbf{30.2} & 0.14 & 0.25 & 0.47 & 1.11 & 1.30 & 1.03\tabularnewline
\textbf{30.4} & 0.19 & 0.29 & 0.37 & 1.11 & 1.19 & 1.01\tabularnewline
\textbf{30.6} & 0.23 & 0.33 & 0.29 & 1.09 & 1.18 & 0.99\tabularnewline
\textbf{30.8} & 0.26 & 0.35 & 0.20 & 1.06 & 1.28 & 1.02\tabularnewline
\textbf{31.0} & 0.29 & 0.38 & 0.11 & 1.05 & 1.26 & 1.07\tabularnewline
\textbf{31.2} & 0.32 & 0.38 & 0.03 & 1.12 & 1.16 & 1.04\tabularnewline
\textbf{31.4} & 0.32 & 0.38 & 0.07 & 1.20 & 1.10 & 1.00\tabularnewline
\textbf{31.6} & 0.32 & 0.37 & 0.15 & 1.17 & 1.12 & 1.01\tabularnewline
\textbf{31.8} & 0.31 & 0.35 & 0.24 & 1.17 & 1.11 & 1.00\tabularnewline
\textbf{32.0} & 0.30 & 0.32 & 0.30 & 1.07 & 1.16 & 1.05\tabularnewline
\textbf{32.2} & 0.29 & 0.30 & 0.37 & 1.02 & 1.23 & 1.04\tabularnewline
\textbf{32.4} & 0.26 & 0.25 & 0.42 & 1.04 & 1.35 & 1.02\tabularnewline
\textbf{32.6} & 0.23 & 0.22 & 0.46 & 1.08 & 1.35 & 1.00\tabularnewline
\textbf{32.8} & 0.20 & 0.18 & 0.49 & 1.17 & 1.21 & 1.06\tabularnewline
\textbf{33.0} & 0.17 & 0.14 & 0.51 & 1.21 & 1.14 & 1.03\tabularnewline
\textbf{33.2} & 0.14 & 0.10 & 0.52 & 1.19 & 1.15 & 1.02\tabularnewline
\textbf{33.4} & 0.11 & 0.06 & 0.51 & 1.14 & 1.18 & 1.02\tabularnewline
\textbf{33.6} & 0.09 & 0.02 & 0.49 & 1.11 & 1.17 & 1.04\tabularnewline
\textbf{33.8} & 0.07 & 0.02 & 0.46 & 1.17 & 1.20 & 1.04\tabularnewline
\textbf{Average} & 0.2165 & 0.2445 & 0.3500 & 1.1240 & 1.2045 &
1.0280\tabularnewline
\bottomrule
\end{longtable}

As expected, the DFIG contribution is higher, but still within the
technical limits, as compared to SCIG due to the inverter interface at
the rotor side. With connection of more DFIGs, the THD will also
increase unless a higher capacity SC is connected to the network.

Flicker (for the SCIG based system) and Harmonic assessment (for the
DFIG based system) can both be used in determining the hosting capacity
of the hybrid system.

3.2. Cranking up of other WECS

A study is undertaken to determine the effect of starting other WECSs in
the same string as the already operational WECSs. Modelling of the DFIG
starting procedure has been discussed in previous research by the same
author {[}34{]}. SCIG based WECS applies the same torque control scheme.
Figs. 13 and 14 shows the results obtained.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image13/image13}

\end{center}
\end{figure}

\textbf{Figure 13.} Response on SCIG WECS starting on a SCIG based
system.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image14/image14}

\end{center}
\end{figure}

\textbf{Figure 14.} Response on DFIG WECS starting on a DFIG based
system.

From the results obtained, starting of SCIG based WECS first requires
energization of a capacitor bank (at the 60th second) to supply its
reactive power requirements. Without the capacitor bank energization,
the SC as shown by the field current would be driven into overexcitation
prompting action of the OEL. For the case of the starting of the DFIG
based WECS, there is a sluggish response, but the SC is not driven into
overexcitation and in contrast it behaves as a capacitive element
reducing the operating level of SC field current.

3.3. Transformer Energization

Figs. 15 to 18 show the system response on transformer energization at
different excitation levels for both the SCIG and DFIG based hybrid
systems. Change of excitation level for the SCIG system is achieved by
energizing a 0.5MVAr capacitor bank while for the DFIG based system it
is achieved by setting the Q reference to 0.1pu on each WECS.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image15/image15}

\end{center}
\end{figure}

\textbf{Figure 15.} Response on transformer energization at 100\%
excitation level in a SCIG based hybrid system.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image16/image16}

\end{center}
\end{figure}

\textbf{Figure 16.} Response on transformer energization at 90\%
excitation level in a SCIG based hybrid system.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image17/image17}

\end{center}
\end{figure}

\textbf{Figure 17.} Response on transformer energization at 100\%
excitation level in a DFIG based hybrid system.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image18/image18}

\end{center}
\end{figure}

\textbf{Figure 18.} Response on transformer energization at 70\%
excitation level in a DFIG based hybrid system.

From the responses above, energization at 100\% excitation levels led to
the SC field current being above an acceptable technical limit of 2.2pu
{[}35{]}. This was the case for energization of the all the selected
transformer sizes for both SCIG and DFIG based hybrid system. On
energization at lower excitation levels (90\% for the SCIG and 70\% for
the DFIG) the field current transient is kept below 2.2 pu for
energization of the selected transformer sizes. From the voltage
response, the windfarm is assumed to sustain the transients considering
typical low voltage ride through capabilities {[}36{]}. At both
excitation levels, the voltage response from the DFIG system is better,
although slower, in comparison to the SCIG based system. The SCIG based
system could handle the energization better if the excitation is lowered
further while keeping the voltage above 1pu. This is achieved by
energization of larger capacitor banks.

3.4. Underground Cable Energization

Although the cable infrastructure can be assumed energized during
energization of the windfarm auxiliaries before starting of the WECSs if
the energization source is located onshore, a scenario might emerge in
which the initial excitation source is located offshore thus the HV
submarine cable to the onshore substation requires energization.
Response on energization of a 40km cable at 80\% and 100\% compensation
levels is shown in Figs. 19 and 20.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image19/image19}

\end{center}
\end{figure}

\textbf{Figure 19.} Response on HV cable energization for a SCIG based
hybrid system.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image20/image20}

\end{center}
\end{figure}

\textbf{Figure 20.} Response on HV cable energization for a DFIG based
hybrid system.

From above, it is possible to energize a HV cable if there is proper
compensation. In this case there is sufficient capability from the SC
and for 80\% compensation, it is driven to underexcitation and negative
excitation. Negative forcing excitation capability is possible in static
excitation systems if excess short-term reactive power absorption
capabilities are required. However, this prompted the UEL to increase
the AVR voltage reference thus the increase in the RMS voltage to 1.1pu
for the DFIG and 1.15pu for the SCIG which is the higher limit of the
range of voltage at MV level. One of the major challenges associated
with simultaneous energization of compensated transmission lines is the
missing zero phenomenon. This occurs when the current through a breaker
does not cross the zero mark for a few cycles. This is dangerous if
there is a fault that needs to be cleared during the few cycles leading
to circuit breaker (CB) damage when trying to open. In investigating the
missing zero phenomenon on the compensated lines, Figs. 21 and 22 show
the stator current response on cable energization at different
compensation levels.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image21/image21}

\end{center}
\end{figure}

\textbf{Figure 21.} SC Stator Current on HV cable energization (100\%
compensation, 80\% compensation) in a SCIG based hybrid system.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image22/image22}

\end{center}
\end{figure}

\textbf{Figure 22.} SC Stator Current on HV cable energization (100\%
compensated, 80\% compensation) in a DFIG based hybrid system.

At the recommended 100\% compensation level (from the voltage response
of Figs. 21 and 22), there is an increased chance of missing zero
phenomenon which is more pronounced on the DFIG based system as compared
to the SCIG based system. The more capacitive the system at the time of
energization the less the missing zero phenomenon effect. For the SCIG
based system, there are connected capacitor banks (required for normal
operation) which makes the system net capacitive as compared to DFIG
based system that does not require external capacitive elements as it
does its reactive power management internally through the RSC or GSC.

3.5. Overhead Line Energization

Assuming that there is a non-blackstart unit located more than 50km away
from the windfarm, the 132kV overhead lines between the windfarm and the
non-blackstart unit will need to be energized. Figs. 23 and 24 show
results obtained on overhead line energization by both a SCIG and DFIG
based hybrid system. Energization of 100km and 200km lines required some
form of compensation: 0.6 and 0.8MVAr respectively.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image23/image23}

\end{center}
\end{figure}

\textbf{Figure 23.} Response on Overhead Line energization for a SCIG
based hybrid system.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image24/image24}

\end{center}
\end{figure}

\textbf{Figure 24} . Response on Overhead Line energization for a DFIG
based hybrid system.

From above, it was possible to energize at least 50km of overhead line
without any form of compensation. Some form of compensation was required
for lines above this and this is seen in the energization of the 100km
and 200km lines. Since the lines were being energized at 0.9pu, the rise
in voltage was not significant as compared to cable energization earlier
mentioned. On investigating the missing zero phenomenon on the SC stator
current, Fig. 25 shows the results obtained.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image25/image25}

\end{center}
\end{figure}

\textbf{Figure 25.} SC Stator Current on 100kM Overhead Line
energization for a SCIG (left) and DFIG (right) based hybrid system.

As previously explained, the missing zero phenomenon was more
significant for the DFIG based system as compared to the SCIG system as
a result of presence of capacitor banks making the system more
capacitive for the SCIG based hybrid system.

3.6. Cold Load Pickup-Motor Energization

Still on the assumption that there is a non-black start unit located
50km from the windfarm, and with the knowledge of maximum reactive power
pickup of around 2MVAr, a 3MVA motor was assumed to be the largest motor
in the plant. Fig. 26 shows the results obtained for both the SCIG and
DFIG based system.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image26/image26}

\end{center}
\end{figure}

\textbf{Figure 26.} Response on motor energization.

The field current for the DFIG went beyond its limit of 2.2pu because it
was energized at a slightly higher excitation level. With proper setting
of the DFIG reactive power reference, it is possible to energize the
motor at lower excitation thus keeping the energization transient to
below 2.2pu. The voltage dip associated with motor starting last for
about a second and this is permissible as per most low voltage ride
through criterion.

4. Conclusion

Analysis on SCIG and DFIG based hybrid windfarm in emergency restorative
conditions with focus on load pickup and component re-energization is
provided. The range of operation of both the windfarms was first
analyzed from both an active and reactive power response perspective. An
additional pitch control based on dump load power was implemented to
prevent the excess production of power at a time when the dump load
power absorption power has been exceeded. The ability to pick up
inductive load for the SCIG based system is governed by the SC
capability and availability of capacitor banks while for the DFIG
system, it is dependent on the both the SC and DFIG reference settings
which can be adjusted to lower the excitation levels and pick up more
inductive load.

The 2 most common power quality issues related to windfarm operation,
harmonics and flicker, have been used to determine the hosting capacity
of the respective hybrid systems. Due to reduced network sources in
restorative conditions, harmonic and flicker effect will be more
magnified. As expected, flicker has a significant effect on the SCIG
based system while harmonics have a significant effect on the DFIG based
system. For maximum wind speed variation (worst case scenario) the
voltage and frequency of the SCIG based hybrid system could not stay
within its technical limit thus flicker mitigation techniques are
required. This was not an issue for the DFIG based system. The harmonic
levels were well below the limit for the DFIG based hybrid system.

The capability of the hybrid systems to perform re-energization
functions, similar to conventional blackstart units was investigated.
This has been successfully proven for the case of transformer, HV
underground cable, overhead lines and motor energization. Cranking up of
other WECSs in the same windfarm was also examined using previous work
(by the same author) proposed windfarm starting model. Regulation of the
excitation levels is important for proper re-energization. Careful
consideration is required for DFIG based hybrid systems on energization
of compensated lines due to the increased risk of missing zero
phenomenon. Apart from that, the DFIG based hybrid system was superior
in performance and flexibility.

Acknowledgments

The authors acknowledge funding support to carry out this research from
Ministry of Business, Innovation and Employment, New Zealand's National
Science Challenge- Resilience to Nature's Challenge and in particular
the project on Electricity Distribution Resilience Framework informed by
West Coast Alpine Fault Scenario; University of Auckland under Faculty
Research Development Fund (FRDF) Grant 3709532 and QuakeCoRE-New Zealand
Centre for Earthquake Resilience.

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\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Fig-1-Test-System/Fig-1-Test-System}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Fig-2-SCIG-based-WECS-Schematic/Fig-2-SCIG-based-WECS-Schematic}

\end{center}
\end{figure}\selectlanguage{english}
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\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Fig-6-Response-of-both-SCIG-and-DFIG-based-WECSs-on-the-described-scenario/Fig-6-Response-of-both-SCIG-and-DFIG-based-WECSs-on-the-described-scenario}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Fig-7-Response-of-both-SCIG-and-DFIG-based-WECSs-on-the-described-scenario-with-additional-pitch-control-mode/Fig-7-Response-of-both-SCIG-and-DFIG-based-WECSs-on-the-described-scenario-with-additional-pitch-control-mode}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Fig-8-SCIG-based-system-response-on-pure-inductive-load-pickup-with-90--level-of-excitation/Fig-8-SCIG-based-system-response-on-pure-inductive-load-pickup-with-90--level-of-excitation}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Fig-9-SCIG-based-system-response-on-pure-reactive-load-pickup-with-90--level-of-excitation/Fig-9-SCIG-based-system-response-on-pure-reactive-load-pickup-with-90--level-of-excitation}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Fig-10-DFIG-based-system-response-on-pure-reactive-load-pickup-at-100--level-of-excitation/Fig-10-DFIG-based-system-response-on-pure-reactive-load-pickup-at-100--level-of-excitation}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Fig-11-DFIG-based-system-response-on-pure-reactive-load-pickup-at-50--level-of-excitation/Fig-11-DFIG-based-system-response-on-pure-reactive-load-pickup-at-50--level-of-excitation}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Fig-12-Response-of-SCIG-and-DFIG-based-system-to-rapid-wind-speed-changes/Fig-12-Response-of-SCIG-and-DFIG-based-system-to-rapid-wind-speed-changes}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Fig-13-Response-on-SCIG-WECS-starting-on-a-SCIG-based-system/Fig-13-Response-on-SCIG-WECS-starting-on-a-SCIG-based-system}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Fig-14-Response-on-DFIG-WECS-starting-on-a-DFIG-based-system/Fig-14-Response-on-DFIG-WECS-starting-on-a-DFIG-based-system}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Fig-15-Response-on-transformer-energization-at-100--excitation-level-in-a-SCIG-based-hybrid-system/Fig-15-Response-on-transformer-energization-at-100--excitation-level-in-a-SCIG-based-hybrid-system}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Fig-16-Response-on-transformer-energization-at-90--excitation-level-in-a-SCIG-based-hybrid-system/Fig-16-Response-on-transformer-energization-at-90--excitation-level-in-a-SCIG-based-hybrid-system}

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\includegraphics[width=0.70\columnwidth]{figures/Fig-17-Response-on-transformer-energization-at-100--excitation-level-in-a-DFIG-based-hybrid-system/Fig-17-Response-on-transformer-energization-at-100--excitation-level-in-a-DFIG-based-hybrid-system}

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\begin{figure}[H]
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\includegraphics[width=0.70\columnwidth]{figures/Fig-18-Response-on-transformer-energization-at-70--excitation-level-in-a-DFIG-based-hybrid-system/Fig-18-Response-on-transformer-energization-at-70--excitation-level-in-a-DFIG-based-hybrid-system}

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\begin{figure}[H]
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\includegraphics[width=0.70\columnwidth]{figures/Fig-19-Response-on-HV-cable-energization-for-a-SCIG-based-hybrid-system/Fig-19-Response-on-HV-cable-energization-for-a-SCIG-based-hybrid-system}

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\includegraphics[width=0.70\columnwidth]{figures/Fig-20-Response-on-HV-cable-energization-for-a-DFIG-based-hybrid-system/Fig-20-Response-on-HV-cable-energization-for-a-DFIG-based-hybrid-system}

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\begin{figure}[H]
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\includegraphics[width=0.70\columnwidth]{figures/Fig-21-SC-Stator-Current-on-HV-cable-energization-(100--compensation,-80--compensation)-in-a-SCIG-based-hybrid-system/Fig-21-SC-Stator-Current-on-HV-cable-energization-(100--compensation,-80--compensation)-in-a-SCIG-based-hybrid-system}

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\includegraphics[width=0.70\columnwidth]{figures/Fig-22-SC-Stator-Current-on-HV-cable-energization-(100--compensated,-80--compensation)-in-a-DFIG-based-hybrid-system/Fig-22-SC-Stator-Current-on-HV-cable-energization-(100--compensated,-80--compensation)-in-a-DFIG-based-hybrid-system}

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\begin{figure}[H]
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\includegraphics[width=0.70\columnwidth]{figures/Fig-23-Response-on-Overhead-Line-energization-for-a-SCIG-based-hybrid-system/Fig-23-Response-on-Overhead-Line-energization-for-a-SCIG-based-hybrid-system}

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\begin{figure}[H]
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\includegraphics[width=0.70\columnwidth]{figures/Fig-24-Response-on-Overhead-Line-energization-for-a-DFIG-based-hybrid-system/Fig-24-Response-on-Overhead-Line-energization-for-a-DFIG-based-hybrid-system}

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\includegraphics[width=0.70\columnwidth]{figures/Fig-25-SC-Stator-Current-on-100kM-Overhead-Line-energization-for-a-SCIG-(left)-and-DFIG-(right)-based-hybrid-system/Fig-25-SC-Stator-Current-on-100kM-Overhead-Line-energization-for-a-SCIG-(left)-and-DFIG-(right)-based-hybrid-system}

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\includegraphics[width=0.70\columnwidth]{figures/Fig-26-Response-on-motor-energization/Fig-26-Response-on-motor-energization}

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\begin{figure}[H]
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\includegraphics[width=0.70\columnwidth]{figures/Fig-3-DFIG-based-WECS-Schematic/Fig-3-DFIG-based-WECS-Schematic}

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\begin{figure}[H]
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\includegraphics[width=0.70\columnwidth]{figures/Fig-4-Single-phase-Transformer-Equivalent-Circuit/Fig-4-Single-phase-Transformer-Equivalent-Circuit}

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