Study on Re-Energization Capability of a Hybrid Windfarm under a
Microgrid-based Restoration Strategy
Duncan Kaniaru Maina1, Nirmal-Kumar C Nair 1
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
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.
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.