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.