Low voltage ride-through strategies for a 3-phase grid-connected PV system

Grid codes is a technical specification which defines the parameters a power system that are connected to the national power systems has to ensure safe, secure and eco-nomic proper functioning of the electric system. One of these requirements is to stay connected to the grid during faults. In such scenarios, the generating unit should remain connected to the grid for a certain period and provide reactive power to support the grid. This is called low voltage ride-through capability. At the early stage, low voltage ride-through requirements were imposed for large scale generators connected to the trans-mission network. However, with the increased penetration of distributed generation, such as PV panels implemented in the distribution network, the low voltage ride-through requirements are also required for distributed generation. With the maturity of PV technology, the cost of PV generation has decreased. Therefore, the total installed capacity of grid-connected PV generation has increased; this has cre-ated new challenges to the low voltage ride-through. Power quality and transient per-formance are the most critical aspects of the grid-connected PV systems under grid faults. PV generation is permitted to switch off from the grid during a fault; however, with the high penetration of the installed PV system, it will degrade the power quality if the same method applied. It is necessary to make sure that the inverter currents remain sinusoidal and within the acceptable limits at the instant of the fault, during and after the fault clearance for different types of faults. Accordingly, this thesis proposes two low voltage ride-through strategies for a 3-phase grid-connected PV system in different reference frames. The presented low voltage ride-through control algorithm in the synchronous reference frame, which fulfils a voltage compensation unit and the reactive power injection block is designed to protect the inverter from overcurrent failure under both symmetrical and asymmetrical faults, reduce the double grid frequency oscillations and provides reac-tive power support by applying a voltage compensation unit. The inverter can also inject sinusoidal current during asymmetrical faults. The method does not require a hard switch from the Maximum Power Point Tracking to a non-Maximum Power Point Tracking algorithm, which ensures a smooth transition. The proposed method in the stationary reference frame provides a fast post-fault recov-ery, which is essential to minimize the fault impacts on the loads and the converter. The method, which consists of a new reference currents calculation block and the voltage compensation unit, maintains the converter current within acceptable limits, produces sinusoidal current even during asymmetrical faults, improves the post-fault recovery performance, and provides independent control for active and reactive powers

Power quality enhancement in electricity networks using grid-connected solar and wind based DGs

The integration of DG into utility networks has significantly increased over the past years primarily as a result of growing energy demand, coupled with the environmental impacts posed by conventional fossil fuel-based power generation. The prominent DG technologies which are capable of meeting bulk energy demands and are clean energy sources are wind and solar energy sources. The resources for solar and wind based DG are available in abundance in most geographical locations in South Africa and the rest of Africa. Through the Renewable Energy Independent Power Producer Procurement Programme (REIPPPP) introduced by the South African government in 2011, 3 920 MW of renewable energy has been procured to date. Out of this, solar and wind energy constitute 2 200 MW and 960 MW, respectively. Grid integration of solar and wind-based intermittent DGs may however pose negative impacts on the quality of power supplied by the utility network. Some of the detrimental impacts of DG include voltage fluctuations, flicker, etc. which are in general categorised as power quality (PQ) problems. The proper planning of DG integration is required to mitigate the negative impacts they pose on system's PQ to ensure that the performance of the utility network is enhanced in terms of the overall PQ improvement of the system. This dissertation reviews general PQ problems in utility networks with DG integration and whether poor planning of DG integration affects PQ negatively. The work emphasizes on the impacts of grid integration of wind and solar PV sources on power quality. It investigates the manner in which wind and solar energy systems differ in their impacts and capacity to improve PQ of the network in terms of a number of factors such as point of integration and capacity of DG, type of DG, network loading, etc. The role of grid-integrated DG in PQ improvement in electricity network is also investigated by exploring different PQ improvement techniques. The networks considered for the grid integration of DG for PQ improvement in this work are the IEEE 9-bus sub-transmission network at the nominal voltage of 230kV and the IEEE 33-bus distribution network at the nominal voltage of 12 kV. The aspects essential for facilitating proper planning of DG integration for PQ improvement and total loss reduction are investigated and the comparative analysis is made between grid integration of wind and solar PV based DGs. The simulations of different case studies in this work are done using DIgSILENT PowerFactory version 14.1 as well as coding in MATLAB. The cases studies conducted are aimed at facilitating the proper planning of grid integration of wind and solar PV-based DGs by comparing their PQ improvement capabilities under different scenarios. First the investigation of how their location and capacity affect the network voltage profiles and active power losses is conducted. Their ability to improve the system's PQ is also studied by observing PQ improvement strategies such as voltage control, installation of energy storage and the optimal placement of DGs under different scenarios. In order to account for the weakness of most South African utility grids, PQ improvement in weak networks with DG integration is also studied by investigating how DG integration in networks with different grid strengths affect the system's PQ. The results provide an understanding of the role of grid integration of wind and solar based DGs on PQ which is useful in the planning of grid integration of RE, particularly in South African electricity networks. The results revealed that the location and capacity of integrated DGs indeed affect the quality of power as well as active power losses in the grid. It is also established that a significant improvement in network's PQ and line loss reduction can be achieved in networks with wind and solar integration. The results however indicated that wind and solar PV based DGs differ in their impacts and capacity to improve the quality of power in the network. Furthermore, the results revealed that wind and solar plants integration into weak utility grids may pose adverse impacts on the system's PQ. It was however established that including reactive power control devices such as STATCOM and SVC at the PCC can successfully improve the system's PQ and enable grid code compliance in electricity networks with DG integration

Enhancement of Solar PV Hosting Capacity in a Remote Industrial Microgrid: A Methodical Techno-Economic Approach

To meet the zero-carbon electricity generation target as part of the sustainable development goals (SDG7), remote industrial microgrids worldwide are considering the uptake of more and more renewable energy resources, especially solar PV systems. Estimating the grid PV hosting capacity plays an essential role in designing and planning such microgrids. PV hosting capacity assessment determines the maximum PV capacity suitable for the grid and the appropriate electrical location for PV placement. This research reveals that conventional static criteria to assess the PV hosting capacity fail to ensure the grid’s operational robustness. It hence demands a reduction in the theoretical hosting capacity estimation to ensure grid compatible post-fault voltage and frequency recovery. Energy storage technologies, particularly fast-responsive batteries, can potentially prevent such undesirable scenarios; nevertheless, careful integration is required to ensure an affordable cost of energy. This study proposes a novel methodical techno-economic approach for an off-grid remote industrial microgrid to enhance the PV hosting capacity by integrating battery energy storage considering grid disturbance and recovery scenarios. The method has been validated in an industrial microgrid with a 2.6 MW peak demand in a ready-made garment (RMG) factory having a distinctive demand pattern and unique constraints in remote Bangladesh. According to the analysis, integrating 2.5 MW of PV capacity and a 1.2 MVA battery bank to offset existing diesel and grid consumption would result in an energy cost of BDT 14.60 per kWh (USD 0.1719 per kWh). For high PV penetration scenarios, the application of this method offers higher system robustness, and the financial analysis indicates that the industries would not only benefit from positive environmental impact but also make an economic profit

Development of Robust and Dynamic Control Solutions for Energy Storage Enabled Hybrid AC/DC Microgrids

Development of Robust and Dynamic Control Solutions for Energy Storage Enabled Hybrid AC/DC Microgrid

Resilient Microgrids Using a State Controller

Wind energy technology is fast becoming a major component of renewable energy deployment in electric grids. This technology however, has a major challenge of low machine inertia that could impact the frequency stability of the system when deployed in microgrids. The frequency response rate to abrupt load changes becomes an issue when many wind turbines are connected in a microgrid. This dissertation investigates the impact of this low machine inertia on the nominal frequency and voltage of a microgrid. The impact of varying wind conditions on the electrical power output is also studied. The system is modelled in MATLAB/Simulink using a DFIG wind turbine rated at 1.5 MVA. This thesis studies control strategies to bring the system to a stability irrespective of the wind speeds, load conditions or perturbations. This work further focuses on how the state controller is used to improve the power system reliability, availability and resilience during extreme events such as hurricanes, earthquakes and wildfires