Control and Protection in Low Voltage Grid with Large Scale Renewable Electricity Generation

Distributed Generation (DG) technology especially renewable energy sources such as Wind Turbine Generations (WTGs), photovoltaic (PV) generations, energy storages etc. has played a vital role in the modern power industry due to some environmental and some cost benefits. The important benefits of renewable energy based DGs are reduced CO2 emission, reduced operational cost as almost no fuel is used for their operation and less transmission and distribution losses as these units are normally built near to the load centers. This has also resulted in some operational challenges due to the unpredictable nature of such power generation sources. Some of the operational challenges include voltage variations due to power fluctuations coming from the DG units. On the other hand, it has also opened up some opportunities. One of the opportunities is islanding operation of the distribution system with DG unit(s). Islanding is a situation where electrical system becomes electrically isolated from the rest of the power network and yet continues to be energized by the DG units connected to it. With the increased penetration of DG units, islanded operation of the distribution network is used to improve the reliability of power supply if various issues regarding islanding are properly tackled. Some of the serious issues with islanding are the Island Detection (ID), voltage and the frequency control, load control and protection.
In this dissertation, some of the major technical issues such as voltage control in case of the wind speed and solar irradiation fluctuations are tackled. The CIGRE Low Voltage (LV) network comprising two solar PV generating units of 3 kW and 4 kW, one 5.5 kW fixed-pitch fix speed WTG and two battery units each producing energy of 30kwh and 21kwh has been chosen for the study. The study of this network in the case of transients such as short circuit faults has been made. The mitigation of symmetrical and unsymmetrical voltage sags has also been addressed here. The components of the CIGRE network are modelled in DIgSILENT power factory software 15.0 and the control system for PV and battery inverters have been developed. Two Static Compensator (STATCOM) controllers have been developed for the PV units and two Battery Energy Storage System (BESS) based STATCOM controllers have been developed for battery units. The controllers are developed in such a way that they inject/absorb desired amount of the reactive power in order to maintain the constancy of the voltage in the network.
Some of the protection related issues have also been addressed in this dissertation. The short circuit power of a distribution system changes when it changes its states. Short circuit power also changes when some DG units are disconnected. This may result in elongation of the fault clearing time of the protective devices and might cause unnecessary operation of the protective devices. Therefore, the protection of the distribution system has been developed which is accurate and satisfy the economic criteria as well. The protection of the different components of the CIGRE distribution network has been presented in case of bi-directional flow of the current. It is proposed that the protection of the entire CIGRE network is mainly developed by using fuses due to economic reasons. The voltage based protection is also proposed somewhere in this network for the cases when fuses are not enough to protect, such as in the case of islanding where the flow of the current has the reverse direction. Over speed protection of the WTG in case of loss of grid due to symmetric and asymmetric faults is also developed in this thesis. This is done by using a mechanical braking system in order to stop the generator. The protection of PV and battery inverters is also presented in this study. The protection of the inverters is made by using ultra-fast fuses. The issues of the protection of the solar PV structure against over voltages in case of islanding from the main power grid are also described here. This is achieved by using switch disconnection devices.
The issues of the voltage and the frequency control in case of islanding are addressed in this study and these issues of voltage and frequency in islanded Micro Grid (MG) has been successfully tackled in this dissertation. This is done by developing appropriate controllers. Initially islanding is detected by using an Island Detection (ID) detection technique which is based on voltage phase angle difference between grid side and DG side of the network. This is a more recent ID technique and has some advantages over other ID detection techniques such as false detection in case of transient or over load conditions.
The development of controllers for all the DG units in the islanded MG is different as compared to the grid connected mode. The controllers are developed in such a way that they ensure the constancy of the voltage and the frequency of the entire islanded MG. In case of the grid connected mode, all of the inverter controllers can either be developed in Power/Voltage (PV) mode or Active power/Reactive power (PQ) mode. In case of islanding with several parallel inverters available in the network; one of the inverter controllers must be developed to Voltage-Frequency (VF) mode; and the others in either PV or PQ modes. The operation of the MG with several PV inverters and single VF inverter is similar to the operation of MG with a synchronous machine as slack bus. The VF inverter establishes the voltage and the frequency references for the operation of all other PV/PQ inverters in the case when the MG islanded from the main power grid. Acting as the voltage source, VF inverter is responsible for controlling the voltage and frequency of the island MG. It injects or absorbs active and reactive powers if frequency or voltage in MG decreases or increases respectively. It requires the significant amount of energy available in the power source with very fast response. A VF control cannot be used for wind and solar power generations because they are unpredictable and depend on the weather conditions (i.e. wind speed and solar irradiance). The VF controller can be used for the inverters of the BESS system, especially the inverter of the bigger energy storage capability. Therefore, when the operating mode of the CIGRE network changes from the grid connected to the islanded mode, the control system should switch from the PV/PQ mode to VF mode for this unit. The switching of the control system is based on the Island Detection (ID) technique.
Issues related to grid reconnection have also been described in this thesis. The DG control strategy needs to be changed again when the islanded network is reconnected back to the transmission grid. Hence, grid reconnection detection has been proposed to detect when the islanded distribution system is reconnected back to the transmission grid. One of the grid reconnection detection techniques based on the synchronization of the voltage, the frequency and phase angle between the distribution system and the transmission grid has been proposed here.
The algorithms, models and methodologies developed during this research study have been tested in a CIGRE low voltage distribution network. The simulation results show that they are able to correctly identify the states of the distribution system in different operating conditions of the network, maintain the voltage during stochastic generation changes, mitigate voltage dips in the different fault conditions, control voltage and frequency during island mode. The simulation results show that the distribution system can survive for long time (i.e. up to 45 minutes) in the islanded condition. The results also show the islanded distribution system can successfully reconnect back to the transmission grid.
The research study shows that integration of renewable energy based DGs is a viable, cost effective and environment friendly solution for the modern power network. The distribution network such as CIGRE network and systems like that comprising wide variety of DGs can operate in stable, controlled, reliable and secure manners.