Protection of multi-inverter based microgrid using phase angle trajectory

This thesis presents a simple, yet a clever way of using the current phase angle to develop low bandwidth communication-assisted line protection strategies for medium and low voltage AC microgrids, particularly those with multi-inverter interfaced distributed generators. It is now a trend in both AC transmission and distribution segments of power network that inverters interface renewable energy to the system. Unlike synchronous generators the fault feeding, and control characteristic of these generators are different and mostly influenced by the topology, switching, control deployed in the power electronics interface. The limited and controlled fault current challenges the existing conventional protection schemes. Offering higher power supply reliability and system resilience than conventional radial distribution systems, multi-inverter based microgrids, particularly those with loop and mesh typologies, are characterised by bidirectional power flow. This further constrains traditional protections such that communication-less protection schemes become ineffective for such systems. So unit protection types, such as differential protection, become more technically suitable for such microgrids despite the necessity for a communication system. In this thesis, two current direction based protection schemes for medium voltage islanded microgrids have been developed. The change in current flow direction in a line is detected using the cosine of the positive sequence current phase angle. Expressing the change and no-change of the flow directions as binary states, a low bandwidth communication based protection scheme is proposed comparing the binary states from local and remote ends of the line. To further enhance the scope and reliability of this scheme, a second protection scheme is proposed in Chapter 7 whereby the cosine function is combined with the rate of change of the slope of the phase angle (ROCOSP). This combination allows the detection and isolation of a fault even with the failure of the communication channel between relays protecting a faulted line. Furthermore, these scheme can work together and share the communication infrastructure as primary and backup protections. The performance of these schemes was assessed through simulations of microgrid models developed in Matlab/Simulink.Open Acces

Analysis and Mitigation of Temporary Over-Voltage (TOV) Phenomenon in Unintentionally Islanded Grid-Connected Photovoltaic (PV) Inverters

Grid-connected photovoltaic (PV) solar systems, like other inverter-based distributed generators, can cause temporary over-voltages (TOVs), especially subsequent to faults and unintentional islanding incidents, and can damage equipment and customers within the host distribution network. Thus, this thesis aims to study the phenomenon and propose corrective measures for it. Thus, the thesis first presents detailed models for a conventional single-stage PV system and a modified single-stage PV system. The conventional system uses a Δ/YG isolation transformer, whereas the modified system, proposed in the literature, assumes a Y/YG isolation transformer that is effectively grounded by an additional half-bridge leg energized by the dc-link of the voltage-sourced inverter (VSI) of the PV system. Moreover, the thesis proposes two TOV mitigation schemes that augment the basic controls of the conventional and modified single-stage PV systems, respectively. Further, the thesis models a two-stage PV system that adopts the same TOV mitigation scheme as that proposed for the conventional single-stage system. Then, the TOV caused by the two-stage system is evaluated, with and without the TOV mitigation scheme. It is shown that the proposed TOV mitigation schemes are effective. The thesis also compares the TOVs caused by the three aforementioned PV systems, with and without the TOV mitigation schemes, and concludes that a two-stage PV system without a TOV mitigation scheme produces smaller TOVs than its single-stage counterparts without TOV mitigation schemes. Similarly, a two-stage PV system with its TOV mitigation scheme produces smaller TOVs than its single-stage counterparts with their respective TOV mitigation schemes

An enhancing fault current limitation hybrid droop/V-f control for grid-tied four-wire inverters in AC microgrids

Microgrid integration and fault protection in complex network scenarios is a coming challenge to be faced with new strategies and solutions. In this context of increasing complexity, this paper describes two specific overload control strategies for four-wire inverters integrated in low voltage four-wire alternating current (AC) microgrids. The control of grid-tied microgrid inverters has been widely studied in the past and mainly focused on the use of droop control, which hugely constrains the time response during grid-disconnected operation. Taking into account the previous knowledge and experience about this subject, the main contribution of these two proposals regards providing fault current limitation in both operation modes, over-load capability skills in grid-connected operation and sinusoidal short-circuit proof in grid-disconnected operation. In the complex operation scenarios mentioned above, a hybrid combination of AC droop control based on dynamic phasors with varying virtual resistance, and voltage/frequency master voltage control for grid-(dis)connected operation modes are adopted as the mechanism to enhance time response. The two proposals described in the present document are validated by means of simulations using Matlab/Simulink and real experimental results obtained from CENER (The National Renewable Energy Centre) experimental ATENEA four-wire AC microgrid, obtaining time responses in the order of two-three grid cycles for all cases.Postprint (published version

Protection of Active Distribution Networks and Their Cyber Physical Infrastructure

Today’s Smart Grid constitutes several smaller interconnected microgrids. However, the integration of converter-interfaced distributed generation (DG) in microgrids has raised several issues such as the fact that fault currents in these systems in islanded mode are way less than those in grid connected microgrids. Therefore, microgrid protection schemes require a fast, reliable and robust communication system, with backup, to automatically adjust relay settings for the appropriate current levels according to the microgrid’s operation mode. However, risks of communication link failures, cyber security threats and the high cost involved to avoid them are major challenges for the implementation of an economic adaptive protection scheme. This dissertation proposes an adaptive protection scheme for AC microgrids which is capable of surviving communication failures. The contribution is the use of an energy storage system as the main contributor to fault currents in the microgrid’s islanded mode when the communication link fails to detect the shift to the islanded mode. The design of an autonomous control algorithm for the energy storage’s AC/DC converter capable of operating when the microgrid is in both grid-connected and islanded mode. Utilizing a single mode of operation for the converter will eliminate the reliance on communicated control command signals to shift the controller between different modes. Also, the ability of the overall system to keep stable voltage and frequency levels during extreme cases such as the occurrence of a fault during a peak pulse load period. The results of the proposed protection scheme showed that the energy storage -inverter system is able to contribute enough fault current for a sufficient duration to cause the system protection devices to clear the fault in the event of communication loss. The proposed method was investigated under different fault types and showed excellent results of the proposed protection scheme. In addition, it was demonstrated in a case study that, whenever possible, the temporary disconnection of the pulse load during the fault period will allow the utilization of smaller energy storage device capacity to feed fault currents and thus reduce the overall expenditures. Also, in this dissertation we proposed a hybrid hardware-software co-simulation platform capable of modeling the relation between the cyber and physical parts to provide a protection scheme for the microgrid. The microgrid was simulated on MATLAB/Simulink SimPowerSystems to model the physical system dynamics, whereas all control logic was implemented on embedded microcontrollers communicating over a real network. This work suggested a protection methodology utilizing contemporary communication technologies between multi-agents to protect the microgrid

Real Time Testing and Validation of a Novel Short Circuit Current (SCC) Controller for a Photovoltaic Inverter

About 45% applications from PV solar farm developers seeking connections to the distribution grids in Ontario were denied in 2011-13 as the short circuit current (SCC) capacity of several distribution substations had already been reached. PV solar system inverters typically contribute 1.2 p.u. to 1.8 p.u. fault current which was not considered acceptable by utility companies due to the need for very expensive protective breaker upgrades. Since then, this cause has become a major impediment in the growth of PV based renewable systems in Ontario. A novel predictive technique has been patented in our research group for management of short circuit current contribution from PV inverters to ensure effective deployment of solar farms. This thesis deals with the real time testing and validation of a short circuit current (SCC) controller based on the above technique. With this SCC controller, the PV inverter can be shut off within 1-2 milliseconds from the initiation of any fault in the grid that can cause the short circuit current to exceed the rated current of the inverter. Therefore, the power system does not see any short circuit current contribution from the PV inverter and no expensive upgrades in protective breakers are required in the system. The performance of the PV solar system with the short circuit current controller is simulated and tested using (i) industry grade electromagnetic transients software PSCAD/EMTDC (ii) real time simulation studies on the Real Time Digital Simulator (RTDS) (iii) physical implementation on dSPACE board to generate firing pulses for the inverter. The validation of controller is done on dSPACE board with actual PV inverter short circuit waveforms obtained from Southern California Edison Short Circuit Testing Lab. This novel technology is planned to be showcased on a physical 10 kW PV solar system in Bluewater Power Distribution Corporation, Sarnia, Ontario. This proposed technology is expected to remove the technical hurdles which caused the denials of connectivity to several PV solar farms, and effectively lead to greater connections of PV solar farms in Ontario and in similar jurisdictions, worldwide

Investigation into Photovoltaic Distributed Generation Penetration in the Low Voltage Distribution Network

Significant integration of photovoltaic distributed generation (PVDG) in the low voltage distribution network (LVDN) could potentially pose threats and challenges to the core activity of distribution system operators (DSO), which is to transport electrical energy in a reliable and cost-effective way. The main aim of this research is to investigate the active planning and operation of LVDNs with increased PVDG integration through steady state power system analysis. To address the impacts of voltage profile fluctuation due to power flow modification, this research proposes a probabilistic risk assessment of power quality (PQ) variations and events that may arise due to significant PVDG integration. A Monte Carlo based simulation is applied for the probabilistic risk assessment. This probabilistic approach is used as a tool to assess the likely impacts due to PVDG integration against the extreme-case scenarios. With increased PVDG integration, site overvoltage is a likely impact, whereas voltage unbalance reduces when compared with no or low PVDG penetration cases. This is primarily due to the phase cancellation between the phases. The other aspect of the work highlights the fact that the implementation of existing volumetric charges in conjunction with net-metering can have negative impacts on network operator’s revenue. However, consideration of capacity charges in designing the existing network tariff structure shows incentivising the network operator to perform their core duties under increased integration of PVDG. The site overvoltage issue was also studied and resolved in a novel way, where the active and reactive power of the PVDG inverters at all the PV installed premises were optimally coordinated to increase the PV penetration from 35.7% to 66.7% of the distribution transformer rating. This work further explores how deficiencies in both reactive power control (RPC) and active power control (APC) as separate approaches can be mitigated by suitably combining RPC and APC algorithms. A novel “Q” or “PF” limiter was proposed to restrict frequent switching between the two droop characteristics while ensuring a stabilizing (smoothened) voltage profile in each of the PV installed nodes. This novel approach not only alleviates the voltage fluctuation but also reduces the overall network losses