Learning Schemes for Power System Protection

In this paper, learning algorithms are leveraged to advance power system protection. Advancements in power system protection have come in different forms such as the development of new control strategies and the introduction of a new system architecture such as a microgrid. In this paper, we propose two learning schemes to make accurate predictions and optimal decisions related to power system protection and microgrid control. First, we present a neural network approach to learn a classifier that can predict stable reconnection timings for an islanded sub-network. Second, we present a learning-based control scheme for power system protection based on the policy rollout. In the proposed scheme, we incorporate online simulation using the commercial PSS/e simulator. Optimal decisions are obtained in real time to prevent cascading failures as well as maximize the load served. We validate our methods with the dynamics simulator and test cases RTS-96 and Poland

Universal and Seamless Control of Distributed Resources-Energy Storage for all Operational Scenarios of Microgrids

This paper proposes one control paradigm that can operate in both grid-connected and islanded modes, hence, does not need any sort of islanding detection method. The proposed method automatically and seamlessly rides-through a fault on the grid side, and controls the microgrid’s voltage and frequency during islanded operation. During islanded operation it utilises the combination of distributed generation-energy storage similar to the prime-mover of a synchronous generator to control the frequency. A comprehensive active and reactive power control is proposed that minimises the usage of a local fossil-fuelled auxiliary generator. The method is based on expanding the so called non-detection zone to all operational scenarios including islanded mode, hence, having small, “undetectable” voltage and frequency deviation. As soon as the grid is reconnected the distributed generator is automatically and seamlessly synchronised with the grid. This is achieved through keeping PLL as part of the operation in islanded mode without altering its phase angle. The proposed method is validated using PSCAD/ EMTDC simulation

Flexible grid connection and islanding of SPC-based PV power converters

Peer ReviewedPostprint (author's final draft

Inter-Microgrid Operation: Power Sharing, Frequency Restoration, Seamless Reconnection and Stability Analysis

Electrification in the rural areas sometimes become very challenging due to area accessibility and economic concern. Standalone Microgrids (MGs) play a very crucial role in these kinds of a rural area where a large power grid is not available. The intermittent nature of distributed energy sources and the load uncertainties can create a power mismatch and can lead to frequency and voltage drop in rural isolated community MG. In order to avoid this, various intelligent load shedding techniques, installation of micro storage systems and coupling of neighbouring MGs can be adopted. Among these, the coupling of neighbouring MGs is the most feasible in the rural area where large grid power is not available. The interconnection of neighbouring MGs has raised concerns about the safety of operation, protection of critical infrastructure, the efficiency of power-sharing and most importantly, stable mode of operation. Many advanced control techniques have been proposed to enhance the load sharing and stability of the microgrid. Droop control is the most commonly used control technique for parallel operation of converters in order to share the load among the MGs. But most of them are in the presence of large grid power, where system voltage and frequency are controlled by the stiff grid. In a rural area, where grid power is not available, the frequency and voltage control become a fundamental issue to be addressed. Moreover, for accurate load sharing a high value of droop gain should be chosen as the R/X ratio of the rural network is very high, which makes the system unstable. Therefore, the choice of droop gains is often a trade-off between power-sharing and stability. In the context, the main focus of this PhD thesis is the fundamental investigations into control techniques of inverter-based standalone neighbouring microgrids for available power sharing. It aims to develop new and improved control techniques to enhance performance and power-sharing reliability of remote standalone Microgrids. In this thesis, a power management-based droop control is proposed for accurate power sharing according to the power availability in a particular MG. Inverters can have different power setpoints during the grid-connected mode, but in the standalone mode, they all need their power setpoints to be adjusted according to their power ratings. On the basis of this, a power management-based droop control strategy is developed to achieve the power-sharing among the neighbouring microgrids. The proposed method helps the MG inverters to share the power according to its ratings and availability, which does not restrict the inverters for equal power-sharing. The paralleled inverters in coupled MGs need to work in both interconnected mode and standalone mode and should be able to transfer between modes seamlessly. An enhanced droop control is proposed to maintain the frequency and voltage of the MGs to their nominal value, which also helps the neighbouring MGs for seamless (de)coupling. This thesis also presents a mathematical model of the interconnected neighbouring microgrid for stability and robustness analysis. Finally, a laboratory prototype model of two MGs is developed to test the effectiveness of the proposed control strategies

Control and operation of multiple distributed generators in a microgrid

Small sized synchronous generator based distributed generators (DG) often have low start-up times, and can serve as dispatchable generators in a microgrid environment. The advantage is that it allows the power network to operate in a true smart grid environment. The disadvantage is that such DGs typically tend to have low inertia and the prime movers driving these resources need to be controlled in real time for them to operate effectively in islanded, grid-connected modes and during transition from grid-connected mode to islanded mode and vice versa. When multiple DGs are present in the microgrid, the overall control can become complicated because of the need for sharing the resources. A smart grid environment is then necessary to control all dispersed generation sources in the microgrid. The most common control strategy adopted for multiple DGs connected to a network is droop control. Droop control ensures that the load needed to be served is shared by all the generators in the network in proportion to their generating capability. When DGs operate in a microgrid environment, there is a need for coordinated operation between the DGs, the utility grid and the loads. A MicroGrid Central Controller (MGCC) can keep track of the status from the system standpoint and command the local Microsource Controllers (MC) to ensure system stability. In various modes of operation like grid connected, islanding and during transition, the MGCC can support the MCs by giving them necessary information to contribute towards stable operation --Abstract, page iii

Intentional Islanding of Active Distribution Networks by GenSets: An Analysis of Technical Constraints and Opportunities

The willingness to improve the security and reliability of power supply to end-users, often pushed by prescriptions of national regulatory authorities, is bringing considerable challenges for distribution system operators. Islanding a portion of the public distribution network after a fault is considered a measure to mitigate the effects of service interruptions. This procedure is usually carried out by counterfeeding the grid through a generator set (GenSet). Even if this approach is widely adopted around the world, reenergizing the grid and keeping the electric island stable is not a trivial task. In this framework, the scope of this paper is to provide a set of technical guidelines for the usage of GenSets to supply public grids in emergency conditions. The goal is to highlight the static and dynamic limits of the GenSet operations and simplify their exploitation for the grid operators. The numerical analyses, which have been carried out through the RMS simulation tool of the DigSilent PowerFactory software, also aim to evaluate the technical constraints in the case of active networks, which involve distributed generation implementing regulations according to ENTSO-E and Italian technical standards

Identification and development of microgrids emergency control procedures

Tese de doutoramento. Engenharia Electrotécnica e de Computadores. Faculdade de Engenharia. Universidade do Porto. 200

A survey of islanding detection methods for microgrids and assessment of non-detection zones in comparison with grid codes

Detection of unintentional islanding is critical in microgrids in order to guarantee personal safety and avoid equipment damage. Most islanding detection techniques are based on monitoring and detecting abnormalities in magnitudes such as frequency, voltage, current and power. However, in normal operation, the utility grid has fluctuations in voltage and frequency, and grid codes establish that local generators must remain connected if deviations from the nominal values do not exceed the defined thresholds and ramps. This means that islanding detection methods could not detect islanding if there are fluctuations that do not exceed the grid code requirements, known as the non-detection zone (NDZ). A survey on the benefits of islanding detection techniques is provided, showing the advantages and disadvantages of each one. NDZs size of the most common passive islanding detection methods are calculated and obtained by simulation and compared with the limits obtained by ENTSO-E and islanding standards in the function of grid codes requirements in order to compare the effectiveness of different techniques and the suitability of each one