International White Book on DER Protection : Review and Testing Procedures

This white book provides an insight into the issues surrounding the impact of increasing levels of DER on the generator and network protection and the resulting necessary improvements in protection testing practices. Particular focus is placed on ever increasing inverter-interfaced DER installations and the challenges of utility network integration. This white book should also serve as a starting point for specifying DER protection testing requirements and procedures. A comprehensive review of international DER protection practices, standards and recommendations is presented. This is accompanied by the identifi cation of the main performance challenges related to these protection schemes under varied network operational conditions and the nature of DER generator and interface technologies. Emphasis is placed on the importance of dynamic testing that can only be delivered through laboratory-based platforms such as real-time simulators, integrated substation automation infrastructure and fl exible, inverter-equipped testing microgrids. To this end, the combination of fl exible network operation and new DER technologies underlines the importance of utilising the laboratory testing facilities available within the DERlab Network of Excellence. This not only informs the shaping of new protection testing and network integration practices by end users but also enables the process of de-risking new DER protection technologies. In order to support the issues discussed in the white paper, a comparative case study between UK and German DER protection and scheme testing practices is presented. This also highlights the level of complexity associated with standardisation and approval mechanisms adopted by different countries

Electromagnetic Transient-Transient Stability Hybrid Simulation for Electric Power Systems with Converter Interfaced Generation

abstract: With the increasing penetration of converter interfaced renewable generation into power systems, the structure and behavior of the power system is changing, catalyzing alterations and enhancements in modeling and simulation methods. This work puts forth a Hybrid Electromagnetic Transient-Transient Stability simulation method implemented using MATLAB and Simulink, to study power electronic based power systems. Hybrid Simulation enables detailed, accurate modeling, along with fast, efficient simulation, on account of the Electromagnetic Transient (EMT) and Transient Stability (TS) simulations respectively. A critical component of hybrid simulation is the interaction between the EMT and TS simulators, established through a well-defined interface technique, which has been explored in detail. This research focuses on the boundary conditions and interaction between the two simulation models for optimum accuracy and computational efficiency. A case study has been carried out employing the proposed hybrid simulation method. The test case used is the IEEE 9-bus system, modified to integrate it with a solar PV plant. The validation of the hybrid model with the benchmark full EMT model, along with the analysis of the accuracy and efficiency, has been performed. The steady-state and transient analysis results demonstrate that the performance of the hybrid simulation method is competent. The hybrid simulation technique suitably captures accuracy of EMT simulation and efficiency of TS simulation, therefore adequately representing the behavior of power systems with high penetration of converter interfaced generation.Dissertation/ThesisMasters Thesis Electrical Engineering 201

Impact of Converter Interfaced Generation and Load on Grid Performance

abstract: Alternate sources of energy such as wind, solar photovoltaic and fuel cells are coupled to the power grid with the help of solid state converters. Continued deregulation of the power sector coupled with favorable government incentives has resulted in the rapid growth of renewable energy sources connected to the distribution system at a voltage level of 34.5kV or below. Of late, many utilities are also investing in these alternate sources of energy with the point of interconnection with the power grid being at the transmission level. These converter interfaced generation along with their associated control have the ability to provide the advantage of fast control of frequency, voltage, active, and reactive power. However, their ability to provide stability in a large system is yet to be investigated in detail. This is the primary objective of this research. In the future, along with an increase in the percentage of converter interfaced renewable energy sources connected to the transmission network, there exists a possibility of even connecting synchronous machines to the grid through converters. Thus, all sources of energy can be expected to be coupled to the grid through converters. The control and operation of such a grid will be unlike anything that has been encountered till now. In this dissertation, the operation and behavior of such a grid will be investigated. The first step in such an analysis will be to build an accurate and simple mathematical model to represent the corresponding components in commercial software. Once this bridge has been crossed, conventional machines will be replaced with their solid state interfaced counterparts in a phased manner. At each stage, attention will be devoted to the control of these sources and also on the stability performance of the large power system. This dissertation addresses various concerns regarding the control and operation of a futuristic power grid. In addition, this dissertation also aims to address the issue of whether a requirement may arise to redefine operational reliability criteria based on the results obtained.Dissertation/ThesisDoctoral Dissertation Electrical Engineering 201

Protection challenges in future converter-dominated power systems : investigation and quantification using a novel flexible modelling and hardware testing platform

Error on title page – year of award is 2023.The research work presented in this thesis addresses anticipated (and documented) protection challenges that will be introduced by the domination of power electronics interfaces in future power systems. A flexible and programmable voltage source converter (VSC) model with controllable fault response has been developed and this is tested using realistic network data (including transmission lines and the corresponding power flow/fault level data) from the GB transmission network, provided by National Grid ESO (the research project sponsor). The results of tests, where a range of variations to the converter controllers’ fault-responses have been implemented (e.g. to reflect different detection and initial converter response delays, output current ramp rates and magnitudes), are presented and analysed. The simulated voltage and current waveforms are injected into actual protection relays using secondary injection amplifiers. The responses of the relays are recorded and a number of issues are highlighted, particularly with respect to the response of distance protection. It is shown that, when the system is dominated by converter-interfaced sources (especially where the sources are modelled as being unable to provide “fast” and “high” fault currents, which is typically the case for actual converter systems), the responses of traditional distance protection systems (and other systems relying on measurement of current magnitude) could be delayed, lose discrimination, e.g. by tripping with a zone 2 delay for a zone 1 fault, or may be completely unable to detect faults at certain locations within the system. Based on the test results, potential solutions are then presented relating to changes to relay algorithms and/or the requirements for converters in terms of behaviour during faults. The outcomes of the work will be of interest to grid code developers (publications arising from this work have already been referred to by ENTSO-E guidance document for national implementation for network codes on grid connection [1]), transmission network operators, other researchers and protection/converter manufacturers. An overview of future work, relating to comprehensive studies (using injection and the developed system/converter models) of a range of faults/ infeeds/ converter mixes with a wide range of protection relays including distance and unit-type, and development of a standard commissioning testing method of protection relays under future power system scenarios that are dominated by converters, is included in the concluding section. This will assist in the investigation and resolution of issues associated with protection performance in future converter-dominated power systems.The research work presented in this thesis addresses anticipated (and documented) protection challenges that will be introduced by the domination of power electronics interfaces in future power systems. A flexible and programmable voltage source converter (VSC) model with controllable fault response has been developed and this is tested using realistic network data (including transmission lines and the corresponding power flow/fault level data) from the GB transmission network, provided by National Grid ESO (the research project sponsor). The results of tests, where a range of variations to the converter controllers’ fault-responses have been implemented (e.g. to reflect different detection and initial converter response delays, output current ramp rates and magnitudes), are presented and analysed. The simulated voltage and current waveforms are injected into actual protection relays using secondary injection amplifiers. The responses of the relays are recorded and a number of issues are highlighted, particularly with respect to the response of distance protection. It is shown that, when the system is dominated by converter-interfaced sources (especially where the sources are modelled as being unable to provide “fast” and “high” fault currents, which is typically the case for actual converter systems), the responses of traditional distance protection systems (and other systems relying on measurement of current magnitude) could be delayed, lose discrimination, e.g. by tripping with a zone 2 delay for a zone 1 fault, or may be completely unable to detect faults at certain locations within the system. Based on the test results, potential solutions are then presented relating to changes to relay algorithms and/or the requirements for converters in terms of behaviour during faults. The outcomes of the work will be of interest to grid code developers (publications arising from this work have already been referred to by ENTSO-E guidance document for national implementation for network codes on grid connection [1]), transmission network operators, other researchers and protection/converter manufacturers. An overview of future work, relating to comprehensive studies (using injection and the developed system/converter models) of a range of faults/ infeeds/ converter mixes with a wide range of protection relays including distance and unit-type, and development of a standard commissioning testing method of protection relays under future power system scenarios that are dominated by converters, is included in the concluding section. This will assist in the investigation and resolution of issues associated with protection performance in future converter-dominated power systems

High Penetration of Power Electronic Interfaced Power Sources and the Potential Contribution of Grid Forming Converters

The traditional electrical power system and electricity markets have been designed to work with SGs, and so these have traditionally provided various 'inherent' capabilities to the system critical to ensure the stable operation of the power systems during severe faults and even basic system survival during rare system splits. Due to the potential total absence of SGs approaches during periods of high penetration (HP) of PEIPS infeed, the wider industry has engaged in a closer examination of the lack of these system capabilities [4], [17], [31], [32]. Traditionally, the focus in the context of PEIPS has been on steady state and a limited number of dynamic (faster) aspects recently expanded to include PEIPS contributing fast fault current during system faults and extended contribution to frequency management (although this latter capability has been required from RES for more than 10 years in some countries). Demand side contributions in these contexts are emerging and have significant potential

Control and Stability of Residential Microgrid with Grid-Forming Prosumers

The rise of the prosumers (producers-consumers), residential customers equipped with behind-the-meter distributed energy resources (DER), such as battery storage and rooftop solar PV, offers an opportunity to use prosumer-owned DER innovatively. The thesis rests on the premise that prosumers equipped with grid-forming inverters can not only provide inertia to improve the frequency performance of the bulk grid but also support islanded operation of residential microgrids (low-voltage distribution feeder operated in an islanded mode), which can improve distribution grids’ resilience and reliability without purposely designing low-voltage (LV) distribution feeders as microgrids. Today, grid-following control is predominantly used to control prosumer DER, by which the prosumers behave as controlled current sources. These grid-following prosumers deliver active and reactive power by staying synchronized with the existing grid. However, they cannot operate if disconnected from the main grid due to the lack of voltage reference. This gives rise to the increasing interest in the use of grid-forming power converters, by which the prosumers behave as voltage sources. Grid-forming converters regulate their output voltage according to the reference of their own and exhibit load sharing with other prosumers even in islanded operation. Making use of grid-forming prosumers opens up opportunities to improve distribution grids’ resilience and enhance the genuine inertia of highly renewable-penetrated power systems. Firstly, electricity networks in many regional communities are prone to frequent power outages. Instead of purposely designing the community as a microgrid with dedicated grid-forming equipment, the LV feeder can be turned into a residential microgrid with multiple paralleled grid-forming prosumers. In this case, the LV feeder can operate in both grid-connected and islanded modes. Secondly, gridforming prosumers in the residential microgrid behave as voltage sources that respond naturally to the varying loads in the system. This is much like synchronous machines extracting kinetic energy from rotating masses. “Genuine” system inertia is thus enhanced, which is fundamentally different from the “emulated” inertia by fast frequency response (FFR) from grid-following converters. Against this backdrop, this thesis mainly focuses on two aspects. The first is the small-signal stability of such residential microgrids. In particular, the impact of the increasing number of grid-forming prosumers is studied based on the linearised model. The impact of the various dynamic response of primary sources is also investigated. The second is the control of the grid-forming prosumers aiming to provide sufficient inertia for the system. The control is focused on both the inverters and the DC-stage converters. Specifically, the thesis proposes an advanced controller for the DC-stage converters based on active disturbance rejection control (ADRC), which observes and rejects the “total disturbance” of the system, thereby enhancing the inertial response provided by prosumer DER. In addition, to make better use of the energy from prosumer-owned DER, an adaptive droop controller based on a piecewise power function is proposed, which ensures that residential ESS provide little power in the steady state while supplying sufficient power to cater for the demand variation during the transient state. Proposed strategies are verified by time-domain simulations

Stability of microgrids and weak grids with high penetration of variable renewable energy

Autonomous microgrids and weak grids with high penetrations of variable renewable energy (VRE) generation tend to share several common characteristics: i) low synchronous inertia, ii) sensitivity to active power imbalances, and iii) low system strength (as defined by the nodal short circuit ratio). As a result of these characteristics, there is a greater risk of system instability relative to larger grids, especially as the share of VRE is increased. This thesis focuses on the development of techniques and strategies to assess and improve the stability of microgrids and weak grids. In the first part of this thesis, the small-signal stability of inertia-less converter dominated microgrids is analysed, wherein a load flow based method for small-signal model initialisation is proposed and used to examine the effects of topology and network parameters on the stability of the microgrid. The use of a back-to-back dc link to interconnect neighbouring microgrids and provide dynamic frequency support is then proposed to improve frequency stability by helping to alleviate active power imbalances. In the third part of this thesis, a new technique to determine the optimal sizing of smoothing batteries in microgrids is proposed. The technique is based on the temporal variability of the solar irradiance at the specific site location in order to maximise PV penetration without causing grid instability. A technical framework for integrating solar PV plants into weak grids is then proposed, addressing the weaknesses in conventional Grid Codes that fail to consider the unique characteristics of weak grids. Finally, a new technique is proposed for estimating system load relief factors that are used in aggregate single frequency stability models

Evolution of microgrids with converter-interfaced generations: Challenges and opportunities

© 2019 Elsevier Ltd Although microgrids facilitate the increased penetration of distributed generations (DGs) and improve the security of power supplies, they have some issues that need to be better understood and addressed before realising the full potential of microgrids. This paper presents a comprehensive list of challenges and opportunities supported by a literature review on the evolution of converter-based microgrids. The discussion in this paper presented with a view to establishing microgrids as distinct from the existing distribution systems. This is accomplished by, firstly, describing the challenges and benefits of using DG units in a distribution network and then those of microgrid ones. Also, the definitions, classifications and characteristics of microgrids are summarised to provide a sound basis for novice researchers to undertake ongoing research on microgrids