Effects of swing equation-based inertial response (SEBIR) control on penetration limitsof non-synchronous generation in the GB power system

This paper investigates the limits to penetration levels of non-synchronous generation (NSG) in a power system and how this may be increased. Reduced system inertia, arising from high penetrations of NSG, is one of the main issues that may increase the risk of system instability in various guises. Swing equation - based inertial response (SEBIR) control, often referred to using a variety of terms, is considered to be a potential solution that can enable converter - interfaced generation to support the system during and after disturbances. However, the effects of SEBIR on system operability and its ability to increase the NSG penetration limits and improve system strength under high NSG scenarios has not been fully investigated. The paper presents the implementation of SEBIR control within a simplified model of the future Great Britain (GB) transmission model, created using DIgSILENT PowerFactory. Using the model, the instantaneous penetration level limits of NSG in terms of both transient and steady - state stability are investigated with and without SEBIR control applied to the NSG. The capability of SEBIR in enabling additional active power output from NSG and improving system frequency response under a loss of infeed event is investigated and it is shown how SEBIR can assist in increasing NSG penetration levels, but that further work is required to understand certain phenomena that have been observed

A VSM (virtual synchronous machine) convertor control model suitable for RMS studies for resolving system operator/owner challenges

In recent years, it has become clear that reaching the targeted levels of renewable power generation poses problems, not only for basic infrastructure and generation/load balancing, but also in terms of fundamental network stability. In Ireland, the contribution from convertor-connected generation is already constrained to 50-55%, while recent studies of other networks suggest that any "penetration" of convertors above 65% could lead to instability. The phenomena have been observed both in RMS and high-fidelity EMT simulations of convertor-dominated power systems, and appears to be unavoidable when using the dq-axis current-source controllers within conventional grid-connected convertors. The high control bandwidth (>50 Hz) of these convertors also means that they cannot be effectively included within RMS type large-scale network models. The idea of "synthetic inertia" has been proposed in some publications as a mitigating solution but needs to be considered carefully, since if implemented incorrectly it has been shown to further destabilise the network at the critical small timescales and high frequencies. In this paper we present simple versions of a Virtual Synchronous Machine (VSM) model which is implemented and demonstrated in both transient and RMS based simulations. An important aspect of the VSM is that the controller’s bandwidth is low (<<50 Hz). This means that it can be modelled with reasonable accuracy in RMS simulation with time steps of the order of 2ms. From a system operator perspective, large-scale RMS simulations of entire countries or regions containing hundreds of VSM generators can be carried out with reasonable accuracy

Use of an inertia-less virtual synchronous machine within future power networks with high penetrations of converters

Conventional converter models for wind turbines and Voltage Source HVDC links, as submitted to System Operators, typically use dq-axis controllers with current injection (DQCI). Recent work carried out by the authors has proven that for DQCI converter-interfaced sources there are overall penetration limits, i.e. the 'tipping points' beyond which the system will become unstable. Initial investigations of this "tipping point", based on a reduced model of the transmission system of Great Britain using phasor simulation within DIgSILENT PowerFactory, are reviewed briefly in this paper. The 'tipping points' relating to maximum penetration of DQCI converter-interfaced sources are subsequently investigated in this paper using a higher fidelity three-phase dynamic power system model in Matlab Simulink. Additionally, a new converter controller, termed here as Virtual Synchronous Machine Zero Inertia (VSM0H), is described and implemented in the model. It is shown that, in principle, it is possible to significantly increase the penetration of converter based generation (up to 100% of installed capacity) without reaching a stability constraint

Effects of VSM convertor control on penetration limits of non-synchronous generation in the GB power system

2013 saw the presentation of a paper [1][2] to the wind integration workshop, which demonstrated 26 high convertor penetration scenarios, 17 of which introduced a type of instability in RMS models previously unseen by the researchers. It also provided an indication of the constraints necessary if NSG levels where to be limited, potentially placing practical limits on the amount of NSG which could be accommodated. It demonstrated that Synchronous Compensation (SC) could be used to mitigate these and other problems but this is believed to be an expensive solution. Further publications have demonstrated that convertor instability at high NSG extends beyond RMS models and is believed to occur in real systems [3]. In addition, Swing Equation Based Inertial Response (SEBIR) control, sometimes referred to as "Synthetic Inertia", has been shown to be ineffective as a countermeasure against the instability observed in [1][2] and can in some circumstances make it worse [4][5]. Whilst SEBIR improves RoCoF, its inability to address the wider range of problems resulted in the need for more comprehensive solutions. Several authors have proposed converters using principles aligned with VSM and VSM0H concepts and controllers using these concepts exist within marine power networks. This paper returns to the studies presented in [1][2], which used a reduced 36 node GB model in PowerFactory (PF). However here, some of the convertors are replaced with VSM convertor models described in [6] to investigate the effects on Instantaneous Penetration Level (IPL) limit of NSG in terms of transient stability and steady-state stability. These and further results presented demonstrate the potential of VSM, in mitigating the effects of various challenges associated with high NSG, potentially allowing 100% penetration

Instantaneous penetration level limits of non-synchronous devices in the British power system

The installed capacity of non-synchronous devices (NSD), including renewable energy generation and other converter-interfaced equipment such as energy storage, bi-directional transfer links, electric vehicles, etc., is expected to increase and contribute a large proportion of total generation capacity in future power systems. Concerns have been expressed relating to operability and stability of systems with high penetrations of NSD, since NSD are typically decoupled from the grid via power electronic devices and consequently reduce the “natural” inertia, short-circuit levels and damping effects which are inherently provided by synchronous machines. It is therefore crucial to ensure secure and stable operation of power systems with high penetrations of NSD. This paper will show and quantify the instantaneous penetration level (IPL) limits of NSD connected to a simple example power system in terms of steady-state stability beyond which the system can become unstable or unacceptable, defined as “unviable”. The NSD used in this example will be a conventional dq-axis current injection (DQCI) convertor model. The paper will introduce a set of criteria relating to locking signal in converter phase-locked loop, frequency, rate of change of frequency and voltage magnitude, which will be used to determine the system viability and the IPL limit. It will also be shown that there are several factors that can potentially affect the IPL limits. Frequency and voltage droop slopes and filter time-constant for DQCI converter are varied and it is shown how these settings influence the IPL limits. Finally, to provide additional insight into network viability under high penetrations of NSD, a visualisation method referred here as “network frequency perturbation” is introduced to investigate responses of individual generators to a change in network frequency

Stability challenges & solutions for power systems operating close to 100% penetration of power electronic interfaced power sources : exchange of experience between hybrid and major power systems

In 2013 the authors presented a paper [1] to the wind integration workshop (WIW), the results of which demonstrated high converter penetration (typically 65-70%) at synchronous area (SA) level could introduce a type of super synchronous instability in RMS models previously unseen by the authors and TSO’s. 2016 saw two related papers [2], [3] presented at a further WIW. These provided in-depth analysis of the specific high frequency instability identified in 2013, as well as considering a wider range of future high penetration stability challenges identified by the wind industry [4]. The two papers reported R&D study results using a proposed holistic approach converter control strategy. Extensive system wide studies with a new (for large power systems) control strategy for power electronic sources were used to explore the possibility of stable operation close to 100% penetration of Power Electronic Interfaced Power Sources (PEIPS). The studies demonstrated that implementation of a Virtual Synchronous Machine (VSM) converter control strategy with added stability controls, applied to about 25% of the power sources, could deliver stable operation, even at 100% penetration for a reduced model of the 2030 GB power system. The solutions explored in the WIW 2016 papers are included in the Grid Forming approach in a pan European Connection Network Code (CNC) Implementation Guidance Document (IGD HPoPEIPS) [5]. This contains significant ideas and experiences arising initially from the world of Hybrid Systems, such as marine power networks. In taking these ideas forward, various questions are raised by manufacturing industry experts about the necessity for this dramatic change in the context of main power systems. Some suggest it is a more fundamental change, even than the introduction of Fault Ride Through (FRT)). Also, it has been suggested that both the time needed to implement the new strategies and the associated cost will be extensive. This paper explores the prospect of finding answers to these questions from experience already gained in the world of hybrid systems. What are the prospects for closer collaboration to establish viable solutions applicable to both small Hybrid Systems and main Synchronous Areas (SA), such as the 5 SAs in Europe as the first SAs progress towards operation sometimes close to 100% PEIPS

Enhanced virtual synchronous machine (VSM) control algorithm for hybrid grid forming converters

There has been considerable interest in convertor solutions which to a greater or lesser extent mimic the behaviour of synchronous machines, thus overcoming many of the disadvantages of the existing technology which are potentially destabilizing at high penetration. These solutions are frequently referred to as Grid Forming Convertors (GFC). For offshore installations, where some equipment is on shore, locating equipment offshore is more expensive and carries greater commercial risks, requiring extensive testing and confidence building prior to deployment in real applications. This is time consuming and particularly significant for GB and where there are significant quantities of offshore generation. Onshore solutions to stability are therefore desirable for Off-Shore Transmission Owners (OFTOs) and might also be applied by retrofitting to existing conventional converter plant. This paper presents and discusses findings of the second stage of the research focusing on the enhanced control algorithm for Hybrid Grid Forming Convertors for Offshore Wind Applications and its performance, while the previous paper [3] presents the initial findings comparing various hybrid solutions for offshore networks where the STATCOM onshore is replaced by synchronous compensator and GFC or Virtual Synchronous Machines (VSM) converter of similar rating with the aim to achieve levels of Grid-Forming capability

Dispatching parameters, strategies and associated algorithm for VSM (virtual synchronous machines) and HGFC (hybrid grid forming convertors)

With the increasing drive towards renewable generation, there is no doubt that whilst the modern converter based plant is starting to replace conventional synchronous generation on a MW for MW basis, there is concern that the modern converter based plant is unable to contribute the same features as Synchronous Generation. Inertia, fault level, synchronizing torque which are for example, all fundamental pre-requisites for the design of and operation of a reliable and robust power system. There is therefore considerable interest in VSM and GFC controls which emulate the behavior of synchronous machines. This paper discusses various parameters, general dispatch principals in relation to VSM / GFC and proposes a possible algorithm to demonstrate a method of dispatch which aims to provide a robust level of system security whilst minimizing the plant requirements, particularly in respect of the storage, inertial and converter rating requirements which are seen as costly items as part of a plant design. In particular, the aim of this work is to develop a set of requirements which are sufficiently flexible to enable developers to meet using whatever means they wish to, yet at the same time ensuring that the Power System remains secure and robust against a background of increasing volumes of converter based plant. The solution discussed, is not tied to commercial parameters allowing these to be selected independently. The starting point is the initial requirement from Grid Code Consultation GC0100 Option 1 [9], and this work therefore informs potential implementation of solutions and specifications