The behaviour of virtual synchronous machine (VSM) based converters in front of non-saturable faults

Power converter penetration has increased substantially in the last 20 years bringing new challenges from the system protection perspective. The power network is undergoing a major transformation as the major part of new installed power comes from non-synchronous sources such as wind or solar. These changes might lead to malfunction of the conventional protection schemes such as overcurrent protection or distance protection relays. At the same time, the reduction of the system inertia might cause the tripping of the Loss of Main protection due to a very aggressive Rate of Change of Frequency. To enhance the grid voltage source characteristic and mitigate the loss of inertia, a new set of converter controllers known as Grid forming Converter or Virtual Synchronous Machine has been suggested in recent years. The performance of VSM could provide a potential advantage compared to traditional power converter controllers when a large frequency deviation occurs helping to keep the system stable. This article quantifies and compares the performance of different converter control algorithms including Current Vector Control, Virtual Synchronous Machine and Power Synchronisation Control in front of different frequency events

Analysis of controller bandwidth interactions for vector-controlled VSC connected to very weak AC grids

Stability assessment of conventional vector current control (VCC) of voltage-source converters (VSCs) in weak grids has not been standardized. In this paper, a small signal model is derived to quantify the maximum active power transfer in a very weak grid across a much wider range of controller bandwidths than has previously been investigated. A novel investigation of the VCC-VSC controller bandwidth interactions between inner and outer control loops, including the phase-locked loop (PLL) dynamics, is demonstrated and a stability bubble of safe operating points is established. Robustness of the stability bubble under different SCRs is investigated and dynamic performance considerations are introduced to form a reduced operating region with good transient performance. The controller gains within this region allow rated power transfer in inverting mode and good dynamic performance with no modifications to the conventional VCC structure. For very weak grids, it is recommended that PLL bandwidths between 5 and 30 Hz are avoided. If a slow PLL bandwidth is chosen, the outer loop q-axis should have a fast bandwidth; with a fast PLL, the outer loop q-axis control bandwidth should be reduced. In all cases, the outer loop d-axis should be slowed down to reach the power transfer limit

PN admittance characterisation of grid supporting VSC controller with negative sequence regulation and inertia emulation

This work presents an analysis of converter output admittance for grid supporting VSC controllers in the positive – negative frame (pn-frame). Previously discovered issues in other reference frames are explored to prove the efficacy of analysis in the pn-frame The effect of negative sequence control is often overlooked and the pn-frame offers a useful method for observing the result. The impact of control parameters such as PLL bandwidth was explored which decreased network damping and increased regions of negative incremental impedance. Reduction of unwanted admittance components was achieved by the addition of appropriately tuned voltage feedforward filters. The equivalence of inertia and droops has been documented previously but not utilising converter impedance. Analogous traces of impedances were obtained for each structure with a similar response obtained when changing the respective associated gain indicating an equivalence

Simulating the Future Renewable-Based Power Network : High-Performance Computing for Power Systems Analysis

The standard simulation method for electric networks is Root Mean Squared (RMS) simulations, which models the grid lines at one fixed frequency, ignoring systems that work at different frequencies (e.g., power electronics). ElectroMagnetic Transient (EMT) simulations use time domain differential equations to model the components of the system. Allowing to capture with great accuracy the results of a grid simulation with components that work at different frequencies. However, EMT simulations take much longer time to simulate compared to RMS simulations. This project investigates different methods of simulating grid networks. MATLAB is used to simulate for RMS and EMT methods, investigating compilers ode23tb and ode45. PSCAD supports only EMT simulations, and two versions are used , PSCAD 5 with compiler GFortran 8.1 and educational license and PSCAD 4.6.3 with compiler GFortran 4.6.1 and professional license. Parallel core computing in PSCAD (PNI) is investigated as well. PNI is a technique that allows to split a big system (from one project) to multiple sub-systems (in more, interconnected, projects). Each sub-system is solved in a separated CPU core

A systematic review of DC wind farm collector cost-effectiveness

DC collection systems have been suggested to improve the cost-effectiveness of offshore wind farms but no consensus currently exists on which configurations are the most promising. This paper aims to determine the primary DC wind farm candidates for commercialisation based on cost-effectiveness and technological risk. A systematic review was performed of the literature that formally assesses the cost, losses or reliability of DC wind farm configurations. The optimal configurations were found to be dependent on the methodology and assumptions used by each study, as well as the individual wind farm characteristics. Series and series-parallel DC designs without offshore platform performed well in terms of costs, but have challenges in operation and reliability that limit the short-term opportunity for commercialisation. The standard DC parallel topology has the lowest technological risk, but the mean cost reported in the literature is similar to that of AC topologies. Standard parallel DC wind farms are the primary candidate for the first commercial DC wind farm demonstrators, but the optimal design will likely need to be determined on a case-by-case basis. Guidelines for this assessment are provided

Analysis of optimal grid-forming converter penetration in AC connected offshore wind farms

The modern electricity network is seeing a trend in the replacement of fossil fuel power plants with converter interfaced generation as worldwide efforts are made to combat climate change. New converter control structures such as grid-forming are seen as a key building block for maintaining the stability of the future power system. Moreover, wind power is the fastest growing renewable technology in the UK with ambitious targets set for installed capacity in the coming decade. While the benefits and drawbacks of the technology have been explored, little attention has been given to how many grid-forming converters will be needed to stabilise the modern network. Is there such a thing as too much grid-forming? This paper utilises an impedance-based windfarm model with the capability to include unique control systems on each turbine to present a small-signal based methodology for determining the penetration limits of grid-forming technology. Key stability and screening metrics are applied to identify the penetration that provides the strongest and most stable system. Three key points are specified: the critical, optimal and maximum penetrations. Moreover, findings suggest providing enhanced system strength via converters is only applicable to a certain extent where further interactions cause increased stability issues

Influence of the mission profile on the lifetime modelling of the wind turbine power converter – a review

Offshore wind energy is currently the leading offshore renewable energy technology and plays an essential role in achieving a low carbon economy in Europe. Offshore wind turbines, as a result of the harsher offshore environment, suffer from high O&M costs, and as such a high overall LCOE. These higher O&M costs can be reduced by focusing on the most critical subassemblies of the OWT, by either improving the reliability of the subassembly or by better understanding the failure mechanism, and thus optimizing the O&M strategy. The power converter is identified as one of the most critical subassemblies as risk of the operation of the OWT in terms of maintainability and availability. The reliability of the power converter is heavily influenced by both its steady-state and its dynamic thermal behavior, and as such great attention will be paid to the literature related to the thermal behaviour of the power converter. This paper seeks to provide a comprehensive overview on how the typical mission profile of an OWT influences the thermal loading. The paper seeks to propose acomprehensive and up-to-date literature review related to the lifetime modelling of the power converter, and to highlight some of the main challenges yet to be tackled in the reliability analysis of the power converter, suggesting future areas of researc

Small signal study of grid-forming converters and impact of different control structures and parameters

Towards transitioning to a carbon-free power system, new dynamic phenomena and interactions involving grid-forming converters (GFMs) will become important as their proliferation occurs to support this transition. The multi-loop control incorporating inner cascaded voltage and current controllers (VC and CC) often utilised within GFMs are generally expected to cause interactions in higher frequency ranges than the (well-studied) dynamics of interest associated with a purely synchronous machine-based system. However, the restricted control bandwidth associated with low switching frequency of large power rated VSCs results in the need for much slower control time constants, causing potentially destabilizing, lower-frequency (or even non-oscillatory) interactions. This paper offers an extensive insight into the small-signal, multi-machine interactions involving large power rated GFMs in a transmission network: the IEEE 39-bus New-England test system (NETS). Furthering the contribution of this paper, multi-loop controllers are employed within the GFMs, offering an insight into their interactions with other power system elements to help aid the ongoing discussions on model appropriateness and direct AC voltage control versus multi-loop control. Finally, parametric sweep sensitivity analyses are performed for the GFMs which are implemented as virtual synchronous machines (VSMs)

Performance evaluation of four grid-forming control techniques with soft black-start capabilities

Grid-Forming Converters (GFC) can be controlled as independent, self-starting, voltage sources. This feature is essential for power converters to achieve successful black-start sequence initiation. Conventional grid-following converters are not capable of self-starting an islanded network. GFC control thus exploits wider grid support and network restart potential. This study analyzes and compares four GFC controllers to assess their generic and soft black-start (ramping voltage) capabilities. The compared techniques are: Droop Control, Power Synchronizing Control (PSC), Virtual Synchronous Machine (VSM), and Matching control. These techniques are selected based on their direct voltage reference control flexibility. Various simulations are performed with common parameters to assess the response of each technique under similar conditions against load, DC voltage and active power reference disturbances, in addition to their soft-start readiness. The results demonstrate the high-level compatibility of these four controllers with soft black-start through successful and timely ramping voltage reference tracking. Moreover, the four considered control techniques achieve satisfactory performance, with VSM demonstrating more flexibility due to its tunable virtual inertia parameter (J)

Modified grid-forming converter control for black-start and grid-synchronization applications

The increasing grid integration of converter- based generation and distributed resources is necessitating a review to the classical synchronous generators dominant network models, and to equip converters with the necessary control capabilities such as inertia emulation and black-start to carry on the required ancillary services provision. This paper proposes a modified grid-forming converter control technique based on virtual synchronous machine (VSM). The modified controller uses a ramping voltage reference to achieve soft transformer energization and eliminate high-magnitude inrush current that can traditionally exceed converters rating. Grid-synchronizing functionality is added to the VSM controller to achieve smooth transition from black-start to grid-connected mode while maintaining grid-forming operation. This is achieved by gradually adjusting the power loop reference through a dedicated synchronizing controller, which is disconnected once grid-connection is achieved. A MATLAB/ Simulink case study is presented to illustrate the controller performance with a 40 MVA simulated converter. The results demonstrate successful black-start sequence execution, starting by soft transformer energization, followed by a block load pickup and grid-synchronization. The sequence is achieved with minimal transformer inrush current, and with seamless transition to the grid-connected operating mode