Now showing 1 - 10 of 16
  • Publication
    Parameter Constraints for Virtual Synchronous Generator Considering Stability
    A virtual synchronous generator (VSG) control for converters has been proposed as a method to provide virtual inertia from power electronics connected generation and storage. Most works to date have analyzed VSG control under the assumption that the VSG dynamics are much slower than that the converter. This work shows that when converter and line dynamics are taken into account, the virtual inertia and damping settings are constrained by stability considerations. These conditions for stability are analyzed based on a simple transfer function approach. It is shown that for the VSG to be stable and validly approximated by a second-order system, the ratio of damping to virtual inertia is a key parameter. This letter quantifies how these VSG parameters are constrained by stability. The transfer function analysis is validated using full switching model simulations of stable and unstable cases.
      544Scopus© Citations 121
  • Publication
    Placement of Virtual Synchronous Generator Controlled Electric Storage combined with Renewable Generation
    The virtual synchronous generator (VSG) control of power converters has been proposed to improve the system inertia. The commonly used VSG requires an energy storage (ESS) to provide the emulated inertia power and the frequency droop power. The placement of the ESS can be either in the DC port of the power electronics generation or co-located with the power electronics generation on the AC side. In the former case, the VSG control makes the total generator behave like a synchronous generator, while in the latter one, the VSG control only regulates the ESS in response to the grid frequency. The different placement of the ESS gives rise to different performance. This paper analyzes this performance firstly by means of simple transfer function analysis and secondly by comparing their effects on the grid frequency at the system level in the IEEE 39 bus test system. The results show that the placement of the ESS has no impact on the system transient stability while it has a significant impact on the frequency dynamics especially in the low inertia situation.
      309Scopus© Citations 9
  • Publication
    Neutral current reduction control for smart transformer under the imbalanced load in distribution system
    Imbalanced loads arouse neutral current looping in the distribution system, which increases power loss and results in neutral potential variation. Compared to the conventional power transformer, the smart transformer (ST) has advantages on the downstream voltage regulation. Thus, this paper proposes a voltage control strategy based on ST to reduce the LV grid neutral current according with EN 50160 imbalanced voltage standard. The proposed control has been validated in the Matlab/Simulink, and the system performance under the proposed control has been simulated under the imbalanced loading profile in a 400 kVA, 10 kV/400 V distribution network. The results prove the proposed control can practically reduce the neutral current.
    Scopus© Citations 7  380
  • Publication
    Use of voltage limits for current limitation in grid-forming converters
    (Power System Technology Press, 2020-02-13) ; ;
    Renewable generation interfaced through grid-forming converters are proposed as a replacement for synchronous generators in power systems. However, compared to the synchronous generator, the power electronics converter has a strict limit on the current to avoid overcurrent damage. The grid-forming converter acts like a voltage source, directily controlling the voltage. This conflicts with the operation of the conventional current limit control, which is applied to a current source. The switch between the voltage control and current control aimed to impose the current limit leads to synchronization instability. This paper proposes a novel control scheme which can be applied to the grid forming voltage control in order to enforce current limits. The proposed method has been verified through simulation and hardware tests in both symmetrical and asymmetrical faults to perform current suppression while maintaining synchronization stability in the voltage control mode.
      334Scopus© Citations 50
  • Publication
    100% Converter-Interfaced generation using virtual synchronous generator control: A case study based on the irish system
    The increase in the use of Converter-Interfaced Generation (CIG) in the power system will require these generators to not only feed the power but also establish the voltage and maintain the grid stability. Virtual Synchronous Generator (VSG) control of the CIG is proposed to fulfill this requirement since it mimics the dynamics of synchronous generation. This paper takes the all-Island Irish transmission system as an example to investigate the frequency stability of the system as it migrates towards 100% CIG under VSG control and quantifies the minimum conditions for frequency support to sustain the system under 100% CIG. Simulations are carried out considering the worst contingency in the Irish grid which is the loss of largest infeed, namely, the disconnection of the HVDC interconnector to the UK. The results are compared and discussed considering other scenarios that include primary frequency control of conventional power plants.
      200Scopus© Citations 30
  • Publication
    Impact of Current Transients on the Synchronization Stability Assessment of Grid-Feeding Converters
    The synchronization instability in the presence of a fault is a main issue for the dynamic behavior and control of grid-feeding converters. In the literature, the synchronization stability assessment is carried out considering the dynamics of Phase-Locked Loops (PLL) but the transients of converter currents are neglected. The letter shows that such a simplification leads to inaccuracies and, thus, the current transients cannot be neglected. The letter proposes a model that captures the effect of such current transients on the converter synchronization. This model allows assessing the transient behavior and, hence, the stability, of power electronics converters with high accuracy, comparable, in fact, to EMT models. The fidelity of the proposed model is duly discussed in the case study.
      315Scopus© Citations 46
  • Publication
    Smart Transformer for the Provision of Coordinated Voltage and Frequency Support in the Grid
    Considering the increase in renewable generation and the consequent reduction in power system inertia, the Virtual Synchronous Machine (VSM) control method has been proposed to control power converters to emulate the inertia and other the characteristics of the synchronous machine. However, to achieve the function of VSM control, an extra energy base, typically storage, is required to connect to the controlled converter. In this work we investigate the application of the VSM control to the distribution system demand through the use of a VSM controlled smart transformer. Through control of the demand in this way, the demand itself can be used to emulate inertia and provide frequency support. This paper presents the details of the flexible demand control applied to a smart transformer supplying a low voltage distribution grid. The operation of the control is validated on scaled hardware using real time simulation with hardware in the loop. Simulations on a 400 kVA, 400 V distribution network are used to quantify the demand flexible. IEEE 39 bus is used to verify the benefit of the proposed control in terms of voltage and frequency in the power system.
      443Scopus© Citations 16
  • Publication
    Smart Transformer and Low Frequency Transformer Comparison on Power Delivery Characteristics in the Power System
    Smart transformer is a power electronics-based transformer, offering voltage regulation and DC connectivity. As a transformer, its basic function is still power delivery. Smart transformer with advanced controls can support MV gird voltage by absorbing/injecting reactive power while actively regulate the LV grid voltage. Due to the controllable voltage in both MV and LV side, the power delivery of smart transformer is flexible. This paper focuses on the power delivery characteristic of smart transformer and compares with the conventional low frequency transformer with the help of STACTOM at its primary side or on load tap changer at its secondary side, in the power system by means of maximum deliverable power and power-voltage curve analysis. The Simulink results validate that the smart transformer improves system voltage stability compared to the traditional low frequency transformer with load tap changer.
      591Scopus© Citations 11
  • Publication
    Neutral Current Minimization Control for Solid State Transformers under Unbalanced Loads in Distribution Systems
    This paper analyses the neutral current reduction performance of a three phase four leg solid state transformer (SST) under different degrees of unbalanced load. Several kinds of control strategies are presented, the neutral current elimination controls which rely on phase shifting, voltage amplitude and phase shifting & voltage amplitude combination control. A neutral current minimization control which ensures the SST output voltages complies with the EN 50160 output voltage unbalance standard is also developed. These control approaches simply build on the balanced voltage control providing voltage references which slightly unbalanced the voltage amplitude and phase angle or both. The effectiveness of the proposed strategies is validated through tests on a downscaled prototype. Simulation results for the neutral current minimization control of the SST applied to a real urban distribution network with distributed loads are presented. The results of this analysis show that overall the neutral current minimization results in an energy saving from both reduced losses in the distribution cables and reduced power consumption in the load.
      486Scopus© Citations 11
  • Publication
    Assessment of Grid-Feeding Converter Voltage Stability
    This letter applies voltage stability analysis to grid feeding converters in the presence of the converter stability versus the grid state and its operation. By applying this analysis, it is shown that the converter may become unstable if the converter reference power or current exceeds the line capacity. This letter proposes to use a conventional PV curve to determine the stability of the dynamic response of grid-feeding converters considering both power and current limits.
      453Scopus© Citations 40