Now showing 1 - 10 of 17
  • 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.
      583Scopus© Citations 10
  • 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.
      520Scopus© Citations 118
  • Publication
    Neutral Current Optimization Control for Smart Transformer-fed Distribution System under Unbalanced Loads
    EU In a 3-phase 4-wire LV distribution system, unbalanced loads lead to neutral current (NC) looping resulting in increase of power losses and variation of neutral potential. Compared to the conventional power transformer, Smart Transformer (ST) has strict current limitations to avoid overcurrent. However, its advantages on the downstream LV grid voltage regulation can provides the capability to regulate excessive NC. This paper proposes a closed-loop NC optimization control in order to, on the one hand, minimize the NC current in the normal operation satisfying the standard EN 50160 requirement, on the other hand, suppress the NC current in extreme cases to avoid the overcurrent damage of the ST. The proposed control strategies are validated by experimental tests via the hardware-in-the-loop setup and a case study based on a 350kVA, 10kV/400V, ST-fed distribution network under unbalanced loading profile according to the 3-phase 4-wire distribution grid in Manchester area. The results clearly prove the effectiveness and flexibility of the proposed NC optimization control strategies on the NC suppression and minimization.
      363Scopus© Citations 14
  • Publication
    Smart transformer Modelling in Optimal Power Flow Analysis
    The smart transformer (ST) implemented using power electronics converters, has the capability of independent voltage control and reactive power isolation between its primary and secondary terminals. This capability provides a flexibility in the power system to support the voltage at the primary side and control the demand at the secondary side. Using this flexibility, the system power flow could, for example, be optimized for lower costs. This paper proposes an ST model suitable for OPF analysis. The effects of using multiple STs at different penetration levels, on the daily generation costs in an IEEE 39 bus test system are presented.
      333Scopus© Citations 3
  • 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  379
  • 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.
      191Scopus© Citations 27
  • 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.
      435Scopus© Citations 40
  • 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.
      330Scopus© Citations 50
  • 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.
      475Scopus© Citations 11
  • Publication
    Analysis of virtual synchronous generator control and its response based on transfer functions
    Virtual Synchronous Generator (VSG) control has been proposed as a means to control power electronics converter interfaced generation and storage which retains the dynamics of the conventional synchronous machine. This study provides a comprehensive, transfer function based, analysis of VGS control, which can be used as the basis for the design of VSG transient and steady-state performance. Based on a hardware validated, large signal model, a small signal model and associated transfer functions which describe the changes in real and reactive powers in response to changes in references and grid frequency disturbances. The derived transfer functions are used to obtain insight into the correct design of VSG controllers. The small signal models, transfer functions and associated analysis are validated by comparison with measured results on a scaled hardware system.
      470Scopus© Citations 34