Now showing 1 - 10 of 16
  • Publication
    Methodology for Assessment of the Impact of Smart Transformers on Power System Reliability
    The smart transformer (ST) has been proposed as an alternative to the traditional low frequency transformer as a means to provide extra control functionality in the smart power system. The ST has merits in terms of reactive power decoupling and voltage decoupling at the primary and secondary side. This provides flexibility for reactive power compensation in the transmission system and demand reduction in the distribution system. Using its ability to control demand through voltage regulation, the ST provides the possibility to reduce demand while keeping the entire load online, which can provide an alternative to load curtailment. Thus, it may provide a means to improve power system reliability. However, no of previous research has investigated these potential system reliability benefits of the ST. The paper presents a methodology which can be used to quantify the system reliability impacts of the use of STs
  • 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.
      374Scopus© Citations 82
  • 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.
      275Scopus© Citations 29
  • Publication
    Harmonic stability of VSC connected Low Frequency AC offshore transmission with long HVAC cables
    Low Frequency AC (LFAC) transmission has been proposed as an alternative to HVDC transmission for the integration of offshore wind. The LFAC offshore grid as a fully power electronic grid with a long HVAC cable provides significant challenges to harmonic stability. This paper presents an impedance based stability analysis to determine the stability of the power electronic offshore system across the harmonic frequency range. The stability analysis is introduced and applied to the LFAC system. The impact of different current and voltage control bandwidths and component sizes on the dynamic impedance of the converters is then examined and their impact on harmonic stability of the LFAC grid is determined. It is found that detailed knowledge of the control parameters and the ability to tune the bandwidths can mitigate significant harmonic instability with the presence of a long HVAC cable. Three phase simulations are then used to validated the impedance based stability technique.
      352Scopus© Citations 3
  • 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.
      69Scopus© Citations 21
  • 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.
      208Scopus© Citations 3
  • 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.
      220Scopus© Citations 9
  • 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.
      374Scopus© Citations 10
  • 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.
      320Scopus© Citations 14
  • 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.
      452Scopus© Citations 9