skip to main content

Title: Robust Control of Solid State Transformer using Dynamic Phasor based model with dq transformation
A high frequency solid-state transformer (SST) proposed by FREEDM centre is an interesting alternative to conventional distribution transformer in microgrids as it supports additional functionalities such as active-reactive power flow control, fault current limitation and voltage regulation. This paper proposes a dynamic phasor based robust control of SST through the modular control of each stage. The control problem is formulated in frequency domain by representing the system states with time varying Fourier coefficients or dynamic phasors (DP). This formulation transforms the oscillating waveforms of ac circuits to constant or slowly varying variables, hence allow the use of PI controller to track the sinusoidal references. For rectifier and inverter stages of SST, dq transformation is applied on DP which facilitates the design of PI controller to smoothen out the ripples in the output voltage waveform. The controller gains are tuned to reject input and load disturbances and attenuate measurement noise using loop shaping and pole assignment technique. The robustness of the controller is assured analytically against parametric uncertainties using small gain theorem. Simulation results are provided to support the proposed control scheme. Hardwarein- Loop (HIL) simulation is carried out on critical stages using Opal-RT and dSPACE simulators to confirm the effectiveness of the proposed scheme.
Authors:
; ; ; ;
Award ID(s):
1710944
Publication Date:
NSF-PAR ID:
10163270
Journal Name:
2019 North American Power symposium
Page Range or eLocation-ID:
1 to 6
Sponsoring Org:
National Science Foundation
More Like this
  1. In this study, a four-port solid-state transformer (SST) with decoupled control scheme to control the power flow and the output voltage is proposed. The proposed decoupled control scheme controls all of the four ports' powers independently. In addition, the design of the four-port transformer including core material selection and winding placement is investigated. The designed transformer is modeled in ANSYS-Maxwell and also co-simulated with ANSYS-Simplorer. The operating frequency of the system is designed for 100 kHz; therefore, a very compact size is obtained for the entire multi-port converter. The performance of the proposed system is validated throughout MATLAB/Simulink simulation and experimental studies carried out for a 10kW/port SST prototype. The obtained results show that the four-port SST provides an interface for four-different power supplies or loads. It is seen that the proposed decoupled control scheme can control the output voltage at the desired value and track the reference power signals for each port. It provides as well a good steady state and dynamic performance.
  2. In this study, analysis and control of a highly efficient, high-power full-bridge unidirectional resonant LLC solid-state transformer (SST) are discussed. A combination of pulse frequency modulation and phase-shift modulation is utilized to control this resonant converter for a wide load range. The converter is designed to maintain soft switching by using a resonant circuit to minimize the switching loss of the high-frequency converter. Zero-voltage-switching (ZVS) is achieved for the H-bridge converter. The ZVS boundary for the proposed combined control method is also analyzed in detail. The experimental setup for the suggested configuration was implemented, and the performance of the proposed control scheme and resonant LLC SST have been verified with test results. The proposed combined control scheme improves control performance. The obtained results show that, the proposed system can regulate output voltage and maintain soft switching in a wide range of load. Thus, the efficiency of the system is improved and an efficiency of 97.18% is achieved.
  3. In this study, a sliding mode control (SMC) scheme is proposed for the single-phase cascaded H-bridge (CHB) multilevel active front end (AFE) rectifier with LCL filter. A PI controller is employed to control the DC voltage of the rectifier modules and to obtain the amplitude for the reference grid current. The SMC based current control scheme uses the grid current and filter capacitor voltage feedbacks. The resonance of the LCL filter is damped using the voltage feedback of the capacitor. Therefore, the requirement for additional damping circuitry is removed. Simulation and experimental results are presented to verify the performance of the SMC for the CHB multilevel AFE rectifier. The overall proposed control scheme provides almost unity power factor and fast transient response. It is seen from the results that the current drawn from the grid is in sinusoidal waveform with low THD.
  4. In this paper, design of a compact high frequency four-port transformer for a Solid-State Transformer (SST) arrangement is presented. Unlike other SSTs, the four-port system integrates three active sources and a load port with galvanic isolation via a single transformer core. In addition to this feature, one of the three source ports is designed to operate at Medium Voltage (MV) 7.2kV for direct connection to 4.16kV AC grid, while other ports nominal voltages are rated at 400V. The transformer is designed to operate at 50kHz and to supply 25kW/port. Thus, the proposed system connects the MV grid, Energy Storage System (ESS), PV, and DC load to each other on a single common transformer core. Based on the system power demand and availability of renewable energy resources, utility and energy storage ports can either supply or draw power, while PV port can only supply power, maintaining the required demand for the load. This work focuses mainly on the High Frequency Transformer (HFT) design. An extensive study is carried out to obtain the optimal, compact, cost effective, and high efficiency model. Modeling, mathematical, and simulation results are derived and presented to demonstrate the viability of this design.
  5. In wireless power transfer (WPT) applications, the multi-level switched capacitor topology achieves significant advantages in terms of efficiency, system loading, THD, and output regulation. The topology requires dual-loop control in order to harness these benefits. First, a small signal discrete time model for the 7-level rectifier WPT system is developed. Then, a control loop is designed that enables the rectifier to regulate DC load voltage by varying its modulation scheme. Next, the WPT carrier frequency is sensed and a phase-locked loop is used in combination with the small signal power stage model to design a closed-loop controller that synchronizes frequency and regulates control phase through adjustments of the switching period. Finally, cross-coupling interactions between the two control loops are modeled, and stable dual-loop operation is shown.