Po-Tai Cheng

A new droop control method for the autonomous operation of distributed energy resource interface converters

Microgrid is widely accepted as an effective mean of integrating various distributed energy resources (DERs) through their interface converters to provide electric power of high quality and reliability. These distributed resources interface converters (DICs) are required to operate in an autonomous fashion without any communication for enhanced reliability. Conventionally, the real power-frequency droop control and the reactive power-voltage droop are adopted as the de-centralized control strategies in these DICs for the autonomous power sharing operations. However, the reactive power sharing of Q-V droop control often deteriorates if there are mismatched transmission line impedance characteristics. In this paper, a Q-V dot droop control method with V dot restoration mechanism is proposed to improve the sharing of reactive power. Its operation principle and control method are explained and analyzed. In addition, simulation and experimental results are presented to validate the effectiveness of the proposed method.

Power decoupling methods for single-phase three-poles AC/DC converters

This paper presents a new method for power decoupling of single phase AC/DC converters using the conventional PWM H-bridge converter for bi-directional AC/DC conversion at unity power factor, and a boost converter for the power decoupling function at DC side. The proposed method compensates the pulsating power at twice the grid frequency by absorbing the ripple current to a DC decoupling capacitor through the boost converter operation. This approach can eliminate the need of large filter capacitor in DC side which is required to suppress the low frequency ripples. This paper explains the principles of operations of the proposed method, and experimental results are also presented for validation.

Control strategies for distributed energy resources interface converters in the low voltage Microgrid

Microgrid technologies are very effective in integrating the distributed energy resources for providing high quality and high reliability of electric power. The Microgrid is often deployed as a cable-based and small-scaled distribution network at a low voltage level. As a result, the line impedance is much more resistive. This paper presents a minimum output current tracking control method for interface converters of distributed energy resources to improve the power sharing issues in a low voltage Microgrid system. The proposed technique manipulates the droop control to achieve minimum converter output current under different conditions of loads and transmission line characteristics. The operation principles are discussed below. Simulation and laboratory test results are also presented to validate the proposed method.

A low voltage ride-through technique for grid-connected converters of distributed energy resources

With more and more distributed energy resources (DERs) being installed, the utility requires these generation systems and their interface converters to remain grid connected during voltage sags to ensure the operating stability of the ac power system. These low-voltage ride-through (LVRT) requirements also suggest that the DER generation system injects real power and reactive power to support grid voltages. In this paper, a positive- and negative-sequence current injection method is proposed to meet the LVRT requirement. The proposed method imposes a predefined ampere constraint in its current injection to reduce the risk of overcurrent during the LVRT operation. Its operation principle and control method are explained and analyzed. Experimental results are presented to validate the effectiveness of the proposed method. Comparisons of the proposed method and other LVRT techniques are also presented.

A grid synchronization method for droop controlled distributed energy resources converters

With the high penetration of distributed generation system, many control methods have been widely discussed for managing the power flows between these distributed energy resources converters in islanded or grid-connected operation modes. The grid synchronization method has been also elaborately discussed for single grid-connected converter. However, it is not often explored for the multi-converter oriented system. In this paper, a grid synchronization method for the multi-converter oriented distributed generation system is proposed. The proposed grid synchronization method can cooperate with P - f, Q - V droop controls, and all the distributed energy sources converters regulate their own phase angles and voltage magnitude at the same speed. Thus the original power flow determined by these droop controllers can be maintained during the operation of grid synchronization. Its operation principle is explained, and experimental test results are presented to validate the effectiveness of the proposed grid synchronization method.

Real-time simulations of a laboratory-scale micro-grid system in Taiwan

Real-time Hardware-in-the-Loop (HIL) simulation is an effective technique to include complexity of the micro-grid under control in the testing real-time embedded system. In this paper, we will report recent progresses of real-time simulations of a laboratory-scale micro-grid system in Taiwan. We use the OPAL-RT platform for real-time simulations and HIL control tests. In particular, the following three tasks will be addressed: (i) real-time simulations of the laboratory-scale micro-grid. Dynamical models of various renewable energy sources and converters will be developed. Control strategies under the grid-connected mode and the islanding mode and transitions between these two modes will be studied. (ii) Real-time implementations of protection relays, including differential relays and distance relays, will be developed. Dynamic simulations confirm that specifications of the developed relay models for various operating scenarios. (iii) HIL control of STATCOMs. Double-loop control of the DC bus voltage and reactive power output of the converter will be employed. Closed relationships between real-time simulation results and HIL experiments indicate that implementations of the controller can be simplified by using HIL real-time environments.

Quantitative analysis of system parameters asymmetry on droop-controlled converters

In the microgrid applications, the converters come in all sizes and from different manufacturers, thus their closed-loop dynamics and AC side output filters are not going to be the same. The dispersed locations of these converters also lead to unequal power line impedances seen from the converters. These system parameters asymmetry affects the autonomous load sharing operations both in the steady state and in the transient. This paper discusses the impacts of these parameters asymmetry on the dynamics of the entire system. Based on the dynamic model analysis, this paper also provides some existing methods to address these problems.

An inrush current reduction technique for multiple inverter-fed transformers

This paper introduces a transformer inrush current reduction technique for the conventional double-conversion UPS feeding multiple loads and their transformers. As these loads are engaged or dis-engaged, their transformers are also connected or dis-connected from the power lines. To avoid the inrush current, a closed-loop flux linkage control is proposed to maintain the flux of transformers which are already engaged, and of the transformer which is switched-in. The proposed method can utilize the existing voltage and current sensors feedback of the UPS, and it can be seamlessly integrated with the voltage and current control. Experimental test results are provided to validate the effectiveness of the proposed method.

Implementation of the digital control algorithm for the maximum power point tracking DC module

The distributed maximum power point tracking (DMPPT) has become one of the most attractive structures of photovoltaic (PV) generation system recently. In this improved structure, the maximum power point tracking (MPPT) DC module is one of the most important components, which provides not only the better utilization of PV sources, but also many supplementary features on the reliability and safety. As more and more functions are put together into this MPPT DC module, the digital controller design gives the flexibility to reduce the complexity. Therefore, this paper discusses the implementing issues about the digital control algorithm for the MPPT DC module from the computational viewpoint. With the improved modifications, the computation time of the overall algorithm is reduced, and the discussed algorithm can thus be implemented in a tailor-made digital control core as a result.

UPS flux compensation techniques for transformer inrush reduction

This paper presents two flux compensation designs for the uninterruptible power supply system. The objective of the flux compensation is to reduce the potential inrush current of the uninterruptible power supply-fed load transformer in the event of grid disturbances and load transformer engagement. The proposed methods utilize the existing voltage and current sensors, and voltage and current control loops of the uninterruptible power supply system for easy implementation. The application scenario is explained, and details of the flux compensation techniques are presented. Laboratory test results are provided for validation.