Richard Duke

DC-Bus Signaling: A Distributed Control Strategy for a Hybrid Renewable Nanogrid

A dc nanogrid is a hybrid renewable system since renewable sources supply the average load demand, while storage and nonrenewable generation maintain the power balance in the presence of the stochastic renewable sources. The system is power electronic based, with converters being used to interface both the sources and loads to the system. The nanogrid is controlled using dc-bus signaling (DBS), a distributed control strategy in which the control nodes, the source/storage interface converters, induce voltage-level changes to communicate with the other control nodes. This paper explains the control structure required for the converters to permit the use of DBS, and explains a procedure for implementing a system-wide control law through independent control of the source/storage interface converters. Experimental results are presented to demonstrate the operation of this novel control strategy

The steady-state performance of a controlled current active filter

An active filter that uses a high-frequency D-class asynchronous switching inverter for power system current distortion compensation is described. The distortion compensation technique involves deriving a signal corresponding to the distortion component of load current, inverting and amplifying this signal, and adding it to the supply current to cancel the load current distortion. A synthetic sinusoid is used to determine the distortion component in the time domain. Extensive computed and experimental results, illustrating the systems steady-state performance and ability to reduce the current harmonic distortion components are presented. An intelligent controller is proposed to maintain the active filters performance at the optimal operating point under varying load conditions. >

Stability and Bandwidth Implications of Digitally Controlled Grid-Connected Parallel Inverters

The increasing use of grid-connected inverter systems is resulting in a desire for parallel-connected inverters that offer greater power capacity while maintaining the high control bandwidth achieved by individual inverters. This paper demonstrates that, in addition to the traditional stability and bandwidth limitations of digitally controlled inverters, further stability and bandwidth limitations occur when LCL inverters with a common set point are connected in parallel to a grid. This paper provides detailed discrete-time derivations for parallel grid-connected inverters and uncovers stability and bandwidth limitations that only occur in grid-connected applications and are not apparent if the system is studied in continuous time. This paper demonstrates that, in a typical application, the voltage bandwidth of an LCL parallel inverter array is 25% lower than a single module or LC parallel configuration. Both simulations and hardware demonstrations on a 105-kVA parallel three-module grid-connected system confirm the findings.

A Case Study on the Application of the Nyquist Stability Criterion as Applied to Interconnected Loads and Sources on Grids

As the penetration of complex grid-connected devices, such as power electronic inverters, increases, so too does the complexity of analyzing system stability. A graphical application of the Nyquist stability criterion is presented that indicates how an individual load and source each contribute to the closed-loop system grid eigenvalues. The case study is not limited to particular impedance forms or scenarios like the more common complex torque coefficient method or passivity theory method. From the individual frequency responses of the load and source impedances, the graphical technique indicates how each impedance contributes to the system stability. Examples are provided that successfully indicate the cause of instability for a digitally controlled voltage source inverter (VSI) operating as a microgrid, with a current source inverter as a load. A second example is provided that identifies the potential instability of a VSI running an induction machine. A 42-kW inverter system is used to confirm the findings, showing a close correlation with the theoretical analysis.

Robust High-Performance Inverter Control Using Discrete Direct-Design Pole Placement

A robust high-performance discrete controller, with a simple implementation and characterizable performance that lend it to high-power industrial applications, is proposed. Discrete direct-design pole placement allows the use of common design parameters such as damping ratio and bandwidth (BW) to define the controllers response. High performance is demonstrated with example systems achieving BWs twice as high as those of traditional cascaded current- and voltage-loop controllers. The controller is shown to be robust against component variations and many different load types, including low-impedance grid connections. Comparisons between the proposed controller and a discretized full-state-feedback continuous-time-derived controller are made and show significant improvements in both BW and stability margins. Practical tests on a parallel 2-MVA test system demonstrate a close correlation with the analysis, with an error of less than 1%.

A frequency-domain analytical model of an uncontrolled single-phase voltage-source rectifier

The harmonic currents generated by the single-phase rectifier are well known. As the levels of these currents become larger, the use of power conditioners, such as shunt active filters, to lower the levels is becoming more attractive. In order to analyze the interaction between the condition, AC system and rectifier, it is necessary to have an accurate model of the rectifier. This paper describes a frequency-domain analytical model of the single-phase rectifier. The model includes the dominant frequency transfer mechanisms. These are the direct transfer and that due to the modulation of the switching instants. A small-signal linearized analysis is presented and the behavior predicted is confirmed by perturbation analysis using time-domain simulation. Accurate results are obtained, and the importance of including the switching instant modulation is shown.

Real-time optimization of an active filter's performance

Recent advances in power electronics have meant that many loads now draw a distorted current from the power supply. For the same real power consumed, the apparent power for the distorted load is greater than the equivalent sinusoidal load. A real-time active filter optimization algorithm has been implemented in a TMS320C30 DSP, with the aim of maximizing the monetary saving from active filtering by reducing the apparent power consumed at the point of supply. As the basis for this optimization a savings function which takes into account active filter efficiency, the cost of energy, and the supply and load current distortion before and after filtering, has been derived. A simplex optimization technique, which is able to find the optimum operating point even under varying load conditions, is used to maximize these energy savings. >

An adaptive battery monitoring system for an electric vehicle

In electric vehicles it is important to know the state of charge of the batteries in order to prevent vehicle strandings and to ensure that the full range of the vehicle is exploited. It is also useful to know state of health information about the batteries to predict when the batteries need replacing. This paper describes a battery monitoring system that is able to calculate the state of charge and state of health of multiple batteries in a battery bank. It has been designed specifically to monitor lead-acid batteries in an electric car environment using noninvasive measurement techniques. The monitor incorporates an adaptive monitoring method, which is based on coulometric measurements when the batteries are under load and predicted open circuit voltage measurements under noload conditions.

Autonomous Load Shedding in a Nanogrid using DC Bus Signalling

Including load shedding in the system control strategy for a small standalone hybrid power system such as a nanogrid is beneficial since the peak generation requirement is reduced and the system is protected from a complete collapse under overload conditions. Autonomous load shedding can be implemented in a nanogrid that uses DC bus signalling for source scheduling by shedding loads when the DC bus voltage decreases to a level that signals an overload condition. This paper explains the control requirements for the system interface converters that is required to permit load shedding and explains a procedure for implementing a prioritized load shedding scheme in a practical system. Experimental results are included to illustrate the operation of this control strategy

Three-Phase Phase-Locked Loop Control of a New Generation Power Converter

This paper describes the development of a new generation of power converter, used to power telecommunications equipment. A telecommunications converter must comply with the psophometric noise standard CCIF-1951 and the IEC1000-3-2 harmonic standard. While the IEC1000-3-2 standard is easily met with active power factor correction techniques, a high degree of effort is usually required to meet with the psophometric standard. Therefore, a control methodology utilising a three-phase phase-locked loop is introduced as a method of complying with the psophometric standard under distorted mains conditions. Simulations show that combining this with a novel feedback controller, results in an improved load step response over using a traditional proportional integral type controller