T.C. Green

Control and filter design of three-phase inverters for high power quality grid connection

The trend toward using inverters in distributed generation systems and micro-grids has raised the importance of achieving low-distortion, high-quality power export from inverters. Both switching frequency effects and pre-existing grid voltage distortion can contribute to poor power quality. A well designed filter can attenuate switching frequency components but has an impact on the control bandwidth and the impedance presented to grid distortion. This paper describes a filter designed to incorporate an isolating transformer and the design of a complementary controller that rejects grid disturbance, maintains good waveform quality and achieves real and reactive power control. A realistic discrete time implementation is discussed and validated with experimental results.

Predictive transient-following control of shunt and series active power filters

A novel technique is presented for generation of a contemporary estimate of the fundamental component of the distorted input current or voltage to an uncontrolled three-phase bridge rectifier with a DC link smoothing filter. This allows for accurate calculation of cancellation references for series and shunt active power filters (APF) operating under steady-state and transient conditions. Improved transient performance allows for reduction of the power rating and control system bandwidth of an APF. An artificial neural network (ANN) predictor has been used to directly calculate the mean dq-axis input to the rectifier without filtering. This is a critical stage in separating harmonic distortion from fundamental current or voltage. The technique is developed using simulation data for both series and shunt APFs and validated with experimental results. The predictive harmonic identifier shows good steady-state performance and excellent transient performance that far exceeds that of a conventional identifier using time-domain or frequency-domain filtering.

On optimization for security and reliability of power systems with distributed generation

Electricity market restructuring and supra-national agreements on the reduction of global greenhouse gas emissions have paved the way for an increase in the use of distributed generation - the connection of generation to the lower voltage power system. This paper formulates and discusses a methodology for the optimal siting and sizing of distributed generation a security constrained system can accept. Optimal siting is determined by sensitivity analysis of the power flow equations. The sizing method for a set of loading conditions, generation penetration level and power factor is formulated as a security constrained optimization problem. The information on optimal generation sites is used further to optimize system reliability assessed via reliability indices calculation. A genetic algorithm is designed to solve for optimal recloser positions when distributed generators are deployed in a securely optimal manner.

Estimating rotational iron losses in an induction machine

We present an assessment of the effect of rotational losses on an induction machine. The assessment provides an estimate of the iron losses in an induction machine by three methods, all of which rely on the output data of a two-dimensional finite-element method: 1) calculating iron losses as if they were produced by a purely alternating field; 2) calculating the iron losses by adding the losses produced by the orthogonal components of the flux density, as if the losses produced by these components were independent phenomena; 3) applying a correction factor based on experimental data to improve the rotational loss calculation. The correction factor is a function of the peak flux density value and the ratio of the major to the minor axis of the flux density loci. The third method represents the main contribution of this paper to the field and is explained in detail. Finally, a discussion of the results addresses two aspects: the location where rotational fields occur, and their impact on the total loss calculation.

Hybrid control of multiple inverters in an island-mode distribution system

Inverter-interfaced distributed generation offers the possibility of introducing power quality functions such as suppression of harmonic distortion. However, the traditional voltage- and frequency-droop methods of achieving load sharing work on average values and do not address waveform quality. This paper proposes a hybrid scheme for an island-mode system with many inverters. Inverters in close proximity operate in master-salve mode whereas load sharing between distant groups uses frequency droop. Communication between inverters is used where it can improve performance but not where such links are impractical. The master inverter uses repetitive voltage control at the common node to suppress harmonic distortion. Slave inverters within a group also use repetitive control but in current mode. The performance has been assessed through simulation.

The development of power quality markets

Power quality (PQ) issues in electricity distribution systems are regarded from an economical perspective, starting from parallels in environmental economics. In order to create incentives to efficiently achieve required levels of PQ, an emission permit/right trading system for the different PQ phenomena, each with a different market, is presented. Its implementation and functioning are explained. This system allows the stimulation of the installation of mitigating devices at the customer site or in the grid and makes it possible to tailor the needs of individual customers, who will become more aware of PQ.

Evaluation of repetitive control for power quality improvement of distributed generation

Small-scale distributed generation (DG) is often not a natural 50 or 60 Hz AC source and so employs an inverter for the interface to the utility grid. Here, repetitive control is examined as a means of also using a DG inverter to improve the distortion of a local grid where a large proportion of the load is nonlinear. The proposed controller can offer better waveform quality in balanced and unbalanced conditions than PI controllers in either stationary or rotating reference frames. The inverter also requires a control loop to regulate the exported power. A decoupled P and Q controller is applied and compared to a traditional amplitude and angle controller. The controllers are tested for disturbance rejection of variations in grid voltage, DC-link voltage and load power.

Control of power quality in inverter-based distributed generation

Power quality is an important additional service of inverter-based interfaces for distributed generators. In grid connected applications the power quality depends on the harmonic content of the current injected at the point of common coupling. By careful design of the power converter and its output filter the switching frequency components in the output current spectrum can be reduced to low levels. The effect of the harmonic distortion of the grid voltage on the output current can be minimised by using an appropriate inverter control strategy. Conventional control methods (manipulation of inverter voltage magnitude and phase) offer active and reactive power control, but not the control of the output current quality. This paper describes a new choice of control structure and explains the interaction between the applied control loops. The inverter is used to control the current in the first element of an LCL filter. A further controller is wrapped around this loop to control power export to the grid. The usefulness of this arrangement in providing big power quality is emphasised. Experimental results from a 10 kVA prototype are used to evaluate the distortion rejection properties and the regulation of active and reactive power control. The results show high quality of generated power and excellent transient and steady state-response to both active and reactive power demands.

Assessment of power losses of an inverter-driven induction machine with its experimental validation

Detailed power loss distribution in induction machines is assessed using time-stepped finite element analysis coupled to circuit equations of an inverter. Losses are examined for various load conditions. Iron losses are largely unexplored and so particular attention is paid to them here. The simulation has revealed the division between ohmic, hysteresis, classic eddy current and anomalous losses; and the distribution in the frequency spectrum between fundamental, slotting, and multiples of the switching frequency. Insight is gained into the spatial location of the loss. Experimental validation is provided for several fundamental frequencies from full-load to light-load conditions.

Robust repetitive control of grid-connected DC-AC converters

The paper proposes a possible circuit topology and voltage controller design for connecting a DC-AC converter to the power grid. This converter is meant to operate in conjunction with a small power generating unit. The design of the output voltage controller is based on H/sup /spl infin// and repetitive control. This leads to a very low harmonic distortion of the output voltage, even in the presence of nonlinear loads and/or a distorted grid voltage. The output voltage controller contains an infinite-dimensional internal model, which enables it to reject all periodic disturbances which have the same period as the grid voltage and whose frequency components up to approximately 1.5 kHz are considered.