Utility Interface Research Articles

Electrotechnical requirements for PV on buildings

AbstractWorld‐wide the number of grid‐connected PV systems is growing fast, especially in the built environment. In order to assure the quality and energy output of these systems, a number of electrotechnical requirements need to be fulfilled, at both component and system levels. In addition requirements with respect to electrical safety need to be met. Part of these requirements are covered by international standards, especially with respect to the PV modules. A number of standards is still under development. In the area of systems and utility interfacing local codes are still in use. These local codes differ significantly from country to country. Copyright © 2004 John Wiley & Sons, Ltd.

Application of a sinusoidal internal model to current control of three-phase utility interface converters

Three-phase voltage-source converters are used as a utility interface. In such a case, the converter line currents are required to track sinusoidal references synchronized with the utility grid without steady-state error. In this paper a current control method based on a sinusoidal internal model is employed. The method uses a sine transfer function with a specified resonant frequency, which is called an S compensator. The combination of a conventional PI compensator and an S compensator is called a PIS compensator. The PIS compensator ensures that the steady-state error in response to any step changes in a reference signal at the resonant frequency and zero hertz reduces to zero. An experiment was carried out using a 1-kVA prototype of three utility interface converters, a voltage-source rectifier, an active power filter, and STATCOM. Almost perfect current tracking performance can be observed. © 2004 Wiley Periodicals, Inc. Electr Eng Jpn, 150(3): 54–61, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20064

PIS補償器を用いた系統インタフェイスコンバータの電流制御

PIS補償器を用いた系統インタフェイスコンバータの電流制御

Applications of cascade multilevel inverters

Cascade multilevel inverters have been developed for electric utility applications. A cascade M-level inverter consists of (M-1)/2 H-bridges in which each bridge's dc voltage is supported by its own de capacitor. The new inverter can: (1) generate almost sinusoidal waveform voltage while only switching one time per fundamental cycle; (2) dispense with multi-pulse inverters' transformers used in conventional utility interfaces and static var compensators; (3) enables direct parallel or series transformer-less connection to medium- and high-voltage power systems. In short, the cascade inverter is much more efficient and suitable for utility applications than traditional multi-pulse and pulse width modulation (PWM) inverters. The authors have experimentally demonstrated the superiority of the new inverter for power supply, (hybrid) electric vehicle (EV) motor drive, reactive power (var) and harmonic compensation. This paper summarizes the features, feasibility, and control schemes of the cascade inverter for utility applications including utility interface of renewable energy, voltage regulation, var compensation, and harmonic filtering in power systems. Analytical, simulated, and experimental results demonstrated the superiority of the new inverters.

A novel high-performance utility-interactive photovoltaic inverter system

This paper presents a novel photovoltaic inverter that cannot only synchronize a sinusoidal AC output current with a utility line voltage, but also control the power generation of each photovoltaic module in an array. The proposed inverter system is composed of a half-bridge inverter at the utility interface and a novel generation control circuit which compensates for reductions in the output power of the system that are attributable to variations in the generation conditions of respective photovoltaic modules. The generation control circuit allows each photovoltaic module to operate independently at peak capacity, simply by detecting the output power of the system. Furthermore, the generation control circuit attenuates low-frequency ripple voltage, which is caused by the half-bridge inverter, across the photovoltaic modules. Consequently, the output power of the system is increased due to the increase in average power generated by the photovoltaic modules. The effectiveness of the proposed inverter system is confirmed experimentally and by means of simulation.

Applications of cascade multilevel inverters.

Cascade multilevel inverters have been developed for electric utility applications. A cascade M-level inverter consists of (M-1)/2 H-bridges in which each bridge's dc voltage is supported by its own dc capacitor. The new inverter can: (1) generate almost sinusoidal waveform voltage while only switching one time per fundamental cycle; (2) dispense with multi-pulse inverters' transformers used in conventional utility interfaces and static var compensators; (3) enables direct parallel or series transformer-less connection to medium- and high-voltage power systems. In short, the cascade inverter is much more efficient and suitable for utility applications than traditional multi-pulse and pulse width modulation (PWM) inverters. The authors have experimentally demonstrated the superiority of the new inverter for power supply, (hybrid) electric vehicle (EV) motor drive, reactive power (var) and harmonic compensation. This paper summarizes the features, feasibility, and control schemes of the cascade inverter for utility applications including utility interface of renewable energy, voltage regulation, var compensation, and harmonic filtering in power systems. Analytical, simulated, and experimental results demonstrated the superiority of the new inverters.

Target area and diagnostic interface issues on the National Ignition Facility (invited)

The National Ignition Facility (NIF) is under construction at Lawrence Livermore National Laboratory for the DOE Stockpile Stewardship Program. It will be used for experiments for inertial confinement fusion ignition, high energy density science, and basic science. Many interface issues confront the experimentalist who wishes to design, fabricate, and install diagnostics, and to help this process, a set of standards and guideline documents is being prepared. Compliance with these will be part of a formal diagnostic design review process. In this article we provide a short description of each, with reference to more complete documentation. The complete documentation will also be available through the NIF Diagnostics web page. Target area interface issues are grouped into three categories. First are the layout and utility interface issues which include the safety analysis report, target area facility layout; target chamber port locations; diagnostic interferences and envelopes; utilities and cable tray distribution; and timing and fiducial systems. Second are the environment interface issues which include radiation electromagnetic interference/electromagnetic pulse effects and mitigation; electrical grounding, shielding, and isolation; and cleanliness and vacuum guidelines. Third are the operational interface issues which include manipulator based target diagnostics, diagnostic alignment, shot life cycle and setup, diagnostic controllers; integrated computer control system; shot data archival; classified operations; and remote operations.

The operation of an inverter-assisted single-phase induction generator

The enhancement of the operability range of a stand-alone single-phase induction generator scheme with an inverter-battery system connected to the auxiliary winding is the theme of this paper. The reactive power requirement of the load connected to the main winding is met by the inverter system operating with either of two proposed pulsewidth modulation techniques, in the process of which the load voltage and frequency are regulated. The proposed schemes have potential application as power sources for autonomous systems and for utility interface to single-phase power system grids. Simulation and calculation results of the generator system connected to an impedance and motor loads, confirmed with some experimental results, are also set forth to validate some of the conclusions of the paper.

A phase tracking system for three phase utility interface inverters

The analysis and design of the phase-locked loop (PLL) system is presented for the phase tracking system of the three phase utility interface inverters. The dynamic behavior of the closed loop PLL system is investigated in both continuous and discrete-time domains, and the optimization method is considered for the second order PLL system. In particular, the performance of the three phase PLL system is analyzed in the distorted utility conditions such as the phase unbalancing, harmonics, and offset caused by the nonlinear load conditions and measurement errors. The tracking errors under these distorted utility conditions are also derived. The phase tracking system is implemented in a digital manner using a digital signal processor (DSP) to verify the analytic results. The design considerations for the phase tracking system are deduced from the analytic and experimental results.

A robust three-phase active power-factor-correction and harmonic reduction scheme for high power

This paper proposes a robust three-phase active power-factor-correction (PFC) and harmonic reduction scheme suitable for higher power applications. The proposed system is a unique combination of a low-kilovoltampere 12-pulse rectifier system with a single-phase boost PFC scheme to shape the input current to near sinusoidal waveshape. The voltampere rating of the active PFC converter is 0.05 pu and is not exposed to line transients under varying load conditions. The proposed system is suitable for utility interface of higher power rectifiers employed in power supplies and adjustable-speed drive systems which demand clean input power characteristics in the range of 1-500 kW. The proposed system is rugged and, in the event the active control were to fail, the system reverts to 12-pulse operation with fifth and seventh harmonic cancellation. Analysis and design of the system is examined in detail, and simulation and experimental results on a 10 kVA prototype are shown.

Autoconnected-electronic-transformer-based multipulse rectifiers for utility interface of power electronic systems

In this paper, an autoconnected electronic transformer (ACET) for 12- and 24-pulse rectifier systems is proposed. Considerable reduction in size and weight is achieved by operating the transformer core at a higher frequency. The rating of the autotransformer is a fraction of the output power. Utility line currents are multipulse in nature with the cancellation of several lower order harmonic currents. Furthermore, the magnetic components on the rectifier DC side are also exposed to higher frequency and can be reduced in size and weight. With an operating frequency of 990 Hz and utilizing conventional grain-oriented steel, a size reduction of 1/3 is achieved compared to a conventional 60 Hz design. Finer grades of steel or alternatives, such as amorphous material, would increase the performance advantage even further. Another unique feature of the proposed concept is that the switching frequency components are neutralized at the input side of the rectifier system. A second configuration using an open delta ACET is also presented. The proposed concept is suitable for higher power rectifier systems, which demand clean input power characteristics. This paper presents analysis, simulations, design example and experimental results for a 208 V 10 kVA prototype system.

A new medium-voltage PWM inverter topology for adjustable-speed drives

In this paper, a new pulsewidth modulation (PWM) inverter topology suitable for medium-voltage (2300/4160 V) adjustable-speed drive systems is proposed. The modular inverter topology is derived by combining three standard three-phase inverter modules and a 0.33-pu output transformer. The output voltage is high-quality multistep PWM with low dv/dt. Further, the approach also guarantees balanced operation and 100% utilization of each three-phase inverter module over the entire speed range. These features enable the proposed topology to be suitable for powering constant-torque, as well as variable-torque type loads. Clean power utility interface of the proposed inverter system can be achieved via an 18-pulse input transformer. Analysis, simulation and experimental results are shown to validate the concepts.

Reducing supply current harmonics

In this article, a simple and effective method for reducing the supply current harmonics has been proposed for a parallel connected 12-pulse thyristor converter with an interphase reactor. The proposed method requires only an interphase reactor with an optimal inductance value being much smaller than conventional inductance values, without any additional devices or components. The supply current waveform of the 12-pulse thyristor converter with the optimal interphase reactor is more sinusoidal than that of a 36-pulse thyristor converter. The proposed design concept of the interphase reactor has been discussed, and confirmed by using digital computer simulation. An experimental system has been constructed and tested, and the experimental results have verified the practicability and effectiveness. The authors conclude that the proposed concept provides a clean power utility interface for various industrial/utility applications including superconducting energy storage systems (SMES).

A thyristor phase-controlled voltage-source converter with bidirectional power flow capability

A new line-commutated thyristor phase-controlled voltage-source power converter (VSC) with bidirectional power flow capability is proposed and studied. The proposed power converter has the same structure as the pulsewidth modulation (PWM) VSC, however, the switching devices are not self-commutated devices but line-commutated thyristors, and the converter is operated not with PWM but with six-pulse phase control. As in the case of the PWM VSC, the proposed converter provides a bidirectional DC-side current capability and, hence, the DC-side voltage can be regulated at its desired value. The harmonics are much smaller than those in the conventional thyristor phase-controlled current-source power converter. The proposed converter can be useful as a utility interface power converter, especially in high-power voltage-source inverter motor drive systems with regenerative braking.

A novel three-phase utility interface minimizing line current harmonics of high-power telecommunications rectifier modules

Based on the combination of a three-phase diode bridge and a DC/DC boost converter, a new three-phase three-switch three-level pulsewidth modulated (PWM) rectifier system is developed. It can be characterized by sinusoidal mains current consumption, controlled output voltage, and low-blocking voltage stress on the power transistors. The application could be, e.g., for feeding the DC link of a telecommunications power supply module. The stationary operational behavior, the control of the mains currents, and the control of the output voltage are analyzed. Finally, the stresses on the system components are determined by digital simulation and compared to the stresses in a conventional six-switch two-level PWM rectifier system.

New 24-pulse diode rectifier systems for utility interface of high-power AC motor drives

This paper proposes two new passive 24-pulse diode rectifier systems for utility interface of pulsewidth modulated (PWM) AC motor drives. The first approach employs an extended delta transformer arrangement, which results in near equal leakage inductance in series with each diode rectifier bridge. This promotes equal current sharing and improved performance. A specially tapped interphase transformer is then introduced with two additional diodes to extend the conventional 12-pulse operation to 24-pulse operation from the input current point of view. The proposed system exhibits clean power characteristics with fifth, seventh, eleventh, thirteenth, seventeenth, and nineteenth harmonics eliminated from the utility line currents. The second scheme is a reduced voltampere approach employing autotransformers to obtain 24-pulse operation. The voltampere rating of the polyphase transformer in the second scheme is 0.23P/sub 0/ (PU). Detailed analysis and simulations verify the proposed concept, and experimental results from a 208-V 10-kVA rectifier system are provided.

Polyphase transformer arrangements with reduced kVA capacities for harmonic current reduction in rectifier-type utility interface

In this paper, polyphase transformer arrangements with reduced kVA capacities are presented for harmonic current reduction in high power rectifier utility interface systems. Based on the concept of an autotransformer, a proposed twelve-pulse rectifier system is realized with a resultant kVA rating of 0.18P/sub 0/ (pu). In this arrangement, the rectifier diodes each conduct for 120 degrees per cycle and the fifth and seventh harmonics are absent from the utility input line current. In the second scheme, an eighteen-pulse rectifier is realized with a kVA rating of 0.16P/sub 0/ (pu), rectifier diode conduction of 120 degrees per cycle and fifth, seventh, eleventh and thirteenth harmonic cancellation in the utility line currents. Additional applications for the proposed polyphase transformer arrangements include twelve and eighteen-pulse systems feeding multiple six-pulse nonlinear loads, such as AC motor drives, with an appropriate phase shift, and this arrangement substantially reduces the utility line current harmonics. For example, it is shown that the fifth and seventh harmonics generated by two separate six-pulse nonlinear loads are subtracted and this contributes to reduced harmonic distortion. Simulation results verifying the proposed concept are presented. >

A new active interphase reactor for 12-pulse rectifiers provides clean power utility interface

In this paper, a new active interphase reactor for twelve-pulse diode rectifiers is proposed. The proposed system draws near sinusoidal currents from the utility. In this scheme, a low kVA [0.02 P/sub o/ (PU)] active current source injects a triangular current into an interphase reactor of a twelve-pulse diode rectifier. This modification results in near sinusoidal utility line currents with less than 1% THD. It is further shown that a low kVA, 12-pulse system with an autotransformer arrangement [kVA rating of 0.18 P/sub o/ (PU)] can be implemented with the proposed active interphase reactor. The resulting system draws clean power from the utility and is suitable for powering larger kVA AC motor drive systems. Detailed analysis of the proposed scheme along with design equations is illustrated. Simulation results verify the concept. Experimental results are provided from a 208 V, 10 kVA rectifier system.

Flux-based active filter controller

This paper presents a synchronous frame flux-based control method for a parallel active filter application. The flux-based controller directly implements the inverter switchings in the synchronous reference frame by a hysteresis rule-based carrier-less pulse-width modulation (PWM) strategy to achieve high current bandwidth. This paper addresses the issues and impact on parallel active filtering requirements for utility interface of commonly used harmonic front-ends. The synchronous frame flux-based controller provides additional insights for harmonic current compensation requirements. Simulation results provide the validation of the flux-based active filter controller to meet IEEE Standard 519 recommended harmonic standards for large rated nonlinear loads under balanced and unbalanced supply conditions.

ADVANCED POWER ELECTRONICS FOR REDUCING MAINS POLLUTION — ACTIVE POWER FILTERS AND OPTIMIZED UTILITY INTERFACES

Rapid advances in turn-off semiconductor elements offer new opportunities for reactive power compensation and reduction of harmonics in energy supply systems. Non-linear consumers and also solid state power electronic equipment like diode rectifiers and thyristor converters disturb the utility voltage waveform by injecting harmonic currents into the utility grid. Their need of reactive power results in higher line currents and thereby in an increase of the rated power of the supply system. Active power filters can be used to compensate the harmonic currents, to balance unbalanced systems and to provide reactive power to the load with small storage elements. Optimized utility interfaces are controlled to ensure sinusoidal input currents of the load with unity power factor. Both, the investigated active power filter as well as the optimized utility interface are based on a four-quadrant switch-mode voltage source inverter.