A neutral-point-clamped three-level inverter with small dc-link capacitors is presented in this paper. The inverter requires zero average neutral-point current for stable neutral-point voltage. The small dc-link capacitors may not maintain capacitor voltage balance, even with zero neutral-point current. This may happen due to nonlinearities present in the circuit. This requires a fast control of the neutral-point voltage. A simple carrier-based modulation strategy which allows modeling of the neutral-point voltage dynamics as a continuous function of power drawn from the inverter is proposed. This continuous model shows that the neutral-point current is proportional to the power drawn from the inverter, and it enables the use of a well-established classical control theory for the neutral-point voltage controller design. A simple proportional integral controller is designed for the neutral-point voltage control on the basis of the continuous model. The design method for optimum performance is discussed. The implementation of the proposed modulation strategy and the controller is very simple. The controller is implemented in a 7.5-kW induction machine-based drive with only 14 μF dc-link capacitors. Also, the experimental results show that fast and stable performance of the neutral-point voltage controller are achieved and thus verify the validity of the proposed control approach.

Design of Neutral-Point Voltage Controller of a Three-Level NPC Inverter With Small DC-Link Capacitors

A small dc-link capacitor based drive system shows instability when it is operated with large input line inductance at operating points with high power. This paper presents a simple, new active damping technique that can stabilize effectively the drive system at unstable operating points, offering greatly reduced input line current total harmonic distortion. The proposed method requires only a first-order, high-pass filter with a gain. Active damping voltage terms, linked directly to the dc-link voltage ripple through gain units, are injected to the drive machine for stabilizing the operating points. The stabilizing effect of the active damping terms is demonstrated for an induction machine based drive system. The effects of the added damping terms on the machine current and dc-link voltage are analyzed in detail. A design recommendation for the proposed active damping terms is given. Experimental results verifying the effectiveness of the new active damping method are presented.

An Active Damping Technique for Small DC-Link Capacitor Based Drive System

This paper introduces a cascaded H-bridge multilevel converter (CHB-MC)-based StatCom system that is able to operate with extremely low dc capacitance values. The theoretical limit is calculated for the maximum capacitor voltage ripple, and hence minimum dc capacitance values that can be used in the converter. The proposed low-capacitance StatCom ( LC -StatCom) is able to operate with large capacitor voltage ripples, which are very close to the calculated theoretical maximum voltage ripple. The maximum voltage stress on the semiconductors in the LC -StatCom is lower than in a conventional StatCom system. The variable cluster voltage magnitude in the LC -StatCom system drops well below the maximum grid voltage, which allows a fixed maximum voltage on the individual capacitors. It is demonstrated that the proposed LC -StatCom has an asymmetric V–I characteristic, which is especially suited for operation as a reactive power source within the capacitive region. A high-bandwidth control system is designed for the proposed StatCom to provide control of the capacitor voltages during highly dynamic transient events. The proposed LC -StatCom system is experimentally verified on a low-voltage seven-level CHB-MC prototype. The experimental results show successful operation of the system with ripples as high as 90% of the nominal dc voltage. The required energy storage for the LC -StatCom system shows significant reduction compared to a conventional StatCom design.

Low-Capacitance Cascaded H-Bridge Multilevel StatCom

In grid-connected photovoltaic (PV) power stations, improving the life expectancy and long-term reliability of three-phase PV inverters is urgently needed to match the significantly higher lifetime of the PV modules. A key contribution toward such improvement is replacing the conventional electrolytic film capacitors by metallized polypropylene film ones. This paper presents a detailed evaluation of a conventional three-phase grid-connected PV inverter performance when replacing the electrolytic capacitor with a minimum value of metallized polypropylene film capacitor. Although the minimum dc bus capacitance leads to higher voltage ripples, such ripples were found to be within acceptable limits to operate the inverter satisfactorily. Simulation and experimental results are presented for a 5-kW grid-connected inverter prototype with a nominal dc voltage of 457 V to confirm the theoretical considerations.

Performance Evaluation of Three-Phase Grid-Connected Photovoltaic Inverters Using Electrolytic or Polypropylene Film Capacitors

In this paper, an analytic capacitor voltage filtering scheme for a cascaded H-bridge (CHB) multilevel converter-based static synchronous compensator (STATCOM) is proposed. The scheme is able to filter out a low-frequency ripple of any magnitude and introduces a negligible delay to the control system. Therefore, it facilitates using high-reliability film capacitors with low capacitance instead of the conventional electrolytic capacitors. The proposed analytic filtering scheme is implemented and verified on a single-phase seven-level CHB multilevel converter-based STATCOM. The parameters' sensitivity evaluation is provided to demonstrate practicability of the proposed scheme. Simulation and experimental results are provided to compare the performance of the proposed filtering scheme with the performance of the most commonly used second-order filter-based approach to demonstrate its effectiveness.

Reduced-Capacitance Thin-Film H-Bridge Multilevel STATCOM Control Utilizing an Analytic Filtering Scheme

3-phase AC-drives for HVAC applications (heating, ventilation & air-conditioning) typically employ a passive diode rectifier and an electrolytic DC-link capacitor to filter the rectified line voltage. The DC capacitor may be accompanied by AC or DC-chokes for harmonic line- current reduction. Recently, commercial HVAC drives, where all traditional passive rectifier components are replaced with a "small" DC-link film capacitor, have emerged. These are referred to as slim DC-link AC-drives. This AC-drive is compared to traditional types below.

H.R. Andersen

Papers

3-phase AC-drives with passive front-ends with focus on the slim DC-link topology