Volume 33, Number 9, October, 1999 Download this article in PDF format. (379 KB)
Introduction Recent approaches to development of compensation methods aim to develop compensators of a type that would be able to realize dynamic compensation (in real time) and would also be more resistant to interference caused by the power network or electricity users. Their objectives include optimization of the loads as seen by power sources (power network). According to Fryze`s suggestion [5] and subsequent developments [4,10,12,13], it is necessary to eliminate a differential current (between a distorted load current and an ideal form of current (i.e., in-phase sine wave)) flowing through the power source to achieve such compensation. In concept, this can be done by generating and injecting a current equal to and in opposite phase to the differential current. In practice, obtaining such a source is difficult; what is really called for is an active system with parametric elements or controlled-current power sources. Structure of an Active Filter
Fig. 1. Block diagram of the active filter. The active filter consists of following modules and elements:
The active filter control process occurs in two phases:
The quality and dynamic properties of the compensation process depend mostly on the method used for calculating the reference-current parameters. Akagi et al’s theory of instantaneous reactive power [1] is commonly used to control power active filters. The authors believe that this theory does not fulfill the requirements of optimization of work in an energy source/receiver system. The general aim of optimization is to minimize the out-of-phase component of source current, reduce distortion of sinusoidal waveforms, and minimize active power losses in transmission of energy from source to receiver. To determine a current, which would have such properties, we applied the variational methods of [10]. As a result, we obtained an expression describing an optimal source current (the target reference current) in the following analytical form:
Where: is the voltage source, the equivalent conductance in form: , where: and are instantaneous values of active power and rms voltage source [10]. The frequency and phase of the reference signal correspond to suitable values of the first harmonic of the voltage source, . To effectively realize the whole control process, CM was divided into two sub-modules:
The Hardware and Software The evaluation system was developed with the addition of a universal analog and digital input/output module type ALS100, which had been designed by P.E.P. ALFINE as an extension of the ADDS–2106X–EZ–KIT. This module (Figure 2), designed for power-electronics applications, includes A/D and D/A converters, as well as PWM generators and a System Console (LCD & KBD). Communication with the host PC is established via an RS-232 port under control of a DSPHOST program. Figure 2 shows the hardware and software structure of the Control Module. The main module of the control program was written in C language (ADDS–21000–SW–PC ver. 3.3), and time-critical procedures are writen in Assembler. The Control Module consist of:
Fig. 2. The hardware and software structure of Control Module (CM) The SIM (Figure 3), consists of three independent blocks: software-frequency-identifier of the reference (SFI), software-amplitude-identifier of the reference (SAI) and software synchronizer of suitable values of the reference (SSYNC).
Fig. 3. Software structure of Software Identification Module (SIM) The SFI uses a mains-supply-voltage pre-filtration method, with the aid of a pass-band FIR filter (F1), to eliminate higher harmonics and increase noise immunity of the identification algorithm [14]. Next the signal is subjected to a Hilbert Transformation to obtain its analytical form (complex signal in the time domain). It permits elimination of frequency products on the negative part of the frequency axis and decreases the identification time to under 12 ms. This is a short time in relation to the 20-ms (50-Hz) mains-voltage period of the present design, and would also be considerably shorter than the 16.7-ms period of 60-Hz systems. [14]. The complex signal is subjected to a digital Fourier transform (DFT) to calculate its basic frequency. This is realized by the DFT and MAX blocks. Calculated in this way, the value of basic frequency serves next to control the tuned filter (F5), a high-Q, IIR-type filter. The F5 filter is in fact the reference current generator; its output signal frequency is equal to the mains-voltage frequency . The amplitude of reference current is calculated within the SAI block, which is based on both load-voltage and load-current samples, stored within circular buffers CB2 and CB3. A synchronization block, SSYNC, eliminates effects of different delay times, involved in calculations within the SFI and SAI blocks. Finally, the SSYNC connects suitable values of frequency and magnitude of reference current. The total time of identification and synchronization of the reference current generator (in this design) is about 18 ms. The Decision Module is realized in the form of a 2^{nd} order FIR filter with constant coefficients; its frequency transmittance model is given by following equation:
The basic condition of proper operation of the filter is that the system sample frequency is twice the PWM carrier frequency (in this system: 30 and 15 kHz). The Execution Module is a power-electronics controlled current source, which uses a highly integrated Intelligent Power Module (IPM) type PM50RSA120 (MITSUBISHI) and inductance coil . This coil also limits parasitic products of the PWM. The general source of energy for the current source is a capacitor within the dc circuit of the inverter (IPM). The inverter is coupled with the Control Module with the aid of fast photocouplers. Performance of the Prototype System The waveforms of Figure 4 show the rectangular shape of reference signal , output current of current source and feedback signal (Figure 4a) and results of spectrum analysis of these quantities (Figure 4b). The bandwidth (-3 dB) of the current source was equal to 3.2 kHz with non-uniform amplitude characteristic 0.4 dB. The total harmonic distortion (THD) of output current within this band was 0.7% — and 0.2% within the 0.5-kHz bandwidth.
Fig. 4. Investigation results of current source prototype system for the case of rectangular shape of reference signal: a) waveforms of selected quantities; b) spectrum analysis. Figures 5 and 6 illustrate the workings of the complete active filter. The source of the distorted current (Figure 5) is a simple single-diode rectifier with RL type load (series connection of resistor and inductor). It is a particularly unfavorable case, because it simultaneously generates a strongly distorted current with a dc component and reactive power. The waveforms of source voltage, , and currents of load, , power network, , active filter, and reference signal, are shown in Figure. 5a—and also results of spectrum analysis of selected quantities (Figure 5b). Figure 6 shows similar quantities for an RC-loaded 4-diode-bridge, the typical configuration of most consumer-electronics power packs.
Fig. 5. Investigation results of the active-filter prototype model with astrongly nonlinear passive receiver—a single-diode RL-loaded rectifier: a) waveforms of selected voltage and current quantities; b) spectrum analysis.
Fig. 6. Investigation results of the active-filter prototype model with an RC-loaded 4-diode bridge: a) waveforms of selected voltage and current quantities; b) spectrum analysis. As in the case of a current source, the active compensation’s system of differential current provides good mapping of the reference signal, , which is calculated in the Identification Module. The power network current is in the same phase as the waveform of power network voltage (because of compensation of so-called reactive power), and its higher harmonics values are considerably reduced. The THD value of active filter input current, , was under 1 %. Conclusion References |