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pscad Application Note - Digital Switching Schemes in PSCAD Space-Vector Modulation


Digital Switching Schemes in PSCAD? Space-Vector Modulation

A PPLICA TI O N NO TES

The Challenge Among the most-used power electronic converters are Voltage-Sourced Converters (VSCs). VSCs are used in both high-power apparatus (in power-conditioning application: UPFC and STATCOM), and low-power apparatus (hybrid vehicles). In either case, the basic function of VSC is to convert a DC voltage to an AC, sinusoidal, three-phase voltage of controllable frequency, phase angle, and amplitude. VSCs come in a number of con?gurations. The number of switches in each leg determines the resemblance of the staircase-shaped outputwave-form to an ideal sinusoidal waveform. In a two-level VSC, the output voltage is a sequence of positive and negative pulses. VSCs with higher number of levels, which increases the complexity of the control scheme, have corresponding level of pulses to better approximate a sinusoidal waveform. For the sake of simplicity, we con?ne ourselves to two-level converters. Because of the nature of VSCs’ output, which is comprised of a number of pulses, the control techniques used to generate the voltage are called Pulse-Width Modulation (PWM) schemes. A conventional PWM schemes such as sinusoidal PWM (SPWM) determines the turn-on and turn-off instants of each power electronic switch, hence forming the aforementioned pulses. An alternative method called Space-Vector Modulation (SVM), however, solely determines the duration of each pulse, leaving room for carefully engineering the actual turn-on and turn-off instants. This extra degree of freedom can be used to modify harmonics and/or reduce losses. In SVM, instead of controlling each switch independently (as is done in SPWM), the converter is successively placed in one of the eight states determined by on/off state of switches. Using Park’s transformation, each state translates into one vector as shown in Figure 2. By selecting the sequence of states and their time-shares, SVM is able to construct the output voltage with desired parameters. The Project The PSCAD?/EMTDC? program is used to implement the SVM controller as a new component shown in Figure 3. The project, started in September of 2005, is carried out by Mr. Ali Mehrizi-Sani, a graduate student at the University of Manitoba working under the supervision of Prof. Filizadeh. The project is supported in part by the Manitoba HVDC Research Centre. The Process The developed model is able to generate ?ring pulses for the linear range as well as overmodulation range of modulation. The model also possesses advanced features such as overmodulation

Figure 1 A three-leg, two-level VSC model in PSCAD

?

Figure 2 Six active space-vectors and two zero space-vectors in SVM.

Figure 3 The PSCAD? Component.

Manitoba HVDC Research Centre Inc. 244 Cree Crescent Winnipeg, Manitoba, Canada R3J 3W1 T +1 204 989 1240 F +1 204 989 1277 info@pscad.com www.pscad.com

compensation techniques to virtually increase the linear range of operation. Two simulation cases are used to show the component operation in linear and overmodulation regions, with modulation indices, m, of 0.8 and 1.2, respectively. For the purpose of harmonic performance demonstration, Weighted Total Harmonic Distortion (WTHD) for the ?rst 63 harmonic components is calculated.
Figure 4 Harmonic spectrum for the ?rst 63 harmonics for m = 0.8.

Figure 5 Harmonic spectrum for the ?rst 63 harmonics for m = 1.2.

Generated waveforms have a fundamental frequency of 60Hz. Sampling frequency is chosen to be 48 times fundamental frequency. Because of symmetric ?ring-pulse generation, even and triple-n harmonics are absent in the line voltage. For m equal to 0.8 (linear region) measured WTHD is 1.58%. For m equal to 1.2 (overmodulation region) measured WTHD is 1.69%. In both cases, WTHD is quite acceptable. Harmonic spectra for linear and nonlinear operations are shown in Figure 4 and Figure 5, respectively. Line currents are shown in Figure 6 and Figure 7. Simulation results con?rm that generated voltages show complete symmetry with acceptable harmonic performance. The Bene?t The model implements space-vector modulation in PSCAD?/EMTDC? program. While SPWM is simply based on comparison of two sets of waveforms, implementation of SVM requires writing the appropriate code. The developed component is of fundamental value because not all PSCAD? users are necessarily familiar with FORTAN programming language. The component saves PSCAD? users from the hassle of reinventing the wheel and provides a ready-to-go component with built-in options for implementing different SVM strategies.

Figure 6 Line currents for m = 0.8.

With the provision to accept dynamically-changedSVM parameters, the user is able to do parametric studies (probably using the Multiple-Run component), which may be used to study the degree of the effect of different parameters such as sampling frequency and vector arrangement on the performance. The model can be also readily used for transient simulation of SVM-controlled VSCs used in power systems or industrial drives. This enables investigation of SVM operation in the context of a power system.
Figure 7 Line currents for m = 1.2.

Mr. Ali Mehrizi-Sani and Dr. Shaahin Filizadeh, The University of Manitoba.


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