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Application and Verification 159 7.4 Experimental verification The proposed hysteresis controller has been implemented on the 5-L ANPC low power prototype and its performance has been experimentally verified. Two different operating points shall be shown. In both cases a simplified version of the hysteresis controller is applied. In steady state, a first NP control type is applied; when hitting the tolerance band, a second NP control type is applied until the NP voltage reaches the reference value again. TABLE 67, COMMON PARAMETERS FOR THE EXPERIMENTAL VERIFICATION OF THE NP VOLTAGE HYSTERESIS CONTROLLER (TWO STATE OPERATION) f load U DC U NP_ref Tolerance band base modulation type L load R load 50Hz 240V 120V +/-30V CPDPWM 14.2 mH 0 Ω TABLE 68, PERFORMANCE OF THE HYSTERESIS CONTROLLER FOR MEDIUM MODULATION DEPTH (M=0.5) AND OVER MODULATION FOR PURE REACTIVE POWER m = 0.5, cos(ϕ) = 0 Hysteresis controller: - NP control type 1: - unconstrained DC CM injection - NP control type 2: - 6 th harmonic injection CH1 (yellow): CH2 (cyan): U out1 U out2 CH3 (magenta): U NP CH4 (green): MATH (red): Time/div: I out2 U out1-2 25ms m = 1.1 (over modulation), cos(ϕ) = 0 Hysteresis controller: CH1 (yellow): CH2 (cyan): U out1 U out2 - NP control type 1: CH3 (magenta): U NP - real time NP current function - NP control type 2: - virtual vectors CH4 (green): MATH (red): Time/div: I out2 U out1-2 100ms m = 1.1 (over modulation), cos(ϕ) = 0 Hysteresis controller: CH1 (yellow): CH2 (cyan): U out1 U out2 - NP control type 1: CH3 (magenta): U NP - real time NP current function - NP control type 2: - virtual vectors CH4 (green): MATH (red): Time/div: I out2 U out1-2 10ms
160 Application and Verification The controller based on unconstrained DC injection cannot keep the NP voltage balanced even at m=0.5. The maximum DC CM injection is visible in the phase voltages while the NP voltage is diverging. When the tolerance band is hit (150V), the 6 th harmonic injection scheme brings the voltage back to its set point in one fundamental cycle (TABLE 68, a). In over modulation (m=1.1), the NP voltage cannot be controlled by any of the standard controllers (real time current function used in this case). The NP voltage is drifting away until it hits the tolerance band, then virtual vector operation brings the NP voltage back to its reference. The zoom shows nicely how the NP voltage (purple) can be brought back to its set point within a fraction of a cycle. The virtual vectors type 2 can be recognized both in the line to neutral voltages (cyan and yellow) and in the line to line voltage (red) as double level steps. The distortion introduced is not very significant and hardly visible in the current (green). A significant 3 rd harmonic is imposed on the NP voltage in this operating point. 7.4.1.1 Active power The unconstrained DC CM voltage injection works reliably throughout the linear operating range of the 5-L ANPC. However, its useful NP current range for control depends heavily on the modulation depth. The available current range start from zero for m=0, rises up to a maximum for m=0.5 and then is reduced again to zero for maximum over modulation. Improved NP control concepts are therefore of primary interest for the very high and very low modulation depth (see also TABLE 88 in appendix 9.5.4 and the resulting condition for the application of virtual vectors in TABLE 66). The following experimental results underline the usefulness of alternate schemes in those operating points. TABLE 69, COMMON PARAMETERS FOR THE EXPERIMENTAL VERIFICATION OF A NP VOLTAGE REFERENCE STEP WITH ACTIVE POWER LOAD f load U DC U NP_ref initial U NP_ref end base modulation type L load R load 100Hz 240V 140V 120V CPDPWM 4.2 mH 12 Ω Unconstrained DC CM injection and use of the real time NP current function yield the same performance regarding NP control in active power operation. They both result in a dv/dt of 40V/s in the NP voltage. The corresponding NP current is 160 mA (C effective = 4 mF) which is equivalent to 14% of the peak phase current of 1.15A. The scheme applying modified vectors is significantly more powerful with a dv/dt of 160V/s and a NP current of 640mA (55% of the peak phase current). These values correspond with the calculated data; at even lower modulation depth, the modified vectors will still make use of 50% of the phase current, whereas in the other schemes, the percentage will go down to zero. Note the voltage waveforms in TABLE 70: Unconstrained DC CM injection makes use of a DC CM injection clearly visible in the phase voltages, whereas the modified vector scheme introduces 2 level commutations but no CM offset. In all schemes, the differential output voltage remains undistorted. The distortion seen in the graphs is based on the large NP unbalance and not on the pulse pattern. The same is true for the current transient, which is more prominent for the modified vector scheme, as the NP voltage transient is much faster (Note that the system is operating in open loop and does not actively control the output currents).
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Acknowledgements i ACKNOWLEDGEMENTS
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Introduction 1 2 INTRODUCTION This
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Page 218 and 219: Bibliography 197 [33] P. Steimer,
Page 220 and 221: Bibliography 199 neutral point bala
Page 222 and 223: Bibliography 201 [88] J. Pou, J. Za
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