New Methods for the Active Compensation of Unbalanced Supply Voltages for Two-Stage Direct Power Converters

Abstract

Unbalanced supply voltages can be fully compensated in boost type power converters with active front end if sufficient energy storage is provided by the DC-link capacitor. In converters that have no energy storage, matrix converters for example, the maximum voltage transfer ratio will be reduced if one or more input lines are at a reduced voltage. Traditional control techniques (passive compensation methods) compensate the effect of unbalanced voltage supply for matrix converters but limit the output voltage/powe depending on the unbalance level.This paper proposes two methods to actively (no loss in output voltage capability) compensate the effect of unbalanced voltage supply: the first one shown in Fig. 1(a), utilizes the clamp capacitor, which is normally needed to protect a Direct Power Converter (DPC), to extend the operating range of a Two-Stage DPC during unbalanced supply conditions preserving its theoretic maximum voltage transfer ratio capability; the second one shown in Fig. 1(b) is a new topology based on a hybrid approach, consisting of an H-bridge inverter inserted into the dc-link, which is also able to preserve the output voltage capability under unbalanced supply conditions. Both solutions are validated through simulations.Unbalanced voltage supply cause the input voltage vector locus to become an ellipse, which means that the average over a switching period of the rectified voltage to vary in a much wider range than in the ideal case (sinusoidal and balanced supply voltages). The principle of active compensation is to use energy storage in order to store energy when the rectified voltage is high (positive and negative voltage sequences point in the same direction) which would be used when this is low (the positive and negative sequences point opposite). Fig. 2(a) shows the DC-link voltage seen at the inversion stage terminals when there is no active compensation and the average dc-link voltage reaches a minimum of 440 V, which means that the voltage transfer ratio defined as a ratio between the output voltage divided to the direct sequence of the input voltage is decreased from 0.866 (ideal case) to 0.78. By applying active compensation using the clamp circuit, this is restored back to 0.866 as shown in Fig. 2(b). The price to be paid for improving the converter robustness against grid disturbances is a slightly oversized clamp capacitor, the need to introduce an extra IGBT in the clamp circuit and the degradation of input current quality due to uncontrolled capacitor charging, which is shown in the full paper.Active compensation using an hybrid approach is not only able to restore the voltage transfer ratio, but also stabilizes the average over a switching period and boost its value to a corresponding voltage transfer ratio of 0.909, which means that the voltage delivered to the output side falls within the normal voltage tolerance (±10%) of a motor which will allow this topology to directly compete the well established back-to-back voltage source converters. Non-memberUnbalanced supply volta...

Implementation of a Hybrid AC–AC Direct Power Converter With Unity Voltage Transfer

“…Space vector modulation Implementation of Space Vector Modulation (SVM) for the two-stage matrix converter has been discussed in [3], [10], [13] where a similar approach as in [10] can be used for the HDPC. The space vector diagram for the rectification and inversion stage is shown in Fig.…”

“…These disturbances deteriorate the quality of the output voltage and reduces the voltage transfer ratio. Different techniques to maintain the load voltage quality and maximum voltage transfer ratio during the above mention disturbances were investigated in the literature [7]- [10]. However, these techniques can provide maximum of 86% voltage transfer.…”

Hybrid matrix converter topologies: An exploration of benefits

“…IMCs also offer the possibility to implement hybrid structures and since they have two stages and an intermediary link, it is therefore more effective to insert an auxiliary inverter emulating a controllable voltage source between the rectifier and the inverter stage [10]- [12]. One possibility is to have this auxiliary voltage source implemented by means of an H-bridge inverter, with a floating DC-link capacitor as shown in Fig.…”

“…Because the input and the output voltage vectors have different instantaneous speeds, the inner hexagon cannot touch the ellipse in more than two points in order to guarantee that the locus of the output voltage vector will not be distorted. This restriction is overridden in case of any hybrid MC/IMC which can implement an active unbalance compensation technique [10], [15].…”

“…In order to solve these drawbacks, several solutions have been investigated: methods able to adapt the modulation to non-ideal supply conditions whilst maintaining the output voltage sinusoidal and minimizing the distortion of the input currents [4], which do not require hardware changes or active compensation methods requiring changes in both modulation and hardware [10]- [15], referred as "hybrid". This paper will compare the various hybrid MC/IMC solutions and summarize their benefits in terms of improvement of the voltage transfer ratio (VTR), power losses and installed power in the semiconductors switches.…”