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The hysteresis modulation for power electronic converters is attractive in many different applications because of its unmatched dynamic response and wide command-tracking bandwidth. Its application and beneftis for two-level converters are well understood, but the extension of this strategy to multilevel converters is still under development. This paper summarizes and reviews the various hysteresis modulation approaches available in the literature for multilevel converters. The pros and cons of various techniques are described and compared for tracking the reference signal in order to attain an adequate switching optimization, excellent dynamic responses and high accuracy in steady-state operation. By using the recently developed multilevel hysteresis modulation approaches the advantages of using several accessible dc potentials in a multilevel inverter has been fully exploited. All of these hysteresis modulation approaches are testing for tracking a current reference when applied to a fivelevel inveter. The relevant simulation and experimental result are also presented. This study will provide a useful framweork and point of reference for the future development of hysteresis modulation for multilevel converters.

The flying capacitor multicell inverter (FCMI) possesses natural balancing property. With the phase-shifted (PS) carrier-based scheme, natural balancing can be achieved in a straightforward manner. However, to achieve natural balancing with the harmonically optimal phase-disposition (PD) carrierbased scheme, the conventional approaches require (n-1) x (n-1) trapezoidal carrier signals for an n-level inverter, which is (n-1) x (n-2) times more than that in the standard PD scheme. This paper proposes two improved natural balancing strategies for FMI under PD scheme, which use the same (n-1) carrier signals as used in the standard PD scheme. In the first scheme, on-line detection is performed of the band in which the modulation signal is located, corresponding period number of the carrier, and rising or falling half cycle of the carrier waveform to generate the switching signals based on certain rules. In the second strategy, the output voltage level selection is first processed and the switching signals are then generated according to a rule based on preferential cell selection algorithm. These methods are easy to use and can be simply implemented as compared to the other available methods. Simulation and experimental results are presented for a five-level inverter to verify these proposed schemes.

In this paper, the performance of voltage-source converter-based shunt and series compensators used for load voltage control in electrical power distribution systems has been analyzed and compared, when a nonlinear load is connected across the load bus. The comparison has been made based on the closed-loop frequency resopnse characteristics of the compensated distribution system. A distribution static compensator (DSTATCOM) as a shunt device and a dynamic voltage restorer (DVR) as a series device are considered in the voltage-control mode for the comparison. The power-quality problems which these compensator address include voltage sags/swells, load voltage harmonic distortions, and unbalancing. The effect of various system parameters on the control performance of the compensator can be studied using the proposed analysis. In particular, the performance of the two compensators are compared with the strong ac supply (stiff source) and weak ac-supply (non-still source) distribution system. The experimental verification of the analytical results derived has been obtained using a laboratory model of the single-phase DSTATCOM and DVR. A generalized converter topology using a cascaded multilevel inverter has been proposed for the medium-voltage distribution system. Simulation studies have been performed in the PSCAD/EMTDC software to verify the results in the three-phase system.

This paper proposes a new connection for a unified power quality conditioner (UPQC) to improve the power quality of two feeders in a distribution system. A UPQC consists of a series voltage-source converter (VSC) and a shunt VSC both joined together by a common dc bus. It is demonstrated how this device is connected between two independent feeders to regulate the bus voltage of one of the feeders while regulating the voltage across a sensitive load in the other feeder. Since the UPQC is connected between two different feeders (lines), this connection of the UPQC will be called an interline UPQC (IUPQC). The structure, control and capability of the IUPQC are discussed in this paper. The efficacy of the proposed configuration has been verified through simulation studies using PSCAD/EMTDC.

Flying capacitor multilevel inverter (FCMLI) is a multiple voltage level inverter topology intended for high voltage and power operations with low distortion. It uses capacitors, called flying capacitors for clamping the voltage across the power semiconductor devices. In this paper, the implementation of a distribution static compensator (DSTATCOM) using an FCMLI is presented. A hysteresis current control technique for controlling the injected current by the FCMLI-based DSTATCOM is discussed. A new method for controlling the flying capacitor voltages is proposed which ensures that their voltages remain constant and at the same time maintain the desired current profile under the hysteresis current control operation. Simulation studies are performed using PSCAD/EMTDC to validate the efficacy of the control scheme and the FCMLI-based DSTATCOM.

In this paper, load compensation using multilevel inverter-based distribution static compensator (DSTATCOM) is presented. The DSTATCOM is connected to a three-phase four-wire (3p4w) distribution system. Two different multilevel inverter topologies are used to realize the DSTATCOM. They are diode-clamped multilevel inverter (DCMLI) and flying capacitor multilevel inverter (FCMLI). A compensating technique has been derived, which uses the state feedback control to the multilevel inverters-based DSTATCOMs. A switching strategy for the multilevel inverters that ensures utilization of all output voltage states of the inverter for efficient tracking of references has been proposed. The schemes of dc-link capacitor voltage control for DCMLI and flying capacitor voltage control for FCMLI have been discussed. Comparative studies of performances of the two DSTATCOM topologies are performed. The simulation studies are performed using PSCAD/EMTDC to validate the efficacy of the proposed control scheme.

In this paper, load compensation techniques under unbalanced and distorted voltages have been discussed. Two kinds of compensation methods are considered. In the first category, synchronous detection and modified equal current strategies are used under the unbalanced but sinusoidal source voltages. In the second method, load compensation based on instantaneous symmetrical component theory with positive sequence extraction is proposed and is shown to work under unbalanced and the non-sinusoidal source voltages. To support the proposed method, a 440 V (L-L) three-phase, four-wire distribution system is considered. The compensator is realized using a three-phase voltage source inverter (VSI) operated in the current control mode. Detailed digital simulation and experimental results are presented to validate the approach.

The flying capacitor multilevel inverter (FCMLI) is a multiple voltage level inverter topology intended for high-power and high-voltage operations at low distortion. It uses capacitors, called flying capacitors, to clamp the voltage across the power semiconductor devices. A method for controlling the FCMLI is proposed which ensures that the flying capacitor voltages remain nearly constant using the preferential charging and discharging of these capacitors. A static synchronous compensator (STATCOM) and a static synchronous series compensator (SSSC) based on five-level flying capacitor inverters are proposed. Control schemes for both the FACTS controllers are developed and verified in terms of voltage control, power flow control, and power oscillation damping when installed in a single-machine infinite bus (SMIB) system. Simulation studies are performed using PSCAD/EMTDC to validate the efficacy of the control scheme and the FCMLI-based flexible alternating current transmission system (FACTS) controllers.

The paper discusses the operating principles and control characteristics of a dynamic voltage restorer (DVR) that protects sensitive but unbalanced and/or distorted loads. The main aim of the DVR is to regulate the voltage at the load terminal irrespective of sag/swell, distortion, or unbalance in the supply voltage. In this paper, the DVR is operated in such a fashion that it does not supply or absorb any active power during the steady-state operation. Hence, a DC capacitor rather than a DC source can supply the voltage source inverter realizing the DVR. The proposed DVR operation is verified through extensive digital computer simulation studies.

The paper discusses the concept of a mini custom power park in which the voltage inside the park is tightly regulated by a distribution static compensator (DSTATCOM). It will be shown on the basis of simulations with ideal components that solid state protection devices, along with the DSTATCOM will eliminate many transients in all the loads of the park during disturbances. Moreover, the DSTATCOM, backed by a diesel generator, will supply the most sensitive loads during the total line outages, thereby nearly making them both transient and interruption free. In this paper, a configuration of the park is proposed and then various issues involved with the operation of the park are discussed. The proposed operation of the custom power park is verified through extensive digital computer simulation studies.

Voltage imbalance in capacitors is a well-known problem in compensator topologies which use two or more capacitors. This imbalance may exist even if the load does not contain any DC component, due to practical factors. However, when the load contains a DC part, the voltage imbalance problem becomes critical. In this paper, a two-quadrant chopper has been used to regulate the capacitor voltages in a two-capacitor compensator structure. Two different control strategies for the two-quadrant chopper to equalize the voltage of the capacitors have been proposed. The strategies are validated through detailed simulation studies. Experiments have also been carried out to validate the hysteresis control of chopper.

In this paper, a static synchronous series compensator (SSSC), along with a fixed capacitor, is used to avoid torsional mode instability in a series compensated transmission system. A 48-step harmonic neutralized inverter is used for the realization of the SSSC. The system under consideration is the IEEE first benchmark model on SSR analysis. The system stability is studied both through eigenvalue analysis and EMTDC/PSCAD simulation studies. It is shown that the combination of the SSSC and the fixed capacitor improves the synchronizing power coefficient. The presence of the fixed capacitor ensures increased damping of small signal oscillations. At higher levels of fixed capacitor compensation, a damping controller is required to stabilize the torsional modes of SSR.

This paper presents the operating principles of a distribution static compensator (DSTATCOM) that is used to maintain the voltage of a distribution bus. A three-phase, four-wire distribution system is assumed in this study. A three-phase bridge inverter circuit that is supplied by two neutral-clamped dc storage capacitors realizes the DSTATCOM. Three filter capacitors, one for each phase, are connected in parallel with the DSTATCOM to eliminate high-frequency switching components. The voltage across the filter capacitor is controlled by a dead-beat controller to maintain the ac bus voltage. The magnitude of the bus voltage is chosen as a nominal value, i.e., 1.0 per unit, while its phase angle is obtained through a feedback loop that maintains the voltage across the dc storage capacitors. Through detailed simulation and experimental results it has been shown that the DSTATCOM can maintain the voltage against any unbalance and distortion in either the load or supply side.