R. Davoodnezhad

An Improved Three-Phase Variable-Band Hysteresis Current Regulator

This paper presents an improved variable-band hysteresis current controller for a two-level three-phase voltage source inverter (VSI). The controller takes the average voltages of the phase-leg switched outputs as an approximation to the load back-EMF voltages, and uses these results to vary the hysteresis bands so as to maintain constant phase-leg switching frequencies. The switching frequency control process is then further refined by fine tuning the hysteresis band variations to synchronize the zero crossings of the phase-leg current errors with a fixed reference clock so as to achieve a nearest space vector switching sequence, which further ensures that the switched output spectrum has been optimized. Finally, a technique is proposed to replace the third phase-leg current regulator with a fixed-frequency open-loop pulse-width modulator, where its commanded reference is generated from the average switched output voltages of the other two phase legs. This avoids the hazard of the three independent hysteresis current regulators adversely interacting with each other in a conventional system, resulting from an overconstrained control problem with only two degrees of freedom. Additionally, this approach allows the linear modulation range to be increased by adding a common-mode third-harmonic component to the third phase-leg reference command signal.

A Novel Three-Level Hysteresis Current Regulation Strategy for Three-Phase Three-Level Inverters

This paper presents a new hysteresis current regulation strategy for the neutral point clamped and flying capacitor (FC) three-level inverters. The strategy uses the measured average of the switched phase leg output voltage to adjust the controller hysteresis band as the load back EMF varies to maintain a near constant phase leg switching frequency. The phase leg switchings are then fine tuned to a fixed frequency clock to further improve frequency regulation. Next, the zero-crossings of the measured phase leg average voltages are used to select between positive and negative switched output voltage levels, so that only one hysteresis current regulator is required for the full inverter switched output voltage range. For the FC inverter, a state machine is then added to select between redundant switching states to maintain balanced capacitor voltages. Finally, the controller is extended to a three-phase system by subtracting the common mode interacting current from the total phase leg current error before making any switching decision. The resulting controller achieves a line-to-line harmonic performance that is very close to open-loop phase disposition pulse width modulation, while retaining all of the dynamic benefits of hysteresis current regulation.

An improved three phase variable band hysteresis current regulator

This paper presents an improved variable band hysteresis current controller for a two level three phase voltage source inverter. The controller takes the average voltages of the phase-leg switched outputs as an approximation to the load back-EMFs, and uses this result to vary the hysteresis bands to maintain constant phase leg switching frequencies. The switching frequency control process is then further refined by fine tuning the hysteresis band variations to synchronise the zero crossings of the phase leg current errors with a fixed reference clock, to ensure nearest space vector switching and hence achieve the best possible harmonic switching performance. Finally, a technique is proposed to replace the third phase leg current regulator with a fixed frequency open loop PWM modulator, where its commanded reference is generated from the average switched output voltage of the other two phases. This avoids the hazard of three independent current regulators in a conventional system adversely interacting as an over constrained control problem, and allows the linear modulation range to be increased by adding a common mode third harmonic component to the third phase leg reference command.

A resonant current regulator based microgrid control strategy with smooth transition between islanded and grid-connected modes

Microgrids consisting of distributed generators (DGs) have the ability to operate connected to the utility grid and also in stand-alone (island) mode. In both cases, frequency and voltage regulation across the microgrid must be ensured, and the DGs should share their generation equally. The transition of the microgrid between the two operating modes also needs to be seamless, which is conventionally achieved by switching between current or voltage based control, depending on the system context. This paper presents a new microgrid control strategy that exploits the intrinsic droop characteristics of a PR current regulator to achieve these objectives for both modes of microgrid operation. The strategy incorporates a predictive regulator to achieve dynamic voltage control within one fundamental cycle, a frequency adaptation strategy for islanded mode, and ensures DG power sharing. Simulation and experimental results obtained for a microgrid transitioning between both modes are used to verify the performance of the proposed strategy.

Hysteresis current regulation of three phase flying capacitor inverter with balanced capacitor voltages

This paper presents a new hysteresis current regulation strategy for a three-phase three-level flying capacitor multilevel inverter, that uses the zero-crossing of the averaged phase leg output voltages to select the switched output level for each inverter phase leg. Consequently each phase leg requires only one hysteresis comparator to generate its switching signals, which significantly improves its performance compared to multiple band strategies. Furthermore, since the phase leg average voltage is an estimation of the load back-emf, the same averaged signal can be used to vary the regulator hysteresis bands to maintain a constant switching frequency and achieve better harmonic performance. A state machine is then used to implement active balancing to maintain balanced capacitor voltages. Finally, the paper uses the phase leg switch states to calculate the interacting current between the phases. This current is subtracted from the measured phase leg currents before making the switching decision, to avoid interaction between the phase legs.

Voltage-frequency control of an islanded microgrid using the intrinsic droop characteristics of resonant current regulators

The Distributed Generators (DGs) of an islanded microgrid must be carefully controlled to regulate voltage and frequency, and to share power evenly between the DG sources. This is normally achieved by operating the microgrid Voltage Source Inverters (VSIs) in voltage controlled mode, so that the microgrid voltages and frequency can be explicitly regulated. However, this results in the loss of direct control of the VSI current, and potentially introduces the need for communication between DG units to maintain robust coordination between DG sources. This paper presents a new approach for islanded microgrid operation, where all VSIs are operated in current regulated mode. By taking advantage of the hitherto unrecognised intrinsic droop characteristic of a PR current regulated VSI, frequency regulation, voltage regulation and power sharing can be achieved across the islanded microgrid without requiring either explicit external control or direct communication between the DG units. Simulation and experimental results are presented to verify the performance of the new approach.

Dynamics of droop-controlled microgrids with unequal droop response times

One increasingly popular approach to integrate distributed generation (DG) energy sources into an electrical grid system, is to assemble them into microgrids which then connect to the main utility grid at a single or multiple points of connection. A microgrid can also operate as a standalone islanded network which is physically disconnected from the grid. During island mode the generated power of each DG unit must be carefully controlled to ensure reliable power distribution and modular operation. Droop control is commonly used for this purpose, commanding individual voltage magnitude and frequency for each DG unit to achieve balanced active and reactive power sharing. A droop controller involves several stages of processing, particularly output power calculation and updating the voltage command setpoint. These stages contribute to a time delay defined in this paper as the droop response time (DR time), which can vary between multiple DG units because of different designs and controller implementations. This paper explores the effect of differences in droop controller response times on the ability of a microgrid to balance its commanded DG generations with its required load profile. It is shown that unequal droop response times can substantially degrade the power sharing dynamics and the modularity of the microgrid.

Three-level hysteresis current regulation for a three phase neutral point clamped inverter

This paper presents a new hysteresis current regulation strategy for a three-phase three-level neutral point clamped (NPC) inverter. The strategy uses the zero-crossing of the fundamental component of the phase leg switched voltage to select the output voltage level, with only one hysteresis comparator required per phase leg. The same average voltage is then used to vary the hysteresis band as the load back-emf changes in order to maintain a constant switching frequency. Next, the strategy is extended to a three-phase NPC inverter by subtracting the common mode interacting current from the total phase leg current error before making any switching decision. The three-phase leg current errors are also synchronized to a fixed clock frequency to further improve the frequency regulation and obtain a line to line harmonic performance that is close to open-loop phase disposition (PD) pulse width modulation.

A new current control droop strategy for VSI-based islanded microgrids

Microgrids are distributed generation structures that link groups of Distributed Generators (DGs) to either connect to the utility grid, or to operate as a standalone (islanded) system. However, the injected power from each DG must be carefully controlled to ensure reliable power distribution, balanced modular operation, and regulated voltages across the microgrid. It is commonly accepted that DG units cannot operate in current-control mode when islanded, because there is no stiff grid voltage available. Consequently they are usually operated in voltage control mode, with individual droop controllers regulating the inverter voltage magnitude and frequency to share active and reactive power. This paper presents a new approach to operate islanded microgrid DG units in current control mode, by combining linear current regulators with predictive voltage control. The result achieves a rapid load response while still maintaining well regulated voltage levels. A current magnitude droop controller is also proposed to maintain equal power sharing between the current controlled DG units.

A fully digital hysteresis current controller for current regulation of grid connected PV inverters

Grid connected voltage source inverters require both current regulation and synchronisation to control the power flow and to maintain stability during grid disturbances. For this type of application, non-linear current control of VSIs using techniques such as hysteresis current regulation offer advantages compared to linear current regulation, such as inherent over-current protection, robustness to load/filter parameter variation and very rapid dynamic response. However these techniques suffer from variable switching frequency and their implementation can require significant analog circuitry. Furthermore, sensorless grid synchronisation can improve the inverters robustness to disturbances such as electrical line interference, sensor failure and rectifier commutation voltage notches. This paper presents a fully digital constant frequency three-level hysteresis current regulator for single phase PV inverters that addresses these issues. The proposed control strategy also uses the average inverter voltage to adjust the hysteresis band to maintain a constant frequency, detect the output voltage polarity and estimate the grid voltage. The result is a robust current regulator with increased flexibility and easy adaptability for grid connected inverters.