Baljit S. Riar

Model Predictive Direct Current Control of Modular Multilevel Converters: Modeling, Analysis, and Experimental Evaluation

Modular multilevel converters (M2LCs) are typically controlled by a hierarchical control scheme, which essentially requires at least two control loops: one to control the load current and another to control circulating currents. This paper presents an M2LC with a single controller, which is based on model predictive direct current control (MPDCC) with long prediction horizons. The proposed MPDCC scheme maintains the load current within tight bounds around sinusoidal references and minimizes capacitor voltage variations and circulating currents. An internal prediction model of the M2LC is used to minimize the number of switching transitions for a given current ripple at steady state while providing a fast current response during transient conditions. A state-space model, which is generalized for an N number of modules per each arm of the M2LC, is also presented to investigate the dynamic behavior of arm currents and capacitor voltages. Simulated performance of the converter, under various operating conditions, is presented in comparison to measured performance of a single-phase, three-level 860-VA M2LC prototype to demonstrate the proposed MPDCC philosophy.

Decoupled Control of Modular Multilevel Converters Using Voltage Correcting Modules

Historically, cascaded H-bridge, capacitor-clamped, and neutral point-clamped topologies have been used for medium- to high-voltage applications but the modular multilevel converter (M2LC) is becoming a popular alternative. However, in comparison to other topologies, control of the load current, which is inherently coupled with circulating currents, is more difficult in the M2LC topology. This paper proposes a modified M2LC topology that allows for decoupled control of circulating currents from the load current. Each arm of the modified topology comprises a plurality of half-bridge modules and one full-bridge module. The full-bridge module minimizes harmonic currents within the converter without affecting the load current. A state-space model, which is generalized per arm with an N number of half-bridge modules and one full-bridge module, is presented to accurately predict the behavior of the proposed topology. Theoretical as well as experimental results of a single-phase three-level 800-VA prototype converter are presented with a discussion to demonstrate the viability of both the proposed mathematical model and modified topology. A comparative investigation with respect to a conventional topology reveals that the proposed topology offers superior performance.

Design of a double coupled IPT EV highway

As global reserves of fossil fuels diminish, and the awareness of their harmful effects increases, Electric Vehicles (EVs) are fast becoming attractive alternatives to conventional petrol driven vehicles. Inductive power transfer (IPT) is a method that can transfer power to EVs over an air-gap without physical contact. If IPT systems are incorporated into highways, then EVs can be charged dynamically as they travel. This will dramatically increase the range, convenience and safety of EV charging and will significantly reduce range anxiety and the required battery bank sizes. A novel double coupled system, consisting of two stages of magnetic coupling, is proposed to drive the IPT highway. An AC-AC controller is used to control the current through buried Double-D (DD) inductive couplers to transfer power to a Double-D-Quadrature (DDQ) coupler mounted on the EV. A laboratory scale prototype of the proposed system is capable of transferring 500W over practical air-gaps to power multiple EV classes.

Model Predictive Direct Current Control of Modular Multi-level Converters

Modular Multi-level Converters (M2LCs) are mostly controlled by using a hierarchical control scheme, where at least two control loops are required for controlling the load currents and balancing the capacitor voltages. This paper proposes a single controller, which is based on Model Predictive Direct Current Control (MPDCC) with long prediction horizons, to directly control the load currents within tight bounds around their sinusoidal references and balance the capacitor voltages. MPDCC uses a model of the converter for an online optimization process to deliver the best possible performance during both steady-state and transient operating conditions. A conceptual description and control algorithm of the proposed controller are presented in this paper. To validate the proposed concept, simulated performance of a three-phase, three-level 2 MVA grid connected M2LC is presented with a discussion. A comparison with a vector control (VC) pulse width modulation (PWM) scheme is also carried out to demonstrate the improvements in performance associated with the MPDCC scheme.

A Modular Multi-level Converter (M2LC) based on Inductive Power Transfer (IPT) technology

This paper proposes a new Modular Multi-level Converter (M2LC) topology, which is based on an Inductive Power Transfer (IPT) technology. The modified M2LC topology employs an IPT system to control the capacitor voltage of each module, thereby decoupling the capacitor voltage balancing control from the load current control. The IPT system maintains the capacitor voltages within a narrow band around a nominal value, simplifies the control algorithm and offers new advantages, such as operation at low output frequency, implementation of simple control schemes and reduced energy requirements over the existing topology. The concept is validated using simulations for a single-phase three-level M2LC and compared with the existing topology to demonstrate the performance improvements associated with the modified topology.

A novel Modular Multi-level Converter topology with Voltage Correcting Modules (M2LC-VCMs)

Modular Multi-level Converters (M2LCs) are usually controlled with PWM based schemes to minimize circulating currents but at the expense of increased complexity of a controller and switching losses. A new approach that uses a modified conventional M2LC topology to minimize circulating currents as well as variations in capacitor voltages is proposed. The proposed technique incorporates a low power rated module, referred to as Voltage Correcting Module (VCM), into each arm of the converter to compensate for the voltage difference in capacitors and the dc-bus. The proposed solution simplifies the control, allows for the operation at lower switching frequency and is unaffected by the number of modules of the converter. The paper describes the simplified control concept and demonstrates its validity through simulated performance of a three-phase five-level, 2 MVA M2LC system. A comparison with a conventional topology is also carried-out to show the improved performance of the proposed topology.

Modelling of modular multilevel converter topology with voltage correcting modules

Modular multilevel converter (M2LC), which features modularity, scalability, reduced voltage rating of the switches and redundant switching operations, is becoming popular in medium to high-voltage applications. Circulating current, which is an inherent feature of the M2LC topology, increases both the converter losses and complexity of the control scheme. Recently, M2LC with voltage correcting modules (M2LC-VCM) is proposed as a method to minimize the circulating current. A mathematical model of the M2LC-VCM that is based on the sum of all the capacitor voltages in an arm is derived in the paper. The model can be used either as a tool for investigating the behaviour of the M2LC-VCM or designing the physical system. Simulated results of a three-phase five-level 2-MVA grid connected M2LC are presented with discussion to demonstrate the viability of the mathematical model.

Analysis and control of a three-phase Modular Multi-level Converter based on Inductive Power Transfer technology (M2LC-IPT)

The Modular Multi-level Converter (M2LC) topology promises to outperform other known converter topologies in various medium to high voltage applications. The topology requires a control scheme for controlling the voltage across module capacitors. This paper proposes a variant of the M2LC topology, which is based on Inductive Power Transfer (IPT) technology, to maintain the capacitor voltages within tight bounds. The proposed M2LC-IPT topology has the unique ability of independent control of capacitor voltages and load currents, and is even capable of producing a regulated output at low output frequency. To validate the proposed concept, simulated performance of a three-phase, three-level grid connected 2 kVA M2LC-IPT is presented with a discussion. A comparison with a conventional topology is also carried-out to demonstrate the improvements in performance associated with the M2LC-IPT.

Model Predictive Direct Slope Control for Power Converters

Model predictive control (MPC) schemes have become popular in the field of power electronics due to their intuitive formulation, flexibility, and ease of implementation. Typically, these schemes have been implemented with the prediction horizon limited to one time-step, and extension of the prediction horizon over multiple time-steps remains an ongoing area of research. In this paper, a variant of the MPC strategy is proposed wherein the slope of the output trajectories is used to emulate long prediction horizons. Each of the outputs, e.g., current, voltage, torque, or flux, is regulated within a set of symmetrical bounds. When switching is necessitated due to collision with a bound, the switching state that yields the set of output trajectories with the minimum slope, relative to the reference trajectory, is applied to the converter. The key benefit of this approach is its ability to achieve low switching frequencies with a minimal level of computational burden. The feasibility of the scheme, which can be adapted easily to different case studies, is demonstrated through simulations of both a medium-voltage induction machine drive and a grid-connected converter. Experimental results, which are presented for a 1.68 kVA prototype grid-connected neutral-point-clamped converter, further demonstrate the practical viability of the proposed strategy.

Energy management of a microgrid: Compensating for the difference between the real and predicted output power of photovoltaics

An increasing awareness of energy efficiency has led to the development of several improved converter topologies, semiconductor devices and control schemes for distributed energy resources, and, particularly, for microgrids. Recent advances in energy management systems (EMS) for microgrids have improved upon existing methods in several aspects, including prediction of power generated by photovoltaics (PV), and optimal management of electrical energy storage. However, the actual generated PV power may deviate from predictions for several reasons, such as rapid cloud changes or system faults. This paper contributes to the ongoing research on EMS control schemes by proposing a model predictive control (MPC) scheme that adapts to the difference between the actual and predicted output power of PV. The key benefit of this approach is its ability to rapidly adapt to varying operating conditions of the PV without increasing the computational burden of a typical MPC scheme. The feasibility of the scheme is demonstrated using simulations of 5 kW microgrid system compromising a 5 kW/400 Ah battery, 10 kW PV and 5 kW grid/load connection. The proposed scheme reduces variations in the state of charge (SOC) of a battery. The proposed scheme also reduces the energy taken from grid and this improvement in performance is a function of the difference between the actual and the predicted power.