Showing papers by "Alan Watson published in 2012"

Distributed commutations pulse-width modulation technique for high-power AC/DC multi-level converters

Abstract: This study presents a novel dedicated pulse-width modulation (PWM) technique for use with single-phase multi-level cascaded H -bridge (CHB) converters. The proposed modulation strategy aims to minimise the converter commutations and distribute them, for any amplitude of the voltage reference, among the different converter cells to evenly distribute the heating and stress on the power switches, improving their reliability without compromising the quality of the voltage waveform. Such characteristics are particularly important in high-power grid-connected converters used as an interface for renewable energy sources or smart grid applications. The technique is described in detail along with the verification of the modulation effectiveness provided by a simulation comparison with other common modulation strategies. Finally, the proposed strategy is experimentally validated by means of different tests using a seven level CHB converter.

DC fault ride-through capability and STATCOM operation of a hybrid voltage source converter arrangement for HVDC power transmission and reactive power compensation

Abstract: HVDC transmission systems are becoming increasingly popular when compared to conventional AC transmission methods. HVDC voltage source converters (VSC) can offer advantages over traditional HVDC current source converter topologies; as such, it is expected that HVDC-VSCs will be further exploited with the growth of HVDC transmission. This paper presents the DC fault ride through capability and new STATCOM modes of operation for a recently published alternating arm voltage source converter (AAC) intended for the HVDC market. Operation of the converter during a local terminal to terminal short circuit of the DC-Link is demonstrated; additionally, operation of the converter as a STATCOM during the fault is also demonstrated. (5 pages)

Multi carrier PWM of the modular multilevel VSC for medium voltage applications

Abstract: This paper presents integrated level-shifted and phase-shifted multi carrier modulation schemes that ensures PWM and local capacitor voltage balancing of the Modular Multilevel Converter (M2LC) for medium voltage applications. The integration of either of the modulation schemes with the cell voltage balancing algorithm ensures the floating capacitor voltages are balanced throughout the operation of the M2LC voltage source converter. A comparison of the two schemes is presented based on the harmonic content of the synthesized output waveforms and peak-to-peak ripple of the local capacitor voltages. The converter semiconductor losses are also evaluated and compared for these modulation schemes in a typical medium voltage grid application. It is shown that both schemes are competitive in terms of the synthesized output waveform quality. However the phase shifted scheme offers less capacitor voltage ripple while the level shifted scheme offers low converter loss. The concepts are confirmed with both PLECS simulation package and a 10kVA 9-level experimental prototype.

Cell capacitor voltage control in a parallel hybrid modular multilevel voltage source converter for HVDC applications

Abstract: This paper presents further characteristics of the recently proposed parallel hybrid modular multilevel voltage source converter (VSC) for HVDC applications. Schemes for the control of the cell voltages and the total chain link voltages are also presented. The concept is supported by simulation results from a 17-level 10kW miniature laboratory model under construction. (6 pages)

Frequency control improvement within a microgrid, using enhanced STATCOM with energy storage

Abstract: This paper introduces the use of a STATCOM with Supercapacitor energy storage (SES) to aid frequency control in a weak power supply network. The control of the fast response energy storage is coordinated with the slow response generator to ensure power matching within the power system at all times. Simulation and Experimental results are presented to show the improved frequency control in the presence of large load steps.

An improved Dead-Beat current control for Cascaded H-Bridge active rectifier with low switching frequency

Abstract: The use of high power multi-level converters as an interface for renewable energy sources allows modern power grids to achieve a higher flexibility and reliability. Cascaded H-Bridge Converters (CHB) are particularly suitable in these applications but more advanced control techniques are required. In particular Dead-Beat current control methods are a promising solution for high-power CHB. However in high-power applications, a reduced switching frequency is required to ensure high efficiency operation and the performance of the control, in term of quality of the AC side current, may be reduced. As a result a more accurate approximation of the derivative, used in the discretised control law, may be required to ensure accurate operation. In this paper three different methods for the discretisation of the converter model are analysed and compared. The most suitable one is then chosen to implement an improved Dead-Beat current control for AC current regulation of a 7-level, CHB active rectifier.

A Novel Pulse Width Modulation technique with active DC voltage balancing and device voltage falls compensation for High-Power Cascaded multilevel active rectifiers

Abstract: This paper presents a novel dedicated PWM technique for use with single-phase (or four wire three-phase) multi-level Cascaded H-Bridge Converters. The proposed modulation strategy aims to minimize the unbalance of the DC-Link voltages, for any amplitude of the voltage reference, amongst the different converter cells in order to obtain high-quality waveforms with a low switching frequency. Moreover the device voltage drops and ON resistance, which in high-power multilevel converter have a significant impact on the quality of the produced voltage, are compensated. The technique is described in detail along with the concept of a modular converter prototype for smart grid applications and an outline of its construction. Verification of the modulation effectiveness is provided with simulation and experimental results for a 3kW seven level Cascaded Converter.

Voltage balancing characteristics for a cascaded H-bridge multi-level StatCom employing space vector modulation

Abstract: Employing space vector modulation in a multi-level H-bridge StatCom (H-StatCom) can provide an excellent trade-off between harmonic performance and switching frequency. However the nature of the techniques used to balance the capacitor voltages in this modulation strategy are heuristic in nature, with their stability and balancing characteristics being difficult to quantify. This paper develops a method to investigate the likelihood that the capacitor voltages remain balanced for the operating modes and system parameters typical in H-StatCom applications.

Droop compensation with soft switching for High Voltage Converter Modulator (HVCM)

Abstract: High Voltage Converter Modulators (HVCM) offer significant performance advantages over conventional modulator technologies for long pulse applications. One of the key advantages of HVCM technology is the ability to compensate for capacitor bank droop. Achieving droop compensation without incurring significant additional switching loss has not been possible in existing designs. This paper presents an analysis of the “Y-point” variant of the HVCM topology using the Combined Phase and Frequency Modulation (CPFM) technique. This, combined with the addition of a ‘lossless’ snubber circuit, enables droop compensation while achieving soft switching over the entire pulse duration. The rise time and overshoot of the output voltage is optimised using an iterative extreme seeking algorithm. The optimisation reduces the rise time from 100μs to 50μs with no overshoot. This will lead to an increase in overall accelerator efficiency.

Comparison of predictive control strategies for direct resonant High Voltage DC power supply

Abstract: This paper presents a comparison between two predictive control strategies for a HVDC power supply based on a direct resonant power converter: Input Predictive Control (IPC) and Output Predictive Control (OPC). IPC regulates the input filter capacitor voltage and OPC, the tank capacitor voltage. The resonant converter corresponds to a three phase to single phase matrix converter which feeds a resonant tank (SRPL). Both predictive strategies allow soft commutations, thus, minimizing the switching losses. Previous work has analysed the effects of the control choice on an AC voltage power supply only. The main contribution of this paper is the consideration of the effect of the control choice on the input current as well HVDC quality produced by the power supply. (6 pages)

Optimal switching pattern for space vector modulated Z-source NPC inverter

Abstract: The Z-source inverter is a relatively recent power converter topology that has been demonstrated in the literature to exhibit voltage-buck and voltage-boost capability. The Z-source concept can be applied to two-level and multilevel inverters. However, multilevel inverters offer many benefits for higher power applications compared to two-level inverters. Previous publications have shown the control of a Z-source neutral point clamped inverter using the carrier-based modulation technique. This paper presents the optimal switching pattern for the control of a Z-source neutral point clamped inverter using the space vector modulation technique. This gives a number of benefits, in terms of implementation and harmonic performance. The adopted approach enables the operation of the Z-source arrangement to be optimised and implemented digitally without introducing any extra commutations. Simulation results are presented to verify the concepts.

Non-uniform capacitor voltage ripple in a cascaded H-bridge StatCom

Abstract: Employing space vector modulation in a multi-level H-bridge StatCom (H-StatCom) can provide an excellent tradeoff between harmonic performance and switching frequency. However the nature of the techniques used to balance the capacitor voltages in this control strategy means that capacitor voltage ripple is increased. This can have detrimental effects on the current tracking performance. This paper both compares the capacitor voltage ripple in differing space vector modulation techniques and develops a compensation scheme to minimise the effect of ripple on the current tracking performance. Simulation results are presented that confirm the correct operation of the compensation scheme.