Switching Losses Of Converter Research Articles

Objective optimisation for multilevel neutral-point-clamped converters with zero-sequence signal control

When the converter level count gets higher, the space-vector pulse-width modulation (PWM) will suffer from the calculation burden for synthesising the voltage vectors and the redundant vector selection, which can be overcome by using the carrier-based PWM, as long as the carrier signal and modulation signal can be properly selected. This study proposes a new carrier-based PWM method to achieve some important control objectives in multilevel converters by adding the optimised zero-sequence signal to the reference voltage. The control objective as well as the control method for balancing the DC-link neutral point (NP) potential is presented. Meanwhile, the relationship between the NP potential and zero-sequence voltage is comprehensively analysed and an algorithm for injecting the optimised zero-sequence signal is derived. This study also investigates how to use the zero-sequence voltage to achieve the ‘two-phase’ mode operation for reducing the converter switching losses. A common mode (CM) voltage mitigation method is then proposed which effectively mitigates the CM voltage within 1/6 of the DC-link voltage. Furthermore, this study gives a simple method to directly map the reference voltage to the converter switches by using the concept of voltage level. The experimental results with a scaled 1 kW system validate the proposed NP potential control method and CM voltage mitigation method.

An Improved Representation of FACTS Controller Semiconductor Losses in EMTP-Type Programs Using Accurate Loss-Power Injection Into Network Solution

This paper extends previous approaches that use electromagnetic transient simulation to estimate the switching and conduction losses of insulated-gate bipolar transistor-based voltage-sourced converters embedded in large electrical networks. In contrast to earlier approaches, which merely estimate and report the device loss, the proposed new model also reflects this loss back into the electrical network model using a controlled voltage source. Additionally, an improved representation of the semiconductor switch in the flexible AC transmission system controller is introduced. This new representation has a mathematically simpler form in comparison to earlier approaches and yet permits the use of a relatively large time step.

Comparison of synchronous PM machine types for wide constant-power speed operation: converter performance

A thorough comparison of the converter performance characteristics for four permanent magnet (PM) synchronous machine configurations is presented. Two versions of an interior PM (IPM) machine with distributed windings are included. One version has a maximum back-EMF limit at the top speed while the second does not have any constraint on back-EMF amplitude. Two types of surface PM (SPM) machines are also considered, one with fractional-slot concentrated windings, and another with conventional distributed windings. The target application is an automotive direct-drive starter/alternator requiring a very wide 10:1 constant power speed ratio (CPSR). Detailed comparisons of the converter performance below and above the base speed are presented, evaluating significant issues, including the converter switching and conduction losses, output ripple current, and DC-link current ripple. Study results show that the higher excitation frequencies required by PM machines with high pole numbers have only a modest impact on converter efficiency for comparable output current waveform quality. In constrast, the imposition of maximum back-EMF amplitude constraints at top speed raise the machine rated current, resulting in elevated converter losses and larger DC-link capacitors

An indirect model of SSSC for reducing complexity of coding in Newton power flow algorithm

In this paper, an indirect modeling approach for static synchronous series compensator (SSSC) is proposed for Newton power flow analysis. This model shows that addition of ‘ p’ SSSCs to an existing ‘ n’ bus power system can be represented as an equivalent ‘( n + p)’ bus power system, without any SSSC. As a result, standard power flow analysis of the ‘( n + p)’ bus system can be carried out to calculate the steady state operating point of the original system containing SSSCs. Absence of any SSSC in the transformed system eliminates the need of writing new codes for computing the power flow equations and the Jacobian elements pertaining to SSSCs. This results in a substantial reduction in the programming complexit y. The proposed model can also easily account for converter switching losses and various practical device constraints. Application of the proposed model for multiple SSSCs in the IEEE 30-bus, IEEE 118-bus and IEEE 300-bus systems demonstrates the feasibility of the model and its excellent convergence characteristics.

Design of Pulse Patterns for Matrix Converters

For the matrix converter (Fig. 1) two basic methods are known to calculate the switching states and turn-on times necessary to generate the average output voltage and input current desired. One modulation technique is the direct method of Venturini and Alesina which controls each output phase separately and the other one is the space vector modulation or indirect method proposed by Huber, Borojevic and Burany which treats the complete converter as a whole.The paper presents a new pattern optimisation principle for the space vector modulation which reduces the switching losses of the matrix converter significantly.Already published optimisation techniques try to minimise the number of commutations within a switching period by different choice of the switching states for the zero voltage and by different arrangements of the switching states. These principles lead to pulse patterns which are quite similar to the Flat-Top Modulation or to the symmetrical use of zero switching states known from voltage source converters.The new optimisation method also takes into account the commutation voltage which is different for each individual switching operation and which has a strong influence on the switching losses.To minimize the commutation voltage it is necessary to follow a certain phase sequence, i.e. the input voltages must be connected to each output phase in a certain order. This order depends on the actual values of the input voltages. For the direct modulation technique this loss optimised sequence can be realized quite easily but for the space vector modulation no arrangement of switching states follows the loss optimised sequence.The paper proposes a technique to obtain the Loss Optimised Pulse Pattern out of the result of the space vector modulation by a subsequent phase oriented rearrangement of the switching states. The influence on switching losses and on harmonic distortion both on the input and on the output side will be discussed.

Multipulse converters and controls for HVDC and FACTS systems

For very high power applications, it is imperative to limit switching losses in the power converter. Multipulse converters are made up from several converter modules, switching at the fundamental frequency, but properly phase shifted to produce a nearly sinusoidal output voltage. Robust controls for HVDC and FACTS systems based on multipulse converters were developed. The developed HVDC system makes the rectifier station to control the instantaneous active (real) power through the HVDC voltage link, whereas the inverter station controls the instantaneous reactive (imaginary) power generation. The control of real and imaginary powers is independent one from the other and there is no control signal that is shared by both converter stations. A complete model of a 100 MW, 33 kVac, 50/60 Hz, 24-pulse HVDC system was implemented in the electromagnetic transient program (EMTP/ATP). The static synchronous compensator (STATCOM) is a shunt type of FACTS controller. It regulates the voltage magnitude of a controlled ac bus by drawing variable reactive current from the power system. The fast dynamic of the STATCOM controller avoids high fluctuations of the dc voltage caused by transients. A complete model of a 100 MW, 33 kV, 60 Hz, 24-pulse STATCOM was implemented and the results have confirmed the robustness of the proposed voltage control.

Evaluation of soft switching for EV and HEV motor drives

Soft switching has the potential of reducing switch stresses and of lowering the switching losses as compared to hard switching. To understand the effectiveness of the soft-switching technique, when applied to electric vehicle (EV) and hybrid electric vehicle (HEV) systems, it may be necessary to first evaluate their system requirements and performance. This evaluation process would require knowledge of the vehicle dynamics. The vehicle load requires a special torque-speed profile from the drivetrain for minimum power ratings to meet the vehicle's operational constraints, such as initial acceleration and gradability. The selection of motor and its control for EV and HEV applications are dictated mainly by this special torque-speed requirement. As a consequence, this requirement will have a strong influence on the converter operation. This paper makes an attempt to evaluate EV and HEV running in both standard Federal Test Procedure 1975 city driving and highway driving cycles. A simplified analysis is carried out for several of the most commonly used electric motors operating on the optimal torque-speed profile. Special attention is given to the converter conduction and switching losses, by analyzing the switching losses, and by assuming that an ideal soft-switching scheme will have zero switching losses, one can evaluate the improvement in the system efficiency if a soft-switching control is used. The relative significance of soft switching for EV and HEV systems is then established.