Medium frequency transformer with loss aspects

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Isolated AC/DC Converter

This chapter gives an overview and analysis of a prototype converter studied for power loss modeling. The operation of the converter is explained with emphasis on losses. Apart from this, implementation of the different components is also described.

Introduction to Converter

The prototype converter resides in the class of converters that perform bidirectional AC/DC operation with isolation by a medium frequency transformer. The fundamental structure is presented in figure 2-1. The transformer in the middle is magnetized by a medium frequency AC voltage, supplied by a Voltage Source Converter (VSC), which is equipped with snubber capacitors. On the other winding, the voltage is converted to a Pulse Width Modulation (PWM) voltage by a cycloconverter which is then applied to an inductive filter in order to achieve a desired AC voltage. This configuration is compatible with a commutation sequence that permits soft-switching conditions for all the power semiconductor valves at all the operating points.

Converter Topology

The topology of the converter is shown in figure 2-2. The VSC equipped with the snubber capacitors in parallel to each valve is connected to one of the windings of the medium frequency single phase transformer. The other winding is connected to the 2-phase to 3-phase cycloconverter which is then connected to a passive line filter [1].

Voltage Source Converter (VSC)

In general, a VSC converts a DC voltage to an AC voltage of a desired frequency and amplitude. Usually PWM strategies, due to their simplicity, are used to control the switching of the IGBTs. Normally, the valves in the VSC are designed in order to block the voltage in one direction by using gate command and also to conduct a current in both directions.
The VSC in the studied converter is a bridge converter having two phase legs. Each phase leg has two valves. Each valve consists of an IGBT with an anti-parallel diode. Snubber capacitors are also used in parallel to each valve to reduce the switching stress to a safe level. The output is connected to the midpoint of each phase leg. A DC link capacitor is used at the input in order to by-pass the AC components during a power flow from AC to DC side.
In the studied converter, different control strategy has been adopted for the VSC to produce an “equal width square” wave at the output by a proper switching of the IGBTs. As a result, a square wave of constant frequency having levels , 0 and is produced. Where, is the amplitude of the DC-link voltage.
The valves in the VSC are implemented with “BSM150GB120DLC” IGBT module with the blocking voltage and rated current of 1200 V and 150 A respectively. The snubber capacitors are of the polypropylene type having a capacitance of 220nF per valve. The IGBT modules are connected to a DC link capacitor that has a capacitance of 2.9 mF.

Cycloconverter

In general a cycloconverter converts an AC voltage directly to another AC voltage of desired frequency and amplitude without any intermediate DC stage. In this type of converter, bidirectional power flow is possible with an ability to operate with a load of variable power factors. Generally, the valves in the cycloconverter are designed to conduct the currents in one direction and to block voltage in both directions.
The cycloconverter in the studied converter is a 2-phase to 3-phase converter having three phase legs. Each phase leg has two valves. The bidirectional valves are made by connecting the IGBTs in the common-emitter fashion as shown in figure 2-2. RC-Snubbers are used in parallel with each valve in order to prevent the overvoltage during the commutation process.
The switching of the valves in each phase leg follows the PWM strategy, which results in a rectangular waveform having a variable width.
The valves in the cycloconverter are implemented with “BSM100GT120DN2” IGBT module having the blocking voltage and the rated current of 1200 V and 150 Amps respectively.

Medium Frequency Transformer

A medium frequency transformer is used in the studied converter to couple the VSC with the cycloconverter.
The transformer is of a toroidal type with a ferrite core from AVX, type B2. Four pieces of the toroidal cores are stacked together to make a core. Each piece has an outer diameter equal to 152mm, inner diameter 68mm and a width of 19mm. One of the windings is split into two in order to reduce the leakage inductance. The key data of the transformer is listed in table 2-1.

Output Filter

The filter in the studied converter has been implemented with a simple LC filter. It has an ability to smooth the load current by blocking the harmonics produced by the PWM voltage. Proper values of capacitance and inductance have been chosen to get the desired results. The parameters that specify the filter are given in table 2-2.
The inductor in the filter has an iron core to improve the inductance, the key parameters of which are given in table 2-3. The core material for an inductor core is assumed in this case because the loss data for the real core is not available. However, the assumed core has the same lamination thickness as the one used in the actual core.

Principle of Operation

To analyze the converter, a convenient method used by [1] is considered in which the coupling functions that relate the voltages and currents are used. For ease of analysis, all the voltages and currents are represented with respect to the transformer voltage [1].
The coupling function for a VSC to couple a DC-link voltage with a transformer winding is represented by [3]. For a full bridge VSC, , when the valves in one of the diagonals are conducting and , when the valves in the other diagonal are in conduction state . In this regard, the transformer voltage at any instant is given by equation-2.1. where, is a DC-link voltage.
Similarly, the coupling function defined for each phase leg of the cycloconverter is represented by [1]. Where, , when the phase leg of the cycloconverter is connected to the upper and lower terminal of the transformer respectively. The cycloconverter voltage as a function of coupling function can be expressed as, (2.2) Similarly, the transformer current can be related to the cycloconverter current by using the coupling function as, The turns ratio of the transformer is assumed to be unity. The coupling function is one half because the output voltages are referred to the midpoint of the transformer windings connected to the cycloconverter [1].
From the aspect of losses when the coupling functions are non-zero, the power semiconductor devices are in on-state and present the on-state voltage during current flow resulting in conduction losses. These losses have a major contribution to the total losses and therefore need to be computed.

Commutation Sequence

The commutation process is also analyzed in a simple way by making few assumptions [3].
• The voltage applied to a DC link capacitor is assumed to be essentially constant.
• The inductor filter on the AC side is assumed to be large enough as it has the ability to maintain constant current in each modulation interval and can be modeled by a current source.
• The transformer is modeled by a leakage inductance , i.e. magnetizing current is neglected.
Both converters commutate alternately. VSC undergoes snubbered or zero voltage commutation [1] that reverses the voltage across the transformer and enables a source commutation (natural commutation) of the cycloconverter which in turn reverses the direction of the transformer current.

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Commutation process with an aspect of losses

VSC commutation

The VSC commutation starts by turning-off the IGBTs, thereby diverting the current to the snubber capacitors to recharge them. The voltage across the transformer starts decreasing whereas the current remains constant due to the passive line inductance. When the potential of the phase legs has fully moved to the opposite DC rail, the diodes in the opposite valves start to handle the current. At this instant the power flow is from AC side to the DC side as explained briefly in [1]. The IGBT anti-parallel to the conducting diode can be turned on at a zero – voltage and zero-current conditions. The turn-off is made at a low voltage derivative [1] due to the snubber capacitors.
The VSC commutation time can be expressed as, (2.4)
The VSC commutation process is also illustrated together with different stages in figure 2-3.
Loss perspective
IGBTs
The turn-off of the IGBTs in the VSC occurs at high current, therefore the turn-off losses in the IGBTs need to be considered, whereas the turn-on process takes place at zero voltage and zero current condition therefore the turn-on losses do not exist.
The VSC can be operated with or without snubber capacitors. In both cases the IGBTs turn-on at zero current and zero voltage conditions. On the other hand, the turn-off process takes place at higher current for both operations therefore the turn-off losses need to be computed in both cases.
The turn-off process of an IGBT without snubbers is shown in figure 2-4.
Two distinct parts of the turn-off current have been observed. The rapid drop in the current during interval corresponds to the turn-off of the MOSFET’s part of the IGBT whereas the tail in the current during the time interval is due to a stored charge in the drift region.
The tail in the current has a significant contribution to the power dissipation as the collector-emitter voltage is in its off-state value. The duration of the tail current also increases with increased temperature. The overlap between the voltage and current for the interval indicates the area of the power loss during the turn-off process.
In case of a turn-off with snubbers (soft-switching), the voltage rises linearly with slow rate which depends upon the current that flows through the capacitor to recharge it, figure 2-5. This condition reduces the overlap between the voltage and current and hence the switching losses. Though the overlap between the current and voltage is reduced, still the losses need to be considered due to the tail current that overlaps with the voltage during the voltage rise.
The stress on a switch as well as electromagnetic interference can be reduced by using the soft switching strategy. The switching frequency and the power density can be increased as well.
Diodes
Usually the turn-on losses of the diode are negligible as it turns-on quickly [4]. The diode however, exhibits reverse recovery losses during turn-off. But in this case they are negligible because the current flowing through it goes to zero naturally and at this moment IGBT is turned on at zero voltage and zero current condition. As a result it faces only the on-state voltage of the IGBT during a turn-off process (reverse recovery current flow) that results in a negligible amount of reverse recovery losses.
VSC-Snubbers
In the VSC, lossless snubbers are used as IGBTs turn-on at zero voltage and zero current. During the VSC commutation, the capacitors in one of the diagonals of the VSC transfer their energy completely to the capacitors in the other diagonal after which the IGBTs are turned-on. Therefore the losses due to these snubbers do not exist.
VSC commutation causes voltage reversal at the transformer terminal leading to a source commutation (natural commutation) in the cycloconverter.
Cycloconverter Snubbers
The VSC commutation establishes a condition of voltage reversal across the RC snubbers in the cycloconverter. The voltage across the charged snubbers changes from
respectively that corresponds to the discharge and then recharge of the snubbers. As resistive snubbers are mounted, the stored energy in the capacitors dissipates across the resistors.
The VSC commutation ends up with the turn-off losses of the IGBTs in VSC and RC-snubber losses in the cycloconverter.

Cycloconverter Commutation

In cycloconverter commutation, one phase leg undergoes commutation at a time. It starts by turning-on the non-conducting valve in the direction of the current through the phase terminal with the following condition to be fulfilled [1] (2.5)
The voltage appears across the leakage inductance, and the incoming valve takes over the current [1]. The current in the initially conducting valve goes to zero, and the switch in the valve can be gated off. The current derivative depends upon the transformer’s leakage inductance and is relatively low. The commutation time for each phase leg can be estimated by (2.6)
At the end of commutation for each phase leg, the following condition becomes valid (2.7)

Table of contents :

1. Introduction
1.1 Background
1.2 Motivations and Objectives
1.3 Outline of Thesis
Chapter 2
Chapter 3
Chapter 4
Chapter 5
Chapter 6
2. Isolated AC/DC Converter
2.1 Introduction to Converter
2.2 Converter Topology
2.2.1 Voltage Source Converter (VSC)
2.2.2 Cycloconverter
2.2.3 Medium Frequency Transformer
2.2.4 Output Filter
2.3 Principle of Operation
2.4 Commutation Sequence
2.5 Commutation process with an aspect of losses
2.5.1 VSC commutation
2.5.2 Cycloconverter Commutation
Loss perspective
2.5.3 Resonant commutation
2.5.4 Voltage and Currents
2.5.5 Modulation
2.5.6 Medium frequency transformer with loss aspects
2.5.7 LC filter with loss aspects
2.6 Summary
3. Evaluation of Losses
3.1 Introduction
3.2 Power semiconductor Losses
3.2.1 Conduction Losses
3.2.2 Switching Losses
IGBTs
Diode reverse recovery losses
Total semiconductor losses in VSC and Cycloconverter
Power Loss Modeling of Isolated AC/DC converter
3.2.3 Thermal Characteristics of Power Semiconductors
Thermal resistance from junction to case ( )
Thermal resistance from case to sink ( )
Thermal resistance from sink to ambient ( )
3.3 RC-Snubber Losses
3.3.1 VSC Commutation
3.3.2 Cycloconverter Commutation
3.3.3 Additional Losses due to cycloconverter commutation
3.4 Losses in Transformer and Reactor
3.4.1 Core losses
Original Steinmetz equation
Modified Steinmetz equation
Generalized Steinmetz Equation
Improved Generalized Steinmetz Equation
3.4.2 Core Losses in Transformer
Proposed method for core losses
3.4.3 Core Losses in Inductor
Proposed method for Core loss calculation
3.4.4 Copper Losses
4. Model of Prototype Converter
4.1 Introduction
4.1.1 VSC
4.1.2 Cycloconverter
4.1.3 Transformer
4.1.4 RC Snubbers
4.1.5 Output Filter
4.2 MATLAB Implementation
4.2.1 Simulation
4.4 Simulation Results
5. Calculation of Losses
5.1 Power semiconductor losses
5.1.1 Conduction losses
5.1.2 Switching Losses
Switching losses in IGBTs
Diode reverse recovery losses
Total power semiconductor losses
5.1.3 Thermal model
Cycloconverter
VSC
5.2 Snubber losses
5.3 Transformer Losses
5.3.1 Copper Losses
5.3.2 Core Losses
5.4 Inductor losses
5.4.1 Copper Losses
5.4.2 Core Losses
5.5 Comparison of results
5.5.1 Variable Load current
5.5.2 Low Input DC-link Voltage
5.5.3 Modulation Index Variation
Results and Discussion
6. Conclusion
Future Work
Works Cited
Appendix
VSC Data Sheet
Cycloconverter Data Sheet
Loss Curves of Transformer

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