SMØLA, NORWAY 2005. ASSIGNEMENT ON CONVERTER LOSSES, MULTILEVEL CONVERTER TOPOLOGIES

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Fundamental Study of 2-level and 3-level Frequency Converters Markku Jokinen, and Anssi Lipsanen

Abstract—The need of higher powers in electrical drives has forced the researchers to develop new power source possibilities. Increasing demand for higher powers can be provided through parallel converter systems or multi-level converters. In some cases both the parallelism and the multi-leveling have been used to provide high voltage and high current power sources. This paper describes the fundamentals of 3-level frequency converter topologies and power loss estimation between generic 2-level and 3-level converters. The power loss estimation is based on the analysis of the semiconductor datasheet. Index Terms—converter losses, multilevel converter topologies

I. INTRODUCTION

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N the industry, there are two basic needs for the development of the power electronic devices. The needs are a higher power capability (over 1 MW) and a smoother output voltage. In a classical two-level low power frequency converter increasing the switching frequency and modifying the modulation techniques have been executed for a smoother output voltage. In the case of the high power applications the switching frequency cannot be increased, because of higher switching losses and electronic limitation of the power switches (turn on and off times are bigger with high voltage switches than with low voltage switches). One solution for this problem is a multi-level frequency converter. In the multilevel converters the voltage rating of power switches can be lower than in two-level converters. Lower voltage rating of switches, decrease the switching losses and diminish the electronic limitation of the high voltage switches. Increasing the voltage levels of a converter can solve the problems of a two-level frequency converter. A harmonic distortion decreases when the number of voltage levels increase. This means that there is no need for such filters, which are implemented nowadays. With the multi-level converter, also, dV/dt decrease. Smaller dV/dt decreases the motor failures like bearing falsies and insulation breakdowns. In the case of wind power application, the stress of the transformer decrease and perhaps the lifetime of the transformers will be longer than with classical two-level

converter. At first, in this study three different multi-level topologies are presented. Then losses of frequency converter are analyzed and compared in cases of the three-level converter and of the two-level converter. A. Diode-Clamped A three-level diode-clamped frequency converter has developed in 1981 by Nabae, Takahashi, and Akagi [1]. The structure of the three-level diode-clamped frequency converter (alternatively known as neutral-point clamped) is shown in Fig 1, where only one phase is described to simplify the structure. From the Fig. 1 can be seen that, the DC-link voltage is split into three levels by two series-connected capacitors C1 and C2. The central point n is defined as the neutral point. When the three-level frequency converter is compared to the two-level frequency converter the biggest difference is the diodes D1 and D1’. These two diodes clamp the switching voltage to the half level of the DC-link voltage. A three-level frequency converter can give three different output voltage levels VDC/2, 0, and -VDC/2. With the diodeclamped topology, switches S1 and S2 need to be turned on to output voltage VDC/2 from a and n terminals. In this case, S1’ blocks the voltage over C1 and S2’ blocks the voltage over C2. The diode D1’ balances out the voltage distribution between S1’ and S2’. For voltage level 0, switches S2 and S1’ need to be turned on. And for voltage level -VDC/2, switches S1’ and S2’ V DC

C1 VDC

D1

S2 van

n

C2

Manuscript received June 28, 2005. This work was supported in part by the Lappeenranta University of technology, Finland. The authors are with Lappeenranta University of technology, P.O.Box 20, FIN-53851 Lappeenranta, Finland (Markku Jokinen; phone: +358-5-6216719; fax: +358-5-621-6799; e-mail: [email protected]).

S1

2

V DC 2

D1 '

a

S1'

S2' 0

Fig. 1. The one phase of the three-level diode-clamped frequency converter. The main difference to the two-level frequency converter are the diodes D1 and D1’, which clamp the switching voltage to the half level of the DC-link voltage.

SMØLA, NORWAY 2005. ASSIGNEMENT ON CONVERTER LOSSES, MULTILEVEL CONVERTER TOPOLOGIES need to be turned on. If output voltage is taken from a and 0 terminals, the circuit will turn into a dc/dc-converter and the output voltage Va0 would be then VDC, VDC/2, or 0. [2] The three-level frequency converter needs more components than two-level frequency converter, but the active components can be chosen differently. Every active switch needs to block the voltage level

VDC , m −1

(1)

where m is the number of voltage levels in frequency converter. So, the active components of the three-level frequency converters need to block only half of the DC-link voltage. Voltage rating of the switches can be select to be a half of the normal voltage rating of the two-level frequency converter, but the voltage rating of the clamping diodes cannot be chosen the same way. The diodes must have different voltage rating for reverse voltage blocking, because they need to block the whole DC-link voltage. If each blocking diode has the same voltage rating than active switches, every phase should have (m − 1)(m − 2 ) blocking diodes. As it can be seen from the previous equation, the number of the blocking diodes increase quadratic in function of m. At higher number of m, the system will be very complex and impractical to implement. Also the voltage balance across each of the seriesconnected capacitors in the DC-link is a challenge problem, but it is solvable by suitable control algorithms [3]. B. Capacitor-clamped A Capacitor-clamped or also called a flying capacitor frequency converter gives three different voltage level Van: VDC/2, 0, and -VDC/2 as a diode-clamped frequency converter too. The structure of the capacitor-clamped converter is shown in Fig. 2. The two clamping-diodes of diode-clamped topology are replaced with one clamping capacitor C1, but the rest of the structure is the same as the diode-clamped topology. Although, the structure is almost the same, the same V DC

C2 VDC

n

S2 van

C1

a

S1'

V DC 2

C. Cascaded There are several ways to implement cascaded multi-level frequency converter. In this study two different methods are presented. The first method is based on the series connection of single-phase inverters with separate DC sources and the second method uses standard three-phase two-level inverters. Both methods have their own advantages and disadvantages. If there are a few separate DC sources available, the series connection may be the best decision. In Fig 3 is shown one

C1

C2

C3

-

control method cannot be used within these two topologies, because the combinations of switches for the voltage levels are different with capacitor-clamped and with diode-clamped topology. For voltage level VDC/2 switches S1 and S2 need to be turned on. For voltage level -VDC/2 switches S1’ and S2’ need to be turned on. And for voltage level 0 switches S1 and S1’ or switches S2 and S2’ need to be turned on. When the switches S1 and S1’ are conducting, clamping-capacitor will be charged. And when the switches S2 and S2’ are conducting, clamping-capacitor will be discharged. With choosing the right combination for the zero voltage level, the charge of the capacitor can be controlled. Increasing the level of the capacitor-clamped frequency converter, the number of different switching combinations increases more rapidly than with the diode-clamped frequency converter. One of these different switching combinations for the same voltage level will either charge or discharge the capacitor, so, control of the multi-level capacitor-clamped frequency converter is challenging. As the diode-clamped topology, also the capacitor-clamped topology needs more components than normal 2-level frequency converter. As it can be seen in Fig. 2, there are a lot of capacitors. m-level capacitor-clamped frequency converter requires a total of (m-1)(m-2)/2 clamping capacitors per phase and (m-1) DC-link capacitors. It is to be noticed that voltage rating of each capacitor has to be the same as that of the main power switch.

a

S1

2

2

n

S2' 0

Fig. 2. The structure of the capacitor-clamped frequency converter.

Fig. 3. Series connection based method of the single-phase frequency converter with separate DC sources. The converter has five-levels and is consisted of two cells per each phase.

SMØLA, NORWAY 2005. ASSIGNEMENT ON CONVERTER LOSSES, MULTILEVEL CONVERTER TOPOLOGIES phase leg of a five-level frequency converter with two cells per each phase. The phase voltage of this kind of topology is summation of the voltages generated in the separate cells. One cell can generate three levels of output voltage Van. The voltage levels are VDC, 0, and - VDC. The five-level frequency converter output voltage varies then from -2VDC to 2VDC. As was said before, this method requires several DC sources to operate. If the generators of a wind park would be DC generators, a series connection should be considered. A modern wind park runs, however, with induction generators or permanent magnet generators, which makes this method impractical to use. The second cascade method, which will be discussed in this study, uses standard three-phase two-level converters. The principle of this structure is shown in Fig. 4. The structure is formed by the combination of three frequency converters and three output transformers, which adds the different voltages together. The total output voltage of this topology is three times output voltage of one converter. The main challenge of this topology is to get all the output voltages synchronized to 120° phase shift.

II. TOPOLOGY POWER LOSS COMPARISON In this section, we take a closer look to IGBT losses in 2-level and diode-clamped 3-level frequency converter topologies. To get some kind of idea how large the losses between 2-level and 3-level converters are a simulation model is used. Among engineers, it is most often assumed that the switching losses of IGBTs are always the dominative source of losses in frequency converters. In [4] a comparison of multi-level converters is made, and in which the general assumption is confirmed. This assumption is not totally wrong, but neither totally correct. Through the simulations and component comparison performed for this paper, it can be easily found out that the distribution of losses depends on the power rating of the IGBT, and not exclusively the conduction time or switching frequency.

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A. Simulation model In the reference [5], an analytical methods based on the interpretation of the IGBT datasheets was used. For this paper the same methods are used to compare power losses in 2-level and 3-level converters. The method is based on the mathematical functions of the IGBT characteristics that are all functions of current, frequency and temperature. The characteristic curves can be modeled by using high order functions, which makes the simulation model as accurate as it is possible to form from the datasheet curves. The method employed gives also the advantage to take into consideration the temperature influence in currents, but unfortunately the temperature characteristics are seldom presented in datasheets at more than one temperature. With one temperature value it is impossible to make use of temperature changes in the model. In Fig. 5 a functional operation of the simulation model is presented. The simulation model is applied to one semiconductor component only. This modeling method requires that the shown model must be applied to every component separately, and then summed to the total power loss. Under PWM-modulation the IGBTs in one leg of the frequency converter can be considered as equally loaded, so the total power loss in inverter can be calculated by multiplying the power Ptotal by three. For the power loss estimation the IGBT type SKM 100GB063D (600 V) was used in 3-level converter and type SKM 145GB123D (1200 V) in 2-level converter. The datasheets for the semiconductors were not however detailed enough to perform thermal analysis, which is why it was left out. The model introduced in Fig. 5 was built in SIMULINKTM for both 2-level and 3-level converters. The load, where the inverter is connected to, is considered purely resistive, which makes the free-wheeling diodes useless in power loss estimation point of view. In the model, it is assumed that every other component except IGBTs, do not generate power loss significant enough. The introduced power loss simulation model is still suitable to apply to any other component also even they are neglected in

a VDC

A B C

VDC

b

VDC

c

Fig. 4. A multilevel converter using standard three-phase two-level frequency converters.

Fig. 5. Two models are needed to estimate the power loss in a frequency converter. From the electrical model 5a the current ID and switching frequency fsw are directed to power loss model 5b. The power loss Ptotal in single switch and the temperature T raise are easily determined as functions of time.

SMØLA, NORWAY 2005. ASSIGNEMENT ON CONVERTER LOSSES, MULTILEVEL CONVERTER TOPOLOGIES

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of 1200 V IGBTs much higher. Earlier, it was mentioned that the conduction losses are estimated with second order function and the switching losses as linear functions. These estimation characteristics can also be seen in Fig. 6. Since the 10 kHz and 15 kHz curves of 2-level converter power losses are linear, it can be assumed that switching losses are dominative. In rest of the curves exponential characteristics can be seen, so in those cases the conductive losses are dominative. If we take a look at the table I, we can see different types of IGBT and their power loss distribution between conduction and switching losses. At higher power rates the switching losses become more dominant than conduction losses. The conduction loss is estimated from the datasheet at current point 75 A. The relation between power losses is expressed as loss ratio

E switching ⋅ f switching Pconduction . Fig. 6. Total IGBT power losses generated in one leg of 2-level and 3-level converters at different switching frequencies. The 2-level inverter has 1200 V IGBTs and 3-level inverter 600 V IGBTs.

this particular simulation. Also the mathematical functions, which were determined from the semiconductor datasheet, were simplified so that the switching energy curves were considered linear and conduction curves as second order functions. The greatest error occurs, when the switching energies are being estimated, because the energies are usually presented at certain voltage, which does not equal to the real voltage values present in a converter. The simulations are still suitable enough to present the power loss increase due to high frequency switching and in the general point of view, how large are the losses in 2-level and 3-level converters. The loss distribution between switching losses and conduction losses depends remarkably on the IGBT type. In Fig. 6 it is shown how the total power loss acts in 2-level and 3-level converters at different load conditions and switching frequencies. Only at the lowest switching frequency (1 kHz) the total losses are smaller in 2-level converter than in 3-level converter. The reason for this is that the switching losses for this particular type of 1200 V IGBT are somewhat lower than conduction losses in 600 V IGBT at any given load condition. Higher frequencies, however, will make the total power losses

(2)

III. CONCLUSION Several multi-level converter topologies were introduced and their characteristics were described. Among diodeclamped, capacitor-clamped and cascaded converter topologies a clear comparison can not be made purely through literature research. The control of the capacitor-clamped topology is more challenging than in diode-clamped topology, since different switching combinations are needed for the same voltage. When one combination charges the clamping capacitor, the other combination discharges it. According to this characteristic, the diode-clamped converter topology is more suitable to use than capacitor-clamped converter topology. The introduced simulations performed with two different types of IGBTs showed that conductive losses are more dominative power loss source when the switching frequencies are kept low. At higher frequencies the switching losses become more dominative.

TABLE I IGBT POWER LOSS COMPARISON

SMØLA, NORWAY 2005. ASSIGNEMENT ON CONVERTER LOSSES, MULTILEVEL CONVERTER TOPOLOGIES REFERENCES [1]

[2]

[3]

[4]

[5]

A. Nabae, I. Takahashi, and H. Akagi, “A new neutral-point clamped PWM inverter,” IEEE Trans. Ind. Applicat., vol. IA-17, pp. 518–523, Sept./Oct. 1981. José Rodriquez, Jih-Sheng Lai, and Fang Zheng Peng, ”Multilevel inverters: A survey of topologies, controls, and applications”, IEEE transactions on idustrial electornics, Vol 49, Np. 4, August 2002. D. Grahame Holmes, Thomas A. Lipo, Pulse Width Modulation for Power Converters principles and practice, A John Wiley & Sons, Inc, 2003, ISBN 0-471-20814-0. Richard Lund, Jonas Beverfjord, Sigurd Øvrebø and Roy Nilsen, “Analytical Power Loss Expressions for Diode Clamped Converters,” presented at EPE-PEMC, Dubrovnik & Cavtat, 2002. Uwe Drofenik and Johann W. Kolar, “ A General Scheme for Calculating Switchingand Conduction-Losses of Power Semiconductors in Numerical Circuit Simulations of Power Electronic Systems,” Power Electronic Systems Laboratory (PES), Zurich, Switzherland, 2005.

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