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Proc. of Int. Conf. on Control, Communication and Power Engineering 2010

A New Inverter Topology for Adjustable Speed Drive Systems Amitava Das1, Debasish Lahiri1, and Arup Kumar Goswami2 1

Electrical Engineering Department, TI, Kolkata, India Email: amitavadas@ieee.org, dlsir.ti@gmail.com 2 Electrical Engineering Department, NIT - Silchar, India Email: gosarup@gmail.com inverters (qZSI) [4, 5]. The main advantages of qZSI circuits are improved input profiles and a common DC rail between the Z-source and inverter, unlike the traditional ZSI circuits. Another approach to ZSI concept with use of transmission line model can be found in [7] and [8]. Present research indicates another possibility: that is, to incorporate different passive networks at the input to the inverters, a technique that differs from the symmetrical LC lattice network [1, 2, 3] which is typically used in ZSI. These alternative passive networks have been known from many years from circuit theory.

Utilization of there will open new possibilities for onestep, energy processing Buck-Boost voltage converters.

This paper outlines the most significant alternative passive networks (Fig. 1) and their selected applications in inverters. Simulation results are shown for this approach. The method involves the use of the T-source inverter (TSI). The goal of this paper is to present a topology similar to that of the ZSI with use of impulse transformer with small inductive leakage. The indirect goal is to demonstrate, with the help of above mentioned transformer, that in the topology presented here it is possible to increase the output voltage.

Abstract— This paper presents a new topologies of voltage source inverters with alternative input LC networks for adjustable speed drive system. The basic topology is known in the literature as a Z-source inverter (ZSI). Alternative passive networks were named as T-sources inverter (TSI).

T-source inverter has fewer reactive components in comparison to conventional Z-source inverter. The most significant advantage of the T-source inverter (TSI) is its use of a common voltage source of the passive arrangement.

Simulation results for the TSI are in agreement with theoretical prediction. PSIM software is used for simulation studies.

Index Terms— LC network, ZSI, T-source inverter, motor drive, quasi Z –source inverter, PWM.

I. INTRODUCTION Fossil fuel reserve availability and environmental concerns are now the driving forces behind the use of new clean and renewable energy sources, such as photovoltaic (PV) energy, wind energy, and fuel cell, which are the input sources of alternative generation system [1]. Moreover, the inverter plays an important role in alternative generation system. Inverters with variable voltage which have an input from a low voltage DC source (eg., a PV cell) are mostly realized in the following three basic topologies: (a) PWM VSI+DC/DC boost converter without transformer (b) PWM VSI + DC/DC boost converter with transformer (c) PWM CSI. None of these solutions is fully satisfactory. Therefore there is a continuous effort to find newer and better solutions. A more interesting solution utilizes the Z – source inverter. The distinguishing feature of this inverter is its input symmetrical LC lattice network which has four impedances. In this design, ZSI provides the single stage voltage Buck-Boost operation, which results in lower costs and decreased losses. The ZSI can be made bidirectional by replacing the input diode with a bidirectional conducting, unidirectional blocking switch [3, 4]. There is also interesting research into NPC (Neutral Point Clamped) ZSI circuits which has been presented in detail in the following papers [5, 6, 7, 8, 9].These findings do not differ from those that result when using basic symmetrical LC lattice network [1, 2, 3]. Rather, they focus on changing the topology arrangement of the connections and don’t focus on either changing the basic structure or improving the ZSI. To eliminate the inconvenience of the typically Z-source inverter, there were modifications of its basic structure that consisted mainly in the change of primary source position. These modifications led to quasi-Z-source

Fig 1:Passive networks alternative to basic LC lattice network

II. T – SOURCE INVERTER The LC lattice applied in the ZSI successfully replaces the DC-DC input stage in boost-type voltage source inverters. To minimize the Z-source size, the couple inductors are designed, and the two inductors are built together on one core. To show the possibility of extending the operation range of the ZSI, the use of a low leakage inductance transformer and one capacitor instead of the LC-lattice is proposed here. A high frequency transformer based T-source inverter (TSI) is developed in this section. The topologies of TSI using modifications from Fig. 1(e) and Fig.1 (f) are shown in the Fig.2 (a) and 162

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Proc. of Int. Conf. on Control, Communication and Power Engineering 2010

In practice, the influence of leakage on inductance of the transformer is very important. The lower the inductance, the closer it is to theoretical dependences.

The performance of TSI depends on the precision of the transformer design.

Fig.2 (b), while Fig.3 shows the joint equivalent circuit of the both TSI topologies in two operational modes for transformer ratio n: 1.

IV. MINIMIZING THE IMPACT OF LEAKAGE INDUCTANCE If the transformer which is used to build the TSI has excessive leakage inductance, the efficiency of the Tinverter worsens. An increase of leakage inductance causes lower amplitude of output voltage, worse DC voltage VDC and increase in voltage stress on the transistors.

Regardless of the leakage inductance value the high performance of the T-inverter can be also achieved using additional snubber circuit shown in Fig.4. In order to reduce the overshoot of the devices caused by large leakage inductance an active snubber circuit as shown in Fig.4 (a) is applied in proposed system. The use of an active snubber ensures effective use of energy which might otherwise be lost due to voltage spikes. Moreover the active circuit can be used to control the charging cycle if additional energy storage is connected to the main capacitor of the TSI. The reduction of voltage stress can be accomplished also by using a simple snubber with a passive clamped circuit shown in Fig. 4(b). This circuit is similar to clamped circuits used in bidirectional ZSI. Two capacitors CS1, CS2 and one diode DS1 connected in series are connected right across dc rails of the inverter bridge. The capacitor CS2 is non inductive. The clamped circuit is connected with the middle point of the T-source.

Due to the discharge of the capacitors, the passive clamped circuit will reduce voltage spikes. As the voltage vDC is lowered due to large inductive leakage, selecting a transformer ratio larger than 1 can compensate the effect.

Fig 2:Low leakage induction transformer based T – Source inverter topology (a) shoot-through mode (b) non shoot-through mode

The TSI topology requires a very low leakage inductance transformer which should be made with high precision. In such a way, the number of passive elements is reduced because only the transformer and the capacitor are needed. It should be noted that the function of the input diode can be served by other power electronics systems as well, including a diode rectifier similar to Zsource inverter.

Fig 3: Operation modes of TSI in switching time period T=T0 + TI

III. PRINCIPLES OF OPERATION As with a conventional ZSI, the TSI can handle shoot through states when both switches in the same phase leg are turned on. The T-network is used instead of the LCnetwork for boosting the output voltage by inserting shoot through states in the PWM. The TSI governing equations can be developed for the Fig.3 using Kirchhoff’s laws and voltage averaging [12]. The average voltage through the transformer inductances should be equal to zero for the switching time period T.

VL = vL = [T0 .VC + T1.(VIN − VC ) / n] / T = 0

(1)

Both capacitor voltage VC and output voltage VOUT are functions of the shoot-through coefficient D = T0 / T.

VC (1 − D) T1 = = VIN T1 − n.T0 [1 − ( n + 1).D ]

Fig 4: (a) Active snubber (b) Passive clamped circuit

(2)

V. SIMULATIONS RESULTS Simulations and experimental investigations of the TSI inverter with proposed alternative passive network from Fig.5 were performed using PSIM simulation software.

Where D satisfies a condition D<1/(n+1). Hence the maximum value of D for TSI n>1 is smaller than for the conventional Z-source. This is the advantage of the TSI with n>1 in comparison with ZSI because the same output voltage can be obtained with smaller time period of short-circuits transistor current.

Using (2) the amplitude VDC of voltage vDC in nonshoot through states can obtain from:

VDC = VC + (VC − VIN ) / n = VIN / [1 − (n + 1).D] (3)

Fig 5: Simulation circuit setup

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Proc. of Int. Conf. on Control, Communication and Power Engineering 2010

CONCLUSIONS Simulations results confirm the reliability of the new TSI operation. The new topology uses a transformer with very low leakage inductance. For larger inductances, an active snubber and a passive clamped circuit are proposed. The input arrangement of the passive TSI consists of three impedances, two coupled inductors, and one capacitor. In comparison to the passive ZSI, these are less reactive components. The most significant advantage of the TSI is the extended possibility of manipulation of inverter output voltage and shoot-through coefficient using transformer turns ratio different than 1. The presented results encourage to continue research on TSI with different turns ratios and also demonstrate the other unconventional ZSI topologies using transformers. The best advantage of the TSI is the use of a common voltage source for the passive arrangement and the converter.

This provides grounding of the configuration and solves many problems involving electromagnetic compatibility which are present in the ZSI.

Fig 6: DC voltage vDC on the T-source output

Fig 7: Input diode voltage vD

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[2] F.Z Peng: Z-Source Inverter, IEEE Trans. on Industry Applications, No.2, Vol.39, 2003, pp.504-510.

[3] Xu Haiping, F.Z. Peng, Chen Lihua, Wen Xuhui: Analysis and design of Bi-directional Z-source inverter for electrical vehicles, Proc. of 23rd Annual IEEE APEC’08, 2008, pp. 1252-1257.

[4] J. Anderson, F.Z. Peng: Four Quasi-Z-Source Inverters, Proc. of IEEE PESC 2008, 15-19 June 2008, pp . 2743 – 2749.

[5] J. Anderson, F. Z.Peng,: A Class of Quasi-Z-Source Inverters, in Proc of 43rd IAS Annual Meeting, 2008, pp.

1-7.

[6] R. Strzelecki, M. Adamowicz, D. Wojciechowski: BuckBoost Inverters with Symmetrical Passive Four-terminal Networks. Proc. of 5th Int. Conference-Workshop CPE’07, Gdansk, Poland, 2007, pp. 1-9.

[7] R. Strzelecki, D. Wojciechowski, M. Adamowicz, A. Wilk, I. Mosoń: Three-phase NPC Z-source inverter, Electrotechnical Review, No. 10, 2006, pp. 54-60.

[8] F. Z. Peng: Z-Source Networks for Power Conversion, Proc. of 23rd Annual IEEE APEC’08, 2008, pp. 1258-1265.

[9] A.Das, S.Chowdhury, S.P.Chowdhury, A.Domijan: “Performance analysis of Z –source inverter based ASD system with reduced Harmonics” Proc. of IEEE Power and Energy Society General Meeting PESGM08, USA, 2008. pp. 1 – 7.

[10] M. Fusachika: Synthesis of a One-Port Utilizing a Lattice Construction, IEEE Tran. on Circuit Theory, June 1963, pp.227-234

Fig 8: T – inverter capacitor voltage

Fig 9: Output current

Fig 10: Output line to line voltage

Maximum Constant Boost Control (MCBC) strategy using modulation reference with 3rd harmonics addition was applied in simulations set up. The results of simulations are shown in Fig.6 to Fig.10 Fig.6 shows the output TSI voltage vDC. The input diode voltage vD is shown in Fig.7 and capacitor voltage VC in Fig.8 The output currents shown in Fig.9 is sinusoidal and proper operation of induction motor is achieved [10].

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