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New Type T-Source Inverter 1
Ryszard Strzelecki1, Marek Adamowicz1, Natalia Strzelecka1, Wieslaw Bury2 Gdynia Maritime University (Gdynia, Poland), 2DeVry University (North Brunswick, United States)
[email protected],
[email protected],
[email protected],
[email protected]
Abstract- This paper presents different topologies of voltage inverters with alternative input LC networks. The basic topology is known in the literature as a Z-source inverter (ZSI). Alternative passive networks were named by the authors as T-sources. 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. Experimental results for the TSI are in agreement with theoretical and simulated prediction.
I.
INTRODUCTION
Inverters with variable voltage which have an input from a low voltage DC source (eg., a PV battery) are mostly realized in the following three basis 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 a Zsource inverter (ZSI) which was proposed by Professor F.Z. Peng in 2002 [1, 2, 3]. The distinguishing feature of this inverter is its input symmetrical LC lattice network which has four impedances. In this design, ZSI provides the singlestage voltage Buck-Boost operation, which results in lower costs and decreased losses. Since the first publication involving the ZSI, there have been many other works which describe different applications of the basic solution as well as its modification. The ZSI can be made bidirectional by replacing the input diode with a bidirectionally conducting, unidirectionally 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 of the ZSI. Just recently, to eliminate the inconvenience of the typically Z-source inverter, there were modifications of its basic structure whose consisted mainly in change of primary source position. These modifications led to quasi-Z-source inverters (qZSI) [10, 11]. 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 [12] and [13]. A
transmission line is a circuit which naturally satisfies the Zsource concept’s requirements. 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 [14, 15]. Utilizing them will open new possibilities for one-step, 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 this approach. The method involves the use of the T-source inverter (TSI), as named by the authors. This paper also shows experimental results of the TSI which support the possibility of practical application of alternative passive networks. 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. For transformers characterized larger leakage inductances two circuits of active snubber and passive snubber are proposed to achieve minimization of voltage spikes in DC voltage caused by these larger leakage inductances. If an IC power module with integrated extra brake transistor not connected to DC bus is used in the TSI the active snubber can utilize such transistor.
Fig. 1. Example of the passive networks alternative to basic LC lattice network.
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II. ALTERNATIVE PASSIVE NETWORKS AND EXAMPLE OF APPLICATIONS Symmetrical passive four-terminal networks have been applied in power electronics for the rejection of the electromagnetic conducted disturbance for many years. However, only recently, F.Z. Peng [1] published a paper in which he discussed the possibility of applying this specific type of four-terminal network – symmetrical LC lattice network (Fig.1a) – in order to create buck-boost inverters. It can be seen from the frequency characteristics [12] that the LC lattice network is characterized by low impact of output current on the output voltage and the influence of input voltage on the input current. Additionally, for a frequency higher than self-resonance, the LC lattice network reverses the phase but does not attenuate the output function. These are the main reasons to use the LC lattice network in the system of ”supply pump”, as in a typical dc-dc buck-boost converter. Interestingly, ZSI systems result from the fact that it is possible to replace the basic lattice network (Fig.1a) with other equivalent four terminal networks (Fig. 1b) – (Fig.1f) which only differ in topology and applied components. Moreover, the application of transformer with transformer ratio different than 1:1 allows additional possibilities for the output voltage manipulation. Similarly to classical Z-source the alternative passive networks (Fig.1b)-(Fig.1f) can be also applied in four-wire and multilevel inverters [12]. Fig. 2 shows example of application of the alternative network from Fig. 1d in four-wire inverter. Additionally, Fig. 3 shows examples of application of the alternative networks from Fig. 1b and Fig.1c in multilevel inverters supplied from one DC source (Fig. 3a) and from two different DC sources (Fig. 3b).
a)
b)
Fig 3. Examples of application of the alternative networks in the NPC inverters: a) from one DC source, b) from two different DC sources. a)
b)
Load
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
Load
NEW TYPE T-SOURCE INVERTER
III.
Fig. 4. Low-leakage-inductance-transformer-based TSI topologies. (a) Shoot-through mode (duration T0)
(b) Non shoot-through mode (duration T1)
D +
V IN
S2
S3
S’2
S’ 3
LC Filter
S1 uf
vDC
va, b,c
Fig. 5. Operation modes of the TSI in switching time period T=T0+TI -
S’1
Fig 2. Example of application of the alternative network in four-wire inverter.
Fig. 1e and Fig. 1f are shown in the Fig. 4a and Fig. 4b, while Fig. 5 shows the joint equivalent circuit of the both TSI topologies in two operational modes for transformer ratio n:1.
POWER QUALITY, ALTERNATIVE ENERGY AND DISTRIBUTED SYSTEMS
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 Z-source. As with qZ-source inverters [10, 11], the TSI topology features a common dc rail between the source and inverter, which is unlike traditional ZSI circuits. Moreover, use of a transformer with other than a 1:1 transformer ratio allows for a change of output voltage Z-source converters, as contrasted with the voltage resulting from the shoot-through index or the modulation index.
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Fig. 6. Decrease of TSI voltage amplitude VDC in non-shoot-through mode.
inductance causes a lower amplitude of output voltage, worse DC voltage VDC and increase in voltage stress on the transistors. As with a conventional ZSI, the TSI can handle shoot- Fig. 6 shows the decrease of vDC voltage as the effect of through states when both switches in the same phase leg are negative impact of increasing leakage inductance. The constant turned on. The T-network is used instead of the LC-network magnetizing inductance 600 μH was set during the simulation. for boosting the output voltage by inserting shoot through The input voltage was UIN = 60 V and VDC denotes DC voltage states in the PWM. amplitude in non shoot-through mode. The TSI governing equations can be developed for the Fig. 5. Regardless of the leakage inductance value the high using Kirchhof’s laws and voltage averaging [12]. The average performance of the T-inverter can be also achieved using voltage through the transformer inductances should be equal to additional snubber circuit (Fig.7). zero for the switching time period T. In order to reduce the overshoot of the devices caused by large leakage inductance an active snubber circuit as shown in VL = vL = [T0 ⋅ VC + T1 ⋅ (VIN − VC ) n] T = 0 (1) Fig. 7a is applied in proposed system. The seventh additional transistor (brake chopper) of used IC power module functions Both capacitor voltage VC and output voltage VOUT are as an active element of switching in proposed active snubber. This is because in particular IC power module this transistor is functions of the shoot-through coefficient D=T0/T. not connected with common DC terminals. The use of an VC VIN = T1 (T1 − n ⋅ T0 ) = (1 − D ) [1 − (n + 1) ⋅ D ] (2) active snubber ensures effective use of energy which might otherwise be lost due to voltage spikes. Moreover the active where D satisfies a condition D1 is smaller than for the conventional energy storage is connected to the main capacitor of the TSI. The reduction of voltage stress can be accomplished also by Z-source. This is the advantage of the TSI with n>1 in using a simple snubber with a passive clamped circuit shown in comparison with ZSI because the same output voltage can be Fig. 7b. This circuit is similar to clamped circuits used in obtained with achieved smaller time period of short-circuit bidirectional ZSI. Two capacitors CS1, CS2 and one diode DS1 transistor current. connected in series are connected right across dc rails of the Using (2) the amplitude VDC of voltage vDC in non-shootinverter bridge. The capacitor CS2 is non inductive. The through states can obtained from: clamped circuit is connected with middle point of the T-source. VDC = VC + (VC − VIN ) n = VIN [1 − (n + 1) ⋅ D ] (3) 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 In practice, the influence of leakage on inductance of the larger than 1 can compensate. transformer is very important. The lower the inductance, the b) closer it is to theoretical dependences. The performance of TSI a) depends on the precision of the transformer design. IV.
PRINCIPLES OF OPERATION
V. MINIMIZING THE IMPACT OF LEAKAGE INDUCTANCE If the transformer which is used to build the TSI has excessive leakage inductance, the efficiency of the T-inverter worsens. As indicated in Fig. 6, an increase of leakage
Fig 7. Application of active snubber (a) and passive clamped circuit (b)
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VI.
SIMULATIONS AND EXPERIMENTAL RESULTS
Simulations and experimental investigations of the TSI inverter with proposed alternative passive network from Fig.1f were performed using PSIM simulation software and an actual prototype, as presented in Fig.8. The prototype was built using digital signal processor ADSP21065L. Optical user interface was applied. System parameters are listed in Table I. An active snubber was used to utilize the energy from voltage spikes. Maximum Constant Boost Control (MCBC) strategy using modulation reference with 3-th harmonics addition was applied
both in simulations and in the experimental setup. The results of simulations and experiments are shown in Fig. 9 – Fig. 13. Fig. 9 shows the output TSI voltage vDC. The input diode voltage vD is shown in Fig. 10 and capacitor voltage VC in Fig. 11. The output currents shown in Fig. 12 are sinusoidal and proper operation of induction motor is achieved. Simulation results
Experimental results
Fig. 9. DC voltage vDC on the T-source output.
Fig. 10. Input diode voltage vD.
Figure 8. The prototype of proposed T-inverter TABLE I PARAMETERS AND VALUES OF T-INVERTER Fig. 11. T-inverter capacitor voltage VC. Parameters
Values used in simulation T-source inverter
DC Supply Voltage
100 V
T-source capacitance
48 μF
T-source magnetizing inductance
1mH
T-source leakage inductances
1.5 μH
Transformer Turns Ratio
1:1
Switching frequency
10 kHz
Fig. 12. Output current (induction motor stator current)
Load – induction motor Rated power
5.5 kW
Rated voltage
380 V
Power factor
0.86
Fig. 13. Output line to line voltage
POWER QUALITY, ALTERNATIVE ENERGY AND DISTRIBUTED SYSTEMS
VII.
CONCLUSIONS AND FUTURE WORK
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REFERENCES
Simulations and experimental results confirm the reliability of the new TSI operation. Experimental results for the TSI are in agreement with theoretical and simulated predictions. 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. In the configuration utilizing the TSI, the voltage is the same. Because of this factor, the configuration is efficient to use with different levels of NPC VSI. The advantage to this is that when there are decreases in voltage levels, there are fewer transistor failures.
[7]
ACKNOWLEDGMENT
[13]
The authors would like to thank all those who worked on the Project of Polish National Centre for Research and Development (NCBiR), 13491/IT1-C/U/08 and those who contributed to this publication.
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