Paper presentation: Z-SOURCE INVERTER FOR ADJUSTABLE SPEED DRIVES

 

Z-SOURCE INVERTER

FOR

ADJUSTABLE SPEED DRIVES

(A Novel ASD System)

 


ABSTRACT
This paper presents a Z-source inverter system and control for adjustable speed drives (ASD). The Z-source inverter employs a unique LC network to couple the inverter main circuit to the diode front end. By controlling the shoot-through duty cycle, the Z-source can produce any desired output ac voltage, even greater than the line voltage. As results, the new Z-source inverter system provides ride-through capability under voltage sags, reduces line harmonics, and extends output voltage range. Simulation results will be presented to demonstrate the new features.




1. INTRODUCTION
The Traditional ASD system is based on a voltage-source inverter (V-source inverter), consisting of a diode rectifier front end, dc link capacitor, and inverter bridge as shown in Fig. 1. Because of the V-source inverter, the ASD system suffers the following common limitations and problems.
• Obtainable output voltage is quite limited below the input line voltage. The V-source inverter is a buck (step-down) inverter. For example, Fig. 1 illustrates
voltages of a 3-phase 230 V drive system, where the diode rectifier powered by the 230–V ac line produces about 310–V dc, under which the inverter can only produce a maximum 190–V ac in the linear modulation range. For a 230–V motor, the low obtainable output voltage significantly limits output power that is proportional to the square of the voltage. This is a very undesirable situation for many applications where the motor and drive system has to be oversized.
• Voltage sags can interrupt an ASD, thus shutting down critical loads and processes. Over 90% of power quality related problems are from momentary (typically 0.1–2 s)
voltage sags of 10–50% below nominal. The dc capacitor in an ASD is a relatively small energy storage element, which cannot hold dc voltage above the operable level under such voltage sags. Lack of ride-through capacity is a serious problem for sensitive loads driven by ASDs.
• Inrush and harmonic current from the diode rectifier can pollute the line.
A recently developed new inverter called Z-source inverter has a niche for ASD systems to overcome the above problems. A Z-source inverter-based ASD system can
• produce any desired output ac voltage, even greater than the line voltage;
• provide ride-through during voltage sags without any additional circuits;
• reduce in-rush and harmonic current.
This paper presents the basic idea of an ASD system using the Z-source inverter, its main circuit configuration, an equivalent circuit, and control. Simulation results are included to demonstrate the idea and features of the new ASD system.

2. Z-SOURCE ASD SYSTEM
Fig. 2 shows the main circuit configuration of the proposed Z-source inverter ASD system. Similar to that of the traditional ASD system, the Z-source ASD system’s main circuit consists of three parts: a diode rectifier, dc-link circuit—Z-source network,  and an inverter bridge. The only difference is the dc link circuit (or Z-source network: C¬1 and C2 and L¬1 and L2) and small input capacitors (Ca, Cb, and Cc) connected to the diode rectifier. Since the Z-source inverter bridge can boost the dc capacitor (C¬1 and C2 ) voltage to any value that is above the average dc value of the rectifier, a desired output voltage is always obtainable regardless the line voltage. Using the 230 V ASD system as an example, the dc capacitor voltage is boosted to 350 V in order to produce 230–V ac output as shown in Fig. 2. Theoretically, the dc capacitor voltage can be boosted to any value above the inherent average dc voltage (310 V for 230V ac) of the rectifier, by using shoot-through zero switching states when a higher output voltage is needed or during voltage sags. The maximum dc capacitor voltage will be limited by the device voltage rating in practical use, however.

3. EQUIVALENT CIRCUIT:

        OPERATING PRINCIPLE AND CONTROL

In the proposed ASD system in Fig. 2, a diode rectifier bridge with input capacitors (Ca, Cb, and Cc) serves as the dc source feeding the Z-source network. The input capacitors are used to suppress voltage surge that may occur due to the line inductance during diode commutation, thus requiring a small value of capacitance. At any instant, only two phases (of the three-phase diode bridge) that have the largest potential difference (i.e., the two phases cross the one of the input capacitors that has the highest voltage) may conduct, carrying current from the ac side to the dc side. Therefore, viewed from the Z-source network the diode bridge can be modeled as a dc source (i.e., one of the input capacitors) in series with two diodes as shown in Fig. 3.

The two diodes (D pa, b or c and D na, c or a) conduct as a pair with the capacitor (C a,b,or c). Note the suffix combinations that indicate diodes D pa and Dnb form a pair with capacitor Ca when voltage cross capacitor Ca (i.e., the voltage cross phases “a” and “b”) is the highest; and with D pa and Dnc with Cb when voltage cross capacitor Cb (i.e., the voltage cross phases “b” and “c”) is the highest; Dpc and Dna and Cc when voltage cross capacitor Cc (i.e., the voltage cross phases “c” and “a”) is the highest, respectively. Further, the two diodes conduct in a pair and in series acting as one when viewed from the Z-source network. Therefore, the proposed Z-source ASD system is reduced to a Z-source inverter.
The traditional three-phase V-source inverter has six active states in which the dc voltage is impressed across the load and two zero states in which the load terminals are shorted through either the lower or upper three devices, respectively. However, the three-phase Z-source inverter bridge has additional zero states when the load terminals are shorted through both the upper and lower devices of any one phase leg (i.e., both devices are gated on), any two phase legs, or all three phase legs. These shoot-through zero states are forbidden in the traditional V-source inverter, because it would cause a shoot-through. There are seven different shoot-through states: shoot-through via any one phase leg, combinations of any two phase legs, and all three phase legs. The shoot-through zero states boost dc capacitor voltage while producing no voltage to the load. It should be emphasized that both the shoot-through zero states and the two traditional zero states short the load terminals, produce zero voltage across the load, and thus preserve the same PWM properties and voltage waveforms to the load. The only difference is that shoot-through zero states boost the dc capacitor voltage, whereas the traditional zero states do not. For the proposed ASD system, the three-phase inverter bridge is controlled the same way as the traditional pulse width modulation (PWM) inverter without shoot-through when a desired output voltage is less than 190 V ac, which is the maximum voltage obtainable from 230 V line using the linear PWM. The diode rectifier functions such as the traditional one producing about 310Vacross the dc capacitors (C1 and C2 ). When a higher output voltage is required or when the line voltage sags, the shoot-through zero states are employed to boost the dc capacitor voltage. The longer time the shoot-through zero states are used, the higher the voltage one gets. By controlling the shoot-through zero state interval, a desired dc voltage can be maintained.

4. SIMULATION VERIFICATION OF THE ASD SYSTEM
Simulations have been carried out to confirm the operating principle of the new ASD system. In order to show clearly the output voltage obtained from the inverter, an LC filter with 1 kHz cutoff frequency is placed in-between the inverter bridge and the motor. The simulation parameters are as follows:
1) three-phase line voltage: 230–V, line impedance: 3%;
2) Load: three-phase 230–V 20 KW induction motor;

3) Input capacitors ( Ca,Cb , andCc ): 10 micro farads ;
4) Z-source network: L1 =L2=160 micro Henry, C1=C2= 1000 micro farads;
5) Switching frequency: 10 kHz.
Figs. 4 and 5 show simulation waveforms under the nominal line voltage of 230 Vac. The output inverter voltage is just like the traditional PWM waveform with a modulation index of 1.0. After the 1 kHz LC filter, the voltage becomes sinusoidal, indicating a 230 V rms value, which is not obtainable by the traditional ASD system.

Fig. 5 shows the inductor current and dc capacitor voltage, which has been boosted to 343 V. The maximum dc voltage was boosted to 376 V and should be limited below the device voltage rating, which can be 450 V for a 600 V IPM. The boost factor was 1.21. Also it is noted that the line current contains fewer harmonics because of the Z-source network and input capacitors.

Figs. 6 and 7 show simulation waveforms during 50% voltage sag (the line voltage drops to 115 Vac). The waveforms clearly demonstrate that the dc capacitor voltage can be boosted and maintained to a desired level, which in this case is above 300 V. The boost factor was 2.8 and the modulation index was 0.82.

5. CONCLUSIONS
This paper has presented a new ASD system based on the Z-source inverter. The Z-source inverter ASD system has several unique features that are very desirable for many ASD applications:
• produce any desired output ac voltage, even greater than the line voltage;

• provide ride-through during voltage sags without any additional circuits;
• reduce in-rush and harmonic current.

6. BIBLIOGRAPHY
      1. B.K.Bose."Adjustable speed ac drive systems."

       2. Y. Kim and S. Sul, “A novel ride-through system for adjustable-speed
drives using common-mode voltage,”