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Concept, N/W Topology and Working of Z-Source Inverter

Prepared by:Kishan M. Darji

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Concept of a Z-Source InverterA Z-Source Inverter(ZSI) is a type of power

inverter that converts direct current into alternating current. It works as a buck-boost inverter without making use of DC-DC converter bridge due to its unique circuit topology.

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Study PurposeNowadays the use of non-conventional energy

sources is emerging.In electric vehicles and in photo-voltaic cell

panels the output dc voltage is converted into ac voltage. The level of this voltage is much lower than required in the actual practices. That is why a Z-source is provided in the inverter circuit to boost the level of output ac voltage.

So, ZSI is being employed in almost all modern renewable energy sourced electricity generating stations.

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Basic OperationNormally three phase inverters have 8 vector

states (6 active states and 2 zero states).But ZSI, along with these 8 vector states, has

additional 7 shoot through states.In this state the energy is stored in the

impedance network and this stored energy is transferred to the load when the inverter is in one of its active states, thus providing the boost operation.

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Network Topology

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As shown in the figure, the diode Ds and the impedance network is the main difference in the power circuit of conventional VSI and ZSI.

The impedance network is symmetrical which consists of two identical pair of inductors and capacitors.

The Z-source allows the VSI to be operated in the shoot through state in which two switching devices of a same inverter leg is turned on simultaneously to effect a short-circuit to the dc-link.

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The capacitors may be charged to higher voltages than the input source voltage during the shoot through states which necessitates the provision of diode Ds in order to prevent the discharging of them into input sources.

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Working of ZSIThe ZSI operates in three different modes as

explained below:Mode 1:

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The mode 1 is known as an active mode.In this mode the inverter is operating in one

of its 6 active states.The Z-source circuit always forces diodes to

conduct and carry the current difference between the inductor current and inverter dc current as shown in Fig. (a).

Note that both inductors have an identical current value because of the circuit symmetry.

This unique feature widens the line current conducting intervals, thus reducing harmonic current.

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Mode 2:

In this mode the inverter bridge is operating in one of its two zero vectors by switching all upper or lower 3 switches simultaneously.

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Thereby acting as an open circuit .The diodes conduct and carry currents. Fig.

(b) shows the circuit for this mode. Again, under this mode, the two diodes have to conduct and carry the inductor current, which contributes to the line current’s harmonic reduction.

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Mode 3:

In this mode the inverter bridge is operating in one of its 7 shoot through states.

During this mode, both diodes are off, separating dc-link from the ac line.

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The line current flows to the capacitor (Ca). Fig. (c) shows the resultant circuit. This is the shoot-through mode to be used in every switching cycle during the traditional zero vector period generated by the PWM control.

Depending on how much a voltage boost is needed, the shoot-through interval (T0) or its duty cycle (T0/T) is determined. It can be seen that the shoot-through interval is only a fraction of the switching cycle; therefore it needs a relatively small capacitor to suppress voltage.

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These shoot-through zero states are forbidden in the traditional V-source inverter, because it would cause a short circuit.

There are seven different shoot-through states: three shoot-through states via any one phase leg, three shoot-through states from combinations of any two phase legs, and one shoot-through state by all the 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 and produce zero voltage across the load, thus preserving the same PWM properties and voltage waveforms to the load.

The only difference is the shoot-through zero states boost the dc capacitor voltage, whereas the traditional zero states do not.

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For the proposed system, the three-phase inverter bridge is controlled the same way as the traditional 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 region PWM.

When a higher output voltage is required or when the line voltage is experiencing 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 voltage one gets. By controlling the shoot-through zero state interval, a desired dc voltage can be maintained.

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The circuit shown below is an equivalent switching circuit which comprises of all the three operational modes of Z-Source Inverter.

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Advantages of ZSI over VSI and CSIThe source can be either a voltage source or a

current source. The dc source of a ZSI can either be a battery, a diode, rectifier or a thyristor converter, a fuel cell stack or a combination of these.

The main circuit of ZSI can either be traditional VSI or the traditional CSI.

It works as a buck-boost inverter.The load of a ZSI can either be inductive or

capacitive or another Z-Source network.It can produce any desired output ac voltage,

even greater than the line voltage.

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It reduces the in-rush and harmonic currents.VSI and CSI are vulnerable to the EMI noise

and the devices get damaged in either open or short circuit conditions.

The combined system of DC-DC boost converter and the inverter has lower reliability.

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ConclusionDue to its unique network topology, ZSI can

provide output ac voltages higher or lower than the input source voltage without using any additional converter.

Owing to its higher reliability and ease of its circuitry, ZSI’s are used in various fields, like renewable energy sources, electric vehicles and motor drives.

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ReferencesFang Z. Peng, “Z-source inverter”, in IEEE

Transactions on Industry Applications, vol. 39, no. 2, March/April 2003, pp. 504-510.

http://ieeexplore.ieee.org/xpl/articleDetails.jsp?tp=&arnumber=5447792&queryText%3Dsteady+state+analysis+and+designing+impedance

http://en.wikipedia.org/wiki/Z-source_inverter'Power Electronics' by M Rashid

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