LCCT-Z-Source Inverters Marek Adamowicz1,2 1)
Abstract— Many renewable generation units (e.g. PVs, fuel cells, small wind turbines) require integrated DC-AC converters providing efficient high step-up power conversion. On the basis of recent developments in impedance source (Z-source) inverters the present paper proposes an integration of quasi-Z-source inverter (qZSI) with a built-in high frequency transformer. Two new type LCCT-Z-Source inverters are presented by author. Proposed inverters characterize continuous input current, improved relationship between boost coefficient and modulation index and improved EMI performance. Application of four element (Inductor - Capacitor - Capacitor - Transformer) LCCT impedance network provides higher voltage gain than obtained in quasi-Z-source inverter. The advantage of proposed topology over other recently developed trans Z-source inverters (TZSI) is that two built-in capacitors block DC currents in transformer windings and prevents core saturation. Simulation and experimental results are shown to verify the proposed topologies. quasi Z-source inverter, renewable sources, buck-boost inverter, single stage energy processing, integrated passives.
I.
INTRODUCTION
Z-source inverters (ZSIs) are one stage energy processing buck-boost inverters that contain unique passive input circuits (impedance networks) [1] and utilize the shoot-through of the inverter bridge to boost DC input voltage. The impedance network of ZSI serves as power storage and guarantees double filtration grade at the input of the inverter. The ZSI was expected to be suitable interface for renewable generation units as its LC Z-network allows the input DC voltage to be varied as desired and its voltage boost gain can be theoretically infinite. Unfortunately, the ZSI boost ratio and the modulation index are interdependent. Therefore in some low DC voltage renewable applications (e.g. PV, Fuel Cell, ultracapacitors) that require a high voltage gain a disadvantageously small modulation index has to be used. Moreover, basic ZSI topology [1] suffers from discontinuous input current characterizing large di/dt and requires additional input filter which increases the element count and costs. Several improvements of the basic ZSI topology have been recently obtained [2] - [10]. These improvements in different topologies have been obtained in four following ways (examples in brackets): •
2)
Department of Ship Automation Gdynia Maritime University Gdynia, POLAND
[email protected]
rearrangement of the basic LC Z-network structure resulting in features which include continuous input
National Centre for R&D Programme 'LIDER' Faculty of Electrical and Control Engineering Gdansk University of Technology, POLAND www.ely.pg.gd.pl/lider
current and improvement in electromagnetic interference (EMI) performance (embedded Z-source inverter (EZSI) [2] and quasi-Z-source inverter (qZSI) [3]), •
replacement of basic LC Z-network with equivalent three port transformer-capacitor network - resulting in features which include reduction in element count and improved relationship between boost factor B (or voltage gain G=MB) and modulation index M when the transformer voltage-turns ratio n>1 is applied (T-source inverter (TSI)[3], [4] and trans-Z-source inverter (TZSI) and transquasi-Z-source inverter (TqZSI) [5]),
•
series connection of several LC qZ-networks - resulting in features which include continuous input current, improved EMI performance and improved dependence between voltage gain and modulation index MB-M (extended boost ZSI (EBZSI) [7] and cascaded quasi Z-source inverters (CqZSI) [8], [9])
•
replacement of two inductors of basic LC Z-network with switched inductor (SL) circuits resulting in improved dependence between voltage gain and modulation index MB-M (switched inductor ZSI (SIZSI) [10]).
Table I summarizes the improvements listed above, obtained in recently developed impedance source inverters. TABLE I.
IMPROVEMENTS OBTAINED IN RECENTLY DEVELOPED IMPEDANCE SOURCE INVERTERS EZSI [2], qZSI [3]
TSI, TZSI, TqZSI [3][5]
EBZSI [7], CqZSI [8]-[9]
SIZSI [10]
Continuous input current
Yes
No
Yes
No
Improvement in EMI performance
Yes
No
Yes
No
Number of elements
unchanged
reduced
significantly increased
significantly increased
Improvement in relationship MB-M
No
Yes
Yes
Yes
The improvement in EMI performance obtained in qZSI, relies on an application of a common DC rail between the source and inverter bridge. The input DC source of qZSI, one capacitor and the inverter bridge can share therefore the same ground. Moreover, the qZ-network inductor connected to the
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inverter bridge, is paralleled by a capacitor during non-shootthrough-state that provides the protection of the transistors against the undesirable voltage spikes. Similar EMI improvements are obtained in EBZSI and CqZSI. The aim of the work presented in this paper is the development of new type impedance zource inverter combining the advantages of qZSI listed in Table I with a desired advantage of improved MB-M relationship and without significant increase in element count. II.
Z-SOURCE AND QUASI Z-SOURCE INVERTERS
The ZSI topology features a DC link consisting of a symmetrical lattice network with two inductors and two capacitors. Two topologies of of 3-phase ZSI and qZSI are shown in Fig. 1a and Fig. 1b. The impedance source inverters can assume nine states that is one more than in conventional voltage source inverter (VSI). Additional ‘ninth’ state is the third ‘null’ state occurring when the load is being shorted simultaneously by lower and upper group of transistors. This state is defined as a ”shoot-through” state and may be generated in seven different ways.
a)
b)
c)
d)
was mentioned earlier the disadvantage of ZSI and qZSI is that two control variables: shoot-through interval Tshoot during switching period T and modulation index M are interdependent. During boost operation both ZSI and qZSI utilize shoot-through duty ratio D = (Tshoot/T)>0. Longer shootthrough period provides higher DC link voltage but also decreases a period of active PWM states. That imposes limitation on boosting of output voltage as the increase in boost factor B results in lower modulation index M. The voltage gain G mentioned in the introduction of both ZSI and qZSI can be written as [11]:
G=
Vout = M ⋅B VDC / 2
(1)
where Vout denotes amplitude of AC output line-to-line voltage. Using the maximum constant boost control (MCBC) [11] the boost factor B can be defined as:
B=
1 1 = 1 − 2D 3M − 1
(2)
III. NOVEL LCCT-QZSI AND LTTC-ZSI This chapter discusses the integration of LC qZ-network shown in Fig. 2a with a high frequency transformer (Fig. 2b). The proposed technique combines the pair of inductor L2 and capacitor C1 into unique two port network of high frequency (HF) transformer (a pair of coupled inductors LT1 and LT2) and a blocking capacitor CT. The HF transformer can be represented by an equivalent circuit with three uncoupled inductors. The T-equivalent for transformer from Fig. 2b is shown in Fig. 2c.
a)
b)
Fig. 1. Circuit schematics of ZSI (a) and qZSI (b) and waveforms of input voltage VDC, DC link inverter voltage vi, input current iIN, diode current iD and inductors currents iL1 and iL2 of ZSI (c) and qZSI (d) for shoot-through duty ratio D=0.19.
During the shoot-through state of ZSI no energy flows from the source to the load. Energy within the Z-source network is re-orientated with electrostatic energy from the DC electrolytic capacitors transferred to magnetic energy stored in the Zsource inductors, before the stored energy is released for output voltage boosting during the next non shoot-through interval [2]. Both topologies from Fig. 1 characterize DC current of two inductors L1 and L2. The qZSI, when compared to the ZSI, features lower DC voltage on capacitor C2. Both ZSI and qZSI can be controlled using the same control methods [6], [7]. As it
c) Fig. 2. Proposed technique of replacing LC qZ-network (a) with novel LCCT Z-network (b) and a T-equivalent of transformer (c).
The novel topology from Fig. 2b is named LCCT-qZnetwork. From Fig. 2b and Fig. 2c it can be written [13]:
L A = LM = k LT 1 ⋅ LT 2
(3)
LB = LT 2 − LM
(4)
LC = LT 1 − LM
(5)
where LM denotes mutual inductance of the pair of windings and 0
