parallel operation of one-cycle controlled three …medinid/energy-gordon/smedley/pocc...parallel...

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UCI Power Electronics Lab

Parallel Operation of One-Cycle Controlled

Three-Phase Inverters

Yang Chen and Keyue Smedley

Power Electronics LaboratoryUniversity of California, Irvine, CA 92697, USA

E-mail: [email protected]

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Outline

• Introduction;

• Review of three-phase OCC gird-connected inverters;

• Concept for OCC inverters in parallel;

• Implementation circuit of the controller;

• Experimental verification;

• Conclusions.

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Introduction

Three-phase grid-connected inverters:

• Converting photovoltaic or fuel cell DC to AC;

• to the present utility power grids;

• with high power quality, high efficiency, and low switchinglosses.

Advantages of paralleled inverters over a single one with a large power rating:

• Higher expandability due to modular configuration;

• Higher reliability due to redundancy design;

• Cost-effectiveness.

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Reported methods

• Current Sharing: Droop method; Average current sharing method; Master-Slave (MS) method )

Dedicative MSAutomatic MS (Democratic)

• Circulating Current limiting Inductors or current-balancers; Instantaneous current deviation cancellation; Control the duty ratios of zero vectors in discontinuous SVM.

Two major concerns related to parallel inverters in a three-phase system:

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Proposed method

• Good current balance capability;

• Limited circulating current;

• Low switching losses;

• Minimized communication burden;

• Simple circuitry;

• Modular design and flexible installation.

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OCC Grid-Connected Inverter

Six-switch bridge topology

Switching-cycle average model 5

Six regions in each line cycle

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€

23 − 1

3 − 13

− 13

23 − 1

3

− 13 − 1

323

⋅

dapdbpdcp

=1E⋅

vavbvc

Input - output relationship:

Since the matrix is singular, non-unique solutions exist.

€

dap = K1+vaE

dbp = K1+vbE

dcp = K1+vcE

€

2 /3 −1/3−1/3 2 /3

⋅dapdcp

=

1Evavc

Two possible solutions:

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OCC with Bipolar Operation Mode

OCC controller with bipolar mode

• All three-phase currents are used togenerate duty ratios ;

• It doesn’t require the condition ofia+ib+ic=0;

• All the switches are operated inswitching frequency, the switchinglosses are high.

Control equations :

−=⋅−

−=⋅−

−=⋅−

)1(

)1(

)1(

1

1

1

K

dViRKv

K

dViRKv

K

dViRKv

cpmcsc

bpmbsb

apmasa

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• Only two phase currents areused for generating twoindependent duty ratios in eachregion ;

• ia+ib+ic=0 is always satisfied inan individual inverter;

• The phase current with themaximum absolute value is notswitched at high frequency. Theswitching losses are lower.

OCC with Vector Operation Mode

⋅−⋅

⋅−⋅=

⋅

⋅

cpmcb

apmab

c

as dVvK

dVvK

i

iR

21

12

Control equation in Region I:

OCC controller (vector mode)6

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OCC Inverters in Parallel Operation

Parallel operation of two inverters

• Current Sharing and Power Distribution

8

js

m

j vREV

Ki

−=

23 rmss

m

total VRE

VK

P−

=

Phase current( j = a, b, c )

Output power

• ij is only affected by Rs, provided that Vm, E and vj are equal for each module;

• Output power could be unevenly distributed to each module which has a different Rs.

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• Current crosslink is unavoidable;

• Circulating currents always go through the phases which are not switched at switching frequency.

Equivalent circuit of circulating current loop

• Circulating current loop behaves as a first-order system;

• The circulating iz has an initial value anddecays during that region with a very largetime constant.

• Circulating Current Control

Equivalent circuit of two paralleled inverters in Region I

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• Theoretical analysis of iz:

2

12

2 cz R

EEi

−=

satisfied by current sharing

• DC bus voltage control can hardly detect iz,

• Sharing Vm can’t limit the circulating current.

τ/tzoz eIi −=

42

21

cc

bb

RRLL

++

=τ

•Time constant is very large due to a small value of Rc2;• If Izo is reduced at starting points, iz will be decaying in that region. Thus the circulating current is limited;• The idea is: to use bipolar mode to limit iz; and use vector mode to reduce switching losses.

From equivalent circulating loop:

2121

2121

,

,

cpcpapap

ccaa

dddd

iiii

==

>>=<<>>=<<

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Proposed OCC Controller For Parallel Operation

• Three current sensors are used for detecting circulating currents;

• When iz is over pre-set limit, controller switches to bipolar mode; otherwise, it remains in vector mode operation;

• Only simple add-on is required to the original control core. It remains simple.

Diagram of OCC controller

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Experimental Results

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va

ia1

ia2

va

ia1,ia2(overlapped)

iz1

iz2

iz1

iz2

Without proposed control method With proposed control method

Circulating currentswith a gain of 5

(5A/div)

Phase A voltage(140V/div)

Input currents(5A/div)

Two OCCinverters witheven powerdistribution.

The circulating current is reducedto 25%.

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Alternation between vector and bipolar operation

• The controller switches to bipolar mode when iz is greater than the pre-set limit;

• When iz decays to below this limit, the system will be switched back to the vector operation;

• Pre-set limits are chosen so that the controller is in vector mode for most of the time; bipolar mode only happens for limiting circulating currents.

Vector

Bipolar Sap (2V/div)

ia1 (5A/div)

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va

ia1

ia2

Phase A voltage(140V/div)

Input currents(5A/div)

• 2 sets of OCC inverters with uneven power distribution;

• By letting Rs2=1.5Rs1 , the output power is 1.5kW for Inverter1 and 1kW for Inverter2

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Conclusions

A new control method is proposed for parallel operation of OCC grid-connected inverters;

Current sharing is achieved by sharing a DC signal among paralleled modules, which is very flexible for installation;

The circulating current is limited and the system preserves the advantages of low switching losses;

There are only a small add-on to the original OCC controller; so the circuit remains simple and low-cost.

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parallel operation of one-cycle controlled three …medinid/energy-gordon/smedley/pocc...parallel...

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