Event under the auspices of the Ministry of Economy of Slovak Republic

Fault identification at microgrid-connected converter and its ride-through capability

*Viktor Valouch
Department of Electrical Power Engineering, Faculty of Elect

Petr Šimek
Institute of Thermomechanics, Academy of Sciences of the Czech Republic

Jiří Škramlík
Institute of Thermomechanics, Academy of Sciences of the Czech Republic

Martin Čerňan
Department of Electrical Power Engineering, Faculty of Electical Engineering, CTU in Prague


     Last modified: 2017-06-17

Abstract
I. THE PURPOSE OF THE RESEARCH

Fault tolerant converters have been intensively studied in several last years. Among main problems analyzed and tested belong fault diagnosis and reconfiguration of converter power circuitry and of its control strategy after a fault.
In the contribution, a model of the three-phase voltage converter for simulation of its modes of operation in standard as well as fault states is presented, which makes their analysis possible.
The contribution presents voltage and current responses at dc as well ac converter side for typical cases of faults. Based on analysis of these responses an algorithm of fault localization and its place has been developed. The strategy is based on evaluation of a converter ac current trajectory.
Further, rules for reconfiguration of the converter topology are presented. Transient responses in the interval between an instant of a fault and the HW and SW reconfiguration are discussed as well.

II. THE PRINCIPAL RESULTS AND ACCOMPLISHMENTS AND THEIR SIGNIFICANCE

A. Converter model
Based on the converter scheme in the full B6 and reduced B4 configuration, its general mathematical model will be presented. In case of a fault, respective load phase is connected to midpoint 0 of two condensers at dc side of the B6 converter.
The converter generates a PWM (Pulse Width Modulated) voltage, where switching functions Sj+, Sj-, j = a, b, c of individual elements determine whether a firing pulse is applied to the respective switching element.
We can introduced generalized switching functions that determine to which dc bar a load phase j (j = a, b, c) is connected. The generalized switching functions consider also directions of phase load currents and the fact whether the switching element in respective converter phase is faulted or not.
The model can be extended by including the cases of combined fault of a switching element and its free-wheeling diode or even of the whole phase damaged. Next, we can describe the converter circuit by differential equations and simulate its behaviour either in regular operation or in faulty modes.

B. Converter voltage and current responses
Examples of responses of the load phase currents ia, ib, ic for all three possible faults: a) fault of the switching element Sa+ only; b) fault of the upper leg Sa+, Da+; c) the whole phase a is out of the order, will be presented.

C. Fault identification
If we want to evaluate a fault, it is recommended to observe the vector of filtrated phase currents. If only the switching element Sa+ is damaged, the current vector draws only the left side part of the circle (ia < 0) in a steady state, because ia > 0 can not flow through the element Sa+. But, if also the diode Da+ or even the second switching element Sa- is in fault, the current vector will move along a line perpendicular to the axis of the phase a. In case of a fault in the phase b or c, the current vector trajectory will look similarly, but will be rotated by the angle ±120˚.

D. Converter reconfiguration and operation in regime B4
If a fault occurs, it is necessary to change the converter topology to the B4 structure where only two healthy phases are active. The phase under the fault should be connected to the midpoint 0 of the pair of two dc condensers. In a time interval between the occurrence of the fault and its identification a transient process appears. Only after that a new steady state in the structure B4 is reached.
It will be demostated that the current transients in the B6 and B4 structure can be, under a proper converter control strategy, almost identical, but the power fluctuation of the frequency of 100 Hz is more pronounced in the B4 configuration than that in the full B6 structure.

III. THE MAJOR CONCLUSIONS
The presented model of the three-phase converter can be used to simulate steady-states and also transients at dc and ac side of the grid-connected converter. The model allows simulate also failure states. Based on simulation, an algorithm was designed to identify and localize a fault in some phase of the three-phase converter. The algorithm was experimentally verified.

 

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