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P. 125

Study of control strategies of 7

power electronics during faults

in microgrids

Jundi Jia*, Guangya Yang*, Arne Hejde Nielsen*, Peter Rønne-Hansen**
*Technical University of Denmark, Kgs. Lyngby, Denmark; **Siemens A/S, Aarhus, Denmark

1 Introduction

Conventionally, power systems have centralized generation units that are intercon-
nected to the users through transmission and distribution networks. This top-down
structure has deficiencies namely limited resilience to failure in transmission sys-
tems, high power losses, low penetration of renewable energy, etc. The integration
of distributed energy resources (DERs) including distributed generations (DGs),
energy storage, and electric vehicles into conventionally passive distribution networks
is playing an important role in modern power systems. One of the practices is the
use of microgrids, which can be regarded as low-voltage distribution networks con-
sisting of a group of DERs and controllable loads with the ability to either operate
in conjunction with the main grid (grid-connected mode) or operate autonomously
(islanded mode). As an active distribution network, microgrids not only help reduce
environmental pollution, but also behave like an aggregated load and can be designed
according to special needs namely high efficiency, lower line losses, continuous power
supply, etc. [1]. However, most of these DERs are interfaced to the microgrid through
power electronic converters. The disconnection of these converters under grid fault
conditions may pose threat to the reliability and stability of the microgrids especially
when they are operating under the islanded mode. Future requirements such as low
voltage ride through (LVRT) may need converters remain connected to microgrids and
provide short-circuit current under fault conditions. Unlike conventional synchronous
generators that are able to naturally provide high short-circuit current, the short-circuit
responses of converters are mainly governed by their control systems and the avail-
able short-circuit current contribution is also restricted by the overload capabilities
of their semiconductors. As the short-circuit analysis cannot be conducted using con-
ventional methods for a converter-dominated microgrid, the design of the converter
control systems becomes a crucial aspect for short-circuit analysis and for the design
of mircogrid protection systems.
As most forms of DGs (e.g., photovoltaics, Type-IV wind turbines, and batteries)
interface with the microgrids through voltage source inverters (VSIs), this chapter focuses
on the control systems of VSIs for both grid-connected and islanded modes. Under grid
unbalanced faults, the control systems of VSIs are sensitive to the negative-sequence
signals, which may propagate in the whole control system affecting control variables

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