Page 161 - Hybrid-Renewable Energy Systems in Microgrids

Study of control strategies of power electronics during faults in microgrids 145

In contrast, as Fig. 7.17 illustrated, the relative phase relationship of the short-
circuit current varies significantly under unbalanced faults with different converter
control strategies. For example, the short-circuit current is phase C is the highest for
an A-g fault in Fig. 7.17H and J, which makes it impossible to identify the faulty phase
using a conventional way. Even though constant active power and balanced current
control strategies gives a relatively better result in terms of identifying faulty phases,
the short circuit current is still too slow to distinguish load current from short circuit
current for an over-current relay because they are typically set to operate at 2–10
times the full load current [14]. By comparing Figs. 7.18 and 7.19 to Fig. 7.17C and
D, the characteristics of the short-circuit current under unbalanced faults also differs
if the prefault conditions change. As a microgrid has dynamic topologies and usually
penetrates with a high level of renewable energy, whose generation is fluctuating, the
short circuit current will also change correspondingly in terms of the direction, ampli-
tude, and phase. Therefore, nondirectional over-current relay with a fixed setting is not
realistic for a microgrid environment.
Most of the DERs in microgrid are interfaced with the microgrid via power
converters, whose short-circuit response can be significantly different from con-
ventional synchronous generators. As a result, the reliability of other traditional
protection methods may also be adversely impacted by power electronic devices.
According to Ref. [15], directional over-current may fail to identify the correct fault
direction and distance relay may miscalculate the impedance from the relay to the
fault location. As the short circuit response of VSI varies with different control strat-
egies under unbalanced faults, the evaluation and the design of microgrid protec-
tion system should not ignore the impact of control strategies and converter current
limit. For microgrid application, it has been approved that traditional protection
methods based on local measurements may be inappropriate. Therefore, the future
trend for microgrid protection would turn to the use of adaptive relays, high-speed
communication, and other smart devices according to the topology of the microgrid
to guarantee a fast, selective, and reliable operation.

References

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