§ Ensure rapid interruption minimize the
impact fault currents.
However, system voltage increases, becomes
increasingly difficult for purely mechanical circuit
breakers interrupt current due the lack
of periodic zero crossings.1.32
MVDC technologies and operational considerations
For MVDC power grids with even shorter fault time
constants, this requirement can tightened a
few milliseconds.1. This method has
been experimentally validated.3 Fault current interruption
MVDC power system protection can broadly
classified into circuit breaker-based protection
and breaker-less protection.
MVDC circuit breakers must address several
technical requirements interrupt MVDC current
effectively:
§ Create artificial current zero points facilitate
current interruption.3.2 Fault current detection
Rapid fault current detection and fault location
identification are significant challenge MVDC
systems. Such
techniques remain under active research and
are not yet commercial use.
For voltages under several kilovolts, mechanical
circuit breakers such moulded case circuit
breakers air circuit breakers, which are similar to
AC circuit breakers can used for current
interruption without additional components.
§ Dissipate energy stored the power system to
prevent damage.
§ Rate-of-change-of-current protection: Faults
are identified setting threshold for the
current’s rate change. The main approaches detection and
identification include:
§ Protection from the network.
§ Signal processing-based methods: Techniques
such wavelet transforms short-term
Fourier transforms can identify anomalies in
system operation that indicate faults.
4. For MVDC
systems linking grids, the simplest
protection method limit the grid’s
contribution the fault current using AC
circuit breakers.
§ Handle the overvoltage that occurs during
switching.
. Directional protection limited
to relatively simple MVDC link applications.1.
4. Thus, MVDC circuit
breakers must employ different mechanisms
to interrupt fault currents. multi-terminal
MVDC systems, adding inductance can modify
the current profile’s rate change across the
grid, enhancing fault selectivity. These mechanisms
often involve creating artificial current zero points
or utilizing power electronics manage the
interruption process. Other measures,
such fuses and explosive-operated switches,
have also been proposed for MVDC systems; but,
broadly speaking, these have drawbacks including
single-use operation and difficulty setting reliable
operating points.
4.
Because the rapid rise fault currents in
MVDC systems, implementing differential
protection requires highly accurate, fast current
sensors and reliable communication channels. But the main drawback of
this approach the relatively long operation
time the circuit breakers; MVAC circuit
breakers typically take 1-3 cycles (16-60 ms)
to isolate faulted section, which too slow
to adequately protect MVDC power system
components.
§ Differential and directional protection: These
approaches are based similar techniques in
MVAC systems.1 MVDC circuit breakers
As introduced earlier, periodic current zero-
crossing highly beneficial for interrupting
currents, mechanical circuit breakers can utilize
these natural zero points effectively interrupt
fault currents
