comparison of practical fault ride-through capability for mv - PSCC

Reduction in fault clearing time can be reached by reducing
time grading of two successive relays or applying
differential protection.
The nowadays undervoltage protection settings in the
Netherlands are such that at voltage levels below 0.8
p.u. all DG-units are disconnected. Comparing the current
settings with the CCT-curves displayed in Figure
13, it can be concluded that the settings of the undervoltage
protection can be altered without instability of
the DG-units.
Based on the data in Figure 12 with the current settings,
the frequency of disconnection is 0.37 times per
annum or 9.3 % of all the voltage dips (calculated by
summing up all the frequencies of the voltage dips below
0.8 p.u.). By applying the developed CCT-curves in
the undervoltage protection the frequency of disconnection
can be reduced to 0.02 times per annum or 0.5 % of
all the voltage dips. This figure holds for the wind turbine,
while for the CHP-plant there are no disconnections
at all. Thus, the availability of the DG-units can be
increased significantly by adjusting the under-voltage
protection in accordance with the CCT-curves.
The fault ride-through capabilities of the wind turbine
can be improved by adding a crowbar to the DFIG. The
crowbar separates the rotor side converter from the rotor
circuit. In [10] the operation of the crowbar is simulated
and it is demonstrated that fault ride through during and
after a deep voltage dip (0 V remains, 150 ms) is possible.
6 CONCLUSION
In this paper the ride-through behavior of a CHPplant
and a DFIG-based wind turbine is studied. Based
on the data of an existing power system including reliability
data a voltage dip profile at the generator terminals
is established. This profile is combined with the
fault clearing times of the test system which have led to
a plot where the depth and duration of the voltage dips
are indicated. Using the single machine infinite network
scheme the CCT-curves of the DG-units are derived.
From stability point of view these curves show that it is
not necessary to disconnect the DG-units at a voltage
level of 0.8 p.u. (the present setting of DG undervoltage
protection in the Netherlands), but at lower levels, which
are dependent on the critical clearing times. These CCTcurves
can be implemented in the undervoltage relay
and prevent switching off during a voltage dip caused by
a fault in a HV- or neighboring MV-grid. In some cases
disturbances in the 20 kV grid lead to unstable operation.
This can be prevented by adjusting the fault clearing
time or applying differential protection. For DFIG
wind turbines transient stability can be significantly
improved by introduction of crowbar protection and
switching between different control strategies for normal
operation and fault conditions.
The method presented in this paper can be followed
in all locations of the grid and prevent ambitious disconnection
of DG.
REFERENCES
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