Answer Posted / birendra singh mehra
Vector shift and ROCOF are basically the same. It is a
function that will disconnect the generator from the grid
in case the grid disappears (black out of the grid/ loss of
mains/ mains failure).
Vector shift and ROCOF are basically the same. It is a
function that will disconnect the generator from the grid
in case the grid disappears (black out of the grid/ loss of
mains/ mains failure).
This function is necessary because in case the grid
returns, it is likely that the generator will not be
running synchronously with it anymore. Thus this would
cause a short circuit.
This type of protection is called either Rate of change of
Frequency (ROCOF or df/dt) or vector shift/ vector surge.
The only difference is the way the set point is expressed.
ROCOF type is used in UK while the rest of Europe usually
uses vector shift.
ROCOF Relay
This relay will trip the generator in case the frequency
changes more than a certain set point within a certain
amount of time. Thus the adjustment is done in Hz per
second [Hz/s] (if the frequency changes more that XHz per
second the relay will trip). The frequency change appears
because the load situation on the generator will change
suddenly in case the grid is lost. This load change causes
a change in the engine speed which is equal to a frequency
change.
Function principle of the ROCOF Relay:
Practically the ROCOF relay will detect zero crossings of
the generator voltage. It measures the time between the
zero crossings and calculates a new frequency after each
zero crossing. In case the frequency changes too much since
the last zero crossing, the relay will trip.
Vector Shift Relay
This relay will trip the breaker in case it detects a phase
shift in the generator voltage. For example if a zero
crossing occurs some degrees earlier or later than
expected. Thus the adjustment is done in degrees.
The vector shift is again depending on the change of the
generator frequency. In case the grid is lost, this will
result in a sudden change of the generator frequency. A
change in frequency will move the zero crossing of the
generator voltage earlier (in case the generator load has
dropped) or later (in case the generator load has
increased). This shift is expressed in degrees. If this
vector shift is larger than a certain set point the relay
will trip.
Function principle of the Vector Shift Relay:
As you can see from above the Vector Shift relay will also
detect zero crossings of the generator voltage. It compares
the calculated zero crossing from the previous periods with
the latest zero crossing. If the difference is larger than
the set point, the relay trips.
Conclusion 1
As you can see from the function principles of both relays,
they are actually the same. Both relays rely on detections
of zero crossings of the generator voltage. The only
difference is the way how the trip level is expressed.
The utility companies require certain trip levels (just as
marine classification societies). In the UK they request
this in Hz per second, in the rest of Europe in degrees.
This goes so far that one of our customers in Switzerland
who used a T3500 ROCOF relay was forced to buy an
additional Vector Shift Relay in order to meet the utility
companies’ requirements (so they could adjust it in
degrees, not in Hz/s).
Conclusion 2
You can see from above that both relays require a sudden
load change of the generator in order to detect the loss of
the grid (mains failure). In case the generator would be
running parallel with the grid but only supplying as much
power as is used by the local consumers, a loss of grid
would not result in a load change on the generator. The
frequency would not change and thus neither the ROCOF nor
the Vector Shift Relay could detect this.
Therefore the utility companies require a minimum import or
a minimum export in case a generator is to run in parallel
with the grid.
Rate of change of frequency relay (ROCOF)
| Is This Answer Correct ? | 78 Yes | 9 No |
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