4.5.8.3
Functionality
The voltage vector shift protection function 78V, also known as vector surge or delta phi
function, measures continuously the duration of a voltage cycle. At the instance of
islanding, the duration of measured voltage cycle becomes shorter or longer than the
previous one, that is, the measured voltage cycle shifts with time. This shifting of voltage
is measured in terms of phase angle. 78V issues an instantaneous trip when the shift in
voltage vector exceeds the set value.
The function can be blocked with
BLOCK
input. Blocking resets timers and outputs.
4.5.8.4
Operation principle
The function can be enabled and disabled with the
Operation
setting. The corresponding
parameter values are “Enable” and “Disable”.
The operation of 78V can be described by using a module diagram. All the modules in the
diagram are explained in the next sections.
V_A_AB
TRIP
V_C_CA
V_B_BC
Vector
shift
detector
BLOCK
INT_BLKD
Pulse timer
GUID-7482410F-B8F0-4922-881C-F641139DBC5A V1 EN
Figure 334:
Functional module diagram
Vector shift detector
This module measures the duration of each cycle of the voltage signal phase. The duration
of the present cycle is compared to the previous cycle, considered as reference. When the
mains is lost, a sudden change is seen in the cycle length, if loading of the generator
changes suddenly and power mismatch or unbalance (generation vs. load) in the islanded
part of the network is large enough. The cycle shifts with time, that is, the frequency may
not change but a vector shift is seen in phase as shown in
.
This step is measured in degrees for each voltage signal defined by the
Phase supervision
setting. The
Phase supervision
setting determines which voltage is used for detecting
vector shift. The available
Phase supervision
options are “All” and “Pos sequence”. If the
calculated value of Δδ exceeds the set
Pickup value
setting for all the defined phases, the
module sends an enabling signal to start the Pulse timer.
The
Voltage selection
setting is used to select whether the available voltage signal is
phase-to-ground or phase-to-phase voltage.
1MAC059074-MB A
Section 4
Protection functions
615 series ANSI
667
Technical Manual
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