CHAPTER 6: SETPOINTS
S4 CONTROLS
350 FEEDER PROTECTION SYSTEM – INSTRUCTION MANUAL
6–145
Synchrocheck (25)
The Synchrocheck element is used for monitoring the connection of two parts of the circuit
by the close of a breaker. This element verifies that voltages (VL and VB) at both sides of the
breaker are within the magnitude, angle and frequency limits set by the user. VL and VB
are the Line and Busdbar voltage values measured by the device.
IMPORTANT:
Calculated voltages are not suitable for synchrocheck. Under the following conditions,
the Synchrocheck function is disabled:
•
Wye is selected under “VT Connection” and a Delta voltage input (V
ab
VT, V
bc
VT,
V
ca
VT) is selected under “Auxiliary VT input”
•
Delta is selected under “VT Connection” and a Wye voltage input (V
an
VT, V
bn
VT,
V
cn
VT) is selected under “Auxiliary VT input”
IMPORTANT:
Due to the discrete computation of the analog incoming data measurements, fixed at a
frequency rate of a quarter of power cycle, the next constraint between settings must
be taken into account:
•
At 50 Hz: 1.8 x
Δ
F
max
<Max Angle Diff
•
At 60 Hz: 1.5 x
Δ
F
max
Angle Diff The non-compliance of this requirement could
cause a synchrocheck close condition out of the limits set by settings.
The non-compliance of this requirement could cause a synchrocheck close condition
out of the limits set by settings.
Synchrocheck, or Synchronism Check (25), is defined as the comparison of the voltage
difference of two circuits with different sources to be either linked through an impedance
element (transmission line, feeder, etc.), or connected through parallel circuits of defined
impedance. The voltage comparison between both sides of a breaker is performed before
closing the breaker, in order to minimize internal damage that could occur due to the
voltage difference, both in magnitude and angle. This is extremely important in steam
generating plants, where reclosing output lines with angle differences could lead to severe
damage to the turbine axis.
The difference in voltage level and phase angle in a given moment is the result of the
existing load between remote resources connected through parallel circuits (load flux), as
well as a consequence of the impedance of those elements connecting them (even if there
is no load flux in parallel circuits, or because sources to be connected are completely
independent and isolated from one another).
In interconnected systems, the angle difference between both ends of an open breaker is
usually negligible, as its sources are remotely connected through other elements
(equivalent or parallel circuits). However, in isolated circuits as in the case of an
independent generator, the difference in angle, voltage levels and relative slip of voltage
phasors can be very important. It can also be that the relative slip of voltage values is very
low or null so that they will be rarely in phase. Luckily, due to the changing conditions of a
power system (connection-disconnection of loads, sources, and new inductive-capacitive
elements) the relative slip between phasors is not null and they can be synchronized.
In the first case, even if we must take into consideration the length of the line whose ends
(sources) will be connected for determining the angle difference between them, this is not
enough to fix the synchronism conditions before closing the breaker. Experience tells us
that the window of angle difference between voltage phasors must be fixed to a value of
15º to 20º.
VOLTAGE INPUTS
