Alstom SHPM 101 - Section 5. PRINCIPLES OF OPERATION; 5.1 The comparator; 5.1.1 Fundamentals of the comparator

SERVICE MANUAL R5888C
QUADRAMHO Chapter 2
Page 33 of 74
Section 5. PRINCIPLES OF OPERATION
5.1 The comparator
The important requirements of high speed and high stability have both been
satisfied in the comparator design. Usually these two requirements are in
contention because the faster the operating speed, the greater the risk of false
operation caused by contaminated relay input signals. Signal contaminations
include harmonic components, switching surges, lightning impulses, travelling
waves, exponential decays, saturated current transformer waveforms, coupling
capacitor voltage transformer transient errors and interference voltages induced on
low voltage wiring due to switching on the high voltage system.
The comparator resolves the speed/stability contention by checking its own input
signals to verify that they are dominated by components consistent with power
system frequency waveforms. If verification is obtained, full operating speed is
allowed. If verification is not obtained, the comparator demands more data before
tripping can be allowed, thereby automatically extending the signal processing
time sufficiently to ensure that no maloperation can occur. By suitable filtering and
preconditioning of the comparator input signals, the relay design ensures that the
comparator is able to operate at its highest speed for the majority of transmission
line faults.
5.1.1 Fundamentals of the comparator
The comparators in Quadramho are used to produce a variety of different
characteristic shapes, such as quadrilateral, mho, offset mho, lenticular etc.
The easiest to explain is the mho (or circular) characteristic, so this will be
described first.
For simplicity describing a self-polarising characteristic, the comparator inputs are
as shown in Figure 29, such that:
A = V –
IZ
B = V
/– 90°
and the condition for operation is that A lags B by 0 to 180°. As the operation of
the comparator must be independent of the magnitude of A and B, these two
quantities are changed to square waves using high gain amplifiers before being
supplied to the comparator The squared up signals convey only the phase angle
information of the original signals.
The comparator treats the input square waves as logic variables which can each
have a high or a low logic state at any time. To facilitate the following explanation,
signal A will be described as A or
A
depending on its logic state at a particular
instant of time and signal B will be described as B or
B
. There are four possible
combinations of state, A.B,
A
.B,
A.B
, and A.
B
. If both signals have unity
mark/space ratios and equal periods but different phases, then the four
combinations occur in a cyclic manner.
There are only two possible sequences of these combinations as shown in
Figure 30. These are:
1) If signal A leads signal B: A.B,
A
.B,
A.B
, A.
B
, A.B
2) If signal A lags signal B: A.B, A.
B
,
A.B
,
A
.B, A.B
From these the following logic statements can be deduced.