Note: Descriptions are shown in the official language in which they were submitted.
1 50,508
SIGNAL QUALITY MONITOR FOR
PROTECTIVE RELAY SYSTEM
BACKGROUND OF THE INVENTION
Field of the Invention:
The invention relates in general to protective
relay systems, and more specifically to new and improved
signal quality monitoring apparatus useful for determining
if a protective relay signal received over a communication
channel is suitable for use.
Description of the Prior Art:
Protective relay systems for protecting elec-
trical power transmission lines must be reliable, operat-
ing correctly when needed, and avoiding unnecessary opera-
tion. When a protective relay decision function receives
a protective relay signal, or signals, rom a communica-
tion channel, it is of critical importance that the signal
quality be of such a level that the desired line between
dependability and security is maintained.
In general, communication channel problems can
be classified as follows:
(1) Dead channel;
; 20 (2) Signal strength outside the normal limits
due to malfunctions or improper calibration of channel
equipment;
(3) Noisy channel due to failing equipment,
signal mixups, EMI, and the like; and
:
.~
779
2 50,508
(43 Invalid signals due to unannounced channel
equipment servicing, testing and calibration.
It would thus be desirable to provide a new and
improved signal quality monitor for communication signals,
which reliably and economically monitors all important
signal parameters, and provides a go, no-go type of re-
sponse which indicates whether or not all monitored para-
meters meet the prescribed standards.
SUMMARY OF THE INVENTION
Briefly, the present invention relates to new
and improved signal quality monitor apparatus useful in
protective relay systems, which systematically monitors
all of the important parameters of a communication signal
usîng AC and DC signals from the automatic gain control
stage of the communication receiver. The AC signal used
is the signal which appears at the output of the automatic
gain control (agc) amplifier stage, and the DC signal is
the gain control voltage which is developed to control the
gain of the agc amplifier. The AC signal is processed to
develop a noise signal indicative of the channel noise,
and its waveform is also squared and processed to deter-
mine if the signal frequency is within the proper range.
The DC signal is processed to determine if its magnitude
is within the proper range, and it is also compared with
the noise signal to insure that the signal-to-noise ratio
(S/N) exceeds a predetermined value. If all of the moni-
tored parameters meet the required standards, the signal
quality monitor provides an "enable" signal, which allows
an associated protective relay system to function normal-
ly. If any monitored parameter is below standard, the
signal quality monitor provides a '1disable" sîgnal, which
disables at least those functions of the associated pro-
tective relay system which utilize the monitored commun-
ication signal.
The noise signal developed for the S/N function
is additionally utilized in the trip circuits of the
circuit interrupter associated with the monitored protec-
.~,
' . '7
3 50,508
tive relay function to modify the trip decision function
which utilizes the monitored communication signal. There
is a direct relationship between the noise in the communi-
cation channel and the noise in the demodulated signal
used in the protective relay function. The demodulated
noise ultimately shows up in the voltage signal sent to
the protective relay trip circuits for comparison with a
reference voltage. In accordance with the teachings of
the present invention, the noise signal is effectively
added to the reference voltage. This arrangement main-
tains the accuracy of the trip circuit, automatically
adapting or modifylng the sensitivity of the trip circuit
function in direct response to the level of noise in the
communication channel.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention may be better understood, and
further advantages and uses thereof more readily apparent,
when considered in view of the following detail descrip-
tion of exemplary embodiments, taken with the accompanying
drawings in which:
Figure 1 is a schematic diagram of a protective
relay system which may be constructed according io the
teachings of the invention;
Figure 2 is a schematic diagram of a receiver
constructed according to the teachings of the invention;
Figure 3 is a partially block and partially
schematic diagram of a signal quality monitor constructed
according to the teachings of the invention;
Figure 4 is a detailed schematic of the signal
quality monitor shown in Figure 3;
Figure 5 is a graph which includes the agc
transfer characteristic, and which illustrates preselected
relationships which are established between the agc signal
and the gain control voltage for the agc amplifier;
Figure 6 is a graph which illustrates a step in
the development of the noise signal in Figures 3 and 4;
and
~LZ4~ 73
4 50,508
Flgure 7 is a schematic diagram of the evaluation
function shown in block form in Figure 1, which illustrates
an application of the signal quality monitor signal to a
protective relay system.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
_
For purposes of example, the signal quality monitor
of the invention will be applied -to the protective relay
system of U.S. Patent 4,275,429, which is assigned to the
same assignee as the present application. The relay system
of this patent performs the functions of an electromechanical
pilot-wire relay, without the necessity of having a continuous,
metallic conductor connected between the points to be compared.
Many different types of communica-tion links may be used to send
protective relay signals from one terminal to another terminal
for comparison with the local protective relay signal, such as
an optical link, microwave, power line carrier, or telephone
channels. For purposes of example, the dedicated Bell Tele-
phone System 3002 channel will be assumed to be the communi-
catlon link. Certain U..S. Patents assigned to the same
assignee as the present application may also be referred to,
if more information is desired relative to certain functions
shown in the drawings. For example, U.S. Patent 4,408,246
sets forth another embodiment of an evaluation function which
may be used, instead of the one shown in U.S. Patent 4,275,429.
25 In like manner, U.S. Patent 4,380,746 discloses a pulse modu-
lator which may be used in the transmitter of the protective
relay system. U.S. Patent 4,510,453 entitled "Demodulator",
discloses a pulse demodulator which may be used in the
receiver circuits of the protective relay system. U.S. Patent
30 4,464,697 entitled "Protective Relay System", discloses a
direct transfer trip (DTT) function, which may also be used
in the protective relaying system of U.S. Patent 4,275,429.
2~0~t79
5 50,508
Only the portions of the incorporated protective
relay system which are necessary in order to understand
the present invention are repeated herein. The reverence
numerals ox the repeated portions have been retained in
Figure 1. Portions from the incorporated system which
have been modified in Figure 1 are identiied with their
prior reference numerals plus a prime (') mark.
Referring now to the drawings, and to Figure 1
in particular, there is shown a new and improved protec-
tive relay system 10' for providing pilot protection for a
transmission line section 12. The protected section 12
may be a two or a three terminal line, with a two terminal
line being shown for purposes of example. Transmission
line section 12 includes a local or near terminal 14,
which includes a circuit breaker 16. Circuit breaker 16
interconnects one end of line section 12 with a high-
voltage, three-phase AC electrical power system having
conductors a, b and c. Transmission line section 12
further includes a first remote or far terminal 18 which
includes a circuit breaker 20. Circuit breaker 20 inter-
connects another end o line section 12 with a high-
voltage, three-phase AC electrical power system having
conductors a', b' and c'.
Terminals 14 and 18 additionally include similar
protective relaying apparatus 22 and 24, respectively.
Since the protective relaying apparatus 22 and 24 at each
terminal may be similar, only the protective relaying
apparatus 22 associated with the near terminal 14 will be
described in detail.
Protective relaying apparatus 22 includes means
26 for obtaining a protective relay signal, such as a
current derived single-phase composite sequence voltage
signal VN responsive to the three-phase currents 10wing
in conductors a, b and c, and the 3Io or ground current.
~L2~'7~
6 50,508
Means 26 includes current transformers 28, 30 and 32, and
a composite sequence filter 34 which mixes predetermined
percentages of positive, negative and/or zero sequence
currents from the three phases to obtain a power fre-
quency, e.g., 60 Hz, single-phase composite sequence
voltage whose phase is responsive to the direction of
power flow, and whose magnitude is responsive to the
current magnitudes in the three phases. The same com-
posite sequence filter now used by the prior art electro-
mechanical pilot-wire relays may be used, with U.S. Patent
2,183,646 describing a composite sequence filter which may
be used; or the composite sequence filters may be solid
state, constructed of operational amplifiers.
The current derived composite sequence signal or
voltage VN is applied to a transmitter 38. Transmitter 38
includes a modulator 38' and a communication interface
38" for the type of communication link utilized. The
waveform of voltage signal VN is used as the modulating
waveform in modulator 38' for the type of communication
selected for transmitter 38. For example, transmitter 38
may produce pulses at a predetermined nominal rate in
response to a modulating signal of zero magnitude, with
the pulse rate changing as signal VN changes from zero.
Pulse period modulation is the preferred form of communi-
cation in the present invention, and the invention isaccordingly described relative to this form of modulation.
The center or nominal frequency may be chosen for the
specific type of communication link 40 employed. Since
the attenuation and envelope delay versus frequency will
be known for the specific communication channel selected,
the nominal pulse rate should be selected to minimize both
attenuation and envelope delay. For example, in a dedi-
cated Bell Telephone System 3002 channel, a narrow band of
approximately +300 Hz around a center frequency of approx-
imately 1.7 KHz provides minimum attenuation and envelopedelay. As hereinafter stated, the present invention will
be described assuming the use of the 3002 channel in the
communication link 40.
7 50,508
Protective relaying apparatus 22 also includes a
receiver 42 connected to the communication link 40, which
includes an interface 42' for the specific type of commun-
ication link 40 utilized, and a demodulator 42''. Demodu-
lator 42 " may be the PPM demodulator of the hereinbefore-
mentioned incorporated application. Receiver 42 receives
a communication signal responsive to the power frequency,
current derived, single-phase composite sequence voltage
signal VF from the far terminal 18. Receiver 42 demodu-
lates the communication signal to provide signal V'E~which is similar to signal VF, except delayed by the
channel delay time.
Signal VN from sequence filter 34 is also ap-
plied to a delay equalizer 35, which provides a signal VIN
which is similar to signal VN, except delayed by the same
time as the channel delay. Signals VIN and VIF are now in
I; suitable form for direct comparison, and they are applied
I' to an evaluation circuit 36"for this purpose.
For normal through current, i.e., no fault in
the protected transmission line section 12, and using the
ct connections shown in Figure 1, signal VIN will, ideal-
ly, be 180 out of phase with signal V'F When a fault
occurs in the protected line section 12, current flow will
be into the polarity marked terminals of the ct's and
signals VIN and VIF will, ideally, be in phase.
The evaluation circuit 36', and its counterpart
in protective relay apparatus 24, compare the single-phase
voltage waveforms of the current derived signals VIN and
V'F~ and if a fault is detected within the protected line
section 12, trip signals TLCB are applied to their asso-
ciated circuit breakers 16 and 18 to clear the trans-
mission line section 12.
A direct transfer trip request for tripping
circuit breaker 20 is initiated at the near terminal 14 by
means 44 which includes a source 45 of electrical poten-
tial, such as the station battery, a contact 46, and a DTT
8 50,508
request function 47. Contact 46 may be manually actuated,
or it may be under automatic control via an appropriate
protective relay circuit.
DTT request means, similar to means 44, is
provided at the remote terminal 18. When this remote
s request means enter~a DTT request to direct the tripping
of circuit breaker 16, the request is recognized by a DTT
recognition function 48. When function 4~ detects a DTT
request signal it provides signals DT and DTQ which modify
the operation of the evaluation circuit 36". When means
44 initiates a DTT request signal, DTT recognition means
located at the remote terminal 18, which is similar to
means 48, processes the request. The DTT functions shown
in block orm in Figure 1 are described in detaii in a
hereinbefore-mentioned incorporated patent application.
A signal quality monitor function 49, construct-
ed according to the teachings of the invention, monitors
the incoming communication signal from the other terminal,
or terminals, to determine if the signal quality is suffi-
ciently Good in order to enable the local-remote compari-
son function, and the DTT function.
While the signal quality monitor 49 is shown as
an individual block in Figure l, it utilizes signals
developed in the communication or receiver interface 42l,
and thus the development of these signals is shown in
detail in Figure 2.
Figure 2 is a schematic diagram of a circuit
which may be used for the communication interface 42l
shown in Figure 1. As hereinbefore stated, the communica-
tion channel 40 is assumed to be a telephone channel. The
input and output impedances are commonly matched to 600
ohms, with the 0 dBm reference, used relative to audio
power, corresponding to a 1 milliwatt power level across
600 ohms.
More specifically, the signal received from
communication channel 40, which will have a frequency of
1.7 KHz + 200 Hz, when modulated with the 60 Hz protective
9 50,508
~'~ relay signal V~, or 1.7 KHz + 220 Hz, when modulated withthe 400 Hz DTT command modulating signal, respectively, is
applied to a 1:1 telephone interface transformer 52 via AC
coupling capacitors 54 and 56. Transformer 52 includes
primary and secondary windings 60 and 62, with a varistor
58 being connected across the primary winding 60, to
protect against voltage spikes. The signal at the output
of the secondary winding 62 is applied to an operational
amplifier (op amp) 64 connected for common mode rejection,
to further condition the communication signal.
The conditioned signal is then applied to a
scaling stage, which includes an adjustable resistor or
potentiometer 66, a switch 67, and an op amp 68 connected
as an amplifier. Connecting the conditioned signal to the
inverting input of op amp 68, via switch 67, provides up
to 26 dB amplification, while connecting the signal to
the non-inverting input provides up to 26 dB attenuation.
The conditioned and scaled signal is then applied to a 1
K~Iz-2.5 KHz band pass filter 69, which may have a first
stage having an op amp 70 connected as a low pass filter,
which provides the 2.5 KHz limit, and a second stage
having an op amp 72 connected as a high pass filter, which
provides the 1 KHz limit.
The band passed signal, referred to as signal
Vin, is then applied to an automatic gain control (agc)
unction 73. The agc function 73 includes an agc ampli-
fier having a current controlled, variable gain amplifier.
The amplifier, for example, may be a transconductance
operational amplifier 74, such as RCA's 3080, and an op
amp 76 connected to amplify the output of the transduct-
ance operational amplifier 7~.
Transconductance operational amplifier 74 in-
cludes a gain control input (terminal 5) which controls
the gain of the amplifier. A gain control voltage of zero
volts provides maximum gain, with the gain decreasing as
the control voltage becomes more negative.
-" ~.2~
50,508
The gain controlled signal, which is an AC
signal referenced AGC, is available at the output of op
amp 76. The gain control voltage, which is a unidirec-
tional voltage, is referenced GCV. Voltage GCV is devel-
oped from signal AGC via an absolute value circuit, whichmay include a rectifier 78 and an operational amplifier 80
connected as a low pass filter. When there is no signal
at the output of op amp 76, the control voltage GCV is
zero. As signal AGC increases, control voltage GCV be-
comes more negative, reducing the gain of amplifier 74.Figure 5 is a graph which illustrates the development of
the gain control voltage GCV. The AGC output voltage
versus the AGC input voltage curve is deliberately forced
to increase substantially linearly across the range of
interest, to provide a change in the control voltage to be
obtained which is suitable for con-trolling the amplifier
gain. Thus, the output of the AGC amplifier is said to be
substantially constant, instead of constant. Both the AC
signal AGC, and the undirectional control voltage, GCV are
uniquely utilized by the signal quality monitor 49, as
will be hereinafter described.
Figures 3 and 4 set forth a signal quality
monitor which may be used for the monitor 49 shown in
block form in Figure 1, with Figure 3 being a partially
schematic and partially block diagram, in order to clearly
identify the various functions. Figure 4 is a more de-
tailed schematic of the circuit portions shown in block
functional form in Figure 3.
More specifically, control voltage GCV, which is
a direct, unidirectional indication of signal strength, is
monitored to insure that the signal magnitude is within
the desired limits. High and low magnitude monitoring
functions 82 and 84 compare voltage GCV with appropriate
references to implement the magnitude monitoring func-
tions.
A signal-to-noise ratio (S/N) monitor 86 com-
pares control voltage GCV, which is representative of the
11 50,508
desired signal plus channel noise, with a signal Vx devel-
oped from signal AGC. channel noise is substantially
constant across the freqllency range of the band passed
signal, as shown in Figure 6. Thus, a signal proportional
to noise in the frequency range of the signal, i.e., 1.7
KHz + 220 KHz, may be obtained by developing a signal
proportional to the channel noise output in the band-
passed frequency range. Any frequency band in the band-
passed signal, outside the communication signal band,
which will provide a signal of usable magnitude may be
selected. As illustrated in Figure 3, the signal AGC is
succe6sively subjected to 1.5 KHz, 1.9 KHz and 1.7 KHz
traps, referenced 88, 90 and 92, respectively. Any DC
level in the signal is removed with a capacitor 94, and
the resulting signal, which, as shown in Figure 6, in-
cludes the frequency ranges of about 1 to 1.5 KHz and
about 2 to 2.5 KHz, is processed in function 96 to provide
a unidirectional signal Vx, whose magnitude represents the
noise portion of the signal received from the communica-
tion channel 40.
The waveform of signal AGC is squared in blockfunction 120 and used to determine if the signal frequency
is in the proper range via high and low freguency monitor-
ing functions 122 and 124, respectively. If all of the
monitored parameters of the signal, i.e., magnitude, S/N,
and frequency are within acceptable limits, function 49
provides a signal EN at the low or logic zero level, which
at this level is an enabling signal. If any one of the
monitored parameters is outside its prescribed acceptable
range or limit, signal EN will go high or to a logic one,
which at this level is a disabling signal, as will be
hereinafter explained. Suitable alarms and/or indicators,
shown generally at 125, may be included to latch in and
thus identify which parameter has triggered the signal
quality monitor.
2~
12 50,508
Referring now to the detailed embodiment of the
signal quality monitor 49 shown in Figure 4, the high
magnitude monitor may be provided by an op amp 126 con-
nected as a comparator, with the control voltage GCV being
connected to its non-inverting input, and a suitable,
relatively high, negative reference voltage is connected
to its inverting input. If voltage GCV becomes more
negative than the reference, it indicates that the signal
has exceeded the upper magnitude limit, and the output of
op amp 126 will switch from a logic one to a logic zero,
as a result of this inverting input becoming more positive
than the non-inverting input.
In like manner, the low magnitude monitor 84 may
be provided by an op amp 128 connected as a comparator,
with the negative control voltage GCV being connected to
its inverting input, and a suitable, relatively low,
negative reference voltage is connected to its non-
inverting input. If voltage GCV becomes less negative
than the reference, it indicates that the magnitude of the
communication signal has dropped below the lower magnitude
limit, and the output of op amp 128 will switch from a
logic one to a logic zero, as a result of its inverting
input becoming more positive than the non-inverting input.
Signal Vx, which is proportional to noise in the
communication channel, is produced by a signal voltage
stripping circuit which includes the three traps 88, 90
and 92, with only trap 92 being shown in detail, since
they are of similar construction. Trap 9~ may include an
,i act
I` op amp ~3~ connected as an active twin T band reject
filter, to block signals having a frequency of 1.7 KHz,
and those immediately adjacent thareto, while passing
signals above and below this small frequency band. Con-
necting the three traps in series provides a signal at the
output of op amp similar to that shown in Figure 6.
35 The noise processing function 96 may include op amps 132
; and 134, with op amp being connected as a precision
rectifier, and op amp 134 connected as a summing ampli-
.
7~
13 50,508
f 3/
-I fier. Op amps ~3~ and 134 provide a full-wave rectifica-
tion of the signal applied thereto, with the output signal
VX of Op asp 134 being negative.
Signal Vx, represents the magnitude of the
S channel noise, and the voltage GCV, which represents the
useful signal plus channel noise, are applied to S/N
monitor 86. S/N monitor 86 may include an op amp 136
connected as a comparator, with the resistive and capaci-
tive reference components being selected to provide about
a-20 dB reference, i.e., as long as the noise signal Vx is
less than l/lOth the signal plus noise, represented by
vo,ltage GCV, the output of op amp will be high. If the
noise signal Vx becomes a greater percentage than 10
percent of the total signal, the inverting input will
become more positive than the non-inverting input, and the
output of op amp 136 will switch to a logic zero level.
The effectiveness of the S/N monitor 86 may be
readily observed from the following relationships, where:
Vin = The AC voltage applied to the agc
function;
f(Vin) = The transfer characteristic of the agc
function;
GCV = The agc control voltage (DC);
AGC = The AC output voltage from the agc
function;
¦Vin¦ = The absolute value of the voltage Vin;
Kl = A constant (unitless);
K2 = A constant (unit of 1/volt); and
K3 = The reference in the S/N monitor
I
The agc function implements the following rela-
tionship:
AGC = f(Vin) Vin (1)
The transfer function f(Vin) may be written:
f(Vin) l~K2¦Vin¦ (2)
77~
14 50,508
Thus, equation (1) may be written:
KlVin
l+K2¦Vin¦ (3)
The input signal Vin includes a signal voltage
Vsi and a wide-band noise voltage Vni, and thus Vin may be
written:
Vin = VSi + Vni
Thus, equation (3) may be written:
AGC = l+K2 IVi +V il (5)
The control voltage GCV may thus be written:
GCV = IVsi Vnil (6)
After signal stripping, the resultant noise
signal Vx is thus:
K1 ¦Vni¦
l K2lVsi Vnil
The signal to noise comparator 136 looks for the
relationship:
K3¦GCV¦ = Vx (8,
Substituting equations (6) and (7) into (8)
provides:
, I si Vnil = Kl (9
20Since Vsi is many times greater than Vni:
v + v I IV I
si ni _ sl K3 (10)
Equation (10) indicates that S/N monitoring may
be readily accomplished by choosing the reference K3 for
~z~ 9
-15- 50,508
The desired S/N ratio. It is also evident that the S/N
funtion is relative, i.e., it is not tied to any specific
magnitude of Vin.
The frequency monitoring function provides carrier
frequency verifica-tion by inspecting every carrier frequency
cycle. If the incoming signal is outside the allowed frequen-
cy deviation, a flag is raised similar to the high/low signal
and S/N detections to signal the protective relay circuit for
appropriate action. Unlike the other two detections (high/
low and S/N), this detection is fast responding. Any incorr-
ect frequency cycle, upon detection, results in the immediate
sending of an appropriate logic signal. The high/low signal
and S/N detections have built-in time delays due to the
absolute value signal processing. All three detections ulti-
mately merge into one logic output to go to the protective
relay circuit. However, separate alarm indications are
provided for identifying the nature of a channel problem.
More specifically, alternating signal AGC is squared
in waveform squaring function 120, which may include an op
amp 137 connected as a threshold squarer, and the output of
the waveform squaring function 120 is applied to a high
frequency monitoring function may be performed by a multivi-
brator (MV) 139. MV 139, for example, may be one of the
retriggerable, monostable multivibrators in Mo-torola's dual
package MC14538B. Capacitor 142 and resistor 144 are selec-
ted such that the Q output of MV 140 will provide a pulse
train, untll the high frequency limit is reached, at which
time the Q output will be continuously low, or a logic zero.
The low frequency monitoring funtion 124 may be
provided by MV 145, which may be the other multivibrator
in the hereinbefore-mentioned dual package, with MV 145
being connected to be responsive to the Q output of MV
139. Capacitor 146 and resistor 147 are selected such
" 2 ~7
16 50,508
that if the Q output of 139 is providing a pulse train
having a repetition rate which indicates signal AGC has a
frequency above the lower limit, then the Q output of MV
145 will be continuously held low, providing a true or low
enabling signal I. Should the pulse rate, and thus the
signal frequency, fall below the lower limit, the output
of MV 145 will provide a pulse train, which functions as a
disabling signal, in the same manner as a continuously
high signal would.
If the communication signal fails to pass the
high frequency test provided by function 122, the output
of MV 139 will be low, instead of a pulse train, and the
output of MV 145 will be high, which also functions as a
disabling signal.
The magnitude and S/N tests are applied to an
appropriate logic function. For example, they may be
OR'ed via diodes 150, 152, 154 and 156, resistors 158, 160
and 161, and a positive source of unidirectional poten-
tial. The output of the OR function is applied to the
reset input R of MV 145. Should any of the OR'ed func-
tions go low, to signify an out-of-limit parameter, the Q
output of MV 145 will be forced high, which is the dis-
abling level for signal I.
Figure 7 is a schematic diagram which illus-
trates how the evaluation circuit 36'' may be made re-
sponsive to the enable/disable signal EN and also how the
trip circuit may be made adaptive by the noise signal Vx.
Certain of the components in Figure 7 were first intro-
duced in the patents related to the DTT function, and
they are retained in order to illustrate how the DTT
function and signal quality monitoring functions may both
be easily added to the evaluation function 36''.
~2 9
17 50,508
Signal EN is connected to an input of NOR gate
104 via inverter gates 162 and 164. Should signal EN go
high, the output of NOR gate 104 will go low and bilateral
switch 102 will open to disconnect the remote protective
relay signal VF from the evaluation circuit. An SPDT
switch 166 may be provided to manually select whether or
not the signal EN, when high, is to block all trips, or
just the comparison trip. Signal EN is connected to the
"block" terminal of switch 166, and its "unblock" terminal
is not connected to the circuit. The switch actuator of
switch 166 is connected to an input ox NOR gate 106 via a
diode 168. If switch 166 is in the "unblock" position
illustrated in Figure 7, only the local-remote comparison
function of the relay will be affected by the high EN
signal. If switch 166 is changed to the "block" terminal,
a high disable signal ON will prevent NOR gate 106 from
going high to initiate a true trip signal, regardless of
the function which is attempting to provide a trip signal.
Signal EN is also connected to MV 116, which may
be the same type of multivibrator as MV 139 or MV 145, via
inverter gates 162 and 164 and NOR gate 114. When signal
EN goes high, the Q output of MV 116 provides a logic one
pulse of predetermined duration, such as 20 ms, which
pulse is applied to an input of NOR gate 106 via diode 169
to block any trip during this period of time. Another
multivibrator MV 170 is provided which is also responsive
to signal EN via inverter gates 162 and 164. MV 170
applies a trip blocking pulse of predetermined duration,
! such as 20 ms, to an input of NOR gate lQ6 when signal EN
goes from a logic one to a logic zero, to override system
transients at the time the communication channel returns
to normal.
As described in U.S. Patent 4,408,246, the
output signal Vop~VR/G from
`' ~2~71~
18 50,508
the evaluation circuit 36' is compared with a preset
pick-up or bias voltage provided by reference 130. When
signal Vop~VR/G is less than the reference 130, signal T
is high, indicating no fault in the protected transmission
line section. When signal Vop~VR/G exceeds the reference,
signal T will yo low to indicate a fault in the protected
line section. The demodulated noise in the communication
channel, which is proportional to Vx, shows up as an
addition to signal VOp-VR/G. The greater the noise the
greater signal VOp-VR/G. If the trip reference is made
proportionally incremental with the channel noise, the
accuracy of the trip circuit can be maintained. If de-
sired, an even greater proportion of Vx may be used to
modify the reference, to produce a desensitizing effect.
Thus, the noise signal Vx may be applied to one of the
inputs of op amp 138, such as to the non-inverting input,
via a summing resistor 172 and a bilateral switch 174.
Signal EN is connected to the control input of switch 174
via inverter gate 162. Thus, when the signal quality of
the communication signal is good, EN is low and switch 174
is closed to connect the noise signal Vx to the tripping
comparator circuit 132. The noise signal Vx, being nega-
tive, is effectively subtracted from the signal Vop~VR/G.
The greater -the channel noise, the higher the noise con-
tent in signal Vop~VR/G. Thus, by adding a proper amountof Vx to the trip reference 130, the accuracy of the trip
can be maintained. This feature is entirely adaptive,
providing a very desirable function which is otherwise
difficult to obtain. If the signal quality is poor and
signal EN goes high, switch 174 opens to remove the noise
signal Vx from the tripping comparator 132. Thus, over-
current trips may be made without the sensitivity reduc-
tion introduced by the noise signal modification. If
desired, the Q output of MV 116 may be connected to the
control input of switch 174, to maintain switch 17a
19 50,508
closed, and thus maintain the less sensitive mode for the
period of the blocking pulse, to override any system
transients during this transition period.
In summary, there has been disclosed a new and
S improved protective relay system including signal quality
monitoring apparatus suitable for monitoring communication
signals used in such relay systems. The signal quality
monitor apparatus utilizes both AC and DC signals from the
receiver agc stage to provide signal frequency, signal
magnitude and S/N checks, with the failure of any para-
meter to satisfy predetermined minimum standards resulting
in a disable signal at the output of the monitor. The
disable signal is used in the protective relay system to
discontinue at least those protective relay functions
dependent upon the sub-standard communication signal.
Further, a signal representative of channel noise, devel~
oped for the S/N monitoring function, is used in the trip
circuits to discount the noise present in the signal
VOp-VR/G. In other words, the comparison signal, which
initiates tripping of the associated circuit breaker when
it reaches a predetermined reference level, is reduced by
the amount of the noise contained in signal VOp-VR/G.
