Transformer circuit and method with saturation prevention

The disclosure is directed to a transformer circuit apparatus which receives an input signal and generates an output signal representative of the input signal. For example, the input signal may be an AC current, and the output signal to be generated may be a voltage that is proportional to the current. A transformer is provided, and has a core, a primary winding for receiving the input signal, a sense winding, and a feedback winding. A first circuit is coupled to the sense winding, and is responsive to the magnetic flux sensed by the sensed winding, for applying a signal to the feedback winding that tends to reduce the flux sensed by the sense winding. A feedback circuit is provided, and includes a low-pass filter, for feeding back a signal that depends on the output signal to the first circuit means. A circuit is provided for determining the presence of input signal offsets, and for generating offset indication signals in response thereto. A control means, responsive to the offset indication signals, is provided for controlling the low-pass filter. In the illustrated embodiment, there is disclosed current transformer circuit suitable for utilization in a protective relaying system. The low-pass filter includes a capacitor, and the circuit for controlling the low-pass filter includes a switch for discharging the capacitor, the switch being controlled by the offset indication signals. In this embodiment, the offset indication signals depend upon the time between zero-crossings of the input signal and the amplitude of the input signal. By discharging the capacitor in response to offsets at the input, the problem of saturating the input amplifier of the sense winding circuit is eliminated, and this permits use of a transformer core that need only be large enough to handle the small flux that is uncancelled by the feedback winding.

FIELD OF THE INVENTION 
This invention relates to improvements in apparatus employing a transformer 
and, more particularly, to preventing saturation of the transformer core 
when there are substantial offsets at the transformer primary. 
BACKGROUND OF THE INVENTION 
There are various applications in which an input signal is applied to the 
primary of a transformer whose secondary is coupled to an output circuit 
that produces a signal having a specified relationship to the input 
signal. An example of such a circuit is a current transformer circuit 
employed in applications such as so-called protective relaying for power 
transmission lines. 
AC power transmission lines are often protected by protective relaying 
systems which operate upon occurrence of a fault to trip circuit breakers 
that protect the transmission line from damage and isolate the faulted 
portion of transmission line from the rest of an overall transmission 
system. Typically, the section of transmission line to be protected 
extends between terminals called local and remote terminals, and 
substantially identical protective subsystems are located at the remote 
and the local terminals. Current on the transmission line (generally, on 
individual phases thereof) is sensed at both the local and the remote 
terminals, and information concerning the current is transmitted over a 
communications channel from the remote terminal to the local terminal, and 
vice versa. Each subsystem includes means for comparing the local and 
remote current readings and for generating trip control signals as a 
function of the comparison. The trip control signals operate, under 
certain conditions, to trip circuit breakers at the respective locations 
when the subsystems detect a condition that indicates an internal fault; 
i.e., a fault within the protected section of transmission line. A prior 
art protective relaying system is disclosed, for example, in U.S. Pat. No. 
4,939,617, assigned to the same assignee as the present application. 
The accurate sensing of current (typically, for each individual phase of 
the transmission line) is essential to proper performance of the 
protective relaying system. Each current transformer circuit operates to 
produce an output voltage that is proportional to the current in the 
current transformer primary winding. The output voltages are then used in 
subsequent processing for determination of the absence or presence of a 
fault, as previously described. The current transformers are required to 
operate over a wide range of current magnitudes and, when a fault occurs, 
a sharp exponentially decaying DC offset can result. If the transformer 
core isn't large enough to handle the flux that results from the offset, 
it will saturate. A substantial recovery time may then be needed before 
the transformer will again operate properly, and this could result in 
serious consequences in an application such as protective relaying, since 
the system will be effectively "blinded" until the transformer recovers. 
[As an example, in differential type protective relaying, should 
saturation occur at one station and not the other, or at both stations, an 
inaccurate signal comparison will be made and could result in a false trip 
in a thru-fault situation, or a failure to trip upon a line fault.] In 
extreme cases, the transformer could be permanently disabled. 
The use of a large core can prevent the stated problem, but the attendant 
size, weight and cost are generally undesirable. Air gaps can make the 
core more resistant to saturation, but generally do not permit core size 
reduction. An approach to the core saturation problem which permits use of 
a relatively small core in certain applications has been to employ a flux 
cancellation circuit. In this approach, the transformer is provided with a 
primary winding, a sense winding, and a feedback winding. A sense winding 
circuit is responsive to the flux sensed by the sense winding and applies 
a signal to the feedback winding that tends to reduce the flux sensed at 
the sense winding. An output signal depends on the signal that is applied 
to the feedback winding and a feedback circuit feeds back the output 
signal to the sense winding circuit to provide stability. The feedback 
circuit includes a low-pass filter, which comprises an RC circuit. The 
circuit operates to reduce the flux in the transformer, but suffers a 
serious problem, as follows: A substantial current offset at the primary 
will cause a DC component in the feedback circuit that tends to saturate 
the input amplifier of the sense winding circuit. When this happens, 
control of the feedback loop is lost, the flux cancellation is not 
effective, and the transformer can saturate. 
It is among the objects of the present invention to provide an improved 
circuit and method for preventing saturation of the core of a transformer 
when there are substantial offsets at the transformer primary. 
SUMMARY OF THE INVENTION 
The present invention is directed to a transformer circuit apparatus which 
receives an input signal and generates an output signal representative of 
the input signal. For example, the input signal may be an AC current, and 
the output signal to be generated may be a voltage that is proportional to 
the current. A transformer is provided, and has a core, a primary winding 
for receiving the input signal, a sense winding, and a feedback winding. A 
first circuit means (also referred to as a sense winding circuit) is 
coupled to the sense winding, and is responsive to the magnetic flux 
sensed by the sensed winding, for applying a signal to the feedback 
winding that tends to reduce the flux sensed by the sense winding. A 
feedback circuit means is provided, and includes a low-pass filter, for 
feeding back a signal that depends on the output signal to the first 
circuit means. Means are provided for determining the presence of input 
signal offsets, and for generating offset indication signals in response 
thereto. A control means, responsive to the offset indication signals, is 
provided for controlling the low-pass filter. 
In an illustrated embodiment, the invention comprises a current transformer 
circuit suitable for utilization in a protective relaying system. The 
low-pass filter includes a capacitor, and the means for controlling the 
low-pass filter includes switch means for discharging the capacitor, the 
switch means being controlled by the offset indication signals. In this 
embodiment, the means for generating offset indication signals comprises 
means for producing signals that depend upon the time between 
zero-crossings of the input signal and the amplitude of the input signal. 
By discharging the capacitor in response to offsets at the input, the 
problem of saturating the input amplifier of the sense winding circuit is 
eliminated, and this permits use of a transformer core that need only be 
large enough to handle the small flux that is uncancelled by the feedback 
winding. 
Further features and advantages of the invention will become more readily 
apparent from the detailed description when taken in conjunction with the 
accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, there is shown a diagram of an AC transmission line 
section 100 which, in the present example, is a three phase electrical 
network having individual transmission conductors 101, 102 and 103 which 
respectively carry phases A, B and C of the network. As in the 
abovereferenced U.S. Pat. No. 4,939,617, there is illustrated a two 
terminal system with the "local" (or "first") terminal shown on the left 
and a "remote" (or "second") terminal shown on the right. The section of 
transmission line between the local and remote terminals is protected, as 
in the abovereferenced Patent, with a system that utilizes circuit 
breakers designated 111, 112 and 113 for the local terminal, and 121, 122, 
and 123 for the remote terminal. The circuit breakers may be of any 
suitable construction and are well known in the art. Main current 
transformers 131, 132 and 133 are provided to sense current on conductors 
101, 102 and 103, respectively, at the local terminal, and main current 
transformers 141, and 142 and 143 are provided to sense current on the 
conductors 101, 102 and 103, respectively, at the remote terminal. In the 
present embodiment, auxiliary current transformer circuits 131A, 132A and 
133A are provided at the local terminal and are respectively coupled to 
the main current transformers, 131, 132 and 133, as shown, to obtain 
voltages (generally designated V.sub.out) that are coupled to local 
terminal control subsystem 150. Similarly, auxiliary current transformer 
circuits 141A, 142A and 143A are provided at the remote terminal and are 
respectively coupled to the main current transformers 141, 142 and 143, as 
shown, to obtain voltages (also generally designated V.sub.out) that are 
coupled to remote terminal control subsystem 170. The common returns of 
the main current transformers 131, 132 and 133 and the respective 
auxiliary transformer circuits 131A, 132A and 133A at the local terminal 
flow through a common (ground current) auxiliary transformer circuit 134A, 
which provides a voltage representative of total ground current to the 
local terminal control subsystem 150. Similarly, the common returns of the 
main current transformers 141, 142 and 143 and the auxiliary transformer 
circuits 141A, 142A and 143A at the remote terminal flow through a common 
(ground current) auxiliary transformer circuit 144A, which provides a 
voltage representative of total ground current to the remote terminal 
control subsystem 170. The local terminal control subsystem 150 is coupled 
with a local terminal transmitter/receiver subsystem 160. Similarly, the 
remote terminal control subsystem 170 is coupled with a remote terminal 
transmitter/receiver subsystem 180. 
Briefly, as described in the referenced U.S. Pat. No. 4,939,617, in 
operation of the apparatus of FIG. 1, the current values [for each phase, 
and for ground current] sensed by the current transformer circuits at the 
local and remote terminals are time integrated between zero crossings to 
obtain signal parameter values that are stored in association with 
respective time indications. Signal parameter values are communicated 
between the local and remote transmitter/receiver subsystems (160 and 180, 
respectively), and are used to determine the status of the transmission 
line (100) being monitored. The status indications can be used, for 
example, as control signals to control the tripping of circuit breakers 
(111-113, and/or 121-123), as appropriate. Reference can be made to said 
U.S. Pat. No. 4,939,617 for further detail. The present invention relates 
to improvements in transformer circuits, such as the current transformer 
circuits 131A-134A and 141R-144R used in FIG. 1. 
Referring to FIG. 2, there is shown a block diagram of an improved circuit 
in accordance with an embodiment of the invention. The circuit can be 
utilized as a current transformer circuit (e.q. 131A, or any of the other 
current transformer circuits 132A-134A or 141A-144A) of the FIG. 1 
apparatus, and in various other transformer applications. A current 
transformer 210 is provided, and has a primary winding 211, a sense 
winding 212, and a feedback winding 213. An input current, i.sub.in, such 
as a sampled current from a phase of the AC power transmission line of 
FIG. 1, is received by the primary winding. The sense winding 212 has one 
end coupled to ground reference potential, and its other end coupled to a 
sense winding circuit represented by the block 220. The output of sense 
winding circuit 220 is coupled to one end of the feedback winding 213, the 
other end of which is coupled to an output circuit represented by the 
block 240. The output of circuit 240 is utilized as an output signal 240A 
which, in the present embodiment, is a voltage V.sub.out that is 
representative of the input current, i.sub.in. Proportionality is 
generally desirable in applications of this type. The output signal is 
also coupled to a feedback circuit 250 which is coupled back to sense 
winding circuit 220 and is represented in FIG. 2 as including a low-pass 
filter that comprises a series resistor R1 and a capacitor C1 coupled 
between an end of resistor R1 and ground reference potential. Signals 259, 
to be described, are received by a control signal generator circuit 260 to 
produce a control signal 260A that controls an electronic switch 270. The 
switch 270 is operative, when closed, to couple the capacitor C1 to ground 
reference potential for a duration that depends upon the duration of the 
control signal 260A. In the present embodiment, the signals 259 are 
available in the local terminal control subsystem 150, as described in the 
U.S. Pat. No. 4,939,617, which is incorporated herein by reference. [They 
are also, of course, available in the substantially identical remote 
terminal control subsystem 170 for the associated transformer circuits.] 
The signals 259 include an indication of the time between successive zero 
crossings, or a signal derived therefrom. If the half-cycle time is 
outside a predetermined range, (e.g. 7.5 ms to 9.0 ms, for a nominal 60 
Hz) there is a likelihood of a power line disturbance with a substantial 
DC component, so a half-cycle duration signal can be used, together with a 
measure of peak amplitude, to obtain an indication of a high probability 
of an offset of at least a given magnitude at the primary of transformer 
210. When the condition is present, the control signal generator 260 (to 
be described in conjunction with FIG. 4) produces a control signal 260A 
that discharges capacitor C1 for a predetermined time (or sequence of 
times) to prevent the problem of saturation of the input amplifier of the 
sense winding circuit, as will be described further hereinbelow. It will 
be understood that other circuits can be utilized for generating an 
indication of offsets at the transformer primary. 
Referring to FIG. 3, there is shown a more detailed diagram of the FIG. 2 
embodiment. The transformer 210 includes primary winding 211, sense 
winding 212, and feedback winding 213, as first shown in FIG. 2. The sense 
winding circuit 220 includes an operational amplifier 221 which may be, 
for example, an OP77 sold by Precision Monolithics Inc. of Santa Clara, 
Calif. The operational amplifier 221 has its inverting input terminal 221i 
coupled to the ungrounded end of sense winding 212, and its non-inverting 
input terminal 221n coupled to the junction between the resistor R1 and 
the capacitor C1 of the low-pass filter of feedback circuit 250. 
Protective diodes CR1 and CR2 are connected, with opposite polarity, 
between the inverting input of operational amplifier 221 and ground 
reference potential. The operational amplifier 221, and other amplifiers 
hereof conventionally have power supply pins (not shown) coupled to a 
power supply and have decoupling capacitors (not shown) coupled with the 
power supply pins. The output of operational amplifier 221 is coupled to 
the input of a buffer amplifier 222 whose output is protected, as shown, 
by coupling via diodes CR3 and CR4 to supply voltages +Vbb and -Vbb. An AC 
feedback link, comprising capacitor C2 and resistor R2, is coupled between 
the output of buffer amplifier 222 and the inverting input 221i of 
operational amplifier 221, and provides AC stability to the circuit. The 
output of buffer amplifier 222 is coupled, via analog switch 290, to one 
end of feedback winding 213, the other end of which is coupled to output 
circuit 240. 
In the output circuit 240, the feedback winding 213 is coupled to the 
inverting input 241i of an operational amplifier 241. The non-inverting 
input 241n of operational amplifier 241 is coupled to ground reference 
potential via a resistor R3. Protective diodes CR7 and CR8 are coupled 
between ground reference potential and the inverting input 241i of 
operational amplifier 241. The output of operational amplifier 241 is 
coupled to the input of a buffer amplifier 242. The output of buffer 
amplifier 242 is the previously indicated circuit output 240A (FIG. 2), 
and is also fed back to the inverting input 221i of operational amplifier 
241 by the parallel combination of variable resistor R4 and limiter 
network 244. The limiter network may comprise a zener diode bridge, and is 
operative to limit the output voltage to the range of interest. 
The circuit of FIGS. 2 and 3 allows use of a small lighter-weight 
transformer core to perform a function that would ordinarily require a 
much heavier transformer, and operates without the previously described 
problem of saturation of the input amplifier by DC offsets. In an example, 
transformer 210 has a single turn primary winding 211, a 1000 turn sense 
winding 212, and a 4000 turn feedback winding 213. Of course, other 
numbers of turns and turn ratios can be used. Overall accuracy depends on 
the sense/feedback turns ratio, but not on the actual number of turns. The 
larger this ratio, the larger the error component and the larger the 
amount of uncancelled flux in the transformer core. There must be some 
uncancelled flux because, as is generally the case for a feedback control 
loop, a minimum error signal is needed in order to develop a feedback 
control signal. In operation, the sense winding circuit, which includes 
operational amplifier 221 and buffer stage 222, amplifies the signal 
developed across the sense winding 212. The output of buffer 222 provides 
a current that drives feedback winding 213 in a manner to reduce the 
signal on the sense winding to near zero. A small uncancelled flux 
remains, and the core selected for the transformer will only be required 
to handle this uncancelled flux without saturating. 
The current required in the feedback winding is equal to the primary 
current multiplied by the primary/feedback turns ratio. The buffer 
amplifier 222 provides this current. The output stage, which includes 
operational amplifier 241 and buffer 242, acts to convert the feedback 
winding current into the output voltage signal V.sub.out. Even though the 
limiter 244 controls the level of the output signal, the current through 
the feedback winding will continue to increase in a linear fashion. If 
this were not the case, the input transformer would saturate at the point 
where linear operation ceased. Resistor R4 is used to scale the system. 
Diodes CR1, CR2, CR7 and CR8 are used to protect the inputs of the 
amplifiers 221 and 241, respectively, against possible high voltages. 
High voltage may exist in this circuit if DC power is removed from the 
amplifier stages while primary current continues to flow in the 
transformer 210. Analog switch 290, along with diodes CR3-CR6 are used to 
protect the output of the buffer 222 against this condition. When DC power 
is removed, the analog switch 290 will open, disconnecting the buffer 
stage output from the high voltage that can exist on the feedback winding. 
CR5 and CR6 will shunt the feedback winding voltage to ground to protect 
the analog switch. Resistor R2 and capacitor C2 provide frequency 
compensation for the input stage to protect against oscillations. Resistor 
R1 and capacitor C1 provide an overall DC-stabilizing feedback signal 
along with the low pass filtering required to remove the AC component of 
the output signal from the amplifier input. 
Referring to FIG. 4, there is shown a block diagram of an embodiment of the 
circuit 260 for generating the control signal 260A that controls the 
switch 270 (FIGS. 2 and 3). At each zero crossing of the signal being 
monitored, the block 150 (FIG. 1) produces a count representative of the 
time elapsed since the previous zero crossing. This count is indicated as 
being available on one of the lines 259, and is coupled to comparators 261 
and 262. The comparator 261 receives, as its other input, a reference 
signal that represent 7.5 ms, and is operative to produce an output when 
the received count represents less than 7.5 ms. The comparator 262 
receives as its other input a reference signal representative of a count 
of 9.0 ms, and produces an output when the received count represents more 
than 9.0 ms. The outputs of comparators 261 and 262 are coupled to an OR 
gate 263 whose output is, in turn, one input to an AND gate 268. The other 
line 259 received by the circuit 260 is the output voltage V.sub.out (from 
output circuit 240), which is coupled to a peak detector 265. The output 
of the peak detector is coupled to a latch 266 whose output is one input 
to a comparator 267. The other input to comparator 267 is a predetermined 
reference level. The output of comparator 267 is the other input to the 
AND gate 268. The zero crossing that results in a new count also enables 
read-out by the peak detector 265, resets the peak detector, and enables 
read-in by the latch. 
In operation, the circuit 260 produces an output when the just-completed 
half-cycle has a duration outside the range 7.5 to 9.0 ms and the peak 
voltage output during said half-cycle indicates a primary current of at 
least a certain predetermined peak value. In the present embodiment, this 
is used as an indication of a high probability of a substantial current 
offset at the transformer primary. The control signal will be present 
during the subsequent half-cycle, and will operate to discharge the 
capacitor C1 to prevent saturation of the input amplifier 221. The control 
signal will persist, on a half-cycle lagging basis, for as long as both 
necessary conditions are present. In particular, it is seen that arrival 
of a new count (immediately after occurrence of a zero-crossing) will 
reset the peak detector 265, and the previously stored peak will be read 
and held by latch 266 for comparison with the reference by comparator 267. 
Also, if the count is outside the prescribed limit, one of the comparators 
261 or 262 will produce an output which will, in turn, result in a high 
output from OR gate 263, at least until the next count arrives. Thus, the 
output of AND gate 268 will be present for every half-cycle that 
immediately succeeds a half-cycle during which the indicated conditions 
are present. It will be understood that for counts longer than 9.0 ms, a 
specified maximum count can be used or a separate timer can be utilized to 
trigger an output when a predetermined time has been exceeded. 
The invention has been described with reference to a particular preferred 
embodiment, but variations within the spirit and scope of the invention 
will occur to those skilled in the art. For example, it will be understood 
that other techniques can be used to obtain the output signal, for example 
more directly from the signal sensed at the sense winding. Also, it will 
be recognized that other suitable techniques can be utilized for sensing 
the presence of offsets at the transformer primary, and that other circuit 
arrangements can be utilized to implement discharging of the capacitor 
when offsets of a particular magnitude are detected. Applicant's have 
noted that there are various ways in which the control signal can be 
successfully applied (for example, based on continuously updated 
half-cycle application, as shown, or for a specific period after detection 
of the offset, etc.). The duration of the control signal is found to not 
be critical, as long as there is sufficient discharge of the capacitor 
upon occurrence of a substantial offset. Further, while an embodiment has 
been described in terms of a current transformer circuit, the principles 
of the invention are also applicable to potential transformer circuits.