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Description  |
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FIELD OF THE INVENTION
The present invention relates generally to electric power load management
and, more particularly, to methods and apparatus for detecting tampering
with remotely installed load management terminals.
BACKGROUND OF THE INVENTION
A serious problem confronting electrical utility companies today is the
inability to handle the excessive demand placed on an electrical power
distribution system during peak demand hours. Periods of excessive demand
occur, for example, during very hot summer days when simultaneous usage of
air conditioning units is widespread. The extreme demand placed on a power
distribution system during such peak demand periods can lead to service
interruptions, such as "brown-outs." In an effort to prevent or minimize
service interruptions during peak demand periods, utility companies are
beginning to employ remotely controllable load management terminals at
selected customer locations.
A typical load management terminal has a relay connected to the power line
in series with the load. The types of loads most often targeted in a load
management system are large appliances such as water heaters and air
conditioning units. During peak demand periods, a utility can transmit a
command to a load management terminal causing that terminal to open the
relay and prevent current from flowing to the load, thereby "shedding" the
load from the power line. Some load management terminals are adapted to
receive commands directly over the power line, for example, on a high
frequency carrier. Other load management terminals employ radio receivers
for receiving remote commands.
Participation by a consumer in a utility's load management program is often
voluntary; that is, the consumer agrees to let the utility install a load
management terminal in return for some form of credit or rebate on the
consumer's electric bill. Once a load management terminal has been
installed for a particular load, the utility can begin to remotely shed
that load at various times during a peak demand period. Utilities try to
perform load shedding in such a way as to minimize the discomfort and
inconvenience to the customer. However, some discomfort and inconvenience
is inevitable.
Unfortunately, some customers will accept a load management terminal in
order to obtain financial benefit from the utility and then will tamper
with the load management terminal in an attempt to disable it.
Consequently, utilities have realized the need to detect such tampering
efforts easily without significantly adding to the cost of the terminal.
As can be expected, there are many ways to tamper with a load management
terminal, and therefore, tamper detection methods and apparatus depend on
the particular form of tampering that the utility is trying to detect.
Virtually all load management terminals have some form of outer housing
that surrounds the load management relay and other internal components.
Tampering by adjusting or circumventing the internal components,
therefore, will require opening the terminal housing. Consequently, most
load management terminals provide some sort of tamper indication when the
outer housing has been opened or breached in some manner. For example,
Rudden et. al., U.S. Pat. No. 4,977,515, describes the use of a sensor to
detect opening of the housing. Stanbury et. al., U.S. Pat. No. 4,850,010,
also describes detecting the opening of a terminal's housing.
Additionally, many current load management terminals are microprocessor
based and contain some amount of electronic memory. Subjecting such a load
management terminal to a severe magnetic field can disrupt the electronic
memory circuits and otherwise interfere with the ability of the load
management terminal to shed its load. Aforementioned U.S. Pat. No.
4,997,515 discloses a means for detecting this form of tampering.
Essentially, a Gauss detector is used to measure magnetic fields present
in the terminal and to signal a microprocessor whenever a magnetic field
strong enough to disrupt the terminal is detected.
Another form of tampering with load management terminals involves the
connection of a by-pass link to the power line in parallel with the load
control relay of the load management terminal. Such by-pass links
effectively take the load control relay out of the circuit because current
is shunted around the relay. Thus, when the normally closed contacts of
the load control relay are opened in response to a load shed command,
current will still be supplied to the load through the by-pass link. This
last form of tampering is not adequately addressed in the prior art of
load management. In the somewhat related area of electrical energy
consumption meters, however, the problem of meter tampering using a
by-pass link around the meter has been addressed. However, the solutions
have been complex and expensive, and therefore, unsatisfactory for use in
load management terminals.
For example, Fielden, U.S. Pat. No. 4,331,915, describes one method for
detecting the presence of a by-pass link across the live connection
through an electrical watt-hour meter. According to the method of Fielden,
a voltage transformer in the meter is used to induce a small voltage in
the live connection in opposition to the normal supply voltage. In the
absence of tampering, the induced voltage has a negligible effect. When a
by-pass link is connected, however, the induced voltage produces a
circulating current in the loop formed by the live connection and the
by-pass link. This induced current will be in phase opposition to the
normal supply voltage. A phase comparator is used to compare the phase of
the induced current with the phase of the supply voltage. A detected phase
opposition indicates tampering. While this solution may be satisfactory
for electrical watt-hour meters, it is not satisfactory for use in load
management terminals. Fielden's method requires too much additional
hardware, and therefore, is relatively expensive. Cost is more of a
concern with load management terminals than with electrical watt-hour
meters because, in addition to the cost of the terminal, utilities must
also provide a credit to consumers who agree to have such terminals
installed. Accordingly, the tamper detection method of Fielden does not
provide a satisfactory solution for load management terminals.
Hurley, U.S. Pat. No. 4,532,471, describes another tamper detection method
for electrical meters. Again, however, this solution is unsatisfactory for
use with load management terminals due to its complexity and resulting
cost. Hurley employs a current transformer and associated electronics to
sense a change in impedance that results from the connection of a by-pass
link around the meter. The current transformer is coupled to the power
line conductor within the meter such that the power line forms a primary
winding for the transformer and defines a primary circuit. The multi-turn
secondary winding on the current transformer defines a secondary circuit.
A by-pass link connected across the meter will cause a change in the
impedance reflected from the primary circuit into the secondary circuit. A
measuring device is connected to the current transformer for detecting
such changes. Although this method achieves tamper detection, it too is
not cost efficient for use in load management terminals due to its
complexity.
A need arises, therefore, for methods and apparatus for detecting tampering
with load management terminals that does not significantly add to the cost
of manufacturing such terminals. In particular, cost efficient methods and
apparatus are needed for detecting the connection of a by-pass link to the
power line in parallel with the load control relay of the load management
terminal in an effort to "short-circuit" around the relay. The present
invention satisfies these needs.
SUMMARY OF THE INVENTION
Briefly stated, the present invention comprises methods and apparatus for
detecting tampering with a load management terminal. Load management
terminals have a load control relay connected to the power line for
selectably decoupling the load from the power line by opening the relay.
According to one embodiment of the present invention, apparatus for
detecting tampering with a load management terminal comprises a current
sensor for sensing whether load current is flowing through the load
control relay contacts when the contacts are closed. A timer measures the
elapsed time during which no load current is sensed flowing through the
closed contacts of the load control relay. An indicator is provided for
indicating tampering when the elapsed time measured by the timer exceeds a
pre-determined time limit. The timer and indicator may comprise a
microprocessor coupled to the sensor. The indicator may further comprise a
light emitting diode coupled to the microprocessor. The tamper detection
apparatus is preferably positioned within the housing of the load
management terminal.
Preferably, the tamper detection apparatus further comprises a non-volatile
memory for storing the elapsed time measured by the timer and for storing
any tamper indication in the event of a power failure. Storage of these
parameters in a non-volatile memory enables tamper detection to resume
after power is restored. The tamper detection apparatus may further
comprise means for resetting the timer if load current is sensed before
the measured elapsed time exceeds the pre-determined limit.
In a most preferred embodiment, the current sensor comprises a voltage
detector coupled to the control coil of the load control relay for
detecting an AC voltage in the control coil. Load current flowing through
the closed contacts of a load control relay will induce an AC voltage in
the un-energized control coil of the relay. The control coil operates as a
current-to-voltage transducer. Detecting an AC voltage in the control coil
indicates that load current is flowing through the relay. Preferably, the
voltage detector comprises an AC millivolt-level zero crossing comparator.
Alternatively, the current sensor may comprise a current transformer
coupled to the power line proximate the load control relay for producing
an output voltage proportional to the current flowing through the power
line, and hence, through the relay. A comparator may be coupled to the
current transformer for comparing the output voltage to a pre-determined
threshold voltage and for providing an indication of the presence of load
current when the output voltage is greater than the threshold voltage.
In operation, the apparatus performs the following steps: (a) sensing
whether load current is flowing through the contacts of the load control
relay; (b) maintaining a measure of elapsed time during which no load
current is sensed flowing through the contacts of the load control relay;
and (c) generating a tamper indication when the elapsed time exceeds a
pre-determined time limit. As mentioned, load current is preferably sensed
by measuring the voltage, if any, induced in the control coil of the load
control relay by that load current. In a microprocessor based embodiment,
data loss in the event of a power failure is avoided by storing the
measured elapsed time in the non-volatile memory prior to complete power
failure. Any tamper indication is also stored in the non-volatile memory
at this time. If load current is sensed flowing through the closed
contacts before the measured elapsed time exceeds the pre-determined
limit, then the timer is reset.
According to a second embodiment, the tamper detection apparatus of the
present invention comprises a voltage sensor coupled to the load control
relay for sensing whether voltage is present across the relay contacts
when the relay contacts are open. A timer is provided for measuring
elapsed time during which no voltage is sensed across the open relay
contacts. An indicator provides an indication of tampering when the
elapsed time measured by the timer exceeds a predetermined time limit.
Preferably, the sensor, timer and indicator are positioned within the load
management terminal housing.
The timer and indicator may comprise a microprocessor coupled to the
sensor. The indicator may further comprise a light emitting diode coupled
to the microprocessor. Preferably, the microprocessor is optically
isolated from the sensor. A non-volatile memory may further be provided
for storing the elapsed time measured by the timer and for storing the
tamper indication provided by the indicator in the event of a power
failure. As explained above, storage of these parameters in the
non-volatile memory enables tamper detection to resume after power is
restored.
According to the second embodiment, the tamper detection apparatus may
further comprise means for holding the relay open until a voltage is
sensed across the relay. Additionally, means may be provided for resetting
the timer if a voltage is sensed before the predetermined limit expires.
In operation, tamper detection is performed by: (a) opening the relay; (b)
sensing whether voltage is present across the open relay contacts; (c)
maintaining a measure of elapsed time during which no voltage is sensed
across the open relay contacts; and (d) generating a tamper indication
when the measured elapsed time exceeds a pre-determined time limit. If
voltage is sensed before the time limit expires, then the timer is reset.
Again, to avoid data loss due to a power failure, the elapsed time
measured by the timer is stored in the non-volatile memory just prior to
complete power failure. Any tamper indication is also stored in the
non-volatile memory.
As mentioned in the Background, most residential and commercial loads, such
as water heaters and air conditioners, have a mechanical thermostat that
controls the load. Consequently, current through the load is periodically
interrupted by the thermostat. According to a third embodiment, the tamper
detection apparatus of the present invention comprises a resistance;
connected in parallel with at least the load thermostat, and a current
sensor for sensing current flow through the load control relay and for
detecting interruptions in the current flow. In some implementations, the
resistance may be connected to the power line so as to be in parallel with
the series combination of the load and thermostat. In either case, the
resistance operates to ensure at least a nominal current flow through the
load control relay even when the load thermostat contacts are open. Thus,
current flows through the load control relay continuously. An indicator is
provided for indicating tampering when an interruption in the continuous
current flow through the relay is detected by the sensor. When a utility
customer has an electronic thermostat, a constant current sink is already
provided in the load management terminal to ensure the nominal current
flow. In such cases, no additional resistance is needed.
Additional features and advantages of the present invention will become
evident hereinafter.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing summary, as well as the following detailed description of the
preferred embodiments, is better understood when read in conjunction with
the appended drawings. For the purpose of illustrating the invention,
there is shown in the drawings, embodiments that are preferred, it being
understood, however, that the invention is not limited to the specific
methods and instrumentalities disclosed. In the drawings:
FIG. 1 is a block diagram illustrating an exemplary application of a prior
art load management terminal;
FIG. 2 shows an apparatus for detecting tampering with a load management
terminal in accordance with a first embodiment of the present invention;
FIG. 2A shows a load control relay having a portion of one of the relay's
contact leads wrapped around the outer housing of the relay in accordance
with an aspect of the present invention;
FIGS. 3A and 3B comprise a flowchart illustrating details of the operation
of the apparatus of FIG. 2;
FIG. 4 shows an apparatus for detecting tampering with a load management
terminal in accordance with a second embodiment of the present invention;
FIG. 4A shows an alternate implementation of the relay control circuitry of
FIG. 4;
FIG. 4B illustrates a tamper indication signal that is provided to the
relay control circuitry of FIG. 4A;
FIGS. 5A and 5B comprise a flowchart illustrating further details of the
operation of the apparatus of FIG. 4;
FIG. 6 shows an apparatus for detecting tampering with a load management
terminal in accordance with a third embodiment of the present invention;
FIG. 7 is a detailed circuit diagram of the current sensor of FIG. 6; and
FIG. 8 illustrates an application of the third embodiment of the tamper
detection apparatus to a load management terminal that is connected to a
residential air-condition unit having an electronic thermostat.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to the drawings wherein like numerals indicate like elements
throughout, there is shown in FIG. 1 an exemplary application of a prior
art load management terminal 10. As shown, the terminal 10 is coupled to a
power line 12 for controlling the supply of power to a load 16. Power is
normally supplied to the load across terminals 14a and 14b. The load. 16
may comprise any type of power line load, such as a residential water
heater or an air conditioning unit, and may include a thermostat 18
connected in series with the load.
The load management terminal 10 comprises a load control relay 20 coupled
to the power line 12 so as to be in series with the load 16. Typically,
the relay 20 is of the normally-closed type. Relay control circuitry 26 is
coupled to the control coil 24 of the relay 20 for opening and closing the
contacts 22 of the relay 20 in response to a signal from a microprocessor
28. Microprocessor 28 controls the overall operation of the terminal 10.
As mentioned, the load control relay's contacts 22 are normally closed.
When the load 16 demands power (i.e., the load's thermostat 18 closes)
current will flow through the relay's contacts 22 to the load 16. Load
shedding is achieved by opening the contacts 22 and interrupting the flow
of current to the load 16.
In operation, the load management terminal may receive a "shed" command
from the utility company during periods of peak demand. As indicated
previously, many techniques are known for communicating with a load
management terminal to provide the terminal with a "shed" command. For
example, several techniques exist for communicating with a load management
terminal via the power line itself. Other techniques involve the use of an
R-F receiver in the terminal 10 for receiving signals transmitted from a
central station. In the example shown, shed commands are received using
any well known method and provided, via line 30, to the microprocessor 28.
In response to a "shed" command, the microprocessor 28 will provide a
signal to the relay control circuitry 26 which, in turn, will energize the
control coil 24 of the relay 20 thereby causing the contacts 22 to open.
Thus, the load 16 is effectively decoupled (i.e., "shed"!) from the power
line. The duration of the load shed interval is also determined by the
utility.
As explained in the Background of the Invention, utility companies often
provide a financial incentive to customers who are willing to have a load
management terminal installed. Unfortunately, unscrupulous customers may
try to disable the load management terminal to prevent load shedding while
retaining the financial incentive provided by the utility company. As
shown in FIG. 1, and as described in the Background, one form of tampering
not adequately addressed in the prior art involves the connection of a
by-pass link 32 to the power line 12 in parallel with the contacts 22 of
the load control relay 20. By-pass link 32 effectively removes the load
control relay 20 from the power line circuit because load current is
shunted around the relay contacts 22. Thus, when the normally closed
contacts 22 are opened in response to a load shed command, current will
still be supplied to the load 16 through the by-pass link 32. Detection of
this form of tampering is of paramount importance to utility companies. As
explained in the Background, cost efficient methods and apparatus are
needed for detecting the connection of such by-pass links. The present
invention satisfies these needs. Preferred embodiments of the present
invention are described hereinafter.
First Embodiment
FIG. 2 shows apparatus 40 for detecting tampering with a load management
terminal (LMT) in accordance with a first, most preferred embodiment of
the present invention. For purposes of illustration, FIG. 2 shows a load
control relay 20 (e.g. relay 20 of FIG. 1) coupled to a power line 12.
Preferably, the apparatus 40 shown in FIG. 2 is positioned within the,
housing of the LMT to prevent unauthorized access. According to the first
embodiment, tampering (i.e., a by-pass link 32 connected in parallel with
the load control relay 20) is detected by sensing whether current is
flowing through the normally closed contacts 22 of the load control relay
20. As explained above, if a by-pass-link 32 is present, little or no
current will flow through the closed contacts 22. Applicants recognize,
however, that an absence of current may be the result of an open
thermostat, and not the result of a by-pass link. Accordingly, tamper is
only indicated if current does not appear within a predetermined time
limit. To give the customer the benefit of any doubt, the predetermined
time limit may be relatively long, such as thirty days. If no load current
is sensed during that period, however, then tampering is probable.
According to the present embodiment, the tamper detection apparatus 40
comprises a current sensor for sensing load current flowing through the
closed contacts 22 of the load control relay 20. Preferably, the current
sensor comprises a voltage detector 42 coupled to the control coil 24 of
the load control relay for detecting an AC voltage in the coil 24. Load
current flowing through the normally closed contacts 22 of the relay 20
will induce a small AC voltage in the control coil 24. In this regard, the
control coil 24 operates as a current-to-voltage transducer with the relay
contacts 22 defining a single turn primary and the coil 24 defining the
secondary. Detection of an AC voltage in the control coil 24 indicates
that load current is flowing through the closed contacts 22 of the relay
20 and that, therefore, no by-pass link is present. As shown, the output
of the sensor (i.e., the output of the voltage detector) is coupled to a
microprocessor 28.
Because the induced voltage in the control coil 24 will be small, e.g.
10-20 millivolts, it may be desirable to increase the induced voltage.
According to a further aspect of the present invention, one or both of the
relay contact leads, or a portion of the power line connected to the
contact leads, may be wrapped around the outside of the relay 20 in a
manner that increases the induced AC voltage in the coil. FIG. 2A shows a
load control relay 20 in which a portion of a contact lead 23 is wrapped
around the outer housing 27 of the relay 20 in accordance with this aspect
of the present invention.
Referring again to FIG. 2, in the present embodiment, the voltage detector
42 comprises an AC milli-volt level zero crossing detector that employs an
operational amplifier 44. Operational amplifier 44 functions as a
comparator. The bias voltage applied to the non-inverting input 48 of the
amplifier 44 is determined by resistors R4, R5, and R6. The voltage at the
inverting input 46 is determined by resistors R1, R2, R3 and the nominal
resistance of the relay control coil 24. The switching threshold of the
amplifier 44 is determined by the relative biasing at the inputs 46, 48.
Because the voltage, if any, induced in the relay coil 24 by current
flowing through the contacts 22 may be on the order of 10-20 millivolts,
it is desirable to provide equal biasing at each input. Consequently,
resistors R1 and R5 are preferably of equal value, and the value of
resistor R4 in parallel with resistor R6 is equal to the combined values
of resistors R2 and R3 and the nominal resistance of the control coil 24.
With equal biasing, the switching threshold is well below the 10-20
millivolt level. According to one embodiment, the resistors and capacitors
have the following values: R1=100 k.OMEGA.; R2=4.9 k.OMEGA.; R3=4.9
k.OMEGA.; R4=10 k.OMEGA.; R5=100 k.OMEGA.; R6=10 .OMEGA.; C1=0.01 .mu.F
and C2=10 pF.
In operation, the output of fire amplifier 44 will switch between
logic-high and logic-low for each cycle of AC voltage in the coil 24 that
exceeds the switching threshold. When current is continuously flowing
through the contacts 22, therefore, a continuous pulse train will appear
at the output 50 of the amplifier 44. When current is not flowing through
the contacts 22, the output 50 of the amplifier 44 will remain at
logic-high. Microprocessor 28, therefore, is able to sense current flow | | |
