Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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CA 02542193 2006-04-06
ELECTRICAL POWER DISTRIBUTION CONTROL
SYSTEMS AND PROCESSES
TECHNICAL FIELD
[00011 The disclosure relates to electrical power distribution
systems, processes and apparatus and power management in power
distribution systems. More particularly, the present disclosure relates
to power conservation and selective power regulation in power
distribution systems.
BACKGROUND
j0002] In electrical power distribution systems, several needs
compete and must be simultaneously considered in managing electrical
power distribution. A first concern has to do with maintaining
delivered electrical power voltage levels within predetermined limits.
A second concern relates to overall efficiency of electrical power
generation and distribution. A third concern relates to these and other
concerns in light- of changing electrical loading of the system and
variations in the character of the loading. A fourth concern relates to
power system management under conditions associated with an
increased probability of compromise of large scale ability to deliver
appropriate power.
[00031 It is generally desirable to manage a power grid to reduce
overall power consumption while maintaining adequate delivered
voltage minimum and maximum levels across the system. In other
words, the voltage levels actually delivered to various users need to be
kept within predetermined limits while delivering power efficiently,
without undue power loss in the delivery system or power grid,
including the power generation equipment. As power usage within the
system changes, in accordance with diurnal, weekly and seasonal
factors, among others, need for regulation of power distribution
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changes as well. To an extent, some of these- changes are reasonably
predictable, however, other aspects of these changes may not be readily
predictable.
[0004] Predictable changes in system loading are forecast by
integrating power demand over time and considering this draw together
with other factors, such as increased outdoor temperature and known
diurnal variation patterns. For example, when summer heat results in
increased power demand for air conditioning during the course of the
day, fast food power demand associated with the end of the work day
may indicate that a power shortage is imminent. Typically,
measurements of power demand and delivered voltage are made every
few seconds, filtered to reveal variations with periodicities on the order
of a few minutes or longer, and adjustments to voltage are made
perhaps once or twice an hour. This is called "automated conservation
voltage reduction" and is intended to reduce overall energy demand.
[0005] However, compromise of power delivery capability due, for
example, to extreme weather conditions (e.g., gale winds affecting the
distribution system) or unforeseen decrease in available power (e.g.,
generator malfunction) is not necessarily amenable to precise
forecasting but is observable. As a result, there is need for dynamic
system adjustment in response to observed changes in system capacity,
conditions and loading.
100061 Increased probability of compromise of large scale ability to
deliver appropriate power may include increased probability of system-
wide failure or blackout of an area, where "system-wide failure" could
mean either a large grid being shut down or a smaller grid being
isolated from a larger grid, with a potential result that the smaller grid
then would be shut down or malfunction. In some cases, grid -failure
may be caused by automated shutdown of one or more generators in
response to determination of grid conditions ill-suited to the generator
in order to obviate catastrophic generator failure.
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[0007] The conditions associated with an increased probability of
compromise of large scale ability to deliver appropriate power are
varied, and can range from "brownout" situations to complete
disruption of electrical service or "blackouts". Some types of power
consumption relate to relatively vital concerns, such as hospitals,
infrastructural support systems (telephone, police, fire protection,
electrical traffic signals and the like) and others relate to more
quotidian concerns, such as air conditioning, fast food operations and
industrial operations such as aluminum smelters and the like, as
equipment is added to or removed from service, for example.
[0008] The latter types of concerns can present a high electrical
draw at certain times of day. However, interruption of power delivery
to such operations does not usually present life-threatening
consequences when such operations are without electrical power.
[0009] Further, in the event of severe disruption or demand, grid
systems used for delivery of electrical power can experience
catastrophic failure when load conditions presented to generators in the
system are such that one or more electrical generators are automatically
shut down or disconnected from the system. This situation obviously
places increased demand or even less suitable loading conditions on
other generators or grids to which the grid is coupled. As a result,
other generators or grids coupled to the affected grid may disconnect
from the affected grid, potentially resulting in a blackout. Such
blackouts can be extremely widespread in electrical generation and
distribution systems employed multiple coupled grids each having
electrical generation capability.
[0010] Prior art power regulation systems include fusing, opening
switches at a power station or substation to remove load components, or
sending out trucks with technicians to manually open switches to
remove portions of the load from the system, or to manually adjust
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power regulators and set points. These methods are not amenable to
rapid, dynamic load adjustment or rapid, dynamic power management.
[00111 Another prior art system provides equipment at the user site
that disables high load appliances, such as hot water heaters, on
demand. This may be based on forecasting of anticipated excess
demand. Such systems are known as "demand side control" systems.
These tend to be expensive, in part because the number of control
switches is high.
[0012] Needed are systems, apparatus and processes for (i)
optimizing efficiency of power delivery while maintaining delivered
voltage levels within acceptable limits under changing conditions for
electrical power demand and (ii) coping with conditions associated with
an increased probability of compromise of large scale ability to deliver
appropriate power in such a way as to avoid compromise of critical
concerns and to further avoid catastrophic electrical system failure.
SUMMARY
[0013] In one aspect, the present disclosure describes a process for
power distribution regulation. The process for power distribution
regulation includes filtering data from electrical sensors to provide
conditioned data representative of a portion of a power distribution grid
and determining, by a controller and based in part on the conditioned
data, when an increase or decrease in an output parameter from one
regulator of a plurality of regulators in the power distribution grid will
reduce system power consumption. The process also includes
increasing or decreasing the associated output electrical parameter in
response to the controller determining that such will reduce system
power consumption.
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BRIEF DESCRIPTION OF THE DRAWINGS
100141 Fig. I is a block diagram of an electrical power distribution
system, which is an exemplary environment suitable for implementation
of the presently-disclosed concepts.
[0015] Fig. 2 is a block diagram of a power controller useful in the
system of Fig. 1.
[0016] Fig. 3 is a block diagram of an example of a portion of a
power distribution system using the power controller of Fig. 2.
[0017] Fig. 4 is a flow chart of a process for managing the
electrical power distribution system of Fig. 1.
[0018) Fig. 5 is a flow chart of a process for operating the power
controller of Fig. 2.
[0019] Fig. 6 is a flow chart of a process for managing the
electrical power distribution system of Fig. 1.
[0020] Fig. 7 is a flow chart of a process for stabilizing the
electrical power distribution system of Fig. 1.
[00211 Fig. 8 is a graph of amplitude and phase response for a low
pass filter.
[0022] Fig. 9 is a block diagram depicting a control system in
accordance with one embodiment.
DETAILED DESCRIPTION
Introduction
[00231 Methods and apparatus for implementing stabilized closed-
loop control of delivered voltage in electric power distribution systems
are disclosed. The disclosed concepts facilitate regulation of the
delivered distribution voltage within predefined bounds, consistent with
the adjustment capabilities of regulators such as regulating
transformers.
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Environment
[00241 Fig. 1 is a block diagram of an electrical power distribution
system 10, which is an exemplary environment suitable for
implementation of the presently-disclosed concepts. The power
distribution system 10 includes a power station 12, which may be
coupled to a power source or sink via a high voltage bus 14. In one
embodiment, the power station 12 includes one or more generators. In
one embodiment, the power station 12 distributes power delivered via
the bus 14. In one embodiment, the power station 12 delivers power to
other power distribution systems via the bus 14. As will be
appreciated, the role of the power station 12 may change with time and
demand, i.e., it may supply excess power to other systems when local
load conditions permit and it may be supplied with power at other times
when local load conditions require such.
[0025] The power station 12 includes one or more group
controllers 16. Power is distributed via buses 18 from the power
station 12 to one or more substations 20. In turn, each substation 20
delivers power further "downstream" via buses 22. It will be
appreciated that a series of - voltage transformations are typically
involved in transmission and distribution of electrical power via the
various power stations 12 and substations 20 and that the system 10
being described exemplifies such systems that may include additional
or fewer layers of transformation and distribution.
[0026] The substation 20 delivers electrical power via buses 22 to
one or more power regulation devices 24, which may include a local
controller 26. In turn, the power regulation devices 24 deliver
electrical power further downstream via buses 28. Ultimately,
electrical power is coupled to a sensor 30 and/or to a user 32. Serisors
30 tend to be associated with critical loads such as hospitals.
100271 In one embodiment, the electrical power is coupled to a
sensor 30 capable of determining electrical parameters associated with
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power consumption and transmitting those assessed parameters to the
associated local controller 26 and/or to the group controller 16. It will
be appreciated that any medium suitable to data transmission may be
employed, such as radio links, which may utilize spread spectrum
coding techniques or any suitable carrier modulation of spectrum
management methods suitable for data communications, point-to-point
radio links, fiber optical links, leased lines, data signals coupled via
power lines or buses, telephone links or other infrastructural data
communications paths. In some embodiments, such may also be
conveniently collateral to power distribution system elements (e.g.,
coaxial cables employed for data transmission such as are often
employed in cable television systems).
100281 In one embodiment, the sensor 30 measures voltage and is also
part of an electrical meter used for measuring the amount of electrical power
used and thus for determining billing data, such as a conventional Automatic
Meter Reader or AMR. In one embodiment, the sensor 30 is equipped to
assess line voltage delivered to the user 32, or "delivered voltage". In one
embodiment, the sensor 30 is equipped to measure current.
[0029] In one embodiment, the local controller is configured to respond
to several associated sensors. This may be accomplished by dynamically
determining which one or ones of an associated plurality of sensors is
providing data most relevant to determining how to most effectively adjust the
associated output electrical parameter. Effective control of power delivered
by
the associated power regulation device 24 is determined by selecting between
the associated sensors, dependent upon changes in current draw in different
loads controlled by the power regulation device 24, load shifts or voltage
changes. In one embodiment, the selection tends to be responsive to the sensor
that results in optimal power conservation.
[0030] In one embodiment, the sensor 30 is equipped to assess
power factor, also known as VAR or Volt Amperes Reactive, that is,
the phase shift induced by inductive or capacitive loads. Power factor
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can be significant because transmission losses known as 12 R losses can
increase when the currents associated with driving the load increase
without necessarily delivering more total work to the load.
100311 These losses can result in situations where the total power
demanded from the power station 12 or substation 20 actually decreases
when line voltage to the user 32 increases. One example of such a
situation is where the load is highly inductive and the amount of work
accomplished is controlled primarily by the amount of current drawn by
the load, e.g., loads including electrical motors. ,
[0032] Conventional power distribution systems provide some
correction of or management of power factor or VAR by switching
reactive elements, such as shunt capacitors, into or out of the system at
strategic locations. These conventional systems do not attempt to
reduce losses by voltage adjustment.
[0033] Conventional Supervisory Control And Data Acquisition.
(SCADA) systems have not in the past been associated with
incremental voltage controllers. In particular, such systems have not
been affiliated with controllers that are equipped to test for conditions
where a decrease in delivered voltage can reduce overall power
consumption by providing improved power factor.
[00341 In the presently-disclosed system, such a controller
advantageously also effectuates data collection and logging. In one
embodiment, at least the group controller 16 records a conventional
system data log for tracking voltage, current, kiloWatt hours and power
factor or kilo volt-amp reactive power and the like over time. In one
embodiment, at least the group controller 16 records a conventional
event log for tracking load tap control data, voltage regulation data and
breaker operations and the like over time. In one embodiment, at least
the group controller 16 records a conventional status log for tracking
position of load tap controls, voltage regulator setting, breaker settings
and the like over time.
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100351 In one embodiment, the group controller 16 records
minimum and maximum values for conventional electrical parameters
such as voltage, kiloWatt flow, KVAR and the like versus time. In one
embodiment, such conventional data are collected at regular intervals,
such as every thirty seconds or every minute. Other suitable intervals
can also be used. In any case, suitable criteria for determining such a
sampling interval is typically two-fold: a) the magnitude- and
frequency-of-variation of the observed process (i.e., system parameter)
- for example, the variation in distribution system loading, both real
and reactive, and the resulting effect on remote line voltage; and b) the
intended use of the observed process, which can include identification
of, for example, statistical measures or spectra of selected distribution
system parameters (voltage, current, VARS, etc.). In one embodiment,
additional such conventional data logs are recorded by local controllers
26 as well. In this context of control system, "parameter" means a
constant, a coefficient, or other numerical configuration entity that
alters the behavior of a control system in predictable ways. As
discussed immediately above, parameter refers to observed signals,
measurements or the like.
100361 Fig. 2 is a block diagram of a power controller 24 for use in
the system 10 of Fig. 1. The power controller 24 includes the local
controller 26 of Fig. 1. The local controller 26 is linked to the group
controller 16 via a data path 34 and is linked to the downstream sensors
30 of Fig. 1 via a data path 36. The power controller 24 accepts input
electrical energy VIN via a bus 38 that is coupled to a voltage
regulator 40. In one embodiment, the voltage regulator 40 comprises a
conventional autotransformer employing a make-before-break variable
tap that is set in conformance with command signals communicated
from the local controller 16 via a data path 42.
(00371 The power controller 24 also optionally includes a data path
44 coupled to switches 46. The switches 46 couple elements 48 for
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power factor management into or out of the circuit in response to
commands from the local controller 26. In one embodiment, the
elements 48 comprise conventional capacitors that are switched into or
out of the circuit in conformance with commands from the local
controller 26.
[00381 A sensor 50 is coupled to the local controller 26 via a data
path 52. The sensor 50 measures electrical parameters associated with
electrical energy leaving the power controller 24, such as kiloWatt
hours, current, voltage and/or power factor. The power controller 24
delivers electrical energy VOUT for downstream distribution via a
bus 54.
[0039] In one embodiment, the local controller 26 regulates power
delivery subject to overriding commands from the group controller 16.
In one embodiment, the power controller 24 increments (or decrements)
line voltage at the 120/240 volt distribution level. In one embodiment,
the power controller 24 changes output voltage in increments of 5/8 %,
or about .75 volt steps at the 120 volt basis. In one embodiment, when
larger changes in voltage are desirable, the power controller 24 allows a
stabilization interval of between forty seconds and two minutes
between an increment and evaluation of system parameters prior to
making a next incremental voltage change.
[0040] In one embodiment, the power controller 24 maintains
delivered line voltage in band of voltages ranging from about 110 volts
or 114 volts to about 126 volts to 129 volts, with 117 volts being
exemplary, and with a reduced level of about 110 to 100 volts being
applicable in emergency or brownout situations. Relevant standards in
this regard include those of the American National Standards Institute
(ANSI), C84.1-1995, and the Canadian Standards Association (CSA),
CAN-3-C235-83, reaffirmed in 2000.
[0041] In one embodiment, multiple power controllers 24 are
situated downstream of a master controller 24. For example, in
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aluminum smelting plants, such an arrangement may be advantageous
in order to provide a recommended voltage or current to the smelting
cells, and to optimize energy costs.
100421 In silicon refining plants, power control can be crucial to
maintaining the melt at the appropriate temperature and also for
maintaining an appropriate rotation speed in Czochralski crystal growth
apparatus. As a result, the criticality of power regulation depends on
the end use to which the user puts the power. Programming parameters
used in the local controller 26 of the power controllers 24 can be set in
light of these needs to effect the desired power regulation.
(0043) In some power distribution situations, power control is
important because the contractual arrangements between the user and
the service provider result in increased power rates for a period, such as
a year, if a maximum or peak amount of power contracted for is
exceeded even once. Accordingly, such users have incentives to
regulate power use to obviate exceeding that contractual amount.
[0044J Fig. 3 is a block diagram of an exemplary system 60
illustrating application of the power controller 24 of Fig. 2. In the
exemplary system 60, electrical power is distributed at a first voltage,
such as 115 kiloVolts, over bus 62. The electrical power is stepped
down to a reduced voltage, such as 12.5 kiloVolts, by a transformer 64,
and is transmitted downstream via a bus 66. A billing meter 68 may be
coupled to the bus 66. The local controller 26 includes one or more
processors 69.
[0045] Taps 70 and 72 are coupled to a power monitor PM 74 in
the local controller 26 to allow the processor 69 to monitor electrical
parameters associated with the power controller 24. In one
embodiment, the power monitor PM 74 monitors voltage. In one
embodiment, the power monitor PM 74 monitors power factor. In one
embodiment, the power monitor PM 74 monitors electrical power. In
one embodiment, the power monitor PM 74 monitors current. A
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conventional recloser or circuit breaker 76 is coupled in series with the
bus 66 and is coupled to the processor 69 in the local controller 26 via a
data path 78, allowing monitoring and/or control of the recloser 76.
[0046] The processor 69 in the local controller 26 is coupled-to the
group controller 16 (Fig. 1) via data path 34. In this example, a
conventional modem 79 is employed for bidirectional data transfer.
100471 A voltage regulator 80 is coupled in series in the bus 66.
The voltage regulator 80 is responsive to control signals delivered from
the processor 69 in the local controller 26 via a data path 82, and the
local controller 26 also is able to collect status data from the voltage
regulator 80 via this data path.
[0048] Electrical power is then transferred downstream via the bus
66, which may include line voltage monitors LVM 84 deployed at
strategic locations in the distribution system and in data communication
with the local controller 26. In one embodiment, a step-down
transformer, instrument transformer, potential transformer or transducer
86 located near the point of use transforms the intermediate voltage
employed on the bus 66 to voltages suitable for sensing equipment such
as a sensing module 88. The device 86 is calibrated to permit readings
corresponding to user voltages but is not necessarily as precise as
transformers used to transform intermediate transmission voltage levels
to end use voltage levels or in conjunction with power metering
purposes.
[00491 The module 88 for measuring electrical parameters
associated with delivered power and/or voltage is typically located at or
near the transformer or device 86, between or near the transformer or
device 86 and the end user 32 (Fig.' 1), and may include power
measurement devices PMD 89 for billing purposes. The module 88 is
in data communication with the local controller 26 via a data path, in
this example, via a radio 90 that exchanges radio signals with a radio
92 that is coupled to the processor 69 in the local controller 26.
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[0050] Data communications via the various links may be effected
using any known or conventional data transfer protocol, method and/or
infrastructure. Non-limiting examples of transactions protocols usable
under the present teachings include UCATM, ModbusTM , ASCII, DNP3,
etc. Non-limiting examples of usable physical infrastructure include
coaxial cable, twisted pair, RF, infra-red, fiber optic link, etc. UCA is
a registered mark owned by Electric Power Research Institute, Inc.,
Palo Alto, California, 94303. Modbus is a registered mark owned by
Gould, Inc., Rolling Meadows, Illinois, 60008.
100511 Fig. 4 is a flow chart of a process P 1 for managing the
electrical power distribution system 10 of Fig. 1.
100521 The process P 1 begins with a step S 1. In the step S 1, the
local power controller 24 of Figs. 1 through 3 increments or decrements
at least one parameter associated with electrical power that is being
distributed, such as line voltage. The process P 1 then waits for a
predetermined interval for the system to settle, which, in one
embodiment, may range from about forty seconds to two minutes.
[0053] In a query task S2, the process P1 determines if the actions
taken in the step S 1 resulted in a decrease in power consumption.
When the query task S2 determines that.. the actions taken in the step S 1
resulted in an increase in power consumption, control passes to steps S3
and S4. When the query task S2 determines that the actions taken in
the step S 1 resulted in a decrease in power consumption, control passes
to a step S5.
[0054] In the step S3, the actions taken in the step S 1 are reversed.
In other words, when the query task S2 determines that overall power
consumption increases when the voltage decreases, the power controller
24 then returns to that voltage setting initially present and waits for the
system to settle in the step S3. The process P1 then increases the
voltage in the step S4 and again waits for the system to settle.
Similarly, when the query task S2 determines that overall power
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consumption increases when the voltage increases, the power controller.
24 returns to that voltage setting initially present and waits for the
system to settle in the step S3. The process P 1 then decreases the
voltage in the step S4 and again waits for the system to settle.
Following the step S4, control passes back to the query task S2.
(0055) The increments in voltage are subject to predetermined
voltage maximum and minimum values, which may in turn depend on
or be changed in response to system conditions. In other words, if the
voltage is initially at the predetermined minimum, the process P 1 tests
the system with an increase in voltage but not a decrease.
[0056) When the query task S2 determines that the power
consumption has decreased, the process P 1 iterates the steps S 1 and S2
(which may include steps S3 and S4) in a step S5. The iteration of the
step S5 continues until no further decrease in power consumption is
observed. In other words, the process P1 determines a line voltage
consistent with reducing overall power consumption.
[0057] The process P1 then sets the line voltage to the optimum
voltage or the voltage at which minimum power consumption occurred
in a step S6. The process P1 then ends.
[0058] Fig. 5 is a flow chart of a process P2 for operating the
power regulation devices 24 or the local controller 26 of Fig. 2. The
process P2 begins with a query task S21.
[0059] In the query task S21, the process P2 determines when a
predetermined interval has passed without a voltage adjustment
occurring. In one embodiment, the predetermined interval is in a range
of one half hour to one hour.
[0060] When the query task S21 determines that such an interval
has not passed without a voltage adjustment, control passes back to the
step S21. When the query task S21 determines that such an interval has
passed without a voltage adjustment, control passes to a step S22.
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[00611 In the step S22, a first power consumption level is
measured. Control then passes to a step S23.
[00621 In the step S23, the power controller 24 adjusts a line
voltage within predetermined limits and then waits for a predetermined
interval for the system to settle. In one embodiment, the predetermined
settling interval is in a range of from forty seconds to two minutes.
Control then passes to a step S24.
100631 In the step S24, a second power consumption level is
measured. Control then passes to a query task S25.
[0064] In the query task S25, the process P2 determines when the
second power level is less than the first power consumption level.
When the query task S25 determines that the second power
consumption level is less than the first power consumption level,
control passes to a step S26. When the query task S25 determines that
the second power consumption level is greater than the first power
consumption level, control passes to a step S27.
[0065] In the step S26, the process P2 iterates the steps S22
through S25 to determine a line voltage associated with optimal power
consumption levels and set the voltage to this level. The process P2
then ends.
[0066] In the step S27, the process P2 iterates the steps S22
through S25 but with the increment reversed from the increment or
decrement employed in the first instantiation of the step S22. Control
then passes to a step S28.
[0067] In the step S28, the process P2 determines a voltage for
optimal power consumption in the system and sets the voltage to that
level. The process P2 then ends.
[0068] Fig. 6 is a flow chart of a process P3 for managing the
electrical power distribution system 10 of Fig. 1. The process P3
begins in a query task S3 1.
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100691 In the query task S31, a group controller 16 determines
when conditions associated with an increased probability of
compromise of appropriate delivery of electrical power are present.
100701 This may be forecast from observed power consumption
trends and knowledge of prevailing conditions, analogous to situations
invoking conventional power peak demand management techniques
such as demand control, or it may be due to observable emergency
electrical disturbance caused by a catastrophe of one sort or another.
These kinds of situations have been dealt with in the past using
ON/OFF switching of one sort or another for shedding portions or all of
the load.
[0071] When the query task S31 determines that such conditions
are not present, the process P3 ends. When the query task S31
determines that such conditions are present, the group controller 16
transmits signals to local controllers 26 to cause them to set 'the power
controllers 24 to predetermined values consistent with reduction of
system power requirements in a step S32. Control then passes back to
the query task S3 1.
[0072] For example, when the system is subject to severe loading,
delivered voltage reduction may be implemented. The initial delivered
voltage might, for example, have been 117 volts. As the voltage is
being incrementally reduced towards I10 volts (representing the lower
setpoint), and the system is being monitored, a minimum in power
consumption might occur at 112 volts. The controller of the present
disclosure will locate this minimum and can set the delivered voltage to
that value. When system conditions will not support system loading,
even at the lower setpoint, the setpoints may be reset or other corrective
actions described herein may take place, depending on circumstances.
[0073] The disclosed arrangement provides greater flexibility than
prior systems in that incremental voltage or power adjustment is
possible and practical, and may be automatically implemented. In one
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embodiment, and under appropriate conditions, some users, such as
residential users and some types of commercial users, are denied power
or are provided with reduced power at a first power level, while other
users, such as hospitals, emergency facilities, law enforcement facilities
and traffic control systems, are provided with power at a second power
level that is greater than the first power level or are left at full power.
In one embodiment, multiple tiers of users are provided with various
grades of power reduction or non-reduction.
100741 In some areas, hydroelectric or other electrical power
generation systems have been extensively developed, while other areas
may not lend themselves to such development. One example of the
former occurs in the Pacific Northwest, where hydroelectric power
generation capabilities have been extensively developed. As a result,
power generation facilities in the Pacific Northwest are able to produce
more power than may be needed in that geographical area from time to
time.
100751 A delivery area such as California, on the other hand, has
extensive power needs but has limited ability to produce electrical
power, and is bordered by desert areas that also do not lend themselves
to hydroelectric power production. Thus, power stations in the Pacific
Northwest may be able to, and in fact do, sell electrici-ty generated in
the Pacific Northwest to users in other places, such as California.
[0076] This leads to some fluctuations in demand in the Pacific
Northwest power generation stations. At times, reductions in demand
in the generation area (in this example, the Pacific Northwest) require
that the system dissipate some of the electrical power that is generated
there in order to preserve synchronization of the generators with each
other and with other portions of the grid. In at least some cases, this
need to dissipate electrical power is met by coupling large resistors
across the generators. Typically, these are very large conventional
nichrome wire resistors.
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(0077] In some situations, the need to slew power into these
resistors can arise rather abruptly. For example, when weather-,
earthquake-, fire- or vehicular-driven events damage a portion of the
distribution infrastructure in the delivery area or between the delivery
area and the generation area, rapid changes in system dynamics are
possible.
[0078] However, the controllers 16 and 24 of the present disclosure
can be advantageously employed to increase voltage that is delivered in
the generation area and in other portions of the grid that is serviced by
generators in that area. The controllers 16 and 24 can adjust delivered
voltages upward but stay within the predetermined limits appropriate
for normal power service. As a result, system stability is increased.
[0079] Fig. 7 is a flow chart of an exemplary process P4 for
stabilizing the electrical power distribution system 10 of Fig. i using
controllers such as 16 and 24.
[0080] The process P4 begins with a query task S41. In the query
task S41, the process P4 determines when an increase in delivered
voltage, within the predetermined voltage setpoints, will result in
improved stability for the system 10.
[0081] When the query task S41 determines that an increase in
voltage is appropriate for improving stability of the system 10, control
passes to a step S42.
[0082J In the step S42, a controller in the system such as the group
controller 16 increases the voltage delivered to the users 32. Typically,
the increase in voltage is incremental, as discussed hereinbefore, and is
followed by a predetermined settling period and then data collection
regarding system parameters. Control then passes back to the query
task S41 to determine if another increase in voltage is appropriate for
the system 10.
[0083] When the query task S41 determines that an increase in
voltage is inconsistent with an increase in stability of the system 10, or
18
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.. . . . . .
CA 02542193 2006-04-06
is not appropriate for such system 10, control passes to the query task
S43.
[0084] In the query task S43, the process P4 determines when a
decrease in delivered voltage is appropriate for increasing stability for
the systern. 10 and is consistent with the predetermined setpoints. When
the query task S43 determines that a decrease in delivered voltage is
appropriate for increasing system stability, control passes to a step S44.
100851 In the step S42, a controller in the system such as the group
controller 16 decreases the voltage delivered to the users 32. Typically,
the decrease in voltage is incremental, as discussed hereinbefore, and is
followed by a predetermined settling period and then data collection
regarding system parameters. Control then passes back to the query
task S41 to determine if an increase in voltage is appropriate for the
system 10. The process P4 then ends.
[0086) It will be appreciated that the processes P 1 through P4 are
cooperative with each other and with other processes carried out in the
system 10. For example, when the system 10 no longer poses a
stability issue, the process P4 may be terminated and power control
may be determined by other factors in the system. Additionally, the
processes P1 through P4 are structured to maintain delivered voltage at
an appropriate level, such as within a range determined by
programmable setpoints. Processes P 1 through P4 may employ suitable
methods from the engineering arts of automatic control theory and
signal processing, including filtering, system identification, and
prediction or extrapolation methods.
[0087] From the foregoing, it is apparent the present disclosure
describes systems, processes and apparatus which can be utilized to
monitor and manage electrical power distribution. Further, the
disclosed systems, processes and apparatus permit power conservation
and also can provide more robust power delivery under inclement
power system loading conditions. In addition, the systems, processes
19
CA 02542193 2006-04-06
and apparatus of the present disclosure are cost effective when
compared with other power management devices.
[0088] Empirical studies have shown that overall system operation
may be improved by incorporating signal processing and conditioning
techniques, prediction of load variations based on measured and
recorded system operation parameters and known ambient condition
variation patterns affecting energy demand.
[00891 For example, the voltage regulator 40 of Fig. 2 is generally
capable of a finite number of switching events during the useful life of
the regulator 40.
100901 Typical voltage regulating autotransformers operated by the
electric utilities effect changes to their output voltage by mechanical
selection of predetermined winding taps. The mechanical selection
process limits the effective operating duty cycle and the useful life of
the regulator. As a result, it is desirable to implement a scheme which
controls the delivered voltage such that energy conservation or other
objectives are achieved while operating the voltage regulators in a
manner that is consistent with their limitations.
[0091] Additionally, the response time of such regulators 40 does
not favor attempting to correct high frequency "spikes" such as may
result from switching of high draw loads such as large motors. As a
result, filtering signals derived from the sensors 30 of Fig. 1 to limit
frequency of voltage adjustment by the regulators 40 to about twelve to
fifteen switching events per day extends regulator life while
maintaining control of the delivered voltage. Accordingly filtering
operations may be applied to the sensed signals to improve systein
operation; in the present context, low pass filtering is indicated.
[0092] Delay behavior in filtering operations affects control system
operation and thus design. In many closed loop control applications,
including certain process control problems in which well-behaved step
response is desirable (that is, step response which exhibits neither
CA 02542193 2006-04-06
overshoot nor oscillation), filters manifesting constant group delay in
the passband may be employed. In the present context, delivered
voltage regulation is implemented using discrete tap selection in the
final control element, resulting in small disturbances to the distribution
circuits which are stepwise signals. Since stability of the controlled
variable (the circuit voltage) is a design consideration in the automatic
voltage control systems considered here, constant group delay low pass
filtering may be usefully applied to the measured voltage signals.
100931 In one embodiment, a discrete-time finite impulse response
low pass filter having a linear phase response, a cutoff frequency of
about 3 milliHertz, and a constant total group delay of about 240
seconds is implemented digitally as a cascade of filter sections provides
effective signal conditioning. The cutoff frequency may be varied or
tailored to specific applications based on knowledge of load
characteristics.
10094) A finite impulse response or FIR filter is a filter whose
output signal vn depends only upon prior observations of the input
signal and may be modeled as vn = Eb;võ_;, where bi represents filter
coefficients and võ_; represents input voltages. This type of filter is
conveniently realizable as a two stage filter implemented as software
using reduced numerical precision compared to some other types of
filters. However, such FIR filters are not limited to two stage
implementations - cascade arrangements of three or more stages (i.e.,
multiple-stage) can also be used. FIR filters tend to exhibit less
sensitivity to the numerical precision of the corresponding processor,
and overall better performance (i.e., interpreted as conformance of the
implemented filter to its design parameters), than do other types of
software implemented filters.
[00951 In a multiple stage filter, each successive stage operates at a
slower sampling rate than the preceding stage, with the sampling rate
determined by the spectral cutoff characteristics of the preceding stage.
21
CA 02542193 2006-04-06
For example, the first stage may use a sample rate corresponding to one
sample per fifteen seconds and may be an eighth or ninth order stage.
The second stage may use a sample rate corresponding to one sample
per 60 to 90 seconds, as determined by the cutoff frequency coc of the
first stage, and may be a sixth order stage. The secon.d stage would
then provide an output signal every 240 to 300 seconds without
aliasing. The filter design is motivated by a desire to achieve suitable
spectral cutoff characteristics whilst reducing the overall group delay of
the multistage system. In general, as filter order increases in low-pass
FIR designs, filter delay increases and this may have deleterious effects
on closed-loop system stability, because closed loop control systems
are susceptible to destabilization both by transport and other
measurement delays and by signal artifacts introduced by sensors,
transducers, filters or other signal processing operations in the
measurement process. In this application, linear phase or constant
group delay, whereby all passband spectral components of the measured
voltage signals are delayed equally, corresponds to a lack of "ringing"
that could otherwise result in system instability. In other words, linear
phase finite impulse response filters can inhibit overshoot or ringing
behavior in the filtered signal. In this type of application, a lack of
delay and amplitude distortion is important for stable system operation.
An exemplary infinite impulse response filter characteristic& suitable
for such applications uses the Bessel characteristic, which provides a
good approximation to linear phase response in the passband.
[00961 Fig. 8 is a graph of amplitude 800 and phase 810 response
for a low pass filter. The amplitude response 800 shows a cutoff
frequency wc which is defined as the frequency at which the filter
response is one-half of the peak response value. The phase response
810 depends linearly on frequency.
[0097] Use of linear prediction techniques can improve system
operation when such prediction is employed in order to remove delays
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CA 02542193 2006-04-06
associated with the low-pass filtering of observed and controlled
signals. These techniques model the subject signals as a combination
of moving average and auto-regressive structures, and generally the
coefficients associated with such structures are estimated continually
during the operation of the control system that is connected to the
process or system that is generating the subject signals. The estimation
may be carried out by methods suited to the properties of the subject
signals and the requirements of the aforementioned signal model, and
may include such methods as gradient search, spectral factorization, or
recursive least squares.
[0098] The removal of filtering delay can improve system stability
and facilitate rapid control system response in emergency situations.
Linear prediction techniques treat the input signal as conforming to a
dynamical model comprising both spectral and stochastic components,
in which the stochastic component is assumed to evolve as a Gaussian
process which is stationary over a suitable estimation interval. In the
present case, a signal that is second-order stationary over an interval of
approximately 30 minutes is consistent with prediction capabilities of
approximately 5 to 10 minutes.
[0099] Linear prediction models comprise a class of methods
employed for the temporal extrapolation of stationary signals. In such
models, the one step ahead predicted value yõ+I of a signal yn can be
formulated as a function of a number of prior signal samples, or yõ+1 =
Edjyn_j, where the coefficients dj depend on the statistics extracted from
the signals and on the algorithm being employed, and are estimated
using a process such as those noted supra. Other examples include
autocorrelation methods, such as Levinson-Durbin recursion of a
corresponding Toeplitz matrix.
1001001 Fig. 9 is a block diagrammatic view depicting a closed-loop
control system (system) 100 in accordance with one embodiment of the
present teachings. The system 100 includes an Artificial Neural
23
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CA 02542193 2006-04-06
Network (ANN) 102. The ANN 102 can be defined by any suitable
such neural network configured to receive conditioned input signals
(135, 137 and 139 - described hereinafter), to predict a future value of a
predetermined electrical parameter, and to provide a signal 104 in
accordance with that prediction. As depicted in Fig. 9, such an
electrical parameter is defined by a remote line voltage 132 (discussed
in further detail hereinafter). The system 100 also includes a voltage
controller 106. The voltage controller 106 is configured to receive the
signal 104 from the ANN 102, as well as voltage boundary (or range,
VB) data 110. The voltage controller 106 then derives a signal 107 that
is coupled to regulator control logic 108. Regulator control logic 108
also receives input signals from an operating mode (OM) entity 112 and
a regulator status (RS) entity 114, respectively.
[00101] As used herein, the term Voltage Boundary or Primary
Voltage Bounds (VB) refers to the desired limits within which the
primary voltage, as measured at a remote location, shall be controlled.
As also used herein, the term Operating Mode (OM) is a mnemonic
defining a class of allowable controller actions, specifically including
at least the following:
Idle - control of a remote voltage is inactive, and distribution
voltage is set by other means including, for example, open loop (i.e., no
feedback) control;
Engaged - control of remote voltage is enforced entirely by the
system (i.e., embodiments thereof) described herein; and
Suspended - a temporary condition in which voltage adjustment
is disallowed, pending resolution of dispatcher activity or a system
anomaly such as, for example, communications interference.
As further used herein, the term Regulator Status (RS) refers to a group
of signals that, when taken together, permit determination of the
operational readiness of the regulating autotransformer, load-tap-
changer transformer, and/or their control interface devices.
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. . . .
CA 02542193 2006-04-06
1001021 As also depicted in Fig. 9, the regulator control logic 108
provides an output signal 116. The output signal 116 is properly
formatted and conditioned to be received by a voltage regulator 122
within an electrical distribution substation 120. The voltage regulator
122 can be defined, for example, by a make-before-break variable tap
regulator, configured respond to the output signal 116 by increasing or
decreasing the distribution line voltage 132 that is derived from
transmission line voltage 148. A regulator status signal 118, indicative
of present tap selection or other suitable status information, is provided
back to the regulator status entity 114. As depicted in Fig. 9, the
substation 120 includes a potential transformer 126 and a current
transformer 128 that provide respective electrical parameter signals to a
meter 124. The meter 124 can be selected and/or configured to sense
and/or derive any one or more electrical parameters such as, for
example, line voltage, line current, real power, power factor, VARs,
apparent power, etc. The meter 124 provides a signal 130 indicative of
one or more such electrical parameters.
[001031 Still referring to Fig. 9, the system 100 includes first,
second and- third low pass filters (LPFs) 134, 136 and 138, respectively.
The LPF 134 receives an ambient temperature signal from a sensor 140
and provides a corresponding filtered (i.e., conditioned) temperature
signal 135. The LPF 136 receives a remote line voltage (e.g., line
voltage 132) signal by way of a line voltage monitor 142 and associated
potential transformer 144, and provides a filtered remote line voltage
signal 137. Similarly, the LPF 138 receives the signal 130 from the
meter 124 and provides a filtered signal 139. It is to be understood that
each of the filtered (conditioned) signals 135, 137 and 139 can be
referred to as indicative of am environmental or electrical distribution
system parameter, respectively. Furthermore, other selected parameters
(not shown in Fig. 9) such as, for example, ambient humidity, core
temperature of a transformer, daylight intensity, remote location power
CA 02542193 2006-04-06
factor, VAR compensator activity, etc., can be sensed and provided as
conditioned signals by way of corresponding sensors and low pass filter
arrangements. In any event, the respective conditioned signals 135,
137 and 139 are provided as inputs to the Artificial Neural
Network 102. In this way, the system 100 of Fig. 9 depicts a closed-
loop control stratagem capable of providing stable remote line voltage
to one or more users 146.
[00104] The system 100 of Fig. 9 also includes a training entity 150.
The training entity is defined by any suitable logic configured to
receive the conditioned signal 137 (i.e., representative of remote line
voltage 132) and the Artificial Neural Network prediction signal 104,
and to derive a training (or learning) vector 154 that is communicated
to the ANN 102. In another embodiment (not shown), the training
entity 150 can be configured to also receive either (or both) of the
conditioned signals 135 and 139. In any case, the training entity 150
serves in the ongoing development or "maturity" of the Artificial
Neural Network 102 with respect to (at least) these observed system
variables or parameters. Thus, the ANN 102 evolves by way of an
ever-improving set of observational vectors correlated with predicted
future values of the electrical parameter(s) of interest (e.g., remote line
voltage, remote power factor, local line voltage, etc.). As also depicted
in Fig. 9, at least the ANN 102, the voltage controller 106, the regulator
control logic 108 and the training entity 150 define a single control
entity 109 of the system 100.
[00105] The Artificial Neural Network (ANN) 102 of Fig. 9 is used
as a predictor of measured voltages (e.g., remote line voltage, etc.).
Such an ANN (e.g., 102) can be defined, for example, an ADALINE
formulation, a Multi-layer Perceptron or a Radial Basis Function
Network (RBF). In the case of an RBF, either Euclidean or
Mahalanobis distances can be used in the formulation. In one preferred
embodiment, an RBF Network is used wherein the input variables must
26
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. . . .
CA 02542193 2006-04-06
include measured remote voltage (after low pass filtering), and may
include ambient temperature at remote and/or substation, and/or real
power demand and/or apparent power demand. Also in such an
embodiment, the predicted observed variable (i.e., parameter) is the
measured remote voltage (after filtering). Table 1 below summarizes
an exemplary formulation:
Table 1: Exemplary Neural Network Formulation
MLP: typical activation functions for the input and hidden layers are
sigmoid, hyperbolic tangent, linear;
MLP: typical activation functions for the output layers are linear,
bounded linear, step;
RBF: typical activation functions for the input and hidden layers are
probability density functions;
RBF: typical activation functions for the output layer are sigmoid,
linear.
The inputs to the ANNs are time series vectors from among the
above variables (parameters), and inputs are zero mean, but trends are
not removed. Also, the present teachings anticipate that ANNs can be
trained to precisely eliminate the respective delays introduced through
low pass filtering. Furthermore, ANNs can be trained with respect to
future remote voltage observations (beyond low pass filtering delay), so
that control moves can be anticipated.
[00106] To briefly summarize, one or more suitable embodiments
can be defined wherein an Artificial Neural Network is used to predict
future values of low pass filtered input signals (i.e., variables, or
parameters). Non-limiting examples of such variables include ambient
and/or equipment temperature; local and/or remote line voltage and or
current; locaI and/or remote real power (Watts), imaginary power
(VARs), and/or apparent power (VAs); etc. Other pertinent variables or
operating parameters can also be measured, filtered (if needed) and
used as input signals to the neural network. Additional information
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regarding Artificial Neural Networks, which can be used in accordance
with the present teachings, is obtainable as follows:
ADALINE: B. Widrow and M. E. Hoff Jr., Adaptive Switching
Circuits, IRE WESCON Conv. Rec., part 4, 96-104 (1960); -
Multi-layer Perceptron: F. Rosenblatt, The Perceptron: A
probabilistic model for information storage and organization in the
brain, Psychological Review 65, 386-408 (1958); and
Radial Basis Function Networks: D. S. Broomhead and D. Lowe,
Multivariable functional interpolation and adaptive networks, Complex
Systems 2, 321-355 (1988).
[00107] In contrast to prior art systems, the present systems,
processes and apparatus provide great variability of system parameters,
such as multiple, different delivered voltage levels, within
predetermined limits. For example, all users can be incrementally
adjusted up or down together, or some users may be adjusted to a first
degree while other users are adjusted to another degree or to separate,
differing degrees. Such advantageously provides new flexibility in
power distribution control, in addition to providing new methods of
adjustment.
[00108] In compliance with the statute, the subject matter has been
described in language more or less specific as to structural and
methodical features. It is to be understood, however, that the subject
matter is not limited to the specific features shown and described, since
the systems, processes and apparatus herein disclosed comprise
exemplary forms of putting the disclosed concepts into effect. The
disclosed subject matter is, therefore, claimed in any of its forms or
modifications within the proper scope of the appended claims
appropriately interpreted in accordance with the doctrine of
equivalents.
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100109] As would be known to one skilled in the art, capacitors are
commonly used to correct the lagging power factor (pf) that otherwise results
from electrically energizing an inductive load, such as an A.C. motor. In a
typical such instance, some number of capacitors are selectively and/or
automatically switched into electrically parallel arrangement with the
inductive
load in an atteinpt to restore the overall power factor back toward unity.
This
approach is generally effective because the reactive behaviors of inductors
and
capacitors are respectively opposite one another, thus having a nullifying
effect
on the phase difference between the applied A.C. voltage and corresponding
current. That is, the use of correction capacitors (i.e., VAR compensators)
tends to reduce or substantially eliminate overall VARs within at least a
portion
of an electrical distribution system.
(00110] It is also known however that the engagement and
disengagement of such capacitors affects the voltage within a distribution
system, as well as the phase angle between voltage and current. In turn,
control
of the voltage regulator (e.g., at a substation, etc.) within a distribution
system
also affects the phase angle between voltage and current, such that automatic
VAR compensation systems may respond with a counter-acting effect.
Therefore, it is known that voltage regulation and VAR compensation tend to
be interactive (i.e. the respective control systems are dynamically coupled) --
frequently with undesirable consequences such as, for example, oscillating
distribution voltage, wear on equipment, and overall distribution system
instability. Consequently, as claimed herein, one aspect of the present
invention may be summarized as dynamically decoupling the voltage regulator
controller, whether group or local, from a VAR compensator. This may be
accomplished as known in the prior art through the use of a Relative Gain
Array, as taught by both Bristol and McAvoy: E.H. Bristol, "On a new measure
of interaction for multivariable process control," IEEE Transactions on
Automatic Control AC-I1, p.p. 133-134, 1966; T.J. McAvoy, "Interaction
Analysis," Instrument Society of America, 1983.
29
