EMS FUNCTIONS
System dispatchers at the EMS are required to make short-term (nextday) and long-term (prolonged) decisions on operational and outage scheduling
on a daily basis. Moreover, they have to be always alert and prepared to deal
with contingencies that may arise occasionally. Many software and hardware
functions are required as operational support tools for the operator. Broadly
speaking, we can classify these functions in the following manner:
• Base functions
• Generation functions
• Network functions
Each of these functions is discussed briefly in this section.
Base Functions
The required base functions of the EMS include:
• The ability to acquire real time data from monitoring equipment
throughout the power system.
• Process the raw data and distribute the processed data within the
central control system.
Data acquisition (DA) acquires data from remote terminal units (RTUs)
installed throughout the system using special hardware connected to the real
time data servers installed at the control center. Alarms that occur at the
substations are processed and distributed by the DA function. In addition,
protection and operation of main circuit breakers, some line isolators,
transformer tap changers and other miscellaneous substation devices are
provided with a sequence of events time resolution.
on a daily basis. Moreover, they have to be always alert and prepared to deal
with contingencies that may arise occasionally. Many software and hardware
functions are required as operational support tools for the operator. Broadly
speaking, we can classify these functions in the following manner:
• Base functions
• Generation functions
• Network functions
Each of these functions is discussed briefly in this section.
Base Functions
The required base functions of the EMS include:
• The ability to acquire real time data from monitoring equipment
throughout the power system.
• Process the raw data and distribute the processed data within the
central control system.
Data acquisition (DA) acquires data from remote terminal units (RTUs)
installed throughout the system using special hardware connected to the real
time data servers installed at the control center. Alarms that occur at the
substations are processed and distributed by the DA function. In addition,
protection and operation of main circuit breakers, some line isolators,
transformer tap changers and other miscellaneous substation devices are
provided with a sequence of events time resolution.
Data Acquisition
The data acquisition function collects, manages, and processes
information from the RTUs by periodically scanning the RTUs and presenting
the raw analog data and digital status points to a data processing function. This
function converts analog values into engineering units and checks the digital
status points for change since the previous scan so that an alarm can be raised if
status has changed. Computations can be carried out and operating limits can be
applied against any analog value such that an alarm message is created if a limit
is violated.
information from the RTUs by periodically scanning the RTUs and presenting
the raw analog data and digital status points to a data processing function. This
function converts analog values into engineering units and checks the digital
status points for change since the previous scan so that an alarm can be raised if
status has changed. Computations can be carried out and operating limits can be
applied against any analog value such that an alarm message is created if a limit
is violated.
Supervisory Control
Supervisory control allows the operator to remotely control all circuit
breakers on the system together with some line isolators. Control of devices can
be performed as single actions or a line circuit can be switched in or out of
service.
breakers on the system together with some line isolators. Control of devices can
be performed as single actions or a line circuit can be switched in or out of
service.
Alarm Processor
The alarm processor software is responsible to notify the operator of
changes in the power system or the computer control system. Many
classification and detection techniques are used to direct the alarms to the
appropriate operator with the appropriate priorities assigned to each alarm.
changes in the power system or the computer control system. Many
classification and detection techniques are used to direct the alarms to the
appropriate operator with the appropriate priorities assigned to each alarm.
Logical Alarming
This provides the facility to predetermine a typical set of alarm
operations, which would result from a single cause. For example, a faulted
transmission line would be automatically taken out of service by the operation of
protective and tripping relays in the substation at each end of the line and the
automatic opening of circuit breakers. The coverage would identify the
protection relays involved, the trip relays involved and the circuit breakers that
open. If these were defined to the system in advance, the alarm processor would
combine these logically to issue a priority 1 alarm that the particular power
circuit had tripped correctly on protection. The individual alarms would then be
given a lower priority for display. If no logical combination is viable for the
particular circumstance, then all the alarms are individually presented to the
dispatcher with high priority. It is also possible to use the output of a logical
alarm as the indicator for a sequence-switching procedure. Thus, the EMS
would read the particular protection relays which had operated and restore a line
to service following a transient fault.
operations, which would result from a single cause. For example, a faulted
transmission line would be automatically taken out of service by the operation of
protective and tripping relays in the substation at each end of the line and the
automatic opening of circuit breakers. The coverage would identify the
protection relays involved, the trip relays involved and the circuit breakers that
open. If these were defined to the system in advance, the alarm processor would
combine these logically to issue a priority 1 alarm that the particular power
circuit had tripped correctly on protection. The individual alarms would then be
given a lower priority for display. If no logical combination is viable for the
particular circumstance, then all the alarms are individually presented to the
dispatcher with high priority. It is also possible to use the output of a logical
alarm as the indicator for a sequence-switching procedure. Thus, the EMS
would read the particular protection relays which had operated and restore a line
to service following a transient fault.
Sequence of Events Function
The sequence of events function is extremely useful for post-mortem
analysis of protection and circuit breaker operations. Every protection relay, trip
relay, and circuit breaker is designated as a sequence of events digital point.
This data is collected, and time stamped accurately so that a specified resolution
between points is possible within any substation and across the system.
Sequence of events data is buffered on each RTU until collected by data
acquisition automatically or on demand.
analysis of protection and circuit breaker operations. Every protection relay, trip
relay, and circuit breaker is designated as a sequence of events digital point.
This data is collected, and time stamped accurately so that a specified resolution
between points is possible within any substation and across the system.
Sequence of events data is buffered on each RTU until collected by data
acquisition automatically or on demand.
Historical Database
Another function includes the ability to take any data obtained by the
system and store in a historical database. It then can be viewed by a tabular or
graphical trend display. The data is immediately stored within the on-line
system and transferred to a standard relational data base system periodically.
Generally, this function allows all features of such database to be used to
perform queries and provide reports.
system and store in a historical database. It then can be viewed by a tabular or
graphical trend display. The data is immediately stored within the on-line
system and transferred to a standard relational data base system periodically.
Generally, this function allows all features of such database to be used to
perform queries and provide reports.
Automatic Data Collection
This function is specified to define the process taken when there is a
major system disturbance. Any value or status monitored by the system can bedefined as a trigger. This will then cause a disturbance archive to be created,
which will contain a pre-disturbance and a post-disturbance snapshots to be
produced.
major system disturbance. Any value or status monitored by the system can bedefined as a trigger. This will then cause a disturbance archive to be created,
which will contain a pre-disturbance and a post-disturbance snapshots to be
produced.
Load Shedding Function
This facility makes it possible to identify that particular load block and
instruct the system to automatically open the correct circuit breakers involved.
It is also possible to predetermine a list of load blocks available for load
shedding. The amount of load involved within each block is monitored so that
when a particular amount of load is required to shed in a system emergency, the
operator can enter this value and instruct the system to shed the appropriate
blocks.
instruct the system to automatically open the correct circuit breakers involved.
It is also possible to predetermine a list of load blocks available for load
shedding. The amount of load involved within each block is monitored so that
when a particular amount of load is required to shed in a system emergency, the
operator can enter this value and instruct the system to shed the appropriate
blocks.
Safety Management
Safety management provided by an EMS is specific to each utility. A
system may be specified to provide the equivalent of diagram labeling and paper
based system on the operator’s screen. The software allows the engineer, having
opened isolators and closed ground switches on the transmission system, to
designate this as safety secured. In addition, free-placed ground symbols can be
applied to the screen-based diagram. A database is linked to the diagram system
and records the request for plant outage and safety document details. The
computer system automatically marks each isolator and ground switch being
presently quoted on a safety document and records all safety documents using
each isolator or ground switch. These details are immediately available at any
operating position when the substation diagram is displayed.
system may be specified to provide the equivalent of diagram labeling and paper
based system on the operator’s screen. The software allows the engineer, having
opened isolators and closed ground switches on the transmission system, to
designate this as safety secured. In addition, free-placed ground symbols can be
applied to the screen-based diagram. A database is linked to the diagram system
and records the request for plant outage and safety document details. The
computer system automatically marks each isolator and ground switch being
presently quoted on a safety document and records all safety documents using
each isolator or ground switch. These details are immediately available at any
operating position when the substation diagram is displayed.
Generation Functions
The main functions that are related to operational scheduling of the
generating subsystem involve the following:
• Load forecasting
• Unit commitment
• Economic dispatch and automatic generation control (AGC)
• Interchange transaction scheduling
Each of these functions is discussed briefly here.
generating subsystem involve the following:
• Load forecasting
• Unit commitment
• Economic dispatch and automatic generation control (AGC)
• Interchange transaction scheduling
Each of these functions is discussed briefly here.
Load Forecasting
The total load demand, which is met by centrally dispatched generating
units, can be decomposed into base load and controlled load. In some systems,
there is significant demand from storage heaters supplied under an economy
tariff. The times at which these supplies are made available can be altered using
radio tele-switching. This offers the utility the ability to shape the total demand
curve by altering times of supply to these customers. This is done with theobjective of making the overall generation cost as economic and
environmentally compatible as possible. The other part of the demand consists
of the uncontrolled use of electricity, which is referred to as the natural demand.
It is necessary to be able to predict both of these separately. The base demand is
predicted using historic load and weather data and a weather forecast.
units, can be decomposed into base load and controlled load. In some systems,
there is significant demand from storage heaters supplied under an economy
tariff. The times at which these supplies are made available can be altered using
radio tele-switching. This offers the utility the ability to shape the total demand
curve by altering times of supply to these customers. This is done with theobjective of making the overall generation cost as economic and
environmentally compatible as possible. The other part of the demand consists
of the uncontrolled use of electricity, which is referred to as the natural demand.
It is necessary to be able to predict both of these separately. The base demand is
predicted using historic load and weather data and a weather forecast.
Unit Commitment
The unit commitment function determines schedules for generation
operation, load management blocks and interchange transactions that can
dispatched. It is an optimization problem, whose goal is to determine unit
startup and shutdown and when on-line, what is the most economic output for
each unit during each time step. The function also determines transfer levels on
interconnections and the schedule of load management blocks. The software
takes into account startup and shutdown costs, minimum up and down times and
constraints imposed by spinning reserve requirements.
The unit commitment software produces schedules in advance for the
next time period (up to as many as seven days, at 15-minute intervals). The
algorithm takes the predicted base demand from the load forecasting function
and the predicted sizes of the load management blocks. It then places the load
management blocks onto the base demand curve, essentially to smooth it
optimally. The operator is able to use the software to evaluate proposed
interchange transactions by comparing operating costs with and without the
proposed energy exchange. The software also enables the operator to compute
different plant schedules where there are options on plant availability
operation, load management blocks and interchange transactions that can
dispatched. It is an optimization problem, whose goal is to determine unit
startup and shutdown and when on-line, what is the most economic output for
each unit during each time step. The function also determines transfer levels on
interconnections and the schedule of load management blocks. The software
takes into account startup and shutdown costs, minimum up and down times and
constraints imposed by spinning reserve requirements.
The unit commitment software produces schedules in advance for the
next time period (up to as many as seven days, at 15-minute intervals). The
algorithm takes the predicted base demand from the load forecasting function
and the predicted sizes of the load management blocks. It then places the load
management blocks onto the base demand curve, essentially to smooth it
optimally. The operator is able to use the software to evaluate proposed
interchange transactions by comparing operating costs with and without the
proposed energy exchange. The software also enables the operator to compute
different plant schedules where there are options on plant availability
Economic Dispatch and AGC
The economic dispatch (ED) function allocates generation outputs of
the committed generating units to minimize fuel cost, while meeting system
constraints such as spinning reserve. The ED functions to compute
recommended economic base points for all manually controlled units as well as
economic base points for units which may be controlled directly by the EMS.
The Automatic Generation Control (AGC) part of the software
performs dispatching functions including the regulation of power output of
generators and monitoring generation costs and system reserves. It is capable of
issuing control commands to change generation set points in response to
changes in system frequency brought about by load fluctuations.
Interchange Transaction Scheduling Function
This function allows the operator to define power transfer schedules on
tie-lines with neighboring utilities. In many instances, the function evaluates the
economics and loading implications of such transfers.
the committed generating units to minimize fuel cost, while meeting system
constraints such as spinning reserve. The ED functions to compute
recommended economic base points for all manually controlled units as well as
economic base points for units which may be controlled directly by the EMS.
The Automatic Generation Control (AGC) part of the software
performs dispatching functions including the regulation of power output of
generators and monitoring generation costs and system reserves. It is capable of
issuing control commands to change generation set points in response to
changes in system frequency brought about by load fluctuations.
Interchange Transaction Scheduling Function
This function allows the operator to define power transfer schedules on
tie-lines with neighboring utilities. In many instances, the function evaluates the
economics and loading implications of such transfers.
Current Operating Plan (COP)
As part of the generation and fuel dispatch functions on the EMS at a
typical utility is a set of information called the Current Operating Plan (COP)
which contains the latest load forecast, unit commitment schedule, and hourly
average generation for all generating units with their forecast operating status.
The COP is typically updated every 4 to 8 hours, or as needed following major
changes in load forecast and/or generating unit availability.
typical utility is a set of information called the Current Operating Plan (COP)
which contains the latest load forecast, unit commitment schedule, and hourly
average generation for all generating units with their forecast operating status.
The COP is typically updated every 4 to 8 hours, or as needed following major
changes in load forecast and/or generating unit availability.
Network Analysis Functions
Network applications can be subdivided into real-time applications and
study functions. The real time functions are controlled by real time sequence
control that allows for a particular function or functions to be executed
periodically or by a defined event manually. The network study functions
essentially duplicate the real time function and are used to study any number of
“what if” situations. The functions that can be executed are:
• Topology Processing (Model Update) Function.
• State Estimation Function.
• Network Parameter Adaptation Function
• Dispatcher Power Flow (DPF)
• Network Sensitivity Function.
• Security Analysis Function.
• Security Dispatch Function
• Voltage Control Function
• Optimal Power Flow Function
Topology Processing (Model Update) Function
The topology processing (model-updating) module is responsible for
establishing the current configuration of the network, by processing the
telemetered switch (breakers and isolators) status to determine existing
connections and thus establish a node-branch representation of the system.
study functions. The real time functions are controlled by real time sequence
control that allows for a particular function or functions to be executed
periodically or by a defined event manually. The network study functions
essentially duplicate the real time function and are used to study any number of
“what if” situations. The functions that can be executed are:
• Topology Processing (Model Update) Function.
• State Estimation Function.
• Network Parameter Adaptation Function
• Dispatcher Power Flow (DPF)
• Network Sensitivity Function.
• Security Analysis Function.
• Security Dispatch Function
• Voltage Control Function
• Optimal Power Flow Function
Topology Processing (Model Update) Function
The topology processing (model-updating) module is responsible for
establishing the current configuration of the network, by processing the
telemetered switch (breakers and isolators) status to determine existing
connections and thus establish a node-branch representation of the system.
State Estimation Function
The state estimator function takes all the power system measurements
telemetered via SCADA, and provides an accurate power flow solution for the
network. It then determines whether bad or missing measurements using
redundant measurements are present in its calculation. The output from the state
estimator is given on the one-line diagram and is used as input to other
applications such as Optimal Power Flow.
telemetered via SCADA, and provides an accurate power flow solution for the
network. It then determines whether bad or missing measurements using
redundant measurements are present in its calculation. The output from the state
estimator is given on the one-line diagram and is used as input to other
applications such as Optimal Power Flow.
Network Parameter Adaptation Function
This module is employed to generate forecasts of busbar voltages and loads. The forecasts are updated periodically in real time. This allows the state
estimator to schedule voltages and loads at busbars where no measurements are
available.
estimator to schedule voltages and loads at busbars where no measurements are
available.
Dispatcher Power Flow (DPF)
A DPF is employed to examine the steady state conditions of an
electrical power system network. The solution provides information on network
bus voltages (kV), and transmission line and transformer flows (MVA). The
control center dispatchers use this information to detect system violations
(over/under-voltages, branch overloads) following load, generation, and
topology changes in the system.
electrical power system network. The solution provides information on network
bus voltages (kV), and transmission line and transformer flows (MVA). The
control center dispatchers use this information to detect system violations
(over/under-voltages, branch overloads) following load, generation, and
topology changes in the system.
Network Sensitivity Function
In this function, the output of the state estimator is used to determine
the sensitivity of network losses to changes in generation patterns or tie-line
exchanges. The sensitivity parameters are then converted to penalty factors for
economic dispatch purposes.
the sensitivity of network losses to changes in generation patterns or tie-line
exchanges. The sensitivity parameters are then converted to penalty factors for
economic dispatch purposes.
Security Analysis Function
The SA is one of the main applications of the real time network
analysis set. It is designed to assist system dispatchers in determining the power
system security under specified single contingency and multiple contingency
criteria. It helps the operator study system behavior under contingency
conditions. The security analysis function performs a power flow solution for
each contingency and advises of possible overloads or voltage limit violations.
The function automatically reviews a list of potential problems, rank them as to
their effect and advise on possible reallocation of generation. The objective of
OSA is to operate the network closer to its full capability and allow the proper
assessment of risks during maintenance or unexpected outages.
analysis set. It is designed to assist system dispatchers in determining the power
system security under specified single contingency and multiple contingency
criteria. It helps the operator study system behavior under contingency
conditions. The security analysis function performs a power flow solution for
each contingency and advises of possible overloads or voltage limit violations.
The function automatically reviews a list of potential problems, rank them as to
their effect and advise on possible reallocation of generation. The objective of
OSA is to operate the network closer to its full capability and allow the proper
assessment of risks during maintenance or unexpected outages.
Security Dispatch Function
The security dispatch function gives the operator a tool with the
capability of reducing or eliminating overloads by rearranging the generation
pattern. The tool operates in real-time on the network in its current state, rather
than for each contingency. The function uses optimal power flow and constrains
economic dispatch to offer a viable security dispatch of the generating resources
of the system.
capability of reducing or eliminating overloads by rearranging the generation
pattern. The tool operates in real-time on the network in its current state, rather
than for each contingency. The function uses optimal power flow and constrains
economic dispatch to offer a viable security dispatch of the generating resources
of the system.
Voltage Control Function
The voltage control (VC) study is used to eliminate or reduce voltage
violations, MVA overloads and/or minimize transmission line losses using
transformer set point controls, generator MVAR, capacitor/reactor switching,
load shedding, and transaction MW.
violations, MVA overloads and/or minimize transmission line losses using
transformer set point controls, generator MVAR, capacitor/reactor switching,
load shedding, and transaction MW.
Optimal Power Flow Function
The purpose of the Optimal Power Flow (OPF) is to calculate
recommended set points for power system controls that are a trade-off between
security and economy. The primary task is to find a set of system states within a
region defined by the operating constraints such as voltage limits and branch
flow limits. The secondary task is to optimize a cost function within this region.
Typically, this cost function is defined to include economic dispatch of active
power while recognizing network-operating constraints. An important limitation
of OPF is that it does not optimize switching configurations.
Optimal power flow can be integrated with other EMS functions in
either a preventive or corrective mode. In the preventive mode, the OPF is used
to provide suggested improvements for selected contingency cases. These cases
may be the worst cases found by contingency analysis or planned outages.
In the corrective mode, an OPF is run after significant changes in the
topology of the system. This is the situation when the state estimation output
indicates serious violations requiring the OPF to reschedule the active and
reactive controls.
It is important to recognize that optimization is only possible if the
network is controllable, i.e., the control center must have control of equipment
such as generating units or tap-changer set points. This may present a challenge
to an EMS that does not have direct control of all generators. To obtain the full
benefit of optimization of the reactive power flows and the voltage profile, it is
important to be able to control all voltage regulating devices as well as
generators.
The EMS network analysis functions (e.g., Dispatcher Power Flow and
Security Analysis) are the typical tools for making many decisions such as
outage scheduling. These tools can precisely predict whether the outage of a
specific apparatus (i.e., transformer, generator, or transmission line) would cause
any system violations in terms of abnormal voltages or branch overloads.
In a typical utility system, outage requests are screened based on the
system violation indications from DPF and SA studies. The final approval for
crew scheduling is granted after the results from DPF and SA are reviewed.
recommended set points for power system controls that are a trade-off between
security and economy. The primary task is to find a set of system states within a
region defined by the operating constraints such as voltage limits and branch
flow limits. The secondary task is to optimize a cost function within this region.
Typically, this cost function is defined to include economic dispatch of active
power while recognizing network-operating constraints. An important limitation
of OPF is that it does not optimize switching configurations.
Optimal power flow can be integrated with other EMS functions in
either a preventive or corrective mode. In the preventive mode, the OPF is used
to provide suggested improvements for selected contingency cases. These cases
may be the worst cases found by contingency analysis or planned outages.
In the corrective mode, an OPF is run after significant changes in the
topology of the system. This is the situation when the state estimation output
indicates serious violations requiring the OPF to reschedule the active and
reactive controls.
It is important to recognize that optimization is only possible if the
network is controllable, i.e., the control center must have control of equipment
such as generating units or tap-changer set points. This may present a challenge
to an EMS that does not have direct control of all generators. To obtain the full
benefit of optimization of the reactive power flows and the voltage profile, it is
important to be able to control all voltage regulating devices as well as
generators.
The EMS network analysis functions (e.g., Dispatcher Power Flow and
Security Analysis) are the typical tools for making many decisions such as
outage scheduling. These tools can precisely predict whether the outage of a
specific apparatus (i.e., transformer, generator, or transmission line) would cause
any system violations in terms of abnormal voltages or branch overloads.
In a typical utility system, outage requests are screened based on the
system violation indications from DPF and SA studies. The final approval for
crew scheduling is granted after the results from DPF and SA are reviewed.
Operator Training Simulator
An energy management system includes a training simulator that
allows system operators to be trained under normal operating conditions and
simulated power system emergencies. System restoration may also be
exercised. It is important to realize that major power system events are
relatively rare, and usually involve only one shift team out of six, real
experience with emergencies builds rather slowly. An operator-training
simulator helps maintain a high level of operational preparedness among the
system operators.
allows system operators to be trained under normal operating conditions and
simulated power system emergencies. System restoration may also be
exercised. It is important to realize that major power system events are
relatively rare, and usually involve only one shift team out of six, real
experience with emergencies builds rather slowly. An operator-training
simulator helps maintain a high level of operational preparedness among the
system operators.
The interface to the operator appears identical to the normal control
interface. The simulator relies on two models: one of the power system and the
other represents the control center. Other software is identical to that used in
real time. A scenario builder is available such that various contingencies can be
simulated through a training session. The instructor controls the scenarios and
plays the role of an operator within the system.
interface. The simulator relies on two models: one of the power system and the
other represents the control center. Other software is identical to that used in
real time. A scenario builder is available such that various contingencies can be
simulated through a training session. The instructor controls the scenarios and
plays the role of an operator within the system.
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