PSOC Computer Control Of Power system .ppt

SRI RAMAKRISHNA
INSTITUTE OF TECHNOLOGY
(An Autonomous Institution)
Topic: COMPUTER CONTROL OF POWER SYSTEMS
DR .G.KANNAYERAM,
ASSOCIATE PROFESSOR /EEE DEPARTMENT ,
SRI RAMAKRISHNA INSTITUTE OF TECHNOLOGY,
COIMBATORE – 10.

ENERGY MANAGEMENT SYSTEM (EMS)
Ø The EMS is a software system. Most utility companies purchase
their EMS from one or more EMS vendors.
These EMS vendors are companies specializing in design,
development, installation, and maintenance of EMS within ECCs.
Ø There are a number of EMS vendors in the U.S., and they hire
many power system engineers with good software development
capabilities.
During the time period of the 1970s through about 2000, almost all
EMS software applications were developed for installation on the
control centers computers.
13-06-2024 DR.G.KANNAYERAM,ASP/EEE. SRIT 2

An attractive alternative today is, however, the application service provider,
where the software resides on the vendor‟s computer and control center
personnel access it from the Internet.
Ø Benefits from this arrangement include application flexibility and reliability
in the software system and reduced installation cost.
Ø One can observe from Figure. that the EMS consists of 4 major functions:
network model building (including topology processing and state estimation),
security assessment, automatic generation control, and dispatch.

Ø These functions are described in more detail in the following subsections.
Ø Energy management is the process of monitoring, coordinating, and
controlling the generation, transmission and distribution of electrical energy.
Ø The physical plant to be managed includes generating plants that produce
energy fed through transformers to the high-voltage transmission network
(grid), interconnecting generating plants, and load centers.
Ø Transmission lines terminate at substations that perform switching, voltage
transformation, measurement, and control.
Ø Substations at load centers transform to sub transmission and distribution
levels.

These lower-voltage circuits typically operate radially, i.e., no
normally closed paths between substations through sub
transmission or distribution circuits.(Underground cable networks in
large cities are an exception.)
Ø Since transmission systems provide negligible energy storage,
supply and demand must be balanced by either generation or load.

Production is controlled by turbine governors at generating plants, and
automatic generation control is performed by control center computers
remote from generating plants.
Ø Load management, sometimes called demand- side management,
extends remote supervision and control to subtransmission and
distribution circuits, including control of residential, commercial, and
industrial loads.

EMS FUNCTIONS
1.System Load Forecasting-Hourly energy, 1 to 7 days.
2. Unit commitment-1 to 7days.
3. Economic dispatch
4. Hydro-thermal scheduling- up to 7 days.

5. MW interchange evaluation- with neighboring system
6. Transmission loss minimization
7. Security constrained dispatch
8. Maintenance scheduling
9. Production cost calculation

2. Power System Data Acquisition and
Control
A SCADA system consists of a master station that communicates with
remote terminal units (RTUs) for the purpose of allowing operators to
observe and control physical plants.
Generating plants and transmission substations certainly justify RTUs,
and their installation is becoming more common in distribution
substations as costs decrease.
.

RTUs transmit device status and measurements to, and receive
control commands and set point data from, the master station.
Communication is generally via dedicated circuits operating in
the range of 600 to 4800 bits/s with the RTU responding to
periodic requests initiated from the master station (polling)
every 2 to 10 s, depending on the criticality of the data

The traditional functions of SCADA systems are summarized:
Data acquisition: Provides telemetered measurements and status
information to operator.
b) Supervisory control: Allows operator to remotely control
devices, e.g., open and close circuit breakers. A “select before
operate” procedure is used for greater safety.
c) Tagging: Identifies a device as subject to specific operating
restrictions and prevents unauthorized operation.

d) Alarms: Inform operator of unplanned
events and undesirable operating conditions.
Alarms are sorted by criticality, area of
responsibility, and chronology.
Acknowledgment may be required

*
e) Logging: Logs all operator entry, all alarms, and selected
information.
f) Load shed: Provides both automatic and operator-initiated
tripping of load in response to system emergencies.
g) Trending: Plots measurements on selected time scales

Since the master station is critical to power system
operations, its functions are generally distributed among
several computer systems depending on specific design.
A dual computer system configured in primary and standby
modes is most common.
SCADA functions are listed below without stating which
computer has specific responsibility.

• Manage communication circuit configuration
• Downline load RTU files
• Maintain scan tables and perform polling
• Check and correct message errors
Convert to engineering units

Detect status and measurement changes
• Monitor abnormal and out-of-limit conditions
• Log and time-tag sequence of events
• Detect and annunciate alarms

Respond to operator requests to:
– Display information
– Enter data
– Execute control action
– Acknowledge alarms Transmit control action to RTUs
• Inhibit unauthorized actions
Maintain historical files
• Log events and prepare reports
• Perform load shedding

3. Automatic Generation Control
Automatic generation control (AGC) consists of two major and several minor
functions that operate online in real time to adjust the generation against
load at minimum cost.
The major functions are load frequency control and economic dispatch, each
of which is described below.
The minor functions are reserve monitoring, which assures enough reserve
on the system; interchange scheduling, which initiates and completes
scheduled interchanges; and other similar monitoring and recording
functions.

4. Load Frequency Control
Load frequency control (LFC) has to achieve three primary objectives, which are
stated below in priority order:
1. To maintain frequency at the scheduled value
2. To maintain net power interchanges with neighboring control areas at the
scheduled values
3. To maintain power allocation among units at economically desired values.

Ø The first and second objectives are met by monitoring an
error signal, called area control error (ACE), which is a
combination of net interchange error and frequency error
and represents the power imbalance between generation
and load at any instant.
Ø This ACE must be filtered or smoothed such that
excessive and random changes in ACE are not translated
into control action.

Since these excessive changes are different for different systems, the filter
parameters have to be tuned specifically for each control area.
Ø The filtered ACE is then used to obtain the proportional plus integral control
signal
Ø This control signal is modified by limiters, dead bands, and gain constants
that are tuned to the particular system.
Ø This control signal is then divided among the generating units under control
by using participation factors to obtain unit control errors (UCE).

These participation factors may be proportional to the inverse of the
second derivative of the cost of unit generation so that the units would be
loaded according to their costs, thus meeting the third objective.
Ø However, cost may not be the only consideration because the different
units may have different response rates and it may be necessary to move
the faster generators more to obtain an acceptable response.
Ø The UCEs are then sent to the various units under control and the
generating units monitored to see that the corrections take place.

This control action is repeated every 2 to 6 s. In spite of the integral
control, errors in frequency and net interchange do tend to
accumulate over time.
Ø These time errors and accumulated interchange errors have to
be corrected by adjusting the controller settings according to
procedures agreed upon by the whole interconnection.
Ø These accumulated errors as well as ACE serve as performance
measures for LFC.

The main philosophy in the design of LFC is that each
system should follow its own load very closely during
normal operation, while during emergencies;
each system should contribute according to its relative size
in the interconnection without regard to the locality of the
emergency.

Thus, the most important factor in obtaining good control of a system is its
inherent capability of following its own load.
Ø This is guaranteed if the system has adequate regulation margin as
well as adequate response capability.
Systems that have mainly thermal generation often have difficulty in
keeping up with the load because of the slow response of the units

SUPERVISORY CONTROL AND DATA ACQUISITION (SCADA)
Ø There are two parts to the term SCADA Supervisory control indicates that the
operator, residing in the energy control center (ECC), has the ability to control
remote equipment.
Ø Data acquisition indicates that information is gathered characterizing the state
of the remote equipment and sent to the ECC for monitoring purposes.
Ø The monitoring equipment is normally located in the substations and is
consolidated in what is known as the remote terminal unit (RTU).

Generally, the RTUs are equipped with microprocessors having memory and
logic capability. Older RTUs are equipped with modems to provide the
communication link back to the ECC, whereas newer RTUs generally have
intranet or internet capability.
Ø Relays located within the RTU, on command from the ECC, open or close
selected control circuits to perform a supervisory action.
Ø Such actions may include, for example, opening or closing of a circuit breaker
or switch, modifying a transformer tap setting, raising or lowering generator MW
output or terminal voltage, switching in or out a shunt capacitor or inductor, and
the starting or stopping of a synchronous condenser

Information gathered by the RTU and communicated to the ECC includes both
analog information and status indicators.
Ø Analog information includes, for example, frequency, voltages, currents,
and real and reactive power flows.
Ø Status indicators include alarm signals (over-temperature, low relay
battery voltage, illegal entry) and whether switches and circuit breakers are
open or closed.
Ø Such information is provided to the ECC through a periodic scan of all
RTUs. A 2 second scan cycle is typical.

FUNCTIONS OF SCADA SYSTEMS
1. Data acquisition
2. Information display.
3. Supervisory Control (CBs:ON/OFF, Generator:
stop/start, RAISE/LOWER command)
4. Information storage and result display.
5. Sequence of events acquisition.

6. Remote terminal unit processing.
7. General maintenance.
8. Runtime status verification.
9. Economic modeling.
10. Remote start/stop.
11. Load matching based on economics.
12. Load shedding.

CONTROL FUNCTIONS
Ø Control and monitoring of switching devices, tapped transformers,
auxiliary devices, etc.
Ø Bay-and a station-wide interlocking
Ø Dynamic Bus bar coloring according to their actual operational status.
Ø Automatic switching sequences
Automatic functions such as load shedding, power restoration, and high
speed bus bar transfer
Ø Time synchronization by radio and satellite clock signal

MONITORING FUNCTIONS
Measurement and displaying of current, voltage, frequency,
active and reactive power, energy, temperature, etc.
Ø Alarm functions. Storage and evaluation of time stamped
events.
Ø Trends and archiving of measurements
Ø Collection and evaluation of maintenance data
Ø Disturbance recording and evaluation

4. PROTECTION FUNCTIONS
Substation protection functions includes the monitoring of events like start, trip
indication and relay operating time and setting and reading of relay parameters.
Ø Protection of bus bars. Line feeders, transformers, generators.
Ø Protection monitoring (status, events, measurements, parameters, recorders)
Ø Adaptive protection by switch-over of the active parameter set.

5. COMMUNICATION TECHNOLOGIES
The form of communication required for SCADA is telemetry. Telemetry is the
measurement of a quantity in such a way so as to allow interpretation of that
measurement at a distance from the primary detector.
The distinctive feature of telemetry is the nature of the translating means, which
includes provision for converting the measure into a representative quantity of
another kind that can be transmitted conveniently for measurement at a
distance.
Ø The actual distance is irrelevant.

Telemetry may be analog or digital. In analog telemetry, a voltage,
current, or frequency proportional to the quantity being measured
is developed and transmitted on a
communication channel to the receiving location, where the
received signal is applied to a meter calibrated to indicate the
quantity being measured, or it is applied directly to a control
device such as a ECC computer.
Ø Forms of analog telemetry include variable current, pulse-
amplitude, pulse-length, and pulse-rate, with the latter two being
the most common.

In digital telemetry, the quantity being measured is converted to
a code in which the sequence of pulses transmitted indicates the
quantity.
Ø One of the advantages to digital telemetering is the fact that
accuracy of data is not lost in transmitting the data from one
location to another.
Ø Digital telemetry requires analog to digital (A/D) and possible
digital to analog (D/A) converters, as illustrated.

The earliest form of signal circuit used for SCADA telemetry consisted of
twisted pair wires; although simple and economic for short distances, it
suffers from reliability problems due to breakage, water ingress, and
ground potential risk during faults
Ø Improvements over twisted pair wires came in the form of what is now
the most
common, traditional type of telemetry mediums based on leased-wire,
power-line carrier, or microwave.

These are voice grade forms of telemetry, meaning
they represent communication channels suitable for
the transmission of speech, either digital or analog,
generally with a frequency range of about 300 to
3000 Hz.

SCADA REQUIRES COMMUNICATION BETWEEN MASTER CONTROL
STATION AND REMOTE CONTROL STATION:
Leased-wire means use of a standard telephone circuit; this is a convenient
and straightforward means of telemetry when it is available, although it can
be unreliable, and it requires a continual outlay of leasing expenditures.
Ø In addition, it is not under user control and requires careful coordination
between the user and the telephone company.
Ø Power-line carrier (PLC) offers an inexpensive and typically more reliable
alternative to leased-wire.

Here, the transmission circuit itself is used to modulate a
communication signal at a frequency much greater than the 60
Hz power frequency.
Ø Most PLC occurs at frequencies in the range of 30-500 kHz.
Ø The security of PLC is very high since the communication
equipment is located inside the substations. One disadvantage of
PLC is that the communication cannot be made through open
disconnects, i.e., when the transmission line is outaged.

Often, this is precisely the time when the communication signal is
needed most. In addition, PLC is susceptible to line noise and
requires careful signal-to-noise ratio analysis.
Ø Most PLC is strictly analog although digital PLC has become
available from a few suppliers during the last few years.
Ø Microwave radio refers to ultra-high-frequency (UHF) radio
systems operating above 1 GHz.

Microwave radio refers to ultra-high-frequency (UHF) radio systems operating
above 1 GHz.
Ø The earliest microwave telemetry was strictly analog, but digital microwave
communication is now quite common for EMS/SCADA applications.
This form of communication has obvious advantages over PLC and leased wire
since it requires no physical conducting medium and therefore no right-of-way.
Ø However, line of sight clearance is required in order to ensure reliable
communication, and therefore it is not applicable in some cases.

A more recent development has concerned the use of fiber optic cable, a
technology capable of extremely fast communication speeds. Although cost
was originally prohibitive, it has now decreased to the point where it is viable.
Ø Fiber optics may be either run inside underground power cables or they
may be fastened to overhead transmission line towers just below the lines.
Ø They may also be run within the shield wire suspended above the
transmission lines.
Ø One easily sees that communication engineering is very important to
power system control.

Students specializing in power and energy systems should
strongly consider taking communications courses to have
this background. Students specializing in communication
should consider taking power systems courses as an
application area.

SECURITY ANALYSIS & CONTROL:
Security monitoring is the on line identification of the actual operating conditions
of a power system.
It requires system wide instrumentation to gather the system data as well as a
means for the on line determination of network topology involving an open or
closed position of circuit breakers.
A state estimation has been developed to get the best estimate of the status .the
state estimation provides the database for security analysis

· Data acquisition:
1. To process from RTU
2. To check status values against normal value
3. To send alarm conditions to alarm processor
4. To check analog measurements against limits.

Alarm processor:
1. To send alarm messages
2. To transmit messages according to priority
Status processor:
1. To determine status of each substation for proper connection.
·
Reserve monitor:
1. To check generator MW output on all units against unit limits

State estimator:
1. To determine system state variables
2. To detect the presence of bad measures values.
3. To identify the location of bad measurements
4. To initialize the network model for other programs

Security Control Function:
Ø Network Topology processor-mode of the N/W
Ø State estimator.
Ø Power flow-V, δ,P,Q.
Ø Optimal power flow.
Ø Contingency analysis.
Ø Optimal power flow.
Security enhancement-existing overload using corrective control action. Preventive action.

System Security
1. System monitoring.
2. Contingency analysis.
3. Security constrained optimal power flow

Security Assessment
Ø Security assessment determines first, whether the system is currently residing in an
acceptable state and second, whether the system would respond in an acceptable manner and
reach an acceptable state following any one of a pre-defined contingency set.
Ø A contingency is the unexpected failure of a transmission line, transformer, or generator.

Ø Usually, contingencies result from occurrence of a fault, or short-circuit, to one
of these components.
Ø When such a fault occurs, the protection systems sense the fault and remove
the component, and therefore also the fault, from the system.
Ø Of course, with one less component, the overall system is weaker, and
undesirable effects may occur.
Ø For example, some remaining circuit may overload, or some bus may
experience an under voltage condition. These are called static security problems.
Ø Dynamic security problems may also occur, including uncontrollable voltage
decline, generator overspeed (loss of synchronism), or undamped oscillatory
behavior

Security Control
Ø Power systems are designed to survive all probable contingencies.
Ø A contingency is defined as an event that causes one or more important
components such as transmission lines, generators, and transformers to be
unexpectedly removed from service.
Ø Survival means the system stabilizes and continues to operate at acceptable
voltage and frequency levels without loss of load.

Operations must deal with a vast number of possible conditions experienced by
the system, many of which are not anticipated in planning.
Ø Instead of dealing with the impossible task of analyzing all possible system
states, security control starts with a specific state: the current state if executing
the real-time network sequence; a postulated state if executing a study
sequence.
Ø Sequence means sequential execution of programs that perform the
following steps:

Determine the state of the system based on either current or postulated conditions.
2. Process a list of contingencies to determine the consequences of each contingency on the
system in its specified state.
3. Determine preventive or corrective action for those contingencies which represent
unacceptable risk.

Security control requires topological processing to build network models
and uses large-scale AC network analysis to determine system conditions.
Ø The required applications are grouped as a network subsystem that
typically includes the following functions:
Topology processor:
Processes real-time status measurements to determine an electrical
connectivity (bus) model of the power system network.

State estimator:
Uses real-time status and analog measurements to
determine the „„best‟‟ estimate of the state of the power
system. It uses a redundant set of measurements; calculates
voltages, phase angles, and power flows for all components
in the system; and reports overload conditions.

Power flow:
Determines the steady-state conditions of the power system
network for a specified generation and load pattern. Calculates
voltages, phase angles, and flows across the entire system.

Contingency analysis:
Assesses the impact of a set of contingencies on the state of the power system
and identifies potentially harmful contingencies that cause operating limit
violations.
Optimal power flow: Recommends controller actions to optimize a specified
objective function (such as system operating cost or losses) subject to a set of
power system operating constraints.

Security enhancement:
Recommends corrective control actions to be taken to alleviate an existing or potential overload
in the system while ensuring minimal operational cost.
Bus load forecasting:
Uses real-time measurements to adaptively forecast loads for the electrical connectivity (bus)
model of the power system network

Preventive action:
Recommends control actions to be taken in a “preventive” mode before a
contingency occurs to preclude an overload situation if the contingency were to
occur.
Transmission loss factors:
Determines incremental loss sensitivities for generating units;
calculates the impact on losses if the output of a unit were to be increased by 1
MW.

Short-circuit analysis:
Determines fault currents for single-phase and three-phase
faults for fault locations across the entire power system
network.

VARIOUS OPERATING STATES:
1. Normal state
2. Alert state
3. Emergency state
4. Extremis state
5. Restorative state

Normal state:
A system is said to be in normal if both load and operating
constraints are satisfied .It is one in which the total demand
on the system is met by satisfying all the operating
constraints.

Alert state:
Ø A normal state of the system said to be in alert state if one or more of the
postulated contingency states, consists of the constraint limits violated.
Ø When the system security level falls below a certain level or the probability of
disturbance increases, the system may be in alert state .
Ø All equalities and inequalities are satisfied, but on the event of a disturbance, the
system may not have all the inequality constraints satisfied.
Ø If severe disturbance occurs, the system will push into emergency state. To bring
back the system to secure state, preventive control action is carried out.

Emergency state:
Ø The system is said to be in emergency state if one or more
operating constraints are violated, but the load constraint is satisfied
.
Ø In this state, the equality constraints are unchanged.
Ø The system will return to the normal or alert state by means of
corrective actions, disconnection of faulted section or load sharing.

Extremis state:
Ø When the system is in emergency, if no proper corrective action is taken in
time, then it goes to either emergency state or extremis state.
Ø In this regard neither the load or nor the operating constraint is satisfied, this
result is islanding.
Ø Also the generating units are strained beyond their capacity .
Ø So emergency control action is done to bring back the system state either to
the emergency state or normal state.

Restorative state:
Ø From this state, the system may be brought back either to alert state or
secure state .The latter is a slow process.
Ø Hence, in certain cases, first the system is brought back to alert state and
then to the secure state .
Ø This is done using restorative control action.

Thank you

PSOC Computer Control Of Power system .ppt

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PSOC Computer Control Of Power system .ppt