Power System Operation & Control

(KEN-070)

Unit – I

Introduction to PSOC

Mr. Gaurav Srivastava

Asst. Professor
EN Department
AKGEC, Ghaziabad
 Contents
• Structure of Power System
• Significance of Voltage and Frequency Control
• Energy Control Centre

• Level Decomposition in Power System

• SCADA

• EMS

• Real Time Computer-Control

• PMU- concept of Synchrophasor

• Power System Security

• Operational Stages of Power System
• Power Scenario in Indian Grid
• Installed Capacity in India
 Structure of Power System
• Generating stations, transmission lines and the distribution systems are the main
components of an electric power system.
• Generating stations and a distribution system are connected through transmission lines,
which also connect one power system (grid, area) to another.
• A distribution system connects all the loads in a particular area to the transmission lines.
• For economical and technological reasons, individual power systems are organized in
the form of electrically connected areas or regional grids (also called power pools).
• Each area or regional grid operates technically and economically independently, but
these are
• Eventually interconnected to form a national grid (which may even form an international
grid) so that each area is contractually tied to other areas in respect to certain
generation and scheduling features.
• Power station siting depends on many factors like—technical, economical and
environmental.
• As it is considerably cheaper to transport bulk electric energy over extra high voltage
(EHV)
• transmission lines than to transport equivalent quantities of coal over rail road, the
recent trends in India has been to build super (large) thermal power stations near coal
mines.
• Bulk power can be transmitted to fairly long distances over transmission lines of 400/765
 Structure of Power System cont..
•

Schematic diagram depicting

power system structure
 Significance of Voltage and Frequency Control
• The stability of electrical grid is maintained by frequency and voltage control.
• The only possible ways of storage.
– Transform AC into DC and then store it.
– Transform the electric energy into the form of energy.
• In the long run, storage in hydrogen could contribute to the management of electric
system.
• Since storage is difficult, two separate equilibriums should be kept on the grid.
• Frequency Control = Active Power Control
• The active power generated should always equal to the active power consumed. A
deviation from this equilibrium results in a deviation from the 50 Hz frequency. So
keeping this equilibrium between active policy and generation means maintaining
frequency.
• Consumption of active power varies strongly according to the time of the day, the
season, or weather conditions.
• The reserve capacity is kept in power plants to be able to react quickly and deliver extra
power when necessary.
Significance of Voltage and Frequency Control cont..
• In a power system the load demand continuously changes, in accordance with it the
power input also varies.
• If the input - output balance is not maintained a change in frequency will occur.
• The control of frequency is achieved primarily through speed governor mechanism
aided by supplementary means for precise control.
• Voltage Control = Reactive Power Control
• The objective of system voltage control is to maintain a satisfactory voltage profile in the
system during both periods of maximum and minimum loadings.
• The reactive power on the grid should be kept in equilibrium as well. Reactive power is
an extra load for the grid, leaving less capacity for active power, resulting in a local
voltage drop. So keeping reactive power in equilibrium means maintaining voltage.
• Reactive power is inextricably related with active power, and oscillates between
generator, inductive elements (motors, transformers) and capacitive elements (capacitor
batteries) on the grid.
• Capacitor banks, synchronous generators are responsible for compensating reactive
power.
 Energy Control Centre
• The energy control center (ECC) has traditionally been the decision-center for the
electric transmission and generation interconnected system.
• The ECC provides the functions necessary for monitoring and coordinating the
minute-by-minute physical and economic operation of the power system.
• In order to have an efficient power system operation and control, various control centers
have to be operated in a hierarchical manner.
• There has been level decomposition of control centers in the power system.
• There are 4 types of control centers.
1. Local Control Centre
2. Area Load Dispatch Centre
3. State Load Dispatch Centre
4. Regional Control Centre.
 Level Decomposition in Power System
• National Load Dispatch Centre
(NLDC) has been setup at New
Delhi and became operational
in January 2014.
• Below this, five Regional level
Load Dispatch Centers (RLDC)
have been shown.
• The role of the NRLDC is to
monitor and supervise the grid
and power generation of the
region.
• It focuses attention on the
regional interconnected
network.
• By using 'Energy Management
System' (EMS) and advanced
application programmes,
NRLDC coordinates with all
inter-region and inter-state
power exchange.
 Level Decomposition cont..
• Below NRLDC, State level SLDCs and Central Project Coordination & Control Centre
(CPCC) have been shown.
• The primary role of SLDCs is to monitor, control and coordinate the generation,
transmission and distribution of power within the State while ensuring safety and
continuity of its transmission and sub-transmission power networks.

• CPCC (North) coordinates with all Central sector projects of northern region such as
those of NTPC, NHPC, Power Grid, Tehri, etc. CPCC gets data from Central Sector
projects and that data is added at regional level.

• Each RLDC has the ability to exchange data with other RLDCs as well as with NLDC,
but direct data transmission does not take place between SLDC of one State with SLDC
of another State.
 Responsibilities of NLDC
• The National Load Dispatch Centre shall be Apex Body to ensure integrated operation
of the national Power System and discharge the following functions.
• Supervision over the RLDCs.
• Scheduling and dispatch of electricity over inter-regional links in accordance with Grid
standards specified by the Authority and Grid Code specified by the Central Commission
in coordination with RLDCs.
• Coordination with RLDCs for achieving maximum economy and efficiency in operation
of National Grid.
• Monitoring of operations and grid security of the National Grid.
• Supervision and control over the inter regional links as may be required for ensuring
stability of the power system under its control.
• Co-ordination with Regional Power Committees for regional outage schedule in the
national perspective to ensure optimum utilization of power resources.
• Coordination with RLDCs for the energy accounting of inter-regional exchange of power.
• Coordination for restoration of synchronous operation of National Grid with RLDCs.
• Co-ordination for trans-national exchange of Powers.
• Providing operational feed-back for National Grid planning to the Authority and the
Central Transmission Utility.
• Levy and collection of such fee and charges from the Generating Companies or the
licensees involved in the power system as may be specified by the Central Commission.
 Responsibilities of NRLDC
• NRLDC: Northern Region Load Dispatch Center
• To ensure the integrated operation of the power system in the Northern Region.
• Monitoring of system parameters and system security.

• Daily scheduling and operational planning.

• Facilitating bilateral and inter-regional exchanges of power.

• Analysis of tripping/disturbances and facilitating immediate remedial measures.

• System studies, planning and contingency analysis.

• Augmentation of telemetry, computing and communication facilities.

• Computation of energy dispatch and drawls values using SEMs.

 Responsibilities of SLDC
• SLDC: State load Dispatch centre
• Be responsible for optimum scheduling and dispatch of electricity within a State in
accordance with the contracts entered into with the licensees or the generating
Companies operating in that State.
• Monitor grid operation.

• Keep accounts of the quantity of electricity transmitted through State grid.

• Exercise supervision and control over the inter-State transmission system.

• Be responsible for carrying out real time operation for grid control and dispatch of
electricity within the State through secure and economic operation of the State Grid in
accordance with the Grid standards and State Grid Code.
 RTU
• RTU: Remote Terminal Unit
• The system gets information from remote terminal unit (RTU) that encode measurement
transducer outputs and opened/closed status information into digital signals which are
sent to the operation center over communications circuits
• Main operations of RTU:
• Each subLDC collects data from various RTUs, installed at important sub-stations
(400KV, 220KV and few 132KV) and powerhouses.
• So far in UPPTCL, 72 RTUs have already been integrated with the system.
• Each RTU automatically picks up required information (MW, MVAr, KV, Hz, Circuit
breaker & isolator status) of the sub-station/powerhouse and transmit it to its subLDC
through communication system.
• This information is processed in the data Server of subLDC.
• Data in the form of binary stream of pulses are sent by RTU at the speed of 300, 600 or
1200 bits per second rate (baud).
• At subLDC, the information is updated within 10 sec.
 Components of Energy Control Centre
• The system control function traditionally used in electric utility operation consists of three
main integrated subsystems:
– The energy management system (EMS),
– The supervisory control and data acquisition (SCADA),
– The communications interconnecting the EMS and the SCADA (which is often
thought of as part of the SCADA itself).
• Figure on next slide provides a block diagram illustration of modern Energy
management system comprising of Initial load forecast and scheduling, SCADA,
Security assessment and analysis; and finally the optimal power flow/constrained
economic dispatch
SCADA
 SCADA cont..
• In SCADA system measured values, i.e. analogue (measured value) data (MW, MVAR,
V, Hz Transformer tap position), and Open/Closed status information, i.e. digital data
(Circuit Breakers/Isolators position i.e. on/off status), are transmitted through
telecommunication channels to respective sub-LDCs.
• Secondary side of Current Transformers (CT) and Potential Transformer (PT) are
connected with 'Transducers’.
• The output of transducers is available in dc current form (in the range of 4mA to 20mA).
• A/D converter converts this current into binary pulses.
• Different inputs are interleaved in a sequential form and are fed into the CPU of the
RTU.
• The output of RTU, containing information in the form of digital pulses, is sent to sub
LDC.
• At sub LDC end, data received from RTU is fed into the data servers.
• In general, a SCADA system consists of a database, displays and supporting programs.
 SCADA cont..
1. Communications - Sub-LDC's computer communicates with all RTU stations under its
control, through a communication system. RTU polling, message formatting, polynomial
checking and message retransmission on failure are the activities of 'Communications'
functional area.
2. Data Processing - After receipt of data through communication system it is processed.
Data process function has three sub-functions i.e. (i) Measurements, (ii) Counters and
(iii) Indications.
i. 'Measurements' retrieved from a RTU are converted to engineering units and linearized,
if necessary. The measurement are then placed in database and are checked against
various limits which if exceeded generate high or low limit alarms.
ii. The system has been set-up to collect 'Counters' at regular intervals: typically 5 or 10
minutes. At the end of the hour the units is transferred into appropriate hour slot in a
24-hour archive/history.
iii. 'Indications' are associated with status changes and protection. For those statuses that
are not classified as 'alarms', logs the change on the appropriate printer and also enter it
into a cyclic event list. For those statuses, which are defined as an 'alarms' and the
indication goes into alarm, an entry is made into the appropriate alarm list, as well as in
the event list and an audible alarm is generated in the sub-LDC.
 SCADA cont..
3. Alarm/Event Logging - The alarm and event logging facilities are used by SCADA data
processing system. Alarms are grouped into different categories and are given different
priorities. Quality codes are assigned to the recently received data for any 'limit violation'
and 'status changes'. Alarms are acknowledged from single line diagram (or alarm lists)
on display terminal in LDCs.

4. Manual Entry - There is a provision of manual entry of measured values, counters and
indications for the important sub-station/powerhouse, which are uncovered by an RTU
or some problem is going on in its RTU, equipment, communication path, etc.

5. Averaging of Measured Values - As an option, the SCADA system supports averaging

of all analogue measurements. Typically, the averaging of measured values over a
period of 15 minutes is stored to provide 24 hours trend.
 SCADA cont..
6. Historical Data Recording (HDR) - The HDR, i.e. 'archive', subsystem maintains a
history of selected system parameters over a period of time. These are sampled at a
pre-selected interval and are placed in historical database. At the end of the day, the
data is saved for later analysis and for report generation.
7. Interactive Database Generation - Facilities have been provided in such a way that an
off-line copy of the SCADA database can be modified allowing the addition of new
RTUs, pickup points and communication channels.

8. Supervisory Control/Remote Command - This function enables the issue of 'remote

control' commands to the sub-station/powerhouse equipment e.g. circuit breaker trip
command.
9. Fail-over - A 'Fail-over' subsystem is also provided to secure and maintain a database
of devices and their backups. The state of the device is maintained indicating whether it
is 'on-line' or 'failed'. There is a 'backup' system, which maintains database on a backup
computer and the system is duplicated.
 EMS & Real Time Computer-Control
• EMS: ENERGY MANAGEMENT SYSTEM
• For energy management of the power system, control personnel and application
software engineers use SCADA data available in the database by using EMS software.
Important features are as below:
1. The Data Base Compiler provides a consistent source of data usable for the
applications in an efficient form. The Data Base Compiler does final checking for
completeness and consistency of the entries for a specific application and prepares
those special tables which are needed for the efficiency of specific application
programmes.
2. Recording of 'Sequence of Events' (SOEs) is the most innovative feature provided in
this system. A RTU has the ability to accurately time tag status change and report this
information to sub-LDC. All RTUs in the system are 'time synchronized' with the master
station. In the event of any tripping, sequence of events can be well established on time
scale with a resolution of 10 milliseconds.
 EMS & Real Time Computer-Control cont..
3. Normally, 'Automatic Generation Control' (AGC) function issues control commands to
generating plants using the concept of Area Control Error (ACE). It is based on
deviations in 'standard frequency (50 Hz)' and 'scheduled area interchanges' from that
of the 'actual frequency' and 'actual area interchanges' In the event of unavailability of
sufficient generation to satisfy the AGC requirement, the System Control Officer can
enforce required quantum of load shedding.
4. For 'Operation Scheduling' the application software has 'short-term' and 'long-term'
'System Load Forecasting' functions to assist dispatching Engineer/control Officer in
estimating the loads that are expected to exist for one to several days in advance. This
function provides a scientific and logical way of scheduling of resources in a very
effective manner.
∙ Under 'Short-term Load Forecasting' function, application software engineers are
able to forecast weekly peak demands and load duration curves for several months
into the future.
∙ Under 'Long-Term Load Forecasting' function, forecasting of monthly peak demands
and load duration curves for several years into the future can done for the use of
'Power System Planner'.
 EMS & Real Time Computer-Control cont..
5. The other functions like economic dispatch, reserve monitoring, production costing, inter
system transactions scheduling, etc. are available to guide System Control Officer to
optimally use available resources.

6. Power System Control Officer/Analyst would be able to use contingency analysis

function to assess the impact of specified contingencies that would cause line (s)
overloads, abnormal voltages, and reactive limit violations.
7. The EMS software system may have many other applications for use, which include
network topology, performing of state estimation, optimal power flow (OPW) programme,
stability programme, power flow displays, help and instructional displays, tabular
displays, single line diagram displays, etc.
 Power System Security
• An overriding factor in the operation of a power system is the desire to maintain system
security.
• System security involves practices designed to keep the system operating when
components fail.
• All equipment in a power system is designed such that it can be disconnected from the
network. The reasons for these disconnections are generally divided into two categories:
scheduled outages and forced outages.
• Scheduled outages are typically done to perform maintenance or replacement of the
equipment, and, as its name implies, the time of disconnect is scheduled by operators to
minimize the impact on the reliability of the system.
• Forced outages are those that happen at random and may be due to internal component
failures or outside influences such as lightning, wind storms, ice build-up, etc.
 Power System Security cont..
• If a forced outage occurs on a system that leaves it operating with limits violated on
other components, the event may be followed by a series of further actions that switch
other equipment out of service. If this process of cascading failures continues, the entire
system or large parts of it may completely collapse. This is usually referred to as a
system blackout.

• Most large power systems install equipment to allow operations personnel to monitor
and operate the system in a reliable manner.

• System security can be broken down into three major functions that are carried out in an
operations control centre:
1. System monitoring

2. Contingency analysis
3. Security-constrained optimal power flow
 System Monitoring
• System monitoring provides the operators of the power system with pertinent up-to-date
information on the conditions on the power system.
• Generally speaking, it is the most important function of the three.
• From the time that utilities went beyond systems of one unit supplying a group of loads,
effective operation of the system required that critical quantities be measured and the
values of the measurements be transmitted to a central location. Such systems of
measurement and data transmission, called energy management systems (EMS), have
evolved to schemes that can monitor voltages, currents, power flows, and the status of
circuit breakers and switches in every substation in a power system transmission
network.
• The power system as seen by power system operators, whether at the highest level or
individual level at a small electric company, all have to deal with the power system in
what has been characterized as one of four modes:
– Normal
– Alert
– Emergency
– Restoration
 System Monitoring cont..
• Normal usually means that there are no alarms being presented and contingency
analysis is not reporting any contingencies that would cause overloads or voltage
violations.
• Alert means that either an alarm has been presented to the operator or the contingency
analysis programs have presented the possibility of a contingency problem.

• Emergency would indicate serious alarm messages that the operators must act on
immediately and threaten to cause major shutdowns of power system equipment or
even parts of the system.

• Restoration comes if the system does in fact lose equipment or part of the system or
even most of it is shut down or blacked out. In restoration, equipment must be
investigated to see if it can be brought back on line and then switched back into the
system. Loads that were dropped are brought back on line, sometimes in small blocks.
Restoration can take many hours especially if large generators are involved.
 Contingency Analysis
• The results of this type of analysis allow systems to be operated defensively.
• Many of the problems that occur on a power system can cause serious trouble within
such a quick time period that the operator cannot take action fast enough once the
process is started. This is often the case with cascading failures.
• Because of this aspect of systems operation, modern operations computers are
equipped with contingency analysis programs that model possible system troubles
before they arise.
• These programs are based on a model of the power system and are used to study
outage events and alarm the operators to any potential overloads or out-of-limit
voltages.
• For example, the simplest form of contingency analysis can be put together with a
standard power flow program together with procedures to set up the power flow data for
each outage to be studied by the power flow program.
• Several variations of this type of contingency analysis scheme involve fast solution
methods, automatic contingency event selection, and automatic initializing of the
contingency power flows using actual system data and state estimation procedures.
 Security-Constrained Optimal Power Flow
• In this function, a contingency analysis is combined with an optimal power flow that
seeks to make changes to the optimal dispatch of generation, as well as other
adjustments, so that when a security analysis is run, no contingencies result in
violations.
• To show how this can be done, power system is divided into four operating objectives.
• Normal state dispatch: This is the state that the power system is in prior to any
contingency. It is optimal with respect to economic operation, but it may not be secure.
• Post-contingency: This is the objective after a contingency has occurred. We shall
assume here that this condition has a security violation (line or transformer beyond its
flow limit or a bus voltage outside the limit).
• Secure dispatch: This is the objective with no contingency outages is to correct the
operating parameters to account for security violations.
• Secure post-contingency: The objective is to re-mediate the contingency as applied to
the base-operating condition with corrections.
 Operational Stages of Power System
• A normal (secure) state is the ideal
operating condition, wherein all the
equipment operate within their design
limits.
• Also, the power system can withstand a
contingency without violation of any of
the constraints.

• The system is said to be in the alert

(insecure) state, if voltage and frequency
are reaching beyond the specified limits.
The system is "weaker" and may not be
able to withstand a contingency.
 Operational Stages of Power System cont..
• Preventive Control actions like shifting generation (re-scheduling), load shedding are
required to get the system back to the normal state.
• If preventive control actions do not succeed, a power system remains insecure (in the
alert state).
• If a contingency occurs, the system may go into the emergency state where overloading
of equipment (above the short term ratings of the equipment) occurs.
• The system can still be intact and can be brought back to the alert state by Emergency
Control actions like fault tripping, generator tripping, load tripping, HVDC power control
etc.
• If these measures do not work, integrated system operation becomes unviable and a
major part of the system may be shutdown due to equipment outages.
• Load shedding and islanding is necessary to prevent spreading of disturbances and a
total grid failure.
• The small power systems (islands) are reconnected to restore the power system to
normal state (Restorative Control).
 Power Scenario in Indian Grid
• Total Installed Capacity (As on 31.05.2023) - Source : Central Electricity
Authority (CEA)
• INSTALLED GENERATION CAPACITY (SECTOR WISE) AS ON 31.05.2023

Sector MW % of Total
Central Sector 1,00,055 24.0%
State Sector 1,05,726 25.3%
Private Sector 2,11,887 50.7%

Total 4,17,668 100 %

Installed Capacity in India
Installed Capacity in India
Installed Capacity in India
 Cont.…
Installed Generation Capacity (Fuel
wise) as on 31.05.2023
• PERFORMANCE OF GENERATION CATAGORY INSTALLED % of
FROM THERMAL, HYDRO, GENERATION SHARE IN
CAPACITY (MW) Total
NUCLEAR Fossil Fuel
• The electricity generation target Coal 2,05,235 49.1%
(Including RE) for the year 2023-24 Lignite 6,620 1.6%
has been fixed as 1750 Billion Unit Gas 24,824 6.0%
(BU). i.e. growth of around 7.2% Diesel 589 0.1%
over actual generation of Total Fossil Fuel 2,37,269 56.8%
1624.158 BU for the previous year
Non-Fossil Fuel
(2022-23). The generation during RES (Incl. Hydro) 1,73,619 41.4%
2022-23 was 1624.158 BU as
Hydro 46,850 11.2 %
compared to 1491.859 BU
Wind, Solar & Other ER 1,25,692 30.2 %
generated during 2021-22,
Solar 67,078 16.1 %
representing a growth of about
8.87%. BM Power/Cogen 10,248 2.5 %
Waste to Energy 554 0.1 %
• The electricity generation target of
Small Hydro Power 4,944 1.2 %
thermal, hydro, nuclear & Bhutan
Nuclear 6,780 1.6%
import for the year 2021-22 has
Total Non-Fossil Fuel
been fixed as 1356 Billion Unit 1,79,322 43.0%

(BU). i.e. growth of around 9.83% Total Installed Capacity

(Fossil Fuel & Non-Fossil Fuel)
4,17,668 100%
 Cont.…
• Total Generation and growth over previous year in the country during 2009-10 to 2023-24 :-
• The electricity generation target for the year 2023-24 was fixed at 1750 BU comprising of 1324.110 BU
Thermal; 156.700 BU Hydro; 46.190 Nuclear; 8 BU Import from Bhutan and 215 BU RES (Excl. Large Hydro)
Total Generation
Year % of growth
(Including Renewable Sources) (BU)
2009-10 808.498 7.56
2010-11 850.387 5.59
2011-12 928.113 9.14
2012-13 969.506 4.46
2013-14 1,020.200 5.23
2014-15 1,110.392 8.84
2015-16 1,173.603 5.69
2016-17 1,241.689 5.80
2017-18 1,308.146 5.35
2018-19 1,376.095 5.19
2019-20 1,389.102 0.95
2020-21 1,381.855 -0.52
2021-22 1,491.859 7.96
2022-23 1,624.158 8.87
2023-24* 286.176 -0.72
Cont.…