Power System Operation and Control Notes:15EE81
2019_20
MODULE 1: Introduction
Structure
1.1: Operating States of Power System,
1.2 Objectives of Control
1.3 Key Concepts of Reliable Operation, Preventive and Emergency Controls
1.4 Energy Management Centres.
1.5Supervisory Control and Data acquisition (SCADA): Introduction to SCADA and its
Components, Standard SCADA Configurations,
1.6 Users of Power Systems SCADA, Remote Terminal Unit for Power System SCADA
1.7 Common Communication Channels for SCADA in Power Systems
1.8 Challenges for Implementation of SCADA
1.9 Unit Commitment: Introduction
1.10 Simple Enumeration Constraints
1.11 Priority List Method
1.12 Dynamic Programming Method for Unit Commitment
Objectives
1.
To describe various levels of controls in power systems and the vulnerability of the system.
2.
To explain components, architecture and configuration of SCADA.
3.
To define unit commitment and explain various constraints in unit commitment and the solution
methods.
1.1 Operating States of Power System
The objective of the control strategy is to generate and distribute power in an
interconnected system as economically and reliably as possible while maintaining the
frequency and voltage within permissible limits. Changes in real power mainly affect
the system frequency.
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The system operation is governed by equality and inequality constraints. Equality constraints
are power balance between generation and load. Inequality constraints set the limits on
different operating parameters such as voltage, generation limits, currents etc.
The system states are classified as:
1. Normal state
2. Alert state
3. Emergency state
4. Extremis state
5. Restorative state
1. 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.
2. 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.
3. 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.
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4.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.
5. 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.
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Fig.1.1: Operating states of a power system
1.2 Objectives of Power system Control
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Fig.1.2: Various Controls in a power system
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1.3 Key Concepts for Reliable Operation, Preventive and Emergency Controls
•
Stability : Continued interact operation of a system following a disturbance.
•
Security: Power system stability to survive contingencies without interruption to the
consumer.
•
Reliability: Probability of s satisfactory operation over a long period.
Key Concepts for reliable operation
•
Balance the generation and Load: AGC is used to match the generation with
demand.
•
Balance reactive power generation and demand :FACTS devices are used for
reactive power control
•
Ensure thermal limits are not exceeded: Heating limits of overhead lines must not
be exceeded
•
Maintain system stability: Regain state of equilibrium after subjected to disturbance.
•
Meet N-1 reliability criteria: System should remain operational and secure even
after the loss of large generator.
•
Plan, Design and maintain to Operate reliably: safe limits planning(short term and
long term)
•
Prepare for emergencies: weather fluctuation, operator error, software failure,
equipment failure. Operators must be trained
Preventive and Emergency Controls
•
Preventive control : Means maintain system in normal state or bring it back to
normal state.
1. Rescheduling of active power generation, to maintain the changing loads.
2. Start up of generation and maintaining adequate spinning reserve.
3. Change of reference points of FACTS controllers, phase shifting transformers
4. Change in the reference voltage points of generation and voltage control devices.
5. Change in bus bar configuration like bus bar splitting
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Emergency Controls:
1. Tripping of generators
2. Load shedding
3. Controlled disconnection of interconnected systems
4. Fast HVDC power transfer control
5. Application of braking resistors
6. Controlled islanding to create local generation load balance
1.4 Energy Management Centres.
Electrical energy management systems (EMS) are an important function for the reliable and efficient operation
of power systems. EMS is related to the real time monitoring, operation and control of a power system. The
information from the power system is read through Remote Terminal Units (RTUs), an integral part of SCADA
to an EMS or Energy Control Centre (ECC).
1.
EMS consists of both hardware and software. Hardware part of EMS consists of RTU, Intelligent
Electronic Device (IED), Protection, Computer networking, .etc. Software part of EMS consists of
Application programs for network analysis of power systems.
2.
In EMS, application programs are run in a real time as well as extended real time environment to keep
the power system in a secure operating state.
3.
EMS is an integral part of any power system. It is used as a part of Substation Automation System
(SAS), Demand Side Management (DSM), Protection, and Distribution Management Systems (DMS)
for renewable energy and so-on.
4.
An energy management system (EMS) is a system of computer-aided tools used by operators of
electric utility grids to monitor, control, and optimize the performance of the generation and/or
transmission system. The monitor and control functions are known as Supervisory Control and Data
Acquisition (SCADA), followed by several on-line application functions.
5.
A.Primary Objectives: Security and Stability of the system
B.Secondary Objectives: Economic Operation and Control
C.Tertiary Objectives: Optimization, Operational Planning and Maintenance Scheduling
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1.4.1 Primary Objectives:
1. Maintaining the power system in a secure and stable operating state by continuously monitoring the power
flowing in the lines and voltage magnitudes at the buses.
2. Maintaining the frequency within allowable limits.
3. Maintaining the tie-line power close to the scheduled values.
1.4.2 Secondary Objectives:
1. Economic Operation of the power systems through real time dispatch and Control.
2. Optimal control of the power system using both preventive and corrective control actions.
3. Real time Economic Dispatch through real power and reactive power control
1.4.3 Tertiary Objectives:
1. Optimization of the power system for normal and abnormal operating scenarios.
2. Optimal control of the power system by appropriate using both preventive and corrective control actions
3. Maintenance scheduling of generation and transmission systems.
1.5 Supervisory Control and Data acquisition (SCADA): Introduction to SCADA and
its Components, Standard SCADA Configurations
1.5.1 Introduction
SCADA is an acronym that stands for Supervisory Control and Data Acquisition. SCADA refers to a
system that collects data from various sensors at a factory, plant or in other remote locations and then sends this
data to a central computer which then manages and controls the data. SCADA systems are used not only in
industrial processes: e.g. steel making, power generation (conventional and nuclear) and distribution, chemistry,
but also in some experimental facilities such as nuclear fusion.
The size of such plants range from a few 1000 to several 10 thousands input/output (I/O) channels.
Definition of SCADA: A collection of equipment that will provide an operator at remote location with enough
information to determine the status of a particular piece of a equipment or entire substation and cause actions to
take place regarding the equipment or network.

What is SCADA? How : By collecting Information from plant / Load centre bend reducing it to the
EMS

Where : Control Centre. Both at Plant and Load
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Why : To gather Information as here (voltage, current, frequency, power, circuit breaker status) and To
perform online actions
Fig.1.3: Functional Units
The figure shown above is a simple SCADA system with a single computer, which represents the simplest
configuration of SCADA with a single computer. The computer receives data from remote terminal units
through the communication interface. One or more CRT terminals for display is controlled by operators. With
this system, it is possible to execute supervisory control commands and request the display of data in
alphanumerical formats.
The I/O SCADA programming is used to change the supervisory software. In the basic SCADA system, all the
data and programs are stored in the main memory. The more complicated version of SCADA has additional
secondary memories in the form of magnetic disc units.
1.6 Users of Power Systems SCADA, Remote Terminal Unit for Power System SCADA
1.6.1 Users of Power Systems SCADA
SCADA can be used to manage any kind of equipment. Typically, SCADA systems are used to automate
complex industrial processes where human control is difficult. For example in systems where there are more
control factors unable to be managed by operators in a control centre. SCADA systems are widely used for
control in the following domains
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1. Electric power generation, transmission and distribution: Electric utilities use SCADA systems to
detect current flow and line voltage, to monitor the operation of circuit breakers, and to take sections of the
power grid online or offline.
2. Water and sewage: State and municipal water utilities use SCADA to monitor and regulate water
flow, reservoir levels, pipe pressure and other factors.
3. Buildings, facilities and environments: Facility managers use SCADA to control HVAC,
refrigeration units, lighting and entry systems.
4. Manufacturing: SCADA systems manage parts inventories for just-in-time manufacturing, regulate
industrial automation and robots, and monitor process and quality control.
5. Mass transit: Transit authorities use SCADA to regulate electricity to subways, trams and trolley
buses; to automate traffic signals for rail systems; to track and locate trains and buses; and to control railroad
crossing gates.
6. Traffic signals: SCADA regulates traffic lights, controls traffic flow and detects out-of-order signals.
1.6.2 Remote Terminal Unit for Power System SCADA
A remote terminal unit (RTU) is a microprocessor-controlled electronic device that
interfaces objects in the physical world to a distributed control system or SCADA (supervisory
control and data acquisition) system by transmitting telemetry data to a master system, and by using
messages from the master supervisory system to control connected objects.[1] Other terms that may be
used for RTU are remote telemetry unit and remote telecontrol unit.
1.6.2.1 Architecture
An RTU monitors the field digital and analog parameters and transmits data to the Central Monitoring Station. It
contains setup software to connect data input streams to data output streams, define communication protocols,
and troubleshoot installation problems.
An RTU may consist of one complex circuit card consisting of various sections needed to do a custom fitted
function or may consist of many circuit cards including CPU or processing with communications interface(s),
and one or more of the following: (AI) analog input, (DI) digital (status) input, (DO/CO) digital (or control relay)
output, or (AO) analog output card(s).
a) Power supply
A form of power supply will be included for operation from the AC mains for various CPU, status wetting
voltages and other interface cards. This may consist of AC to DC converters where operated from a station
battery system.
RTUs may include a battery and charger circuitry to continue operation in event of AC power failure for critical
applications where a station battery is not available.
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b) Digital (status) inputs
Most RTUs incorporate an input section or input status cards to acquire two state real world information. This is
usually accomplished by using an isolated voltage or current source to sense the position of a remote contact
(open or closed) at the RTU site. This contact position may represent many different devices, including
electrical breakers, liquid valve positions, alarm conditions, and mechanical positions of devices. Counter inputs
are optional.
c) Analog inputs
A RTU can monitor analog inputs of different types including 0-1 mA, 4–20 mA current loop, 0–10 V., ±2.5 V,
±5.0 V etc. Many RTU inputs buffer larger quantities via transducers to convert and isolate real world quantities
from sensitive RTU input levels. An RTU can also receive analog data via a communication system from a
master or IED (intelligent electronic device) sending data values to it.
The RTU or host system translates and scales this raw data into the appropriate units such as quantity of water
left, temperature degrees, or Megawatts, before presenting the data to the user via the human–machine interface.
d) Digital (control relay) outputs
RTUs may drive high current capacity relays to a digital output (or "DO") board to switch power on and off to
devices in the field. The DO board switches voltage to the coil in the relay, which closes the high current
contacts, which completes the power circuit to the device.
RTU outputs may also consist of driving a sensitive logic input on an electronic PLC, or other electronic device
using a sensitive 5 V input.
e) Analog outputs
While not as commonly used, analog outputs may be included to control devices that require varying quantities,
such as graphic recording instruments (strip charts). Summed or processed data quantities may be generated in a
master SCADA system and output for display locally or remotely, wherever needed.
e) Software and logic control
Modern RTUs are usually capable of executing simple programs autonomously without involving the host
computers of the DCS or SCADA system to simplify deployment and to provide redundancy for safety reasons.
An RTU in a modern water management system will typically have code to modify its behavior when physical
override switches on the RTU are toggled during maintenance by maintenance personnel. This is done for safety
reasons; a miscommunication between the system operators and the maintenance personnel could cause system
operators to mistakenly enable power to a water pump when it is being replaced, for example.
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Maintenance personnel should have any equipment they are working on disconnected from power and locked to
prevent damage and/or injury.
f) Communications
A RTU may be interfaced to multiple master stations and IEDs (Intelligent Electronic Device) with different
communication media (usually serial (RS232, RS485, RS422) or Ethernet).
1.7 Common Communication Channels for SCADA in Power Systems
•
Power line Carrier Communication
1. Used for speech and data transmission
2. Simple technique, easy to maintain.
3. Speed of data transmission is limited
4. Modem used at sending and receiving end to modulate & demodulate power and
data.
Disadvantage: Since conductor acts as a medium, there is attenuation in the transmitted
signal
•
Microwave Communication
1. Frequency Range: 1GHz to 1000GHz
2. 10GHz will limit the transmission distance to 5 miles
3. Data carrying capacity is high due to high bandwidth and data are protected
from noise
•
Fiber Optic Communication
1. Wide bandwidth and high transmission over long distance
2. No emission outside the cables and is not affected by external radiation.
3. Preferred where security is an issue
4. Immune to EMI, corrosion, noise
•
Satellite Communication
1. Reliable alternative to traditional network
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2. Benefits: Broadcast networks( Multiple stations can receive a single message), cost
effectiveness, reliable with world wide coverage and easy to integrate with RTUs
1.8 Challenges for Implementation of SCADA




Maintaining System Reliability
Continuos service deliverability
Maintenance
Least cost of operation
1.9 Unit Commitment: Introduction
Power systems have grown in size and complexity. In power system, the total generation on
the system will generally be higher than total load on the system. The total load on the system
will generally be higher during the day time and early evening and lower during the early
morning and late evening. It is not economical to run all the units available all the time. So,
the commitment of a generating unit is difficult. The cost of the system can be saved by
turning off generators when they are not needed. Generating units can be broadly divided into
two groups namely thermal and hydro plants. The operation of thermal units involves both
full and maintenance cost but no fuel cost is required for hydro plants. Thermal units include
steam plants, nuclear plant, diesel and gas turbine plants are used as peak loads.
Need for Unit Commitment:
Enough units will be committed to supply the system load
1. To reduce the loss or fuel cost
2. By running the most economic unit, the load can be supplied by that unit
3. operating closer to its best efficiency. Difficulties to find unit commitment solution
4. Time consuming process
5. If the number of units is more, the number of combinations is more in a complex
6. bus system If n be the number of units, then the number of combinations will be 2n-1.
1.10 Simple Enumeration Constraints
Each individual power system may impose different rules on the scheduling of units depends
on generation make up and load curve characteristics etc. The constraints to be considered for
unit commitment are
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Spinning reserve Thermal constraints: -Minimum uptime -Minimum down time -Crew
constraint Other constraint: -Hydro constraint -Must run constraint -Fuel constraint Spinning
Reserve: Spinning Reserve is the total amount of generation available from all units
synchronized on The system minus the present load and losses being supplied Spinning
Reserve= total amount of generation - present load +losses Spinning Reserve must be
established so that the loss of one or more units does not cause drop in the system frequency.
i.e. if one unit is lost, the Spinning Reserve unit has to make up for the loss in a specified
time period.
Spinning Reserve is the reserve generating capacity running at no load. Reserve has To
avoid transmission system limitations or bottling of reserves
1. To allow some parts of the system to run as islands (some area
electricallydisconnected)
Reserve capacity: Capacity in excess of that required to carry peak load
Cold reserves: It is that reserve generating capacity which is available for service but is not
in operation.
Hot reserves: It is that reserve generating capacity which is in operation but is not in service.
Reserve generating capacity: The amount of power that can be produced at a given point in
time by generating units that are kept available in case of special need. The capacity may be
used when unusually high power demand occurs or when other generating units are off-line
for maintenance, repairs or refueling.
Reserve generating capacity include quick-start diesel or gas turbine unit or hydro units
and pumped storage hydro units that can be brought on-line, synchronized and brought upto
full capacity quickly. Automatic generation control system is used to make up for a
generation unit failure and to restore frequency and interchange power through tie-line
quickly in the event of generating unit outage.
Reserve margin: The percentage of installed capacity exceeding the excepted peak demand
during a specified period. Typical rules for spinning reserve set by regional reliability council
Reserve must be given percentage of forecasted peak demand Reserve must be capable of
making up the loss of the most heavily loaded unit in a given period of time Calculate
reserve requirements as a function of the probability of not having sufficient generation to
meet the load.
Thermal unit constraints: A thermal unit can withstand only gradual temperature changes
and is required to take some hours to bring the unit on-line. For thermal plants, one hour is
the smallest time period that should be considered for unit commitment solution as the startup and shut-down time for many units is of this order.
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The thermal unit constraints are minimum up time, minimum down time and crew constraints.
Minimum up time: Once the unit is running, it should not be turned off immediately.
Minimum down time: Once the unit is decommited, there is a minimum time before it can be
recommitted. Crew constraints: If a plant consists of two or more units, they cannot both be
turned on at the same time. Since there are not enough crew members to attend both units
while starting up. Start-up cost: It is independent upon the down time of the unit i.e. the time
interval between shut down and restart. Start up cost=0, if unit is stopped and started
immediately.
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1.11 Priority List Method
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1.12 Dynamic Programming Method for Unit Commitment
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Outcomes
At the end of the course the student will be able to:
CO1: Analyze various levels of controls in Power systems, SCADA and Solve unit commitment problems [L4]
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