Control System in Power System Automation

Power system Control

• Control refers to sending command messages to a device to operate
the I&C and power system devices. Traditional systems rely on
operators to supervise the system and initiate commands from an
operator console on the master computer.

Power system control functions

Control functions are initiated by the operators or automatically from
software applications and directly affect power system operation.
They can be grouped into four subclasses:
 Individual device control: which represents the direct open/close
command to an individual device.
 Control messages to regulating equipment that requires the
operation, once initiated by the control room, to automatically be
conducted by local logic at the device to ensure operation remains
within predetermined limits. Raising or lowering tap changer taps
is a typical example or sending of new set points to power
generators.
 Sequential control covers the automatic completion of a linked set
of control actions once the sequence start command has been
initiated. A set of sequential switching steps to restore power
through a predefined backup configuration typifies sequential
control.
 Automatic control is triggered by an event or specific time that
invokes the control action. Automatic control of voltage through on
load tap changing responding automatically to the voltage set point
violation is a common example. Time switched capacitor banks are
another.
 PSC objectives
Reconfigure after events like:
Permanent fault
Open – phase conductor
Miscoordination
Multiple faults
Overload condition
Observe Constraints:
Abnormal conditions
Power system limits (V,p.f.)
Protection considerations
 Types of Control System
1. Central Control System(CCS):
A control system in which the power system operation, online
control and management (monitoring) are coordinated at a single
point (Central Control Center) with information from different
sensors, IEDs, plants, substations and feeder system.
To implement the functions of CCC, all online measured data,
including voltages and active and reactive load flow, are required to
be transmitted to CCC in a very tiny time limit.
CCC is the eye and brain of the integrated PS.
DCSs are subsets of CCS.
The measured parameters that are transmitted to CCC could be:
–Online measured data (V,I,p.f.,…)
–Reactive and active power flow
–Status of the protection and controlling devices
–Control commands
–Switch status open/close
–Fault current indication
–Abnormal condition
indication
 In CCC, we should have reliable network, high performance
computers, efficient and robust system.(large amounts of data are
to be processed in real time)
With more and more functionalities deployed to support the
operation of power systems at CCC, the computational burden of
CCC becomes heavier, and many CCCs have to expand their
hardware and software facilities to meet online requirements.
2. Distributed Control System(Aerial CS):
 A control system in which the control elements are not
central in location but are distributed throughout the system
(Substation and power plants) with each component sub-
system controlled by one or more controllers.
 It is featured with monitoring and controlling of the regional
PS.
 The entire system of the controller is connected by networks
for communication and monitoring with the aid of a number
of distributed centers.
 The computational burden of a central control center can be
greatly reduced.
• The difference between the two is that CCC monitors and controls
the entire power system whereas ACC is only responsible for the
monitoring and control of a regionalsystem.
• ACCs appeared prior to CCCs because most CCCs of largescale
power systems are the results of a merger of several regional power
systems that were previously monitored and controlled by ACCs.
• The structure can provide a large saving on capital investment for
the construction of expensive, long distance communication links.
 SCADA/EMS:

• Both DCS and CCS perform their monitoring and control through
SCADA/EMS that are installed at the control centers.
SCADA: responsible for remote measurements and control.
EMS: represents a set of senior online applications software for
power system optimization and control.
Functional structures of SCADA/EMS