11-02-2013, 01:43 PM
POWER SYSTEM AUTOMATION
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OVERVIEW
Power providers constantly deal with demands to increase productivity and reduce costs. This translates into the need for administrators, engineers, operators, planners, field crews, and others to collect and act on decision-making information. Power system vendors are following a trend to make devices smarter so they can create and communicate this information. The term “power system” describes the collection of devices that make up the physical systems that generate, transmit, and distribute power. The term “instrumentation and control (I&C) system” refers to the collection of devices that monitor, control, and protect the power system.
Power system automation refers to using I&C devices to perform automatic decision making and control of the power system.
Data Acquisition
Data acquisition refers to acquiring, or collecting, data. This data is collected in the form of measured analog current or voltage values or the open or closed status of contact points. Acquired data can be used locally within the device collecting it, sent to another device in a substation, or sent from the substation to one or several databases for use by operators, engineers, planners, and administration.
Power System Supervision
Computer processes and personnel supervise, or monitor, the conditions and status of the power system using this acquired data. Operators and engineers monitor the information remotely on computer displays and graphical wall displays or locally, at the device, on front-panel displays and laptop computers.
Power System Control
Control refers to sending command messages to a device to operate the I&C and power system devices. Traditional supervisory control and data acquisition (SCADA) systems rely on operators to supervise the system and initiate commands from an operator console on the master computer. Field personnel can also control devices using front-panel push buttons or a laptop computer.
Power System Automation
System automation is the act of automatically controlling the power system via automated processes within computers and intelligent I&C devices. The processes rely on data acquisition, power system supervision, and power system control all working together in a coordinated auto-
matic fashion. The commands are generated automatically and then transmitted in the same fashion as operator initiated commands.
I&C System IEDs
I&C devices built using microprocessors are commonly referred to as intelligent electronic devices (IEDs). Microprocessors are single chip computers that allow the devices into which they are built to process data, accept commands, and communicate information like a computer. Automatic processes can be run in the IEDs, and communications are handled through a serial port like the communications ports on a computer. IEDs are found in the substation and on the pole-top.
Instrument Transformers
Instrument transformers are used to sense power system current and voltage values. They are physically connected to power system apparatus and convert the actual power system signals, which include high voltage and current magnitudes, down to lower signal levels.
Programmable Logic Controller (PLC)
As the name implies, a programmable logic controller (PLC), is an IED that can be programmed to perform logical control. As with the RTU, a dedicated pair of copper conductors for each contact and transducer value are terminated on panels within the PLC. Personnel familiar with the PLC development environment can program PLCs to create information from sensor data and perform automation. The PLC can transfer collected data to other devices and receive data and control commands from other devices through a serial port.
POWER SYSTEM COMMUNICATIONS
Communications Media
Many different types of communications media can be used to conduct the data between IEDs in a power system. They include copper communications cables, power line carrier (PLC), land line telephone, fiber, and wireless. Wireless includes FM and microwave radio as well as cellular telephone and satellite communications.
Direct copper - A copper communication cable dedicated to power system communications between two devices.
Land line telephone - Conventional dial-up or leased lines dedicated to power system communications.
Power line carrier (PLC) - A method of passing data on the power line conductor at high frequency.
Fiber - Fiber applications communicate data in the form of light conducted over a single direct
connection or multiple direct connections bundled together.
Communications Connections
Direct connect and multidrop are the two types of communications connections available to create networks. In a direct connection, there are only two devices connected to each other. The network media, or conductor, used for passing data can be metallic, wireless or fiber. Each interface consists of a separate transmit and receive connection at each device. Since there are only two devices, each of them can constantly control the connection on which they are transmitting and both can know implicitly to which other device they are connected. Having several individual direct connections to many IEDs would allow each of them to communicate simultaneously. A system of many direct connections originating from one device is called a star network topology. Figure 14 illustrates the star topology. Many star networks can be connected together.
Any protocol, including those designed for multidrop applications, can be used for direct connec-tions in a star topology. Virtually all microprocessor-based relays, LTCs, and meters have a simple EIA-232 serial port connection to support direct connections. Fiber, wireless, and PLC can be used in a direct connection as well.
Star network designs support a wide range of IED capabilities. Simple, slow communicating devices can coexist with more complex, fast communicating relays. Devices from different manufacturers with different protocols can coexist in the same star network because each has a dedicated direct connection.
Conclusion:-
The benefits of monitoring, remote control, and automation of power delivery include improved employee and public safety, and deferment of the cost of purchasing new equipment. Also, reduced O&M costs are realized through improved use of existing facilities and optimized per-formance of the power system through reduced losses associated with outages and improved volt-
age profile. Collection of information can result in better planning and system design, and increased customer satisfaction will result from improved responsiveness, service reliability, and power quality.