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Supervisory Control and Data Acquisition (SCADA) Optimizes Power Systems Operation
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Supervisory Control and Data Acquisition (SCADA)


The SCADA System

An optimized power distribution and transmission system could have never been made possible without Supervisory Control and Data Acquisition SCADA), since it minimizes the incidence of power outages by proper monitoring and control of critical electrical equipment in the grid. This reduces costly downtime and improves the reliability and utilization of generated, transmitted and distributed energy.

Advantages of SCADA System in the Power Sector

Increased reliability, lower costs; forecasting accurate demand supply management; faster restoration of power in case of a break down; better active and reactive power management; reduced maintenance cost, conditioning monitoring; reduced human influence and errors; assists operator for faster decision making and automated meter reading

Smart Grids

Smart meters depend much on the operation of communications systems. Smart meters monitors theft and various other loses. Transmission and distribution systems equipped with smart meters are called "smart grid".

Scada also minimizes labor cost, since they can be easily programmed to perform automatic tasks that results to smoother operation and minimizing power outages. In case a big power plant trips, the Scada system immediately sends a tripping command to "load dropping" circuits that was pre-programmed in order to avoid possible drop in the system frequency. A drop in frequency say below 57hz will be detected by frequency relays which would trip a generating station. The tripping of the generating station is picked up by the Scada system to implement load shedding or load dropping scheme to prevent possible cascading tripping of power plants. Please read details below.

SCADA systems, however, do more than simply collect data. They also deliver automated control that greatly benefits utilities. Their alarms detect problems in the system, and analysis of these problems enables adjustments and corrections, often preventing an outage. In the event of power disruption, Scada’s data gathering system provides immediate identification of the cause and its location which is relayed to field personnel by power management personnel for proper action.

SCADA Operation

Scada is a computer-based software program fed with data collected from generation, substation, transmission and distribution system through a complex network of controllers and operator interfaces.

Programmable Logic Controllers (PLC) enable the control and monitoring of power systems by specialized set of devices, circuit breakers and power monitoring systems. As an example, data collected by the PLC and its associated devices in a substation are transmitted to a PC based scada system within the substation (local scada network) which is linked to a distant Central Scada Station by fiber optics, microwave link (use in long distances) or Ethernet-based systems implemented via the Internet in a secure “cloud-based” system.

Aside from gathering information, Scada systems enable the power management personnel in the central station to issue circuit breaker opening or closing commands as might be dictated by the network condition. Taken in a simple term, Scada provides better power management of power generation, transmission and distribution system.

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Load Dropping or Load Shedding

Pre-programmed Scada operation include load dropping schemes which stabilizes the grid frequency in the event of tripping of a large capacity power plant or tripping of a major transmission line circuit that subjects some power plants to become overloaded; and some plants becoming "under-loaded" as they abruptly lose a big part of their loads which causes them to trip, too (over frequency condition). A frequency relay (Device#81) is the device use during under/over frequency system condition. As an example, a frequency relay is set to trip a generator below 57Hz and above 63Hz. A frequency relay functions on a predetermined value of frequency either under or over, or on a normal system, frequency or rate of change frequency. System frequency drops during an overload and rises up above 60Hz (rotates faster) when the generating station suddenly loses a substantial part of its load. The tripping of the generating station is picked up by the Scada, which implement load dropping measure to prevent a serious power outage as a result of cascading tripping of power plants. Cascaded tripping happens when the load supplied by a "disproportionately" large power plant trips and the load shedding scheme operation is delayed or insufficient. This will cause other power plants in the grid to become overloaded, since their running reserves cannot immediately be put to use: water in a hydro-electric plant, steam, in case of geothermal or coal-fired power plant, have to build up pressure gradually in order to cope with the additional demand on their system.

On the other hand, when an un-looped major or backbone transmission line, say a 230kV bundle of 4-795 ACSR double circuit transmission line interconnecting region to region's power plants in the grid - for example - the Southern region and the Northern region - trips; the Northern region suffers from under frequency that can trip several plants or all of the plants, if it has a larger number of industrial/commercial loads and no automatic load shedding was implemented. If the Southern region supplies a substantial load to the Northern region, but by itself have lesser number of connected loads, power plants in that region suffers from over frequency that can reach the tripping threshold of several power plants or all of the power plants. It is called cascaded tripping when all the plants supplying power to the grid trips. That's a big disaster unless proper measures have been diligently implemented to prevent it from occurring. These sequence of events happen before you can even blink your eyes twice or thrice.

The ability of a Scada system to automate the full operation of a power plant and substation reduces the number of field operations personnel. Compared to human beings, computer acts with dispatch, since parameters in all possible events sequence that can occur are pre-programmed into the PCs data system.

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The SCADA Management System

The management of SCADA system has three classifications:

1. Input

Analog:

Analog input are used for monitoring of electrical parameters such real or active power (MW), apparent power (KVA), reactive power (KVAR), voltage (kV), system frequency in (Hz), power factor and many many more parameters that operates steadily such as a continuous water flowing in the cooling system of power plants.

Digital:

Digital input are used for monitoring electrical installation system status such as an open or close circuit breakers, load break switches, isolators and many more. This parameters are translated to either 0/1 (open/close).

2. Signal Processing

The signals gathered from both the analog and digital inputs are then converted to digital format which implement a protocol between master and slave and operates on real time OR Real Time Operating System (RTO)

3. Output

In the Scada station where they are viewed on monitoring screens, the single-line-diagram of the electrical installation being monitored will show which switching element is close or open.

Application of SCADA in Transmission Lines

Courtesy of : IOSR Journal of Electrical and Electronics Engineering (IOSR-JEEE) e-ISSN: 2278-1676, p-ISSN: 2320-3331, pp 67-71 - www.iosrjournals.org

Automatic Switching

Emergency Load shedding; Re-routing services for station maintenance; Automatic transfer schemes; Load sectionalizing; Custom, automatic reclosing schemes; Automatic service restoration; Circuit breaker control and interlocking; Feeder automation and fault recovery.

Protection and Control

Circuit breaker lockout; protective relay interface/interaction; Dynamic protective relay setting for dynamic station topology.

Voltage Regulation Management

Load Tap Changer (LTC) control; Voltage regulator control; Capacitor control; Transformer Management Parameter monitoring and alarming; Real-time modeling; Interface to existing transformer monitors.

Automatic System Diagnostics

Power apparatus health monitoring; LC and communications self monitoring; Report and alarm on IED self diagnostics.

Maintenance and Safety

Kirk Key interlocks management; Maintenance „Lock-out/Tag-out management; Automatic circuit isolation control

Station MMIs – Graphical User Interface (GUI)

Interface real-time single-line displays; Interactive real-time breaker and switch control display; On-line operation and maintenance logs; Sequence of events recording; IED detail displays; Parameter trending displays

References

SCADA Systems Automate Electrical Distribution - A White Paper from InduSoft

SCADA in Transmission Line - Shabnam Rukhsar

Guide to Supervisory Control and Data Acquisition (SCADA) and Industrial Control Systems Security - National Institute of Standards and Technology (NST) US Department of Commerce


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