Improving Network Performance Through Automation And Improved Protection Grading

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Extracted from "Application of Advances in Magnetic Actuator, Vacuum Interrupter and Microprocessor Technology to Improve Network Performance Through Automation and Better Protection Grading of Auto Reclosers". This paper was presented to the IEEE conference in Acapulco, Mexico in 2004 by Neil O'Sullivan, Managing Director, NOJA Power and Tony Stacey, Sales Manager, NOJA Power. A complete copy of this paper is located on the NOJA Power website.

Today's electricity utilities are challenged with achieving greater performance and reliability levels than ever before, generally with fewer resources available to them. The use of modern auto reclosers with Intelligent Electronic Device (IED) controllers is one of the most effective tools available to today's utility engineer to increase overhead network performance.

Modern recloser design has seen the application of many new technologies to the problem of increasing network reliability. Single coil magnetic actuators and vacuum interrupters have made it possible to simplify the mechanism design. This has resulted in faster operations, greater reliability and a longer lifetime.

NOJA Power combines these simpler mechanisms with compact Rogowski current sensors and capacitively coupled voltage measurement techniques to build a recloser with reduced size, lighter weight and increased effectiveness in overhead network automation and protection schemes. All this is achieved without using oil or SF6 insulants within the recloser.

Solid di-electric reclosers are commonly constructed using cyclo-aliphatic epoxy. These suffer from a few shortcomings:

1) No Arc-Fault Containment

It can be argued that with three separate poles it is not possible to achieve a phase to phase arc fault and therefore arc fault containment is not required. However, it is certainly possible to achieve a phase to ground fault that could propagate. In this case arc fault containment is essential.

2) Power Measurement

Solid dielectric reclosers utilising cyclo-aliphatic epoxy resin for both the mechanical support structure and insulation suffer from an inability to provide voltage measurement on all six bushings. The ability to measure voltage on all six bushings and current on all three phases is essential in order to provide full distribution automation functionality.

NOJA Power's reclosers use a combined insulation system to eliminate the two problems above. Combined insulation allows current and voltage measurement devices to be embedded into all six bushings. The dead tank housing means arc-fault containment can be integrated into the design.

NOJA Power reclosers use Rogowski sensors for current measurement. Unlike traditional Current Transformers they do not saturate at high current levels, are not influenced by hysteresis or DC components, do not generate dangerous voltages when open circuit and are very light weight.

The combined insulation system allows the use of conductive rubber screens inside the bushing for voltage measurement. These are not affected by temperature variation making their accuracy more reliable than the more traditional method of moulding a screen into cyclo-aliphatic epoxy bushings.

Pole mounted switchgear must always strive to use the lowest amount of energy possible to operate the device. This is to allow battery operation in the event of a loss of HV power. Enough power must be left over to operate the controller and remote communication equipment. Therefore the design of the actuator mechanism is critical.

Magnetic actuators for use in recloser designs were first developed more than a decade ago. Most relied on rare earth permanent magnet latching. Eliminating these magnets results in a simplified, longer life and more energy efficient actuator.

NOJA Power's design ensures the magnetic actuators and vacuum interrupters are closely matched to provide the maximum force with a minimum of energy. Magnetic actuators such as these allow for faster auto-reclosing with duty cycles as fast as CO-0.1s-CO-1s-CO-1s-CO.

Modern reclosers take advantage of recent advancements in micro-electronics. Features such as power quality monitoring, harmonic analysis, sag and surge monitoring are now possible.

Protection grading can now be significantly reduced. Typically in the past two reclosers in series would have required a minimum of 200ms of grading time. Products using Rogowski sensors and fast micro-electronics sampling can now be reliably graded to within 100ms. This allows for up to eight devices on a single feeder while still allowing the substation circuit breaker to operate in one second. This allows more devices to be used on a feeder to offer more points of fault isolation and increase feeder reliability.

Real time calculation of symmetrical components and their utilisation in protection algorithms allows better discrimination for a more diverse range of fault types. This is only possible if current and voltage measurement together with symmetrical component calculation is utilised to provide more intelligent protection algorithms. An example of this is full directional protection for overcurrent earth fault and sensitive earth fault elements.