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05/05/2005
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Tom Shelley reports on a technology that allows current sensing to be greatly improved, particularly that pertaining to fault currents and transients
A new generation of air circuit breakers can be made 25 per cent smaller than their predecessors with improved performance thanks to their ability to detect sudden current rises using air cored sensing coils and electronics instead of traditional current transformers.
Although first described in an article published in Germany prior to World War I, the coils are still not used as widely as they might be, despite their eminent suitability for monitoring current in most kinds of electrical devices and systems, whether they be rotating machines, power supplies, transmission equipment, electromagnetic launchers or railways.
The new circuit breakers are the EMAX range from ABB and the coils are the latest embodiment of the Rogowski coils first described by Professor Walter Rogowski in an edition of the Archiv fur Electrotechnik published in 1912.
Current transformers are toroidal coils wound on an iron core, with the conductor whose conductor is to be sensed running through the hole in the middle. Rogowski coils, on the other hand, are similarly wound but with only an air core. While they cannot deliver the current output used to operate the older generations of meters and trip devices, they do not saturate, and deliver a voltage linearly related to current over a very wide range of frequencies. Coupled to modern integrating electronics, they are thus ideal for detecting the early onset of potentially damaging transient conditions.
The EMAX breakers still use current transformers to provide operating power for the electronics, and to power the trip. The E1 model, however is now able to offer current ratings up to 1600A instead of 1250A and the E2 version S can break 85kA at 440V, up from 65kA. ABB has also incorporated features in the range that are conventionally supplied by external relays. These include directional and reverse power protection, restricted earth fault and metering including harmonics, further reducing the size of switchboards. The devices also include protection units including a data logger function able to record 20 trips and up to 60 events in non volatile EEPROM. A Bluetooth interface allows trip parameters to be remotely set from PDAs and laptops. The devices conform to all the requirements of EN 60947-2: 2003 for low voltage switch gear and control gear.
A British company with particular expertise in the design and development of Rogowski coil sensors is Rocoil Rogowski Coils based in Harrogate. The company claims that it can make transducers to measure alternating currents whose frequencies range from less than 0.1Hz to 1MHz, at currents ranging from a few mA to over 1 million amps. Applications listed include: measuring the starting transients of electric motors, fault testing uninterruptible power supplies, tracing earth faults in large and complicated conductor systems, measuring weld currents and those in rail guns, and monitoring railway traction current to ensure that locomotive power systems do not produce frequencies that could interfere with signalling.
ABB
ABB enquiries
Rocoil Rogowski Coils
Pointers
* Rogowski Coil sensors do not saturate, respond to low or high frequencies, and produce a voltage output proportional to current
* Their use in circuit breakers allows them to respond more quickly to fault currents, improving performance and allowing a significant reduction in size
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Author
Tom Shelley
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