Sensors for Conditioning Monitoring High Voltage Switching

Condition monitoring on high voltage switchgear for the supply of mains electricity is an important function for preventative maintenance to ensure continuity of supply to consumers. The high voltage switchgear at major power distribution facilities operates in a very harsh environment with very high voltages and very fast movements.

The condition of the switchgear and mechanism is assessed by checking the profile of the movement.  Because the switch closes so fast, within a few milli seconds, the sensors measuring the movement has to have very good frequency response.  The P500 rotary sensor has a frequency response of over 10KHz which is sufficiently fast to give an analogue output with sufficiently small phase lag.  The sensor also provides a 4-20mA output signal which is more convenient for connection in facilities which are a long way from the control centre.

The P500 sensor has been tested for a long period and has proven to be very reliable in the difficult application.  The customer is specifying the P500 sensor across the whole range of switchgear produced in Europe.

For more information contact:

Sales at Positek Limited, Tel:  +44 (0)1242 820027 Fax:  +44 ( 0)1242 820615

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British Sensors Feedback Position of Water Jets for USA Support Vessel

The first support vessel in the USA to be powered by waterjets is using a Rotary Inductive Position Transducer (P500) from British company Positek. New Zealand manufacturer Hamilton Jet supplied the quadruple HM571 waterjet systems for the 140ft vessel, which can reach speeds of up to 28 knots. The many benefits of waterjet propulsion, including outstanding manoeuvring capabilities - necessary for crew and equipment transfer - place high performance demands on the sensors within the steering and reverse control system.

The Hamilton waterjets provide manoeuvring by using a reverse duct that can deflect the jet stream anywhere between maximum forward and full reverse thrust. In addition a steering deflector controls the lateral direction of the jet stream. By combining reverse duct and steering positions, full 360 degree thrust vectoring is obtained.

Two P500 sensors are used in each hydraulic system to sense the position of the reverse duct and steering cylinders. Making very small adjustments of the reverse duct and steering deflector allows very precise manoeuvring and the ability to 'hover' the vessel in a given position.

Initially, Hamilton Jet incorporated two potentiometers into the system but soon found them to be completely inadequate, as Dick Borrett - Chief Engineer, Control Systems at Hamilton Jet explains: "In this installation, the potentiometers were failing after just two weeks in operation as they weren't robust enough to stand up to the vibration effects. This was causing mechanical wear on the conductive plastic tracks."

This led him to search for a non-contact position sensor, and Borrett discovered the Positek P500 product, which offers increased operational durability compared with potentiometers and better performance and at lower cost than other inductive sensors.  The P500 is supplied with an internal micro electronic interface incorporating Positek's unique single chip integrated circuit design.

"We needed to move quickly and soon bought four of the devices.  Subsequent testing proved them to be extremely satisfactory in terms of performance and durability. We decided to move over to the P500 as standard and will continue to do so for future projects." The accuracy levels required were within 1mm in terms of actual hydraulic cylinder position which was well within the capabilities of the P500.

The other problem with potentiometers was that we had to construct a housing that was oil and water proof to protect them , in what can be a fairly dirty environment" added Borrett.

"Positek provides a complete 'ready to go' unit. The P500 is totally sealed and enclosed which makes our manufacturing process a lot easier."

Hamilton Jet was one of the originators of the waterjet system and has now developed the more powerful HM range of waterjets for larger sized vessels.

The success of the 140ft support vessel is set to become a significant landmark for the company, and result in further business for UK based Positek which has already supplied 650 P500 products to Hamilton.

"We are going through a phase where work is increasing dramatically, with orders for similar waterjet propulsion systems coming in from the USA and many other countries around the world." concludes Borrett.

When valve actuator control supplier Domgas International, now Limitorque, was looking to utilise an unusual system of flow control. As Domgas Special Projects Manager, Mike Thurlow, explained, the actual valve is a "little bit unique" - a ball valve is being used for flow control and Domgas was appointed to supply 60 hydraulic systems, complete with micro-processor for networking, health monitoring and control.

"Conventionally a hydraulic servo-system would be used, which is a relatively expensive system. We have designed and engineered a system which provides the accuracy of a typical servo for half the cost."

For this project, Domgas had developed a system whereby a highly accurate sensor was required to feed information into the plc programme - which Limitorque had revised to match the hydraulic systems to the electric actuator systems.

"Basically the sensor looks at the position of the valve. The program compares this with the incoming signal - which tells it where the valve should be. This signal is transmitted to drive the hydraulic actuator and the valve into the correct position," says Mike Thurlow.

"The accuracy levels required were highly demanding, 0.4% for over 90 degrees movement, which means that less than half of one degree was the level of accuracy demanded." Having discounted several other options, Mike Thurlow found the answer to his requirement in a highly accurate position sensor - the Positek Rotary Inductive Position Sensor.

Launched by Positek in 1994, RIPS® sensor is a non-contacting sensor which has been developed with an "internal" electronic interface, utilising unique micro-electronic technology.  It has proved particularly suited to control valve applications, providing operational durability along with excellent performance in terms of both accuracy and stability compared with the other inductive sensors.

"We had looked at straightforward potentiometers and also at shaft encoders, but neither were suitable as effective solutions for the accuracy required," says Mike Thurlow.

"Potentiometers are just not accurate enough, particularly when we were faced with big ambient temperature changes.  Shaft encoders are very expensive and also require additional electronics - and cheaper versions did not supply the levels of accuracy we needed.

I first came across Positek at an exhibition symposium and began discussing the possibilities of utilising the RIPS® sensor within this application," he adds.

"It provided the accuracy we required, and also had significant advantages in its price - which seemed very cost-effective.  It is also very compact and well protected. We initially ordered fifty of the sensors from Positek for the first exercise and have recently ordered a further twenty."

Mike Thurlow is enthusiastic about the future possibilities for this system, with the Positek RIPS® sensor an important component.  "We were not previously aware of position sensors such as RIPS® sensor and believe there is considerable potential for this application."

For more information contact:
Sales at Positek Limited  Tel:  +44 (0)1242 820027 Fax:  +44 ( 0)1242 820615

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