First-Hand:A Hidden Voltage Source
Submitted by R. H. Rehder, LSM, Peterborough, Ontario, Canada
In the 1970’s Canadian General Electric Company Limited (CGE) in Peterborough, Ontario, Canada, was faced with the problem of measuring conductor temperatures at site during commissioning of a 30,000 A, 23 kV , isolated phase bus duct at a large nuclear generating station. In the terms of the bus duct contract CGE had guaranteed the limit for conductor and enclosure losses and Dr. E C Elgar had calculated a temperature profile of the bus run as support. The customer agreed that if the temperatures of the conductors and enclosures during the commissioning heat run were equal to or less than the calculated predictions then the losses were equal to or less than the guarantees. Once the heat run had been on line and approaching a leveling off point Dr. E. C. Elgar was notified in Peterborough that the heat run was at 27,760 amperes and details on air flow on cooling system and ambient temperature. Dr. Elgar then recalculated the temperature profiles and these were used for comparison with actual temperatures when the heat run was terminated.
The conductor temperature measurements were made using thermocouples located on the inside surface of the tubular conductors. They were connected to a central instrument package that included automatic stepping point to point and a digital readout of the temperatures. This instrument package was also located on the inside of the tubular conductors and batteries supplied its power requirement. A window opening in the high voltage conductor and a corresponding window opening in the ground potential bus enclosure permitted technicians to safely visually read the temperature values. For convenience a video tape camera was mounted at the window in each phase and a display was monitored at a central control desk.
There was concern that the battery in the instrument package could fail due to high ambient temperatures and how would you change batteries without shutting down the main generator. Also it would be an advantage to leave the measurement system inside the conductor as it could be used to monitor temperatures during future normal operations if the batteries did not have to be changed.
A hidden voltage source for the instrumentation was found. There is practically no magnetic field inside a tubular conductor. An insulated wire was run along the inside of the conductor and connected to the conductor at the generator connection and the transformer terminals. The wire was cut at the instrumentation site and these cut ends provided a voltage source for the instrumentation. The voltage source was equal to the IR voltage drop along the 210 three phase feet of bus duct. With no external magnetic field around the wire there was little or no inductance. This voltage source would be adequate to provide temperature readings whenever the conductor current was above 15,000 amperes. Temperatures would not be of concern if the bus was carrying less than 15,000 amperes.
For the critical heat run D. Boothman the instrument engineer added circuitry to use this IR power source. He also added circuitry so the power source could be switched from battery to IR or back again by a technician shining a flashlight through the windows onto a light sensitive transistor in the instrument package. The performance of this IR voltage source was verified at the end of the heat run and the instrument displayed identical temperature readings no matter which source was used. At the end of the heat run, the forced air cooling was turned off simulating blower failure and the bus duct current was left at 27,760 amperes until the maximum temperature rise was 20 degrees C above the end of run hottest temperature. Then the current was reduced to 15,000 amperes, the self cooled rating of the bus duct. The IR power source was used and continued to give readings at the 15,000 ampere rating.
This hidden voltage source was again used successfully on the commissioning heat run on isolated phase bus on a vertical run of more than 450 feet at a large underground hydro generating station. On this station the temperature sensors were thermistors and the instrument package inside the conductor was modified to use pulsed infra red light to pass the information from the conductor to the outside enclosure. The pulsed light was converted to digital signals to a computer and it could be viewed on a computer screen in the control area.
In both the above generating stations, the temperatures were equal to or less than predictions and the customers accepted this as verification that losses in conductors and enclosures were within the guaranteed limits.