Eliminating the Dummy Loop
The protocol used during thermal cycling testing requires that the cable system undergo a heat cycling voltage test over the relatively long period of 20 days. The test involves heating, soaking and cooling the cable system for 20 cycles while the system is energized according to the specific voltage class of the cable and accessories. Each cycle lasts 24 hours while heating is maintained for 8 hours. During the first 6 hours, the cable conductor must reach a specified temperature and this must be maintained within a 5°C limit over the next 2 hours. The cable is then allowed to cool naturally for 16 hours.
When performing this test, the standards suggest a control loop utilizing an identical cable. This ‘dummy’ loop is heated in the same manner as the test loop and the temperature of its sheath and conductor are continuously recorded. The only difference between the loops is that the dummy loop is not energized and therefore thermocouples can be directly attached to the conductor in order to measure its temperature.
The need for such a dummy loop has effectively been eliminated by implementing a mechanism for transmitting data under voltage using a wireless data logging transmitting system. Basically, conductor temperature is measured by means of a smart link telemetry system installed on a length of the same cable being tested. Thermocouples are attached directly to the surface of the conductor as well as to the sheath of the control cable and connected to a wireless transmitter nearby. The control cable is then installed between the outdoor terminations in series with the test loop such that the conductor in this length of cable carries the same current as the test loop itself.
Use of such a ‘smart link’ allows for electrically isolated temperature measuring points directly on the conductor and on its sheath. At the moment, existing monitoring equipment at Kinectrics during heat cycle voltage testing cannot be used to automatically control the test loop’s heating cycles since it is unable to continuously transfer data under high voltage.
However, new fibre-optic based technologies for monitoring temperature under voltage are available and one has already been identified that has the potential to perform well during the heating cycle voltage test described. The advantage of such a system lies in the fact that the fiber-optic cables are isolated and can be attached directly to the energized conductor. Such a set-up allows temperature reading of the smart loop’s conductor to be captured on a continuous basis, thereby enabling automatic control of the test loop heating current. These minor modifications to the temperature sensing elements combined with custom programming allow the system to be tailored to the application.
A fiber-optic temperature monitoring and control system as described above has already been successfully tested in monitoring mode during a 132 kV cable system type test. Moreover, it is now being deployed as the primary monitoring and control system for type testing of 240 kV cable.
Commissioning Tests for HV XLPE Cable Systems
Traditionally, due to lack of AC high voltage power supplies capable of energizing several kilometers of cable, DC overpotential tests similar to those used for fluid-filled cables have been applied. However, possible problems caused by space charge injection during such testing resulted in DC tests being abandoned for cables insulated with solid extruded polymers. Lack of alternative forms of external energization has led to the so-called ‘soak test’ under which the newly installed cable is put on potential for a 24-hour period. Unfortunately, there have been cases reported of subsequent cable and accessory failures within only a short time after the tested cable system was placed into service.
Availability of variable frequency AC power supplies has since enabled after-laying high voltage testing of XLPE-insulated transmission cables up to 400 kV and over 20 km in length. In this regard, application of AC overpotential testing in conjunction with partial discharge (PD) testing can aid in assuring reliability of a new cable installation.