Zoltan Roman, R&D Director for Conventional Instrument Transformers, confirms that approach. “Initially the current transformer was chosen because a German customer was interested in a pilot project, and we were convinced that we could make a g3 version available rapidly. The 245 kV CT, originally designed for SF6 insulation, was selected as this system voltage is in the mid-range of high voltage transmission voltages and therefore highly representative of the technology.
245 kV g3-insulated current transformer
The gas-insulated high voltage CT, which is used to measure the current and to relay the data, underwent a whole battery of tests successfully – routine, type and special tests as specified by the IEC 61869 standard. “The tests were carried out in independent and certified laboratories to demonstrate the electrical and thermal behaviour of the g3 gas mixture,” adds Roman.
The range of dielectric tests that were performed at p2 pressure (the second alarm pressure, which is the lowest operating pressure of the equipment) included:
- Applied Power Frequency Withstand Voltage (PFWV). This is the highest voltage to which the equipment will ever be subjected, and then only during testing. This test is performed at 50 and 60 Hz, and the test time is one minute. It is designed to ensure that at extreme overvoltages the equipment is still operational. The PFWV is usually 2.5 to 4 times the normal operating voltage. For 245 kV equipment the PFWV is 460 kV, while the maximum operating voltage is 170 kV.
- Wet PFWV.
- Basic Impulse Level (BIL). This is a fast rising and falling, impulse-like signal designed to simulate lightning stress to which the equipment may be subjected during operation. For a 245 kV system, the BIL is at a peak of 1,050 kV.
- Partial discharge (PD) up to 460 kV.
- Capacitance and dielectric factor measurement at 10% and 100% of the nominal line-to-ground voltage.
- Withstand test at -33 °C ambient temperature after thermal equilibrium has been reached, at 1.2 times the nominal line-to-ground voltage for 15 minutes.
Roman adds: “The current transformer passed all these tests successfully and proved that the g3 gas mixture is electrically equivalent to SF6 in our designs of high voltage current transformers, without modification in the design of most of the parts.”
Ageing tests were also conducted with g3. “Here, the sealing material was subject to particular attention,” Roman explains. “g3 is a composition of a new 3M NovecTM molecule and CO2, the latter being the dominant gas in the mixture. Therefore, a larger leakage rate could have been expected with the pre-existing seals. So we opted for a different material for sealing. A new seal was rapidly developed and proven for service, and a new density meter is also being used because of the increased pressure in the g3 CT.”
Novec™ 4710: A new compound developed specifically by 3MTM for dielectric applications, fluorinated nitrile
After the success of the tests on the 245 kV CT, the g3 gas will be tested on ultra-high voltage instrument transformers – first a 420 kV CT, followed by a combined 420 kV current and voltage transformer.