4. Discussion When Acting as a Passive Device at Voltages <2kVpk • The GDT based device drew a very low leakage current. This assumes: • The GDT devices were unaged • The applied voltage is <2kV • The SiC based device had larger leakage currents • Calculations of leakage current and power dissipation need to be made, particularly under fault conditions • Calculating the current sharing between a GDT and other voltage limiting devices is necessary within high impedance relay circuits When Acting as a Passive Device at Voltages >2kVpk • The GDT based device forms an effective short circuit • If a CT is used in a high impedance relay system, such voltages may be present during fault conditions • The short circuiting of the CT may influence/prevent the operation of the protection systems Influence of Ageing • The GDT based devices age quickly when connected across an O/C CT • Ageing effects significantly reduce the firing voltage • Could influence the operation of protection devices • 100 heat cycles with a cycle time of 30 minutes = CT in an O/C condition for 1 1/2 – 2 days • The SiC based devices did not age after a sequence of O/C events • The devices maintained consistent operation after 100 heat cycles • The devices will act in a passive manner once the burden is replaced across the secondary terminals of the CT Influence of Thermostat Re-opening Temperature • The thermostat reopening temperature of the GDT based device is 40 ºC • Operation is therefore limited to locations where the ambient temperature is <40 ºC as the would not switch from active to passive if the temperature was >40 ºC • Temperatures at SEC can reach 55ºC Influence of RF Noise • Significant amounts of RF electromagnetic noise is generated when the GDT fires • This may influence the operation of electronic systems within substations A Comparison of Current Transformer Secondary Open Circuit Protection System Technologies Dr. Jeff Robertson [email protected] metrosil.com 2. Test Circuit for Comparison of SiC and GDT Devices The test circuit below was developed to compare the CT O/C protection devices. 5. Conclusion To reliably protect CT’s during O/C events, the Metrosil silicon carbide based devices (CTPUs) proved to have a superior performance to the GDT based devices when subject to 100 heat cycles under O/C conditions. 3. Results of Test Circuit SiC Varistor Based Device as a Passive Component • The current increased according to the V-I characteristics of the varistor discs = • The current waveform was non sinusoidal and • in phase with the voltage waveform GDT Based Device as a Passive Component • Below 2kV pk , the current through the GDT was very small and reactive • At voltages of between 2 – 2.2 kV pk the GDTs fired, causing an effective short circuit across the CT SiC Varistor Based Device for O/C Protection • The clamping voltage is determined through the V-I characteristics of the varistor disc • The O/C heat cycle showed that • The heating phase takes around 37 secs • The thermostatic switch closing temperature is around 141ºC • The cooling phase takes around 30 mins • The thermostatic switch re-opening temperature is around 60ºC • Assumes an ambient temperature of 17ºC • 3 devices were subjected to 100+ heat cycles without performance deterioration GDT for O/C Protection • The clamping voltage is determined from the breakdown strength of the gap in the gas discharge tube • Once the tube fires, the device clamps the system to <2kV • The clamping voltages are not stable, and change with the flickering of the tubes • For successful heat cycles, the switching temperature of the devices are consistent • The heating phase takes around 6 mins • The thermostatic switch closing temperature is around 111ºC • The cooling phase takes around 20 mins • The thermostatic switch re-opening temperature is around 40ºC • Assumes an ambient temperature of 17ºC • It was not possible to reach 100 heat cycles • Testing was aborted once the firing voltage <1.5kV pk • 5 devices averaged a total of 29 heat cycles before testing was aborted • The firing voltage decreased with the number of O/C events • Further testing of 2 devices showed the firing voltage to decrease further to <1kV pk Silicon Carbide Varistor (SiC) Based Device Gas Discharge Tube (GDT) Based Device 1. Background A Current Transformer (CT) can become damaged if it becomes open circuited (O/C) whilst the primary is energised. Two technologies commonly used in the open circuit protection of high kneepoint class X CTs used in Gas Insulated Switchgear (GIS) have been compared. These protection systems are largely designed to act as: • A passive device drawing a minimal leakage current when the relay burden is connected across the CT • An active device to limit the voltage across the CT under open circuit conditions Burden (Relay, etc) Current Transformer CT O/C Protection Device V s I s I D I B R B Z D Arrangement of Protection Devices in a Secondary Circuit Typical AC Waveforms of the Silicon Carbide Based Device when Subject to an Applied Voltage Typical AC Waveforms of the Silicon Carbide Based Device when Protecting an O/C CT Typical AC Waveforms of the Gas Discharge Tube Based Device when Subject to an Applied Voltage A Typical Heat Cycle for a SiC Varistor Based Device GDT Acting as a Passive Device at Voltages >2kV pk Typical AC Waveforms of a Gas Discharge Tube Based Device when Protecting an O/C CT Flickering of Gas Discharge Tubes Whilst Clamping an O/C CT Decline in Firing Voltage of Gas Discharge Tube Devices When Subject to a Number of Heat Cycles Flickering of GDT Based Device when Protecting an O/C CT