A Comparison of Current Transformer Secondary ... - Metrosil
Transcript of A Comparison of Current Transformer Secondary ... - Metrosil
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 11/2 – 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
𝑉𝑃𝑘 = 𝐶 𝐼𝑃𝑘𝛽
• The current waveform was non sinusoidal and
• in phase with the voltage waveform
GDT Based Device as a Passive
Component
• Below 2kVpk, the current through the GDT was
very small and reactive
• At voltages of between 2 – 2.2 kVpk 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 <1.5kVpk
• 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
<1kVpk
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
Vs
Is
ID
IB
RB
ZD 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 >2kVpk
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