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 JeffRobertson@mimaterials.com 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