Technical information
The 3 essential parameters
Protecting electrical transformers implies to continuously monitor 3 essential parameters:
Gas discharge
Presence of gas in the transformer’s tank has two disctinct origins. Gas discharge is caused by pyrolysis of the dielectric fluid, which is generally caused by small shorts due to ruptured insulation. Significant decrease of dielectric level occurs because of a leak on the transformer (e.g. a leaking drain tap).
Gas discharge is a slow phenomenon in comparison to excess pressure.
Monitoring
The monitoring principles are simple and avoid any risk of leaking. A single-piece floating widget with a magnet on its bottom side is housed in a tank in the device; immediately underneath it sits a magnetic bulb with a changeover contact. Since the bulb is not in contact with the dielectric fluid, there is virtually no risk of leaking.
If gas is present in the transformer’s tank, it rises to the device’s tank, thus lowering the floating widget. When the widget gets as low as 3 to 5 mm (.1 to .2 inches) from the bottom of the tank, the magnet switches the contact of the magnetic bulb.
The gas discharge detection contact is not a one-way switch. If the level rises again in the tank (e.g. after having purged the tank), the bulb’s contact will return to its initial state, which will cancel the fault.
Pressure
In case of a short-circuit in the transformer, the electric arc causes an instantaneous shockwave. The excess pressure rapidly rises and swells the tank, which may lead to an explosion.
Excess pressure is the most important safety-related phenomenon. It is also extremely rapid.
Detection
A pressure switch is connected to the transformer’s body with a capillary tube.
The pressure threshold is not defined by Automation 2000, it depends on specifications of the manufacturer of the transformer. The Client provides the value in a technical specification. If a transformer is upgraded with a DGPT2 device, it is set to a standard value of 0.2 bars (29 psi) except if specified otherwise.
We strongly recommend that the exact value for the pressure threshold, which depends on the tank’s elasticity, be obtained directly from the manufacturer of the transformer.
Operation
The pressure switch is a direct actuation bellows switch. The threshold may be modified, in case of On-site testing for example. Its response time is 5 milliseconds with an oil dielectric fluid.
The pressure contact is not a one-way switch: the fault is cleared when the pressure drops below the trigger value.
Temperature
The rise in temperature of the dielectric fluid can be caused either by an internal defect causing overheating, or by overuse of the transformer’s nominal power (heat is dissipated by Joule effect).
Temperature rise is a relatively slow phenomenon.
Monitoring
The temperature of the dielectric fluid is monitored by a thermometer and two independent thermostats (bulb/capillary and liquid expansion with temperature compensation). The bulbs are housed in the well of the attachment flange, which is permanently immersed in the dielectric fluid.
Temperature trigger values are not set by Automation 2000 but can be obtained from the transformer manufacturer. A technical specification from the Client defines these values. If a transformer is updated by adding a DGPT2, it is supplied with the following standard settings: 90°C (194°F) for T1 and 100°C (212°F) for T2. We recommend that the Client contact the transformer manufacturer in order to obtain the exact trigger values.
Operation
An increase in temperature of the thermometer’s bulb causes the liquid in it to expand. The expansion is transmitted along the capillary tube to a mobile device which in turn activates the trigger that controls the electric inverter contact of the thermostat.
T1 is set at a lower value than T2 and acts as a pre-threshold thermostat. T2 is set at the maximum allowed temperature for the transformer.
The thermometer indicates the local temperature of the dielectric fluid and can be used to check the thermostats’ thresholds. The thermometer can be equipped with a maximum temperature marker needle (see Options).
Temperature compensation
This type of temperature detection device is influenced by ambient temperature. When the ambient temperature is high, the measurement is less accurate for high temperatures; conversely, if the ambient temperature is low, the measurement is less accurate for low temperatures. The discrepancy can reach ±10°C (±18°F), which significantly affects the trigger values.
To overcome this, the devices are equipped with an antagonist double strip which compensates for the discrepancy. Therefore, both the temperature reading and the thermostat trigger values will be exact (within tolerance limits).
Using the device
Our device provides the Client with non-polarised dry electrical contacts that can be used to monitor and protect an electric transformer, in compliance with current standards and laws.
Because we have no information concerning the environment up or downstream from the transformer, we cannot define standard uses for the three DGPT2 detection functions as either alarms or triggers. We can however provide simple, reasonable advice as to the possible uses of the device’s detection functions.
Gas discharge is usually a slow phenomenon, and the detection function is usually set as an alarm, so that the transformer can be checked on site and the necessary repairs carried out. In case of a very violent gas discharge, the pressure detection will act before the gas discharge detection does. It should be noted that standards NF C 13-100, NF C 13-200, and NF C 17-300 state that gas discharge detection should be set as a trigger; this is in reference to the “Buchholz”-equipped transformers which have no excess tank-pressure contacts. Furthermore, these standards refer to both liquid-cooled and dry transformers.
Excess pressure in the tank is a rapid and dangerous process. It is advisable to use its detection for a trigger.
Because excess pressure usually occurs in the event of serious damage, the transformer should rapidly be isolated in order to disconnect the fault. If the primary winding’s disconnecting device does not allow for in-charge disconnection, power should be cut down from the secondary winding, then from the primary with a contact auxiliary from the secondary winding.
If either one or both of the circuits does not have a automated circuit breaker, the client must make careful use of the equipment or make modifications. Whatever the case, a rapid fuse with a high breaking power is mandatory (French regulations NF C 13-200, section 551).
If the secondary wiring is automatically cut on a secured transformer, care should be taken that the secondary wiring is not re-supplied by a parallel transformer.
Regarding temperature detection, T1 can be used as an alarm that is set to warn the user that the transformer is operating close to its nominal power rating (level of guarantee). T2 should be used to protect the transformer when it is operating beyond the maximum authorized temperature as defined by the manufacturer. It is usually used as a trigger that cuts the consumption side, but it can also be used to cut off the supply to the primary winding.