Though transformers are equipped with precautionary methods to ensure plant’s performance and longevity, it is important to be updated constantly with what is happening within the apparatus.
Reacting to faults is simpler when the substation crew is kept informed about the transformer’s internal function and the logic behind it. With such information, they could react effectively and immediately to transformer faults.
For instance, when substation transformers are tripped off on fault due to poor wiring conditions, they would not energize it because combustible gases are present in transformer tank and the voltage levels must be limited. In such a situation, the substation operator must inspect relay panels and transformer visually. The control room staff must be duly informed about the situation, along with the reason for the fault.
Faults and protections
The faults that occur in the transformer include earth faults, core faults, interturn faults, phase-to-phase faults, tank faults and other external reasons.
Six common protections for transformers are against overheating, over current, earth fault, gas detection relay or Buchholz, over fluxing and differential protection of transformer.
Transformer relays provide control, basic protection and backup protection including current differential and restricted ground differential and over current protection.
The most common relays used for transformer protection include differential relay, over flux relay, Buchholz relay, OLTC Buchholz relay, PRV relay, OT Trip relay, WT Trip relay and overload alarm relay.
Thermal overload relay protection measures thermal time constant of transformer’s windings and provides protection accordingly.
Checklist for substation operators when transformers trip off on fault
Check all relevant relay panels and follow protection flags
Inspect the transformer visually
Contact control room and report the fault
Low oil level alarm
Two types of oil level indicators are attached to transformers in which one detects temperature and another detects level. In one type, conservator gauge temperature is ensured to be identical with transformer oil temperature gauge.
In the second type, a float arrangement is made inside the transformer with the help of Buchholz relay so that an alarm is activated when either oil lowers below the preset level or if gas is built up inside the transformer, and informs the control center.
When a gas surge is formed due to internal fault in the transformer, the Buchholz trip is activated and oil level falls below the Buchholz relay.
Such arrangements are made because speedy removal of faults reduces damages. The concerned staff must immediately collect gas samples employing Buchholz bleeder valves and send those samples for chemical analysis. Such analysis would enlighten on the type and intensity of the fault, which in turn would help to make decisions in future.
In exceptional cases, an additional Buchholz pressure activated relay is found in tap changer tank.
Winding Temperature Indicator
The winding temperature indicator is programmed with a maximum temperature indicator which is associated with the loaded winding. Heating resistance applied to the current transformer must be proportionate to the winding.
The winding temperature indicator (WTI) and oil temperature indicator (OTI) operates the alarm, trip and cooler control and functions with the help of thermal imaging.
Overload of transformer or malfunction of cooling equipment is identified by WTI and provisions for extra cooling are made before the alarm trips.
Oil temperature Indicator
The oil temperature indicator or OTI initiates alarm and trip settings in such a way that winding temperature gauge is lowered and heat generated through cooling medium so that alarm settings of the oil gauge and winding gauge are same.
The oil protection and winding protection complement one another and ensure efficiency in the absence of pumps and fans.
Malfunction of Earthing Compensator
When the earthing compensator gives earth reference to delta winding, earth faults in the connected instruments are detected. When fault occurs, return path for earth currents are activated and fault current level is mitigated. The earthing compensator also measures earth current for transformers and limits rise in voltage when fault occurs.
Over excitation protector of transformers
When magnetic flux increases above the designated level in transformer, over excitation occurs in the transformer as a result of which magnetizing current increases and damages the transformer.
However, such a situation could be averted or taken care of by balancing voltage and frequency is appropriately balanced. In simple words, if voltage is increased frequency should be increased, so that the transformer is not overexcited.
Though not an internal fault, such a situation can lead to overexcitation of transformer which can be avoided with differential protection.
The differential protection must be connected to the windings in the side of the transformer without tap changer. Since the side with tap changer tolerates voltage changers, it is not suitable for excitation protection.
Besides products such as oil purification systems, oil recycling plants and drying ovens, transformer alarms play a prominent role in protecting the apparatus, none the less. While other entities ensure protection on process, alarms call for action in emergency cases, which should be immediately attended.