IDM – Intelligent Electronic Device OLTC Torque Monitor

Because it has moving parts, which switch high voltages and currents, the on-load tap changer (OLTC) is statistically a major source of faults in transformers. In order to detect defects in the OLTC at an early stage and reduce the likelihood of unexpected downtime, the IDM monitors the OLTC torque online, alerting the operator to changes in its typical signatures that indicate developing failures.

Energy for the OLTC operation is supplied by a motorized mechanism, which applies variable applicable torque for each commutation stage, thus creating a typical “signature”, which under normal conditions, is repeated at each commutation. Incipient mechanical problems in the OLTC change this signature, facilitating early detection.

Since the torque developed by the motor is proportional to the electrical power, the IDM detectsproblems indirectly by measuring motor power and provides alarms when the commutation signature or time changes. Hence,incipient commutator mechanical failures ae detected early. In order to perform this task the IDM Torque Monitor monitors several variables in real time:

• Current Oscillographs, voltages and power consumed by the motor during operations, either in single or three-phase mode;
• OLTC tap position, through resistive disc or current input (e.g. 4-20mA);
• Driving mechanism temperature;
• Mechanism anti-condensation heater current;
• Line Current interrupted by the commuter contacts;
• Motor circuit breaker auxiliary alarm contact.

In order to diagnose the OLTC, the IDM correlates these measurements in specific engineering algorithms, extractingvaluable information for the diagnosis and prognosis, as applicable:

• Power and energy signature of the motor during the operation;
• Motor starting current;
• Commuter operation time signature;
• Minimum and maximum voltages in the motor during operation;
• Number of OLTC operations and service time, total and after the last maintenance;
• Integration of the commuted current, indicating wear of the commuter contacts, total and after the most recent maintenance;
• Time remaining until to next service/maintenance, number of operations and integration of the commuted current;
• Heater mechanism operation, avoiding water condensation and oxidation;
• Under or over temperature of the operation mechanism;
• Motor power supply under or over voltage;

In the event of abnormalities in the measurements and calculated valuesfrom the engineering algorithms, the IDM providesvisible alarmscomplimented by the activation of output contacts(as well as maintenance schedules in advance for the user) via a signalling system that allows you to quickly diagnose the OLTC state,:

• Green – Transformer operating correctly. No maintenance required;
• Blue – Transformer working correctly. Maintenance reminder;
• Yellow – Minor abnormality;
• Red – Major abnormality.

PT100ΩW Temperature Sensor at 0ºC

A temperature sensor installed in a thermowell on the transformer’s cover usually measures the top oil temperature in a power transformer.  The sensors used with the IDM are Pt100W at 0ºC. These sensors are supplied with the appropriate size and fittings to suit the thermowells.

We also supplyweather shields for ambient temperature measurement.

External Clip-on CTs (Split Core)

The use of external clip-on CTs with a split core window allows the installation of the temperature monitors without directly connecting the bushing CT secondary to the IDM. This eliminates the risk of accidentally opening the secondary and allows installation during transformer operation.

Outdoor cabinets

The OLTC Torque Monitor must always be installed in a shelter against the weather. The IDM OLTC can be installed in the transformer control panel. When this is inconvenientas when retrofitting old transformers the IDM can be supplied in a weatherproof cabinet.