SDM Smart Device Expert OLTC Torque Monitor

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

The energy for OLTC operation is supplied by a motorized mechanism, which applies the appropriate torque at each commutation stage, thus creating a typical “signature”.Under normal conditions, this signature is repeatedduring every commutation. Incipient mechanical problems in the OLTC change this signature which can be detected early.

Since the torque developed by the motor is proportional to the electrical power, the SDM detects problems 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 SDM Torque Monitor monitors several variables in real time:

• Current Oscillographs, voltage 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, extracting valuable information for the diagnosis and prognosis, as applicable:

• Power and energy signature of the motor during operation;
• Motor starting current;
• Commutator operation time signature;
• Minimum and maximum voltage 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 the next service time maintenance, number of operations and integration of the commutated 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 values from the engineering algorithms, the IDM provides visible 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.

DNP3.0 AND IEC 61850 PROTOCOLS

The IEC 61850 communication protocol is available for the communication between the SDM and the user’s data acquisition systems.The user may choose, during configuration, if he wants to use the Modbus, DNP 3.0 or IEC 61850 protocols.

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 supply weather 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 inconvenient as when retrofitting old transformers the IDM can be supplied in a