What is On Load Tap Changer Monitoring? Why is OLTC Needed?

On Load Tap Changers (OLTCs) are a specific type of voltage regulator that can be fitted to Power Transformers. OLTC regulate connection points from the Transmission line for high-voltage or low-voltage and will vary the transformer ratio while the transformer is energized and carrying load.

What does an On Load Tap Changer (OLTC) Do?

An OLTC voltage regulator opens and closes switches that connect and disconnect taps on specific transformer windings.  One may consider the OLTC to be a very large rotary switch whose mechanism is driven by a gear motor. Principally there are two different types of load carrying contacts used:

1. Physical contacts that open and close in an insulating fluid
2. Physical contacts that open and close in a vacuum bottle that itself is immersed in insulating fluid

Both share what is known as a reversing switch that are load carrying and always in the insulating fluid.

The advantage of using OLTCs over Step Voltage Regulators is that it has a smaller footprint in the substation than individual single-phase step voltage regulators. However, it has the disadvantage, should there be an issue with the OLTC, that will require the entire Power Transformer to be taken out of service to repair. It is estimated that roughly 50% of all Power Transformers are fitted with an OLTC.

OLTC Failure Modes

It is estimated that 33% of all Power Transformer failures are caused by a failure of its OLTC.  There are several reasons why failures occur:

1. Coking contacts

Coking is the process that typically occurs when there are carbon particles in the insulating fluid that may get trapped between the contacts. As the carbonization above shows, the I²R heating becomes so severe, the hot metal gases begin to build as the contacts become hotter to a point that the contacts may melt. As the pressure increases in the head space the tank may rupture releasing the oil within. The photo below illustrates an infrared photo taken before the transformer was removed from service:

2. Failing Motor drive mechanism

An improperly operating mechanism has been observed when the contact of the OLTC become melted together, but may be caused by a faulty motor, loose or binding mechanism.

OLTC Condition Monitoring

Advanced Power Technologies has the most comprehensive monitoring packages for OLTCs. It includes:

1. Monitoring the differential Temperature between the Main transformer Tank and the On Load Tap Changer Tank.
2. A methodology to eliminate false alarms due to sunlight heating of the OLTC tank.
3. Magnetic mount temperature sensors that are not thermally influenced by direct sunlight.
4. Mechanism timing to let you know when it is not operating at its top efficiency.
5. Recording of Differential Temperature and Tap position (US Patent 7417411).
6. Simultaneously detects and reports alarms from slowly evolving OLTC events from coking rapid rises of temperatures in the OLTC tank (US Patent 7323852).

The above features provide Electric Utilities with unparalleled capability to:

1. Detects coking as well as a quickly evolving On Load Tap Changer from a rapid temperature increase
2. Guidance on when to schedule your outage
3. Knowledge of the location of the faulty contacts before scheduling the outage
4. When it is impossible to take an outage, you will have sufficient information to know the range where the OLTC may continue to operate by limiting the range of the tap changer via the OLTC controller
5. Replace unreadable tap change counters with our built-in tap position tracking

OLTC Monitoring Solutions

Total Eclipse Transformer Monitor

ECLIPSE Transformer Monitor

TTC 1000 Transformer Monitor

Note:

1. All can monitor OLTC mechanism timing
2. ECLIPSE and Total ECLIPSE can be programmed for OLTC bus voltage regulation