LumaSMART
®
User Manual
Installation of Probes and Accessories
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The FOT fiber must not block any oil duct that would interfere to any degree with oil flow. Routing of the fiber
cable must be carefully considered. During block assembly, a spacer at the hottest spot is removed and a modified
spacer with the FOT tip is installed. Functional tests should be conducted after initial installation, after
compression of the coils is applied and after assembly in the tank. Also, during factory testing, the measurement
system should be operational to determine the temperatures at the hot spots. Each fiber must be identified to its
location using a technique similar to wire tags. The fiber leads should be routed out of the windings and along the
leads and then into protective fiber tubing that takes the fiber to the tank wall penetration plate. Sufficient slack
should be left to account for expansion and contraction in the transformer. Never use tie wraps to hold fiber cable
in place. Cotton tape is preferred. The sample specification from LumaSense addresses most of the problem issues.
3.2.2
FOT Installation Guidelines for Common Designs
The number of probes to be installed is dependent upon the information that the user desires and how the
transformer is loaded.
Three Phase Wye-Wye or Delta-Wye Sub-Transmission Transformers Feeding Distribution Systems
For transformers that may have unbalanced loading and/or feeding distribution systems, at least one FOT probe is
recommended for the hottest spot in each phase of the transformer. This ensures that the hottest spot will always
be available to the remote operator and will control the cooling, alarms and protection.
In cases where transformers have a tertiary winding, the following considerations must be made:
Loaded Tertiary (3rd Winding)
In this case, the secondary and tertiary windings will have an MVA rating such that the sum of the two is equal to
the primary winding MVA rating. It is important to monitor both the secondary and tertiary winding since the load
could subject either or both to an overload without exceeding the primary winding MVA rating. In this case, the
two windings that are loaded should have an FOT probe installed in the hottest spots of each winding.
Consideration should be given to installing an FOT probe into the primary winding as overloads on two separate
secondary windings are additive in the primary.
If the load on the tertiary is fixed and no additional load is possible, installation of an FOT probe is probably not
required for temperature measurement. In this case, the tertiary is usually rated for smaller load than the
secondary windings.
Buried Tertiary that is Delta Connected
In almost all cases, the tertiary will not require an FOT probe installation as the tertiary duty will be limited to zero
sequence currents that should be within the design of the transformer.
Note:
For HV and EHV three phase transmission transformers, most operators will install an FOT
probe in each phase in the hottest spot, allowing the controller to decide which phase controls.
Some users will also install redundant FOT probes into the windings, so that if one fiber optic probe
is damaged during maintenance, continuous operation with all three phases will be possible until
the repair is made.
Three Phase Wye-Wye, Delta-Wye, Autotransformer Used for Transmission Step Down or Step Up and
Generator Step-Up Unit (GSU) Transformers
In all of these cases the load on the transformer is usually balanced between phases, and selection of only one
phase hotspot may be adequate for controlling and protecting the transformer. Usually this will be the center
phase. If this option is selected, it is highly recommended that redundant FOT probes be installed as a contingency
during service. If the user does not install FOTs into all windings, protection for blocked cooling ducts or loss of
cooling in the unprotected phase will not be detectable.
For transmission transformers with tertiary windings, the same recommendations apply as in the previous section.
