Faq

1. Why do I need to monitor winding temperature of a transformer ?

For economic reasons now-a-days the transformers are operated at “near limits” of temperature ratings. If suitable temperature monitoring system is not installed on the transformer, then there is a very high risk of failure of insulation leading to transformer failure and associated hazards and loss. Hence there is a need for specialized and reliable temperature surveillance system. Although the dry / cast resin transformers are safer than oil cooled ones from the point of fire hazards, they can also cause disasters, if they exceed the rated temperature limits.

2. Do I need to monitor all three windings of the transformer ?

As dry type / cast resin transformers are usually installed at utilities where large unbalanced load is a normal condition, it is necessary to monitor all the three phase windings of the transformer. In case of large oil cooled power transformers, they are usually installed at places where the loads are normally balanced and so in that case, monitoring only one winding’s temperature is sufficient. However, It’s always better to monitor temperature of all the three windings.

3. What is a thermal image and how does it help to estimate the winding temperature ?

Due to high voltages associated with the windings and due to insulation considerations, direct access to the windings for installing the temperature sensor is not possible. Therefore, indirect means of estimating this temperature has to be done. One of the most reliable  methods to estimate winding temperature is by using “THERMAL IMAGE” formation technique. This image is formed digitally by our ( OTI&WTI ) models, by
measuring top oil temperature & current flowing through the winding and doing calculations considering
temperature gradient and time constant. And this image is displayed as Winding Temperature. The RTD
sensor is installed on the transformer to measure “Top Oil” temperature. Winding current / load current is
measured using a C.T. installed in the winding of the transformer. Temperature gradient ( dT ) and Thermal
Time constant of windings are estimated by design consideration and set accordingly.
Please note that the Thermal image gives the estimated winding temperature and is not actual.

4. What is the difference between Winding Temperature Scanner (WTS) & Normal Temperature
Scanner?

WTS are built specially for transformer application & the features provided in this series of scanners are in line
with the requirements of transformer industry.
The operating conditions are more severe for equipments meant for transformer / switchyard application. The
design for these scanners is more focused towards reliability, vibration handling, noise immunity, mounting
ease, higher ambient temperatures etc. Normal temperature scanners are designed for control rooms where
environmental conditions, are much less severe.
This is the reason we call these temperature scanners as “Transformer Protection Relays”. At PECON, we
manufacture products for both transformer as well as for control room application. We therefore insist and
suggest our customers to use appropriate equipment for specific need and application.

5. What features are essential for a Transformer protection relay (WTS, OTI&WTI) ?

For the safety, reliability & long operational life of a transformer, a good temperature surveillance system is
necessary. A good temp. surveillance system should provide the following features:

• Accurate temperature measurements (windings, core, oil etc etc).
• Capability of warning the user of excessive transformer temperature (alarm).
• Capable of switching off and isolating the transformer from line in case the transformer reaches a
  limiting temperature level (trip).
• Cooling fan control.
• Alarm, trip and fan controls settable over operating range.
• Sensor Fault monitoring and indication.
• Maximum temperature registering.
• Signal for remote indication & data logging.
• Should be suitable for switchyard environmental conditions.

All PECON winding temperature scanners ( Transformer protection relays ) are designed to meet these basic
requirements. Most of them will have many additional features incorporated.

6. What is a RTD?

Resistance Temperature Detector or RTD for short, is a device whose resistance changes with temperature.
Higher the temperature more the resistance. This change in resistance of these sensors allow us to measure
the temperature.

7. Why RTD ? Why not thermistor ?

The main reasons which make RTD based instruments a better choice for transformer winding temperature
measurements are :

• RTDs are fully interchangeable.
• They have very good accuracy, stability & linearity.
• They can be easily be tested for their healthiness, while it is very difficult to check the
  healthiness of thermistors.
• With thermistor based equipments,
  • Temperature indication is not available.
  • Only fixed temperature cut offs are possible and no settability can be given.
• RTDs offer higher temperature operating range.
• RTDs come at reasonable cost and are easily available.

Thermistor are mostly used in consumer goods. Their industrial use are limited to, where no indication is
required and where fixed temperature cut offs with no settability is acceptable.

8. How to check RTD sensor S-02 ?

S-02 can easily be checked with ordinary multimeter. Sensor S-02 being a simple resistance, will show about
110 to 115 Ohms corresponding to room temperature between White and Red
lead. Further, both the RED leads should show near zero ohms of resistance.
Table below shows the resistance values at different temperatures

All S-02s coming to you from PECON are checked and calibrated to international standards and traceable to
national standards.

9. Do I have to extend all the three leads of RTD to WTS?

Yes, this is necessary to minimize errors in temp. measurement due to lead resistance of the sensor.

10. What type of wire leads should I use for extending RTDs to WTS?

Standard field cables (copper) of 1, 1.5 or 2.5 mm². (as per plant standard) can be used for RTD lead
connections. It is recommended to keep RTD leads as small as possible to keep lead resistance errors
minimum. Also, try to avoid intermediate joints and terminations. However, this distance in some installations
can be as long as few hundred meters. With three wire leads extension, 250 meters of 1.5 mm² extension
leads will cause less than 1 °C error. 2.5 mm² wire would cause even lower errors.

11. Is it safe to mount WTS directly on a transformer?

WTS is designed for marshalling box application. However, WTS can be mounted on the transformer
enclosure directly, if the transformer is installed in a covered area. Make sure that the surrounding ambient
conditions are within the specified limits.
Never mount WTS inside the transformer enclosure, as the temp. inside the enclosure is likely to be more than
the permissible maximum ambient temperature limit.

12. What is Fan exerciser function ?

Some of the WTS & ( OTI&WTI ) are provided with this feature. This feature turns on the cooling fan for short
time at regular predetermined interval (settable & selectable by the user), irrespective of the temperature of
the transformer. This keeps the fan in healthy & working condition.

13. What is Anticipatory fan control function (related to OTI&WTI only) ?

This function helps in better thermal management of a transformer in comparison to conventional WTI
systems. By enabling this function, WTI can anticipate the winding temperature rising above the FAN
operating set point, much before it actually rises & operates the fan. Studies have proven that this leads to
longer life of the transformer.

14. What is fault relay and how do I use it ?

Fault relay is provided to indicate abnormal or fault condition in WTS. These conditions are,

• No power to the scanner (only for WTS supplied with Electrical Fail fault)
• Sensor RTD open or short circuited.

The exact operation of this relay is configured in a different way for different models. Further, it is sometimes
configured for specific requirements. Please refer the engineering manual & test report for exact configuration of fault relay in a unit supplied to you.
This is a very important feature and when used will prevent transformer operating in “unprotected” condition.
This relay contact is to be used either as a permissive contact in interlocks or is to be wired in alarm system.

15. What is Electric Fail Fault ?

Absence or failure of auxiliary power to WTS is “Electric Fail Fault”. Under this condition, the transformer is
under no protection of WTS.
To avoid this dangerous situation, Fault relay is energized all the time, when power to WTS is available. This
relay turns off, whenever there is an absence or failure of power. This contact can be used externally in
interlocking circuits to avoid this situation of running a transformer without temperature monitoring.
Please note that,

1. This relay will energize after a few seconds after application of power to WTS.
2. This fault relay will also de-energize, in case of sensor circuit errors.
3. Against some specific demands, some of WTS are supplied without this feature.

16. Relay contact diagram

As a standard engineering practice, relay contacts are shown as in “Un-energized” state. During working of the
equipment, the actual relay state may be different than that shown on the label / in the diagram. Therefore, it is
necessary to confirm this from the engineering manual while making the wring diagrams.
This is particularly important in understanding the Fault relay contact conditions in some of our models.

17. What is the ratings of output relay contacts ?

The relay contacts on WTI / WTS & Latch-relay are control contacts. These are primarily meant for
interlocking and controlling the logic circuitry of the power circuits. These contacts are to be wired in control
panel logic scheme / in indicating circuitry / for annunciation / for PLCs / in logic circuitry controlling power
devices like contactors or breakers.
These contacts are not meant for direct motor operation. These being control relays, their specifications, do
not indicate any motor duty like AC1, AC2 or AC3 etc as is indicated for contactors.
The specifications of relay contacts on PECON WTI / WTS are as below :

• – for res. load : 5 A @ 230 VAC, 0.5 A @ 125 VDC.
• – for inductive load : 5 A @ 230 VAC ( Cos Φ = 0.4 ) & 0.3 A @ 125 VDC ( L/R = 7 msecs )

18. Remote Output Signal ( 4-20ma )

Most of our Winding temperature scanners are provided with a current signal for indication of temperature at
remote location. This current signal corresponds to the measured temperature. Say for example, TR-7544
gives output signal of 4 ma for 0 °C, 12 ma for 100 °C and 20 mA for 200 °C. Current signal facilitates error free
temperature signal over long lead between the scanner and the temperature indicator ( Remote winding
temperature indicator – RWTI ) mounted at remote place / in control room. This current signal is also used on
many occasions for DAS application. However, it is advisable and suggested to use signal isolators for this
signal while connecting it to DAS. This is important for safe and reliable operation for overall DAS system.

19. What is DAS ?

DAS stands for Data Acquisition System. It is computerized plant / system monitoring system where signals of
metering / current / temperature / level / vibration / flow etc etc from various places in plant are collected and
monitored by supervisory staff in control room.

20. What is the difference between RS-232 and RS-485 ?

RS-232 is the most popular serial communication protocol but has its own limitations. the length of cable
cannot be longer than 10 – 12 feet and only one equipment can be connected to each port. RS-485 is the most
popular serial communication protocol for industrial environments. It has many advantages above the
conventional RS-232 protocol. The length of cable can be as long as 1 km. It is a 2 wire connection and many
devices ( upto 32 nos. ) can be connected on the same two lines. The distance and number of devices can be
increased by using “Repeaters”.

21. What do I do, if I have 110VDC or 220 VDC or 120VAC or 60 Hz supply ?

Most of the PECON WTS manufactured are with auxiliary power rating of 85 to 270 VAC / DC. Further, they
can accept any supply frequency from 40 Hz to 400 Hz, therefore, you really do not have to worry about this.