Winding Temperature Indicators

Oil Temp Indicators 4

Microswitches Making and Breaking Capacity

Microswitches Making and Breaking Capacity

 Standard MicroswitchesHigh-Performance Microswitches
VoltageResistive LoadInductive LoadResistive LoadInductive Load
125 VAC5 A5 A10 A10 A
250 VAC5 A5 A10 A10 A
30 VDC5 A3 A10 A10 A
50 VDC1 A1 A3 A2,5 A
75 VDC0,75 A0,25 A1 A0,5 A
125 VDC0,5 A0,1 A0,5 A0,1 A
250 VDC0,25 A0,1 A0,25 A0,1 A

 

Introduction

The winding is the transformer component with the highest temperature and, above all, the one subject to the fastest temperature changes as the load increases. Thus, to have total control of the temperature parameter inside of the transformer, the temperature of the winding must also be measured. An indirect system is used to measure this latter since it is dangerous to place a sensor close to the winding due to the high voltage. The indirect measuring is done by means of a Thermal Image. This Winding Temperature Indicator is designed to measure the temperature of the winding by means of a special bulb surrounded by a heating resistance through which passes a current proportional to the current passing through the transformer winding subject to a given load and immersed in insulating oil at temperature Toil. Itís possible to adjust the heating system by means of a potentiometer whose knob is located on the winding temperature indicatorís dial. In this way the value of the winding temperature indicated by the instrument will be equal to the ones planned by the trafo manufacturer for a given transformer load. The winding temperature indicators can be fitted with up to five change-over microswitches suitable to control cooling equipments and protection circuits (alarm and trip) of the transformer. This sector of our production is the result of considerable research and experiment commitment which has led to internationally patented new concept instruments design and construction. The component designs of our instruments are protected by :

  • ITALIAN PATENT No. 208603
  • ITALIAN PATENT No. 89113
  • E.E.C. PATENT No. 0245212
  • U.S. PATENT No. 4,727,227

Effectiveness of these instruments must be stressed, both as regards measuring/commutation precision and extreme simplicity of operation. Special attention has been paid to design of each single part resulting in extreme high reliability of our instruments and ensuring long-lasting accurate operating. We have designed the setting system, the mounting devices and the dimensions of the cable boxes to consent the operator to easily install the indicator and to save time in setting and making cable layout. Besides the exact constructional and severe quality control we adopt, the high performances of our instruments are further assured by the employ of the best products supplied by European technologyís more advanced company names. In particular:

  • the INDICATING SHAFT is mounted on 2 micro ball bearings to reduce the frictions and to grant right working under vibrations;
  • the AWG 22 CABLES we adopt are silver plated and protected with Teflon according to MIL – W – 16878- 4 Standard;
  • the TERMINAL BLOCKS (WEIDMULLER – Germany) grant very high performances and are certified in accordance with European standards;
  • the POWDER PAINT grants total protection against corrosion and increases the insulation of the device;
  • the SENSING SPRING TUBE is manufactured with a special bronze alloy that avoid any plastic deformation and histeresis of the spring.

Description and General Specifications

Temperature sensing system : expansion type compensated for ambient temperature changes by means of a built-in compensating device. To avoid to many checks after setting into work and periodical re-calibrations we adopt particular cares in testing the components. In particular the sensing system is subject to 3 different tests :

  1. Vacuum test : the sensing system is connected to a vacuum plant. The plant pressure is decreased to 2×10³ mbar (hpa) to verify the quality of the welding and the porosity of the material;
  2. Pressure test: the sensing system is put under pressure up to 280 bar to verify the welding and that the spring is not subject to any plastic deformation;
  3. Overheating test : once the sensing system is completed, its bulb is located in a heating plant controlled by a microprocessor based temperature monitoring system. The temperature is increased up to a value that is 20% higher than the maximum range value of the sensing systems ( i.e. for an indicator whose range is 0 / 150°C the overheating test temperature is 180°C ). The temperature remains at that value for 8 hours in this way simulating 1 year life under normal working conditions ( i.e. for an indicator whose range is 0 / 150°C —> 110°C ). In this way we train the spring and verify that the precision remains the same.

Capillary tube protection : rilsan tubing / stainless steel armouring / steel + PVC armouring.

Bulb : bronze .

Casing : aluminium alloy powder painted (RAL 7035) suitable to withstand to any climate and to heavy polluted atmosphere in as well tropical or artic climates ( -40 / +70°C ). All components are made of corrosion resistant or surface treated materials. The case is provided with a breather device to avoid dew on the lens. To make cable layout quick and easy, the case is equipped with a large junction box that is completely separate from instrumentís sensing system. Cable glands PG 16 – M20 – 3/4″BSP.

Mechanical protection degree : IP 65.

Working temperature : -40 / +70°C.

Lens : glass or polycarbonate.

Locking ring : Nickel plated brass. Transparent coated.

Standard measuring ranges : 0 / +150°C; 0 / +160°C.

Measuring tolerance : 1,5% of full scale value.

Commutation tolerance : 2% of full scale value.

Commutation differential : 4% of full scale value. On customerís request the differential can be increased.

Insulation : 2000V 50Hz between terminals and earth for a 60 seconds time.

Options

Elastic suspension (Drwg. No.1231) : itís a vibration damping system able to minimize the effects of a machine vibrations on the instrument.

Earthquake proof version : done by equipping the instrument with the elastic suspension and suitable internal components.

PT 100 sensor : the oil temperature indicator can be equipped with one or two PT 100 sensors that convert the temperature values in resistance values and transmit them to a receiver or to a monitoring system. Up to 2 PT 100 sensors can be mounted on the WTI.

Receiver (Drwg. No.1479) : we can supply a digital receiver to display the temperature signal received from the sensor.

Transducer 4…20mA (Drwg. No.1707) : we can supply a wide range of transducers 4Ö20mA that convert the resistance values ( input Pt100 ) into current values ( output 0..20mA or 4..20mA ) or into voltage values ( 0..5V or 0..10 V). These devices can be mounted on a DIN rail inside of the transformer marshalling box.

Operating Instructions and Maintenance

Mounting: mount the instrument on its machine or plant :

  • rigid locking screw M14 (Drwg. No.1242/B) located on the top of the thermometer;
  • elastic suspension (Drwg. 1231) that is mounted on the top of the thermometer with a screw M14 that consents to install the instrument to the plant,
  • rear flange (Drwg. No.1242/B-F) for wall mounting to the oil tank.

Removing of the terminal box cover : by unscrewing the 4 stainless steel screws.

Cable layout : the numerations 1-2-3-4-5 indicate the microswitches progression (red, blue, green, yellow, white pointer). Close to the terminals you will find the following abbreviations :

  • C = common
  • NO = normally open
  • NC = normally closed

that allow the operator to choose the desired cable layout. Connect the microswitches terminals and the earth terminal. If the thermal image is equipped with the PT 100 probe you find also the PT 100 terminals with a clear label that indicates how you can connect the probe to the display or to a transducer. After having done all the connecting operations re-position the terminal box cover taking care to put the flat gasket in the right position and screwing the 4 stainless steel screw.

Regulation of the value of ▵T : within the instrumentís terminal board there are, as well as earth and microswitches connection terminals, the terminals T-T and the terminals A-A. Procedure for regulating the instruments :

  1. insert the ammeter probes in terminals A-A;
  2. remove jumper A-A;
  3. connect terminals T-T to the current transformer. AFTER having checked that the value of the power supply current printed on the dial (above the knob for regulating overheating) is, in fact, the same as that of CT (current transformer);
  4. regulate the current on the basis of curve I – T attached;
  5. replace jumper A-A;
  6. remove the ammeter probes;
  7. wait a few minutes to allow Tw to stabilise;
  8. check the exactitude of Tw;

N.B.: the bulb of the thermometer for the thermal image must be filled with oil to accelerate the heat interchange occurrences. The bulb must be inserted in a well filled with transformer oil : the oil will rise through a suitable hole located in the bottom of the bulb itself until it covers the resistance.

It’s very important to stress that the I – T curves are only valid if the bulb is immersed in oil.

Setting : to set microswitches pls., follow exactly the instructions :

  • remove the locking ring;
  • remove the glass or polycarbonate lens (take care to the O-ring);
  • stop the microswitches setting dial (small black dial) with two fingers and slide the frictioned microswitches setting pointers until they are located at the desired temperature. Note that to reduce errors you have to slide the pointers towards higher temperature value;
  • Replace the lens taking care that the max. temperature indicating pointer is located on the right side of the temperature indicating pointer and that the lens itself is correctly positioned over the sealing O-ring;
  • Lock the lens screwing the locking ring.

Maintenance : No particular maintenance is required. Only periodical inspections ( typical interval 6 months ) to verify precision, functions and electrical connections. In case of working test effected with thermostatic bath, please note that WTI bulb MUST NOT be immersed in water. The WTI bulb is surrounded by the heating resistance and water may cause serious damages to the heating system. The calibration test must be done with oil or hot air only. When the instrument is equipped with polycarbonate lens, cleaning must be done with care in order to avoid scraps on the surface. Use water and soap only.

Finished Product Quality Control Tests

Instrument calibration : carried out through thermostatic baths controlled by a computer system. The procedure varies according to instruments scale. Example of procedure for a thermal image scale 0/150°C: the calibration is made using 5 different baths set at the following temperatures :

  • bath 1 = 0°C
  • bath 2 = 20°C
  • bath 3 = 50°C
  • bath 4 = 100°C
  • bath 5 = 125°C

Calibration procedure :

Step 1: a check is carried out to see whether the temperature taken by the instrument under test differs from that taken through the sample sensor by more than the 70% of the maximum allowed instrument reading tolerance value. This test is performed by sequentially plunging the Winding Temperature bulb into successive temperature increasing thermostatic baths: 0°C / +20°C / +50°C / +100°C / +125°C.

Step 2: the instrument is heated until the instrument pointer exceeds by 20% the angular full scale value.

Step 3: step 1 is repeated, but inversely.

Microswitches actuation test : performed through a computer controlled testing unit. The bulb is immersed in a thermostatic bath. The computer changes the temperature inside the bath and by means of suitable sensors verifies the commutation tolerance, the commutation differential, the electrical circuits of each microswitch. At the end of the test a test report is directly printed by the computer.

Check of instrument protection degree : IP 65. This test is carried out by means a lance-sprinkled water jet on all sides of device

Insulation test : carried out by means of a microprocessor controlled testing unit.

Heating system functionality test : to verify trimming functionality of potentiometer and total resistance values of the circuit.

Note : all the collected data are immediately transferred, by means of the computer net, to the quality control and to the design departments to be supervised and evaluated. In our files, we keep all the above mentioned informations and we can supply to the customer detailed reports regarding the performances of each instrument delivered.