ONLOAD TAP CHANGING TRANSFORMER

ABSTRACT On-load tap-changers (OLTCs) are indispensable in regulating power transformers used in electrical energy networks and industrial applications. This project explains the technological developments of resistor-type OLTCs and reactor-type OLTCs. The general switching principles for OLTCs are discussed and OLTC applications are presented. Today’s OLTC design concepts, including the new generation of vacuum type OLTCs, are described. The vacuum switching technology used in OLTCs is the “state of the art” design now and in the foreseeable future. Examples of OLTC designs and the respective switching principles show the range of the usage of vacuum interrupters.

INTRODUCTION Power transformers equipped with on-load tapchangers (OLTCs) have been the main components of electrical networks and industrial applications for nearly 90 years. OLTCs enable voltage regulation and/or phase shifting by varying the transformer ratio under load without interruption. From the start of tap-changer development, two switching principles have been used for load transfer operation – the high-speed resistor- type OLTCs and the reactor-type OLTCs. Over the decades both principles have been developed into reliable transformer components which are available in a broad range of current and voltage applications.

SWITCHING PRINCIPLE The OLTC changes the ratio of a transformer by adding or subtracting to and turns from either the primary or the secondary winding. The transformer is therefore equipped with a regulating or tap winding which is connected to the OLT Simple changing of taps during an energized status is unacceptable due to momentary loss of system load during the switching operation . The “make (2) before break (1) contact concept”, is therefore the basic design for all OLTCs. The transition impedance in the form of a resistor or reactor consists of one or more units that bridge adjacent taps for the purpose of transferring load from one tap to the other without interruption or appreciable change in the load current. At the same time they limit the circulating current (IC ) for the period when both taps are used.

THE OPERATING CONDITION OF OLTC The storage ambient temperature of OLTC is from -25℃ to 40℃. The storage humidity of the OLTC should be no more than 85 percent. The service temperature of standard designed OLTC is -25℃ to 40℃ If the temperature exceeds the range of above (-25℃ to 40℃), please specify when ordering. To meet the ordering requirements and comply with the operating environment, if the requested service temperature is out of the range of -25℃ to 40℃, the material and accessories of the OLTC will be specially designed and selected. The non-perpendicularity of OLTC on the transformer with the ground level should not exceed 2% The space for mounting OLTC should be free from serious dust and other explosive and corrosive gases.

INSTALLATION METHOD OF OLTC 1. Put the diverter switch and tap selector of the tap changer separately on a level surface. 2. Remove the connecting screws (6×M12) between the diverter switch and the tap selector. 3. Remove the red painted dowel pin from the sliding connector of the tap selector's step-by-step Geneva wheel mechanism. Do not move the connector. 4. The conductor of the tap selector has been installed during factory delivery. 5. Lift the diverter switch and put it on the tap selector. Take care not to damage the sliding connector of the step-by-step Geneva wheel mechanism.

6. Tighten six M12 recessed cap screws between the supporting stand of the tap selector‘s Geneva wheel mechanism and cylinder bottom of the diverter switch. Pay attention to the perpendicularity of the diverter switch and tap selector. 7. Thoroughly clean the bottom surface of the tap changer head flange and the sealing surface of the mounting flange. Put an oil resistant sealing gasket on the mounting flange. 8. Lift the complete tap changer and carefully insert it into the transformer through the mounting hole on the transformer tank cover. Take care not to damage the terminals on the tap selector and corona rings on the diverter switch compartment. 9. Check the head position and its setting position. Secure the tap change head flange to the mounting flange. At last, remove the red painted dowel pin from the intermediate gear wheel connecter at the cylinder bottom of the diverter switch.

OPERATING TEST OF THE TAP CHANGER Mechanical operating test Before voltage is applied to the transformer, 5 complete cycles of mechanical operating test ( no less than 200 times) must be performed. There should be no damage to the tap changer and motor drive unit. The position indication of the motor drive unit, its remote position indication and the position indication of the tap changer should be the same. Both the mechanical and electrical limit protection should be reliable.   Final oil filling Final oil filling is done after the operating test of the tap changer. Before oil filling, loosen the bleeding screw on the suction pipe and the top cover of the tap changer. Use a spanner to pry up vent oil overflowing in on the top cover of tap changer.

ADVANTAGES - allows changes in voltage ratio without deenergizing the transformer  - if the application involves interconnecting generation, a transformer with on-load tap changing capability provides the ability to adjust both voltage magnitude AND reactive flow. Without on-load tap changing, only one of these can be controlled, and then not very well.  - provides finer control of voltage than would be possible with off- load taps 

DISADVANTAGES transformer is more expensive requires more maintenance transformer reliability suffers - load tap changer failures are far more frequent than failures of the transformer itself

CONCLUSION A conventional static resistance measurement can be used to check the winding as well as all of the fixed internal connections. In some cases, however, defects could not be detected using the standard winding resistance measurement [9]. Therefore, the DRM as a supplementary measurement has proven to be beneficial for analysing the switching process, and mobile contacts, of OLTCs on power transformers. By using the same test setup as for static resistance, the DRM function enables insight into the fast switching process of the diverter switch to detect mechanical wear and tear of contacts, leads and commutating resistors without additional wiring effort. As a result, the reliability of the OLTC assessment can be improved, maintenance costs can be reduced and most importantly, unexpected and expensive outages can be avoided.