18 May 2011

Replacing an AVR

Automatic Voltage Regulator (AVR) is one of the main component of an Alternator. Main functions of AVR is to built-up the rated voltage and maintain the output voltage within the limit at no-load and on-load conditions. There will not be any output or abnormal output when an AVR fails. What are to be checked before replacing an AVR
  1. Disconnect the  AVR. Then check all windings of alternator for continuity and insulation resistance. If any defect is found then rectify it. Never replace the AVR if any winding is having a very low IR value or winding is defective. Connect the AVR after rectifying the defect and then start the machine. If there is no output voltage go to step 2.
  2. Check the condition of diodes in RRA. If found defective then replace the defective diode of Rotating Rectifier Assembly (RRA). Start the DG Set. If alternator does not develop  voltage go to step 3.
  3. Then run the alternator at idle speed without connecting AVR and note the output voltage. The output voltage must be balanced (If unbalance output voltage is found check the armature winding  and lead wires as explained in step 1). Then run the alternator at rated speed and again check the output voltage. Most of the AVRs need 5V output  between phase and neutral (or on the input power termlias of AVR). If the output is very low (<5V) then connect a battery (at least 12V, also check the resistance of exciter field coil. While connecting a battery please confirm current through the exciter field do not exceed the maximum excitation current of alternator. To know the maximum excitation current please refer name plate of alternator or product catalogue) at the exciter field lead wires of the alternator in correct polarity. Leave it for few minutes and then disconnect the battery. Start the DG set and run it at rated speed. If the output between neutral and phase is more than 5V then stop the engine. If the output does not improve then repeat this process for few more time. If the alternator does not develop voltage the we can try with another AVR.
  4. Replace the AVR. While replacing with other make AVR please confirm that this AVR suits with the alternator. Exciter field resistance & rating of the alternator must be with in the limit as prescribed in the catalogue of AVR.
  5. Check the output voltage, frequency and excitation voltage. Output voltage must be balanced and frequency must be slightly (1Hz) more than the name plate value. To set the frequency adjust the engine speed. Also the no-load excitation voltage must be within the limit. Switch OFF DG. While doing so note that  under-frequency indicator on AVR glows and voltage droops.
  6. Re-start the DG. Put load on DG. Check the output voltage, frequency and excitation voltage. Maximum permissible droop in frequency is 3%. If frequency droops more than this value check the engine. Excitation voltage must not be more than the alternators maximum excitation voltage.at trated load (refer name plate to get maximum excitation voltage)
  7. Operate the DG Set for few time. After switching OFF alternator check for any abnormal heating.

09 May 2011

KEL Alternator

Kerala Electrical & Allied Engineering Company Ltd (KEL) is a Public Sector Undertaken company fully owned by government of Kerala. Their main products are Alternators and Transformers. All alternators manufactured by KEL (in collaboration with Leroy Somer, France) are of Brush Less Excitation type. In lower rantings (<100kVA) no electronic regulator (AVR) is required to regulate the output voltage. In this type of alternator the voltage is build up and regulated with compound excitation system. In higher rating compound excitation & electronic voltage regulators are used to regulate the output voltage. This dual regulation gives high reliability. In this type of alternator one advantage is that we can operate the machine even after an AVR failure. With AVR we will get a 2% voltage regulation and in the second mode it will be 5%. The alternator manufacturing division of KEL is situated in Kasaragod district of Kerala.

The above picture shows the regulating part of a KEL alternator. The stator winding also have a 3phase star connected auxiliary winding. This winding generates a 40V output at rated speed. This output is fed to the secondary winding of compounding transformer. The secondary of compounding transformer has three independent winding. The second and third set have 15% and 5% turns of first set. By changing the way we connect theses windings we can change the effective number of turns of secondary winding from 80% to 120%. Any increase in the number of turns in the secondary winding will decrease the output voltage. (Increase in the number of turns will increase the impedance and thus decrease the output). Primary winding (outer winding) of the compounding transformer have only a few turns wound in opposite direction of secondary winding. This winding carries a portion of output current (load current). As this is wound in opposite direction flux generated by this winding will oppose flux generated by secondary winding. Thus as the load increases output voltage of each transformer will increase and this will help to regulate the output voltage of alternator. Both primary and secondary winding of each compounding transformer must be in phase. Output from the compounding transformer goes to a three phase bridge rectifier unit (4KD4). The output of this rectifier goes to the compounding field coils of exciter field. (in low rating there is only compounding system)

In higher rating (>100kVA) there is an electronic voltage regulator. Input to this regulator is from the the auxiliary winding. It monitor the output of the alternator (two phases). DC output of the AVR goes to regulating field coils on exciter field. The regulating field and exciter field are wound in opposite direction. The output generated by compounding field will be 10% higher than the rated output voltage. Flux created by regulating field opposes that created by compounding field and reduces the output voltage of alternator to normal value. Winding resistance of compounding field is less than that of regulating field. If there is an AVR failure (no output from AVR) then the output of the alternator will increase. Then disconnect the AVR and connect the regulating field to the full wave bridge rectifier unit (fixed above 4KD4, output are available at 6+ and 7- on the connector tray, just change link on connector tray). Then adjust the output voltage by adjusting the POT for manual voltage adjustment. (on picture just above AVR). 

Cost of KEL AVR is very high (mostly around Rs. 25,000/-) comparing to that of other make. Now KEL  is owned by Bharat Heavy Electricals Ltd (BHEL). Rotating Rectifier Assembly (RRA) have 6 diodes. 3 forward (violet tail colour) and 3 reverse (red tail colour). Getting genuine spares in time is always a problem with KEL. All orders are to be send to Kasaragod and some times they do not supply spares directly to the customer. In some models they are using imported diodes and there is always delay in getting these spares.