Sub: Few probables for Load Hunting

There is a requirement to have a checklist sort of thing useful for proceeding for analysing load hunting observed occasionally in both units.

For this purpose data from PIS charts were utilised. The behaviour of the systems which are interlinked was studied during the normal operation as well during recent disturbances on 12.2.2014 and 18.6.2014 in Unit-1. The history trend in the recent past (say within 10 days) was also reviewed in COPIS utilities history menu. Summarised are the observed variations with probable causes that can be fit into to the data to explain the normal behaviour.The following pattern exists normally in both units.

The generation always gets corrected as per speeder rotation through Governor oil pressure impulse.Boiler pressure control system maintains Boiler pressure and corrects speeder position which follows reactor power.Reactor power is decided by Delta Twhich in turn follows feed water flow.However it is also to be noted that RRS and BPC are servo controlled systems with integral action repeats as per design once error is created. Hence traces of the variations of the parameters for a large duration were collected and studied. ParameterU-1U-2

Generation 6-84-6

LinN 1-1.2.8 -1.0

Feed water 60-80 40-60

level1-1.2 cm0.8 - 1.2 cm

Few anomalies which are at present and as such not causing any additional hunting are.

1. Boiler delta T -1, 3 and 6 pattern are not exactly matching and sometimes slipping and not maintaining phase. However it is not causing any additional hunting. This is because the two channels are deciding the correction of RRS.2. The variation in Lin Nand generation are not equal magnitude wise probably due to BPC system gain. 3. Disturbances are also noted during other occasions due to BPC median shifting due to delta T signal variation between three controlling boilers being non- uniform occurring coincidentally with other causes for disturbances.

Hence disturbances occurring for few cycles with higher magnitudes alone were taken in the past and checked for the causes as below.

The disturbance on 18.6.2014 was analysed

The traces of PIS charts attached.

There is a gap min from reactor power change to pressure change and further min delay to generation change. This is evident in charts 1, 2, 3. But normally the delay between neutron power change and generation change is to be less and this is a precursor for this type of problem.Only when the measured pr of channel B (which controls generation being the median) slows down to respond prominently visible at instant 13.34 onwards to reactor power changes, speeder rotates more time due to error and causes generation correction more than required. Reference Charts 4, 5. This over correction of generation in the opposite direction initiates rolling over or cycling.Event on 11.2.2014/12.2.2014BLC controlling boiler changed from Boiler 3 to boiler 1 at around 00.00 hrs after observing hunting pattern in the level of highest boiler and load hunting stoppedCharts 6, 7 are attached for some duration

Event on 29.7.2014 in Unit-2.

Channel C Delta T got disturbed and median got affected. MWe got shifted to new low level as per the next existing higher delta T as per BPC program. This will initiate a load hunting as delta T values will settle for the new MWe later 6 sec and that will ask for chnage in genertion and cycle continues.

The median got shifted by almost 1 ma and caused the disturbance

Charts 8 attached.Under normal condition the above pattern can get disturbed under the following cases.Sl.NoFaulty parameterIdentification process

1Field observations Air leaks in feed water CV Here positioner bleeding and CV diaphragm leaks

2Governor valve routines checks ??

3Highest boiler level has fluctuationsTaking on manual/changing over to other boiler for checking this parameter.. Level hunting will be happening due to other reasons and level hunting would not stop by taking to manual.(12.2.2014)

4Speeder rpm /dead bands both sides not equal Taking speeder to manual will solve if any of this value is not properly tuned.(earlier occasions)

5Any lagging or in measured signal which is median or shift in the median signal due to channels outputs having largerdifferences. (Boiler Delta T or boiler pressure)This will cause movement of final control element till error is made zero (as integral action is built in servo motor)and this causes over correction of the process and initiates second half of cycle of hunting(18.6.2014)/(29.07.14)