Distribution Transformer (DTR) plays a crucial role in the power distribution network. Failure of distribution transformer results into interruption of power supply to consumers (major concern), and this disruption affects the Company in the form of loss of revenue and material, apart from incurring heavy repairing charges etc.
Following are the causes of failures of DTRs, prevention, timely attention & checks on a daily basis.
IMPROPER INSTALLATION/SHIFTING OF DTRs TO LOAD CENTER
The Distribution Transformers, in the first place, should be erected at a place where loads can be evenly distributed i.e., a place where no. of LT feeders can be connected. If the already existing DTR is not in a convenient place to do so, the transformer should be shifted to a load center. Same time over loaded and under loaded DTRs may be identified and shifted to the appropriate locations.
SINGLE PHASE LOADING
It is a practice observed that the field/FOC staff convert one or two phases into three phases at the distribution box to avoid interruption and give temporary solution especially during night time, when the lugs, crimping tool etc. are not available, this should be strictly avoided, as the overloading of one or two phases out of three causes damage to the DTR.
Practically it is difficult to maintain a perfect balance on the three phases, but, it is necessary to observe on a timely basis by taking the load currents with a tong tester, and instructing the staff to change the service connections from one phase to the other. The imbalance in loading creates current to flow through neutral, which is a loss, and also the circulating currents in the core damages the insulation levels.
Most of the LT distribution boxes, the cable terminations, including the cable insulation is found damaged in most locations. This is caused mainly because the LT cable used is generally XLPE Under Ground cable, but when used outdoors the insulation gets brittle and breaks-up causing dead short in between the phases/neutral or the box itself being metal box. Now that synthetic LT distribution boxes are being supplied, the beakdowns are slightly minimized. Also, the bi-metallic clamps, and lugs causes carbon accumulation, which results in poor contact and loose contact, again creating fluctuations and imbalances.
In some places it can be observed that the LT fuses are fixed by consumers, especially in rural areas. Aluninium conductor lead is put instead of a proper LT fuse wire of copper and lead mixed. Also the HRC fuse carriers, at times are supplied with an inferior quality, wherein the metal female contact, becomes loose and worn out causing loose connection/fluctuation and damage to the DTR.
Size of TC fuse wire (in SWG) is as follows….
DTR KVA 11KV SIDE LT SIDE
S.Ph. 10 & 15 – 4
25 38 20
50 33 18
63 33 2X20
100 33 2X16
160 30 2X14 or 3×13
200 28 3×14
250 26 4×14
315 20 4×14
500 20 4×12
HORN GAP FUSE (HG Fuse)
On the HV side of the DTR, the horn gap and the proper rated fuse wire has to be maintained, or else each blow-out of fuse reduces the life of the DTR. Preferably, the HG Fuse wires need to be replaced with new fuse wires at least once in six months.
HORN GAP FOR 33KV – 385 mm or 15 Inches
HORN GAP FOR 11KV – 205 mm or 8 Inches
Where ever the lightning arrestor fails it is generally not replaced, but just kept disconnected. Lightning damages the DTR winding and insulation, so providing of LAs to all the DTRs is a must.
POWER THEFT AND HOOKING OF MAINS
This situation is observed in the rural belt where the LT lines are hooked with pvc & other types of wires and indulge in theft. Here, not only there is loss of revenue but also while hooking and removing the wires, the LT phase wires get short which is a dead short and results in failure of DTR, and also conductor snapping in most of the cases. This situation can be avoided by maintaining the LT loose spans by doing re-stringing, and also avoid the hooking.
The nylon bushes, male & female contacts ( moving and fixed contacts), of the Switches generally ware out mechanically. In such instances the staff & CBD personal do temporary repair like binding the contacts with aluminium wire, which causes loose contact and red hot spot is created. This leads to fluctuation in the delta circuit of the DTR, due to which the winding gets heated up and the insulation fails. Thus this problem can be avoided by supplying sufficient spare parts of the A.B. Switch to field staff and take up repairing or AB Switches at least twice in a week.
The earthing of any electrical appliance is very very important for saving life and property of all living beings. The earth electrodes viz., GI Pipes and CI Pipes along with the earth pits should be maintained regularly. The earth faults etc., need to be grounded offering least resistance otherwise the equipment will get damaged.
TREE & BUSH CLEARANCE
Another major cause of concern is Tree branches touching the HT & LT lines, especially when there is wind, gale and rainy weather. This could be avoided by regularly getting the clearance done ***not by the tree cutting CBD gang, but, arranging the field staff & contract workers of the Operation section.
SPM FIELD STAFF
Field staff of SPM can be used where ever, minor repairs are required, like opening top cover, checking for lug cutting at core ends, oil level & oil top-up where ever required. If not possible on field the DTR can be taken to the SPM shed and get minor repairs done (make sure that the transformer is opened in the presence of the field AE)
Apart from the above causes, the most important aspect which plays a key role in failure of DTRs is the existence of “Harmonic distortion in the system”. Harmonics are caused by non-linear loads like bridge rectifiers, electronic circuits and to some extent by TVs and Computers also. In the industrial belt it is caused mostly by DC welding machines. The harmonic distortion affects the capacitive current and the frequency causing disturbance to the sinusoidal wave, thus heating up the winding and other components of the DTR reducing its life. A DTR’s expected life is >35 years if maintained properly. Although not much can be done to reduce harmonics, to some extent the distortion can be reduced by requesting the consumers to maintain proper Power Factor, by the consumers.
It can be summarized that apart from the technicalities mentioned above, managing & maintenance of works and organizing the staff is of UTMOST IMPORTANCE. There should be two persons in a section, one a lineman preferably & a contract workers attending to Cleaning of Yards, Checking LT fuse wires & ascertaining the correct size of wire depending on the capacity of the DTR., reporting to the AE about the damaged distribution boxes, replacing of HG fuses according to programme, tree branch and bush clearance invariably, cleaning and tightening of transformer bushings and PG clamps. Loose line stringing by ALL STAFF as per program etc. Maintenance of a register for noting the works done and requirement of material for the next day should be logged by the lineman concerned. This register if observed by the AE on a daily basis one can avoid not only interruptions to the consumers, but also can enhance the life of the DTRs.
THE ABOVE ASPECTS WERE PRACTISED AT BALANAGAR SECTION IN HYDERABAD NORTH CIRCLE RESULTING IN 0% FAILURE OF DTRs FOR A PERIOD OF 3 AND HALF YEARS. THANKS TO THE THEN ADE SRI. K.K. RAMAKRISHNA, WITH WHOSE SUPPORT THE ABOVE ACHIEVEMENT WAS POSSIBLE….
By Er. Vincent Joseph
Addl. Assistant Engineer/Masterplan I, SD IV, Hyderabad Circle