IEC 56 Rated operating
4.7.18 STANDARD VECTOR GROUP
Group Phase Displace Symbols Group (1)
Group (2) Group (3) Group (4)
00 1800 -300; 3300
+ 300
Yyo, Ddo, Dzo, Zdo Yy6, Dd6, Dz6, Zd6 Dy1, Yd1, Yz1, Zy1 Dy11,Yd11,Yz11, Zy11
4. 7.19 PARALLEL OPERATION OF TRANSFORMER 4. 7.19.1 Terminal Marking (Viewed from H.T. Side)
(1) 1φ transformer: - Subscripts are marked in DESCENDING ORDER.
FROM LEFT TO RIGHT.
(2) 3φ transformers: - Neutral is on extreme left and then phases are in sequence (R, Y, and B).
(3) Autotransformer: Neutral is on extreme left and then phases are in sequence (R, Y, and B).
4. 7.19.2 CONDITIONS FOR PARALLEL OPERATION
i. Same inherent phase angle difference between primary and secondary terminals.
(ii) Same voltage ratio (iii) Same frequency (iv) Same polarity (v) Same phase rotation
4. 7.20 LOAD SHARING BY TRANSFORMERS IN PARALLEL 4. 7.20(A) For two Transformers
PA = P (QA.ZB) / (ZA.QB + ZB.QA), PB = P (QB.ZA) / (ZA.QB + ZB.QA) Where PA = Load shared by TFR (A),
PB = Load shared by TFR (B) P = Total Load, QA = Rating of transformer (A)
QB = Rating of transformer (B),
ZA = % of impedance of transformer (A) ZB = % impedance of transformer (B)
Note. Currents are also shared by same proportionate as loads.
4. 7.20(B) For 3 Transformers
PA = P (QA.ZB ZC) / , PB = P (QB.ZC ZA) / , Pc = P (QC.ZA.ZB) /
Where = (QA.ZB ZC + QB.ZC ZA + QC.ZA.ZB)
Note: - Currents are also shared by same proportionate as loads.
4. 7.21. CIRCULATING CURRENTS 4. 7.21.(A) Two Transformers in Parallel
(i) For impedance having same ratio (R /X) ICr = (VA-VB) / (ZA+ZB)
(ii) For impedance having different ratio (R/X) ICr = (VA-VB)/Z,
Where Z = (RA +RB)2+(XA +XB)2
VA = Secondary terminal voltage of transformer (A) (Lower ratio) VB = Secondary terminal voltage of transformer (B) (Higher ratio), ZA = (VZA.VA) / (100 X IA), IA = F.L. current
VZA = % Imp. Voltage drop at F.L. Rating
4. 7.21 (B) Three Transformers in Parallel (i) For impedance having same ratio (R/X) ICrA = (VA – M ) / ZA ICrB = (VB – M) / ZB
ICrC = (VC-M) / ZC
Where M = (VA.ZB.ZC + VB.ZC.ZA +VC.ZA.ZB)/(ZA.ZB+ZB.ZC+ZC.ZA) 4. 7.22. TRANSFORMER TAPPINGS
Note: - (1) For 2 – winding transformer, tap changer is generally provided on the H.T. side i.e.
L.T. side voltage remains constant.
(2) For Autotransformer, tap changer is generally provided on the L.T. side i.e. H.T. side voltage remains constant.
Transformer % Winding under gone Tap change
Standard No. Of tap from normal tap
1. Distribution Transformer (11/0.4 or
11/6.6 KV) 2. 33/11KV
3. 132 / 33 KV 132 / 11KV 4. 220 / 132KV
220 / 33KV
10%
15%
20%
20%
20%
20%
20%
20%
+5% Up, -5% Below +5% Up, -10% Below
+10% Up, -10% Below +5% Up, -15% Below +10% Up, -10% Below
+5% Up, -15% Below +10% Up, -10% Below
+5% Up, -15% Below
4. 7.23 TRANSFORMER OIL DATA
4. 7.23.1 CHARACTERISTIC REQUIREMENTS OF IS, IEC, & BS SPECIFICATION FOR UNIHIBITED TRANSFORMER OIL DATA
Sl No
CHARACTERISTICS IS:335 1993 1 APPEARANCE Oil should be clear, transparent, free from suspended mater and sediments
2 DENSITY gm/cc Max@ 4 Inter facial Tension( N/m
Min. 1. Neutralization Value (mg KOH/ gm Max) 11 Neutralization Value
1. Total Acidity(mg KOH/
gm )
2. Inorganic Acidity
0.03 Max
12 Corrosive Sulphur Non- Corrosive
Non- corrosive Non- Corrosive Non- corrosive Non- Corrosive 13 Oxi. Inhibitor 0.05% Max Not-Detectable Not-Detectable Not-Detectable Not-Detectable 14 Water Content (ppm) Max
Bulk / Drum Delivery
50 40 40 20/30 20/30
15 S.K. Value Under
Consideration
- - - -
16 Accl. Ageing Test (Open Beaker Method with Copper Catalyst)
1. Tan Delta @ 90 0 C Max.
2. Resistivity Ohm-cm. @ 90 0 C
3. Resistivity Ohm-cm. @ 27 0 C
4. Total Acidity 5. Sludge Cont. (% / wt
20 Poly cyclic Aromatic Wt%, Max
- - - 3.0 3.0
4.7.23.2 CHARACTERISTIC REQUIREMENTS OF IS, IEC, & BS SPECIFICATION FOR INHIBITED TRANSFORMER OIL DATA
Sl No
CHARACTERISTICS IS:
12463/1988 1 APPEARANCE Oil should be clear, transparent, free from suspended mater and sediments 2 DENSITY gm/cc Max@ 29.5 4 Interfacial Tension
( N/m Min.
11 Neutralization Value 1. Total Acidity (mg KOH/ gm Max) 2. Inorganic Acidity
0.03
12 Corrosive Sulphur Non-
Corrosive
Non- Corrosive
Non- Corrosive Non- Corrosive
Non- Corrosive Oxi. Stability 164 Hrs
1. Neutralization Value ( mg KOH/ gm Max) 2. Sludge Cont.
(% / wt Max)
3. Volatile Acidity Max
0.4
Oxi. Stability 500 Hrs 1. Neutralization Value (mg KOH/ gm Max) Bulk / Drum Delivery
50 30/40 30/40 20/30 20/30
15 Accl. Ageing Test (Open Beaker Method with Copper Catalyst)
4. Total Acidity
5. Sludge Cont. (% / wt Max)
19 Poly cyclic Aromatic Wt%, Max
- - - 3.0 3.0
4.7.23.3 +HIGH QUALITY TRANSFORMER OILS FOR POWER TRANSFORMERS
Sl No
CHARACTERISTICS ASTM D 3487
TYPE-1
1 APPEARANCE CLEAR &BRIGHT
2 DENSITY gm/cc Max 0.91 @ 15 0 c 0.885 @ 20 0 c 0.895 @ 200 c 0.895 @ 200 c Min. Needle Neg. to Sphere grounded (25.4mm gap)
145 150 - - 2 Ageing Resistance as per
Badder (140 Hrs / 110 0 c) Saponification No. mg KOH /mg 3 Oxi. Stability Max. (Rotating
Bomb Test ) Minutes Min.
195 - - -
4 Oxi. Inhibitor % Mass/Max 0.08-0.3 - - 0.15-0.4
5 Corrosive Sulphur Non- Corrosive Non- Corrosive Non- Corrosive Non- Corrosive
6 Water Content (ppm) Max 35 10 - 30(Bulk)/40
4.7.23.4 TEST ON TRANSFORMER OIL IN SERVICE
Sl.
No
CHARACTERISTICS Voltages Test Method PERIODICITY PERMISSBLE LIMIT refilling prior to energizing then after 3 Months and after one
year 30 KV ( Min. ) refilling prior to energizing then after 3 months and after one Refilling prior to Energizing then after 3 Months and after 2
IS 6262/1971 After filling or Refilling prior to Energizing then after 2 years
1.0
5. Neutralization Value 1. Total Acidity MAX
ALL refilling prior to Energizing then after 3 Months and after one year
REFER IS 10593/1993/DGA
CHART
4. 7.24 DISOLVED GAS ANALYSIS
4.7.24.1 INTERPRETATION OF RESULTS 4.7.24.1.1 DOERNENBURGE RATIO METHOD
SUGGESTED FAULT DIAG.
CH4/H2 C2H2/C2H4 C2H2/CH4 C2H6/C2H2 Thermal Decomposition > 1.0 < 0.75 <0.3 >0.4 Corona (Low intensity P.D) <0.1 - <0.3 > 0.4 Arching (High intensity
P.D)
>0.1
<1.0
>0.75 >0.3 <0.4
4.7.24.1.2 ROGER’ S RATIO METHOD Method-I
SUGGESTED FAULT DIAGNOSIS
C2H2/C2H4 CH4/H2 C2H4/C2H6
Normal 0.1to 1.0 < 0.1 <1.0
Low Energy Density Arching <0.1 <0.1 <1.0 Arching (High intensity P.D) 0.1 to 3.0 0.1 to 1.0 >3.0 Low Temp. Thermal <0.1 0.1 to 1.0 1.0 to 3.0 Thermal >700 0 c <0.1 >1.0 1.0 to 3.0 Thermal <700 0 c <0.1 >1.0 >3.0 4. 7.24.1.2 ROGER’ S RATIO METHOD
Method-II
SUGGESTED FAULT DIAGNOSIS
CH4/H2 C2H6/CH4 C2H4/C2H6 C2H2/C2H4 If CH4 /H2 <0.1, then P.D
other wise Normal Deterioration
0 0 0 0
Slight Over Heating Below 150 0 c
1 0 0 0
Slight Over Heating Below 150 0 c to 200 0 c
1 1 0 0
Slight Over Heating Below 200 0 c to 300 0 c
0 1 0 0
General Conductor Overheating
0 0 1 0
Circulating Currents / Overheated
1 0 1 0
Flashover without power flow current
0 0 0 1
Tap Changer selector breaking current
0 1 0 1
Arc with Power flow 0 0 1 1
REMARKS
Ratio <1.0 is taken as 0, Ratio >1.0 is taken as 1
A given Ratio can be taken for Diagnosis if the concentration of one gas is at least equal to the limit values as below
Value in ppm
H2=200, CH4= 50, C2H6=15, C2H4=60, C2H2=15
4.7.24.2 KEY GAS METHOD
Decomposition of Oil
C2H4 > 150ppm 3. .Electric Discharge
H2 (60-70%), 2. Partial Discharge
H2 (80 –90 %)
2. Thermal ageing of oil
CO (90 –95 %)
> 350 ppm
H2 (5-10 %), C2H6 (1 –2 %), CH4 (2 - 5 %)
6. 1. Arching in Cellulose 2. Decomposition of Insulation
4.7.24.3 TOTAL DISSOLVED COMBUSTIBLE GAS LIMITS (TDCG) TDCG LIMITS in ppm INTERPRETATIONS
(0 – 720) Satisfactory operation – Unless individual gas acceptance values are exceeded
(721 – 1920) Normal Ageing / Slight Decomposition Unless individual gas acceptance values are exceeded (1921 – 4630) Significant Decomposition fault is to be
monitored
>4630 Very Substantial decomposition of oil.
Immediate action to be taken
4. 7.24.4 PERMISSIBLE LIMIT (IN ppm) OF GASSES FOR SERVICE TRANSFORMERS
GASSES TRANSFORMER
WITHIN 4 YEARS
TRANSFORMER 4-10 YEARS
TRANSFORMER AFTER 10 YEARS
C02 2000-3000 3000-4000 4000-6000
CO 150-300 300-450 400-550
C2H4 100-150 150-200 200-300
C2H2 20-30 30-50 30-60
H2 100-150 150-250 200-300
CH4 50-70 70-150 70-150
4.7.24.5OTHER INFORMATIONS DISINTIGRATION OF
TRANSFORMER OIL AT ORD. TEMP
RELATION OF EVOLVED GAS WITH TEMPERATURE
NATIONAL SPECIFICATIONS
GAS % TEMP. IN 0 C GAS COUNTRY SPECIFICATIONS
C02 1.17 >120 METHANE (CH4) USA ASTM D 1040 –73
Heavy Hydro Carbon
4.86 >120 ETHANE (C2H6) FRANCE N.F Cir. C 1.03
O2 1.36 >150 ETHYLENE
(C2H4)
GERMANY VDE – 0370
CO 19.21 >700 ACETYLENE
(C2H2)
INDIA IS 335
H2 59.10 SWEDEN SEN-14
N2 10.10 ITALY A.E.1.7
CH4 4.2 SWITZERL
AND
S.E.V.124
Total 100 UK BS 148
USSR GOST-981
IEC IEC: 296
4.7.25 TYPICAL VALUE OF CAPACITANCE & TAN OF TRANSFORMERS Voltage Rating Configuration Capacitance in nF Tan
HV – LV 4 - 5 0.002 - 0.005 HV - Tank 13 - 15 0.007 - 0.009 400/220 KV
LV - Tank 23 - 24 0.004 - 0.008
HV – LV 5 - 7 0.003 - 0.006
HV - Tank 10 - 12 0.005 - 0.010 220/132 KV
LV - Tank 19 - 22 0.004 - 0.010
HV – LV 5 - 7 0.003 - 0.006
HV - Tank 10 - 12 0.005 - 0.010 132/33 KV
LV - Tank 19 - 22 0.004 - 0.010
HV – LV 5 - 7 0.003 - 0.006
HV - Tank 10 - 12 0.005 - 0.010 132/11 KV
LV - Tank 19 - 22 0.004 - 0.010
HV – LV 5 - 6 0.003 - 0.006
HV - Tank 2.9 - 3.5 0.006 - 0.008 66/11 KV
LV - Tank 6 - 10 0.005 - 0.008 HV – LV 8 - 10 0.015 - 0.018 HV - Tank 11 - 13 0.015 - 0.020 33/11 KV
LV - Tank 15 - 17 0.015 - 0.020