0 Issued for Approval 13.04.

2017

REV. DESCRIPTION DATE STATUS

CUSTOMER:

TANZANIA ELECTRIC SUPPLY COMPANY LIMITED,

TANZANIA.

CONSULTANT:

FICHTNER GmbH & Co. KG., TANZANIA.

CONTRACTOR:
NATIONAL CONTRACTING CO. LTD.
TRANSMISSION & DISTRIBUTION DIVISION,
POST BOX No. 90, AL-KHOBAR - 31952,
KINGDOM OF SAUDI ARABIA.
NAME OF PROJECT:

LOT 2 : SUPPLY AND INSTALLATION OF PLANT AND EQUIPMENT FOR REHABILITATION

OF FIVE 33/11kV SUBSTATIONS IN DAR ES SALAAM.
CONTRACT No.: PA/001/10/HQ/W/013
TITLE:

RELAY SETTING CALCULATION FOR PROPOSED & EXISTING TRANSFORMER BAY

CHANGOMBE SUBSTATION

SH. No/TOTAL SHTS.

PREPARED BY R.Laxman Rao SIZE A4

CHECKED BY - SCALE - 15

APPROVED BY Bhaskar Sahu DATE 13.04.2017

REV No.
DOC /DWG No.:
DOC./DWG No NCC TAN CH ELE DOC 008
NCC-TAN-CH-ELE-DOC-008 0

297 X 210 - A4
 CONFIDENTIAL
The information contained in this document is not to
be communicated either directly or indirectly to any
person not authorized to receive it.

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REVISION HISTORY RECORD

Rev. Date Create/Updating

0 .0.17 First Submission
 15MVA Transformer
REV 0
Differential Protection
A SYSTEM DETAILS
UT rating in MVA = 15.0
HV Voltage rating in kV = 33
LV Voltage rating in kV = 11
Gen Transformer % Impedance = 9.40%
HV CT primary = 400
HV CT secondary = 1
LV CT primary = 1250
LV CT secondary = 1
B HV SIDE :
= MVA/(√3*kV)
Full Load current of HV side in A = 15 MVA/(√3*33kV)
= 262
= 262 / (400/1)
Equivalent current in Secondary
= 0.656
= 1/0.656
Amplitude matching factor
= 1.52
C LV SIDE :
= MVA/(√3*kV)
Full Load current of HV side in A = 15 MVA/(√3*11kV)
= 787
= 787/(1250/1)
Equivalent current in Secondary
= 0.630
= 1/0.630
Amplitude matching factor
= 1.59
1.00 Differential protection relay :
Make = Schneider Electric
Model = MiCOM P642
1.10 System Configuration
Winding Config = HV+LV
Winding Type = Conventioanl
HV CT Terminals = 001
LV CT Terminals = 100
Ref Power (MVA) = 15.00
HV Connection = Y-wye
HV Grounding = Grounded
HV Nominal Voltage = 33
HV MVARating = 15.00
Reactance (HV-LV) = 9.4%
LV Clock vector = 11
LV Connection = Y-wye
LV Grounding = Grounded
LV Nominal Voltage = 11
LV MVA Rating = 15.00
Phase Sequence = Standard ABC
VT Reversal = No Swap
CT1 Reversal = No Swap
CT2 Reversal = No Swap

15 MVATransformer 3DJHRI M/s Schneider Electric Infrastructure Ltd,

Differential Protection Poonamallee, Chennai
 1.20 Differential protection relay, MiCOM P642 has the following parameters for setting:
1) Pick up value of Differential current, Is1. (Operate value of the differential protection function as referred to the
reference current of the relevant end).

2) Slope-1 of tripping characteristic, K1. (Gradient of the tripping characteristic of differential protection in the range).

3) Slope-2 of tripping characteristic, K2. (Gradient of the tripping characteristic of differential protection in the range).

4) Base point for slope-2 characteristic, Is2. (This setting defines the second knee of the tripping characteristic. Above this
knee, the gradient is m2).
5) Threshold value of the differential current for deactivation of the inrush stabilization function (harmonic restraint) and of
the overfluxing restraint, Is-HS1. (If the threshold is set too high, it is possible for the P64x not to trip in the presence of
internal faults with transformer saturation).

6) Threshold value of the differential current for tripping by the differential protection function independent of restraining
variable, harmonic restraint, overfluxing restraint and saturation detector, Is-HS2. (If the threshold is set too low, the P64x
can trip in the presence of external faults with transformer saturation).

1.30 Differential Protection setting

Trans Diff = Enabled
Set Mode = Advance
Is1 in p.u = 0.20
K1 = 30.0%
Is2 in p.u = 2.00
K2 = 80.0%
tDIFF = 0s
Is-HS1 (1/Xt) = 14.29 PU
Is-HS2 (1.3*1/Xt) = 18.57 PU
Zero seq filt HV = Enabled
Zero seq filt LV = Enabled
Ih(2)%> = 20%
Cross Blocking = Enabled
5th Harm Blocked = Enabled
Ih(5)%> = 35%
Circuitry Fail = Disabled
1.40 The stability check is performed for the following cases :-
A. The UT is at -10% tap and loaded at rated capacity
B. The UT is at +10% tap and loaded at rated capacity
C. The UT is at +10% & -10% tap with a through fault at rated capacity.
UT Rating in MVA = 15.00
Full Load current of the UT in HV side in A = 262
1.50 Stability of Differential protection when UT at -15% tap at the above lDifferential:
Reference Power = 15.00
= 15MVA* 1000 / (√3 *0.85*33kV)
Transformer HV Side current in A @ -15% tap
= 309
= 309/(400/1)
Equivalent secondary current at HV Side in A
= 0.772
= 0.772*1.52
Transformer HV Side current in Iref, Amp I
= 1.18
Transformer LV Side current in A = 787
= 787/(1250/1)
Equivalent secondary current at LV Side in Iref, A
= 0.630
= 0.630*1.59
LV side current in Iref, Amp I2
= 1.000
= I1-I2
Differential current @ -15% tap position
= 0.177
= (|I1+I2|)/2
Restraining current @ -15% tap position
= 1.088

15 MVATransformer 3DJHRI M/s Schneider Electric Infrastructure Ltd,

Differential Protection Poonamallee, Chennai
 Characteristic Equations
Characteristic equation: For Ibias< (Is1/K1) ; Diff current required is Idiff ≥ Is1
Characteristic equation: For Is1/K1 < Ibias < Is2 ; Diff current required is Idiff ≥ K1*Ibias
Characteristic equation: For Ibias ≥ Is2 ; Diff current required is Idiff ≥ K1*Is2 + K2*(Ibias-Is2)
Considering the above refered setting the operating current calculation.
Differential current = 0.177 Bias Current = 1.088
The Differential current required for the relay to operate = K1 x Ibias
= K1 x Ibias
The Differential current required for the relay to operate in A
= 0.326
The operating point lies below the characteristic curve . Differential element is restrained and stable
1.60 Stability of Differential protection when UT at +10% tap at the above lDifferential:
= 15MVA* 1000 / (√3 *1.1*33kV)
Transformer HV Side current
= 239
= 239/(400/1)
Equivalent secondary current at HV Side in A
= 0.596
Transformer HV Side current in Iref, Amp = 3.732*1.52
I1 = 0.91
Transformer LV Side current in A = 787
= 787/(1250/1)
Equivalent secondary current at LV Side in Iref, A
= 0.630
= 0.630*1.59
LV side current in Iref, Amp I2
= 1.000
= I1-I2
Differential current @ +10% tap position
= 0.091
= (|I1+I2|)/2
Restraining current @ +10% tap position
= 0.955
Considering the above refered setting the operating current calculation.
Differential current = 0.091 Bias Current = 0.955
The Differential current required for the relay to operate = K1 x Ibias
= K1 x Ibias
The Differential current required for the relay to operate in A
= 0.286

The operating point lies below the characteristic curve . Differential element is restrained and stable

1.70 Through fault stability of Differential protection:

UT MVA = 15.00
= MVA/%Imp
The Fault MVA
= 159.57
HV System Voltage in kV = 33.00
LV System Voltage in kV = 11.0
HV Side CT - primary in A = 400.00
HV Side CT - secondary in A = 1.00
LV Side CT - primary in A = 1250.00
LV Side CT - secondary in A = 1.00
= 159.57MVA* 1000 /(√3 *33kV)
Through Fault current on the HV side in A
= 2792
= 2792/(400/1)
Equivalent secondary current on the HV side in A
= 6.98
= 6.98*1.52
Through fault Current of HV Side in Iref, Amp I1
= 10.64
= 159.57MVA* 1000 /(√3 *11kV)
Through fault current on the LV side in A
= 8376
= 8376/(1250/1)
Equivalent secondary current on the LV side in A
= 6.70
= 6.70*1.59
Through fault LV side current in Iref, Amp I2
= 10.64

15 MVATransformer 3DJHRI M/s Schneider Electric Infrastructure Ltd,

Differential Protection Poonamallee, Chennai
 1.80 Through Fault Stability @ Tap Position +10%
= 159.57MVA* 1000 /(√3 *1.1*33kV)
Through fault current on HV side at Max. tap position in A
= 2538
= 2538/(400/1)
Equivalent secondary current on the HV side in A
= 6.35
Through fault Current of HV Side in Iref in A, I1 = 9.67
Through fault Current of LV Side in Iref in A, I2 = 10.64
= |I1-I2|
Differential current in Max. tap position, Idiff in A
= 0.97
Restraining Current in Max. tap position in A = 10.16
Considering the recommended setting the operating current calculation.
Differential current = 0.967 Bias Current = 10.155
The Differential current required for the relay to operate = K1 x Is2 + K2(Ibias-Is2)
Is2 in A = 2
= 0.3 x 2 + 0.8 (10.16-2)
The Differential current required for the relay to operate in A
= 7.12
The operating point lies below the characteristic curve . Differential element is restrained and stable

1.90 Through Fault Stability @ Tap Position -15%

= 159.57MVA* 1000 /(√3 *0.85*33kV)
Through fault current on HV side at Min. tap position in A
= 3285

= 3285/(400/1)
Equivalent secondary current on the HV side in A
= 8.21
Through fault Current of HV Side in Iref in A, I1 = 12.516
Through fault Current of LV Side in Iref in A, I2 = 10.64
= |I1-I2|
Differential current in Max. tap position, Idiff
= 1.88
Restraining Current in Min. tap position in A = 11.58
Considering the recommended setting the operating current calculation.
Differential current = 1.88 Bias Current = 11.577
The Differential current required for the relay to operate = K1 x Is2 + K2(Ibias-Is2)
Is2 in A = 2
= 0.3 x 2 + 0.8 (11.23-2)
The Differential current required for the relay to operate in A
= 8.26
The operating point lies below the characteristic curve . Differential element is restrained and stable

15 MVATransformer 3DJHRI M/s Schneider Electric Infrastructure Ltd,

Differential Protection Poonamallee, Chennai
 33/11 kV, 15 MVA TRANSFORMER 1(:
Rev 0
HV Backup OC & EF Protection
1.00 BACK UP OVER CURRENT & EARTH FAULT PROTECTION: HV SIDE
Relay type used = C264P
Transformer capacity in MVA = 15
HV Voltage rating in kV = 33
Trafo % Imp = 9.40%
CT Ratio primary = 400
CT Ratio secondary = 1
Rated current in Amps = 15MVA/1.732*33kV
= 262.4396389
HV Through Fault Current in Amps (I Fault)
Max current (A) = 2791.911052
Overcurrent Protection:
Stage 1
= 125% of rated current
Pick up Setting in Amps, I>1
= 1.25*262.44
= 328.0495486
= 0.82
Delay Type
= 0.45
Downstream relay operating time in sec = 0.15
= 0.60
= (0.14 * TMS)/((I Fault/ I>1)^0.02-1)
Operating time at TMS = 1, 't1'
= 3.200
= treq/t1
TMS to be set in the relay TMS>1
= 0.19
Stage 2
Pick up Setting in Amps, I>2 = 1.25 times of Through fault current
= 1.25*2791.91
= 3489.889
Pick up Setting in secondary, I>2 = 8.724722037
Delay Type = DT
Delay Time = 0ms

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 Earth Fault Protection
Stage 1
Earth fault pickup in Amps In>1 = 20% of full load current
= 52.48792778
Equivalent secondary current in Amps = 0.131219819
Delay Type = IDMT - SI
Downstream relay operating time in sec = 45.0%
Grading margin = 0.15
Required operating time 'treq' in sec = 0.60
Operating time at TMS = 1, 't1' = (0.14 * TMS)/(I fault/ IN)^0.02-1
= 1.692421597
TMS to be set in the relay TMS>1 = treq/t1
= 0.35

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A SYSTEMDETAILS
UTratinginMVA = 15.0
HVVoltageratinginkV = 33
LVVoltageratinginkV = 11
GenTransformer%Impedance = 9.40%
HVCTprimary = 300
HVCTsecondary = 1
LVCTprimary = 800
LVCTsecondary = 5
B HVSIDE:
= MVA/(3*kV)
FullLoadcurrentofHVsideinA = 15MVA/(3*33kV)
= 262
= 262/(300/1)
EquivalentcurrentinSecondary
= 0.873
= 1/0.873
Amplitudematchingfactor
= 1.15
C LVSIDE:
= MVA/(3*kV)
FullLoadcurrentofHVsideinA = 15MVA/(3*11kV)
= 787
= 787/(800/5)
EquivalentcurrentinSecondary
= 4.919
= 1/4.919
Amplitudematchingfactor
= 0.20
1 00
1.00 Differentialprotectionrelay:
Differential protection relay :
Make = SchneiderElectric
Model = MiCOMP642
1.10 SystemConfiguration
WindingConfig = HV+LV
WindingType = Conventioanl
HVCTTerminals = 1
LVCTTerminals = 100
RefPower(MVA) = 15.00
HVConnection = Ywye
HVGrounding = Grounded
HVNominalVoltage = 33
HVMVARating = 15.00
Reactance(HVLV) = 9.4%
LVClockvector = 11
LVConnection = Ywye
LVGrounding = Grounded
LVNominalVoltage = 11
LVMVARating = 15.00
PhaseSequence = StandardABC
VTReversal = NoSwap
CT1Reversal = NoSwap
CT2Reversal = NoSwap

15MVATransformer(Existing)Page9of14 M/sSchneiderElectricInfrastructureLtd,
DifferentialProtection Poonamallee,Chennai
 1.20 Differentialprotectionrelay,MiCOMP642hasthefollowingparametersforsetting:

1)PickupvalueofDifferentialcurrent,Is1.(Operatevalueofthedifferentialprotectionfunctionasreferredtothereferencecurrentofthe
relevantend).

2)Slope1oftrippingcharacteristic,K1.(Gradientofthetrippingcharacteristicofdifferentialprotectionintherange).

3)Slope2oftrippingcharacteristic,K2.(Gradientofthetrippingcharacteristicofdifferentialprotectionintherange).
4)Base point for slope2 characteristic, Is2. (This setting defines the second knee of the tripping characteristic. Above this knee, the gradient is
m2).
5)Thresholdvalueofthedifferentialcurrentfordeactivationoftheinrushstabilizationfunction(harmonicrestraint)andoftheoverfluxing
restraint,IsHS1.(Ifthethresholdissettoohigh,itispossiblefortheP64xnottotripinthepresenceofinternalfaultswithtransformer
saturation).

6)Threshold value of the differential current for tripping by the differential protection function independent of restraining variable, harmonic
restraint, overfluxing restraint and saturation detector, IsHS2. (If the threshold is set too low, the P64x can trip in the presence of external
faultswithtransformersaturation).

1.30 DifferentialProtectionsetting
TransDiff = Enabled
SetMode = Advance
Is1inp.u = 0.20
K1 = 30.0%
Is2inp.u = 2.00
K2 = 80.0%
tDIFF = 0s
IsHS1(1/Xt) = 14.29PU
IsHS2(1.3*1/Xt) = 18.57PU
ZeroseqfiltHV = Enabled
ZeroseqfiltLV = Enabled
Ih(2)%> = 20%
CrossBlocking = Enabled
5thHarmBlocked = Enabled
Ih(5)%> = 35%
CircuitryFail = Disabled
1.40 Thestabilitycheckisperformedforthefollowingcases:
A.TheUTisat10%tapandloadedatratedcapacity
B.TheUTisat+10%tapandloadedatratedcapacity
C.TheUTisat+10%&10%tapwithathroughfaultatratedcapacity.
UTRatinginMVA = 15.00
FullLoadcurrentoftheUTinHVsideinA = 262
1.50 StabilityofDifferentialprotectionwhenUTat15%tapattheabovelDifferential:
ReferencePower = 15.00
= 15MVA*1000/(3*0.85*33kV)
TransformerHVSidecurrentinA@15%tap
= 309
= 309/(300/1)
EquivalentsecondarycurrentatHVSideinA
= 1.030
= 1.030*1.15
TransformerHVSidecurrentinIref,AmpI
= 1.18
TransformerLVSidecurrentinA = 787
= 787/(800/5)
EquivalentsecondarycurrentatLVSideinIref,A
= 4.919
= 4.919*0.2
LVsidecurrentinIref,AmpI2
= 0.984
= I1I2
Differentialcurrent@15%tapposition
= 0.196
= (|I1+I2|)/2
Restrainingcurrent@15%tapposition
= 1.084

15MVATransformer(Existing)Page10of14 M/sSchneiderElectricInfrastructureLtd,
DifferentialProtection Poonamallee,Chennai
 CharacteristicEquations
Characteristicequation:ForIbias<(Is1/K1);DiffcurrentrequiredisIdiffIs1
Characteristicequation:ForIs1/K1<Ibias<Is2;DiffcurrentrequiredisIdiffK1*Ibias
Characteristicequation:ForIbiasIs2;DiffcurrentrequiredisIdiffK1*Is2+K2*(IbiasIs2)
Consideringtheabovereferedsettingtheoperatingcurrentcalculation.
Differentialcurrent=0.177 BiasCurrent=1.088
TheDifferentialcurrentrequiredfortherelaytooperate=K1xIbias
= K1xIbias
TheDifferentialcurrentrequiredfortherelaytooperateinA
= 0.326
Theoperatingpointliesbelowthecharacteristiccurve.Differentialelementisrestrainedandstable
1.60 StabilityofDifferentialprotectionwhenUTat+10%tapattheabovelDifferential:
= 15MVA*1000/(3*1.1*33kV)
TransformerHVSidecurrent
= 239
= 239/(300/1)
EquivalentsecondarycurrentatHVSideinA
= 0.797
TransformerHVSidecurrentinIref,AmpI1 = 0.797*1.52
= 1.21
TransformerLVSidecurrentinA = 787
= 787/(800/5)
EquivalentsecondarycurrentatLVSideinIref,A
= 4.919
= 4.919*0.2
LVsidecurrentinIref,AmpI2
= 0.984
= I1I2
Differentialcurrent@+10%tapposition
= 0.23
= (|I1+I2|)/2
Restrainingcurrent@+10%tapposition
= 1.097
Consideringtheabovereferedsettingtheoperatingcurrentcalculation.
Differentialcurrent=0.091BiasCurrent=0.955
TheDifferentialcurrentrequiredfortherelaytooperate=K1xIbias
= K1xIbias
TheDifferentialcurrentrequiredfortherelaytooperateinA
= 0.287

Theoperatingpointliesbelowthecharacteristiccurve.Differentialelementisrestrainedandstable

1.60 ThroughfaultstabilityofDifferentialprotection:
UTMVA = 15.00
= MVA/%Imp
TheFaultMVA
= 159.57
HVSystemVoltageinkV = 33.00
LVSystemVoltageinkV = 11.0
HVSideCTprimaryinA = 300.00
HVSideCTsecondaryinA = 1.00
LVSideCTprimaryinA = 800.00
LVSideCTsecondaryinA = 5.00
= 159.57MVA*1000/(3*33kV)
ThroughFaultcurrentontheHVsideinA
= 2792
= 2792/(300/1)
EquivalentsecondarycurrentontheHVsideinA
= 9.31
= 9.31*1.15
ThroughfaultCurrentofHVSideinIref,AmpI1
= 10.70
= 159.57MVA*1000/(3*11kV)
ThroughfaultcurrentontheLVsideinA
= 8376
= 8376/(800/5)
EquivalentsecondarycurrentontheLVsideinA
= 52.35
= 52.35*0.2
ThroughfaultLVsidecurrentinIref,AmpI2
= 10.47

15MVATransformer(Existing)Page11of14 M/sSchneiderElectricInfrastructureLtd,
DifferentialProtection Poonamallee,Chennai
 1.80 ThroughFaultStability@TapPosition+10%
= 159.57MVA*1000/(3*1.1*33kV)
ThroughfaultcurrentonHVsideatMax.tappositioninA
= 2538
= 2538/(300/1)
EquivalentsecondarycurrentontheHVsideinA
= 8.46
ThroughfaultCurrentofHVSideinIref,Amp I1 = 9.73
ThroughfaultLVsidecurrentinIref,Amp I2 = 9.52
= |I1I2|
DifferentialcurrentinMax.tapposition,IdiffinA
= 0.21
= 9.625
Consideringtherecommendedsettingtheoperatingcurrentcalculation.
Differentialcurrent=0.21 BiasCurrent=9.625
TheDifferentialcurrentrequiredfortherelaytooperate=K1xIs2+K2(IbiasIs2)
Is2inA = 2
= 0.3x2+0.8(9.6232)
TheDifferentialcurrentrequiredfortherelaytooperateinA
= 6.70

Theoperatingpointliesbelowthecharacteristiccurve.Differentialelementisrestrainedandstable

1.90 ThroughFaultStability@TapPosition15%
= 159.57MVA*1000/(3*0.85*33kV)
ThroughfaultcurrentonHVsideatMin.tappositioninA
= 3285

= 3285/(300/1)
EquivalentsecondarycurrentontheHVsideinA
= 10.95
ThroughfaultCurrentofHVSideinIrefinA, I1 = 12.593
ThroughfaultCurrentofLVSideinIrefinA, I2 = 12.31
= |I1I2|
DifferentialcurrentinMax.tapposition,Idiff
= 0.28
RestrainingCurrentinMin.tappositioninA = 12.45
Consideringtherecommendedsettingtheoperatingcurrentcalculation.
Differentialcurrent=0.28
Diff ti l t 0 28 BiasCurrent=12.45
Bi C t 12 45
TheDifferentialcurrentrequiredfortherelaytooperate=K1xIs2+K2(IbiasIs2)
Is2inA = 2
= 0.3x2+0.8(12.452)
TheDifferentialcurrentrequiredfortherelaytooperateinA
= 8.96

Theoperatingpointliesbelowthecharacteristiccurve.Differentialelementisrestrainedandstable

15MVATransformer(Existing)Page12of14 M/sSchneiderElectricInfrastructureLtd,
DifferentialProtection Poonamallee,Chennai
 N909$75$16)250(5(;,67,1*
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1.00 BACKUPOVERCURRENT&EARTHFAULTPROTECTION:HVSIDE
Relaytypeused = C264P
TransformercapacityinMVA = 15
HVVoltageratinginkV = 33
Trafo%Imp = 9.40%
CTRatioprimary = 300
CTRatiosecondary = 1
RatedcurrentinAmps = 15MVA/1.732*33kV
= 262.4396389
HVThroughFaultCurrentinAmps(IFault)
Maxcurrent(A)=2791.911052
OvercurrentProtection:
Stage1
= 125%ofratedcurrent
PickupSettinginAmps,I>1
= 1.25*262.44
= 328.0495486
= 0.82
DelayType
= 0.45
Downstreamrelayoperatingtimeinsec = 0.15
= 0.60
= (0.14*TMS)/((IFault/I>1)^0.021)
OperatingtimeatTMS=1,'t1'
= 3.200
= treq/t1
TMS t b
TMStobesetintherelayTMS>1
t i th l TMS>1
= 0.19
Stage2
PickupSettinginAmps,I>2 = 1.25timesofThroughfaultcurrent
= 1.25*2791.91
= 3489.889
PickupSettinginsecondary,I>2 = 8.724722037
DelayType = DT
DelayTime = 0ms

15MVATransformer(Existing)Page13of14 M/sSchneiderElectricInfrastructureLtd,
OCEFProtection Poonamallee,Chennai
 EarthFaultProtection
Stage1
EarthfaultpickupinAmpsIn>1 = 20%offullloadcurrent
= 52.48792778
EquivalentsecondarycurrentinAmps = 0.131219819
DelayType = IDMTSI
Downstreamrelayoperatingtimeinsec = 45.0%
Gradingmargin = 0.15
Requiredoperatingtime'treq'insec = 0.60
OperatingtimeatTMS=1,'t1' = (0.14*TMS)/(Ifault/IN)^0.021
= 1.692421597
TMStobesetintherelayTMS>1 = treq/t1
= 0.35

15MVATransformer(Existing)Page14of14 M/sSchneiderElectricInfrastructureLtd,
OCEFProtection Poonamallee,Chennai