LT XLPE Cable & HT ABC Redesign: Distribution Utility Reliability Improvement

Initiative

2

Cables are like Artery & Veins of a distribution utility

Failures of cables hamper reliability & customer satisfaction

Cables

Inner Sheath

Armour

Outer Sheath

Conductor

XLPE Insulation

Construction of LT XLPE Cable as per IS 7098- I

About LT XLPE

Customer Supply Interruption & Analysis Data (FY 11-12, 12-13 & 13-14)

FY: 11-12 FY: 12-13 FY: 13-14300000305000310000315000320000325000330000335000340000345000

No Current Complaints

Nos

.

S/L Broken/

Burnt

Feeder Trip Cons. Int. Ckt. Fault

Supply Failure/BD

0.00

5.00

10.00

15.00

20.00

25.00

30.00

Top 4 Highest contributor

%

Service Cable Failure

30% of No Supply attributed to Service Cable Failures

Financial Loss (FY 13-14) Revenue Lost due to unserved power : INR 2.5

Cr/Yr Repair & Replacement Cost : INR 2 Cr/Yr

Reasons for failure :- Cracking of insulation in exposed portion

Probable Reasons of CRACKING :-

1. Improper Bending but not so as Cracking is throughout the exposed surface only

2. Ultra Violet Rays

Trigger

UV Rays are penetrable in RYB Pigmented XLPE Resulting in Cracking & Failure.

* Refer IEEE Paper: UV Ageing

Need to Re-look LT Cable DesignIs IEC 60502(I)/IS:7098(I) adequate ??

How to make XLPE UV retardant?

I0 = intensity of incident light

I = intensity of transmitted lighte = molar absoptivity coefficient in cm2 mol-1

c = concentration in mol L-1

b = pathlength of absorbing solution in cm-1

Beer-Lambert Law Photo-chemical ageing due to UV Rays

Describes ageing of XLPE under UV Light as per following equation

UV Effect on XLPE (Scientific Theory & Field Observation)

Carbon Additive for all

3 coresUse of TiO2 for

all 3 cores throughout

Use of UV retardant

termination kit

Cost Escalation by 30 %

UV test in hot silicon oil also confirmed that UV is penetrable

through RYB coloured XLPE but not penetrable through Black Pigment

LAB Test with UV Source

Lab Test & Probable Solution

EUREKA:UV Retardant by redesigning all

XLPE cores with black pigment at

Zero Cost Conventional design of LT XLPE as per IS: 7098-I

New Design with all black core & extruded colour coding

EUREKA/New XLPE Cable Design

Technical Evaluation

Parameter "As Is"(RYB & Black Pigment

Neutral)Proposed Solution (Black Pigment for

all cores)UV Retardant No Excellent Water Ingress (Probability)

High Very Low

UV retardant Accessories Required No Requirement

Probability of exposed insulation failure

Very High No

Safety of Man & material Unsafe Safe

Economic Evaluation

Parameter "As Is " Design (RYB Pigmented Core)

Proposed Design (All Black Pigments Cores)

Cost of UV Protective accessories

High, approx. Rs. 50 Lacs/ Annum Zero

Repair Cost (Mat. + Lab) High, Approx. Rs. 1.5 Cr. / Annum Zero

FY: 11-12 FY: 12-13 FY: 13-14 FY: 14-15300000305000310000315000320000325000330000335000340000345000

No Current Complaints

Nos

.

S/L Broken/

Burnt

Feeder Trip Cons. Int. Ckt. Fault

Supply Failure/BD

0.00

5.00

10.00

15.00

20.00

25.00

30.00

Top 4 Highest contributor

%

Technical & Economic Analysis of New Design

HT ABC

A very novel concept as

compared to the conventional over

head bare conductor.

ABC provides higher safety and reliability.

Best choice in congested urban areas with narrow lanes & bi-lanes.

ABC: Aerial Bunched Cable

Conductor

Semi-conducting Conductor Screen

Insulation

Semi-conducting Insulation Screen

Semi-conducting water blocking tape

Copper Screen

Outer Sheath

No IS available for HT ABCIS 7098-II (HT U/G XLPE) is followed by

OEMAll distribution utilities use copper screen

Construction of HT ABC

FY:09-10 FY:10-11 FY:11-12 FY:12-13 FY:13-140

500

1000

1500

2000

2500

Nos. of ABC Failure

Nos

.

FY:09-10 FY:10-11 FY:11-12 FY:12-13 FY:13-140

0.2

0.4

0.6

0.8

1

1.2

MU Loss

MU

Increasing trend of ABC Failure

High MU Loss

High interruption & Customer Dissatisfaction

Trigger

Sparking spot

Outer sheath burnt

Insulation damage due to sparking

Cause of Sparking

Sparking & Hissing sound is developed between Copper screen & Messenger wire due to floating screen(Disconnected from messenger)

Initial stage

Outer Sheath burnt along with damage of XLPE insulation due to heat & sparking.

Later stage

The design of HT ABC is such that a substantial capacitive voltage is induced at copper screen if metallic screen is in floating state, resulting in sparking & discharging sound and capable to puncture the outer sheath.

Issues with ABC

When screen is not connected with messenger/earth @ NO LOAD Voltage Measured at conductor of ABC : 6.06 KVVoltage measured at copper screen: 4.49KV (Approx. 80% of Ph Voltage)

MF: 1000

Design Snag

11 KV 150mm2

Aluminium Conductor

Messenger 100 mm2

6.5 KV

8 Volts100 mm2 Aluminium Screen

11 KV 150mm2

Aluminium Conductor

Messenger 100 mm2

6.5 KV

4.8 KV2.5 mm2 Copper Screen

Design Snag

• Faults generally occurs due to substandard material/Design snag

INTERNAL FAULTS

• Birdage /tree fault/equipment/Network failures.

EXTERNAL OR

THROUGH FAULTS

Our fault analysis reveals that around 70% faults are of Internal fault in nature.

Type of ABC Failure

Screen

Messenger

Fault CurrentOwing to its lower resistance, messenger wire will carry about

36 times more current than copper screen

Conductor

Fault Study: External Fault

(f ile test2.pl4; x-v ar t) c:XX0054-XX0002 c:XX0054-XX0004 0.00 0.04 0.08 0.12 0.16 0.20[s]

-10.0

-7.5

-5.0

-2.5

0.0

2.5

5.0

7.5

10.0[kA]

Earthing of HT ABC Screen - Simulation

Messenger Current

Screen Current

Fault current distribution in case of External Fault

Fault Analysis: Ext. Fault

Conductor

Screen

MessengerFault Current

In case of internal fault, almost total fault current travels through the copper screen thus Burning the screen as well

as damage of XLPE Insulation

Fault Study: Internal Fault

Earthing of HT ABC Screen - Simulation

Fault current distribution in case of Internal Fault

Screen Current

Messenger Current

Fault Analysis

Missing/Damage/ disconnected Earthing

Capacitive Voltage: 80% of Phase Voltage on Screen during floating earth

Insufficient cross section of Copper screen to withstand high fault current

Shortcomings

Existing short circuit current rating of the Cu tape which is 2.5mm² was reviewed so as to withstand a short circuit current of 10kA for 1sec.

70 mm ² Copper is required for 10KA which will increase the cost by 4 times & weight by 2 times

So 100mm² Aluminium is opted instead of copper

Reduction in capacitive voltage due to increased cross section

Old HT ABC with copper tape screen for 0.25KA for one sec fault current rating

New HT ABC with Al wire screen for 10KA for one sec fault current rating

Eureka: New ABC Design

When screen is not connected with messenger/earth @ NO LOAD Voltage Measured at conductor of ABC : 6.06 KVVoltage measured at Aluminium screen: 8V

Voltage Measured with new ABC Design

Technical Evaluation

Parameter Conventional ABC ABC with Al Screen

Fault Current capacity 0.25kA for 1sec 10kA for 1sec

Probability of insulation failure

Very High Low

Induced Voltage at Cu Screen @no load

80% of Ph Voltage Close to Zero

Aging of cables Very Fast Very Slow

FY:09-10 FY:10-11 FY:11-12 FY:12-13 FY:13-14 FY:14-15 0

500

1000

1500

2000

2500

Nos. of ABC Failure

Nos

.

FY:09-10 FY:10-11 FY:11-12 FY:12-13 FY:13-14 FY:14-15 0

0.2

0.4

0.6

0.8

1

1.2

MU Loss

MU

Economic Evaluation

Parameter Conventional ABC ABC with Al Screen

Cost of Cable/m(150sqmm) INR 600 INR 650

Repair/Replacement Cost Very High Low

Technical & Economic Analysis of New Design

Cab

le &

AB

C R

edes

ign

Specification revised for both LT XLPE Cable & HT ABC

Patent Filed for LT XLPE Cable Design. Reg. No. 1301/DEL/2014

Replicable across all distribution utility

Proposal sent to BIS to revise IS: 7098-I for LT XLPE Cable

Already Shared with nearby utility & implemented at DHBVNL,Haryana

Journey So Far

Thank You