TECHNICAL

WRITE-UP

ON

VOLTAGE DROP CALCULATION FOR HT

MOTORS OF AMMONIA-II & CT-II

Prepared by – Gagan Goel, Assistant Engineer (Electrical)

IFFCO AONLA UNITBareilly, UP

VOLTAGE DROP CALCULATION FOR HT MOTORS OF

AMMONIA-II & CT-II

AONLA UNIT

1. OBJECTIVE

To calculate the voltage drop in cables, transformers, switchgears from 11KV E1 Bus upto motor terminal of 11KV motors & 3.3KV motors in Ammonia-II and Cooling Tower-II.

2. GENERAL CONSIDERATIONS

2.1. Brief overview of Ammonia-II Power flow:

2.1.1. 11kV system: At Ammonia-II MESS 11kV switchgear, two incomers are coming from Power plant E1-Bus & E2-Bus which is directly connected to GTG-1 & GTG-2. Power flows from E1-Bus to MOCB (outgoing feeder to Amm-II) then it flows through 11kV/11kV (UE) XLPE insulated Aluminium conductor 3nos.-3core x 300 mm2 Power cable. At AMM-II MESS this cable terminates at 11kV VCB switchgear and AMM-II 11kV bus-bar is charged. From here the power goes through various transformers & motor feeders through VCB.

2.1.2. 3.3kV system: At Ammonia-II MESS 3.3kV switchgear, two incomers are coming from LT side of two nos. 4MVA 11/3.45kV transformers which are fed by Ammonia-II 11kV switchgear. These incomers charges AMM-II 3.3kV bus-bar and from here the power goes through various 3.3kV plant motors through VCB.

2.2. Single Line Diagram of Ammonia-II, 11KV & 3.3KV Switchgear: Attached as Annexure-1

2.3. System Description: Electrical power is available at B.L. of Ammonia Expansion Plant at 11 KV, 3 phase and 50 cycles/sec.

Power Distribution Levels:11 kV 3-phase with resistance earthed neutral, Fault level 750MVA or

40 kA for 3seconds3.3kV 3-phase with resistance earthed neutral, Fault level 150MVA or

26.4 kA for 3seconds0.415kV 3-phase with solidly earthed neutral0.24kV Single phase with solidly earthed neutral

The above voltages have a tolerance of ±10% and a 50 Hz frequency with a tolerance of +3%, - 5%.

The general criteria applied in selecting and sizing the cables, are:

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VOLTAGE DROP CALCULATION FOR HT MOTORS OF

AMMONIA-II & CT-II

AONLA UNIT

- For system utilization voltage 11 kV, cables shall be three core XLPE insulated, rated 11/11 kV.

- For system utilization voltage 3.3 kV, cables shall be three core XLPE insulated, rated 3.3/3.3 kV.

- For system utilization voltage 0.415 kV, cables shall be PVC insulated, rated for 1.1 kV.

- Cable sizing calculations shall include current derating for installation conditions, voltage drop limitations during normal and starting conditions, and short circuit verification(11kV & 3.3kV only).

- The short circuit rating of cables shall be based upon switchgear short circuit rated values. Time duration to size 11kV & 3.3kV cables shall be 0.25s for motor feeders with Vacuum Circuit Breaker.

3. MATERIAL INSTALLATION SPECIFICATION

3.1 Site conditions

Site conditions for design purposes are as per basic engineering data, in particular:

Temperature

- Short time max. value +46 C (Design Temperature)- Average max. value +46 C- Minimum -1.5 C

3.2 11kV & 3.3 kV systems

Main characteristics of cables:

- Aluminium conductors, Three core XLPE insulated, extruded PVC inner-sheathed, armoured, PVC outer-sheathed, flame retardant type (FRLS).

- Rated insulation:- 11 kV system Uo/U = 11/11 kV 3.3 kV system Uo/U = 3.3/3.3 kV

- Max conductor temperature for normal operation for XLPE: 90C- Max conductor temperature during short circuit for XLPE: 250C- Conductor cross section: three core XLPE-185, 240 & 300 mm2

Type of installation:

- Cables run above ground in ladder type trays, at a distance of 1D - Number of equally loaded cables taken into account to determine the

grouping correction factor K2: n = 9 & no. of rack is considered 6 for maximum derating factor.

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VOLTAGE DROP CALCULATION FOR HT MOTORS OF

AMMONIA-II & CT-II

AONLA UNIT

- Value of the corresponding grouping correction factor K2, with reference to IS 3961 (Part II): K2 = 0.66 as per table-3 in annexure-3.

4. BASIS OF CALCULATION

4.1 Allowable current carrying capacity

The allowable current carrying capacity Iz is calculated by multiplying the cable rated ampacity In, by the following correction factors:

Iz = K1 x K2 x In (A)

where:

K1 =

K1 = correction factor for cable at ambient temperature Tm = maximum allowable temperature of the cable under rated conditionsT = ambient temperatureTn = reference ambient temperature for cable rated currentK2 = correction factor for grouped cables, according to typical arrangement of

cables (as defined in para 2.2 above)

K1 = Correction factor for cable in air & at 460 C ambient temperature

=√ ((90-46)/(90-40))= 0.938

4.3 Voltage drop

The voltage drop on the cable line is computed as a percentage value of the rated feeding voltage, as follows:

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VS

90-θ

IR*XVR

IR*Rθ

θ

VOLTAGE DROP CALCULATION FOR HT MOTORS OF

AMMONIA-II & CT-II

AONLA UNIT

VS = Sending end voltage VR = Receiving end voltageIR = Current flowing in cable X = Reactance of the cable R = Resistance of the cable

VS = VR + IR x Rcosθ + IR x Xcos(90-θ)

= VR + IR x Rcosθ + IR x Xsinθ

VS - VR= IR x Rcosθ + IR x Xcos(90-θ)---------- ------------------------------------- VR VR

(Rcosθ + Xsinθ) 100 X L X I % Volage drop= -----------------------------------------

V1where:

V1 = for 3 ph system

V1 = V/2 for 1 ph systemV = line to line voltage of the system (V)R = cable electrical resistance at rated operating temperature

(ohm/km)X = cable reactance (ohm/km)L = line length (km)I = line current (A)

For motor feeders, voltage drop is calculated for both operating and starting conditions, to verify that the acceptable limits are not exceeded.

Taking into account voltage drop limitation at motor terminals, as per para. 1.3, the following max. voltage drop values on the line are allowed to size the cable:

ΔVn% ΔVs%M.V. motor feeders: 4.0 13.0L.V. motor feeders (75kW to 199 kW): 4.0 13.0L.V. motor feeders (<75kW): 5.0 15.00.240 kV lighting points feeders: 1.0other feeders: 1.0

Page 4 of 11

VOLTAGE DROP CALCULATION FOR HT MOTORS OF

AMMONIA-II & CT-II

AONLA UNIT

4.4 Short circuit (for 11kV & 3.3kV only)

Short circuit verification is performed by use of the formula:

S =

where:t = fault duration (S)S = cross-sectional area (mm2)I = effective short circuit current (kA)K = 0.094 for aluminium conductor insulated with XLPE

The above formula is verified for the typical disconnection time t of the protective switchgear provided, and for the fault current I at both ends of the cable line (max and min short circuit current).

Here,t = 0.25sI = 26.24 kAS = 26.24 x (0.25) 1/2

0.094 = 139.6 sq. mm

The selected cross sectional area is 185 sq. mm.

For HT motors of ammonia-I the effective Short circui current(kA) is calculated by assuming a typical protective fault clear time of 0.3 second .

Sr.No. Motor Tag Effective Short circuit capacity in

kA1 MK3201 41.22 MK3202 41.23 MK3203 31.74 MK3301 31.75 MP3302B 31.76 MK4011 31.77 MP3601C 41.28 MP3301C 41.29 MP3801C 41.2

10 MP4801C 41.2

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VOLTAGE DROP CALCULATION FOR HT MOTORS OF

AMMONIA-II & CT-II

AONLA UNIT

5 VOLTAGE DROP CALCULATION

Voltage drop calculation based on para. 4 is calculated. Refer Annexure-2 for detailed calculation. Cable parameters viz current rating, resistance & reactance is based on M/s CCI Ltd. tables which is attached in Annexure-3.

6 RESULTS

Motor Tag

Total Voltageavailabl

e at Motor terminal

(Running)

Total Voltageavailable

at Motor terminal (Start)

%VD at at Motor terminal wrt to E1 Bus Voltage

(Run)

%VD at at Motor terminal wrt to E1

Bus Voltage (Start)

MP3601C 10977 10962 0.209 0.348MP3301C 10977 10967 0.206 0.304MP3801C 10977 10965 0.207 0.315MP4801C 10976 10953 0.222 0.424

Motor Tag

Total Voltageavailable

at Motor terminal

(Running)

Total Voltageavailable

at Motor terminal (Start)

%VD at at Motor terminal

wrt to 3.3kV (Run)

%VD at at Motor terminal wrt to

3.3kV(Start)

MK3201 3300 3285 0.190 0.649MK3202 3301 3286 0.182 0.621MK3203 3299 3279 0.246 0.835MK3301 3300 3283 0.210 0.711MP3302B 3299 3284 0.221 0.702MK4011 3297 3276 0.294 0.934

Voltage drop is within the permissible limit in all 11kV & 3.3kv motor of Ammonia-II & CT-II.

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