How to calculate voltage regulation of distribution line

Introduction to voltage regulation

Voltage (load) regulation is to maintain a fixed voltage under different load.Voltage regulation is limiting factor to decide the size of either conductor or type of insulation.

In circuit current need to be lower than this in order to keep the voltage drop within permissible values. The high voltage circuit should be carried as far as possible so that the secondary circuit have small voltage drop.

Voltage Regulation for 11KV, 22KV, 33KV Overhead Line

% Voltage Regulation = (1.06 x P x L x PF) / (LDF x RC x DF)

Where:

P – Total Power in KVA
L –  Total Length of Line from Power Sending to Power Receiving in KM.
PF – Power Factor in p.u
RC – Regulation Constant (KVA-KM) per 1% drop.

RC = (KV x KV x 10) / ( RCosΦ + XSinΦ)

LDF – Load Distribution Factor.
LDF = 2 for uniformly distributed Load on Feeder.
LDF > 2 If Load is skewed toward the Power Transformer.
LDF = 1 To 2 If Load is skewed toward the Tail end of Feeder.

DF – Diversity Factor in p.u

Permissible Voltage Regulation (As per REC)

Maximum  Voltage Regulation at any Point of Distribution Line
Part of Distribution System	Urban Area (%)	Suburban Area (%)	Rural Area (%)
Up to Transformer	2.5	2.5	2.5
Up to Secondary  Main	3	2	0.0
Up to Service Drop	0.5	0.5	0.5
Total	6.0	5.0	3.0

Voltage Regulation Values

The voltage variations in 33 kV and 11kV feeders should not exceed the following limits at the farthest end under peak load conditions and normal system operation regime.

• Above 33kV (-) 12.5% to (+) 10%.
• Up to 33kV (-) 9.0% to (+) 6.0%.
• Low voltage (-) 6.0% to (+) 6.0%

In case it is difficult to achieve the desired voltage especially in Rural areas, then 11/0.433 kV distribution transformers(in place of normal 11/0.4 kV DT’s) may be used in these areas.

Required Size of Capacitor

Size of capacitor for improvement of the Power Factor from Cos ø1 to Cos ø2 is:

Required size of Capacitor (Kvar) = KVA1 (Sin ø1 – [Cos ø1 / Cos ø2] x Sin ø2)

Where KVA1 is Original KVA.

Optimum location of capacitors

L = [1 – (KVARC / 2 KVARL) x (2n - 1)]

Where:

L – distance in per unit along the line from sub-station.
KVARC – Size of capacitor bank
KVARL – KVAR loading of line
n – relative position of capacitor bank along the feeder from sub-station if the total capacitance is to be divided into more than one Bank along the line. If all capacitance is put in one Bank than values of n=1.

Voltage Rise due to Capacitor installation:

% Voltage Rise = (KVAR(Cap) x Lx X) / 10 x Vx2

Where:

KVAR (Cap) – Capacitor KVAR
X – Reactance per phase
L – Length of Line (mile)
V – Phase to phase voltage in kilovolts

Calculate % Voltage Regulation of Distribution Line

Calculate Voltage drop and % Voltage Regulation at Trail end of following 11 KV Distribution system:

• System have ACSR DOG Conductor (AI 6/4.72, GI7/1.57)
• Current Capacity of ACSR Conductor = 205Amp,
• Resistance = 0.2792Ω and Reactance = 0 Ω,

Permissible limit of % Voltage Regulation at Trail end is 5%.

Method-1 (Distance Base)

Voltage Drop  = ( (√3x(RCosΦ+XSinΦ)x I ) / (No of Conductor/Phase x1000))x Length of Line

Voltage drop at Load A

• Load Current at Point A (I) = KW / 1.732xVoltxP.F
• Load Current at Point A (I) =1500 / 1.732x11000x0.8 = 98 Amp.
• Required No of conductor / Phase =98 / 205 =0.47 Amp =1 No
• Voltage Drop at Point A = ( (√3x(RCosΦ+XSinΦ)xI ) / (No of Conductor/Phase x1000))x Length of Line
• Voltage Drop at Point A =((1.732x (0.272×0.8+0×0.6)x98) / 1×1000)x1500) = 57 Volt
• Receiving end Voltage at Point A = Sending end Volt-Voltage Drop= (1100-57) = 10943 Volt.
• % Voltage Regulation at Point A = ((Sending end Volt-Receiving end Volt) / Receiving end Volt) x100
• % Voltage Regulation at Point A = ((11000-10943) / 10943 )x100 = 0.52%
• % Voltage Regulation at Point A =0.52 %

Voltage drop at Load B

• Load Current at Point B (I) = KW / 1.732xVoltxP.F
• Load Current at Point B (I) =1800 / 1.732x11000x0.8 = 118 Amp.
• Distance from source= 1500+1800=3300 Meter.
• Voltage Drop at Point B = ( (√3x(RCosΦ+XSinΦ)xI ) / (No of Conductor/Phase x1000))x Length of Line
• Voltage Drop at Point B =((1.732x (0.272×0.8+0×0.6)x98) / 1×1000)x3300) = 266 Volt
• Receiving end Voltage at Point B = Sending end Volt-Voltage Drop= (1100-266) = 10734 Volt.
• % Voltage Regulation at Point B= ((Sending end Volt-Receiving end Volt) / Receiving end Volt) x100
• % Voltage Regulation at Point B= ((11000-10734) / 10734 )x100 = 2.48%
• % Voltage Regulation at Point B =2.48 %

Voltage drop at Load C

• Load Current at Point C (I) = KW / 1.732xVoltxP.F
• Load Current at Point C (I) =2000 / 1.732x11000x0.8 = 131 Amp
• Distance from source= 1500+1800+2000=5300 Meter.
• Voltage Drop at Point C = ( (√3x(RCosΦ+XSinΦ)xI ) / (No of Conductor/Phase x1000))x Length of Line
• Voltage Drop at Point C =((1.732x (0.272×0.8+0×0.6)x98) / 1×1000)x5300) = 269 Volt
• Receiving end Voltage at Point C = Sending end Volt-Voltage Drop= (1100-269) = 10731 Volt.
• % Voltage Regulation at Point C= ((Sending end Volt-Receiving end Volt) / Receiving end Volt) x100
• % Voltage Regulation at Point C= ((11000-10731) / 10731 )x100 = 2.51%
• % Voltage Regulation at Point C =2.51 %

Here Trail end Point % Voltage Regulation is 2.51% which is in permissible limit.

Method-2 (Load Base)

% Voltage Regulation =(I x (RcosǾ+XsinǾ)x Length ) / No of Cond.per Phase xV (P-N))x100

Voltage drop at Load A

• Load Current at Point A (I) = KW / 1.732xVoltxP.F
• Load Current at Point A (I) =1500 / 1.732x11000x0.8 = 98 Amp.
• Distance from source= 1.500 Km.
• Required No of conductor / Phase =98 / 205 =0.47 Amp =1 No
• Voltage Drop at Point A = (I x (RcosǾ+XsinǾ)x Length ) / V (Phase-Neutral))x100
• Voltage Drop at Point A =((98x(0.272×0.8+0×0.6)x1.5) / 1×6351) = 0.52%
• % Voltage Regulation at Point A =0.52 %

Voltage drop at Load B

• Load Current at Point B (I) = KW / 1.732xVoltxP.F
• Load Current at Point B (I) =1800 / 1.732x11000x0.8 = 118 Amp.
• Distance from source= 1500+1800=3.3Km.
• Required No of conductor / Phase =118 / 205 =0.57 Amp =1 No
• Voltage Drop at Point B = (I x (RcosǾ+XsinǾ)x Length ) / V (Phase-Neutral))x100
• Voltage Drop at Point B =((118x(0.272×0.8+0×0.6)x3.3)/1×6351) = 1.36%
• % Voltage Regulation at Point A =1.36 %

Voltage drop at Load C

• Load Current at Point C (I) = KW / 1.732xVoltxP.F
• Load Current at Point C (I) =2000 / 1.732x11000x0.8 = 131Amp.
• Distance from source= 1500+1800+2000=5.3Km.
• Required No of conductor / Phase =131/205 =0.64 Amp =1 No
• Voltage Drop at Point C = (I x (RcosǾ+XsinǾ)x Length ) / V (Phase-Neutral))x100
• Voltage Drop at Point C =((131x(0.272×0.8+0×0.6)x5.3)/1×6351) = 2.44%
• % Voltage Regulation at Point A =2.44 %

Here Trail end Point % Voltage Regulation is 2.44% which is in permissible limit.