Capacitor connection at LT side of Transformer

• The power factor compensation can be provided either LT or HT side of the distribution transformer. 
• The electricity board measured power factor at the HT side for penalty calculation. 
• The power factor compensation provided by the HT capacitors and LT capacitors. 
• We will discuss the advantages and disadvantages of LT capacitors over HT capacitors.

 

Let us consider two cases in which capacitor are connected at the HT side and LT side.

Capacitor connected to HT side

• When a capacitor connected to HT side of transformer, the reactive power of capacitor does not flow through transformer. 
• This will does not affect the power factor of LT side.

Capacitor connected to LT side

• When a capacitor connected to LT side of transformer, the reactive power flows through the transformer and therefore the power factor improves. 
• This will result in reduced current flows through HT side as well as LT side.

Example

Consider a 1000 kVA transformer in which full load copper losses are 10 kW.  When 500 kW load is connected to the secondary side. Calculate power saving when capacitor is connected at the LT side. Consider that the energy charges is Rs. 6 per kWh and power factor of transformer is 0.8 lagging

Solution

When capacitor is connected at HT side

The operating power factor of the transformer say Cos Φ1 = 0.8

When capacitor is connected at LT side

The operating power factor of the transformer say Cos Φ2 = 0.98

K = Load in kW / kVA rating of transformer

   = 500 / 1000

   = 0.5

Power saving

= Wcu × K × ( 1 / Cos Φ1 – 1 / Cos Φ2 )

= 10,000 × 0.5 × ( 1/ 0.8 – 1/0.98 )

= 10,000 × 0.5 × ( 1.25 – 1.020 )

= 10,000 × 0.5 × ( 0.23 )

=  1150  watt

Monthly saving = 1150 × 24 × 30 /1000 = 828 kW

Let us consider that the energy charges is Rs. 6 per kWh

Monthly saving = Rs 6 × 828 = Rs. 4968

Yearly saving = Rs.4968 × 12 = Rs.59616

Transformer Capacity

• When capacitor is connected to HT side, the maximum load that can be connected to transformer is 1000 kVA × 0.8 = 800 kW
• When capacitor is connected to LT side, the maximum load that can be connected to transformer is 1000 kVA × 0.98 = 980 kW

Advantages of capacitor connected at LT side

• The cost of LT capacitor is less as compared to HT capacitors
• Maintenance of LT capacitor panel is easier as compared to HT capacitors