1. Why DC supply used as control supply in all protection and control panels?
DC supply is storable and if any problem in HV side AC voltage then Alarm and protection is easy by using DC supply.
2. Why CT & VT wires are bigger in size comparatively other control cables?
The I²R loss is more when small Sq.mm cables used.
Resistance is inversely proportional to cross section area of the wire or cable.
When area increase Resistance decrees.
R ∞ 1/a
Example :
VT secondary voltage = 115v
Burden of the VT secondary circuit = 300 VA
2.5 Sq.mm cable RL = 2 Ω
Current flow = 300 / 115 = 2.6 Amps
Loss per meter = I² RL = 35 watts
Voltage drop = I X RL = 2.6 * 2= 5.2 volts
3. What is Inrush current in Transformer or motor?
During commissioning time the residual magnetic flux in the
Transformer and motor coil is almost zero. There is no back emf for opposing
the main emf in the coil. So the magnetic current very high during starting
stages.
During charging time the inrush current value is less when
HV primary voltage frequency cross the zero value and comparatively high during
frequency cross its peck value.
Inrush current contain 3rd and 5th harmonics and it is detected by protection relays and block the tripping.
4. Why most of the Power Transformer primary side connection arrangements Star connection?
Per Phase voltage value of star connection = V/ √3
But per phase voltage in Delta connection is same as Phase
to Phase voltage.
The insulation requirement of transformer is depending open the voltage value. If voltage reduces then insulation requirement reduced and size & cost of transformer reduced.
