Z Bus Calculation

RL circuit on a AC source.

In the previous post we had seen RL circuit on DC source. Now lets see it on AC source

Transients in power system

Transients in simple circuit

DC Sources:

• Resistance only
• Inductance only
• Capacitance only
• 
  
• RC Circuit
• 
  
• RLC Circuit:

How to control voltage in power system

We have seen total grid collapsing twice recently. How voltage is controlled? Lets see in the above diagram


Sources and sink of Reactive power

• Transmission line
• Cables

• Synch. Machines
• Shunt capacitors and reactors
• Series capacitors

Per Unit Impedance

Grounding

Advantages of neutral grounding

• Voltages of phases are limited to "phase to ground" voltage.


• High Voltages due to arcing grounds or or transient line to ground faults are eliminated.


• Sensitive protection relays against L-G faults can be used.


• Overvoltages due to lightening are discharged to ground.

Advantages of isolated neutral

• It is possible to maintain supply with fault on one line.


• Interference with communication lines is reduced because of no Z sequence currents.

Q) Find out level calculation at 11 KV bus . ( % Z = 9.88 )

Fault level at 110 KV bus = 1517 MVA ( from utility's fault study )

Take base MVA as 100

System % Impedance = ( Base MVA x 100 )/ fault MVA

% source impedance = (100 x 100 ) /1517 = 6.59 %

% Impedance of T/F at Base MVA = (9.88 x 100)/12.5 = 79.04 %

Effective Impedance since T/F are in parallel = 79.02/2 = 39.52 %
Fault level at 11 KV Bus = ( base MVA x 100 )/% Z upto 11 KV bus
= (base MVAx100)/(6.59+39.52)
= 100x100/46.11

= 216.87 MVA

Fault current on 11 KV side = ( 216.87 x 10^6 )/ sq.root 3 x 11000 KA = 11.39 KA

Calculation for cross section of conductor

A = 11.1 x Is x sq.rt t

Is = SC Current , t= duration of fault in seconds.

Transmission Systems

Hybrid transmission systems ( HVDC+ FACTS) offer many technical and reliability advantages. There are now ways of transmitting 3-4 GW over large distances with only bipolar DC transmission system. This is usefull in offshore wind projects.
HVDC provides reduction in transmission costs due to less losses, and for sea cable transmission with distances over 80 km, DC is the only technically feasible solution.
Today's maximum rating of 6 GW ( at DC voltages of =/- 800 kv) - with UHV AC at voltages of 1000 KV, power transmission on one 3-phase circuit with 10 GW is feasible.
HVDC at present are limited to 600 KV, in order to enable bipolar ratings of 6 GW, we need 750 KV HVDC.