Friday, March 31, 2017

Sunday, September 18, 2016

Plant Design: Motor Protection Parameters

The table below is an example of a working protection parameters of an electrical system. A parameter sheet such as this will enable the designer to tabulate necessary initial values and  calculations.

NO.
Equipment name
Nominal power
(kW)
Circuit calculation current(A)
CT trans. ratio
Zero sequence CT trans. ratio
Circuit type
1
Motor Pump No.1
3400
355.6
1000/1
2000/1(Differential)
100/1
Vacuum breaker
2
Motor Pump No.3
1400
146.4
500/1
100/1
 Vacuum breaker
3
Motor Pump No.3
1000
104.6
400/1
100/1
Vacuum breaker
4
Motor Pump No.4
400
41.8
200/1
100/1
F-C circuit
5
Motor Pump No.5
900
94.1
300/1
100/1
F-C circuit
6
Motor Pump No.6
710
74.3
200/1
100/1
F-C circuit

First is to work out in calculating the circuit current of a three phase motor circuit:

Circuit Current=kW/(√3∗kV∗pf )

Instrument and protection CTs are governed by standard IEC 60044-1 . The matching of CTs with protection relays calls for a thorough knowledge of CTs. The following section gives a few reminders in determining   the CT ratio, the table below will give the recommended primary CT ratio for various circuit current in the  motor protection  section. See (link) for complete list of CT ratio per application.

CT Primary=kW/(√3∗kV∗pf *η )


If you do not know exact values for ϕ and η as a first approximation, you can assume that: cos ϕ = 0.8 ; η = 0.8. T. The secondary circuits of a CT must be suitable for the constraints related to its application for  or protection purposes.

CT Secondary  
For use in a local situation Isn = 5 A
For use in a remote situation Isn = 1 A

The use of 5 A in a remote situation increase the cross section of the line or the sizes of the transformer (lost in line). IEEE C57.13 Table 8 - Standard multi-ratio current transformer taps gives the standard ratings for instrument transformers. Sequence CT Ratio are constant 100/1 in 4.160 and 6.9kV Circuit.

For the circuit Type in 6.9kV , there are two recommended circuit types. Use  Vacuum Circuit when the Current reached above 100 Amps or Fused Contractor when dealing with currents below 100 Amps.

Sunday, September 4, 2016

Power System Study for CADPI

Our freelancer have been involved in the Analysis of Electrical System of Central Azucarera De Don Pedro (CADPI) in Nasugbu , Batangas Philippines. The scope of the projects includes Short Circuit Analysis, Load Flow and Protective Device Coordination evaluation of the Plant.