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Harmonics Generation in a Thyristor Soft Starter

An analysis of harmonic created by Thyristor based Soft Starters

The troubles created by harmonics : Overheating of motors, transformers and cables, buzz/excessive noise of transformer, malfunction of electronic devices , malfunction of relays and contactors, breakdown of the whole power supply system

Voltage withstanding capacity of thyistors,

For a solid starter of 6kV, as at least 7 thyristors have to be connected in series to equally share the voltage of 6kV, any defects or any unevenness of voltage share of the thyristors may results in the breakdown of all the thyristors.

Calculation of harmonics

A series of thyristors are connected in series between motor and power supply system to reduce the voltage in the process of start to limit the start current. But when thyristors are used to serve the purpose of soft start, a great mount of harmonic is created and injected into power supply system. Engineering survey shows that the percentage of 5th harmonic is 25% and the 7th is 17% while the total harmonic distortion is 35%.

The following is the analysis on the harmonic created when a 3000kW motor is started by solid starters:

Parameters given:

  • Minimal short-circuit capacity of generator set:200MVA,
  • Rated capacity of motor: 3000KW,
  • rated current of motor:170.2A 
  • start current of motor: 6.4 times the rated current

Suppose the start current is 4 times the rated current---- 680.8A when solid starter is used, then,

  • total harmonic created: 680.8×35%=238.28A
  • 5th harmonic: 680.8×25%=170.2A
  • 7th harmonic: 680.8×17%=115.6 A

 

Compare with National Criteria for tolerable harmonic(6.6 KV,100MVA)equivalent of IEEE 519 ( International Electric and Electronic Engineering Committee)

I3i

I5i

I7i

I9i

I11i

I13i

22.80

22.80

17.10

7.75

10.60

9.01

I15i

I17i

I19i

I21i

I23i

I25i

4.67

6.84

6.16

3.31

5.13

4.67

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