DOL vs Star delta type of starting 4

[navin34]

Hi,
I would like to know how in an industrial plant the boundary limits for selection of DOL and star delta type of starter are fixed. I understand that for deciding this we need to have information about Minimum fault level at MV level and rating of main trnaformer (MV/LV).It would be of help if somebody can be little desciptive alongwith the supporting calculations.

 

[bigamp]

It is all to do with acceptable voltage dip caused by the impact load of your motor starting. The main reason for using a star-delta starting system or any other reduced voltage starting system, as opposed to DOL starting, is to reduce the impact load and allow larger motors to be started than would be possible with DOL starting.

There will be a limit, imposed by the network you are connected to, as to the maximum voltage dip you are permitted to cause at the point of common coupling (PCC) i.e. at the part of the network that other users are connected to. This may be low, say no more than 3% up to 9 times per hour (you would need to check with the network you are connected to).

Other limits may include what you are prepared to tolerate on your own system. This may be as high as 10%.

A quick and reasonably accurate method of estimating voltage dip at any point is to divide the impact load (in MVA) by the available fault level (in MVA) i.e:

voltage dip (%) = 100xMVA(Impact)/MVA(fault level)

MVA fault level should not include contribution from in-service motors.

For more accuracy you can model the supply network, your own network and your impact load as a set of R's and jX's and manually calculate the voltage (and voltage dip) at any desired point. It's even easier if you have the use of an electrical analysis package.

 

[rbulsara]

The limitation on a system may be based on the permissible voltage dip which is a fucntion of the inruch currents and the impedances, but not necessarily the type of a starter used.

A 300 HP motor with a star/delta starter will cause the same voltage dip as the 100HP motor with a DOL starter. The system will limitation should be not only the type of starter but by a combination of the HP of a motor and a given type of starter.

Also the selection of a starter has to pass another test also, that the starting torque procuded during the reduced voltage starting is adequate for the load to accelerate and reach the rated speed within acceptable time.

 

[jbartos]

Suggestions to navin34 (Electrical) Dec 26, 2003 marked ///\\I would like to know how in an industrial plant the boundary limits for selection of DOL and star delta type of starter are fixed.
///It depends
1. Where you are
2. Voltage level
3. Type of industry
4. Etc.
Sometimes, the limits on DOL are set by standards or practices, e.g. motor max kW may be set to 5kW or max motor HP may be set to 10HP in low voltage power distribution systems. However, some industry, e.g. power generating stations with their fine-tuned auxiliary system may start medium voltage motors DOL as high as thousands of HPs.
The criteria should be set conservatively in an industrial plant. One aspect that has not been addressed yet is a possibility that large motor starts are not mutually exclusive. It means that two or more large motors can start simultaneously. Then, this worst case scenario has to be considered to avoid any trips of upstream protective devices.
The star-delta starters should be compared with modern electronic soft starters since the latter tend to be electrically and mechanically smoother starters.
The DOL limit is set by the motor starting transient voltage at the motor terminal. Depending on the motor design, it can be 85% of the motor terminal (or rated) voltage. The motor can be designed to start DOL with 75% of the motor terminal voltage. It is more expensive. The star-delta starter will reduce the motor starting inrush current approximately to 1/3. This will allow starting the larger HP motor connected to the same power supply.
Visit

http://www.cbibreakers.com/papers/20/Motor.pdf

http://www.engineeringtalk.com/news/frf/frf102.html

http://www.saftronics2s.co.uk/softstarts/manuals/pdf/productguide.pdf

(for: Figure 3.3 Current/speed characteristics)

http://www.e-publishing.af.mil/pubfiles/af/32/afpam32-1192/afpam32-1192.pdf

(for: Figure A3.5. Comparison of Current and Torque Reduction With Different Starting Methods.)
etc. for more info.\\ I understand that for deciding this we need to have information about Minimum fault level at MV level and rating of main transformer (MV/LV).
///And mainly, sizes of conductors, i.e. feeders, busways, etc.\\It would be of help if somebody can be little desciptive alongwith the supporting calculations.
///The calculation mainly concentrates on voltage drops across various equipment, e.g.
1. Transformers
2. Cables
3. Overload relays
4. Contacts
5. Power supply internal impedance
6. Etc.
Item Re[volt drop] + jIm[volt drop]
transf
cable
overload
contacts
System Z
etc.
Total = SumRe[volt drop] + jSumIm[volt drop]
Total = |Total| x exp[j atan(sumIm/sumRe)]\\

 

[Marke]

Hello navin34

The selection is based on the supply impedance and local regulations. In many areas, there are standard supply rules in place that have been developed to minimise disturbances on the supply. For example, locally, you must use a reduced voltage starter for all motors greater than 5.5KW if you are taking supply at 400 volt.
For this purpose, a star/delta starter is/was considered to be a reduced voltage starter. The reality however, is that generally, a star/delta starter produces more supply disturbances than a DOL starter. (- I generaly refer to the star/delta starter as a "Political" starter. It gets around the regulations!!)
The bottom line is that provided the reduced voltage starter/installation is properly engineered, you can minimise supply disturbances on a common bus. Rules such as "use a reduced voltage starter" or "use a star/delta starter" do not necessazrily achieve the desired results. Rules based on the maximum supply disturbance are a much better means of control.
Best regards,

Mark Empson

http://www.lmphotonics.com