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Topic Title: Power Quality Analysis
Topic Summary: PQA connection on 11 KV system
Created On: 24 February 2010 02:42 PM
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 24 February 2010 02:42 PM
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Badshah

Posts: 38
Joined: 22 July 2009

Is it Possible to connect Power Quality Analyzer( which can measure up to 1000V) across the VT secondary of 11 KV system in order to monitor the quality of the supply. By doing so, can we get the accurate data? i.e. fluctuations, transients, Harmonics etc.
 24 February 2010 05:51 PM
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ArthurHall

Posts: 735
Joined: 25 July 2008

Yes you can, its the only place you could make a connection.
Remember to allow for the change in phase angle when doing power measurements. Harmonics may be slightly attenuated by the VT winding.
 28 February 2010 07:32 PM
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Badshah

Posts: 38
Joined: 22 July 2009

VTs in 11 KV system are configured in open Delta connection i.e.with B-phase grounded? Why it is grounded?..does this affects the Power quality measurement?
 01 March 2010 12:13 AM
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sfchew

Posts: 589
Joined: 10 December 2002

This VT connection is made with 2 VTs instead of 3. Earthing one end is necessary to tie the electrical connection to a reference.

It will not affect the measurement .

Regards
Chris Chew
 01 March 2010 11:44 AM
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ArthurHall

Posts: 735
Joined: 25 July 2008

VT secondarys are usually star connected. In most parts of the world the star point is earthed to provide a reference as Chris said. Brittish practice was to earth the Yellow phase and fuse the neutral. The reason the yellow was earthed was because VT's have a high impedance so fault currents are low and the fuses may not opperate, by earthing the yellow you should get the line voltage (110V) driving the current rather than the phase voltage (63.5V). Brittish practice is gradualy becoming obsolete as there are few Brittish makers and most VT's are imported from mainland Europe.
An open delta winding on a VT is to pick up inbalances in the primary voltages. If you think of a delta winding but with one corner left unconnected and a voltage sensing relay connected across the open terminals. When the primary voltages are equal the secondary voltages will be equal and no voltage will be present across the relay. Any unballance in the primary voltages will consequently cause a voltage to appear at the relay. One of the VT secondary terminals will be connected to earth as a refferance.
 01 March 2010 07:55 PM
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alancapon

Posts: 5770
Joined: 27 December 2005

I think we need more information here to define your "open delta" connection. Conceivably, Chris & Arthur could both be correct.

In UK standard practice, VTs are star-connected on the Primary and Secondary sides, usually with the yellow phase Secondary connection earthed. In some circumstances, a further Secondary winding is connected in "open delta". This takes the form of three VT windings, with the connection at the last "corner" disconnected. The voltage across the "open corner" gives an indication of how out of balance the Primary voltage is. Connection is made with two wires, (one of which is earthed) across the open corner.

In USA standard practice, an "open delta" connected secondary winding is taken from two VTs, with the third VT not being installed. A three phase three-wire connection is taken from the VT. Usually the "B" phase would be earthed, with the "missing winding" being between "A" and "C" phases.

Regards,

Alan.
 07 March 2010 10:14 AM
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Badshah

Posts: 38
Joined: 22 July 2009

Thanks to all,

Yes, the star connected VT's Y phase is grounded and we connected the PQA and got the trend. We are going to connect and try in different Substations.

Regards,
Badshah
 12 March 2010 09:24 AM
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qpa77

Posts: 1
Joined: 12 March 2010

Hi Sirs

Just curious. With 2 VTs, one can only get V12, V32. How would the PQA calculate out the third phase-to-phase voltage?

With the third phase-to-phase voltage being a calculated value rather than a measured value; would there be any implications on the PQ data obtained (eg, voltage unbalance?)

thanks
 12 March 2010 02:36 PM
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sfchew

Posts: 589
Joined: 10 December 2002

The 2 VT connection can still display the secondary voltage perfectly. Therefore measurement of the primary system is not affected.

Regards
Chris Chew
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