METERING CHANGE OVER.

[apprenteng]

We do have a client metered on 11kV. The facility is fed via two 11kV lines, initially supplied via a single feeder,A. Feeder B was on stand by but could be connected to the bus via an airbreak switch when A had a permanent fault. A changeover using voltage as the actuator was used. Both lines are now energized upto the respective breakers on request by the client,and a manual change over is carried out to the other feeder when onegoes off. The facility has one meter and the utility is not ready to instal another meter. I am trying to come out with a new autochangeover for the VTs; I have parralled the currents from CTs on both feeders to the meter but I need to have an autochange-over for the VTs.

The tie breaker is ever on and using loss of voltage as a change over mode is not possible anymore since both lines are usually energized upto the respective breaker. I thought of using the breaker contacts but the utility may not allow this for the purpose of integrity,lest they steal power... The diagrams are attached. Any suggetions?!

Note: The incoming cable confuration was ahd to be modified as shown in the diagram for the purpose of phasing out with the clients loads.

 

[slavag]

Hi
Small things, don't use two separated contactors.
Preferd use only one with changeover contacts CO.
Normal works from main, change over operated only by loss of voltages of main VT. Please take in account, it's same voltages if tie breaker is closed.
One of the option three small contaktors supplied by Voltages A, B, C from main VT, and sries connection of contacts of those relays controlled CO contactor.
Regards.
Slava

 

[apprenteng]

Slavag,
We may not be communicating.. A similar changeover has failed since the CT currents on the reserve feeder don't match the PT voltages on the main feeder. Currents are shifted by 120 degrees and the Red phase current is zero. From the phasor diagrams I got from a power analysor the vectors are as follows:

Main feeder Voltage Reserve feeder CTs

Red phase---------------- Yellow Phase
Yellow Phase------------- Blue phase
Blue phase--------------- Red phase

Please see the corrected diagram after a thorough investigation on the connection of the feeders. Feeder A is the main feeder.

Thanks guys

 

[jghrist]

I'm afraid I can't make any sense out of your CT connections. You have ØR from Breaker A paralleled with ØB from Breaker B, ØB from Breaker A paralleled with ØY from Breaker B, and ØY from Breaker A and ØR from Breaker B shorted. What is the reasoning for this?

 

[apprenteng]

The metering is 3phase 3wire. The yellow phase CT is shorted. The problem came in when the reserve feeder was connected to the bus. To phase out with the bus voltage the XLPE cable had to be connected as shown. The problem arsing from thr connections is that the Currents from the reserve CTs don't match the Main feeder PT voltages. The final diagram actually depicts the situation as it is; it makes sense... I got the lay-out from the Electrician at the installation

 

[slavag]

Sorry, I don't puted attention on the connectin between phases.
Agree with Jghrist, isn't clear.
R must be connect to R, Y to Y and B to B.
( by the way, a long time I don't see this naming).
Same for the voltages.
You only sum currents from both infeeds. Voltages are same for both infeeds ( I assume it, becouse you can paralelling them). We have such application for directional overcurrent protection 67.
Best Regards.
Slava

 

[slavag]

Sorry Aperenteng again. We send posts together.
I have think.
Slava

 

[apprenteng]

Slavag and jgrhist
The termination is done as simple as it is in the diagram. I have not altered anything. Study the diagram keenly and you will understand where the problem arises and your changeover scheme may not work. In addition to this, the info on the vectors as I gave in my previous post is very clear!!! The attachment has once again been edited.

 

[Marmite]

You need to swap connections on the CT secondaries on breaker B so that you are summating quantities derived from the same primary phase out on the network. Also swap the VT secondary connections on Breaker B VT so that Red phase derived from breaker A VT corresponds to Red phase derived from breaker B VT and so on. If you do that the metering should summate correctly and the voltage changeover will work.
Regards
Marmite

 

[slavag]

Hi.
Apprenteng, your last drw is not opened.
But isn't important.
As Marmite wrote, you have swap all your connections.
Please dont pay attention on the R, Y, B of stand by infeed. You must connect CT and VT according to the main breaker and rename phases of stand by accordantly too.
Very simple - continiue with phase R from main across tie to stand by and rename this phase to R in the stand by side, now connect both CT's in parallel, connect same R of VT to CO contact. Same for all phases.
I still strongly recommend use one contactor with CO contacts and not two contactors, for avoid option of connection both VT's ( we had this issue twice in the synch systems)

Good Luck.
Slava

 

[ScottyUK]

Hi Slava,

One option would be to use mechanically interlocked contactors for reversing duty. When one contactor pulls in an auxiliary contact it disables the coil of the other providing an electrical interlock. This solution has the benefit of three stable states: none selected; 'A' selected; 'B' selected. Here's an example:

https://mall.automation.siemens.com...21-5222-5223-5233-5234-5235-5236-&jumpto=5236

If the link screws up, it is the Siemens 3RA13 series if you want to have a look on their web site.


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If we learn from our mistakes I'm getting a great education!

 

[slavag]

Thanks a lot Scotty.
We used this interlock too, for many cases.
But, for the VT changeover we decide used only one contactor
with CO contacts, more simple control for one coil only.
Best Regards.
Slava
Have a nice weekend guys.

 

[waross]

A couple of comments folks;
Using one contactor for voltage. Many contactors and relays are not rated for opposite polarities on the same contact set. A 120 degree phase shift would be similar to opposite voltages. I have seen many devices that were rated for 600 volts, yet opposite polarity use at 120 volts was forbidden.
If you are paralleling CTs, why not connect the PTs on the bus and cross connect the CTs so that the CTs match the PTs.
When you consider the financial implications of a failed voltage contact on a revenue meter, I strive to avoid any switching on revenue meters.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[scottf]

Why wouldn't you just use 2 meters?

 

[slavag]

Bill, Scott you are right 100+%.
But in OP: "The facility has one meter and the utility is not ready to instal another meter"
Possible add new one or used multyfeeder meter.
BTW, this "utility is not ready to instal another meter"
isn't clear, it's interes of utility.
Best Regards.
Slava

 

[scottf]

Would be willing to bet if facility would buy the new meter, the utility would allow it to be installed, but who knows.

 

[apprenteng]

Waross,
I am sure you mean connection like the one in the attachment?..

 

[PUNGLU]

The change-over may work but consicer the commercial losses when the contactor fails. Before one get to discover the utility company may have lost quite some KWHrs.. A seperate meter may be the best solution commercially

 

[apprenteng]

Guys,
The solutions are good but bear in mind too that an outage to may be check and swap the secondaries of CTs is not an option. Bear in mind the bus cannot be shut down and any work MUST be carried out live...

 

[ScottyUK]

any work MUST be carried out live...

Try defending that statement in court. The branch of government responsible for workplace safety would have you over a barrel because it is almost indefensible. It is very likely to be commercially beneficial to avoid an outage, planned or otherwise, and you might hear various manager types claiming that it 'MUST' be done without an outage, but in reality the work can and should be done with the circuit dead. The company's bottom line is rarely viewed as a sound reason for unsafe working practices.


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If we learn from our mistakes I'm getting a great education!