Can I use WYE PT connection for 2-elemet 3-wire power measurement

[ters]

A power measurement device (transducer) used to be connected to the open delta (2 PTs) scheme, using 2 current elements (A, C) 3 voltage wires scheme (A-B, B-C). The PT configuration, however, is going to change from an open delta to 3 PTs wye-wye, whereas the secondary L-L voltage will remain the same as before.

Is it possible to somehow continue using present 2 current elements 3 voltage wires scheme, connecting 3 line voltages in the same way (A-B, B-C) from the new PTs to the device.

Or, with the new PT scheme, I have to use 3 element 4 wire measurement, in which case I have to replace device as well (it can be reconfigured to 3 element 4 wire scheme, but seems to expect the same L-N voltage as previously L-L was).

 

[Skogsgurra]

A change in the supply configuration should not influence the load and how power is measured.

If you consider the three phases A, B and C only, without knowing anything about what the transformers are, it is perfectly OK to use a two current element and three phase wire measurement.

So, even if the supplying transformers are changed, you should be able to measure the same way as you did before.

Only thing you need to make sure is that there isn't any star point that is being loaded, or that you don't have a midpoint tap in any of the windings. That usually screws up two element measuring. But you do not seem to have that complication.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[ters]

Thank you very much Gunnar. The application is actually the measurement of active and reactive power needed for the generator control system and the PTs will be on the generator bus.

Sounds I can use the same connections as before. There are about 10 devices needing PT signal, and while for some of them it was easily conclusive that nothing changes regardless what primary PT configuration I have, for some of the transducers I was not sure.

 

[waross]

Will yo be tying the wye point to a solid neutral? A floating wye point on the PT primaries may be problematic.
respectfully

 

[ters]

Yes, the the star point of PTs will be solidly grounded at both sides, primary and secondary.

 

[rcw retired EE]

One problem to check is a sneak connection to ground in the metering or relaying. On the open-delta PT's, B phase is usually grounded. On the wye PT's B phase will be at 67 volts to ground when the wye point neutral is grounded. A ground left on the B phase wiring will blow the PT fuse.

This problem usually occurs if a device, like a synchroscope, connects B phase to neutral or ground.

There is no problem using line-to-line voltages for synchronizing with one set of voltages from Y-Y PT's and the other from open delta as long as the ratios are the same and the voltage leads are not tied together. On many synchscope circuits the "neutral “ side of the voltage coils are tied together creating the sneak ground connection.

 

[waross]

You may wish to brush up on the difference between a "Neutral" and a "Ground". They are not always the same. If you ground your wye point and you have a resistance grounded generator you will have interesting issues with your devices any time there is any current flowing in the grounding resistor.
If you are using a synchroscope off of these PTs be very careful of your phase relationships. As rcwilson states, line to line should be no problem. However some synchroscopes require 120 volts. If you replace 120 volt delta connected PTs with 120 volt wye connected pTs you may have a problem.
I have seen a plant run with a mix of wye PTs and delta PTs on the synchroscope. I was amazed. Every time they brought a machine on line, they closed the breaker with a 30 degree phase displacement. After closing the breaker, the syncroscope would snap to the 2:00 O-clock position. A lot of the various meters on the board would twitch unhappily.
From time to time an operator would close the breaker a little late and blow a coupling off.
respectfully

 

[ters]

Waross, thank you for comments. My generators are grounded through a grounding transformer, so the issues you mentioned may probably still arise...

Here is some more background. This is an LM6000 project with additional complications – conversion from 60 to 50 Hz. In the original 60 Hz scheme both generator and bus PTs were primary open delta, with 120 V secondary. However, for the 50 Hz application, system designers for some reason decided to change/reconfigure both PTs to WYE-WYE with 100 V secondary L-L (100/1.73 L-N), while all (or most) devices using PT voltage are still the same as originally installed for 60 Hz.

There are about 20 different devices needing either generator or bus PT voltage or both. So I have now all kind of cases. Major devices such as generator protection, AVR and automatic synchronizers don’t seem to mind either 50 Hz or 100 V, most if not all devices seem to be OK with 50 Hz too, however, several devices (such as synchroscopes and synchro-check relays) are declared to be designed for nominal 120 V with tolerance +-10%. And while I have “feeling” that they will work at 100 V too, I cannot really be sure. So I’m thinking of buying small interposing PTs 100/120 V, maybe only 10 VA, and use them for the devices that “refuse” to operate at 100 V... And, yes, we will be using L-L for synchronization on both PTs.

 

[ScottyUK]

Ters,

Do the customer's operation and maintenance guys a favour and replace the 60Hz US-origin equipment with correctly specified equivalents from the 50Hz world. We have a similar machine and we have so many frustrating problems with obscure 60Hz instruments from manufacturers unheard of in Europe operating on the ragged edge of their performance envelope because the original project was done cheaply instead of specifiying equipment designed for the service conditions. It is poor, poor engineering, of a standard I would expect from accountants, not from engineers! I do hope you are not going to give someone a PoS electrical package like we have received: Those responsible for ours should be ashamed!


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Sometimes I only open my mouth to swap feet...

 

[ters]

Scotty

Thank you for the reply. I’m unsure what Client’s preferences are, we are still to cross that bridge...

And I’m afraid that I’m dealing with similar equipment as your is... What we have is so called GE Standard Package (SP). LM6000 SP consists (in our case) of turbine, generator (GE or Brush), enclosures, skids, control system and everything else excluding BOP mechanicals (such fuel and water supply) and LV and MV switchgear.

The 60 Hz Standard Control Package is sort of convertible to 50 Hz, but when it comes to generator P&C equipment, which is together with GT controller, originally designed for 60 Hz 120 V, that is a bit fuzzy, some are clearly OK for 50 Hz 100 V but some may not be (most manuals are not very clear, or say nothing). That equipment in our case includes:

Gen Protection - Beckwith 3425 , not sure why GE is using Beckwith and not their own protection (Multilin), maybe the package was manufactured before GE acquired Multilin... OK for 50 Hz and 100 V, but requires some kind of recalibration for 50 Hz (sort of magic conversion )

Synchronizer – Woodward DSM, appears to be OK too.

AVR – Brush Prismic, also doesn’t seem to mind 50 Hz 100 V

Synchro-check relays - Basler, inconclusive, seem to be good for 120 V only

Power meter - SATEC, it sort of looks OK too, manual fuzzy…

Various W, VAr, f, I and V transducers. Some OK for 100 V 50 Hz, some may not be...

Synchro-scopes, Yokogawa, don’t look promising...

Auxiliary voltage relays, so-so...

 

[slavag]

Hi Ters.
Before two days I asked my friend (now he work in GE local office) about why GE use Backwith relay, he told me: is not GE, it's package supplier, many years he used this relay, all design, setting are ready, change is very expensive.

If you have SATEC, you can bay ,as I know, some extention module with 4-20mA outputs (free programmable).

For EQ with 120V you can add inter. VT 100/120V .
PS
As I remembare, before some time you ask about BFP ( maybe in same project), from my last job,I'm strongly recommend you, in all cases put BFP, w/o any " special" logik and send BFP with dedicate cable not via FO for cable diff relay.
Good Luck.
Slava

 

[ters]

Thank you very for the info much Slava. Yeah, that is still the same project and it is not about to go away . There is nothing wrong with Beckwith relay although when converting from 60 to 50 Hz (i.e, converting from ANSI to IEC world), Multilin relays would offer one advantage - they have both 5 Amp and 1 Amp CT inputs while Beckwith relays don't - they have to be ordered for either current. In our case this created an additional complication - the relay is made for ANSI markets and has 5 Amp input while switchgear is an IEC product and CTs are 1 Amp... So to accommodate Beckwith, we have to install another set of 5 Amp CTs on the line side (natural CTs are part of the Standard Package and are 5 Amp), or mickey-mouse with some interposing CTs what is not so recommendable...

 

[slavag]

Hi Ters.
Please pay attention also on VA curves of new CT (also on ratio). Beckwith have also another "problem ", both of CT sets must have same ratio. Interposing CT, today with modern relays,not, not, not. I think more chipper change protective relay and meas eq. also, but what is GE policy
about it. Other Q: Beckwith is only generator protection, what about unit gen-trafo protection. From my point of view you need coordinate all with end-user. Change of type relay on desgn stage is not so big eng. issue.
Where are you? I'm also many work in ME.
Regards.
Slava

 

[ters]

Thanks Slava, we don't need to change relays - we will just buy another set of low voltage toroidal (window) CTs and put them on the cable at the line side. New CTs will be exactly the same make and model as those on the neutral side.

We are not at the initial design phase, conceptual designs were done and agreed upon with the Client earlier, but we are still dealing with some compatibility and conversion issues.

As for other protections, there are actually no new generator transformers, the power will be injected into the existing medium voltage grid, and eventually to HV network through exiting transformers. The generator breaker/switchgear will be about 300-500 m away with MV cable in-between.

So the entire protection scheme is as follows:

- generator protection - Beckwith M-3425
- cable protection is pilot differential sheme - two ABB RED 670 linked by FO
- ABB REF 542 feeder relay which looks for the faults at switchgear and MV grid side

In addition, on the cable between the generator and MV switchgear, there will be an ultra fast "fuse", current limiter, which is needed to reduce SC current, and which will be much faster than any other protection in case of a short circuit, so we seem to have plenty of protections... But the limiters will make selectivity nearly impossible - when a short circuit happens they will operate first in any case.

 

[slavag]

Hi Ters.
Good luck you in this project. I understand you very well.
You have "good" mix of companies and HMI's.
Ultra fast "fuse" I asumme is also ABB, Is limiter of Calor Emag. ABB RED670 for MV cable 300-500m is not bad (good client).

Erection of this system will not easy.

I think about future of this customer: stock of spare parts!!!!
Regards.
Slava