Question - Is that correct wiring for ION7650? MV measurement 3PTstar, 2CT L1,L3 - volts mode 4W-Wye

[jary_svab]

Typical wiring from Installation guide ION7650

If we use wire the Voltage and Current Inputs like bellow is there any impact for billing and power quality measurement with setting of volts mode = 4W-Wye ? This wiring is not mentioned in Installation guide, but for mesuremnet in MV site are very offten used 3PT(star) and 2CT(L1,L3).

Thank you in advance for estimation and answers.

[jimbradley]

Hi,

I have sen this approached used by utilities in HV and MV installations as CTs for these voltage levels are expensive.

The flaw in using 2 CTs is that the load must be balanced for accurate power and energy results.  The wiring assumes that Ia + Ib + Ic = 0 (vectorally).  Any unbalanced current is missed.  In utility MV distribution the assumption that the load is balanced is often incorrect due to single phase distribution legs.  In environments with harmonic currents such as non linear loads like VFDs, UPSs,, etc. the unbalanced current. can be significant especially in regards to harmonic currents.

Jim

[DanL · Accepted solution]

Short answer: Yes.

The volts mode should always be configured to match how the voltage terminals are connected (on this platform, processing of current channels is not affected by volts mode).

The 2CT configuration shows actually generates the residual current in the CT secondary, so as long as load current only flows on 3 wires (up to one more wire than you have CTs), all readings will be correct (assuming perfectly matched CTs).

The term "balanced" is a little misleading. The residual current generated by the CT secondaries will be a proportional representation of the line current so long as all the residual current is flowing in that line. Technically theis means that the zero-sequence magnidue (only measureable with 3CTs) is near zero. Problems typically arise when load current returns on a 4th neutral conductor or in ground as a fault.

[frans-willem]

Hi All,

Further to this discussion:

Can the ION meter estimate the neutral current given a 3 CT installation?

I'm sceptical about vector calculations as currents waveforms are typically very far from sinusoidal.

Numerical, subcycle waveform addition and RMS calculations should do better but I think the neutral will typically be phase shifted leading to an innacurate estimation.

Any thoughts?

Frans-Willem Vermaak

South Africa

[DanL]

The ION 7650 can of course perform vector or many other user specified calculations.

The best way to determine the residual neutral current on a 4-wire system with three CTs (especially where harmonics are present) is to sum the three CT secondaries and pass the result through the I4 terminals on the ION7650.

[giuliano]

Estarei ausente do escritório a partir de 06/04/2015 e não retornarei até

16/04/2015.

Responderei à sua mensagem quando retornar.

Em caso de urgência entrar em contato com:

Bruno Galfo (bruno.galfo@schneider-electric.com) ou Sandra Haro

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Je serai absent(e) à partir du 20/12/2012 de retour le 07/01/2013.

I'm out of office and will be back on January 7th . My e-mail access will

be limited .

Best regards

[aaron_azzo_com_]

Hi Dan.  Would it not be best practice to use a fourth CT for I4 rather than relying on secondary shorting?  Also, if the S2 is earthed as it's supposed to be for reference, how would that affect the S1's of the three phases being brought together; would this not create a short?

[DanL]

This technique of summing CT secondaries (from 2, 3 or 4 CTs) is most commonly used when low price is more important than accuracy.

On a 3-wire system (3 conductors between the source and load), there is no fourth wire in the system to put the fourth CT on... The wiring diagram above effectively uses the meter's I4 terminals to measure the return current in the CT secondary common wire (typically 3 CTs are wired with one wire each plus a shared common wire that is grounded at one point).

There are also special purpose CTs (zero sequence CTs or Earth leakage CTs) that could possibly be used instead - but I'm not sure they have much of an advantage in this situation.

I'm not familiar with the terms "S1" and "S2"? On the ION7650 the S1, S2, S3... S8 terminals are the digital Status Inputs.

Or did you mean the common wire of the CT secondaries? Yes, the CT common (non-dot side of CT output) is grounded at the shorting block for safety - it prevents the CT secondaries from floating to coupled voltages (usually primary-side voltage).

[aaron_azzo_com_]

Hi Dan, thanks for clearing that up.  I was referring to the wye connection, not the delta, which is the one in question; my mistake.

S1 and S2 refer to the high and low (grounded) leg of CTs on the secondary; S2 should always be the earthed terminal.  The main purpose is to ensure a solid reference, rather than referring to itself, which will cause erroneous readings.  Where I was initially going with this was exactly as you said, which is accuracy: if a customer is purchasing a very costly PQ meter (ION7650), holding back on accuracy defeats the purpose of having a PQ meter on their system in the first place.  If four CT's can be used on the system (some utilities do have wye HV:MV transformer), then I think it's best to use them for the sake of accuracy, and the sake of ensuring the high-end meter is installed in a high-end fashion.

[engineer_langfo]

Dan,

Of course; the "other" reason there is no neutral CT is when an ION7650 is retrofitted into an existing installation where there are only three CTs.  We would then have to do a residual current calculation like on the ION73x0 meters using an arithmetic module in the ION7650. 

Can you confirm what algorithm should be programmed into an arithmetic module in order to calculate the residual current?
I am guessing I residual =  I1*cos(I1 angle) + I2*cos(I2 angle) + I3*cos(I3 angle)  ...

.. where I1 is from the power meter module and I1 angle is from the Power Harmonics Module.

Is this valid?

[edward_lim_uon_]

Hello Jaroslav,

Did you end up wiring the 3PT and 2 CT and using 4W mode? Was everything OK?

I am trying to work out the correct way to wire this ABB metering TX to ION8800.

Regards
Edward