1730   Polarity of neutral in WYE is unclear

Created: 28 Oct 2020

Status: Conformance Test Preparation

Part: Part 7-3 (2020; Edition 2.1)

Links:

Page: 55

Clause: 7.4.6

Paragraph: CDC (WYE)

Issue

Figure 16 seems to request that Ineut = -(Ia + Ib + Ic). However in table 33, the definition of Ineut specifies the "algebraic sum of the instantaneous values of currents flowing through all live conductors."

Proposal

61850 shall define un-ambiguously the Neut both for Current and Voltage systems, so that it is clear what is the interpretation of the associated samples, espcially when coming to the instrument transformer communication and process bus.

Backward compability shall consider potentially a polarity setting in order to change the polarity of neutral over the ethernet based on the utility philosphy for wiring.

Discussion Created Status
A subscriber today needs to be able to be configured based on the PICS Svp12 to expect either a plus or minus 3I0

Forward compatibility:
- the subscriber shall be configured to receive 3I0 when he subscribes to a stream from a new MU (based on knowledge that new publisher always sends 3I0)

Backwards compatibility
- Compatibility rule: a new subscriber still needs to be able to be configured to receive either +3I0 or -3I0 based on Svp12 from an old publisher
22 Nov 22 Conformance Test Preparation
No objection - Move forward 22 Nov 22 Analysis Of Compatibility
Agreement reached: IN=3I0

Proposed changed

Harmonize Fig 16 with text in Table 33 to un-anbiguously specifiy the Ineut is the algebraic sum of the phase current.

The PIXIT entry Svp12 needs to be deprecated and the test sSvp8 shall consider the sign of Ineut.
11 Oct 22 Verify Draft Implementation
Agreement reached: IN=3I0

Proposed changed

Harmonize Fig 16 with text in Table 33 to un-anbiguously specifiy the Ineut is the algebraic sum of the phase current.

The PIXIT entry Svp12 needs to be deprecated and the test sSvp8 shall consider the sign of Ineut.
11 Oct 22 Drafting Implementation
The only good definition we have in this context is the statement in table 33, saying that IN shall be the "algebraic sum of the phase values".
This is also what has been assumed for 9-2LE and has been implemented in the SV publisher test procedures and used in all MU certifications so far.
For the sake of interoperability, it is necessary that two MUs applied at the same measurement point deliver equivalent IN values, regardless if IN is derived or measured. There is no point to deliver 3I0 in one case and -3I0 in the other.
Sadly, the figures disturb our picture, since they suggest IN=-3I0. We would have been better off if we had named the fourth current just I4 and not IN, removing this connection to a physical model. This IN=-3I0 is deeply embedded in the thinking of the protection engineers (although there is no issue with accepting UN=3U0). It is questionable if the SVs should be seen as a digital twin of this schematic. Also, the formulas in the figures (somehow) suggest that there is a CT in the N line, which is typically not the case anyway. The "IN" is measured either through a Holmgreen circuit or a dedicated residual current CT (often also called zero sequence CT!), which both deliver 3I0 in the first place. Note that the "derived" case corresponds to the Holmgreen circuit. The "desired direction" is then achieved by connecting this in the appropriate way at the relay.
The SVs are a communication protocol and what is in the fourth current can be defined as +3I0 without restrictions for the applications. A MU needs to have a setting to deliver this, independent of the actual wiring of the residual current CT.
So I propose to stick with IN=3I0 and make the documents (in particular the figures) consistent with this.
The other option (which I do NOT recommend) would be to go for IN=-3I0 and change the current wording "algebraic sum of the phase values" and the SV publisher test procedures and waiving interoperability of all MUs certified until now. And I do not see that we would define UN=-3U0 just to make this consistent with IN=-3I0.
13 Nov 20 Accepted

 

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