Tariff Metering With Earthed Voltage Transformers

Can you add a schematic diagram to clarify what exactly are you referring to ?

Hi

Below is a brief draft paper I have prepared in the past on this issue, including the requested diagrams. As is stated in the paper, the conclusions are based on forensic empirical data after the event (and is therefore speculative as to the root cause). Hopefully, this will explain the context of my question.

TARIFF METERING CONNECTED FROM EARTHED VOLTAGE TRANSFORMERS(EVT)

UNDERSTANDING & PREAMBLE:This Document is based on experience gained in the field since 1986.Cases of what was believed to be the result of inversion (neutral point displacement) were examined & tested from 600mW 24kV generators, 132kV, 33kV & 11kV municipal EVTs.Remedies as described below were applied and the results obtained thereafter in the main supported the theory.Readings of actual inversion were never obtained on the site as the plant was locked off under operation and no possible interruptions of supply were tolerated.The document therefore is in the main theory & speculation.

INVERSION (NEUTRAL POINT DISPLACEMENT)

EVT neutral displacement, Transient Oscillations or EVT Neutral oscillation is mentioned in this document as Inversion.

Ferroresonance versus inversion

Some authors place inversion under the general heading of ferroresonance. This is not strictly correct as:"Ferroresonance is normally initiated under some type of switching event such as load rejection, fault clearing, transformer energization, single-phase switching or loss of system grounding." ("Examples of Ferroresonance in a High Voltage Power System" David A.N. Jacobson, Member, IEEE, Power Engineering Society General Meeting, 2003, IEEE, Volume: 2, p 1212-, 13-17 July 2003 [see the fourth to last paragraph above heading entitled "II. WOUND PT-CB GRADING CAP"]. Also available as a download from: http://www.ece.mtu.edu/faculty/bamork/FR_WG/Panel/paper03gm0984.pdf)

Unlike in a ferroresonant condition, Electromagnetic Earthed Voltage Transformer Inversion can be summarised as follows:An Earthed Voltage Transformer (EVT) Primary (and Secondary) neutral can be displaced as a result of the imbalances of the line to Earth (L-E) capacitance's of the system and the return path impedance to the system neutral point. Inversion causes higher magnetisation currents due to core losses. The resultant primary magnetisation current due to Inversion can cause severe overheating of the primary coils and large voltage errors, the phase angle accuracy can also be severely distorted.

From "Introduction to Instrument-Transformers" by Brian D Jenkins, B.Sc, A.M.I.EE Published 1967-George Newnes Ltd: "In the context of a YNyn connected voltage transformer with the N earthed on an otherwise unearthed system, then due to the third harmonic generated there is oscillation of the neutral point and almost certain neutral point inversion. This inversion leads to over-voltages occurring accompanied by high exciting currents due to core saturation and considerable departure from sinusoidal waveform. A simple method of preventing (excessive) inversion is to use a ballast load on each secondary winding, (enough to shunt the core excitation)."

Inversion, therefore, does not strictly fall within the definition for ferroresonance. There are similarities between inversion and ferroresonance, especially in the remedies used to alleviate both of these conditions.

There are many papers dealing with ferroresonance and ways it can be addressed. See "Ferroresonance" Philippe FERRACCI (Supra), "Examples of Ferroresonance in a High Voltage Power System" David A.N. Jacobson (Supra), and Art & Science of Protective Relaying, C. Russell Mason, General Electric Company, Chapter 8 Voltage Transformers p 124.

"Ferroresonance" Philippe FERRACCI, No 190, Cahier technique [1998], Group Schneider, available from:
http://www.schneider-electric.com.tr/schneider_tr/pdf/publications_ect/ECT190.pdf at p 17

Despite the seemingly semantic difference between inversion and ferroresonance, there are important considerations for inversion that do not apply in ferroresonance analysis in dealing with the function and specification of EVTs and their earthing systems. This is explored in more detail below. Tariff Metering on a Solid Earth system connected from an EVT.

1. The EVT should have a Rated Voltage Factor of 1.5/30s, Accuracy Class 0.2, Extended range 10VA burden (IEC60044-2 12.2):

2. BS7625 4.23 defines this EVT connection onto a system with an effectively earthed neutral as having low impedance (to earth) enough to "Reduce Transient Oscillations".

3. On an EVT subjected to inversion the Ph-N (L-N) primary could induced secondary phase voltages which are generally expected to be small in error(<6%) to the true system neutral position with a earth return path & solid earthed system neutral as per the vector diagram 1 below;

4. Ph-Ph (L-L) EVT accuracy may remain in class (0.2) as the iron losses would not be excessively high enough to affect Ph-Ph accuracy to any large extent. Therefore 3Ph3W connection are acceptable for Tariff Metering & are accurate to the power consumed;

5. However 3Ph4W connection is not correct for Tariff Metering as the EVT secondary neutral position cannot be aligned to the true system neutral position & therefore is not within the accuracy class(0.2) required to measure the power consumed;

6. Inversion can further be stabilised (reduced) in a EVT (RVF 1.5/30s) on a solidly earthed system with a separate secondary Broken Delta(0-1.5vsn) with a damping resistor for better stability;

Tariff Metering on a Impedance/Resonance/Isolated Earth system connected from an EVT.

1. The EVT should have a Rated Voltage Factor of 1.9/30s or 1.9/8H, Accuracy Class 0.2, Extended range 10VA burden (IEC60044-2 12.2):

2. BS7625 may by omission suggest that these types of neutral connection systems are not low enough in impedance (to earth) to "Reduce Transient Oscillations (inversion)".

3. On an EVT subjected to inversion the Ph-N (L-N) primary could induced secondary phase voltages which are generally expected to be larger than 120% vn & in error to the true system neutral position with a earth return path & earthed system neutral as per the vector diagram 2 below;

4. Ph-Ph (L-L) EVT accuracy may not remain in class 0.2, as the iron losses would be excessively high and will affect Ph-Ph accuracy.

5. The 3Ph3W connection may not be stable or accurate enough for Tariff Metering. A 3Ph3W(L-L) connection from a "1.9VF EVT in Inversion" will give an Ph-PH low induced secondary voltage due to the iron losses;

6. The 3Ph4W(L-N) connection will not be stable or accurate enough for Tariff Metering as the 1.9VF EVT secondary neutral position and voltage(L-N) would be to large in error to the true system neutral position and voltage(L-N) with a earth return path & impedance/ resonance/isolated earthed system neutral as per the vector diagram 2 below;

7. EVT Inversion may only be damped (reduced) enough to prevent iron loss overheating on these earthed systems (RVF 1.9/….) & cannot improve accuracy with a large secondary phase-neutral burden 2 (>100VA) & a Broken delta with damping resistor;