Capacitive vs. Inductive Voltage Transformers 4

[Power0020]

I came to know that inductive voltage transformers are usually expensive for voltages higher than 100 kV, that is why a lot of utilities prefer to go for capacitive VTs at higher voltages but, is there any operational aspects? Would the added capacitance be susceptible to ferroresonance issues?

 

[Bill West]

Until we built a microwave back bone, Power line carrier was our communication link. 138kv & 230kv stations were the ones that had full supervisory (SCADA) and regularly their protections used tones for line permissive tripping or transformer transfer trips. So the extensive use of PLC meant coupling capacitors were going in anyway. I think we only had a few 138kv PTs and I don't remember any 230kv ones except perhaps a billing metering set. Our 230kv system history went back to the late 40's. Oh I did see mentions in the IEEE Transactions of transformer and breaker bushing potential devices being used in the 30's because the protection needs couldn't justify the cost of PTs.

Later on we had a 500kv line junction, no transformers, that needed just a bit of station power but was miles from any outside source. Our CCVT supplier built us a special set that could supply, I don't remember, maybe 5-15kva per phase. At that size it had concerns over resonance between the load and the capacitors to be sorted out at the design stage. I looked into a simple relay to detect any oscillations but it was messy and we let it go.

I don't recollect hearing any general drawbacks to CCVTs.

Bill

 

[cuky2000]

CVT has several advantages. However, some of the Operational issues are:

* CVT outputs should be measured for drift periodically (typically every 6 months) for safety reasons. Secondary voltage drift is an indication of the possible failure of the CVT.

* CVT could create challenger for high-speed tripping distance relays, particularly during line faults, when the primary voltage collapses and the energy stored in the stack capacitors and the tuning reactor of a CVT needs to be dissipated, the CVT generates severe transients that affect the performance of protective relays.

* Grading coupling capacitors (see C1 & C2 in the sketch below) are used to step down voltage divider from HV to MV and are usually classified as “high capacitance” or “extra-high capacitance,” depending on their value selected.

* Specifying CVT with high capacitance will help to reduce the magnitude of the transient. Also, higher capacitance could help reduce the TRV imposed by the system and allow better circuit breaker interruption performance.

* To mitigate the potential dangerous & destructive transient overvoltages, CVT is furnished with a ferroresonance-suppression circuit.

....

 

[crshears]

Operationally, PTs have the advantage of being capable of draining trapped charges when pure cables are switched out of service, as once the AC supply is removed the trapped charge becomes a simple DC flow that promptly passes to ground via the wound PT's galvanic connection; my utility has numerous pure cables not readily connectable to high-side-wye trafos, and we use this ability of PTs whenever possible so as to preclude whenever we can the pitting and gouging of grounding device contacts that otherwise inevitably results from de-energizing [grounding / earthing] pure cables.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[scottf]

cuky2000 - may I ask what the picture you posted is from? I believe I generated most of those graphics at one time or another

I agree with the points made above.

Concerning ferro-resonance...as a general statement, with CVTs there can be ferro-resonance between the capacitor divider and the inductive components in the base tank which is why units from with ferro-resonance suppression circuits, normally across one of the secondary windings. For inductive VTs, the concern is with system ferro-resonance (between VT and some system capacitance) that can cause the VT to fail...normally violently.

In regards to transient response of CVTs and the impact on distance protection...the residual voltage rating (normally stated as a % after 1 cycle) is heavily dependent on the connected burden. All of the ratings listed on literature/in the standard are based on transient response at the rated burden. For example in the IEEE world, if the CVT accuracy is rated 0.3WXYZ, the transient response is rated at a ZT ("T" standards for transient) burden....or 200VA. For most modern applications for electronic meters and relays, the actual connected burden is typically in the range of 10VA or less. The transient response change versus burden is not proportional, but it's not too far off.

So, for example, if the CVT's transient response is rated <9% @ ZT (200VA) after 1 cycle and the actual connected burden is 10VA, the actual performance will be much better, something like <2% after 1 cycle.

 

[cuky2000]

Hi Scottf:

I did use various reference as described in the bottom of the graph:

a) The circuit and capacitance table are from the OTCF CVT Catalog from GE _{(former Alstom & originally Ritz).}
b) I believe the picture Catalog for CVT & CC from Trench Electric _{(now Siemens)}.
c)The figure of % of residual voltage vs time is edited from the article “Transient Response of CCVT Designs CCVT Transients Revisited, R. Hedding, ABB Inc. Dousman, WI”.
d) Oscilogram Signal from RTDS taken from the drain coil of CVT. S. Khan, Institute of Graduate Studies, University of Malaya.

_{I also reviewed several references such as:
• IEEE Std C57.13-2016 - IEEE Standard Requirements for Instrument Transformers
• IEC Std 60044-5: Additional Requirements for CVT
• ABB Instrument Transformer Application Guide , Edition 3, 1 HSM 9543 400 –
• CCVT Failures & Their Effects on Distance Relays. S. Gray, CenterPoint Energy D. Haas, & Ryan McDaniel, SEL
• CVT: Transient Overreach Concerns & Solutions for Distance Relaying., D. Hou and J. Roberts SEL
• Personal Notes from various sources, conferences, training & seminars.
• Various Internet Search}

 

[scottf]

Nice...the picture in the upper left is a Ritz (GE) OTCF unit. Looks similar to the current Ritz CVO style (attached), except current Ritz CVO style is currently only available with polymer insulators.

 

[cuky2000]

Scottf: Good catch with the picture.

_{It is interesting to see how dynamic are the changes in the power industry in the past few decades. In the late 80s, we witnessed the transformation of Westinghouse integrated into ABB & Siemens. Also, the changes in GE, Altom, Areva, and other worldwide recognized manufacturers.}

Those mergers and acquisitions also impacted the Instrument Transformers that is hard to track. Here is an example of a time table and evolution of the CVT Model OTCF originally designed by Ritz:

• August 2006: _{RITZ High Voltage became part of AREVA T&D.}
a) CVT Catalog OTCF: by Ritz/Areva

https://www.steamforum.com/pictures/ccvt.pdf

• June 2010: _{Alstom and Schneider Electric finalized the acquisition of the T&D business of Areva}
b)CVT Catalog OTCF: by Astom

• November 2015: _{GE completed the acquisition of Alstom’s power and grid businesses.}
c) CVT Catalog OTCF: by GE

• End 2020: GE schedule _{to close Waynesboro plant} built-in 1991 as Ritz Instrument Transformers by German-owned Ritz Messwander.

 

[RRaghunath]

Cuky2000,
"* CVT outputs should be measured for drift periodically (typically every 6 months) for safety reasons. Secondary voltage drift is an indication of the possible failure of the CVT."
You are right we experienced this phenomenon in no. of CVTs.
Now, since the communication for protection, SCADA, voice etc. is all through optic fibers, the thinking is to go in for inductive VTs instead of CVTs.
The system is 132kV and the inductive VTs cost nearly same as the CVTs, it is found.

 

[scottf]

cuky2000-

I am all too familiar with the Ritz time line

I was in Waynesboro from 1994 to 2006 and hired back by Ritz Germany to help build our current factory in Lavonia, Georgia in 2008 and then our entrance back into HV in 2018 and now I head up Ritz USA.

The current Ritz CVO is designed by many of the same engineers behind the OTCF design. We would like to think we've made some good improvements on an already solid design.

On the issue of CCVT accuracy drift, it "could" be the an indication of failure of the CVT, but more commonly it is a result of capacitance drift of the capacitor divider due to settling of the capacitor elements over time in certain designs. The settling changes the spacing factor of some of the elements, which changes their capacitance, and therefore changes the accuracy of the unit.

The former Ritz OTCF design and the current Ritz CVO takes measures to prevent this kind of capacitance drift by breaking up the capacitor divider into small sub-sections that are rigid and stable in capacitance over time.

The other reason for accuracy drift is due to an element failure (akin to insulation failure) and they can be tougher to detect, depending on where the failed element is (C1 or C2) and the voltage class of the CVT.