A common mistake when replacing the ТПЛ-10

This article is not written for designers, but for ordinary users (operational staff). In practice, we face often with such a problem like to replace current transformer  ТПЛ-10 (ТПЛМ-10) not passed metrological verification, or to perform other advises (most requirements) of energy-distribution companies related to the reduction of energy consumption by the consumer (such as a reduction in transformation ratio or an increase of the current transformer accuracy class).
Lets consider the case where current transformer of ТПЛ-10 type, Soviet-style, made usually in last century, 80-90th years, is a subject to replacement. The fact that  ТПЛ-10 was operated for a long time, talks about pretty well quality of a transformer, but this transformer still have a significant design flaws, the main one being - is an open magnetic circuit assembled from individual plates of electrotechnical steel. Because of this design feature it`s quite difficult to achieve a higher accuracy grade of the transformer and to protect the magnetic core from corrosion.
At the moment, current transformer ТПЛ-10 -type is not made and is not included to the СИТ Register (measuring devices). New ТПЛ-10 does not exist. Replacement is only possible on the transformers from storage, if they passed the last metrological verification or certification in metrological center.As a result, we obtain the replacement of the old transformer, which did not pass verification, for another old transformer, which passed verification. The question suggests itself: how long this transformer will meet to GOST? Before the next scheduled calibration, maybe if you're lucky, a bit longer, but anyway not for scheduled 25 years average lifespan.
Such replacement can without exaggeration be considered as a big mistake.
So, the ТПЛ-10 failed verification have to be replaced for what ?
"ELIZ" factory makes more perfect, with the best technical characteristics, corresponding to the last ГОСТ, certified current transformers ТПЛ-Э-12-02 type, which dimensions and seatings are fully coincide with the dimensions and seats of ТПЛ-10.
These transformers are cast-type, ie all the elements of the transformer design, excluding outlet contacts, has no touch with atmospheric air, it`s not afraid of high humidity, which guarantees the service life of 25 years or more. Accuracy class of the transformers are standard - 0,5S, but at the request of the customer can be made current transformers with 0.2 and even 0,2S accuracy class, using of amorphous steel cores.

What is the partial discharge ?

The main cause of partial discharges in cast resin insulation is the presence of air bubbles in a hardened epoxy compound, as a consequence of non perfect manufacturing transformers process.

Partial discharges appears in the voids of solid (paper or plastic) insulation, in multi-layer systems of electrodes / conductors having a several layers of solid insulation and also in gas bubbles (in case of liquid insulation) or around the electrodes in the gas, and in most cases are air or oil gaps in the insulation.
The presence of partial discharges in the current and voltage transformers with cast insulation will eventually lead to the insulation breakdown, so the Partial discharge test can determine the reliability of the product over its entire life. Any other acceptance tests can determine specifications only at the moment, but just Partial discharge test allows to predict the stability of future performance and reliability of the product as a whole. That is why the Partial discharge testing is crucial.

As per ГОСТ 7746-2003, used in Russia, determines the partial discharge level not more than 20pKl, measured at voltage 7,63kV. Ukrainian DSTU IEC 60044-1:2008 requires the same Partial discharge value at more voltage 8,32kV and additionally - at increased voltage 14,7kV - not more than 50 pKl. Thus, the requirements (and therefore quality) to be met by current (CT) and voltage (VT) transformers, manufactured in Ukraine is higher than CT and VT of Russian production.
However, if the transformers are manufactured in Russia are certified in Ukraine and are in СИТ register, they are considered to be compliant with the requirements of ДСТУ IEC 60044-1: 2008.

Marking of current transformers.

Current transformers, by installation way, are produced for indoor and outdoor electric installations, as well as built-in versions for power transformers and vacuum circuit breakers.
Current transformers are divided by installation way onto passing design, installed in the openings of walls, ceilings or switchgears metal fences, and supporting design, mounted on the supporting structures.

The current transformers are divided by design of primary winding:
- single-turn, rod-type with primary winding as linear rod with contact clips on the ends;
- single-turn, busbar-type, where busbars of electric installations are primary windings, passing while mounting through internal opening of the current transformers;
- multi-turn, loop-type, chain-type and coil-type.
Each type of current transformers have its literal & digital nominal designations:
Т — current transformer;

П — passing (absence of П letter means the transformer is supportive type);
В — embedded to a vacuum switch;
ВТ — embedded to a power transformer;
О — single-turn;
Л — with cast insulation;
Ш — busbar type;
М — compact (for current transformers for internal installation);
К — coil-type;
Ф — with porcelian insulation; ,
3 — for protection of short-circuit grounding;
У — enforced type (with increased electrodynamic resistance);
ФЗ — porcelian housing with link type primary winding;
Н — for outdoor installation;
Р — with core for relay protection;
Д — with secondary winding for feedinf the differential protection;
М — oil-immersed (current transformers for outdoor installation).
the first number after literal designation — the transformer`s rated voltage in kV;
the next group of numbers «through slash» — cores accuracy class (instead numbers, the letters Р or Д may be used);
two numbers «through slash» — primary and secondary currents;
the digit after the rated current — the design version;
the letter after the design version — climatic performance;
the last digit — accomodation category.