Measuring current transformer
Measuring current transformers TOP-N-0,66 and TSP-N-0,66 are designed for measurement information signal transmission to instruments in 50-60 Hz AC units with nominal voltage up to 0.66 kV inclusive. Transformers 0,2S and 0,5S accuracy class are applied in schemes of commercial accounting of electric power for settlements with consumers, and in measurement and protection schemes.
Measuring current transformer (CÒ) is a transformer designed for transformation of current to values suitable for measurement. Primary winding of current transformers is put in series into circuit with alternating current to be measured, measuring instruments are brought into the secondary winding. Current in the secondary winding of a current transformer is proportional to the current in its primary winding.
Current transformers are widely used for electric current measuring and in protective relay devices for electrical power systems. Therefore, requirements as to the accuracy of such transformers are very high. Current transformers ensure measurement safety by isolating measuring circuits from the primary circuit with high voltage often up to hundreds of kilovolt.
Measuring current transformer
for through-type current transformers
TPP-N-0.66 and TPP-0,66 accuracy class 0,2S
|N||Type of transformer||Unit price,|
|1||Transformer TPP-N-0,66-0,2S-1000/5-3-N||given after request|
for through-type current transformers
TPP-N-0.66 and TPP-0,66 accuracy class 0,5S
|N||Type of transformer||Unit price,|
|1||Transformer TPP-N-0,66-0,5S-150/5-1-N||given after request|
- Prices are given without VAT, including metrological verification.
- Current transformers TPP-N-0,66, TPP-0,66 meet the requirements of technical regulations of the Customs Union "On safety of low voltage equipment" CU TR 004/2011 (GOST 12.2.007.0-75).
- Two variants of fastening are possible:
- with usage of clamping screws;
- with usage of plastic clamp;
To buy measuring transformer send us a request in a convenient form.
►measuring current transformer application
Requirements to transformers’ accuracy are very high. Current transformers are often built with two or more secondary windings: one is used for protective devices connection, the other which is more accurate – for accounting and measuring devices (e.g. electricity meters).
Loading of current transformers secondary windings is obligatory. In case secondary windings are not loaded voltage high enough for transformer insulation failure is generated on them, which leads to transformer breakdown and endangers lives of operating personnel. Moreover, due to increasing losses in the core transformer magnetic core starts overheating, which can also lead to damage (or at least to insulation wear and its subsequent failure). By these reasons CT secondary winding should not be kept open during transformer operation.
Normal CT operating mode is short-circuit duty of its secondary circuit (e.g. for a CT with nominal power of secondary load S2n=5 VÀ and nominal secondary current I2n=5À, maximal external load in the secondary circuit should not exceed the nominal load: Z2max < Z2n = S2n/I2n2 = 5/52 = 0.2 Ohm). Maximal secondary circuit load Z2max equals the sum of resistances of wires Z2wires (in mode short-circuit wires resistance should not be neglected) and resistance Z2i of series circuits connected to instrument CT: Z2max= Z2wires+Z2i. In this mode induced current I2 flows in CT secondary circuit. The magnetomotive force of this current generates secondary magnetic induction flux F2 in the magnetic circuit directed according to the law of electromagnetic induction counter the magnetic induction flux F1 generated by the magnetomotive force of the primary circuit current I1. As a result at steady-state conditions relatively low total nominal magnetic induction flux F0=F1-F2 (2-3% of F1) is established in the core. This flux induces certain EMF (not over 1 V) in the secondary winding, which maintains the current in the secondary circuit at the range (0-100)% of the nominal current 12n in proportion to the value of the primary circuit current I1= (1-100)% I1n. Primary circuit current does not depend on the secondary circuit load and can range from zero to nominal, and in case of short circuit in the primary circuit (Z1=0) it can transcend the nominal current tenfold. In such a case secondary circuits and their loads safety is ensured due to CT core saturation, at that allowable overload is determined by CT safety coefficient, which is normally 3…5.
Scheme of interconnection of the primary and secondary CT circuits (W1<<), where W is the number of windings)
If CT secondary circuit is disconnected (emergency mode) disappearance of the secondary current I2 and of the magnetic flux F2 generated by it will lead to significant increase of magnetic flux F0=F1 from magnetomotive force of the primary circuit current, and, respectively, to the increase of EMF in the secondary winding (up to several kilovolt), which can cause insulation failure and shock hazard. Moreover, at high magnetic flux that significantly differs from the nominal one core losses increase dramatically and the transformer starts vibrating (humming) and heating, which in particular is one of the reasons of preliminary wear of its magnetic core. Therefore, disconnection of CT secondary circuit in presence of load at the user (Z1<>0) should be excluded, and when it is necessary to replace the meter connected to CT, CT secondary winding should be short-circuited (state-of-the-art CT contain coupled terminals for this purpose).
It follows by the theory of CT operation that its errors (current error or actual transformation ratio error, or phase displacement – phase difference between primary and secondary circuit currents) are determined by two factors: limited magnetic permeability ? of magnetic core and finite nonzero value of the secondary load. If magnetic permeability ? of the core were infinite (which would mean that its magnetic resistance equals zero), or the secondary load were zero (short-circuit conditions), then errors would be zero. In practice both the conditions are not met.
At that CT errors are the least the less is the magnetic resistance of the magnetic core, that is the more is the magnetic permeability of the material, the larger is the cross-section of the core, and the less is its length, as well as the less is its secondary load. It is important to take into account that magnetic permeability ? of ferromagnetic material depends on magnetic field intensity (depending on its size it is possible to speak of weak, medium and strong fields), and such dependence curve is bell-shaped: with a small ?n value in weak fields, ?max – in medium fields, and again minimal – in strong fields. Since CTs operate in steady-state conditions in weak fields it is important to use material not only with considerable maximal magnetic permeability, but also with high initial magnetic permeability.
Nanocrystalline alloys fully ensure these characteristics. They are the high initial magnetic permeability, linearity of magnetization properties and narrow hysteresis loop that explain the resistance of metrological characteristics of CTs with magnetic cores of nanocrystalline alloys to magnetization with direct current: complete magnetization reversal of the core at supply of alternate current occurs even at weak field intensity and at values of initial current of 1-2% I1n. With reference to cores of electric steel this can hardly be achieved even by means of increasing magnetic core cross-section. In general nanocrystalline cores are characterized by lower materials consumption, lower weight and smaller size in comparison with electric steel cores for analogous CTs.
►main advantages of current transformers with amorphous nanocrystalline allow cores
With transition of commercial electricity accounting to using electronic counters requirements to nominal load of CTs are reduced: it can be restricted to the value of 5 VA (for CTs for accounting with induction counters it was 10-20 VA and more), which eventually proportionally reduces technical energy losses to instrument accounting. This is of special importance since CT efficiency (relationship of the active takeoff power from the secondary winding of a transformer, to the active input power to the primary winding), compared with voltage transformers efficiency is low due to copper losses in the magnetic core: the efficiency does not even reach 50% at nominal currents. It is easy to calculate that if a power system has 100 thousand pieces of CTs, then saving of 10W of power on each of them will yield total saving of 1 MW, and annual energy saving will be 8,760 MW-h or about 440 thousand Dollars (at 0.05 Dollar per 1 kWh).
In case operating conditions require that counters be located at a distance from CT (e.g. 25 meters and more) it is necessary to use CT with higher nominal load power, or at the same power with nominal current of 1A (at that the secondary circuit allowable external resistance is increased 25-fold). In the latter case it is required to apply counters for nominal power of 1A, not 5A respectively.
High magnetic properties of nanocrystalline alloy cores make transformers built with them sensitive in terms of metrological characteristics to load increase (load resistance increase) in the CT secondary circuit in excess of the nominal value at maximal initial current, which in practice necessitates strict fulfillment of all the above-stated overload requirements. Overload capacity of such CTs may be increased due to core power rising, which is not always economically feasible for the manufacturer as nanocrystalline alloy cores are 1.5-2 times more expensive than those of electric steel.
Measuring transformers with nanocrystalline cores have the following advantages compared with CTs with electric steel cores:
1) resistance of metrological properties to magnetization with direct current;
2) high electrical resistance of material and reduction to 4-10 times of eddy current losses and core magnetization reversal;
3) increased (double) accuracy class margin;
4) longer service life with retention of metrological properties (and, thus, a potentially larger calibration interval);
5) lower material consumption for the core and windings, smaller sizes, lower core weight and CT weight in general;
6) better resistance to energy stealage (with user load less than 50% of the nominal) and to increase in commercial losses with reduction of technological energy losses and operating costs.
►prospects and economic efficiency of amorphous iron transformers application
In November 2008 in Minsk first session of the “Energy Club” was held. The session addressed the concept of energy security of the Republic of Belarus till 2020. 4 new lines for its strengthening were distinguished:
1) Energy independence – maximal abandonment of electric power import;
2) Diversification of supplies by resource types and by countries;
3) Reliability of energy supply (fuel safety-stock investment);
4) Energy efficiency (improvement of energy saving promotion mechanisms, reduction of energy content of GDP, improvement of efficiency in heat and power resources use based on latest research results, increase in expenditures on energy saving).
In conditions of energy market formation there is a necessity to improve commercial energy accounting. Commercial energy accounting with the use of measuring CTs in 0.4 kV distribution nets for energy supply organizations and consumers at present requires radical modernization and includes replacement of measuring CTs of 0.5 class for CTs of 0.5s class (Instructive letter by Belenergo Concern No 09/171 dd. 19.02.2002). The mentioned current transformers according to interstate standard GOST 7746-2001 ensure the allowable tolerances margins with a larger range of primary current measurement: their current error is 0.5% for 20-120% I1nom, 0.75% for 5-20% I1no and 1.5% for 1-5% I1nom. This allows at load drop and variation to reduce the share of commercial losses that is determined by instrument accounting failures. Energy measuring and accounting devices market in the republic presents various models of ICTs of the mentioned class that are included in the State Instrument Register of the Republic of Belarus. These products are to a large extent similar in terms of their declared technical characteristics, but in fact, according to tests and operation practice they are not of equal worth in the long-term perspective of their application. On instructions by Belenergo Concern (Order No 112 dd. 29.05.2003) Republican Unitary Enterprise “BelTEI” together with the accredited test center at the Republican Unitary Enterprise for Dispatch and Process Supervision “Grodnoenergo” have tested a range of ICTs from national and foreign manufacturers. 25 specimens of ICTs manufactured in Russia, Belarus, Lithuania and the Ukraine were tested.
Based on the results of the tests and their analysis undeniable advantages of ICTs on nanocrystalline alloy cores versus ICTs on electric steel cores have been proved.
à) stability of metrological characteristics to magnetization with direct current;
á) high electrical resistance of material and reduced to 4-10 times losses to eddy currents and core magnetization reversal;
â) double accuracy margin;
ã) long service life with retention of metrological characteristics (thus, a potentially larger calibration interval);
ä) smaller dimensions and weight of the core.
Taking into account the conducted tests and based on the experience of measuring current transformers operation in 0.4 kV distribution nets, Belenergo Concern specifies technical requirements to measuring current transformers in addition to GOST 7746 – 2001 requirements.
►technical requirements specified by Belenergo Concern to measuring current transformers in addition to GOST 7746 – 2001 requirements
1. Measuring transformers of low-voltage system current designed for commercial energy accounting should be of accuracy class 0,5 S at the least and should have nominal secondary load not over 5 VA.
2. All ICTs should be included in the State Instrument Register of the Republic of Belarus and pass technical examination at Belenergo Concern.
3. All ICTs should be metrologically calibrated, sealed and have a corresponding note in their certificates.
4. Requirements to current transformers design
4.1 ICT body parts should have a fire resistant design of at least PV-0 category.
4.2 ICTs should have primary buses for connection to copper and aluminum buses and wires (bus length should correspond to the order sheet).
4.3 ICTs should have fastenings for bus connection and transformer attachment to electricity-generating equipment including Din Rail (as per order sheet).
4.4 ICTs should allow to change bus alignment for transformers with primary nominal current (200-600) À.
4.5 Connection dimensions of transformers should correspond to previously used dimensions (as per order sheet).
4.6 Data plate (material and lettering) should guarantee preservation of the information during the whole service life (25 years).
5. Current transformers protection from energy stealage.
5.1 Eliminate the possibility of data plate replacement and disassembly of current transformers without their body, protective parts, and seals damage.
5.2 After mounting and sealing of current transformers access to the secondary winding leads should be eliminated.
5.3 There should be a possibility for sealing each current transformer with two independent seals (a seal of a metrological service and that of an energy supply organization).
5.4 Current transformer should have a sealed voltage circuit contact having a permanent connection with the primary bus.
5.5 Transformation coefficient should be indelibly marked on the transformer body.
On the basis of the above-stated Belenergo Concern obliged:
1. RUE – Oblenergo and JSC "Belenergosnabkomplekt":
1.1 to select only state-of-the-art ICTs corresponding to the present technical requirements in bidding for purchasing measuring current transformers of 0.5S class for distribution nets of 0.4 kV voltage.
1.2 to ensure purchasing of test batches of transformers on nanocrystalline alloys cores with the amount of at least 10% of the total purchased quantity with the purpose of accumulation of experience on such transformers operation.
2. RUE – Oblenergo: to ensure that the given technical requirements are taken into account at approval of design decisions on organization of customer energy accounting.
3. The present Directive should be brought to knowledge and execution of the employees of RUE-oblenergo departments.
4. Control of the execution of the present Directive shall be imposed on Energy Distribution Department at Belenergo Concern, at the local level – on Chief engineers at RUE – oblenergo.
Thus, currently the integrated power system of the Republic of Belarus within the framework of modernization makes provisions for replacement of class 0.5 CTs for class 0.5S CTs, as well as, in accordance with scientific and technical progress, replacement of low-voltage CTs on electric steel cores with CTs on nanocrystalline alloy cores. Moreover, since at the present time in the Republic of Belarus there are domestic manufacturers of three-phase electricity meters of transformer connection with accuracy class 0.2S Gran-Electro ÑÑ-301(Ê) (manufacturer R&D LLC "Gran-System-S", Minsk, Republic of Belarus) and measuring current transformers TOP-N-0.66 and TSP-N-0.66 on nanocrystalline alloy cores with accuracy class 0.2s (manufacturer LLC "Yudzhen" , Novopolotsk, Republic of Belarus), replacement of CTs of accuracy class 0.5 and 0.5S with CTs of class 0.2S and appearance of more efficient and accurate Automatic Systems for Electric Power Control and Accounting on their basis is around the corner.