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Low-Voltage Measuring Current Transformers

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Catalogue of low-voltage measuring current transformers VIKODEK

Current transformers catalog VIKODEK

Current transformers in our life: on the shopfloor and at home

In the present state of humankind development there is a big and growing reliance on electricity in industry, transportation as well as in household use. In the same manner of increase financial expenditures on energy products are growing as well. Looming electric energy efficiency problem can be resolved in two different ways:


Expansion in the number of electricity by building new power generating complexes of new large-size power stations (thermal, hydroelectric, nuclear, solar). It is obvious that this is costly approach and it cannot be done within the scope of one or even several enterprises. To surplus this method leads to aggravating of ecological problems.


Optimization and enhancement of electricity usage without further harnessing of planet nonrenewable resources. That is to say, the priority in our efforts has to be put on raise of energy effectiveness rather than increasing the power stations' capacities. On the top of everything else this approach does not carry the potential to harm ecology, what is currently important nowadays.


It is well known that in order to be effectively transmitted from generating capacities electric energy have to pass the whole range of power lines before it can be safely used by enterprise or household and it pass several layers of high-voltage and low-voltage current transformers (for illustration please refer to fig.1).

 Fig. 1. Electrotransmission schematics “Power plant - to - consumer” 

Current transformers: measuring (metering, instrument) CTs from EU

Electric energy is the most utilizable resource in current industry. Without electricity can not work even the smallest enterprise.


Nowadays electric power is primary produced by big power plants such as nuclear facilities, thermal and hydroelectric power stations. All these enterprises produce millions of kilowatts of energy per day. Transmission of this energy to the end user require high-voltage power lines. Electric machinery of any enterprise does not require and in most cases succumb under electricity of high voltage. This is why any enterprise install relay protection systems intended for constant control of operational conditions of the equipment, metering as well as equipment protection on an emergency basis. An integral part of any relay protection system and correspondingly an important part of modern electrification system are measuring current transformers (or just simply current transformers).

Main principle of work of current transformer is transformation (pulldown) of current to the value under which it is possible metering.

Current transformers are grounded and connected to low-voltage winding that is why they also provide isolation of relay metering equipment from high-tension circuit, securing work of people. Due to this current transformers are used even in cases when there is no necessity of voltage reduction.

Low-voltage current transformers together with high-voltage current transformers are very important elements of short-circuit protection therefore they are an important part of hardware security of any enterprise.

Measuring current transformer (Metering CT, Instrument CT) is operationally necessary up-to-date element of energy consumption of any enterprise because it serves for efficiency upgrading in measuring of electric energy usage and as a result minimization of financial expenditures for production, distribution and consumption of the energy as well as for definition of capacity requirements aiming optimization and prognosis of utilization of electric energy.


Current transformers types: advantages of toroidal measuring CT

In the operation of power supply systems, it is often necessary to convert certain electrical quantities into similar analogs with proportionally scalable values.  It allows modeling of some processes in electricity-generating equipment and safely making current measurement. Such opportunity is provided by voltage and current transformers, that lower the index of  voltage or current strength,  which usually are so high, that the electric appliances and counters are simply destroyed.


Both types of current transformers work on the general physical principles of energy transformation but they have different design and ways of wiring connection. Main distinctive feature of construction of step-down current transformers from step-down voltage transformers is the larger number of turns of the secondary winding in comparison to the primary winding.


Such electric signal transducers are used for correct equipment operation in order to liquidate power spikes in electrical networks. Metering equipment ensure safety of not only the facility on which it is located but also people who works at or near the danger zone. Additionally current transformers allow standardize and systematize work of  electrical facilities within the company.


Schematically current and voltage transformers connection patterns to electric lines depicted below:


           current transformers connection patterns



Use of measuring current transformers

When using different electric power systems arise necessity in transformation of some electrical values into similar with proportionally scalable values.

Such operation allow reproducing a processes in electrical accessories, ensuring their safe consumption records. For successful realization of such a problem the most suitable equipment is measuring current transformer of outdoor installation.

When we need and have to use measuring current transformers (measuring CTs)?
Measuring (metering, instrument) CTs intended for measuring parameters limited by nominal current. Last value vary from 0.66 to 750 kV. Such current transformers are broadly used for different purposes:

• for separation low-voltage metering devices and relay from primary current in electrical grid, ensuring safety of electrical appliances during maintenance and diagnostics.
• by the instrumentality of current transformers a chain of relay protection get power. In case of short circuit or problems with electric equipment operation current transformer provide correct and timely activation of relay protection.
• CT are used for electric power metering by means of electrical energy meters.


In practice there are different models of measuring current transformers, both in compact electrical appliances with small device cases as well as in fully-featured power units with huge dimensions.


Calculation and selection of current transformer should be started from classification of present instruments at the marketplace. All current transformers first of all fall into two big categories depending on intended end-use:


• for measurement of electricity by electric counters;
• for electrical equipment overvoltage protection.


Those two categories of current transformers in their turn fall into subcategories, according to connection type:

• outdoor CTs - for open-air work;
• indoor CTs - for work in the building ;
• built-in CTs, that are used as built-in elements of electrical equipment;
• attachable CTs - for feedthrough insulator ;
• portable CTs - allow metering on the go.


What current transformers’ parameters say?

Below you can find main current transformers’ parameters that are essential for both high-voltage and low-voltage current transformers as well as voltage transformers.

1. Nominal primary current is the value of nominal current of primary winding of current transformer when given current transformer works in the best way. This is optimal value of primary current under which the transformer error is minimal and degree of efficiency is the highest, this determines the best productivity under condition of optimal heating of current transformer.


2. Nominal secondary current The same as nominal primary current is the value of current of secondary winding when transformer errors are minimal. In the other words nominal current of current transformers’ secondary winding is such a current value under which is obtained maximal productivity of the device.


3. Secondary load of current transformer is a total circuit resistance of its external secondary winding (amperemeters, windings of electric meters, current relays of protective relay, different current converters). Its value is calculated in Ohms (Ohm). It is also possible to express secondary load of current transformer in terms of full output that is measured in volt-amperes (VA) under certain power factor and nominal secondary current.


4. Nominal loading of current transformers. This parameter reflects maximum output value, connected to the secondary winding of current transformer. In the other words it is nominal power of the given device. Usually it is expressed in Ohms (Ohm) or as a product of current by voltage (V*A).


5. Nominal frequency of current transformer Is such value of system frequency under which measuring current transformer is working. Usually it is 50-60 Hz.


6. Accuracy class of current transformer. This parameter reflects the highest total acceptable error in percentage under nominal current. Standard accuracy classes are 0.1, 0.2, 0.5, 1, 3 and 5. For example, if the accuracy class or simply the class of measuring current transformers is 0.1, then it means that maximal acceptable error is 0.1%. That is to say if to measure 100A with current transformer with the accuracy class of 0.1, then measured value can be either 100.1 or 99.9 or any other value within this range. For specific goals are used current transformers with different accuracy classes: e.g. for commercial electric counters – 0.5; for electrical measuring instruments – 1; for current protection relay - 3; for laboratory instruments – 0.2.


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