How to choose a current transformer - by power

The total load current on the line of a residential, commercial facility or enterprise in some cases may exceed its actual capabilities. The correct calculation of the current transformer will help ensure the quality of the linear conversion, control and protection of the power grid.

Reasons for installing current transformers

Current transformer RTP-58

The device is designed to transform the primary current value to a safe one for the network. Transformers are also used for the purpose of:

  • differentiation of low-voltage accounting equipment and relays thrown into the secondary winding, if there is a primary high voltage in the network;
  • increase or decrease in voltage indicators;
  • measuring the state of the power grid and AC parameters;
  • ensuring the safety of repair and diagnostic work;
  • fast activation of relay protection in case of short circuits;
  • metering of energy costs - an electric meter is usually combined with them.

To measure, you will need to connect a CT in the wire break, and connect a voltmeter or ammeter combined with a resistor to the secondary mark.

Varieties of current transformers

It is necessary to choose a device suitable for the mains voltage or specific work on the basis of classification according to various criteria.

Appointment

There are such transformers:

  • measuring - measure the parameters of the circuit;
  • protective - prevent overloads, equipment failure;
  • intermediate - are connected to a circuit with relay protection, equalize currents in differential protection circuits;
  • laboratory - are highly accurate.

Laboratory models have more conversion factors.

Mounting type

For a private house and apartment, you can choose a device mounted inside or outside the room. Some modifications are built into the equipment, and also put on the bushing. Portable models are used for measurement and laboratory tests.

Primary winding design

There are bus, single-turn (with a rod) and multi-turn (with a coil, loop-type winding and "eight") devices.

Insulation type

There are the following converters:

  • dry insulation - based on cast epoxy, porcelain or bakelite;
  • oil-paper - standard or condenser;
  • gas-filled - inside there is inorganic SF6 gas with high breakdown voltage;
  • compound - inside there is a filling of thermoactive and thermoplastic resin.

The compound has the highest moisture resistance.

Depending on the number of transformation stages, one-stage and cascade models can be selected. The entire line has an operating voltage of over 1000 V.

Accuracy class

The accuracy class of the current transformer is prescribed in GOST 7746-2001 and depends on its purpose, as well as the parameters of the primary current and secondary load:

  • Under conditions of low resistance, almost complete shunting of the magnetized branch occurs. The device works with a large error.
  • As the resistance increases, the error also increases. The reason is the operation of the device in the saturation area.
  • At the minimum value of the primary current, the transformer works in the lower part of the magnetized curve, at the maximum - in the saturation section.

The exact selection of the transformer according to the accuracy class can be made on the basis of the table.

Accuracy classPrimary current rating in%Secondary load limit in%
0,15, 20, 100-20025-100
0,2
0.2 S1,5, 20, 100, 120
0,55, 20, 100, 120
0.5 S1, 5, 20, 100, 120
15, 20, 100-120
350-12050-100
5
10

For protection devices, the accuracy class is also determined from the table.

Accuracy classLimit errorPercentage of ultimate secondary load
heatcorner
minWed
5P±1±60±1,85
10P±3No norm10

For energy metering, models with an accuracy class of 0.2S - 0.5 are used, for ammeters with a minimum sensitivity - with 1 or 3, for relay protection - 5P and 10P.

Features of choice

In the process of choosing a current transformer, it is necessary to be guided by the basic parameters:

  • Mains voltage rating. The rating must be greater than or equal to the operating voltage.
  • Primary and secondary winding current. The first indicator depends on the transformation ratio, the second depends on which counter.
  • Conversion factor. It is selected according to the load in emergency cases, but the PUE establish the need to install devices with a coefficient greater than the nominal one.
  • Accuracy class. Depends on the intended use of the meter. In a commercial enterprise, 0.5S devices are justified, in a private house - 1S.

The design is determined by the type of meter. For models up to 18 kV, a single-phase or three-phase device is suitable. If the value is greater than 18 kV, a single phase transformer is used.

Selection of a current transformer for the organization of relay protection

The relay current transformer differs in accuracy class 10P and 5P. In the PUE it is established that its error should not be more than 10% in current and 7 degrees in angle. If the error is exceeded, additional equipment is installed.

Under normal conditions, the transformer relay detects the type of failure (low voltage, over / under current or frequency). After measuring the parameters and detecting deviations, protection is activated - the network is de-energized.

The nuances of choosing devices for the metering chain

For correct measurements, devices with an accuracy class of no more than 0.5 (S) can be connected to the metering circuit. In the presence of fluctuations and accidents, the graphs of the current and voltage flow are incorrect. Failure to comply with the accuracy class can lead to an overestimation of the meter readings.

In clause 1.5.17 of the PUE it is established that with an overestimated coefficient, the transformer for the metering circuit must have a secondary current:

  • at maximum load - no more than 40%;
  • at minimum load - no more than 5%;
  • accuracy class - from 25 to 100% of the nominal.

The CT power factor is from 1 to 5% of the primary.

Current transformer preselection table for power and current

It is advisable to make a tabular selection of equipment after specifying the technical parameters of the apparatus. If they are known, it is worth choosing a CT according to the table, where the power, load and transformation coefficient are indicated.

Maximum power when calculating, kVA380 V network
Load, ATransformation ratio, А
101620/5
152330/5
203030/5
253840/5
355350/5 or 75/5
406175/5
507775/5 or 100/5

For a network with a voltage of 1.5 kV, a similar table applies.

Maximum power when calculating, kVA1.5 kV network
Load, ATransformation ratio, А
100610/5
160910/5
1801010/5 or 15/5
2401315/5

With the tabular method, it must be borne in mind that the secondary current of the device should not exceed 110% of the nominal.

Reliability of measuring voltage transformers in a network with isolated neutral

A simple measuring device is designed to lower the voltage ratings, which is supplied to meters and protective relays connected to a 6-10 kV network. The transformer works properly only when the neutral is grounded.

In case of ferroresonant reactions (loss of the power line phase, touching branches, dripping dew drops along the wires, incorrect switching), there are risks of damage to voltage transformers. The failure rates are 17 and 25 Hz.Under these conditions, an overcurrent flows through the primary winding and it burns out.

If the Zvezda-Zvezda scheme is used, the induction of the magnetic circuit rises when the voltage rises. The device burns out. You can prevent this process with:

  • decrease in indicators of working induction;
  • connection in the network of devices damping resistance;
  • creation of a three-phase device with a common magnetic five-rod system;
  • operation of devices connected to the network when the triangle is opened;
  • neutral grounding by means of a current-limiting reactor.

The simplest option is to use special windings or relay circuits.

Calculation of the current transformer by power

The current transformer is placed on 3 wire cores, but models with an accuracy class of 0.5S, where one ring goes to one phase, can be connected to a single-core cable. Before installing the device, its calculation is performed.

Calculation example for 10 kV

10 kV models are suitable for custody transfer metering. For calculations, you can use an online calculator program. After entering data in the fields and pressing the calculation button, the necessary information will appear.

If there is no program, you can calculate the parameters of the device yourself. It will be necessary to convert a three-second thermal current into a one-second one. For this, the formula is used I3s = I1s / 1.732.

The complexity of using this device is the minimum, about 10 A, power current of the circuit.

Current transformers installed in production or in a residential apartment building are not independently calculated. You will need to contact the power supply company to obtain a technical specification with a model of the metering unit and the type of device, the nominal value of the machines. This eliminates the complexity of independent calculations.

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