Technical solutions of modern homes abound with devices that create a load on the network. Electric hobs, ovens, boilers and boilers lead in consumption. Demands of modern induction cookers reach 11000 VA, and accounting equipment does not connect directly at 100+ A. An alternative choice is to use current transformers (CT) for electric meters.
TT device
Transformers convert the measured value from a larger to a smaller or vice versa. They act with the help of electromagnetic induction. At the heart of the device is a magnetic core assembled from rectangular steel frames, and on it are fixed turns of insulated wires - windings. The input coil is connected to the source and the CT is represented by only one turn. Depending on the transformer model, the primary winding may be occupied by:
- core winding;
- a fixed bus with a connecting screw that passes through the housing;
- a step or rectangular hole to let the tire through and fasten during installation;
- round window under the cable core for contactless connections (household relays with built-in transformers).
The difference between measuring transformers and power transformers is that the secondary circuit current remains constant regardless of the consumer's resistance - the voltage changes. With a current transformer connected to the network, the secondary winding cannot be opened. It should always be shorted to the measuring device, if not, short-circuit with jumpers. If the produced current disappears, the voltage reaches kilovolts. A jump will provoke a failure of the equipment (semiconductor devices are especially sensitive), insulation damage and fire, a short circuit, and injury to personnel. For safety reasons, grounding of each winding at one point is mandatory.
Key parameters of measuring transformers
The rated voltage defines the circuits in which the transformer can operate. There are two large groups: up to 1kV and higher. Converters of 0.66 kV class are common.
Transformation coefficient is the ratio of the rated primary and secondary currents. At the input, the values vary depending on the parameters of the mains: 1, 2, 5, 10, 15, 20, 30, 40, 50, 75, 80, 100, 150, 200, 300, 400, 500, 600, 750, 800 , 1000, 1200, 1500, 1600, 2000, 3000, 4000, 5000, 6000. At the output, it is unified under the scale of measuring instruments 1, 2, 5. The marking with the designation looks like a fraction (50/5, 100/5, 200 / 5, etc.).
The accuracy class indicates the maximum permissible error in accounting for energy in percent. The most accurate instruments are used for commercial purposes:
- 0.2s;
- 0.5s.
The s symbol indicates that accounting is possible within the minimum division. For other models, this is a blind spot.
In measuring chains of different directions:
- 0,1;
- 0,2;
- 0,5;
- 1;
- 3.
Relay protection: 10P.
If the number of windings is more than one, for each accuracy class is determined separately. Up to 1000 V it is customary to connect simple CTs in series, and above 1000 V it is unprofitable, therefore one converter with several windings is installed. For example, the first may be on the protection circuit - 10P, the second 0.5, the third - 0.5s.
If the rated load power specified in the characteristics of the transformer (5 VA, 10 VA, 15 VA, 30 VA, etc.) is not observed, the accuracy class drops relative to the declared one.
Accounting node equipment
For an account cabinet of a unit over 100 A, a minimum set of equipment is defined.
Introductory circuit breakerthrough which the power line enters the internal network. From its lower part to transformers, access for unskilled personnel is closed by standards. A simple protection option is represented by plexiglass fixed with sealed studs.
Current transformers. The transformation ratio depends on the power that is allocated to the network user. The calculation is made by employees of Energosbyt and provide technical specifications (technical conditions).
A single-phase meter does not involve the use of converters. In three-phase networks, the load distribution can be uneven, therefore, accounting is carried out for each phase separately. It is necessary to choose all 3 CTs from one manufacturer, with the same set of properties.
Technical passports must be kept before registering the site. The inspector will not accept the transformer, after the release of which more than a year has passed. For a seal on the device’s case there is a special plug with a screw. Under it can be a second pair of terminals for grounding and a mount for the mains voltage.
Measuring terminal block KKI (test panel) consists of 2 sectors. The current has 7 pairs of terminals. 1 - grounding. To the other 6, wires from the secondary windings of the CT are suitable. Between them, you can install pairwise jumpers to close the network before disconnecting the accounting device. The phase A, B, C cables and the neutral conductor N enter the voltage sector. Slider jumpers allow you to open the circuit with a screwdriver.
Counters can be electromechanical (disk), electronic (with LCD display, remote control), combined. Energosbyt prescribes the requirements for the device in TU individually. The connection diagram for each model is located on the cover or in the attached passport.
The universal meter has 10 terminals, grouped in 3 for each phase, the last one is zero. The first, third terminal is the output from the secondary winding of the transformer I1, I2; the second is a phase wire.
Manufacturers produce similar direct counters and there is no connection. When selecting, you need to carefully study the labeling. On the phase meter, instead of the maximum permissible current value, the transformation coefficient is indicated (for example: 5 (7.5), 3X150 / 5 A)
Wires use rigid cross-sections of 2.5+ mm2, forming rings for connection. Soft with insulated tips are possible. In the meter, the core is clamped with two screws.
Cartridge with an electric lamp through a key switch from condensate in outdoor switchboards.
Boxing with windows under the scoreboard of the accounting device and levers of automatic machines.
The complete set is supplemented by protective automation in accordance with the power grid project.
To select a transformer for a three-phase meter, you should draw up the desired plan for the electrical wiring, approve it with the regional representative of Energosbyt and get the technical conditions. Choose a model strictly according to the characteristics indicated in the document.
