Energy consumption is monitored using an electric meter. The connection of current transformers in the network is used to measure high-power alternating current, reducing its strength to 5 A. The meter is connected to the power supply system, while three-phase and single-phase devices are used, which are connected in different ways.
Content
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Instrument transformers
- Three-phase device
- Work in 380 V networks
- Installation of a single-phase device
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Installing a multi-turn meter
- Ten-wire circuit
- Other connection systems
Instrument transformers
Current transformers (instrument transformers) convert high initial load currents to safe values for measurement in the secondary coil. They operate at a frequency of 50 Hz and a rated current of 5 A. A transformer with a conversion factor of 100/5 can withstand a maximum current of 100 A, and a measuring one - 5 A. The meter readings with such a transformer are multiplied by 20.
The current with the initial values flows through the series-connected primary winding of the transformer. The directional movement of electrons in the secondary circuit with a connected coil is created thanks to an electromagnetic induction, as a result of which the current values become several times lower than the initial ones, which is fixed electric meter.
Three-phase device
It is difficult to make a three-phase meter for a loaded network with a current of more than 100 A, since a large cross-section of the primary winding is required. Single-turn and multi-turn transformers have been developed, which are installed in front of the coils. Such a circuit structure eliminates the need for the production of powerful measuring devices, protects the device from short circuits and overloads.
There are two types of three-phase meters available:
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Devices for indirect connection of a three-phase meter through current transformers - this type is built into the circuit using a flow converter. They are more often installed in production facilities in order to control the energy of high-voltage lines. - Direct connection - the appliance directly checks the electricity consumption. It operates at an allowable power up to 60 kW, while the maximum current value is up to 100 A. The device is used to connect wires with a cross section of 1 mm².
The magnetic drive is made of split or solid type with a bar or branched winding. Multiphase devices contain a link or loop sheathing in the structure. The choice of the measuring device depends on the rated current and voltage at the beginning and when passing through the secondary winding. The device consists of a core, a group of primary turns and a secondary coil with a high wire speed.
Work in 380 V networks
In three-phase networks with a power of more than 60 kW and a current of more than 100 A, when taking into account the energy consumption, an indirect connection scheme is used.
After reading the reading, a conversion factor is applied.
This connection method also has disadvantages. If the user is consuming a small amount of energy, then the measurement current may be lower than the value required to start the meter. The device is inoperative - this effect occurs when installing induction meters of the old model, which are distinguished by high energy consumption for their own needs. In the models of recent years, this drawback is almost always absent.
Features of inclusion in the 380 V circuit:
- When connecting transformers, the polarity at the input and output is observed, using the cable selected by the load calculation.
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A wire with a cross section of more than 2.5 mm² is used for the secondary circuits. - Connections to the terminals are made with conductors marked with the corresponding color.
- The connection of the subsequent circuit is used by means of an intermediate terminal block with a seal.
- Installation of three-phase transformers with different conversion factors on one meter is not allowed.
The meter is replaced without stopping the power supply to users and removing the voltage. The technical inspection takes place in a safe mode, the verification of the errors of the metering devices is also carried out on the operating line without shutting down.
Before choosing a circuit for connecting a current transformer to a meter, check the suitability of the device for this use case. If the device is designed for direct installation, then it is prohibited to use it in conjunction with a transformer. The operating conditions of the meter are described in the technical passport, which contains the recommended connection diagram.
Installation of a single-phase device
The power conductor acts as a primary winding in single-phase transformers, the nominal current value reaches 100 or more amperes. A current not exceeding 5 A passes through the secondary winding. A single-phase device is installed in the gap between the supply line.
Consumers should not be connected to the line in front of the installed meter.
An automatic machine is mounted in the supply circuit - it is necessary when replacing the device when the mains need to be de-energized. The same circuit breaker is installed after the measuring group of devices in order to turn off the consumer in case of malfunctions on its side. On the back of the device, there is a diagram of the installation of an electric meter.
The device has 4 clamps for contact. Phase and zero are connected according to the scheme:
- clamp 1 is attached to the phase terminal;
- terminal 2 is connected to the outgoing phase contact;
- clamp 3 is connected to the supply zero cable;
- terminal 4 is connected to the outlet zero.
Such a scheme is used when connecting a measuring device in a private building, an apartment in a multi-storey building or a small pavilion for trade.
Installing a multi-turn meter
Three-phase electricity meters are installed in multi-wire networks using a current converter. Such measuring devices are called transformer meters because they work together. Converters are used for indirect connection to a network with high power, the primary winding is replaced with an electric wire. The device controls the charge of electricity when electrons move along the secondary winding, which occurs on the basis of electromagnetic induction.
Ten-wire circuit
A ten-wire switching system is used with isolation of powerful power circuits, which is provided by transformers. Galvanic isolation is used in industrial or domestic conditions and guarantees the safety of equipment operation. The disadvantage of this scheme is the large number of cables.
The connection diagram occurs in the following sequence:
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terminal 1 - to the phase input (A); - terminal 2 - to the input of the measuring circuit of the phase device (A);
- contact 3 - to the output of the measuring drive (A);
- terminal 4 - phase input terminal (V)
- terminal 5 - input of the measuring circuit of the phase (V);
- pin 6 - phase output (B);
- terminal 7 - phase input (C);
- pin 8 - input of the phase measurement loop.
Transformers are connected using contacts L1 and L2 to the line break. If single-turn devices work only with a voltage of 220 V, then multi-rod devices measure the power consumption in circuits with a power of 380 V.
Advantages of a three-phase meter over a single-phase device:
- allows you to save electricity at night (about 50%);
- measurement error is 2-2.5%;
- automatic saving and analysis of indicators in the event log is provided;
- You can export readings over the network over a distance, for this there is a built-in electric power modem.
Of the shortcomings, the requirement of experience in installing electrical equipment and the large dimensions of the device are noted.
The difficulty arises in the fact that when connecting a single-phase metering device, there is one fundamental system. And before connecting the current transformer to a three-phase meter, you need to choose one version among similar schemes.
Other connection systems
A simplified diagram is a star connection. This makes it easier to install the meter as fewer external wires are used, which is due to the complex design of the internal connection system. In the structure of the transformer there is a magnetic circuit containing 3 rods. Each of them has a primary and secondary winding. The primary is at high voltage, while the secondary circuit supplies low voltage electricity to the users.
The ends of each winding are joined together and the consumer phases emerge from the start of the turns. A neutral (zero) is derived from the connection of the ends, which is used as a load equalizer to eliminate power surges in the network. A three-phase 4-wire circuit is formed, often used for overhead electric lines.
It turns out two types of voltage: linear and phase. The voltage of the first type exceeds the phase voltage by √3 times, therefore, multiplying 220 V by √3, we obtain the linear power of the network 380 V. The phase current is equal to the linear current, they pass the windings in the same way.
When connecting the windings according to the delta method, the phases are combined according to the scheme:
- end A - with beginning B;
- end B - with beginning C;
- end C - with beginning A.
The windings are connected in series. This method is used for lines with balanced load, where there is no change depending on the phase. The phase and line voltage are the same in value, and the line current exceeds the phase voltage √3 times.
To select the connection scheme, it is taken into account that when connected according to the star type, two types of voltage are created, and the triangle scheme allows only 380 V.
The outputs of the secondary circuits are connected, and grounding is arranged, but in the accompanying documents for the meter (passports) such a requirement is not always voiced. If the commission for the acceptance of the equipment into operation insists on removing the grounding cable, then the loop will have to be removed. Installation of electrical equipment is carried out only in accordance with the developed project for the production of works.
Combined circuits in highways are rarely used due to large measurement errors and difficulties in identifying breakdowns on the windings of a three-phase transformer. In circuits with an insulated neutral wire, a circuit with the connection of two measuring transformers (incomplete star type) is used. Such an inclusion reacts sensitively to a break in a phase cable.
Secondary circuits are always loaded, their operation takes place in a mode close in characteristics to a short circuit. A break in the secondary circuit causes a loss of the compensating effect of electromagnetic induction from the current passing through the secondary turns. Such malfunctions lead to overheating of the magnetic circuit.
The three-phase transformer is selected taking into account the conversion factor according to the rules of PUE1.5.17. It is determined that the current in the secondary circuit when connecting the maximum load should not fall below 40% of the rated current of the measuring device. The correct distribution of contacts and the alternation of phase terminals A, B and C are controlled by a phase meter.
