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- The principle of operation of the induction meter.
- Wiring diagrams of electricity meters and their verification, a description of the schemes.
- Basic definitions (nominal voltage and current of the meter, meter sensitivity, gear ratio and meter constant, meter error)
The principle of operation of a single-phase induction counter of active energy.
The meter is a measuring power meter system and is an integrating (summing) electrical measuring device. The principle of operation of induction devices is based on the interaction of variable magnetic fluxes with the currents induced by them in the moving part of the device (in the disk). Electromechanical interaction forces cause the movement of the moving part. The schematic device of a single-phase meter is shown in fig.
Its main components are electromagnets 1 and 2, an aluminum disk 3 fixed on axis 4, axle bearings 5 and bearing 6, permanent magnet 7. Axis is connected by means of a gear 8 to a counting mechanism (not shown in the figure), 9 - counter pole of electromagnet 1. Electromagnet 1 contains a W - shaped magnetic circuit, on the middle core of which there is a multiwire winding made of thin wire connected to the mains voltage U parallel to the load N. This winding is called parallel winding or obm in accordance with the switching circuit voltage drop. At a nominal voltage of 220 V, the parallel winding usually has 8-12 thousand turns of a wire with a diameter of 0.1–0.15 mm. The electromagnet 2 is located under the magnetic system of the voltage circuit and contains a U-shaped magnetic conductor, with a winding located on it from a thick wire with a small the number of turns. This winding is connected in series with the load and is therefore called a series or current winding. A full load current flows through it. Usually the number of ampere turns of this winding is between 70 and 150, i.e. at a rated current of 5 A, the winding contains from 14 to 30 turns. A complex of parts consisting of serial and parallel windings with their magnetic cores is called a rotating element of the counter.
The current flowing through the voltage winding creates a common alternating flow of the voltage circuit, a small part of which (working flow) suppresses the aluminum disk located in the gap between both electromagnets. Most of the instantaneous flow of the voltage circuit is closed through shunts and side rods of the magnetic circuit (non-working flow), which is divided into two parts and is necessary to create the required phase angle between the magnetic fluxes of the voltage circuit and the load circuit (current circuit). The magnetic flux of the voltage circuit is directly proportional to the applied voltage (mains voltage).
The load current flowing through the current winding creates an alternating magnetic flux, which also crosses the aluminum disk and closes along the magnetic shunt of the upper magnetic core and partially through the side rods. A small part (non-working flow) is closed through the anti-pole on the intersecting disk. Since the magnetic core of the current winding has a U-shaped design, its magnetic flux intersects the disk twice.
Thus, all through the disk drive pass three variable magnetic flux. According to the law of electromagnetic induction, the alternating magnetic fluxes of both windings when crossing a disk induce an emf in it (each its own, that is, two), under the action of which the corresponding eddy currents flow around the traces of these currents (we remember the “drilled hole” rule). As a result of the interaction of the magnetic winding of the voltage and eddy current from the magnetic flux of the current winding and on the other side of the magnetic flux of the current winding and eddy current from the voltage winding, electromechanical forces arise that create a torque acting on the disk. This moment is proportional to the product of these magnetic fluxes and the sine of the phase angle between them.
The active power consumed by the load is defined as the product of the current and the applied voltage and the cosine of the angle between them. Since the instantaneous flows of both windings are proportional to voltage and current, it is possible to achieve in a constructive way the equality of the sine of the angle between the flows and the cosine of the angle between the current and voltage vector to achieve proportionality of the meter torque with the coefficient of the measured active power. The sine of one angle is equal to the cosine of the other angle if there is a 90 degree shift between them, which is achieved in the construction of meters (use of short-circuited turns, additional windings closed by adjustable resistance, movement of the screw terminal, etc.). A torque proportional to the power of the network drives the counter disk in rotation, the frequency of which is set when the torque is balanced by the braking torque. To create the braking moment in the meter, there is a permanent magnet that covers the disk with its poles. Crossing the disk, the magnetic field lines induce an additional emf in it proportional to the frequency of rotation of the disk. This EMF, in turn, causes the flow of the eddy current in the disk, the interaction of which with the flow of a permanent magnet leads to the appearance of an electromechanical force directed against the movement of the disk, i.e. leads to the creation of braking torque. Adjustment of the braking torque, and hence the frequency of rotation of the disk is produced by moving the permanent magnet in the radial direction. As the magnet approaches the center of the disk, the rotational speed decreases.
Thus, having achieved a constant counter disk rotation frequency, we obtain that the amount of energy measured by the counter is obtained from the product of the number of revolutions of the counter disk and C-coefficient. proportionality, constant counter.
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The principle of operation of a single-phase electronic active energy meter.
The counter is an analog-to-digital device with a preliminary conversion of power into an analog signal with the subsequent conversion of an analog signal into a pulse repetition frequency, the summation of which gives the amount of energy consumed.
Structurally, the meter consists of a case, a measuring current transformer and a converter made on the printed circuit board and a charging module. Structurally, the counter consists of the following nodes:
• LCD driver
• secondary power supply
• optical port
• telemetric output
• real time clock
The converter is an analog-to-digital device with preliminary conversion of power into an analog signal by the PWM-AIM method with subsequent conversion of the analog signal into a pulse signal proportional to the electricity consumed. The secondary power source converts the alternating input voltage to the value required to power all the nodes of the meter. The microcontroller counts the input pulses, calculates the energy consumed, controls and exchanges information with other nodes and meter circuits. The supervisor generates a reset signal when the power is turned on and off, and also generates a power failure signal when the input voltage drops. The memory stores data on consumed electricity and other parameters. Real-time clock is designed to count the current time and date. The LCD driver receives information from the microcontroller and issues control signals on the LCD. LCD is a multi-digit indicator and is designed to indicate operating modes, information about the electricity consumed and temporary parameters. The optical port is designed for reading and programming the counter. The microcontroller receives signals from the buttons on the counter panel and signals from the converter proportional to the consumption of electricity. The microcontroller stores information in memory and sends a pulse signal about the power consumption to the telemetric output.
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COUNTERS INCLUSION SCHEMES AND THEIR CHECK. DESCRIPTION OF CIRCUITS
A counter is a device that reacts not only to the value of energy, but also to the direction of its transmission. The property of the counter to react to the direction of energy makes it imperative to turn on the current circuit of the meter and the voltage circuit in a coordinated manner, so that with a positive energy direction the disk rotates in accordance with the arrow. Before considering specific schemes for the inclusion of counters, we list a few general provisions
The terminals of the current winding of the meter and the voltage winding connected from the side of the power source are conventionally called unipolar. In the diagrams, the unipolar conclusions of the windings of the meter (the beginning of the windings) are indicated by an asterisk. The unipolar clamp of the voltage circuit is always located next to the corresponding clamp of the current winding and is connected to the current clamp by a removable jumper for direct connection meters.
Earlier, when describing the meters, it was noted that the terminals of the current windings are denoted by the letters T (generator) and H (load). In this case, the generator terminal corresponds to the beginning of the winding, and the load terminal - to its end. When connecting the meter, it is necessary to ensure that the current through the current windings passes from their beginnings to the ends. For this, wires from the power supply side should be connected to the generator terminals (terminals D) of the windings, and the wires extending from the meter towards the load side should be connected to the load terminals (terminals H). For meters included with measuring transformers, the polarity of both the CT and the TH must be taken into account. This is especially important for three-phase meters with complex switching circuits, when incorrect polarity of measuring transformers is not always immediately detected on a running meter. If the meter is switched on through a CT, a wire from the current terminal of the CT, which is unipolar with the primary terminal, is connected to the beginning of the current winding winding connected to the power supply side. With this switch-on, the direction of the current in the current winding will be the same as with direct switching. For three-phase meters, the input terminals of voltage circuits, unipolar with the generator terminals of current windings, are designated by the numbers 1, 2, 3. This determines the specified phase sequence 1- 2-3 when connecting meters. It should be noted that when connecting the circuit of internal connections should not cause any doubts or ambiguities, since all the required internal connections are made in the manufacture of meters. It is important to monitor only the correctness of external connections. Fig. A.6.c shows typical schemes for switching on active and reactive energy meters both with their direct connection to the electrical network and with measuring transformers. Fig. A, b, c are schematic diagrams of switching on a single-phase active energy meter with an indication of the polarity of the measuring transformers. The secondary windings of the TT and TH for safety reasons are grounded. It makes no difference in principle that grounding is the beginning or the ends of the windings of measuring transformers.
Fig. Switching on single-phase active energy meter: a - with direct switching; b - with semi-indirect inclusion; - with indirect connection;
The schematic diagrams of switching on a three-phase three-wire two-element active-energy meter of the SAZ type (АSZU) are shown in fig. a B C. Here we especially note that the middle phase is necessarily connected to the terminal with the number 2, i.e. that phase, the current of which is not supplied to the meter. When the meter is turned on with a TH, the terminal of this phase is grounded. In the diagram of Fig. In T1, the terminals on the power supply side (i.e., clamps .and 1) are grounded, but the clamps could also be grounded on the load side. Counters like SAZ are mainly used with measuring transformers, and therefore the diagram shown in fig. in is the main when accounting for active energy in electrical networks of 6 kV and above.
Fig. The inclusion of a three-phase three-wire two-element active energy meter type SAZ (SAZU):
and - at direct inclusion;
b - with semi-slave inclusion;
in - with indirect inclusion
Circuit diagrams of switching on a three-phase three-element active energy meter of the type CA4 (СА4У) are shown in Fig. E, while in Fig. a, b, c are three-wire connection schemes, and in fig. g, d-four-wire counter.
Fig. Three-phase three-element CA4 (SA4U) active energy meter switching schemes:
and - at semi-indirect inclusion in a three-wire network;
b - with indirect connection to a three-wire network;
c - with direct connection to the four-wire network;
d - at semi-indirect inclusion in a four-wire network
In fig. shows a circuit with three single-phase voltage transformers, the primary and secondary windings of which are connected in a star. In this case, the common point of the secondary windings is grounded for safety purposes. The same applies to the secondary windings of the CT. In fig. c, d it is necessary to pay attention to the presence of the obligatory connection of the neutral conductor of the network with the zero terminal (0) of the counter. It was noted above that the absence of such a connection may cause an additional error when energy is taken into account in networks with asymmetry of voltages. Diagrams of the inclusion of reactive energy meters with a 90th shift of type СР4 (СР4У) into a four-wire network are shown in fig. a B C. The order of supplying voltages and currents to the meter is the same as that of the active energy meter. Diagram of indirect inclusion of the same meter in a three-wire network is shown in Fig. d. Since there is no TT in the middle phase of the network, instead of the current Ib, the geometrical sum of the currents Ia + Ic is summed to the current winding of the second element of the counter, which, as is known, is equal to -Ib.
Fig. Three-element reactive energy meter switching circuit with a 90 ° th shift of type СР4 (СР4У):
a - with direct connection to the four-wire network;
b - at semi-indirect inclusion in a four-wire network;
in - at indirect inclusion in a four-wire network;
d - with indirect connection to the three-wire network
In fig. a diagram of a semi-indirect connection of a two-element reactive energy meter with separated series windings of the type CP4 (CP4U) into a four-wire network is presented.
In three-wire networks, where there are only two CTs, this counter can be switched on according to a circuit using the geometric sum of the currents of the two phases, similar to the circuit in fig. In fig. There are diagrams of switching on a reactive energy meter like SRZ (SRZU) with a 60 ° th shift into a three-wire network.
Fig. Scheme of semi-indirect connection of a two-element reactive energy meter with separated consecutive windings of СР4 (СР4У) to a four-wire network
Fig. The scheme of inclusion of a two-element reactive energy meter of the type SRZ (SRZU) with a 60-m shift to a three-wire network:
and - at direct inclusion;
b — when semi-activated;
in - with indirect inclusion
Due to the fact that the counters of active and reactive energy are usually used together, in Fig. As an example, the schemes of their joint inclusion are given. In fig. diagrams of semi-transparent switching of meters into a four-wire network (380/220 V) are given. The diagram in Fig. Requires the installation of a smaller
Fig. The scheme of semi-linear connection of three-element active and reactive energy meters into a four-wire network with combined current and voltage circuits.
quantities of wire or control cable. When it is assembled, the risk of incorrect switching on of the counters is significantly reduced, since a mismatch of the phases (A, B, C) of the current and voltage is excluded. Check the correctness of the scheme can be simplified ways without removing the vector diagram. To do this, it is sufficient to measure phase voltages, determine the order of the phases and check the correctness of switching on the current circuits by alternately outputting two elements of the counters from work and fixing the correct rotation of the disk. The disadvantage of the circuit is that it is necessary to check the current circuits disconnect customers and take special safety measures during the work, as the secondary circuits of the TT are under the potentials of the phases of the primary ti. Another serious disadvantage of the considered scheme is that its use conflicts with the OLC (clause 1.7..46), which states that it is necessary to ground or ground the secondary windings of measuring transformers. Unlike the previous diagram in fig. It has separate current and voltage circuits, so it allows checking the correctness of switching on the meters and replacing them without disconnecting the consumers, since in this circuit the voltage circuits can be disconnected. In addition, it complies with the requirements of the ПУЭ for decreasing and grounding the secondary windings of the CT.
Fig. The scheme of semi-direct connection of three-element active and reactive energy meters into a four-wire network with separate current and voltage circuits.
In fig. shows a diagram of the indirect inclusion of meters in the network over 1 kV. In this scheme, a two-element four-wire counter with separated windings is used as a reactive energy meter. It was indicated above that since there is no TT in the middle phase of the network, instead of the current Ib,
Fig. The scheme of indirect connection of two-element active and reactive energy meters into a three-wire network of more than 1 kV.
the corresponding current windings of this counter are summed by the geometric sum of the currents Ia + Ic equal to - Id. Instead of the indicated reactive energy counter in this scheme, a counter with a 90-degree shift can be used. In this case, the geometrical sum of the currents Ia + Ic is also supplied to the current winding of the second element. In fig. shows the wiring diagram using a three-phase TN type NTMI, which is grounded secondary winding. In practice, a three-phase voltage transformer can be used and the secondary winding of phase B can be grounded. Instead of a three-phase voltage transformer, two single-phase voltage transformers connected in an open triangle can also be used. In conclusion, we note that the meter switching circuit is usually applied on the lid of the junction box. However, under operating conditions, the cover may be removed from the counter of another type. Therefore, it is always necessary to make sure that the scheme is reliable by checking it with a typical scheme and marking of clamps.
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Basic concepts and definitions associated with the design and maintenance of electricity metering circuits.
The main purpose of electricity metering is to obtain reliable information on the amount of electricity produced and power, its transmission, distribution and consumption in the wholesale and retail consumption markets in order to solve the following technical and economic tasks at all levels of energy management:
• financial payments for electricity and power between the subjects of the wholesale and retail consumption market
• power management
• determination and prediction of all components of the electric power balance (production, output from tires, losses, etc.)
• determining the cost and cost of production, transmission, distribution of electricity and power
• control of technical state and compliance with the requirements of normative-technical documents of electricity metering systems in installations
The rated voltage and rated current of three-phase meters is indicated as the product of the number of phases and the rated values of voltage and current, and the voltage is assumed to be linear, for example: 3 * 5; 3 * 380 V. Three-phase four-wire meters indicate linear and phase voltages, separated from each other by a slash, for example: 3 * 5 A; 3 * 380/220 V. For transformer counters the nominal transformation ratios are indicated: 3 * 6000/100 V; 3 * 200/5 A. On the front panels of direct-on meters, in addition to the rated current, the maximum current is indicated (usually in brackets): 5-20 A or 5 (20) A.
In addition to the requirement of the absence of a self-moving device, the counter is also subject to the requirement of sensitivity , which is determined by the lowest current value, expressed as a percentage of the nominal, at nominal voltage and cos f = 1, which causes the disk to rotate without stopping. At the same time no more than two rollers of the counting mechanism are allowed to move simultaneously. The threshold of sensitivity should not exceed: 0.3% for meters of accuracy class 0.5; 0.4% for accuracy class 1.0; 0.46% for single-phase meters of accuracy class 2.0; 0.5% for three-phase meters of accuracy classes 1.5 and 2.0. The threshold of sensitivity for an accuracy class of 0.5 with a backstop should not be more than 0.4% of the rated current.
Gear ratio is the number of revolutions of its disk corresponding to the unit of measured energy. The gear ratio is indicated on the front panel of the meter by the inscription, for example: 1 kWh = 1280 disk revolutions.
The constant of the counter shows the number of units of electricity, which the meter takes into account for one revolution of the disk. It is customary to determine the counter constant as the number of watt-seconds per one disk revolution. That is, the counter constant is 36000000 divided by the gear ratio of the counter.
In practice, due to a number of reasons specific to the counters of a certain type, and sometimes random factors, the counter actually takes into account the energy value that differs from the value that it had to take into account. This is the absolute error of the counter and is expressed in the same magnitudes as being measured, i.e. kWh The ratio of the absolute error of the meter to the actual value of the measured energy is called the relative error of the meter. It is measured in percent.
The largest permissible relative error, expressed as a percentage, is called the accuracy class. In accordance with GOST, active energy meters should be manufactured with accuracy classes: 0.5, 1.0, 2.0, and 2.5. Reactive energy meters - 1.5, 2.0 and 3.0. The accuracy class of the counter is indicated on its front panel as a number enclosed in a circle. It should be noted that the accuracy class is set for the normal working conditions of the meter, namely:
• direct phase rotation
• uniformity and symmetry of the load
• sinusoidal current and voltage
• nominal frequency (50 Hz and 0.5%)
• rated voltage (deviation up to 1%)
• rated load
• cosine or sine of the angle between current and voltage (must be equal to 1 (for active or reactive energy meters, respectively))
• ambient temperature
• absence of external magnetic fields (no more than 0.5 mT)
• vertical counter location (no more than 1% from vertical)
All of the above working conditions have a different effect on the error of the meter and cannot be neglected. This issue is discussed in detail in the section
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