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- The principle of operation of an induction electric meter.
- Wiring diagrams of electric meters and their verification, description of circuits.
- Basic definitions (rated voltage and current of the meter, meter sensitivity, gear ratio and meter constant, meter error)
The principle of operation of a single-phase induction active energy meter.
The counter is a measuring wattmeter 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 forces of interaction cause the movement of the moving part. A schematic diagram of a single-phase meter is shown in Fig.
Its main nodes are electromagnets 1 and 2, an aluminum disk 3 mounted on the axis 4, the axis supports are a thrust bearing 5 and a bearing 6, a permanent magnet 7. A counting mechanism (not shown) is connected to the axis with a gear 8, 9 - the opposite pole of electromagnet 1. Electromagnet 1 contains a W-shaped magnetic circuit, on the middle core of which is a multi-turn winding of thin wire connected to the mains voltage U parallel to load N. This winding is called a parallel winding or coil in accordance with the switching circuit otkoy tension. At a rated voltage of 220 V, the parallel winding usually has 8-12 thousand turns of wire with a diameter of 0.1 - 0.15 mm. Electromagnet 2 is located under the magnetic system of the voltage circuit and contains a U-shaped magnetic circuit, with a small wire winding located on it number of turns. This winding is connected in series with the load and is therefore called a series or current winding. The full load current flows through it. Typically, the number of ampere turns of this winding is in the range of 70 - 150, i.e. at a rated current of 5 A, the winding contains from 14 to 30 turns. The complex of parts, consisting of serial and parallel windings with their magnetic circuits, is called the counter rotating element.
The current flowing through the voltage winding creates a common alternating matrix flow of the voltage circuit, a small part of which (the working flow) suppresses the aluminum disk located in the gap between both electromagnets. Most of the magnetic flux 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 circuit and partially through the side rods. An insignificant part (non-working flow) closes through the pole to the intersecting disk. Since the magnetic circuit of the current winding has a U-shaped design, its magnetic flux crosses the disk twice.
Thus, in total, three variable magnetic fluxes pass through the counter disk. According to the law of electromagnetic induction, the alternating magnetic fluxes of both windings when crossing the disk induce an EMF in it (each of them is two), under the action of which the corresponding eddy currents flow around the traces of these flows in the disk (we recall the “gimlet” rule). As a result of the interaction of the magnetic flux of the voltage winding and the eddy current from the magnetic flux of the current winding and on the other hand the magnetic flux of the current winding and the 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 the indicated magnetic fluxes and to the sine of the phase angle between them.
The active power consumed by the load is defined as the product of the current strength by the applied voltage and the cosine of the angle between them. Since the magnetic fluxes of both windings are proportional to voltage and current, it is possible to achieve by constructive equality of the sine of the angle between the flows and the cosine of the angle between the current and voltage vector, to make the counter torque proportional to the measured active power coefficient. The sine of one angle is equal to the cosine of the other angle if the shift between them is 90 degrees, which is achieved in the meter designs (using short-circuited turns, additional windings closed by adjustable resistance, moving the screw clamp, etc.) A torque proportional to the power of the network drives the counter disk into rotation, the rotation frequency of which is set when the torque is balanced by the braking torque. To create a braking torque, the counter has a permanent magnet, which covers the disk with its poles. The magnetic field lines crossing the disk induce an additional EMF in it, proportional to the disk rotation frequency. This EMF, in turn, causes the eddy current to flow in the disk, the interaction of which with the flux of a permanent magnet leads to the emergence of an electromechanical force directed against the movement of the disk, i.e. leads to the creation of a braking torque. Adjustment of the braking torque, and hence the rotational speed of the disk, is carried out by moving the permanent magnet in the radial direction. As the magnet approaches the center of the disk, the speed decreases.
Thus, having achieved a constant frequency of rotation of the counter disk, 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.
The principle of operation of a single-phase electronic meter of active energy.
The counter is an analog-to-digital device with preliminary conversion of power to an analog signal with subsequent conversion of the analog signal to the pulse repetition rate, the summation of which gives the amount of energy consumed.
Structurally, the meter consists of a housing, a measuring current transformer, and a converter and a charging module made on a printed circuit board. Structurally, the counter consists of the following nodes:
• LCD driver
• secondary power source
• optical port
• telemetric output
• real time clock
The converter is an analog-to-digital device with preliminary conversion of power to an analog signal using the PWM-AIM method with subsequent conversion of the analog signal to a pulse signal proportional to the consumed electricity. The secondary power source converts the alternating input voltage to the value necessary to power all nodes of the meter. The microcontroller calculates the input pulses, calculates the energy consumed, controls and exchanges information with other nodes and counter circuits. The supervisor generates a reset signal when the power is turned on and off, and also gives a power failure signal when the input voltage decreases. The memory stores data on the consumed electricity and other parameters. The real-time clock is designed to count the current time and date. The LCD driver receives information from the microcontroller and issues control signals to the LCD. The LCD is a multi-bit indicator and is intended to indicate operating modes, information on consumed electricity and time parameters. The optical port is for reading and programming the meter. The microcontroller receives signals from the buttons on the meter panel and signals from the converter are proportional to the energy consumption. The microcontroller stores information in memory and provides a pulsed signal of energy consumption to the telemetric output.
WIRING DIAGRAMS OF THE METERS AND THEIR CHECK. DESCRIPTION OF SCHEMES
The counter is a device that responds not only to the value of energy, but also to the direction of its transmission. The property of the counter to respond to the direction of energy leads to the mandatory need to include the current circuit of the counter and the voltage circuit in a consistent manner, so that with a positive direction of energy, the disk rotates in accordance with the arrow. Before considering specific schemes for including meters, we list a few general points
The clamps of the current windings of the meter and voltage windings connected from the power supply side are conventionally called unipolar. In the diagrams, the unipolar outputs of the counter windings (start 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, at the meters of direct connection, is connected to the current clamp by a removable jumper.
Earlier, when describing the counters, it was noted that the clamps of the current windings are indicated by the letters G (generator) and H (load). In this case, the generator clamp corresponds to the beginning of the winding, and the load clip corresponds 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. To do this, the wires from the power supply side must be connected to the generator terminals (terminals G) of the windings, and the wires extending from the counter to the load side must be connected to the load terminals (terminals H). For meters included with measuring transformers, the polarity of both CTs and VTs must be taken into account. This is especially important for three-phase meters having complex switching circuits, when the wrong polarity of the measuring transformers is not always immediately detected on a working meter. If the meter is switched on via a CT, then a wire is connected to the beginning of the current winding of the CT secondary winding, which is unipolar with the primary output windings connected from the power supply side. With this switching on, the direction of the current in the current winding will be the same as with direct switching on. For three-phase counters, the input terminals of the voltage circuits, unipolar with the generator terminals of the current windings, are indicated by the numbers 1, 2, 3. This determines the specified sequence of phases 1- 2-3 when connecting meters. It should be noted that when connecting, the internal connection diagram should not cause any doubt or ambiguity, since all the required internal connections are made in the manufacture of meters. It is important to monitor only the correctness of external connections. Figure a.6.c shows typical schemes for including active and reactive energy meters both when they are directly connected to the electric network and with measuring transformers. Fig. A, b, c shows the schematic diagrams of switching on a single-phase active energy meter indicating the polarity of the measuring transformers. The secondary windings of CTs and VTs are earthed for safety reasons. It doesn’t matter in principle what to ground - the beginnings or ends of the windings of measuring transformers.
Fig. Schemes of inclusion of a single-phase meter of active energy: a - with direct inclusion; b - with half-turn on; c - with indirect connection;
Schematic diagrams of the inclusion of a three-phase three-wire two-element active energy meter type SAZ (SAZU) 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. the phase whose current is not supplied to the meter. When the meter is switched on with a VT, the clamp of this phase is grounded. In the diagram in fig.v at T1, the terminals on the side of the power supply are grounded (i.e., the terminals .and 1), but it would be possible to ground the terminals on the side of the load. SAZ-type meters are mainly used with measuring transformers, and therefore the diagram shown in Fig. in is the main when accounting for active energy in electric networks of 6 kV and above.
Fig. Schemes of inclusion of a three-phase three-wire two-element active energy meter type SAZ (SAZU):
a - with direct inclusion;
b - with semi-indirect inclusion;
c - with indirect inclusion
Schematic diagrams of the inclusion of a three-phase three-element active energy meter type CA4 (CA4U) are shown in Fig. E, while in Fig. a, b, c are three-wire switching circuits, and in fig. g, d is a four-wire counter.
Fig. Schemes for switching on a three-phase three-element active energy meter type CA4 (SA4U):
a - with semi-indirect connection to a three-wire network;
b - when indirectly connected to a three-wire network;
c - for direct connection to a four-wire network;
g - with half-turn on the four-wire network
In fig. shows the connection diagram with three single-phase VTs, the primary and secondary windings of which are connected to a star. In this case, the common point of the secondary windings is grounded for safety reasons. The same applies to the secondary windings of the CT. In fig. c, d it is necessary to pay attention to the presence of a mandatory 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 can cause an additional error when energy is taken into account in networks with voltage asymmetry. The schemes for including reactive energy meters with a 90th shift of type CP4 (SR4U) in a four-wire network are shown in Fig. a B C. The procedure for applying voltages and currents to the meter is the same as for the active energy meter. The scheme of indirect inclusion of the same meter in a three-wire network is shown in Fig. d. Since there is no CT in the middle phase of the network, instead of the current Ib, the geometric sum of the currents Ia + Ic, which, as is known, is -Ib, is connected to the current winding of the second counter element.
Fig. Schemes of inclusion of a three-element counter of reactive energy with a 90 ° shift of type CP4 (SR4U):
a - for direct connection to a four-wire network;
b - with semi-indirect inclusion in a four-wire network;
c - when indirectly connected to a four-wire network;
g - when indirectly connected to a three-wire network
In fig. a diagram of the semi-indirect inclusion of a two-element reactive energy meter with separated serial windings of the CP4 (CP4U) type in a four-wire network is shown.
In three-wire networks, where there are only two CTs, this counter can be turned on according to the scheme using the geometric sum of the currents of two phases, similar to the scheme in Fig. In Fig. schemes for the inclusion of a reactive energy meter of the SRZ (SRZU) type with a 60 ° shift into a three-wire network are presented.
Fig. Diagram of the semi-indirect inclusion of a two-element reactive energy meter with separated serial windings of CP4 (SR4U) tin in a four-wire network
Fig. The circuit for the inclusion of a two-element reactive energy meter type SRZ (SRZU) with a 60th shift into a three-wire network:
a - with direct inclusion;
b - with half-turn on;
c - with indirect inclusion
Due to the fact that active and reactive energy meters are usually used together, in fig. As an example, schemes for their joint inclusion are given. In fig. The schemes of semi-integrated switching of meters into a four-wire network (380/220 V) are given. The diagram in Fig. Requires the installation of a smaller
Fig. Scheme of the semi-indirect inclusion of three-element active and reactive energy meters in a four-wire network with combined current and voltage circuits.
the amount of wire or control cable. When assembling it, the risk of incorrect switching on of the meters is significantly reduced, since the mismatch of the phases (A, B, C) of the current and voltage is eliminated. You can check the correctness of the scheme in simplified ways without removing the vector diagram. To do this, it is sufficient to measure the phase voltages, determine the phase sequence and verify that the current circuits are turned on correctly by alternately taking the two counter elements out of operation and fixing the correct rotation of the disk. The drawback of the circuit is that checking that the current circuits are turned on correctly requires three times disconnect consumers and take special safety measures during work, since the secondary circuits of CTs are under the potentials of the phases of the primary tee. Another serious drawback of the considered circuit is that its use contradicts the PUE (clause 1.7..46), which states the need for grounding or grounding of the secondary windings of measuring transformers. In contrast to the previous diagram in Fig. It has separate current and voltage circuits; therefore, it allows checking the correct inclusion of meters and replacing them without disconnecting consumers, since voltage circuits can be disconnected in this circuit. In addition, it complies with the requirements of the PUE for the grounding and grounding of the secondary windings of CTs.
Fig. Scheme of semi-indirect inclusion of three-element meters of active and reactive energy in a four-wire network with separate current and voltage circuits.
In fig. The scheme of indirect switching on of meters in the network over 1 kV is shown. In this diagram, a two-element four-wire counter with separated serial windings is adopted as a reactive energy counter. It was indicated above that since there is no CT in the middle phase of the network, instead of the current Ib to
Fig. Scheme of indirect inclusion of two-element meters of active and reactive energy in a three-wire network of more than 1 kV.
the corresponding current windings of this counter summed the geometric sum of currents Ia + Ic equal to - Id. Instead of the indicated reactive energy counter, a counter with a 90-degree shift can be used in this circuit. In this case, the geometric sum of the currents Ia + Ic is also supplied to the current winding of the second element. In fig. shows the switching circuit using a three-phase VT type NTMI, in which the secondary winding is grounded. In practice, a three-phase voltage transformer can be used with earthing of the secondary winding of phase B. Instead of a three-phase voltage transformer, two single-phase voltage transformers connected according to an open triangle circuit can also be used. In conclusion, we note that the circuit for switching on the meter is usually applied on the cover of the terminal box. However, under operating conditions, the cover may be removed from a different type of meter. Therefore, it is always necessary to verify the reliability of the circuit by reconciling it with the typical circuit and with the marking of the clamps.
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 about the amount of electric energy and power produced, about its transmission, distribution and consumption in the wholesale and retail consumption markets to solve the following technical and economic problems at all levels of management in the energy sector:
• financial settlements for electricity and capacity between the wholesale and retail consumption market entities
• power management
• determination and prediction of all components of the balance of electricity (generation, release from tires, losses, etc.)
• determination of the cost and cost of production, transmission, distribution of electricity and capacity
• control of technical condition and compliance with the requirements of regulatory and technical documents of electricity metering systems in installations
The rated voltage and rated current of three-phase counters is indicated as the product of the number of phases by the rated values of voltage and current, and the voltage is assumed to be linear, for example: 3 * 5; 3 * 380 V. For three-phase four-wire meters, linear and phase voltages are indicated, 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-connected meters, in addition to the rated current, the maximum current value is indicated (usually in brackets): 5-20 A or 5 (20) A.
In addition to the requirement that there is no self-propelled device, the meter is also required to have a sensitivity , which is determined by the lowest current value, expressed as a percentage of the rated voltage, at the rated voltage and cos f = 1, which causes the disk to rotate without stopping. In this case, the simultaneous movement of no more than two rollers of the counting mechanism. The sensitivity threshold must 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 sensitivity threshold of the accuracy class 0.5 meters equipped with a backstop should not exceed 0.4% of the rated current.
The gear ratio of the counter 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 with an inscription, for example: 1 kWh = 1280 rpm.
The counter constant shows the number of units of electricity that the meter takes into account per revolution of the disk. It is customary to define the counter constant as the number of watts per second per disk revolution. That is, the counter constant is 36000000 divided by the gear ratio of the counter.
In practice, for a number of reasons specific to counters of a certain type, and sometimes random factors, the meter actually takes into account the value of the energy different from the value that it should have taken into account. This is the absolute error of the counter and it is expressed in the same quantities as the measured one, i.e. kWh The ratio of the absolute error of the counter to the actual value of the measured energy is called the relative error of the counter. 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 must be manufactured in 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 operating conditions of the meter, namely:
• direct phase rotation
• uniformity and symmetry of the load
• sinusoidality of current and voltage
• rated frequency (50 Hz and 0.5%)
• rated voltage (deviation up to 1%)
• rated load
• cosine or sine of the angle between current and voltage (should be equal to 1 (for meters of active or reactive energy, respectively))
• ambient temperature
• absence of external magnetic fields (not more than 0.5 mT)
• vertical arrangement of the counter (from a vertical no more than 1%)
All of these operating conditions have a different effect on the accuracy of the meter, and they cannot be neglected. This issue is discussed in detail in the section.
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