Current transformers are used as matching devices. The consumer is characterized by high power. Switching the meter directly to the circuit is dangerous: able to burn. Applies to measuring instruments equipped with a low-impedance input. The current grows, the process must be limited. Consider today how to connect a three-phase meter through current transformers, why you need it. Let's try to explain on the fingers.
Measurement of current, voltage
On an industrial scale, current consumption is large. If the local network considers the voltage to be constant, the value in the workshop sector as a whole may differ greatly. Because it happens that rarely the load of the phases is the same. Power supplies are designed according to opposite conditions. The uneven loading of the phases will adversely affect the supplier, consumers think little. One winding of the transformer of the substation is able to “dry out”.The current will not disappear completely, just lower the voltage. But the factory pays for power! If the current consumption is equal to, say, 100 A, the voltage determines a lot.
Suppose the effective voltage value is 230 volts. Power consumption will be: 230 x 100 = 23 kW.Should the voltage drop by 15%, with the same current, the consumer benefit will decrease proportionally. In industrial networks of 400 V, there are three phases with an effective voltage of 230 volts each, it is customary to measure both parameters at once. The meter multiplies, finds how much to pay for the use of electrical energy.
Specificity is detected. Three-phase counters include two components within themselves:
- The current coil is used to estimate the speed of movement of electrons. Performs an ammeter. It is important to protect against high currents, if any. Otherwise, the three-phase meter could be turned on without transformers.
- Voltage coil is connected in parallel with the transformer, evaluates the voltage.
The question arises: if you connect a three-phase meter through current transformers, the readings will change in comparison with the case when the meter cuts directly? Exactly! Therefore, it is time to say: three-phase meters may be suitable to work with instrument transformers. It is easy to learn, having considered marking, decoding according to GOST 25372-95.The first useful information appears. Look at the figure, the type of current transformer of the electric circuit is shown:
- . The first difference is immediately noticeable. The current transformer is more like a choke, an ordinary inductance, crossed with a straight line in length.
- coils schematically shows a secondary winding with a low current. The transformation ratio is selected according to the marking indicated by the three-phase meter housing. It is impossible to turn on one chain of devices of a completely different class.
- The load created by electric motors, welding machines, is connected to a fat straight line.
We believe, much is clear, we will explain fairly. The three-phase current coil is connected to the secondary circuit of the transformer, and it is customary to bring one end to neutral. It is permissible to do outside( with your own hands) and inside the case. How to do it is indicated by the connection diagram of a three-phase meter through current transformers, often referred to by a nameplate. Look carefully at the device, looking for a similar image.
The second moment we call the connection of the voltage coil. Cut in parallel to the supplied primary winding of the current transformer phase. The second end, as with the first coil, forms a neutral( neutral wire).Difficult to understand, see the illustration( figure): they outlined in detail the indicated point: where to dock, you need to connect current transformers. For simplicity, we made wires of different colors, see:
- Green shows the wiring coil connection. It forms a closed circuit with the secondary winding of the transformer. One side is grounded. Helps existing inductances to form a divider through which the current will reach the ground. Easier to measure the desired parameters.
- Blue shows a voltage coil. It is assumed that the circuit contains a neutral. But not necessarily. There are three-phase meters of 380 volt networks without neutral. Often used by electric motors with a star-winding circuit. Parallel connection is considered a typical voltage measurement method. It is assumed that the resistance of the coil should be large in order to avoid energy loss. A different alignment is dangerous three-phase meter. Large currents would heat the coil, the device could burn.
To summarize: in order to connect the meter through current transformers, you need to take into account the marking of the device. There are the necessary factors. According to the figures current transformers are selected. Three-phase meters are often combined. Allow connection without current transformers. The body gives the working values, in parentheses - the limit. For example, 5( 10) A. The rated current( of one phase) will be 5 A, the limiting current - 10.
Now the features of the circuit design. In three-phase networks, devoid of neutral, the current flows away from the consumer through the free phase, where is zero, or the desired polarity. Therefore, take care to take into account the direction of the magnetic field of the coil, correctly assessing the power consumption. There is no guarantee that a three-phase meter for neutral circuits will work correctly. It is recommended to choose the meter strictly according to the existing conditions.
Marking of three-phase meters
We provide information not because readers are too lazy to open GOST 25372-95.For a graduate of a specialized university, the above designations may seem to be a real filter. What to say about most people. Let us briefly analyze the methods of forming the marking of energy meters:
First, a three-phase meter connected via a current transformer is characterized by a special sign( but not only for devices with a primary counter mechanism).We consider in detail, it is this class of devices that falls under the topic of this review. The three-phase meter case is provided with markings of two mutually intersecting circles with schematic taps from each up and down. From GOST we see four types: with a secondary counting mechanism, a mixed counting mechanism( variables can be either current or voltage), a primary counting mechanism.
The definition of the concepts is given, for example, GOST 6570-96.The document became invalid on the territory of the Russian Federation, leaving the value of a capacious reference book, we are interested only in terminology that remains unshakable. When discussing the primary meter of electrical energy, means the device that takes into account the coefficients of current transformers. The testimony is removed directly, to give the supplier for payment. If the measurement mechanism of three-phase meters connected via current transformers is secondary, you will have to take a calculator, do multiplication by a factor. The size of the digit is indicated.
Nature gave birth to three-phase counters of a mixed evidence recording mechanism. It is considered that the transformation ratio of one parameter is taken into account. Voltage or current. Instrument readings need to be adjusted manually before paying bills. Now let's discuss the labeling.
The main shield contains a transformation ratio: an oblique or ordinary fraction, taken into account by the manufacturer( tuner).Summarizing the above, for the counter with the secondary registration mechanism, there is actually nothing to give here. Near the icon of two circles are the same voltage, current. The measurement mechanism is mixed, there is an alternating primary current - in the numerator, the denominator will be the voltage. Otherwise, everything will be the opposite, but in everyday life it is rare. For a counter fitted with a primary counting mechanism, the coefficients are given for current, voltage. It was said above, in this case, ready-made testimonies are given, which are submitted for payment.
Therefore, three-phase electric current meters with a secondary measurement mechanism are preferable from the point of view of ease of use. How to recognize on the store counter( without looking at the passport) should be clear. Now discussed the main shield. The icon formed by circles decorates the dial. On the additional panel, the unaccounted transformation factors are shown, along with the coefficients for multiplying the readings and obtaining the necessary figures. Since the three-phase electricity meters with the primary counting mechanism are all taken into account, the devices in this regard are clean. Additional shield is able to be absent, contains no information.
Caution when purchasing!
Please note that three-phase electricity meters are different in. .. registration parameter. We use the estimated total energy. Is there another? Yes! When a 3-phase meter is connected via current transformers it is especially clearly visible. The presence of inductive, capacitive impedances causes a phase shift. It seems incredible, the current begins to flow from the supplier back. It turns out that the total energy takes into account the passage of reactive current, which does not perform useful work in the load.
A consumer may be. .. a generator. You involuntarily recall a joke about pumping water into the water supply system. Reactive power is completely parasitic. The flow of excess currents causes losses. Active power is reduced. Ideally, reactive power is reduced by special measures. In the store we will find instruments for measuring active, reactive, and total power. In most cases we use the latest type of devices. See exactly what needs to be done. Or do not be surprised later that the connection of a three-phase meter Mercury through current transformers gives an incorrect result( overestimated).
We say goodbye to this. Hopefully, the pictures are useful, the connection scheme of a three-phase meter through current transformers is now clear to readers. Add, each electric circuit is associated with the concept of a coefficient characterizing the value of reactive power. The concept is foggy, not everyone is encouraged to believe the explanation( they were convinced by inspecting thematic sites).