About the use of current transformers and transformers

Here are the questions that are asked about this circuit:

1. The current transformer is a special transformer, and its specifications are clearly about the ratio of primary and secondary turns, and its capacity is specified in the national standard, 5VA, 10VA, 15VA, etc., so its load capacity must be paid attention to, and the line impedance should not be too large, otherwise it will be distorted. So r1 needs to be calculated.

2. If the initial current of the line is not sinusoidal, the loss of the collected signal on the transformer will increase accordingly;

3. What is the voltage on the secondary side of the transformer and how to measure it?

4. When the transformer is applied, it is necessary to look at the load situation of the primary and secondary sides. As the upstream upstream of this transformer, if the transformer cannot provide sufficient excitation current and reactive power, how can this transformer work; So does this count as the power consumption on the secondary side of the transformer?

5. Furthermore, how to select the transformer load resistance r0? How do you determine what the load rating of this transformer is and how much current flows through its secondary side?

6. The PLC analog channel has its terminal impedance, and after connecting this impedance in parallel, it is not equivalent to the total change of transformer load. So how do you determine the value of the current flowing through each resistor?

5. PLC Sampling Question: What do we want to get? is a valid value (a value that is uniquely accurate for any sinusoidal or non-sinusoidal waveform), i.e., rms rms.

The sampling frequency must be much larger than the frequency of the source current, so that as many sampling points as possible can be used to calculate the accurate value.

The above are the questions raised about this circuit, please discuss.

Why do you need a transformer?

The working principle of transformer and current transformer is the same, the difference is that the structure and use are noisy, the transformer is used for the rise and fall of the pulse voltage change, and the current transformer is used for metering. The main differences are as follows:

The current transformer is strictly forbidden to open the secondary side, because this liquid sock will cause the excitation current of the primary current is all the core, so that it will be saturated and induce high voltage on the secondary side, and the insulation breakdown accident will occur, while the transformer has no such limitation.

The impedance of the load connected to the secondary side of the current transformer is very small, which is approximate to short circuit, and the secondary side of the transformer is not allowed to short circuit.

The design value of the magnetic flux density of the core of the current transformer is low, only; The magnetic flux density of the transformer core, cold-rolled silicon steel sheet, hot-rolled silicon steel sheet.

The secondary current of the current transformer varies with the magnitude of the primary current, while the transformer is reversed, and the magnitude of the primary current is determined by the magnitude of the secondary (i.e. load) current.

The current transformer is equivalent to a step-up transformer, which turns the signal of high current and small voltage into a signal of small current and high voltage. However, since the voltage change on the measured wire is small (the current is very large), the voltage after the boost is not high.

Because the purpose of the current transformer is to measure the current, the secondary circuit is connected to the current measuring instrument, which is equivalent to a closed circuit (short circuit).

1. Installing a circuit breaker on the secondary side can make the current transformer more safe and reliable during operation, so the selection of the circuit breaker must be appropriate.

2. In order to avoid the voltage imbalance on both sides of the open triangle winding, it is necessary to make parallel resistors on both sides of the open triangle winding, and when the parallel resistor induces the zero sequence voltage, the zero sequence current is turned on to the equipment. This has the effect of suppressing resonance.

3.In order for the insulation to effectively monitor for faults in relays and grounding devices, the current transformer must have the correct structure. At least one of the two winding resistors on the pressure side of the current transformer must have a grounding device.

4. If the grid voltage is less than 10kV, the center line of the switching power terminal does not need to be grounded immediately, but it needs to be grounded at the neutral point. Doing so avoids damage to the fuse due to the low voltage generated during the power switchover.

5. Reasonable methods can be used when connecting the end screen of the current transformer, such as the end screen is immediately grounded, and once the accumulation occurs, it can avoid damaging the equipment. In addition, if the grounding device is not installed immediately, there will be a certain penetration gap between the end screen and the ground.

Current transformers are widely used in our work and life, and their role is mainly to convert the primary current with a larger value into a secondary current with a smaller value through a certain conversion ratio, which has the purpose of protection and measurement.

When the current transformer is working, the secondary circuit is closed, and if the secondary open circuit occurs, the primary current will be used for excitation, so that the core is seriously saturated. At the same time, the alternating magnetic flux will induce a very high voltage on the secondary coil, the peak value can reach thousands of volts or even tens of thousands of volts, obviously the high voltage acts on the secondary coil and the secondary circuit, which will cause catastrophic consequences to personal safety and the safety of equipment, and the insulation of the coil will also be burned out due to overheating. Therefore, the "Regulations" stipulate that it is strictly forbidden to open the circuit of the current transformer during operation.

The difference between the principle of a current transformer and a transformer:

The principle of the current transformer and the transformer are almost the same, and they are basically the same in structure, both of which are two windings: one with more turns and a thin wire diameter, and the other with a few turns and a thick wire diameter.

If the winding with a large number of turns and a thin wire diameter is connected in parallel with the circuit to be measured as a primary winding, and the winding with a small number of turns and a thick wire diameter is connected to a measuring instrument (voltmeter), then the transformer is a voltage transformer. The voltage transformer is actually a step-down transformer that works in the no-load state (because the voltmeter is a high-resistance meter and the current is very small, it is no-load. And because the number of turns of the primary winding is large and the number of turns of the secondary winding is small, it is a step-down).

If the winding with a small number of turns and a thick wire diameter is connected in series with the measured circuit as a primary winding, and the winding with a large number of turns and a thin wire diameter is connected to a measuring instrument (ammeter), then the transformer is a current transformer. The current transformer is actually a step-up transformer that works in a short-circuit state (because the ammeter is a low-resistance meter and the current is very large, it is equivalent to a short circuit. And because the number of turns of the primary winding is small and the number of turns of the secondary winding is large, it is a boost, and the reason why the secondary winding voltage of the actual current transformer is not boosted is because it works in a short-circuit state).

When the current transformer is working, the secondary winding must not be opened, otherwise it will induce high voltage and endanger the safety of equipment or personnel, and due to the loss of the demagnetizing magnetic potential of the secondary winding, the core will be seriously saturated and the accuracy of measurement will be lost. The current transformer (CT) is not allowed to open the circuit during operation.

The working principle of the voltage transformer is the same as that of the general transformer, only the structural type, the materials used, the capacity, the error range and other aspects are different, the transformer is a device that converts AC voltage, current and impedance, when there is an AC current in the primary coil, the iron core (or core) will generate AC magnetic flux, so that the voltage (or current) is induced in the secondary coil, which is used to change the voltage level, and the load is larger. There are many types of transformers, which are divided into several categories according to cooling method, moisture-proof method, core or coil structure, power phase number, use, etc.; A voltage transformer is a type of voltage conversion device. It converts the high voltage to the low voltage so that the change in the high voltage value is reflected with the low voltage value.

Therefore, voltage transformers can be used to measure voltages directly with ordinary electrical instruments.

1. The current transformer is equivalent to a transformer running in a short circuit on the secondary side;

2. The primary current of the transformer changes with the secondary current, and the primary current of the current transformer has nothing to do with the secondary current;

3. The main magnetic flux of the transformer is determined by the primary voltage, and the main magnetic flux determines the secondary potential, so the main magnetic flux remains unchanged and the secondary electric potential remains unchanged. When the secondary impedance of the current transformer changes, the secondary potential changes;

4. When the secondary load of the transformer increases, it has a great impact on the amount of electricity; Within the rated capacity of the current transformer, a few more current coils or meters are added to the secondary stage, which has little impact on the secondary current.