News | company news | Oct 23,2024

**Analysis of operating characteristics of current transformer**

Characteristic analysis and solution of current transformer secondary open circuit, and specific conditions for current transformer operation.

In electrician manuals and various textbooks, the conclusion on the open-circuit operation of the secondary side of a current transformer is: “The open-circuit secondary side of a current transformer generates high voltages of several hundred volts, 1kV to 10kV, which endangers personal safety; the iron core becomes seriously heated and burns out the current transformer.” This is also a recognized regulation in the electric power industry.

When our technical service team was inspecting a non-indicating ammeter in the school’s power distribution board, they found that the measured voltage of the current transformer in series with it was 2.6V when the secondary open circuit was running. They restored the original closed circuit connection and connected a current transformer of the same model and change in the main circuit. The secondary open circuit ran for a long time without heating. This shows that the operating characteristics of the current transformer in actual application at present are significantly different from the traditional conclusions of more than 100 years, which cannot be ignored.

In order to analyze the operating parameters of the current transformer, with the strong support of the electrical company and the school party branch, we collected two voltage levels of 0.5kV and 10kV, LQJ, LFC, LFCD, and busbar core-type used in substations. 9 series including lMK, LMKl, LMZ, LMZ1, LMZJ1 and LQG which have been eliminated in Pingdu, 56 varieties produced by 20 manufacturers including Beijing, Tianjin, Shangyi, Shenyang and Hefei from 30/5 to 2000/5 After two and a half years of experiments on the current transformer and its 350 variants on the variable current experimental bench, hundreds of thousands of operating data were recorded, and the following conclusions were summarized.

**1.Secondary open circuit voltage characteristics of current transformer**

For each type of current transformer, there is a strict correspondence between the secondary open circuit voltage and the primary current, which is explained only by the secondary open circuit voltage value at the primary rated current. For the busbar through-type 150/5 current transformer, when the through-current is the rated value, the secondary open circuit voltage of different types of current transformers is 2.6~2;4V. The rule of the 80 secondary open circuit voltage characteristic curves is: when the primary through-current increases from 0A to 150A, the secondary open circuit voltage begins to rise sharply, and increases very little after 30A (even if it increases from 150A to 800A, the secondary open circuit voltage only increases by 0.2V on average).

The secondary open circuit voltage of busbar through-type 200/5 current transformer is 3.9~6.5V; the secondary open circuit voltage of busbar through-type 300/5-2000/5 current transformer is 6.5~37.2V.

The LQG series current transformers range from 30/5 to 600/5, and the secondary open circuit voltage is 12.8 to 21.4V.

The LQJ, LFC, LFCD series 50/5~300/5 current transformers used in substations have a secondary open circuit voltage of 18~121.3V.

On the second volt-ampere test bench, all the secondary coils of the current transformer were connected to the output end of the voltage regulator using a voltage regulator, an ammeter, and a wire. When the secondary input current was 5A, the measured secondary voltage and the secondary open circuit voltage measured when the primary current was added with the rated current had a strict correspondence. If calculated by the formula, the secondary open circuit voltage is equal to the increment of the primary voltage from the secondary closed circuit to the open circuit multiplied by the change. The authenticity of the secondary open circuit voltage test value was proved by the conclusions derived from the positive and negative experiments and Faraday’s law of electromagnetic induction.

In summary, there is a huge difference between the traditional conclusions of more than 100 years and the actual secondary open-circuit voltage of modern new current transformers due to technological progress, material updates, structural changes, and process innovations, so the traditional conclusions should be revised.

**Analysis of heating condition when the secondary circuit of current transformer is open**

For busbar through-type current transformer, according to Joule-Lenz law Q=I2Rt, when the secondary circuit is open, I2=0, Q2=0, so the secondary coil will not heat up. However, when the secondary coil changes from closed circuit to open circuit, the power consumption on the busbar increases. When the primary current of the busbar through-type 150/5 current transformer is the rated value, the power consumption increment is calculated to be 9W at the minimum and 17W at the maximum. Because the busbar is thick and long, the heat dissipation tolerance is good and it will not heat up.

All busbar through-type current transformers have been running intermittently on the current conversion test bench for as long as 2 and a half years, and no case of burnout has ever been found, which fully demonstrates that the heating characteristics of the current utility model current transformer are completely different from the traditional conclusions.

The LQG series current transformers that have been eliminated in Pingdu have been proved on the AC test bench that when the current of the primary coil is the rated value, the secondary coil is open-circuited, and the primary current is still the rated current. Although the secondary coil will not heat up, the power consumption of the primary coil will increase. For example, the 44#30/5 current transformer has a primary voltage drop increment of 2.5V, a power consumption increase of 75W, and a slight heating of the primary coil. A large number of actual observations of this series of current transformers show that if there is an overcurrent, the primary coil consumes more power and does heat up. When the current exceeds 50% of the rated value, the primary coil is hot. Due to heat transfer, the iron core and the secondary coil also heat up successively. After long-term operation, a burnt smell can be smelled. If the insulation is burnt, the primary voltage will be added to the secondary coil, which shows that the heating situation of this series of transformers still conforms to the traditional conclusions of more than 100 years ago, but this voltage is the power supply voltage, not the induced secondary open circuit voltage at all.

**Analysis of the secondary open-circuit voltage waveform of current transformer**

When the primary current is 0A, the secondary output voltage waveform is a horizontal line. As the primary current increases, the waveform changes of the 56 current transformers are very complex, and each has its own characteristics. For example, for transformer No. 6, the primary current is 3A, the secondary voltage waveform is a sine wave, and the waveform is distorted when the primary current increases from 4A to 150A. As the primary current increases, the waveform distortion intensifies, and its changes are especially the narrowing of the effective width, the increase of the peak-to-peak value, and the decrease of the duty factor, but the area enclosed by each half cycle increases very little.

The small increase in the enclosed area per half cycle indicates that the iron core has tended to magnetic saturation, reflecting that the secondary open-circuit voltage can only rise slightly with the increase of primary current, while the increase in peak-to-peak value reflects that the rectifier and filter voltage has a large increase, providing a wider control range for automatic control.

**Ratio difference characteristic analysis**

At present, when measuring current in my country, it is required to use 1.5mm2 copper wire to short-circuit the secondary coil of the current transformer and the current coil of the watt-hour meter. This is an application method that reduces accuracy and produces proportional difference. Taking the busbar through-type 150/5 current transformer as an example, we use 1.5mm2 copper wire with a length of 4m to connect its secondary coil in series with the current coil of the 150/5A ammeter and the watt-hour meter respectively. When the primary through-current is 150A, all current transformers can make the ammeter touch the needle. If the primary current drops to 135A, the current display number is 140~150A.

8000 data points measured on the current transformer test bench show that when the primary current exceeds the rated value by 10%, the proportional difference becomes more and more obvious as the primary current increases. When the primary current is below 15A, the current indication number in the secondary measurement of the current transformer has a negative proportional difference.

According to national standards, the total load current should be equal to 75% to 100% of the rated current of the current transformer. When the accuracy of the current transformer is 1 to 3 levels, the load of the secondary circuit should be 50% to 100% of its nameplate load value. For current transformers with an accuracy of 0.1-1, 25% to 100% of the nameplate load should be taken. For current transformers with a load of 0.2fl, which is absolutely dominant in local usage, when the primary current reaches the rated value, the secondary closed-circuit voltage must be 1V, 0.5V and 0.25V according to regulations. my country’s national standards also stipulate that if it is lower than the lower limit of the above limit, the measurement accuracy will be reduced, resulting in positive errors. However, we have measured the distribution boards of many users. When the current transformer used for current indication makes the ammeter display close to the rated value, and the current transformer used for charging, the secondary measurement is directly connected to the current coil of the three-phase watt-hour meter with a specified wire, and the secondary closed-circuit voltage is 0V measured by the pointer multimeter AC voltage 10V gear. Operational data show that the examples all violated the secondary load standards prescribed by the state, which is a common technical problem in power transformation and distribution.

**Methods to eliminate errors**

In two years, we explored the compensation methods for proportional difference by using conductors, fractional turns, capacitors, inductors, and resistors. At the end of 1995, we finally found a technical solution that can significantly reduce the positive and negative ratio differences of current transformers. For example, for the No. 23 200/5 current transformer, when the busbar passes through the core for one turn and the primary current is 180A, the secondary display is 194A, the proportional difference is 14A, the primary current is 5A, the secondary display is 0A, and the negative ratio difference is 5A. Using this technical solution, the proportional difference of the current transformer can be 0A and the maximum negative ratio difference is 1A in the entire range from 0A to the rated current of the primary current, thus discovering a technical solution to reduce the current transformer ratio difference.

**Load characteristics of current transformers**

When the load impedance connected to the secondary measurement is much larger than the impedance marked on the nameplate, the current transformer is a constant voltage source. For example, the impedance of the No. 9 150/5 transformer is marked on the nameplate as 0.2Ω. When the primary current is 150A and the secondary circuit is open, the MF173 multimeter AC 10V range with an accuracy of 9kΩ/V is used, which is equivalent to connecting a 90kΩ resistor in series in the secondary circuit, and the voltage is measured to be 2.6V; when the secondary measurement is connected to a 15Ω300 magnetic field resistor and the resistance values are 15Ω, 7.5Ω, and 3.75Ω respectively, the secondary voltage is still 2.6V; when the resistance value is 1.875Ω, the secondary voltage drops to 2.55V, which strongly proves the above conclusion.

When the external resistance is much smaller than the calibrated impedance, the current transformer is close to a constant current source. For example, for a No. 9 150/5 transformer, adjust the primary current to 135A, connect the 150/5A ammeter in series with 1.5mm/X 30cm aluminum wire and 1.5mm2X4m copper wire respectively, and use 1.5mm2X 30cm aluminum wire and 1.5mm2X 4m copper wire to connect the 150/5A ammeter and the 5A watt-hour meter current coil in series. The current reading is 150A. The above experiment shows that when the external resistance is much smaller than the calibrated impedance, the current transformer is a constant current source and has the same proportional difference.

**Specific application of secondary open circuit of typical busbar through-type current transformer**

Since the secondary open circuit voltage of various current transformers below 400/5 of busbar through-type is very low, the internal impedance is small, and there is no heat when the current is excessive, it has the characteristics of constant voltage source and constant current source, and can be used stably and reliably as an automatic control power signal source. The secondary open circuit voltage value is used to directly control the high input impedance composite tube, single crystal tube, VMOS power field effect tube, unidirectional crystal tube, operational amplifier, comparison amplifier, time base circuit, sensitive relay, etc., to break through the forbidden area of the traditional conclusion of the secondary open circuit application mode, and form a mechatronics and relay protection product. After this method is confirmed by people, a mechatronics and relay protection product is formed. After this method is confirmed by people, it will inevitably be widely used in automation control design, which will produce immeasurable social benefits that will get out of the misunderstanding of traditional structures.

**Specific operating conditions and internal impedance analysis of current transformers**

The maximum primary current of the current transformer in actual application is equal to the full load current, and is strictly controlled by the maximum current allowed by the power transformer, the fuse melting current, the knife tripping current, the current limited by the automatic air switch, the protection current set by the overcurrent protector, and the safe current allowed by the conductor. As we all know, the secondary current rating of the current transformer is 5A. If it is separated from the above conditions, it is unrealistic to try to use the secondary induced voltage formula E=L× di/dt to imagine the infinite current combined change relationship or to imagine the secondary infinite current to calculate the secondary open circuit voltage in order to maintain the traditional secondary open circuit voltage conclusion.

On the current transformer test bench, for each current transformer, the primary measurement injects 0.1, 0.2, …, 1 times the rated current. The secondary operates in open circuit and closed circuit modes, and the primary voltage and primary current are measured. The impedance of the primary measurement under the secondary open and closed circuit conditions can be calculated, and its value is very small. For example, the 44-number 3015LQG series current transformer with the largest primary impedance, when the primary measurement adds 30A current, the secondary is open circuit, and the primary voltage is 3V, then the primary impedance is 0.1Ω. Under the secondary closed circuit condition, the primary voltage is 0.5V and the primary impedance is 0.017Ω. The other current transformers increase with the change. The primary impedance becomes very small, among which the busbar through-type current transformer is the smallest. Similarly, using the secondary volt-ampere characteristics, the secondary impedance of other types of current transformers except the busbar through-type current transformer under the primary open or closed circuit conditions can be directly calculated.

For the busbar through-core current transformer, the secondary impedance can be directly calculated using the secondary volt-ampere method when the primary circuit is open. The secondary impedance under the primary closed circuit condition must be added with a through-core short-circuit ring. The short-circuit ring used is a single-turn or multi-turn short-circuit ring of more than 150mm2 composed of multiple strands of fine copper wire. Under the condition that the secondary input ampere-turn is approximately equal to -100 ampere-turns, the secondary impedance of the busbar through-core current transformer under the primary closed circuit condition can be calculated. For example, for a No. 5 current transformer, after injecting 1~5A current into the secondary and adding a 150mm2 short-circuit ring, its secondary impedance is 0.19Ω, which is approximately equal to the load 0.2Ω or 5VA marked on the nameplate.

**Summarize**

When inspecting the current transformer, it was found that even if the secondary coil of the transformer was open-circuited, it would not cause damage to the equipment. This phenomenon is inconsistent with the traditional electrician manuals and various textbooks that the secondary coil of the current transformer is open-circuited to generate high voltage of more than several hundred volts, which endangers personal safety, and the iron core is seriously heated, burning the current transformer. In order to solve this problem and clarify the operating parameters of the current transformer, the author conducted a series of experiments and reached a convincing conclusion based on a large number of investigations and experiments.

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