Current sensor for 10kv pole mounted circuit breaker

Why can't the current transformer be opened when it is running? What is the principle? What are the adverse effects of opening a circuit?   Most welders often hear this sentence: "The secondary side of the current transformer cannot be open-circuited, and the secondary side of the voltage transformer cannot be short-circuited." At work, everyone must regard this as the "Quran". When disassembling the secondary line of the current transformer, first add a short rotor or short wiring to connect the secondary side, and then perform surgical removal or wiring work. , Only in this way can the safety of the person be guaranteed, which is a very appropriate approach. When I removed the current wire of the electric meter before, I encountered a situation where the wiring was not firm and the terminals were crackling with charge and discharge. So why can't the secondary current transformer be open circuited? What is the principle? What adverse effects will it cause? The following is a detailed analysis for everyone, I hope everyone can understand it. When the current transformer is working normally, the load connected to the secondary coil is an ammeter or meter current coil and a smart transmitter, etc. The impedance of these coils is not large, and most of them operate in short circuit conditions. Fault conditions. In this case, the magnetic flux generated by the primary current of the current transformer and the secondary coil current cancel each other, keeping the magnetic flux density in the core at a moderate level, usually at a few tenths of a Tesla ( The unit of magnetic flux density: T), because the secondary coil resistor is not large, the operating voltage of the secondary coil is also very low. Under normal operating conditions, the magnetic fluxes of current transformers cancel each other, and the relative density of the magnetic fluxes is not large. When the secondary winding of the current transformer is open-circuited, if the primary current does not change, the secondary circuit is broken, or the resistor is very large, then the current on the secondary side is 0, or very small, and the secondary coil Or the magnetic flux of the iron core is not large and cannot offset the primary magnetic flux. At this time, all the primary current turns into self-induced electromotive force, saturating the iron core. This change is sudden, called a sudden change, and its magnetic flux density reaches several Teslas.   Adverse effects of secondary open circuit of current transformer   When this kind of situation occurs, it will cause the following adverse effects:   1. The second generation of thousands of volts of voltage (this has not been certified, it is a copied theory), the high voltage will penetrate the insulation layer of the current transformer, causing the entire substation equipment casing to be induced and electrified, and will also It will cause electric shock accidents to maintenance workers and threaten their lives.   2. A sudden change in the saturation state of the iron core will increase the core loss of the voltage transformer, and the iron core will become hot and damage the voltage transformer.   3. The saturated iron core of the voltage transformer is in a saturated state, the measurement calibration is inaccurate, and the CT ratio difference and angle difference increase. 

The current transformer is a special transformer. Its function is to convert large current into a small standard current, and it works in conjunction with equipment such as measuring instruments, calibration instruments, and relays. This can have the effect of expanding the detection range of the instrument and improving the reliability and safety of the power circuit. The schematic diagram of the wiring circuit of the current transformer is as shown in the figure. The primary electromagnetic coil of the current transformer is connected in series in the primary main power circuit, and the electromagnetic coils of its secondary connected instruments and automobile relays are also connected in series.   Usually when people use current transformers, they mainly consider the two performance parameters of transformation ratio and accuracy.   1. Transformation ratio   The rated voltage value of the secondary side of the current transformer is 5A or 1A. Under normal circumstances, 5A is selected. Common transformation ratios for accurate measurement of current transformers are 5/5, 10/5, 15/5, 20/5, 25/5, 30/5, 40/5, 50/5, 75/5, 100/5, 150/5, 200/5, 250/5, 300/5, 400/5, 500/5, 600/5, 750/5, 800/5, etc. So how to appropriately choose the transformation ratio of the current transformer?   The "Design Specifications for Electrical Measuring Instruments in Electrical Power Installations" requires that "the selection of the detection range of pointer measuring instruments should ensure that the rated current of the power project is marked at 2/3 of the instrument scale." According to this standard, people can use the following The formula is used to calculate the transformation ratio N of the selected current transformer.   In this formula calculation, I is the maximum load current of the control loop, 0.7 means that the maximum load current is indicated at 70% of the instrument panel, and 5 is the secondary rating of the current transformer.   current. Then select the transformation ratio of the relative current transformer based on the measured transformation ratio value. For example, if I is 50A, N=14.29 can be obtained according to the calculation method, so a current transformer with a 75/5 ratio is selected.   2. Accuracy   The accuracy of the current transformer is also called precision, because the ratio deviation and angular deviation of the current transformer are unavoidable. The accuracy of a current transformer is calibrated as a percentage of the deviation limit of this ratio. For example, the maximum ratio deviation of a class 0.5 current transformer is ±0.5%. There are 0.1, 0.2, 0.5, 1, 3, and 5 levels of current transformers for accurate measurement, and 5P and 10P level two current transformers for maintenance. Under normal circumstances, choose level 0.2 for metrological verification, choose level 0.5 for precision measurement, and choose level 1 for general monitoring instruments.   In addition, people must pay attention when using current transformers: the secondary side of the current transformer must be grounded to prevent high voltage from the primary side from flowing into the secondary side, which may endanger the safety of equipment and life; pay attention to the first and second sides of the current transformer Optical rotation, optical rotation will ablate the instrument in serious cases; the secondary side of the current transformer cannot be leaded, and it is forbidden to install a circuit breaker on the secondary side to prevent the secondary side lead from magnetic induction of high voltage, endangering life and road safety.