What is an Inrush Current?

Surge current refers to the current characteristics of electrical equipment at the moment of being turned on. It is important for the safety of power supply networks and electrical equipment. In engineering, it is usually necessary to suppress the inrush current.

Inrush current

Surge current refers to the current characteristics of electrical equipment at the moment of being turned on. It is important for the safety of power supply networks and electrical equipment. In engineering, it is usually necessary to suppress the inrush current.

Chinese name

Inrush current

Foreign name

surge current

Basic explanation

Current characteristics of electrical equipment at the moment of switching on

Classification

Discharge gap, gas discharge tube, etc.

Inrush current refers to the

1. Discharge gap (also known as protection gap):

It generally consists of two metal rods that are exposed to the air with a certain gap. One of the metal rods is connected to the power phase line L1 or the neutral line (N) of the equipment to be protected, and the other metal rod is connected to the ground line ( PE) phase connection. When the transient overvoltage hits, the gap is broken down, and a part of the overvoltage charge is introduced into the ground, which prevents the voltage on the protected equipment from rising. The distance between the two metal rods of such a discharge gap can be adjusted as required, and the structure is relatively simple. Its disadvantage is poor arc extinguishing performance. The improved discharge gap is an angular gap. Its arc extinguishing function is better than the former. It is extinguished by the electric force F of the circuit and the rise of the hot air flow.

2. Gas discharge tube:

It consists of a pair of cold cathode plates separated from each other and enclosed in a glass tube or ceramic tube filled with a certain inert gas (Ar). In order to increase the triggering probability of the discharge tube, a trigger agent is also provided in the discharge tube. There are two types of gas-filled discharge tubes, and three-pole types. The technical parameters of gas discharge tubes are: DC discharge voltage Udc; impulse discharge voltage Up (Under normal circumstances Up (2 to 3) Udc; power frequency The current is In; the current Ip is shock; the insulation resistance R (> 109); the inter-electrode capacitance (1-5PF) gas discharge tube can be used under DC and AC conditions. The DC discharge voltages Udc selected are as follows: Use under DC conditions: Udc 1.8U0 (U0 is the DC voltage for normal line operation); Use under AC conditions: U dc 1.44Un (Un is the effective value of AC voltage for line normal operation).

3. Varistor:

It is a metal oxide semiconductor non-linear resistor based on ZnO. When the voltage across the resistor reaches a certain value, the resistance is very sensitive to voltage. Its working principle is equivalent to the series-parallel connection of multiple semiconductor PNs. The varistor is characterized by good non-linear characteristics (I = non-linear coefficient in CU), large current capacity (~ 2KA / cm2), small normal leakage current (10-7 ~ 10-6A), and low residual voltage (depending on Due to the working voltage and current capacity of the varistor), the response time to instantaneous overvoltage is fast (~ 10-8s), and there is no continuous current.

The technical parameters of the varistor mainly include: varistor voltage (ie switching voltage) UN, reference voltage Ulma; residual voltage Urs; residual voltage ratio K (K = Ures / UN); maximum current capacity Imax; leakage current; response time .

The use conditions of the varistor are: varistor voltage: UN [(2 × 1.2) /0.7] U0 (U0 is the rated voltage of the power frequency power supply)

Minimum reference voltage: Ulma (1.8 to 2) Uac (used under DC conditions)

Ulma (2.2 2.5) Uac (used under AC conditions, Uac is the AC working voltage of the protected circuit)

The maximum reference voltage of the varistor should be determined by the withstand voltage of the protected electronic equipment. The residual voltage of the varistor should be lower than the damaged voltage level of the protected electronic equipment, that is (Ulma) max Ub / K. Where K is the residual voltage ratio and Ub is the loss voltage of the protected equipment.

4. Suppression diode:

The suppression diode has a clamping voltage limiting function. It works in the reverse breakdown region. Because of its advantages such as low clamping voltage and fast response, it is particularly suitable for use as the last few protection elements in multi-level protection circuits. The volt-ampere characteristic of the suppression diode in the breakdown region can be expressed by the following formula: I = CU, where is a non-linear coefficient, for Zener diode = 7-9, and in avalanche diode = 5-7.

The technical parameters of the suppression diode are mainly

(1) Rated breakdown voltage, which refers to the breakdown voltage at the specified reverse breakdown current (usually lma), which is generally in the range of 2.9V to 4.7V for Zener diodes, and avalanche The rated breakdown voltage of a diode is usually in the range of 5.6V ~ 200V.

(2) Maximum clamping voltage: It refers to the highest voltage appearing across the tube when it passes a large current with a specified waveform.

(3) Pulse power: It refers to the product of the maximum clamping voltage across the tube and the equivalent value of the current in the tube under a specified current waveform (such as 10/100 s).

(4) Reverse displacement voltage: It refers to the maximum voltage that can be applied to the two ends of the tube in the reverse leakage area. Under this voltage, the tube should not break down. This reverse displacement voltage should be significantly higher than the highest operating voltage peak of the protected electronic system, that is, it cannot be in a weak conduction state during normal system operation.

(5) Maximum leakage current: It refers to the maximum reverse current flowing in the tube under the effect of reverse displacement voltage.

(6) Response time: 10-11s

5. Choke coil: The choke coil is a common mode interference suppression device with ferrite as the core. It is symmetrically wound around the same ferrite ring core with two coils of the same size and the same number of turns. On the other hand, to form a four-terminal device, it is necessary to suppress the large inductance of the common mode signal, and it has little effect on the leakage inductance of the differential mode signal. The use of choke coils in balanced lines can effectively suppress common mode interference signals (such as lightning interference) without affecting the normal mode differential signal transmission.

The choke coil should meet the following requirements during production:

1) The wires wound on the core of the coil should be insulated from each other to ensure that no breakdown short circuit occurs between turns of the coil under the action of transient overvoltage.

2) When instantaneous high current flows through the coil, do not saturate the core.

3) The magnetic core in the coil should be insulated from the coil to prevent breakdown between the two under transient overvoltage.

4) The coil should be wound in a single layer as much as possible. This can reduce the parasitic capacitance of the coil and enhance the coil's ability to withstand transient overvoltages.

6. 1/4 wavelength short circuit

1/4 wavelength short circuit is a microwave signal surge protector based on the spectrum analysis of lightning waves and the standing wave theory of antenna feeders. The length of the metal short bar in this protector is based on the working signal frequency (such as 900MHZ or 1800MHZ). ) To determine the size of the 1/4 wavelength. For the working signal frequency, the length of the parallel short-circuit bar has infinite impedance, which is equivalent to an open circuit, and does not affect the transmission of the signal. However, for lightning waves, because the lightning energy is mainly distributed below n + KHZ, the short-circuit bar For the lightning wave impedance is very small, which is equivalent to a short circuit, and the lightning energy level is leaked into the ground.

The diameter of the 1/4 wavelength short-circuiting rod is generally a few millimeters, so it has good inrush current resistance, which can reach more than 30KA (8 / 20s), and the residual voltage is very small. This residual voltage is mainly caused by the inductance of the shorting rod The disadvantage is that the industrial frequency band is narrow, and the bandwidth is about 2% to 20%. Another disadvantage is that it cannot add DC bias to the antenna feed facility, which limits some applications.

surge