Distinction of three-phase voltage imbalance

  There are many reasons for the three-phase voltage imbalance, such as single-phase grounding, disconnection resonance, etc., the operation manager can only process it correctly if it is correctly distinguished.

I. Broken wire fault If one phase is disconnected but not grounded, or the circuit breaker and the isolating switch are not connected, the voltage transformer fuse is blown and the three-phase parameters are asymmetrical. When the previous voltage level line is disconnected, the voltage of the next voltage level shows that the three phase voltages are reduced, one of the phases is lower, and the other two phases are higher but the voltage values ​​of the two are close. When the line of this stage is disconnected, the voltage of the disconnected phase is zero, and the voltage of the unbroken phase is still the phase voltage.

2. Ground fault When the line is disconnected and single-phase grounded, although the three-phase voltage is unbalanced, the voltage value after grounding does not change. Single-phase grounding is divided into metallic grounding and non-metallic grounding. Metal grounding, fault phase voltage is zero or close to zero, non-fault phase voltage rises 1.732 times, and lasts forever; non-metallic grounding, ground phase voltage is not zero but decreases to a certain value, the other two phases rise It is less than 1.732 times higher.

Causes of resonance With the rapid development of industry, the nonlinear power load has increased a lot, and some loads not only generate harmonics, but also cause fluctuations and flicker of the supply voltage, and even cause three-phase voltage imbalance.
There are two kinds of three-phase voltage imbalance caused by resonance.

One is the fundamental frequency resonance, the characteristic is similar to single-phase grounding, that is, the voltage of one phase is lowered, and the voltage of the other two phases is increased. It is difficult to find the fault point when looking for the cause of the fault. At this time, the special user can be checked. If it is not grounding, it may be Caused by resonance.

The other is frequency-divided resonance or high-frequency resonance, characterized by a simultaneous increase in three-phase voltage.

In addition, it should be noted that when the air-drop busbar cut-off part line or single-phase ground fault disappears, if a grounding signal occurs, and the one-phase, two-phase or three-phase voltage exceeds the line voltage, the voltmeter pointer hits the head and moves slowly at the same time, or The three-phase voltage alternately rises above the line voltage. In this case, it is generally caused by resonance.
The harm and impact of three-phase imbalance

Harm to the transformer. In the production and living power, when the three-phase load is unbalanced, the transformer is in an asymmetrical operating state. Increased transformer losses (including no-load losses and load losses). According to the transformer operating regulations, the neutral current of the transformer in operation shall not exceed 25% of the rated current of the low-voltage side of the transformer. In addition, the unbalanced operation of the three-phase load will cause the zero-sequence current of the transformer to be too large, and the temperature rise of the local metal parts may even cause the transformer to burn out.

The impact on electrical equipment. The occurrence of a three-phase voltage imbalance will result in several times the current imbalance. The reverse torque is increased in the induction motor, so that the temperature of the motor rises, the efficiency decreases, the energy consumption increases, vibration occurs, and output loss is affected. The imbalance between the phases can lead to shortened service life of the electrical equipment, accelerate the frequency of equipment component replacement, and increase the cost of equipment maintenance. The circuit breaker allows the current margin to decrease, and overload and short circuit are likely to occur when the load changes or alternates. An excessively large unbalanced current flows into the neutral line, causing the neutral line to thicken.

The effect on line loss. Three-phase four-wire system connection method, when the three-phase load balances, the line loss is the smallest; when the one-phase load is heavy, the two-phase load is light, the line loss increment is small; when the one-phase load is heavy, the one-phase load is light, When the load of the third phase is the average load, the line loss increment is large; when the phase load is light, and the load of the two phases is heavy, the line loss increment is the largest. When the three-phase load is unbalanced, the current imbalance is greater and the line loss increment is larger regardless of the load distribution.

Causes of inverter overvoltage

(1) The overvoltage of the breaking transformer is based on the theory of intercepting overvoltage. When the transformer is disconnected, the current in the transformer inductance cannot be abrupt, and the magnetic field energy stored therein oscillates between the transformer magnetizing inductance and the ground capacitance. An overvoltage has occurred.

(2) Overvoltage generated by transformer with load closing In the actual test, the combined no-load transformer has detected several times the overvoltage of the power supply voltage. The physical principle is that the no-load transformer can still be equivalent to a magnetizing inductance and Parallel connection of the equivalent capacitance of the transformer itself. If the neutral point of the transformer is not grounded, the switch is aperiodic closing (one phase or two phase first), due to the inductance of the feeder, the capacitance of the transformer to the ground, the longitudinal capacitance and the inductance of the transformer. As a result, a higher overvoltage is generated, especially in the transformer neutral point overvoltage. Although the transformer is basically loaded with a load, the transformer will also generate an overvoltage when it is loaded with a load, but it is smaller when it is relatively empty. There is a relatively large capacitance in the real load, because the storage of the capacitor does not suddenly increase, and the transmission cable has a distributed capacitance to the ground when transmitting a high frequency oscillating voltage, and these capacitors have an absorption effect on the overvoltage. The combination of the two causes the overvoltage of the transformer during the closing process to be suppressed, but sometimes the value is still high and may even be higher than the withstand voltage of the component, which is very dangerous.

(3) The commutation overvoltage rectifying element of the rectifying element is high in steering, and therefore the steering overvoltage is also high. This not only damages components, but also creates electromagnetic interference.

Frequency converter overvoltage processing method

(1) For the breaking overvoltage of the phase shifting transformer of the frequency converter, the overvoltage absorption circuit is formed by the RC absorption network and the zinc oxide arrester, and good results are obtained.

(2) For the overvoltage generated by the transformer with load closing, a switch with good cycle performance can be selected (the switch will have different periods after long-term operation); a good resistance-capacitance absorption circuit or active suppressor technical solution is adopted; The shielded transformer can also effectively suppress the closing overvoltage. However, the difficulty of making a high-power transformer in the formation of an electrostatic shielding layer will be considerable.

(3) For the overvoltage generated by the commutation of the rectifying element, the point of attention is: the reverse withstand voltage of the rectifying element is sufficient, and the second is that the absorption circuit and the freewheeling circuit must be properly controlled. Otherwise, the rectifier device may be broken down by an overvoltage. (4) Since the overvoltage during the operation of the inverter is basically generated when the transformer is opened, it is necessary to start from the transformer to find a way to suppress the overvoltage of the inverter. Can be used:

1 Increase the magnetizing inductance of the transformer and the capacitance to the ground, increase the excitation inductance to reduce the no-load current, which will increase the cost of the transformer.

2 increase the capacitance of the transformer to the ground: in principle, it is easy to analyze, but in fact, due to the structure and material limitations of the transformer itself, it is impossible to make a transformer with any insulation method or high insulation level, so it is necessary to increase it greatly. The capacitance to ground C of the transformer is also quite difficult.

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