Does a long cable increase or decrease the voltage at the end?

乱世煮酒论天下

Does a long cable increase or decrease the voltage at the end? [Copy link]

 

I read an introduction, part of which is as follows:

Long-distance transmission cable problem: The distance between the inverter and the motor is more than 150 meters. The product is not equipped with an output inductor. Distributed capacitance will be formed between the three wires during long-distance transmission, which will cause the motor terminal voltage to increase. The product harmonic voltage and harmonic current will have certain adverse effects on the motor and IGBT. The specific impact can be further tested and verified. Solution: Add an output inductor.

When the cable is long, the distributed capacitance between the cables and between the cables and the ground will increase the terminal voltage. According to the equivalent circuit of the cable, it is approximately an RC low-pass filter. A cable that is too long will cause the terminal voltage to decrease, which contradicts the above statement. Adding an output inductor can offset part of the influence of the parasitic capacitance. I wonder if my understanding is correct?

led2015

In some cases, the existence of distributed capacitance may cause distortion of the voltage waveform, resulting in voltage increase at certain frequency points.

乱世煮酒论天下

led2015 published on 2024-9-8 22:30 In some cases, due to the existence of distributed capacitance, the voltage waveform may be distorted, and the voltage may rise at certain frequency points.

Is it that at certain frequencies, the parasitic inductance and capacitance of the cable resonate, and only in this way will the terminal voltage increase?

Can you elaborate on this in detail? What is the general frequency point? Is the purpose of adding an output reactor to reduce the terminal voltage or to increase the terminal voltage?

maychang

【I don’t know if my understanding is correct? 】

This is completely wrong. The mistake is in the sentence [According to the equivalent circuit of the cable, it is approximately an RC low-pass filter].

maychang

Power transmission and distribution is a profession that involves a lot of content.

The power industry does not allow terminal loads to be capacitive at all, only inductive. In the power industry, there is a "reactive power compensation" that increases the terminal voltage when the power factor is too low. The method is to connect capacitors in parallel at both ends of the load, or connect a phase regulator in parallel.

maychang

Luan Shi Zhu Jiu Lun Tian Xia published on 2024-9-8 23:15 Is it that at certain frequency points, the parasitic inductance and parasitic capacitance of the cable resonate, and only in this way will the terminal voltage increase? Can you elaborate...

The output reactor you mentioned is only when the inverter is connected to the motor by a cable and the cable is very long. In this case, the cable is capacitive, and the series reactor can offset the capacitance of the cable and increase the voltage at the motor end.

maychang

Cables and overhead lines have different characteristics. Overhead lines have small distributed capacitance and large distributed inductance, while cables have large distributed capacitance and small distributed inductance. This is because overhead lines have large distances between wires (small capacitance) and large surrounding areas (large inductance), while cables have small distances between wires (large capacitance) and small surrounding areas (small inductance).

乱世煮酒论天下

maychang posted on 2024-9-9 08:07 The characteristics of cables and overhead lines are different. The distributed capacitance of overhead lines is small and the distributed inductance is large, while the distributed capacitance of cables is large and the distributed inductance is small. This is because the line of overhead lines...

What I mean is, should the terminal voltage caused by the parasitic capacitance of the long-distance wiring of this motor be reduced or increased? Will the series inductor lead to the effect of offsetting the capacitance?

Also, why doesn't the power system allow loads to be capacitive?

maychang

Luan Shi Zhu Jiu Lun Tian Xia published on 2024-9-9 08:35 What I mean is that the terminal voltage caused by the parasitic capacitance of the long-distance wiring of this motor should be reduced or increased? Will the series reactor cause the offset voltage...

[Will the series reactor cause the effect of offsetting the capacitor? ]

The series reactor is used to offset the influence of the distributed capacitance of long cables.

maychang

Luan Shi Zhu Jiu Lun Tian Xia published on 2024-9-9 08:35 What I mean is that the terminal voltage caused by the parasitic capacitance of the long-distance wiring of this motor should be reduced or increased? Will the series reactor cause the offset voltage...

[Also why does the power system not allow the load to be capacitive? ]

Because generators, transformers, overhead lines... are all inductive.

For a detailed explanation, you need to read the textbooks on power systems and power transmission to understand the difference between active power and reactive power. There must be a certain amount of reactive power in the transmission line, but too much reactive power is not good, as it will reduce efficiency.

led2015

Impact of capacitive load on power system

1. Voltage increase : When there are too many capacitive loads in the system, it will cause excess reactive power in the system, which will cause the system voltage to increase. Excessive voltage may cause damage to power equipment and user equipment, and even cause safety accidents.

2. Resonance risk : Under certain conditions, the capacitive load and inductive load in the system may resonate, resulting in abnormal fluctuations in system voltage and current. Resonance not only affects the stability of the power system, but may also cause damage to the equipment.

3. Reactive power compensation problem : Although capacitive loads can be used as reactive power compensation devices, overcompensation or undercompensation will have an adverse effect on the system. Overcompensation will cause the system voltage to increase, while undercompensation will not be able to meet the system's demand for reactive power.

乱世煮酒论天下

led2015 published on 2024-9-9 20:08 The impact of capacitive load on power system Voltage increase: When there are too many capacitive loads in the system, it will cause the system reactive power to be excessive...

Can you briefly explain from a circuit perspective why capacitive loads increase voltage?