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In the practical application of high-power power transformers in power systems, the treatment of neutral grounding is an important part of the assembly process of the entire transformer system, which will directly affect the stable operation of the power transformer. In today's article, let's take a look at the different types of power transformers, the depth of which line should be buried is the best, and what form of grounding measures are the most suitable.

I believe that what many technicians know is that to make a grounding system for high-power power transformers dedicated to electric power, the first preparation is to ground the pole, which means that it needs to dig a place that is at least 2 meters deep and long not far from the transformer. A 10-meter trench, and then use G80 galvanized steel pipes 2 meters long to distribute 3 equidistantly along the trench, and then punch down the steel pipe until only the pipe head is exposed at the bottom of the trench. After that, three pipe heads need to be welded with 50x5 galvanized flat iron, then lead out to the ground, backfilled and compacted. After the grounding electrode is made, the ground wire of the high-power power transformer can be connected to the 50*5 galvanized flat iron. In the design process of this grounding electrode, the capacity of the high-power power transformer will determine the size of its grounding wire.

In the power system, the neutral grounding methods of high-power power transformers can usually be divided into two categories. One is the neutral grounding directly or through low impedance grounding. This method is also called large ground current system. The other type is that the neutral point is not grounded, and it is grounded through an arc suppression coil or high impedance. This method is also called a small ground current system. Let's take a look at the choices of power transformers that these two methods are suitable for in practical applications.

In practical applications, for 6-10kV power systems, since the insulation level of the equipment has little effect on the cost of the equipment according to the line voltage consideration, in order to improve the reliability of power supply, the neutral point is generally used without grounding or arc suppression. The way the coil is grounded can meet the needs of the safe operation of the equipment. For the power system of 110kV and above, the main consideration is to reduce the insulation level of the equipment, so in order to simplify the relay protection device, the neutral point is generally directly grounded. And adopt measures such as erecting lightning protection lines and installing automatic reclosing devices on the entire transmission line to improve the reliability of power supply.

For the 20-60kV power system, when the one-phase power transformer is grounded, the capacitive current itself is not very large, and its internal network structure is not very complicated. The increase or decrease of the equipment insulation level has no effect on the cost. It is obvious, so the current industry generally chooses to use the neutral point through the arc suppression coil grounding method. However, the neutral point of the power grid below 1KV is mostly operated in an ungrounded manner. But if it is a 380/220V system, the power transformer needs to adopt a three-phase five-wire system.

The output of the fixed power supply is not absolutely constant. For example, when the stable power supply is working in a regulated state, its output voltage will cause a slight change in the output voltage due to the change of the input voltage or the change of the load. When the stable power supply is in In the constant current working state, the output current will also change slightly due to the change of the input voltage or the change of the load. In order to measure the degree of this change, that is, to measure the stability of a stable power supply, the International Electrotechnical Commission (IEC) introduced the concept of source effect and load effect when formulating a stable power supply standard.

The source effect refers to: only when the effect of the change in the output stability caused by the change in the input quantity is called the source effect. For an ordinary regulated power supply: the input quantity refers to the input voltage, that is, AC 220V power frequency voltage. my country's national standards stipulate that when the input voltage changes by ±10%, the relative percentage of the output voltage change is called the source effect.

For example, for a regulated power supply with an output voltage of 100V, the specific value of the output voltage change caused by the change of the input voltage is as follows:

Input voltage Output voltage

AC220V (standard value) 100V

AC242V (change +10%) 100.1V

AC198V (variation -10%) 99.9V

Source effect of power supply=∣△Uo∣/Uo=∣U-Uo∣/Uo=∣100.1-100∣/100=0.1%

Or=∣△Uo∣/Uo=∣U-Uo∣/Uo=∣99.9-100∣/100=0.1%

It can be seen from the above formula that the source effect of this regulated power supply when its output voltage is 100V is 0.1%, that is, when the input voltage changes by 10%, the output voltage changes by one-thousandth. (Note: When measuring the source effect, the load should remain unchanged).

The load effect refers to: the effect of only changing the output stability due to the change of the load is called the load effect.

Let's take another example: a power supply in a regulated working state, its no-load output voltage is 100V, and the maximum load capacity of the power supply is 5A. The output voltage changes when no-load and full-load are as follows:

Power load current Power output voltage

0A 100V

5A 99.7V

Load effect=∣△Uo∣/Uo=∣U-Uo∣/Uo=∣99.7-100∣/100=0.3%

That is, when the power supply works at 100V, its load effect is 0.3%. (Note: When measuring the load effect, efforts should be made to keep the input voltage at the standard value)

In the above description of source effect and load effect, the output stability is given as an example of stable voltage. In fact, when the power supply is in a constant current working state, the output current is a stable quantity, but in fact, when the input voltage or load changes At the same time, the output current will also undergo some slight changes. This change is the source effect and load effect in the constant current state, which will not be described here.