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In addition to electromagnetic catapults, the 003 medium-voltage DC integrated power system is at least a decade ahead of the Ford class

author:Global Archives

Long text warning: the whole dry goods, I believe that after you read it, you will have a clearer understanding of the advantages and disadvantages of the mainland's medium-voltage DC power system and the United States' medium-voltage AC power system.

On June 8, 2020, USNI News (U.S. Naval Research Institute News Network) reported that the electromagnetic catapult system of the Ford aircraft carrier had a serious failure, resulting in the aircraft carrier being unable to eject the aircraft within 5 days, and the failure probability of the ford-class aircraft carrier electromagnetic catapult design was 1/4000, but in the test phase, there were more than 2,000 unmanned ejection tests, the probability of failure was as high as 181, and the successful ejection of F/A-18E began in 2010 to 2018. In total, more than 700 official ejections of fighter jets were carried out, 10 serious failures were encountered, and other problems caused by aircraft carrier nests occurred from time to time.

In addition to electromagnetic catapults, the 003 medium-voltage DC integrated power system is at least a decade ahead of the Ford class

The 003 aircraft carrier Fujian ship that we have just launched has been ejected more than 3,000 times in a row. As a latecomer in the field of electromagnetic catapults, how did China achieve the goal of being a latecomer and at least a decade ahead of the United States in this field?

The basic reason is the gap between the ship's integrated power system, the United States adopts the first generation of integrated power system with medium voltage AC grid connection, while our country directly crosses generations and adopts the second generation integrated power system of medium voltage DC grid connection. AC DC, word difference, will there really be such a huge gap in reality?

There are many articles on the Internet about the "medium voltage DC integrated power system", some of which are relatively shallow, so that people feel that China is very cattle after reading it, but where are the cattle? I don't know. Some say it too deeply, and various technical terms directly pass a large number of readers.

So the curator decided to write an article to explain this matter as clearly as possible with junior high school knowledge.

First of all, what is electricity? What is magnetism?

A stationary charge produces an electric field, which repels the same sex and attracts the opposite sex. The essence of electricity is the movement of the charge.

In addition to electromagnetic catapults, the 003 medium-voltage DC integrated power system is at least a decade ahead of the Ford class

Suppose there is a wire placed horizontally, with the current direction to the left, and a positive charge placed below the wire, will this positive charge be attracted to the wire or repulsed?

Although both positive and negative charges are moving in the wire, the total amount remains the same, so the wire remains neutral and does not have any effect on the stationary charge next to it, so the answer is neither attracting nor repelling.

The following question escalates: If this positive charge moves in the opposite direction of the current, will it be attracted to the wire or repulsed?

According to special relativity, when the observer is in a low-speed reference frame, objects moving at high speeds are compressed; When located in a high-speed reference frame, objects moving at low speeds are stretched.

Assuming that at this point we are moving in sync with the negative charge, we will observe that the positive charge in the wire is moving in the opposite direction and is therefore compressed, and the number of positive charges per unit length exceeds the number of negative charges.

In addition to electromagnetic catapults, the 003 medium-voltage DC integrated power system is at least a decade ahead of the Ford class

Then the magic happens: although the wire as a whole is neutral, in the kinematic frame of reference, each small segment of the wire carries a positive charge, and if the mover carries a positive charge, it will be repelled by the wire; If the athlete carries a negative charge, he will be attracted to the wire. This magical phenomenon is called magnetism.

In addition to electromagnetic catapults, the 003 medium-voltage DC integrated power system is at least a decade ahead of the Ford class

An induced magnetic field is generated around the energized straight wire, and according to the right-hand spiral rule, the direction of the thumb is the direction of the current, and the direction in which the remaining four fingers surround is the direction of the magnetic induction line.

In addition to electromagnetic catapults, the 003 medium-voltage DC integrated power system is at least a decade ahead of the Ford class

Assuming that the current direction in the wire is to the left, then the magnetic induction line above the wire enters the mobile phone screen, we use "fork" to represent, the magnetic line below the conductor is through the mobile phone screen, we use "point" to represent.

In addition to electromagnetic catapults, the 003 medium-voltage DC integrated power system is at least a decade ahead of the Ford class

The charge moving in the magnetic field will be subject to the Lorentz force, according to the left hand rule, we let the magnetic induction line pass through the palm of the left hand, the direction of the four fingers is the direction of the positive charge movement, the direction of the thumb is the direction of the force, when we put an energized wire into the magnetic field, the combined force of the Lorentz force of the countless charges is the ampere force.

In addition to electromagnetic catapults, the 003 medium-voltage DC integrated power system is at least a decade ahead of the Ford class

The above is also the basic principle of the orbital electromagnetic gun, the upper and lower two rails are through the current direction opposite the current, the shell itself is also a conductor, so the current from top to bottom through the shell to form a circuit, according to the right hand spiral rule, the middle part of the wire magnetic field direction is into the mobile phone screen, according to the left hand can determine the direction of the amperometric force of the shell to the right, amperometric size F = ILBsin (I, B), where I is the current strength, L is the width of the shell, B is the magnetic field strength, sin (I, B) is the angle between the magnetic field and the current, in this case the magnetic field and the current angle is 90 degrees, so the sin(I,B) = 1.

It can be seen that the greater the input current, the stronger the induced magnetic field strength, the greater the ampere force received, and the longer the guide rail, the longer the acceleration time of the ampero force on the shell, when the instantaneous power is large enough, the 20KG shell can be accelerated to more than 4000 m / s, according to the kinetic energy formula: Ek = mv^2/2 Can calculate that the kinetic energy carried by the electromagnetic shell is 160 million joules, which is about equal to 40 kg TNT equivalent, equivalent to a C-801 anti-ship missile. The electromagnetic railgun in the Continental Test has completed a 400-kilometer launch test.

What is this concept? A 20 kg solid iron lump, carrying the kinetic energy of an anti-ship missile, at a speed of mach 10 or more to carry out an unquestionable over-the-horizon strike at you, you call this thing "naval gun"?

Let's calculate the cost of firing an electromagnetic shell, the energy of one degree of electricity is equivalent to 3600000 joules, the electromagnetic cannon is accelerated to 160 million joules of energy consumption of 44.4 degrees, plus the loss of energy and conversion efficiency, we give a discount even if 100 degrees of electricity, in 2022, Beijing residents use electricity ladder The most expensive third gear of 7 mao 8 degrees of electricity, we generously calculate 1 yuan a degree, the cost of electricity consumption is 100 yuan, the price of steel is about 4000 ~ 5000 yuan a ton, Even if we pay 5 yuan for 1 kilogram, the cost of 20 kilogram shells is 100 yuan, so the total cost of firing an electromagnetic shell is a huge amount of 200 yuan! Look at the tens of millions of dollars of missiles, sigh, what a fragrance!

What is electromagnetic induction?

In addition to electromagnetic catapults, the 003 medium-voltage DC integrated power system is at least a decade ahead of the Ford class

There is a pair of magnetic poles, the magnetic induction line starts from the N pole on the left side, to the end of the S pole on the right, the wire in the magnetic field to do the movement of the magnetic induction line will produce an induced electromotive force, if a loop is formed, it will form an inductive current, according to the right hand rule, let the magnetic induction line pass through the palm of the right hand, the thumb direction is the direction of the wire movement, and the other four finger directions are the direction of the induced current.

In addition to electromagnetic catapults, the 003 medium-voltage DC integrated power system is at least a decade ahead of the Ford class

As shown in the figure, if the wire moves upwards, the direction of the induced current is a to b, and if the wire is moving downward, the direction of the induced current is from b to a.

For ease of understanding, the curator drew only 5 magnetic lines.

Now we place a toroidal wire in the middle of the magnetic field, rotate it counterclockwise, and you can see that the number of magnetic lines crossing the wire coil is also different depending on the rotation angle of the wire:

In addition to electromagnetic catapults, the 003 medium-voltage DC integrated power system is at least a decade ahead of the Ford class

When the coil is parallel to the magnetic induction line, no magnetic induction line passes through the coil;

In addition to electromagnetic catapults, the 003 medium-voltage DC integrated power system is at least a decade ahead of the Ford class

When the coil is at a 45-degree angle between the magnetic induction line, 3 magnetic induction lines pass through the coil;

In addition to electromagnetic catapults, the 003 medium-voltage DC integrated power system is at least a decade ahead of the Ford class

When the coil is perpendicular to the magnetic induction line, all 5 magnetic induction lines pass through the coil;

We can understand the number of magnetic induction lines through the coil as magnetic flux, if the magnetic flux has changed, then it will generate an inductive electromotive force in the wire, if the wire is connected into a loop, it will form an inductive current, which is the basic principle of electromagnetic induction.

What is AC? What is DC?

Some readers will say, if I hold the coil and continue to rotate in the magnetic field, then the magnetic flux of the coil will continue to change, will there be a continuous current in the wire? Congratulations, you invented a hand-cranked generator.

Corresponding to the performance of the generator output current, the size and direction of the current are constantly changing, presented as a sinusoidal curve, we call this current direction periodically changing with time as alternating current (ABAC). In contrast, the direction of the current that is always the same is called direct current (DC for short).

In addition to electromagnetic catapults, the 003 medium-voltage DC integrated power system is at least a decade ahead of the Ford class

What happens if we put three sets of two-by-two coils in a magnetic field that differ by 120 degrees?

Here is a simplified circuit diagram:

In addition to electromagnetic catapults, the 003 medium-voltage DC integrated power system is at least a decade ahead of the Ford class

The three sets of coils are G1, G2, G3, each set of coils lead out of two wires, and now the curator asks readers a question, which line will be electrocuted, which line will not be electrocuted?

The answer is published below, because these three sets of coils are constantly outputting alternating current, and there is a voltage between each wire and the earth, so no matter which wire you touch, you will be electrocuted.

Here's the problem escalation: What happens if you catch a1, a2, a3, or b1, b2, b3 at the same time?

Let's not rush to announce the answer, let's first see what the three sets of sine curves with a phase difference of 120 degrees look like when superimposed.

In addition to electromagnetic catapults, the 003 medium-voltage DC integrated power system is at least a decade ahead of the Ford class

Let's look for two special phases to calculate:

At 0 degrees, sin0°+sin120°+sin240°=0+0.866-0.866=0;

At 90 degrees, sin90°+sin210°+sin330°=1-0.5-0.5=0;

In fact, after we substitute any angle into the calculation, the sum of the three sine values with a difference of 120 degrees is 0, that is to say, the voltage difference between the three lines of a1, a2, a3 or b1, b2, and b3 is 0, so we will not be electrocuted if we grasp the above three lines at the same time. Since the voltage difference between a1, a2, and a3 is 0, we can intersect these three wires at point O and lead to a wire a.

In addition to electromagnetic catapults, the 003 medium-voltage DC integrated power system is at least a decade ahead of the Ford class

This kind of three sets of phase two two two difference of 120 degrees of the generator, called three-phase generator, where b1, b2, b3 three lines are called phase line or fire wire, O point because there is no voltage difference, so called neutral point, from the neutral point of the lead of the wire a is called neutral line, if the neutral line grounded, then it is called the zero line.

In addition to electromagnetic catapults, the 003 medium-voltage DC integrated power system is at least a decade ahead of the Ford class
In addition to electromagnetic catapults, the 003 medium-voltage DC integrated power system is at least a decade ahead of the Ford class

We often use this kind of three-phase alternating current in our lives, and you can open your own meter box, or go to the community to see if the telephone poles in the household are all three fires and one zero of four lines?

We often say that the voltage at home is 220 volts, which means that the voltage between any fire wire and the zero line is 220 volts, but the voltage between the two fire wires is 380 volts. Friendly reminder, do not read the curator's article after the hand owes to touch the zero line! The zero line is only theoretically 0 potential, once the contact or leakage of electricity is still possible to be charged, life is only once, and it is ok and cherished.

So is there any way to turn the alternating current output by the generator into direct current?

There are ways to do it, but this time we need some additional knowledge reserves.

The movement of free electrons will form an electric current, and the outermost number of electrons in the atom is closer to 8, the less likely it is to conduct electricity, such as inert gas elements; The fewer the number of electrons in the outermost layer, the easier it is to conduct electricity, such as metal elements. Elements like silicon and germanium, which have 4 outermost electrons, are between conductive and non-conductive elements call semiconductors.

In addition to electromagnetic catapults, the 003 medium-voltage DC integrated power system is at least a decade ahead of the Ford class

Taking silicon as an example, each silicon atom shares the outermost electron with four adjacent silicon atoms, satisfying the steady state of the outermost electron number 8.

In addition to electromagnetic catapults, the 003 medium-voltage DC integrated power system is at least a decade ahead of the Ford class

If we dop a silicon atomic clock with a portion of the outermost electron number of 3, such as aluminum or boron.

In addition to electromagnetic catapults, the 003 medium-voltage DC integrated power system is at least a decade ahead of the Ford class

After they bind to the surrounding silicon atoms, the outermost layer has only 7 electrons, forming a hole that can conduct electricity, which we call p-type semiconductors.

In addition to electromagnetic catapults, the 003 medium-voltage DC integrated power system is at least a decade ahead of the Ford class

If the outermost electron number of the element we dop is 5, such as phosphorus or antimony.

In addition to electromagnetic catapults, the 003 medium-voltage DC integrated power system is at least a decade ahead of the Ford class

After binding with the surrounding silicon atoms, there is an extra free electron, and these free electrons can also conduct electricity, which we call N-type semiconductors.

In addition to electromagnetic catapults, the 003 medium-voltage DC integrated power system is at least a decade ahead of the Ford class

But if we stitch together P-type and N-type semiconductors, something magical happens, and free electrons on the material contact surface fill the holes on the other side. So that the part where the two materials touch has neither holes nor free electrons, you can think of it as a non-conductive insulator, we call it a PN junction, the side of the PN junction close to the P type carries a negative electricity due to the absorption of electrons, the side close to the N type carries positive electricity due to the loss of electrons, so the PN junction will form an internal electric field pointed to the P type by the N type, and this internal electric field will repel both the holes of the P type semiconductor and the electrons of the N type semiconductor, thus preventing the further expansion of the PN junction. When the PN junction is stable, the strength of the internal electric field is 0.7 volts.

In addition to electromagnetic catapults, the 003 medium-voltage DC integrated power system is at least a decade ahead of the Ford class

If we put this combined semiconductor material into a powered circuit, P-type connection power positive, N type connected to the power supply negative, then it is equivalent to applying an external electric field opposite to the direction of the internal electric field of the PN junction, when the external electric field strength is greater than 0.7 volts, the PN junction disappears, the holes and free electrons in the semiconductor can move freely, and the current can pass smoothly.

In addition to electromagnetic catapults, the 003 medium-voltage DC integrated power system is at least a decade ahead of the Ford class

If the direction of the external electric field is the same as the direction of the internal electric field, it is equivalent to strengthening the internal electric field, and the PN junction range will be expanded, further preventing the passage of current. This kind of electronic device that allows only one direction of current to pass through is called a diode.

In addition to electromagnetic catapults, the 003 medium-voltage DC integrated power system is at least a decade ahead of the Ford class

We use this symbol instead of the diode, and the direction of the arrow indicates the direction of the current allowed to pass.

If we connect a diode in the output circuit of alternating current, then the alternating current can only pass through the same direction as the diode direction, and the current in the other direction is filtered out, so we get an intermittent direct current, which is called half-wave rectification.

Of course, this rectification method of directly cutting off half of it is too rough and wastes half of the energy, is there a way to use the other half of the alternating current? There is still a way, we can use four diodes combined into the circuit shown on the screen:

In addition to electromagnetic catapults, the 003 medium-voltage DC integrated power system is at least a decade ahead of the Ford class

When the current enters from point a, its path is like this.

In addition to electromagnetic catapults, the 003 medium-voltage DC integrated power system is at least a decade ahead of the Ford class

When the current enters from point b, its path is like this.

In addition to electromagnetic catapults, the 003 medium-voltage DC integrated power system is at least a decade ahead of the Ford class

If we block the diode part.

In addition to electromagnetic catapults, the 003 medium-voltage DC integrated power system is at least a decade ahead of the Ford class

It can be seen that the current output of the generator is always a complete direct current in one direction, and we call this rectification method full-wave rectification. The above figure is the bridge rectifier circuit diagram.

In summary, no matter which method is used to obtain DC, from the simplest principle we can see that DC power is at least one commutation or rectification step to increase the alternating or rectifier than alternating current, and in practice, the production cost of DC generators is also higher than that of alternators.

Direct current and alternating current love each other

In addition to electromagnetic catapults, the 003 medium-voltage DC integrated power system is at least a decade ahead of the Ford class

In history, there was a bloody war between DC and AC, when Edison stood up for DC and Tesla was a fan of AC.

It is said that the gods fight the common people eat melons, "electricity" this kind of fresh stuff who are the first time to see, the two big guys have supporters, regardless of victory or defeat, at that time in New York, the wires are not doing any protective measures horizontally and vertically exposed directly on top of people's heads, there are AC and DC, people do not know the danger of electricity, there is a hand owes to touch it, after touching the experience is not the same, some people are instant heartbeat feelings, some people are endlessly eager to die, some people are dying on the spot, Edison and Tesla, the two big men, naturally know the mystery of this:

Compared with the wire, the human resistance is very large, can be seen as a capacitor, students who have learned the circuit know that the capacitor has the characteristics of "passing ac resistance DC", dc current will not pass through the capacitor, although there is no current in the AC circuit really passes through the capacitor, but it will continue to "charge" the capacitor - "discharge" the process, so people who touch the DC are basically fine, and those who touch the ALTERNA are shocked.

But why do some people want to die, and some people die on the spot? Because the human constitution is different, according to Ohm's law, the voltage is equal to the current multiplied by the resistance (U = IR), in the case of a certain voltage, the greater the resistance, the smaller the current, when the current exceeds 10 mA it will be life-threatening, because the cross-section of the fat man is larger than the thin man, the water ratio is less than the thin man, so the resistance is smaller, so it is easier to be electrocuted. (Just kidding ha, human resistance and many factors are related, interested students can use a multimeter to measure your resistance is, but don't touch the wire!) )

In addition to electromagnetic catapults, the 003 medium-voltage DC integrated power system is at least a decade ahead of the Ford class

When Edison saw that someone had been electrocuted, he immediately thought that this was the scourge of alternating current, and how could the big gifts sent by competitors not be received? So he made a far-reaching invention: the electric chair. When the electric chair is connected to direct current, the people on the electric chair talk and laugh freely, and when the alternating current is turned on, not only will people burp immediately, but even the strong elephant will not be able to resist a few rounds. He wanted to use this to show the public the dangers of alternating current and let people turn to the arms of direct current, but unfortunately, the invention of the electric chair eventually deviated and became a torture device for execution.

In addition to electromagnetic catapults, the 003 medium-voltage DC integrated power system is at least a decade ahead of the Ford class

In the movie Green Mile Miracle, in order to reduce the pain of death row inmates, the hair on the top of the death row inmate's head will be shaved off, and the liquid that is easier to conduct electricity will be poured to reduce the resistance to increase the current, so that the prisoner will die instantly, and then the neurotic prison guard will not water the prisoner's head, resulting in the prisoners being roasted and not dead, I believe it has become a lingering nightmare for many people.

But Edison this trick belongs to the practice of killing a thousand self-damage eight hundred, we look back at this formula "U = IR", although the larger the resistance, the smaller the current, but to ensure that the current is less than 10 mA, the voltage also needs to have an upper limit, the continental standard stipulates that the maximum safe voltage of direct current is 120 volts, more than this voltage, direct current can also be electrocuted, so small partners do not feel that DC electricity does not die because of reading the curator's article, do not have the idea of touching the electric door to death.

For ordinary people, since DC AC can electrocute people, then who I choose is not the same, is it difficult for me to measure the voltage before electrocution?

The real reason why alternating current beats dc is "Money", just now we said that dc generators are more expensive to produce than alternating currents, and another important reason is that in that era, the transmission cost of direct current was also higher than that of alternating current.

First of all, let's look at the resistance power formula: power P is equal to the square of the current I multiplied by the resistance R (P = I^2R), although the resistance of the wire is very low, but when we need to carry out long-distance transmission, the wire must be very long, the longer the wire, the greater the resistance, so the farther the transmission distance, the greater the power lost on the wire.

How to reduce the power loss? The first is to reduce the resistance of the wire, which is no difference between AC and DC, the second is to reduce the current, how to reduce the current, according to P = UI, the voltage and the current are inversely proportional, the larger the voltage, the smaller the current.

Therefore, increasing the voltage can effectively reduce the power loss in the transmission process, and the long-distance transmission lines we see in daily life are high-voltage transmission. When it comes to the difficulty of increasing the voltage, there is a world of difference between direct current and alternating current.

First of all, alternating current, according to the principle of electromagnetic induction, the changing current can produce a changing magnetic field, and the changing magnetic field can produce an induced electric field.

This is an ideal transformer:

In addition to electromagnetic catapults, the 003 medium-voltage DC integrated power system is at least a decade ahead of the Ford class

On the left is the primary coil that inputs alternating current, which is spared n1 turns on the core, and on the right is the auxiliary coil that generates the induced current, which spares the n2 turns on the core. In the case of a loop formed, the induced electromotive force E, equal to the voltage U, equal to the number of turns n of the coil multiplied by the rate of change of the magnetic flux k, because the core binds all the magnetic inductance lines, so the flux rate of change in the two sets of coils is equal, and the voltage ratio in the left and right circuits is equal to the turn ratio of the coil. Therefore, the method of increasing the AC voltage is also very simple, and it is OK to wind the transformer sub-coil a few turns.

In contrast, DC boost is much more troublesome. Therefore, dc power, both infrastructure costs and transmission costs, are lost to alternating current, so it withdrew from the stage of history, and eventually AC became popular.

Let's put aside the cost, in fact, dc and alternating current do not have the problem of who is good and who is bad, they have their own suitable scenarios, the curator first asked you a question, our computers and mobile phones use direct current or alternating current?

There are countless transistors in the chips of computers and mobile phones, representing 0s and 1s with power and power, respectively.

In addition to electromagnetic catapults, the 003 medium-voltage DC integrated power system is at least a decade ahead of the Ford class

Let's look at the AC waveform diagram, there are two zeros with an instantaneous current of 0 in a cycle, and when we need a switch to always be energized to represent 1, this sudden zero point will lead to an error. Both the computer and the mobile phone are used after the 220-volt household AC transformer is rectified to a low-voltage DIRECT CURRENT.

However, the seeming "disadvantage" of the periodic zero point of alternating current can play a huge role in some occasions:

When we disconnect the circuit with a switch in the atmosphere, as long as the voltage exceeds 12 to 20 volts, the current exceeds 0.25'1 amperes, an arc will be generated in the contact gap, and the arc can be disconnected until the arc is completely extinguished, but in the atmosphere, only 10 volts of voltage is needed to maintain a stable combustion of 1 cm long arc without extinguishing, when the current of only 5A is opened at 100 kV voltage, the arc length can reach 7 meters, so it is impossible to extinguish the arc by lengthening the length of the arc, and the direct current is very stable due to the current , can continuously provide energy to the arc, so it is very difficult to cut off the high voltage of the direct current.

In addition to electromagnetic catapults, the 003 medium-voltage DC integrated power system is at least a decade ahead of the Ford class

To show you the arc formed when the DC power is cut off, this sour feeling is simply amazing.

We once again sacrifice the AC waveform diagram, we disconnect the AC at any time, just wait for tens of milliseconds, we can wait until there is no current at the zero point, the arc is extinguished instantaneously.

In addition to electromagnetic catapults, the 003 medium-voltage DC integrated power system is at least a decade ahead of the Ford class

To show you the arc (almost flashing) formed when the AC is cut off, is it much gentler than direct current?

The above two examples are to tell you that AC and DC have different use scenarios in daily life, and it is only right to learn from each other's strengths.

UHVDC transmission

In addition to electromagnetic catapults, the 003 medium-voltage DC integrated power system is at least a decade ahead of the Ford class

Perhaps some readers have seen in the news that our country is vigorously developing UHVDC transmission, and the curator has just said that the transmission cost of DIRECT CURRENT is higher than that of ALTERNA, why should China develop UHVDC transmission?

To answer this question, you need to add several knowledge points:

First, how does direct current convert alternating current?

There are generally three methods:

1, with a DC power supply to drive the DC motor, the mechanical energy is transmitted to the alternator and then emit alternating current, this method slightly takes off the pants and farts, but now there are still people in use, characterized by low cost, easy maintenance. It is still used in high-power conversion;

2, mechanical oscillator converter, the principle is to make the DC current intermittent, through the transformer can be in the transformer secondary output ALTERN, this is a relatively old method, the current basic has been eliminated.

3, inverter, this is a relatively advanced method, high cost, mostly used for low power conversion.

In those days, whether it was DC boost or DC current AC re-boost, it was necessary to pay additional fixed costs, and under the high transportation costs, if you wanted to use DC power, you had to build a power plant at intervals. As a commodity, alternating current is significantly more market-friendly than direct current.

However, the point is that the director of the first half of the article said that the cost of DIRECT CURRENT transmission is higher than that of alternating current, which is based on the higher fixed costs such as power plants and DC boosts. But as the transmission distance increases, a defect in the alternating current gene is reflected.

In addition to electromagnetic catapults, the 003 medium-voltage DC integrated power system is at least a decade ahead of the Ford class

This is a wire with a current direction to the right (the wire is enlarged, the middle part of the two straight lines is the inside of the wire), whether it is direct current or alternating current will induce a circular magnetic field, we will enlarge the longitudinal section of the wire to see that the upper half of the magnetic field direction passes out of the mobile phone screen, the lower half of the magnetic field direction into the mobile phone screen, the magnetic field is distributed inside and outside the conductor.

The difference is that the direct current does not change, so it induces a static magnetic field; The alternating current induces an alternating magnetic field that changes at any time.

In addition to electromagnetic catapults, the 003 medium-voltage DC integrated power system is at least a decade ahead of the Ford class

Let's simplify the problem, take a circular magnetic field area, assuming that this magnetic field is uniformly changed, then the changing magnetic field will induce a vortex electric field in space, and the electric field line is a circle of concentric circles.

In addition to electromagnetic catapults, the 003 medium-voltage DC integrated power system is at least a decade ahead of the Ford class

We noticed that some electric field lines are outside the wire, then there will only be induced electromotive force here, but there is a part inside the wire, where the induced current will be formed, according to the Len ci theorem, the effect of the induced current will always rebel against the cause of the induced current, so the direction of the induced current is the opposite direction of the wire current, thereby canceling out a part of the current.

We skip the derivation process and directly write the formula for the induced electric field intensity E: E = rk/2, where K is the rate of change of the magnetic field and r is the radius of the magnetic field region.

In addition to electromagnetic catapults, the 003 medium-voltage DC integrated power system is at least a decade ahead of the Ford class

It can be clearly seen that the intensity of the induced electric field is proportional to r, that is, the closer to the edge of the magnetic field area, the stronger the induced electric field and the induced current, and the upper and lower parts of the magnetic field edge area intersect at the center of the wire, in other words, the induced current in the center part of the wire is the strongest, and the induced current at the edge of the wire is the weakest. Therefore, during the transmission of alternating current, the current is close to the surface of the wire, and the current in the middle part of the wire is basically zero, which is the skin effect.

Alternating current in the transmission process voltage is a certain, according to U = IR, the smaller the current, the greater the resistance, and the greater the resistance, resulting in the transmission of the power consumed is also greater. Direct current, on the other hand, does not generate induced current, so the current is evenly distributed within the wire. Therefore, under the influence of the skin effect, the loss of alternating current in the transmission process must be greater than that of direct current.

In addition, three-phase AC transmission lines require at least three wires, while DC only needs two wires, which is a third the cost.

In summary, the farther the transmission distance, the more expensive the AC transmission, when the transmission distance reaches a certain point, the power loss in the ALTERNA transmission process equals the construction cost, conversion cost, boost cost and other fixed costs that are higher than the DC power, if you want to continue to extend the transmission distance, then DC is undoubtedly a more cost-effective solution.

The media can often see the mainland in the vigorous development of UHVDC transmission reports, so many people will mistakenly think that our country has completely abandoned alternating current, stud direct current. In fact, this understanding is one-sided, whether it is AC or DC, what can save money is the top stream.

In addition to electromagnetic catapults, the 003 medium-voltage DC integrated power system is at least a decade ahead of the Ford class

Open the homepage of the State Grid, you can clearly see the UHV projects under construction and operation on the mainland. Among them, the blue is an UHVDC transmission line, with a length of more than 1000 kilometers; Green is an UHV AC transmission line with a length of basically less than 600 km. Therefore, it can be reversed that according to the cost accounting of the mainland, it is cost-effective for alternating current below 600 kilometers and direct current for more than 600 kilometers.

At present, only Russia, Japan and Italy have a small number of 1000 kV UHV AC lines in the world, and they are all in operation. In addition to China, UHVDC transmission projects include India's 800 kV transmission project, Africa's west coast HVDC transmission project, and Brazil's Itaipu 600 kVDC transmission project, which is not enough to see in front of the uhVDC transmission project in the mainland, so why is the UHVDC transmission project only in China?

Because several preconditions need to be met:

1, the country must have a vast territory, often more than 1000 kilometers of transmission distance in the mainland is only a province, in Europe is afraid that it is not going to be out of several countries;

2. The geographical distribution of power generation and electricity consumption is uneven, and 76% of the coal resources of the mainland are distributed in the north and northwest; 80% of hydroenergy resources are distributed in the southwest; Most of the land wind and solar energy resources are distributed in the northwest. At the same time, more than 70% of energy demand is concentrated in the eastern and central regions. The distance between the energy base and the load center is between 1,000 and 3,000 kilometers.

Taking coal-fired power generation as an example, the fuel cost of 1 kWh of electricity in the pit mouth power station in the western coal production area is only 9 cents, if the thermal coal is loaded into a truck and transported to Qinhuangdao by land, and then transported to East China by sea and land, the combustion cost of each kWh of electricity will be 3 cents, and the minimum electricity consumption of residents will be 4 to 5 cents per kWh of electricity. If the pit mouth power station in the northwest is transmitted through UHVDC, in addition to various costs, the cost per kilowatt-hour of electricity can still be reduced by 6 cents to 1 cent and 2 cents.

Wind energy on the mainland is mainly concentrated in the Three North Regions, and hydropower is mainly concentrated in the Southwest Region, but the local power consumption is extremely limited, and natural resources will be wasted if they are not utilized, and the UHVDC transmission network has built a large-capacity, long-distance energy transmission channel to transport massive amounts of clean energy to the central and eastern regions.

3, the need to have the ability to concentrate large-scale power generation, such as nuclear power plants, there are many countries in the world with nuclear power plants, but at the same time there are only a few countries with the above conditions, so their attitude towards UHVDC transmission is either no demand, or no ability, or no money to play. The United States has demand and ability and is not bad money, but just to save a few cents to tear down and rebuild the already perfect AC transmission system, starting from 0 to point the HVDC transmission technology tree, mercenary and value the capital recovery cycle capitalists do not have this motivation, nor a strong national will to promote this matter.

Due to the late start of the AC transmission system in our country, the rapid development of the economy has led to a sharp increase in electricity consumption in the central and eastern regions, the uneven distribution of natural resources and other comprehensive factors, the development of UHVDC transmission can be described as a time and place and people.

Originally in the eyes of Western countries, China's crazy point of HVDC transmission skill points, only save so much lost transmission costs, but the construction of UHVDC transmission system stimulated the domestic equipment manufacturing industry to take off, the three major domestic UHV experimental projects used in the equipment almost all provided by domestic enterprises, the project localization rate reached about 95%, equipment localization rate reached about 91%. Through experimental engineering, domestic equipment manufacturing enterprises have been trained, and the strength of scientific and technological research and development has been greatly improved.

Integrated power system for ships

Oh, after talking so much, it can finally be said that the British and American warships are in the premise of the popularity of alternating current, the use of medium voltage AC grid-connected first-generation integrated power system, the essence of the integrated power system is grid-connected power generation, by a power system to supply different equipment.

Ship integrated power system includes: power generation system, transmission system, power distribution system, propulsion system, energy storage system, energy management system, we dismantle one by one in detail:

Power generation system

The US military's power generation system uses medium voltage AC power frequency synchronous power generation, many precision electronic devices on the warship need to be controlled by chips, so direct current is essential, so either add a set of DC power generation system, or the alternating current generated by the alternator is rectified into direct current and then DC power supply.

Warships in conventional cruise, tactical maneuvering, electromagnetic catapult, release of high-energy weapons, the required power is different, when the power consumption increases sharply, multiple generators need to be powered together, alternating current grid conditions are very harsh, it is necessary to ensure that the voltage rms value is equal, the frequency is equal, the phase is equal, if you want to maintain the stability of grid-connected operation, you should also ensure that the range and speed of voltage and frequency regulation meet the requirements of load changes, in order to ensure that the load change will not occur when the grid collapse occurs. DC only needs to consider the amplitude of the voltage.

In the AC power generation system, once there is a problem of multiple engines out of sync, it is easy to oscillate, "inherent oscillation" is a common problem in the generator industry in early years, surface ships are good to say, at most is noisy, noisy, but the inherent shock of the submarine is simply going to kill, huge noise will increase the probability of being found by the enemy sonar, when the oscillation is serious, it will lead to the paralysis of the whole ship's power system, if the generator strikes in the deep sea, the submarine's fragile ecological maintenance system will collapse in a short time. The consequences are unimaginable.

In terms of controlling "inherent oscillation", Germany from six-phase rectifier generator to twelve-phase rectifier generator, has always been at the forefront of the world, when the generator of Chinese ships needs to be imported from abroad, a young man named Ma Weiming participated in the negotiation of a twelve-phase rectifier generator, when he was ready to discuss the inherent oscillation solution with German experts, he was coldly received by German experts, and the other party dismissively said "our equipment has no problems."

The Germans may not have thought that this professional master's degree in ship electrical engineering from the Naval University of Engineering will be a great achievement in the future, he only took 6 years to independently develop 2 twelve-phase generators, not only to overcome the "inherent oscillation" of this world's stubborn disease, but also to make the mainland in the field of generators first, surpassing Europe to the forefront of the world.

In 2001, Ma Weiming developed the world's first AC-DC dual-winding generator on this basis, a generator can output AC and DC at the same time, and can maintain uninterrupted power supply when AC-DC switching, and the problem of medium-voltage AC dissynchronization is also perfectly solved.

In order to meet the needs of large ships and aircraft carriers, increasing power density has become the only way for modern ship power supply systems, and increasing the speed is the most direct way to improve power density. The generator set using alternating current system uses a reduction gearbox to connect the high-speed prime mover with the low-speed generator, its weight, volume increases, and the vibration noise is huge, and the use of medium voltage DIRECT current system, the speed of the prime mover is no longer limited by the AC frequency, not only can more conveniently increase the speed, but also reduce the reduction gearbox. It can be said that the high-speed integrated medium-voltage rectifier generator in the second-generation integrated power system in the mainland comprehensively crushes the first-generation medium-voltage AC power generation system in the United States.

Transmission systems

The United States uses medium voltage AC power frequency transmission, we use medium voltage DC transmission, the first half of the article mainly from the cost of the comparison of AC and DC transmission, warships are the country's heavy equipment, naturally not bad that little money, reliability is the first thing to consider.

Then some of the drawbacks of AC transmission have to be considered, first of all, alternating current will definitely produce an inductive magnetic field, warships are close together on such a large point, the various precision instruments are very compact, the electromagnetic interference generated by AC transmission can not be ignored, in contrast, stable DC transmission does not have to consider these problems at all. And there is no additional energy loss in HVDC transmission, which is more efficient.

In addition, HVDC transmission is not subject to the stability of synchronous operation, which plays a great role in ensuring the stable operation of the AC power grid between different systems. Different parts of the ship's AC systems with different frequencies can be interconnected asynchronously through HVDC transmission.

Propulsion system

First of all, understand how electrical energy is converted into mechanical energy, which is a set of relative permanent magnets, the magnetic field direction from left to right, this is a powered coil, this is the direction of the magnetic field generated by the coil, according to the magnetic field is the energy formula: W =-mBcosθ, where θ is the angle between the coil magnetic field and the external magnetic field.

We all know that objects always tend to move towards places with low potential energy, such as placing a small ball at the bottom of the bowl, where the gravitational potential energy is the lowest, and no matter where you push the ball in any direction around, the combined force will always pull the ball back to the bottom of the bowl, because it is the most stable.

And where in the magnetic field is the lowest potential energy?

In addition to electromagnetic catapults, the 003 medium-voltage DC integrated power system is at least a decade ahead of the Ford class

We draw the cosine function in the formula, and it can be clearly seen that the potential energy at θ = 0 is the lowest and the most stable state; When θ = 180 degrees, the potential energy is the highest and the most unstable.

In addition to electromagnetic catapults, the 003 medium-voltage DC integrated power system is at least a decade ahead of the Ford class

When the coil magnetic field and the external magnetic field have a certain angle, affected by the magnetic field potential energy, the coil will automatically turn to the same direction as the external magnetic field and remain stable.

If we input is alternating current, when the angle between the coil and the external magnetic field is 0 degrees, due to the change in the direction of the current, the direction of the induction magnetic field suddenly turns, at this time the angle between the coil induction magnetic field and the external magnetic field becomes 180 degrees, in the highest state of potential energy, so the coil will continue to rotate 180 degrees, the alternating current continues to enter the direction of the current, the coil will also do continuous rotational movement, if we connect the coil to a transmission shaft, you can drive the machine to do work, So we realized the conversion of electrical energy into mechanical energy, which is also the simplest principle of alternator motors.

In addition to electromagnetic catapults, the 003 medium-voltage DC integrated power system is at least a decade ahead of the Ford class

The principle of the DC motor is the same as that of alternating current, except that two semicircular metal rings are added to the end of the coil's wire, so that every 180 degrees of rotation of the coil will automatically switch the direction of the current to achieve continuous rotation.

Well, after the principle is finished, let's test a question: If you receive an order at this time and find an incoming torpedo, the warship needs to speed up immediately to avoid, please ask the DC motor drive speed or ac motor drive speed up?

We look back at this schematic diagram again, when the coil induction magnetic field and the external magnetic field angle is 0, the coil will continue to deflect a certain angle because of inertia, at this time due to the potential energy becomes higher, and forced to pull back the equilibrium position of the potential energy of 0, that is, if the current direction does not change, the coil can only rotate 180 degrees at most and do not move.

So far, there is no difference between AC and DC motors, and then go down is different, AC motors are because of changes in the direction of current to cause the coil to continue to move, and DC motors because the inertial movement of the coil leads to a change in the direction of the current, which then prompts the coil to continue to rotate.

Do you see the difference? The speed of the AC motor is consistent with the input AC frequency and is fixed, because the frequency of the current direction change determines the frequency of the coil rotation, to change the speed of the paddle, you need to do the frequency conversion process of the ALTERNA first, in order to achieve the speed we need.

The DC only needs to change the armature voltage, you can easily change the speed, the response speed is faster, suitable for the system that requires extensive stepless smooth speed adjustment, we can get any speed we need within a certain range.

Therefore, it can be concluded that the DC motor drive is faster.

The motor needs to accelerate and decelerate, that is, it needs to generate alternating current with an amplitude frequency phase controllable, while the electrical energy of the AC grid is not adjustable, so it is necessary to propel the converter. The propulsion converter is a kind of high-power power electronic converter, which first rectifies the ALTERNA of the AC power grid into direct current, and then reverses the DIRECT current into ALTERNA according to people's needs.

In addition to electromagnetic catapults, the 003 medium-voltage DC integrated power system is at least a decade ahead of the Ford class

The picture above is a picture of a converter randomly searched by the curator on the Internet, you can intuitively feel that just the propulsion system, the alternating current system needs to be N more than the direct current system.

In addition, alternators and motors often work at different voltages, so additional propulsion transformers are needed, limited by physical laws, transformers such a big and heavy thing is essential, if one step in place to use DC, then even the transformer is not needed.

In addition to electromagnetic catapults, the 003 medium-voltage DC integrated power system is at least a decade ahead of the Ford class

When it comes to the ship's drive system, we have to mention another great achievement of Academician Ma Weiming: shaftless pump push. Common ship propulsion system, the motor needs to convert electrical energy into mechanical energy through a transmission shaft and reduction gearbox, energy conversion must be accompanied by a certain loss, and the propeller, transmission shaft and motor are in a straight line, coupled with the huge volume of the transmission shaft and gearbox, which is not conducive to flexible ship layout design, and the noise of the transmission shaft is the main source of noise for submarines.

Academician Ma Weiming's black technology "shaftless pump push" system is to creatively combine the motor and the paddle into one, directly combine the propeller and the rotor on the motor, so that the motor rotates at the same time, the paddle directly rotates the work, the huge drive shaft and the deceleration gearbox are not used, the energy conversion efficiency is improved, not to mention, even the noise problem is also solved, so that the submarine really realizes the sonar "stealth", disappears in the background noise of the vast ocean, it is said that the next generation of our country's submarine will be on the shaftless pump push system, The technical principle seems very simple, is not to do the motor and the propulsion system together, in fact, there are many technical difficulties to overcome, because the motor is placed outside the submarine, once there is a problem in the deep sea, even the maintenance will become impossible, so the reliability of the shaftless pump push system is very critical, few countries in the world dare to try in this regard, otherwise how can it be called black technology?

Energy storage system

Many people have had such a question, the United States aircraft carriers are nuclear power, the power is continuous, why do you need energy storage systems?

Let's first make a calculation, usually about 33 tons of fighter take-off speed needs to reach 200 kilometers per hour, converted into kinetic energy is one hundred million joules, assuming that the fighter is ejected within 2 seconds, then the ejection of an aircraft requires an average of fifty thousand kilowatts of power, and the electromagnetic gun is a large consumer, because the acceleration time is very short, the instantaneous power exceeds 3 million kilowatts.

Even if the entire ship's generators work together, they cannot provide such a large instantaneous power, so they need to store the electrical energy in advance, and when they need to catapult or fire high-energy weapons, they will release huge energy instantaneously.

In addition to electromagnetic catapults, the 003 medium-voltage DC integrated power system is at least a decade ahead of the Ford class

Energy storage system China and the United States take two different routes, the United States through the AC motor to convert electrical energy into mechanical energy stored in the high-speed rotating flywheel, take the physical route, its structure is very simple, a disc generator + a huge flywheel, high-speed rotation will have a strong kinetic energy.

China takes the chemical route of supercapacitors, as mentioned earlier, the capacitor is through AC resistance DC, and can only be charged through DC.

So which of these two energy storage routes is better or worse?

First of all, the flywheel relies on the high-speed rotating rotor for energy storage, the energy storage density is low, because to improve the energy storage density can only be improved by improving the quality or speed of the flywheel, the process of continuous improvement of the flywheel speed, the safety of the support structure puts forward high requirements, to maintain the time of the flywheel energy storage, you need to maintain vacuum, active heat dissipation and other harsh conditions, the energy storage speed is slower, you can imagine, if a minute to eject an aircraft, or a minute to launch 10 electromagnetic guns, It is necessary to complete the process of energy storage and energy output in a very short period of time, whether such a drastic energy conversion can be achieved stably, regardless of the damage to mechanical components.

There is also a problem that cannot be ignored is that the safety risk of flywheel energy storage, unlike electric energy, as long as the circuit is disconnected, the electrical energy cannot be released, and once the flywheel energy storage device is destroyed, the stored mechanical energy will definitely be released, which is basically equivalent to the onboard impact of its own warship, or hit by its own electromagnetic gun, the scene is too beautiful, can not bear to look directly.

The supercapacitor is much better, not only the energy storage density is higher than the flywheel, its charging and discharging speed is also higher than the flywheel, and the energy loss between the electrical energy and the chemical energy is much smaller than the conversion between the electrical energy and the mechanical energy. It is possible to store enough electrical energy at a time to complete several ejections or high-energy weapon launches as needed. At the same time, the supercapacitor occupies a smaller footprint than the flywheel, has no safety hazards at all, and can be installed in any position on the ship. More importantly, the flywheel energy storage route lift space is already very small, and the supercapacitor still has a broad room for improvement, in the long run, the super capacitor energy storage route is the right choice.

The integrated power system of the United States was established in the 1990s, when the level of power electronics technology was not enough to support medium voltage DC, and the US Navy was difficult to meet in the world, and there was no such urgent need to develop new technologies, so it chose a relatively stable medium voltage AC route. When the mainland's integrated power system is established, the relevant research and application of medium voltage DC technology has been enriched a lot, if you start from the medium voltage power frequency AC again, through the medium voltage medium frequency AC, and eventually develop into medium voltage DC, then we will always be in the position of catching up, directly across the generation to take the route of medium voltage DC, the risk is very large, the military is dangerous, the curve overtaking is a helpless move.

Assuming that in another parallel time and space, our medium voltage DC technology has not been broken, then the 003 electric bomb, electromagnetic gun, stealth submarine, we will have to re-inhale the exhaust gas of the American exhaust pipe, what a humiliating experience!

Very fortunate that we have Academician Ma Weiming, the Americans did not make the things we made, the superiority of medium voltage DC over medium voltage AC is clear at a glance, it is Academician Ma's forward-looking vision and the hard work of his team, so that we have the advantage of leading Americans in this field for ten years, I believe that Americans in a sense of urgency, do not hesitate to tear down the reconstruction, but also in the next five to ten years to pull out the medium voltage DC integrated power system.

I believe that Academician Ma Weiming and countless military workers will not waste the leading edge of this position in vain, and will certainly go one step further, and only with this article I would like to express my highest respect and thanks to them!