1. What is a "synchronous" generator? How is the synchronization speed determined?

Answer: Generator is the heart equipment of power plant. According to the power it drives, it can be roughly divided into hydro-generator (hydraulic) and turbo-generator (steam). All the contents involved in this book refer to synchronous generators (vertical hydrogenerators only).

When the generator is running normally, there is a rotating synthetic magnetic field between the stator and rotor air gap, which consists of two magnetic fields: rotor magnetic field and stator magnetic field. The so-called "synchronous" generator means that the rotating speed of the rotor magnetic field (generated by the prime mover) of the generator is equal to the rotating speed of the stator magnetic field (determined by the frequency of the power system).

The rotor magnetic field is generated by DC rotating rotor windings (magnetic poles), and the rotating speed of the rotor magnetic field is also the rotating speed of the rotor, that is, the rotating speed of the whole unit. The rotor is driven by the prime mover, and the speed is adjusted by the unit governor. This speed is clearly marked on the nameplate of the generator. The rotating magnetic field of the stator is generated by the stator three-phase winding (symmetrically arranged at 120) through three-phase symmetrical current, and its rotating speed is determined by the formula (where: p is the rotor pole number; F is the frequency of the power system; N is the unit speed). As can be seen from the formula, for a specific generator, the number of magnetic poles is fixed, and the frequency of China's power system is also fixed, that is, 50Hz (also known as power frequency). It can be seen that the rotating speed of the stator rotating magnetic field of each specific generator is a "fixed value" after the generator is manufactured. Of course, the frequency of power system can't really be stabilized at 50Hz.

Numerically, it is allowed to fluctuate slightly up and down this value, that is, the stator magnetic field actually changes dynamically around the rated speed value during operation. In order to synchronize with the stator magnetic field, the rotor magnetic field should also adapt to this change, that is, the speed of the unit should be dynamically adjusted. If the rotating speed is not consistent with the stator magnetic field, we say that the generator is out of step.

2. What is the flywheel torque of the generator? ? What is its electrical significance?

Answer: The flywheel torque of the generator is the product of the weight of the rotating part of the generator and the square of its inertia diameter. It seems to be a quantity that has nothing to do with electrical parameters, but it is not. It has great influence on the transient process and dynamic stability of power system. Sudden load rejection under various working conditions directly affects the speed increase of the unit and the pressure increase of the water delivery system. First of all, the calculation requirements of regulation guarantee of water conveyance system should be met. When the power system fails and the load of the unit changes suddenly, the speed of the unit increases due to the time lag of the speed regulating mechanism. In order to limit the speed, the unit needs a certain amount. The greater the speed, the smaller the speed change rate of the unit and the better the stability of the power system.

Closely related to the unit cost. The larger the unit, the greater the unit weight and manufacturing cost.

3. What is the short-circuit ratio Kc of the generator? What is the relationship between Kc and generator structure?

Answer: The short-circuit ratio Kc is an important parameter to characterize the static stability of the generator. The original meaning of Kc is the ratio of short-circuit current to rated current in three-phase steady-state short-circuit under the excitation current corresponding to no-load rated voltage, that is, Kc=Iko/IN. Because the short-circuit characteristic is a straight line, Kc can be expressed as the ratio of the excitation current Ifo at rated no-load voltage of the generator to the excitation current Ifk at rated three-phase steady-state short-circuit current, and the expression is: KC = Ifo/IFK ≈ 1/XD. Xd is the reactance of the generator when the three-phase short circuit is stable in operation, that is, the generator direct-axis synchronous reactance (unsaturated value).

If the influence of magnetic saturation is ignored, the short-circuit ratio and direct-axis synchronous reactance Xd are reciprocal. Small short-circuit ratio means that the synchronous reactance is large and the short-circuit current is small when corresponding short-circuit, but the generator voltage changes greatly when the load changes in operation, and the stability is poor when the generators run in parallel, that is, the overload capacity of the generators is small and the voltage change rate is large, which affects the static stability and charging capacity of the power system. When the short-circuit ratio is large, the overload capacity of the generator is large, and the change of terminal voltage caused by load current is small, which can improve the static stability of the generator in system operation. However, Ambassador Kc increased the excitation current of the generator and the copper consumption of the rotor, which increased the manufacturing cost. The short-circuit ratio is mainly put forward according to the transmission distance of power plant, load change and other factors, and the k of general hydro-generator is 0.9 ~ 1.3. Structurally, the short-circuit ratio is approximately equal to

It can be seen that to increase Kc, it is necessary to reduce A, that is, increase the unit area; Or increase the air gap, it is necessary to increase the ampere-turns of the rotor winding.

4. What is the instantaneous reactance Xd' of the generator? What does it have to do with the generator structure?

Relationship?

Answer: XD' is the transient reactance representing the initial time of three-phase sudden short circuit in generator operation (after damping winding current decays). Direct-axis instantaneous reactance is the ratio of the initial value of AC fundamental component in the voltage generated by the stator winding direct-axis total flux linkage to the DC fundamental current that changes at the same time when the generator is running at rated speed. It is also an important parameter of the generator and the whole power system, which has great influence on the dynamic stability limit of the generator and the transient voltage change rate when the load is suddenly added. The smaller XD', the greater the dynamic stability limit and the smaller the transient voltage change rate. However, the smaller Xd', the larger the stator core will be, which will increase the size and cost of the generator. The value of xd' is mainly determined by the leakage reactance values of stator winding and excitation winding.

Structurally, the relationship between Xd' and power load a and polar distance τ is as follows:

K is the proportional coefficient. It can be seen that to reduce XD ′, it is necessary to reduce A or increase τ, which will increase the size of the generator.

5. What is the direct-axis ultra-instantaneous reactance XD "of the generator? What does it have to do with the generator structure? How does the size of XD "affect the system?

Answer: XD "is the transient reactance indicating the initial moment of three-phase sudden short circuit in generator operation. When the generator is suddenly short-circuited, the excitation winding and damping winding of the rotor induce currents that demagnetize the armature reaction flux, and squeeze the armature reaction flux into the leakage paths of the excitation winding and damping winding. The path has large magnetic resistance, that is, small magnetic permeability, so its corresponding direct axis reactance is also small. This equivalent reactance is called the direct-axis super-instantaneous reactance XD ",that is, when the generator with damping winding is suddenly short-circuited, the periodic component of stator current changes from XD.

Structurally, XD "is mainly determined by the leakage reactance values of generator stator winding and damping winding.

For generators without damping windings, xd "= xd'.

Because the size of XD "affects the short-circuit current when the power system is suddenly short-circuited, the value of XD" also affects the selection of high-voltage power transmission and transformation equipment, especially high-voltage circuit breakers, such as dynamic stable current and other parameters. From the choice of electrical equipment, it is hoped that XD "will be larger, so that the short-circuit current will be smaller.

6. What is the function of damping winding?

Answer: The hydrogenerator rotor is designed with AC and DC axis damping windings. The damping winding is structurally equivalent to a short-circuit squirrel-cage ring superimposed on the rotor excitation winding, and functionally equivalent to a "squirrel-cage asynchronous motor" which rotates synchronously with the rotor, and plays a role in regulating the dynamic stability of the generator. When the generator runs normally, the stator and rotor magnetic fields rotate synchronously, so the damping winding does not cut magnetic flux, so there is no induced current. When the generator is disturbed and the rotor speed is lower than the stator magnetic field, the damping winding cuts the stator magnetic flux to generate induced current, and the torque generated by the induced current on the damping winding accelerates the rotor. The greater the speed difference, the greater the torque and the stronger the acceleration effect. On the contrary, when the rotor speed is higher than the stator magnetic field speed, the torque direction is opposite, which makes the rotor slow down. Therefore, damping winding has a good effect on the dynamic stability of generator operation.

Supervision.

7.3 What does wiring mean? Why is the generator connected in a star shape?

Answer: On the nameplate or drawing of the generator, we often see that the connection mode of the generator stator winding is expressed as Y, 3 Y, 5 Y, etc. This means that the generator is wired in a star pattern. 3 Y means that the generator stator winding is 3-way star-shaped parallel connection, which can also be understood as the parallel connection of three star-shaped generators.

Because there is a strong third harmonic in the magnetic flux of the generator, if the generator is connected to river line, the third harmonic will form a loop in the triangle, resulting in additional loss and heat generation. Therefore, the generator stator winding is generally Y-connected, so that the third harmonic can not form a loop.

8. What is the excitation winding? What is armature winding?

Answer: Among the stator and rotor windings of the motor, the winding that produces air-gap magnetic field under no load is called excitation winding (or excitation winding); Another winding that produces power conversion (absorbing or outputting active power) is called armature winding. It can be seen that the excitation group of hydro-generator is rotor winding and the stator winding is armature winding. The excitation winding of asynchronous motor is stator winding, and the rotor winding in short circuit state is armature group.

9. What is laminated winding? What are the characteristics? What is wave winding? What are the characteristics?

Answer: Overlapping means that any two adjacent coils are the last coil stacked on the previous coil. In manufacturing, this kind of winding is mostly made one coil at a time, which is also called frame winding. The advantage of this kind of winding is short torque.

At the end of copper, there are many parallel branches. Its disadvantage is long terminal wiring. In multi-pole large motors, there are many connecting wires, which are inconvenient to arrange and consume a lot, so they are mostly used in small and medium-sized motors. Wave winding is that any two series coils advance in waves along the winding direction. In practice, one coil of this kind of winding is mostly composed of two strip bars, so it is also a strip winding. Its advantage is that there are few connecting wires between coil groups, so it is mostly used for large hydro-generators. In the field, the elements of wave winding are directly called "rods". In this book, "rod" is often used instead of "coil".

10. What is the number of slots per pole and G phase? What is an integer slot winding? What is a slotted winding?

Answer: For a specific generator, the number of slots in the generator stator and the number of poles in the rotor have been determined. One of the important concepts is the number of slots per pole and phase Q. The generator winding consists of three phases: A, B and C, so the number of slots occupied by each phase in the stator is

Wait a minute. , each1/3; Corresponding to each pole of the rotor, the number of stator slots occupied by each phase under each pole is also equal. The number q of slots per pole and phase is the number of slots that each phase should occupy under each pole.

Where z specifies the total number of sub-slots;

2p-number of magnetic poles;

M phase number.

As can be seen from the formula, the q value is easy to get. When q is an integer, the winding is called an integer slot winding; When q is fractional, the winding is called fractional slot winding. If q=3, it means that under a magnetic pole, three phases, A, B and C, each occupy three slots in the stator slot. such as

It means that under a magnetic pole, A, B and C occupy the slot in the stator slot, that is, the fractional slot. However, it is impossible to divide the stator slot into several parts. That is, 1 1/4, that is to say, under every four magnetic poles, the three phases A, B and C each occupy 1l slots in the stator slots, and the total number of corresponding slots under each magnetic pole is still equal.

1 1. What is the number of cycles (or turns) of fractional slot winding? How is it composed and determined? '

Answer: In the parameters of the generator stator winding drawing, we can see the number of cycles or revolutions of the winding. For example, the generator stator has 792 slots, and the number of winding cycles per pole and slot is 3233. This number is the number of revolutions of fractional slot winding, which is closely related to the number of slots in each pole and each phase, which means that the stator three-phase windings are arranged correspondingly.

Number of stator slots.

Add the four digits of the above 3233: 3+2+3+3 =11; Ll is the number of stator slots, and "number of bits" 4 represents four magnetic poles. Obviously, these two numbers are the numerator and denominator of the number of slots per pole and each phase Q = 1 1/4, respectively. The order of all slots of the stator is as follows: A- phase 3 slot, B- phase 2 slot, C- phase 3 slot, A- phase 3 slot (note that one round has been arranged), B- phase 3 slot, C- phase 2 slot, A- phase 3 slot, B- phase 3 slot (note that two rounds have been arranged) ... So all slots of the stator are arranged all the time (see figure) That is, according to the order of 3233, all slots of the stator are divided into three equal parts. If the generator has 792 slots, then arrange 72 rounds according to the order of 3233 (72× 1l=792), and the slots of the stator are all finished, with each phase accounting for 264 slots (see this section 13). They are all 1 1/4. Of course, the number of cycles can also be ranked to 2333 or 3332. The principle of selecting 3233 is that all kinds of arrangements are arranged and displayed on the block diagram with the least connection. That is to say, it is best to choose the wave winding mode with the least number of end joints between winding bars, and the design of winding end connections should minimize the number of connecting wires between poles.

In order to save space, only one branch connection is marked, and the middle slot is omitted.

12. What is the compound pitch of the wave wrap? What does the value of synthetic high school represent?

Answer: The synthetic pitch moment is a parameter used to characterize the connection law of wave windings. It shows that the wave winding connects all coils in series into a complete winding, and the number of slots along the winding direction is the number of slots separated by the corresponding sides of two adjacent coils. For example, on the drawing of generator stator winding, we can see that the parameter column of the winding is marked with a parameter similar to1-7-14, which is the composite pitch of the winding.

Synthetic asphalt y = y1+y2; The pitching moment y 1 means that one bar of the stator coil is below the N pole and the other bar is below the S pole, and the number of stator slots between the two ends is 1-7, which means that one end of the coil is in 1 slot and the other end is in the 7th slot, and y 1=

6: pitch moment y2, which means that the slot number of the next connected coil after the coil comes out of the seventh slot is 14 slots, and y2=7, so the combined pitch moment Y= 13.

14. What are the advantages and disadvantages of fractional slot winding?

Answer: Fractional slot winding is mostly used in large hydro-generators, which has the following advantages: ① It can weaken the higher harmonic potential generated by the non-sinusoidal distribution of magnetic field of magnetic poles; ② It can effectively weaken the amplitude of tooth harmonic potential and improve the electromotive force waveform; ③ Flux pulse amplitude of each pole decreases due to the change of air gap permeability, and pulse vibration loss on the magnetic pole surface decreases.

Its disadvantage is that there are odd and even harmonics in the magnetomotive force of fractional slot winding. In some cases, their interaction with the main pole magnetic field may produce some interference force. When the frequency of some interference force coincides with the natural vibration frequency of the stator frame, it will cause * * vibration, which will lead to the vibration of the stator core. Therefore, improper selection of Q value of fractional tank may also bring many hidden dangers, which is an example of actual generator operation.

15. What is the tooth harmonic potential? What are the methods to weaken the tooth harmonic potential?

Answer: In the analysis of the generator winding potential, it is assumed that the iron core surface of the stator winding is smooth at first, but in fact, due to the existence of iron core slots, the inner circular surface of the iron core is undulating. For magnetic poles, the reluctance of the air gap actually changes. When the magnetic poles are opposite to the teeth, the reluctance is small, but the air gap reluctance at the notch of the core is large. With the rotation of the magnetic pole, the potential will be induced in the stator winding due to the change of air gap reluctance. The potential induced in the winding due to the cogging effect is called tooth harmonic potential.

The methods to weaken the tooth harmonic potential are:

(1) adopts an inclined slot, that is, the stator or rotor slot is not parallel to the axis. Of course, it is impossible for large generators to make stator slots into non-vertical chutes, and it is impossible to make magnetic poles into oblique poles. In small motors, such as asynchronous squirrel-cage motors, the rotor winding adopts chute. In some small and medium-sized generators, stator chutes are also used, and the general slope is equal to one stator slot pitch.

(2) Magnetic slot wedge is adopted, which improves the magnitude of magnetic resistance. But at present, there is no mature technology, which is only used in small and medium-sized motors.

(3) Increasing the air gap between stator and rotor can also effectively weaken the tooth harmonics, but it will make the power factor worse, so it is generally not used.

(4) fractional slot winding is adopted. This is a widely used method for large hydro-generators at present.

16. What are the main losses in the operation of the generator?

Answer: The losses of generators can be roughly divided into five categories, namely stator copper loss, iron loss, excitation loss, additional electrical loss and mechanical loss. In the operation of the generator, almost all the losses are expressed in the form of heat.

(1) The copper loss of the stator is all the losses caused by the stator current flowing through the stator winding.

(2) Iron loss refers to the loss caused by generator magnetic flux in the core, mainly including hysteresis loss and eddy current loss caused by main magnetic flux in the stator core, and additional loss.

(3) Excitation loss is the loss produced by the rotor circuit, mainly the copper loss produced by the excitation current in the excitation circuit.

(4) The additional electrical losses are complicated, mainly including losses caused by magnetic leakage at the end of nearby iron components, losses caused by various harmonic magnetic fluxes, and iron losses caused by tooth harmonics and higher harmonics on the rotor surface.

(5) Mechanical losses mainly include ventilation loss and bearing friction loss.

17. What are the hazards of sudden short circuit of generator?

Answer: (1) When the generator is suddenly short-circuited, the end of the generator winding will be greatly impacted by electrodynamic force, which may deform the end of the coil and even damage the insulation.

(2) Overvoltage occurs in the stator and rotor windings, which adversely affects the generator insulation. The combination of strong surge current and overvoltage in stator winding may lead to the breakdown of weak insulation links.

(3) The generator may generate violent vibration, which may cause strong destructive mechanical stress to some structural components.

18. What is insulation partial discharge? What are the main forms of generator partial discharge?

Answer: Under the action of electric field, the electric field intensity in some areas of insulators in the insulation system reaches the breakdown field strength, and some areas are discharged. This phenomenon is called partial discharge. Partial discharge only occurs in a part of insulation, not the whole insulation.

There are three kinds of partial discharges of generators: internal discharge of winding main insulation, end corona discharge and slot discharge (including slot corona). In addition, the generator has a harmful discharge, that is, the arc discharge caused by the fracture of stator coil strands or joints. The mechanism of this discharge is different from that of partial discharge.

19. What is the cause of partial discharge in the main insulation of the generator? What are the disadvantages?

Answer: In the production process of stator bars of large generators, there may be air gaps or impurities between insulation layers or between insulation layers and strands due to technological reasons; Under the combined action of electricity, heat and mechanical force, insulation will deteriorate directly or indirectly, resulting in a new air gap between insulation layers. Because of the different dielectric coefficients of air gap and solid insulation, the electric field distribution of this sandwich medium composed of air gap (impurity) and insulation is uneven. Under the action of electric field, when the working voltage reaches the initial discharge voltage of air gap, partial discharge occurs. The initial voltage of partial discharge is closely related to the dielectric constant and air gap thickness of insulating materials.

The partial discharge of gas in air gap belongs to streamer high-voltage glow discharge, and a large number of high-energy charged particles (electrons and ions) hit the main insulation at high speed, thus destroying the molecular structure of insulation. In the air gap where partial discharge occurs in the main insulation, the local temperature can reach 1000℃, which deteriorates the insulation of adhesive and stranded wire, leads to loose stranded wire and short circuit between strands, leads to local overheating and hot cracking of the main insulation, and finally damages the main insulation.

The further development of partial discharge is to produce dendritic discharge in insulation, which further deteriorates the main insulation and eventually forms discharge channels and destroys insulation.

20. What is corona? What harm does corona do to generators?

Answer: The corona in the generator is caused by uneven electric field distribution and excessive local field strength, which causes glow discharge in some parts of the insulation surface of the high-voltage winding of the generator stator, resulting in ionization of the nearby air. It can be seen that corona is a partial discharge of generator. It is produced on the insulating surface, which is different from the corona near the conductor in the outdoor high-voltage electric field that we are familiar with.

Compared with other forms of partial discharge, the discharge intensity of corona itself is not very high, but the existence of corona greatly reduces the performance of insulating materials. Surface corona increases the local temperature of insulation surface, and the thermal effect of corona and its compound of O3 and N2 (O3 is easily decomposed and combined with N2 and moisture in the air to form acid) will also destroy local insulation. For yellow insulation, the insulation layer will turn into white powder, and its degree is related to corona action time. After the surface of the material is damaged, the discharge will concentrate in the pits and develop into insulating materials, and in severe cases, it will develop into branch discharges until breakdown. In addition, corona will also produce charged ions around it, and various unfavorable factors will be superimposed. Once the stator winding has overvoltage, it may cause short circuit or bar breakdown. The breakdown field strength of yellow insulation decreases slightly with the increase of temperature, and it breaks down when the temperature exceeds 180℃.

The field strength will drop sharply.

2 1. Which parts of the generator are prone to corona?

Answer: Under normal circumstances, the parts where the generator may produce external corona are: ① the notch of the bar. The winding slot is a typical sleeve structure, and the electric field at the slot is very concentrated, which is most likely to produce corona. (2) ventilation ditch of iron core section. The sharp edge of ventilation channel steel is easy to cause uneven local electric field. (3) The surface of the bar is in poor contact with the iron core groove or there is an air gap. (4) End band bandage. ⑤ Between rods with different ends. The distribution of electric field at the end of winding is complicated, especially at the contact parts and edges of coil, end hoop, binding rope and backing plate. Because of the process, it is often difficult to completely eliminate air gaps, and corona is also easy to occur in these air gaps.

22. What are the related factors of generator corona?

A: (1) is related to altitude. The higher the altitude, the thinner the air and the lower the corona discharge voltage.

(2) It is related to humidity. With the increase of humidity, the surface resistivity decreases and the corona onset voltage decreases.

(3) The terminal high-resistance anti-corona layer is related to temperature. If the resistance of the high-resistance anti-corona layer is very high at room temperature, the corona onset voltage will also increase with the increase of temperature. At room temperature, if the resistance of the high-resistance anti-corona layer is low, the corona onset voltage will decrease with the increase of temperature.

(4) The corona in the groove is related to the gap between the groove walls. The gap between the bar and the wall of the iron core slot will cause the spark discharge between the anti-corona layer of the slot and the iron core. The dangerous gap of epoxy mica powder insulation prone to partial discharge is about 0.2 ~ 0.3 mm At present, the linear expansion coefficient of epoxy mica powder insulation used in high-voltage large motors in China is very small. Under normal operating conditions, the expansion of epoxy mica powder insulating rod can not fill the gap between rod and iron core. This is very different from black insulation.

(5) It is related to the distribution of electric potential and electric field at the position of the rod. The higher it is, the easier it is to get dizzy, and the more uneven the electric field distribution, the easier it is to get dizzy.

23. What is galvanic corrosion? What are internal corrosion and external corrosion? What are the measures to prevent electrical corrosion?

Answer: Electric erosion is a capacitive discharge caused by the loss of electrical contact between the surface of the anti-corona layer of the stator bar in the generator slot and the wall of the stator slot, thus causing corrosion and damage to the surface of the bar. The discharge energy of this kind of capacitor discharge is much larger than that of pure corona discharge, so the strict method and stator slot coupling method are used to monitor the partial discharge of generator on line.