New thinking on the selection of excitation system for large hydroelectric generating units The parameters of the self-excited excitation system of the large-scale hydroelectric generating units of Tsinghua University's power system should be explained as follows: 1. For the Itaipu hydropower station in Brazil, Paraguay, it was adopted in the early stage of operation. A three-phase bridge rectifier circuit with positive and negative excitation currents can increase the phase-in capacity of the generator. In the later stage of power generation, it is said that the positive and negative excitation circuits have been out of operation.
2. For the Sayan-Shushensk hydropower station, most of the units use a separately excited thyristor excitation system, some of which use a self-excitation excitation system with high and low voltage bridge rectifiers, as shown. The parameters related to the contact system are: excitation transformer wiring â–³ / into the secondary voltage secondary current rated strong excitation excitation transformer rated capacity generator rated excitation voltage rated excitation current typical high and low voltage bridge rectifier line rectification voltage and current waveform diagram Shown.
The main components of the high and low bridge rectifier circuit wiring diagram of the Sayan-Shushensk hydropower station are: rectifier transformer TU; thyristor converter U1 (working group) and U2 (strong excitation group) Self-use transformer TB; automatic de-excitation switch (ATH) QE; built-in discharge FV, shunt resistor R1 contactor Q1 and Q2 overvoltage protection; automatic voltage regulator with feedback divider CAH; initial supply from battery excitation means built contactors 04 and Q3, resistors R2, isolation diode V1 / V2. Id- rectified current, Id = 3300A '. working group a current conducting time of the group of strong excitation current conducting time of the control angle ap working group The former Soviet Union developed an excitation system with high and low voltage bridge rectifier lines in the early 1970s, which is widely used in thyristor AC exciter circuits and self-excitation excitation systems. The main purpose is to use the single three-phase bridge type full-controlled rectification line when the excitation system is required to provide a higher maximum value of the top voltage, in order to meet the requirements of the no-load, rated and strong excitation of the generator, the AC exciter or The secondary rated voltage of the excitation transformer will meet the requirement of multiple excitation top value, so that the controlled rectifier of the excitation or self-excitation excitation system will be in the deep control state when the generator is under no load or rated state, which will cause rectification. Severe distortion of the voltage waveform. Harmonic losses and spike overvoltages caused by commutation also increase significantly and jeopardize the safety of the excitation system. If the three-phase high and low voltage bridge rectifier circuit is used as shown, the situation will be improved, because the normal excitation of the generator will be supplied by the low voltage group rectifier bridge in this line, the control angle is small, and the high voltage group rectifier bridge Almost in a closed state, only a small excitation current is supplied. In the case of strong excitation, the high-voltage group rectifier bridge is fully open and the low-voltage group rectifier bridge is locked. According to this working mode, the harmonic loss of the excitation system and the commutation can be significantly improved. The spike overvoltage can fundamentally improve the operating characteristics of the self-excited excitation system.
Second, the selection of the secondary rated voltage of the excitation transformer Currently, in some of the world-class large hydropower stations, such as Brazil's Paraguay Yitaipu, Venezuela's Guri, Russia's Sayan Shushensk part of the unit and China's already put into operation the Three Gorges Hydropower Station The main hydraulic turbine engine unit with a capacity of about 700 MW adopts self-excitation excitation mode.
In large hydroelectric generating sets, the use of self-excitation excitation systems has become mainstream. The following is a brief discussion of some key issues in the selection of the excitation mode.
It is well known that in the self-excitation excitation system, the selection of the secondary rated voltage of the excitation transformer (hereinafter referred to as the anode voltage) depends on the multiple of the strong excitation top voltage, and the higher the top voltage multiplier, the higher the anode voltage value. On the other hand, the thyristor element connected to the power rectifier bridge on the secondary winding side of the excitation transformer is suddenly turned off at the end of the commutation, so that the magnetic energy stored in the secondary winding of the excitation transformer is released, usually The RC resistor-capacitor buffer connected in parallel with the rectifying element is bypassed and needs to be balanced between energy consumption, storage and energy consumption, and has a certain absorption capacity reserve. Otherwise, the RC element absorbing the commutation energy will be burned, thereby causing The occurrence of serious accidents in the phase-to-phase short circuit between rectifier bridges has occurred many times in domestic water and thermal power stations. The energy absorbed by the RC damper is calculated according to the American Westinghouse recommendation: the energy ratio will be (1243) 2 / In addition, the higher the anode voltage of the rectifier bridge, the more the rectifier will be in the deep state under the rated state. The state, which causes the peak overvoltage that jeopardizes the safe operation of the rectifying element, is also higher. From this aspect, it is also expected that the basic guarantee of the safe operation of the excitation system should be taken into consideration while satisfying the stability requirements of the power system. Balanced. From the point of view of the current operation of the unit in China, when the anode voltage of the rectifier bridge exceeds 1000V, it can be considered to have entered the high parameter range. 3. The selection of the insulation method of the excitation transformer In recent years, due to the development and application of new insulation materials and the progress of manufacturing technology, The performance of dry-type transformers has been greatly improved, and their operating characteristics are also safer, more reliable, and meet environmental requirements.
Dry-type transformers can be classified into two types of dry-type transformers: epoxy resin (winding type and wrap type) and Nomex type (open type and encapsulation type) depending on the insulating material selected for the transformer.
The so-called Nomex insulation material refers to the aromatic polyamide insulation material and laminate produced by DuPont of the United States. It is a high quality insulation material with unique electrical and mechanical properties.
The following is a brief introduction to the performance characteristics of various types of dry-type transformers.
1. Epoxy-based dry-type transformers As mentioned above, epoxy-type dry-type transformers include transformers with winding type and wrap-around type of insulation, among which, especially epoxy-wound dry type transformers, since the 1960s Since its inception, it has experienced several development processes including thick insulating tape packing, thin insulating without packing and thin insulating tape packing. After adopting specially treated glass fiber reinforced materials to improve the conductive padding and heat dissipation capability of the winding, The epoxy resin injection type 1000 transformer can be manufactured to a voltage of 35KV, a capacity of 20MVA and an insulation grade of F (155C). It is currently a widely used variety.
In the future, based on the use of special glass fiber braids to improve the thin insulation winding process and flame retardant properties, products with insulation grades up to H (180C) will be developed.
The difference between the epoxy resin package type and the above-mentioned wound type dry type transformer is that the former does not need to wrap the mold, and the winding is wound under vacuum conditions, and the epoxy resin is wound and dried to solidify the Chinese hydropower project. Learned the power excitation conference papers set 203 in Yichang, Hubei, once formed.
2, NOMEX type open type impregnation, dry type transformer (SG) For Nomex type open type dry type transformer, generally, the low voltage winding is mostly made of foil type or multiple layers and the layered structure, and the high voltage winding is a cake type structure, winding Vacuum impregnation or vacuum pressure impregnation process can be used, and the insulation grade can reach H or C.
For the open impregnated dry type transformer, there are currently two types of processes and structures of the US type and the German type, and the differences are shown in Table 2.
The open type dry type transformer has a simple manufacturing process, similar to an oil-immersed transformer, and has a low manufacturing cost, but is not as good as an anti-fouling and moisture-proof performance, such as an encapsulated type (SCR) thousand-type transformer, an open type, and a dry type transformer. As shown.
DuPont ReliafraN technology open-type impregnated dry-type transformers Figure 2 Structure and insulation characteristics of open-type impregnated dry-type transformers American structural type features German structural type features a, turn-to-turn insulation using Nomex paper b, spacers using Nomex cardboard or combs Strip c, impregnation process using vacuum pressure impregnation (VPI) a, inter-turn insulation using glass fiber b, spacer using ceramic sheet c, impregnation process using vacuum impregnation (VI) 3, Nomex-type encapsulated pre-impregnated dry-type transformer ( The SCR) encapsulated pre-impregnated dry-type transformer is a superior dry-type transformer that uses a Class H (180C) insulation system, while the main insulation uses Nomex Class C (220C) insulation. The low-voltage winding of the encapsulated dry-type transformer is foil type, and the high-voltage winding is layered. The products have good performance in anti-pollution, moisture-proof and anti-lightning impact and harmonic overload capability.
It should be emphasized that the Nomex encapsulated dry-type transformer is currently the only one of the thousands of transformers required by all European and international standards, including: In HD464/S1, three special tests for climate, environment and fire resistance were proposed for dry-type transformers. Later, in 1993, the HD464/S1 standard was equivalently adopted in the French NFC-52-726 standard. In 2000, These three special requirements are also included in the draft standard of IEC60073-11, and it is clearly stated that the grades of the three special tests must be indicated on the nameplate of the dry-type transformer, such as climate, environment and fire test.
For this reason, for the excitation transformer with high performance requirements, the use of Nomex-type dry-type transformer is a suitable choice. See the three special performance tests.
The three special performance tests of dry-type transformers have high smoke transparency, low temperature and no harmful gases when burned.
It can operate safely under the condition of quenching and rapid heat. It can be put into operation directly under the environment of 25 °C and can operate safely under severe humidity and pollution. 4. Selection of demagnetization system 1. Magnetic field circuit breaker and demagnetization method At present, there are several types of degaussing methods widely used in China: a DC magnetic field circuit breaker; b AC circuit breaker C AC circuit breaker is connected to the AC voltage demagnetization mode formed by the AC side of the rectifier or the rectification side of the excitation circuit.
The first item a DC circuit breaker is used for the traditional demagnetization mode, and the second item b is used for the AC circuit breaker connected to the AC side. In recent years, the unit has been applied. For example, the 300MW pumped storage hydroelectric generating unit of Guangzhou Pumped Storage Power Station Application, the application of the AC circuit breaker, that is, the AC circuit breaker connected to the AC side of the rectifier is used for de-excitation, the de-excitation starts to perform the inverter de-excitation, and after reaching the set time, the AC circuit breaker is jumped, and the thyristor rectifier is cut off. The pulse forms an alternating voltage demagnetization mode. The third item c, the so-called AC voltage demagnetization method, is not substantially different from the second demagnetization method. The difference is that the AC circuit breaker can be connected to the AC or DC side. The advantage of this demagnetization method is that it can be utilized. The line voltage on the AC power supply side is used as a supplement to the arc voltage of the AC circuit breaker, which is beneficial to the transfer of the de-excitation energy.
In addition, the choice of the magnetic circuit breaker has a greater choice than the DC circuit breaker in the selection of the magnetic circuit breaker.
It should be noted that for a large hydro-generator set with a capacity of about 700 MW, the excitation current is usually above 4000 A. In addition, considering the requirement of de-excitation to the shutdown voltage, it is sometimes necessary to use multiple short-arc. The DC contactor constitutes the requirement of connecting the DC circuit breaker in series. For example, the DC magnetic circuit breaker in the Three Gorges Hydropower Plant is composed of eight DC contactors with a 500V fault voltage connected in series to form a DC circuit breaker of the type CEX-5500. However, excessive mechanical operation mechanism is not desirable from the viewpoint of ensuring the reliability of the demagnetization system, because multiple series fractures are difficult to ensure the synchronization of the break when the fracture is opened and closed, and if the spring pressure is improperly adjusted, It will also cause the closed contact product to rebound and instantaneously break. In the debugging of the excitation device of the Three Gorges Hydropower Plant, there has been a case where the arc contact is closed and then rebounds instantaneously (1-2) ms. For this reason, when the capacity of the DC circuit breaker is limited, the AC voltage de-excitation method using the AC air circuit breaker as the de-excitation circuit breaker is a simple, reliable and easy to implement solution, because in terms of capacity, The capacity of the standard AC air circuit breaker is not limited. The shortage of the voltage of the AC circuit breaker at the time of demagnetization can be solved by the measure of introducing the negative voltage of the secondary voltage of the excitation transformer after the pulse of the power rectifier.
If the demagnetization residual voltage at the time of demagnetization is 3000V, it is assumed that an AC breaker with 4 fractures is connected to the generator excitation circuit side, and the voltage of each fracture is 500V (the arc cover is improved), then the total synthesis The fracture voltage is 4X500=2000V, and the negative half cycle voltage amplitude provided by the secondary side of the excitation transformer is 3000V-2000V=1000V, and the corresponding secondary voltage effective value is 1000/10000 580V, that is, when the secondary voltage of the excitation transformer is greater than this value It can meet the requirements of establishing the corresponding demagnetization residual pressure.
It should be noted that the advantage of connecting the AC breaker to the DC side is that the voltage generated by all the fractures can be fully utilized, and the fracture voltage is not affected by the voltage drop of the power supply side and can be effective when a short circuit fault occurs on the AC side of the rectifier. Demagnetize the ground.
2. Selection of demagnetization resistance capacity For the hydro-generator unit, since the rotor pole of the generator has a salient pole structure, the damping effect of the rotor winding loop is weak. When the demagnetization is performed, all the magnetic field energy is absorbed by the de-excitation circuit. This puts more stringent requirements on the capacity of the de-excitation resistor. The de-excitation energy can be determined by the following normal or fault operation modes, and the maximum value is selected as the basis for selecting the de-excitation resistor valve.
a generator no-load demagnetization b generator rated demagnetization c generator strong de-excitation d generator sudden short-circuit demagnetization e no-load out of control mis-excitation excitation magnetic when the generator and transformer internal fault caused by the de-excitation energy is much smaller than The corresponding value of one of the above five ways. To this end, the above five types of demagnetization can be used as the basis for determining the demagnetization capacity. For the three kinds of demagnetization methods of items a, b and c, the selection of the demagnetization resistance capacity should be based on the respective excitation state of the generator as the main basis for selecting the demagnetization resistance capacity. For the magnetic field energy generated by the generator three-phase sudden short-circuit state flowing through the generator rotor excitation winding circuit, the influence on the de-excitation resistance capacity should be selectively considered. For large hydroelectric generating units with split-phase busbars, the probability of such accidents is extremely small. Secondly, during the sudden three-phase short circuit, the transient time constant Td of the stator-side of the generator is much smaller than that of the generator. The no-load time constant Tdo, the non-periodic component of the rotor decays faster. At the moment of sudden short-circuit, the current can be naturally attenuated by the thyristor rectifier bridge forming a freewheeling loop. The analysis shows that the rotor has a non-periodic component when the generator is suddenly short-circuited. The maximum value Ifrn is about 3IFn, or is obtained according to the following formula: xd, xd-generator steady state and transient direct-axis synchronous reactance (pu). During the sudden three-phase short circuit of the generator stator, the rotor circuit excitation current changes. As shown.
The generator is set to a three-phase sudden short circuit, and the rotor excitation current changes after a certain fault duration, usually not exceeding 0.1S. The magnetic field circuit breaker is connected to the de-excitation resistor, and the rotor non-periodic component DC current will be proportional to the stator winding three-phase. The short-circuit time constant Ta is attenuated by a smaller time constant. For the ALSTOM hydro-generator set of the Three Gorges Hydropower Plant, Tdo=10.1S and Ta is only 0.28S. Based on the above analysis, considering the sudden three-phase short-circuit condition of the stator winding of the generator. The following characteristics should be noted when demagnetizing the resistance capacity.
a, the non-periodic component of the rotor current is attenuated by the Ta time constant, and the total decay time is taken as the example of the ALSTOM model unit of the Three Gorges Hydropower Plant, which is about 4Ta=4X.28=1.12S. When the de-excitation resistor is connected, the decay time will be Less than 4Ta, the voltage change of the magnetic circuit breaker breakage is as shown.
b. The maximum non-periodic DC component value of the demagnetization resistance is 3IfN, and its value corresponds to the time of resection.
Sudden three-phase short circuit at the generator end, the voltage across the main contact breaker after the excitation circuit breaker is faulty. For the e-generator, the unloading of the generator is out of control. This state is the most common choice for the demagnetization resistance capacity. In fact, In essence, this is a very debatable question in the concept of demagnetization. Because the item e de-excitation condition, that is, different from the items a, b, and c, is different from d, the items a, b, and c are the conditions that occur during operation, and the de-excitation system must satisfy these requirements. Although the situation is rare, it may happen after all, and it should be noted that the problem facing the demagnetization of the item d is the sudden change of the current source, and there is no room for foreseeable and early prevention. At this time, only the fault can be removed before the fault is removed. Firstly, the non-periodic current of the rotor is allowed to flow to the rectifier bridge circuit with sufficient capacity to naturally flow and attenuate to avoid its edge. Then, after the current drops to a certain extent, the current is protected by the relay protection action, and the demagnetization resistance capacity is limited. a practical, safe and reasonable level. E erroneous item of load off the strong excitation in terms of the magnetic case, it should be noted that a voltage is transformed by the field excitation current process, coupled with the generator at no load, the rotor time constant Tdo large, when the rotor current increases to rising to a terminal voltage of the generator and the load current value corresponding to 1.3 times, there is a very long process of buffering, as preventive measures countermeasure process, the change process discuss below: first, since the excitation regulator control, All AVR limiting unit will fail, for no-load current error strong growth excitation C, selected magnetoresistive field off capacity taking into account the short-time overload capability of suppression resistance, so that the resistance of the selected nominal capacity demagnetization Can be greatly reduced.
Restrictions can only be completed by the relay.
Before deciding the effectiveness of relay protection, first define the time to limit the growth of the false positive excitation to the final value, which is estimated to be within 35, and the meaning of this determination time is: if the time of the protection action is outside this time, this Protection against the limitation of the strong excitation current will not help, taking the Three Gorges unit as an example, can be used to limit the protection of the false positive excitation current value: generator stator overvoltage protection 120% can be seen from the above data, when the generator voltage rises When 120% has 5sçµ, the voltage will continue to rise to 130% and trip through 0.3s. At this time, the generator excitation current is limited to the value corresponding to 1.31, which is close to or slightly higher than the rated excitation current. For this reason, the corresponding de-excitation capacity is equivalent to the rated state de-excitation capacity. To limit the excitation current when the no-load mis-excitation is around the rated excitation current, the Three Gorges Hydropower Plant has two sets of 130%, 0.5s delay at the generator end. Two sets of double overvoltage protection to improve the reliability of the action. Other protections, such as V/HZ protection and over-current protection of the excitation transformer, are too long to effectively limit the increase in the no-load excitation current.
On the other hand, if the final value current is forced by the no-load error (this value is about 19100A for the Three Gorges unit), it is not operability to select the de-excitation resistor capacity. Firstly, for the magnetic field circuit breaker, under the condition of no-load mis-excitation, the strong excitation voltage of the erroneous excitation current reaches the final value is nearly 2300V, which greatly weakens the reverse voltage value synthesized when the DC magnetic field circuit breaker is de-energized. its value 4000V- 2300V = 1700V, the residual pressure value and the current selection rather Sic suppression resistance, Burgundy P: 8 series DC contactor fracture voltage, the output error when strong excitation Ufin forward voltage of the rectifier: URSic off The residual value of the magnetoresistance flowing through the mis-energized final value current.
Equation (3) shows that at this time, the demagnetization is in the limit state of complete commutation, and the demagnetization current reaches a high parameter state of nearly 4.6 times the rated excitation current. Under this demagnetization condition, for the principle of short arc The arc-extinguishing CEX-5500 circuit breaker will inevitably burn out. The accident of the Gezhouba Power Plant burning the magnetic field circuit breaker under the condition of no-load mis-excitation is equivalent to this situation.
In addition, from the heat capacity limitation condition of the generator rotor excitation winding, it is known that the generator rotor current is limited by the inverse time limit. When t=10s, 2 times the rated excitation current is allowed. If the I2t heat capacity is considered to be a constant value, it is not difficult. When the excitation current is 4.6 times the rated value, the generator rotor is allowed to flow through this current for 1.85 s. During this time, the whole process of de-excitation has not been completed, and the result is that the generator rotor winding is overloaded and jeopardized. Security to the host. If the magnetic field breaker or the generator excitation winding can not bear the no-load mis-excitation final value current value, then what is the significance of the degaussing resistance valve plate selected by this condition up to several tens of M? For the selection of the demagnetization resistance capacity of large hydro-generators, it should be based on objective reality, taking into account not only the requirements of the excitation system, but also the safety of the operation of the main engine. When considering the maximum capacity of the demagnetization resistance, the DC component of the rotor current caused by the sudden three-phase short circuit of the generator can be selected as the basis for selection. At the same time, the value of the DC component should be selected according to the corresponding current value at the time of fault removal, and noticed. After the de-excitation resistor is connected, the influence of the three-phase short-circuit time constant Ta of the generator and the short-time overload capability of the de-excitation resistance are combined, and the rated capacity of the de-excitation resistor is positioned at a reasonable value by comprehensive factors.
As for the no-load mis-excitation state, it is a simple and effective measure to “cut off†the increasing excitation current by means of 1.3 times generator over-voltage protection action, in order to further clarify the rotor when the generator suddenly has three-phase short circuit. The variation law and calculation method of current non-periodic DC component are now combined with the specific parameters of the hydro-generator of the Three Gorges Hydropower Plant to conduct a special study, in order to provide a strong basis for the selection of the demagnetization resistance capacity.
If the linear resistance demagnetization mode is not considered, in terms of varistor, there are mainly Zno and Sic valve plates to choose from.
For the Zno valve piece, China's own development of products with independent intellectual property rights, its superior performance, a wide range of applications, is a fact that is obvious to all, not to describe too much.
The performance of the Sic valve is now combined with the application and test results of the 700MW hydro-generator de-excitation system of the Three Gorges Hydropower Plant as an example for the Sic valve of the ABB 700MW turbine generator set deactivation system of the Three Gorges Hydropower Plant. The VI characteristic curve is as shown.
Sic valve VI characteristic curve aSpec.6361Sic valve basic parameters valve diameter 4>158 thickness 20 each piece rated capacity 75K temperature rise 1051 (ambient temperature 25K individual case 90K temperature rise 130K rated current 250Ab demagnetization resistance composition series number S=2 parallel circuit number P=126 total valve number 252 pieces 4 demagnetization time The definition of demagnetization time of the large hydro-generator excitation system has different definitions.
a is defined as the demagnetization time when the generator terminal voltage drops to near the residual voltage or less than 500V, because the arc of the core ground fault point is automatically extinguished when the stator voltage is less than 500V. b According to the time when the generator rotor current is zero, the interval is defined as the demagnetization time. c Sometimes defined as the demagnetization time when the rotor current drops to the initial value of 10% according to the contract. For example, the de-excitation system of the Three Gorges hydropower plant and the de-excitation system of the Guri hydropower plant in Venezuela adopt this definition standard.
The definition of the demagnetization time should be clear. The demagnetization time is defined by the zero voltage of the generator, which directly reflects the effect of demagnetization. However, the rotor current is zero, and the demagnetization time is not fully reflected. The demagnetization effect is not fully reflected when the rotor current is zero. The machine still has a residual pressure of about 10%, and the basis for defining the demagnetization time with a rotor current of 10% is mainly considering that the energy stored in the rotor winding circuit below 10% has a very small value, and continues to determine the extinction duration. magnetic is no much sense.
The demagnetization test results of the 700 MW unit are used to understand the demagnetization characteristics of the generator without load in the demagnetization system.
The attenuation characteristics of the no-load voltage and the excitation current of the inverter de-excitation generator are shown in 0.
一啊疆ié—º(1)丨(1):丨畔;丨(1) 丨切ç£ç£æ–æ–机 generator no-load degaussing characteristics 0 Inverter de-exciting generator de-excitation characteristics are determined by the generator voltage of 3% rated value magnetic time tm4.0S. 0 demagnetization by the inverter mode and the generator voltage was 3% of the rating determined for the de-excitation time and fall time when the de-excitation of the rotor by a current value of zero 3.55S. press the initial rotor current The demagnetization time determined by 10% of the value is 1.9S. It can be seen from 0 that the generator voltage is also reduced with the decrease of the rotor current during the inverter process, and the inverter voltage is not constant, so the demagnetization effect Compared with the Sic nonlinear demagnetization resistance, the effect of the demagnetization method is equivalent.
V. Power System Stabilizer PSS For large hydro-generator sets using self-excitation system, in order to compensate for the negative damping effect caused by the hysteresis characteristics of the excitation system, the power system stabilizer PSS must be used to solve the low-frequency oscillation problem in the power system. .
Low-frequency oscillations can occur between the generator and the grid, or between the generator groups and the weakly interconnected interconnected grid, essentially due to the relative rocking of the rotor power angle between the associated generator sets under small disturbances. The resulting active oscillation mode.
For the oscillating frequency between the generator and the grid, the value is higher, generally between 1.0HZ and 2.0HZ. Taking the Three Gorges unit as an example, the main frequency of oscillation between the generator and the grid is at (1.2-1.3) HZ, and the power is generated. The frequency of the oscillation mode between the clusters is around 1.0HZ. For the oscillation mode between the weakly connected interconnected grids, the oscillation frequency is generally between 0.2HZ and 0.5HZ, and the transmission capacity is increased with the capacity of the grid-connected grid. The length of the oscillation between the Internet will be further reduced. The Internet-related oscillation frequency of the Ertan Hydropower Station in the Sichuan-Yunnan Power Grid is 0.3HZ. Before the Three Gorges Hydropower Plant was put into operation, it was in the PSS setting of each domestic hydropower plant. The lowest frequency setting. For the units of the Three Gorges Hydropower Plant, the five major power grids in Northeast China, North China, Northwest China, East China and Central China are connected.
With the expansion of capacity, the frequency of oscillation between the Internet will be further reduced, and the value will be reduced to about 0.13HZ. The application of PSS in this low-frequency range is the first application in China or in the world, and it is still in operation. There are many key technical issues to be resolved.
Combined with the application situation of the PSS of the Three Gorges Hydropower Plant, the following several opinions are proposed for selecting the PSS technical specifications in the large hydro-generator set.
The aPSS adopts a single electric power signal, which will cause the reverse regulation of reactive power during the process of increasing or decreasing active power. When the situation is serious, it will cause over-excitation or low-excitation limiter action.
b. In the case of increasing or decreasing active power, such as the use of instantaneous blocking PSS output is an effective measure. It is still feasible to use a single power signal plus reverse lockout procedure for units that do not require high oscillation frequency damping.
C. For units that require high damping frequency in the low frequency range, for example, it is required to dampen the lowest oscillation frequency below 0.2HZ. It is absolutely necessary to use dual signals, such as electric power and speed signals, to improve the reactive power reverse and expand the frequency range of PSS adaptation. .
When the national adjustment of the D is set, the PSS parameters should be carefully monitored, taking into account the improvement of the grid structure of the power grid. The single requirements for the PSS parameters of a regional unit are too high, which often affects the PSS damping. The function of the low-frequency oscillation between the generator and the grid and the generator group is also important for ensuring safe power generation.
1. For the large-scale hydro-generator unit of the Three Gorges unit capacity class, the self-excitation excitation system with inherently high initial response performance has become a mainstream of development, but when selecting the secondary rated voltage of the excitation transformer, it should be taken into consideration. The requirements of the power system and the basic guarantee for ensuring the long-term safe operation of the unit should not be unilaterally pursued with high parameters. As a result, the accident that guarantees the stability of the system does not occur, but the high excitation parameters result in the unit excitation system in daily operation. The accidents continue, and there are many examples of such painful lessons in the introduction of crews.
2. For large hydropower units, high and low voltage bridge rectifier circuits should be developed as soon as possible. This line can significantly improve and avoid the rectifier in the deep control state under rated conditions, and reduce the peak overvoltage multiple, which provides a favorable guarantee for the safe operation of the unit.
3. In the selection of the parameters of the de-excitation system of large hydro-generator units, the basic concepts should be discussed and reflected in depth. Because of the unsuitable concept, for example, the capacity of the de-excitation resistor is selected according to the conditions of no-load mis-excitation, and the result is inevitable. It is a redundant device that is too much to solve the problem of the essence of the demagnetization.
According to the sudden three-phase short-circuit condition of the generator proposed in this paper, and the non-periodic DC component of the rotor current at the time of resolving the fault as the basis for selecting the de-excitation resistance capacity, it should also be noted that after the nonlinear de-excitation resistor is connected, the power generation is performed. The three-phase short-circuit time constant will be further reduced. For the Three Gorges unit, Ta=0.28S. For this reason, the whole process of de-excitation when the generator is three-phase short-circuit will not exceed 1.5S (4Ta=1.12S). The demagnetization resistance capacity can significantly reduce the composition and cost of the device.
4. There are two feasible ways to select the de-excitation system of large hydropower units. One is that the Zon varistor valve is suitable under the premise that the current capacity fracture voltage of the long-arc DC magnetic circuit breaker meets the requirements of de-excitation. Solution, because this scheme can obtain faster demagnetization speed. If there is no corresponding long-arc DC magnetic circuit breaker, you can use DC breakers such as CEX series and multiple short-arc DC contactors, so there are many contactors. When there are series connection, the contacts have different points, no synchronization and rebound.
When the current capacity of the current DC magnetic circuit breaker can not meet the requirements of the excitation system, it is the most common to form an AC voltage de-excitation system based on a series of air-to-air circuit breakers that are mass-produced, inexpensive, and unlimited in current capacity. Good choice, regarding the problem that the arc voltage of the air AC breaker is too low, the AC arc breaker can be connected to the DC test and the four contacts can be connected in series and the arc shield can be modified to achieve a total arc voltage of (2000-2400)V. On this basis, the AC negative half cycle voltage formed after the pull pulse is introduced again; it is feasible to make the reverse voltage reach 4000V and above.
In view of the fact that the arc voltage established by the AC circuit breaker is low, it is easy to convert the current during the demagnetization. In this case, it is suitable to use the Sic de-excitation resistor with soft VI characteristics. This scheme is simple, reliable and easy to implement. The demagnetization time of the three-machine unit with demagnetization time, when Tdo=10S, the achievable demagnetization time is: 5. When conducting the selection of the excitation system of the large hydropower unit, the communication between the communication majors should be fully carried out. AC, such as the coordination between the generator loss of magnetic protection and the excitation low excitation limit, the low frequency oscillation value of the interconnected grid and the frequency range of the host oscillation mode.
6. When selecting the excitation parameters of large hydropower units, the requirements of high parameters should be avoided as much as possible. The safety of the normal operation of the unit should be taken into consideration. The equipment should be simple and reliable. In addition, when introducing foreign excitation equipment, it should not be excessively strong.
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