WO2012130163A1

WO2012130163A1 - Method for generator subsynchronous current calculation, subsynchronous over-current and divergence protection

Info

Publication number: WO2012130163A1
Application number: PCT/CN2012/073327
Authority: WO
Inventors: 张琦雪; 牛红海; 柏传军; 熊良根; 王凯; 陈俊; 严伟
Original assignee: 南京南瑞继保电气有限公司; 南京南瑞继保工程技术有限公司
Priority date: 2011-04-01
Filing date: 2012-03-30
Publication date: 2012-10-04
Also published as: CN102185283A; CN102185283B

Abstract

A method for generator subsynchronous current calculation, subsynchronous over-current and divergence protection comprises: collecting a three-phrase current signal of a generator; converting the positive sequence and negative sequence components of the subsynchronous current of each mode into an equivalent industrial frequency negative sequence current, which is substituted into the original industrial frequency negative sequence over-current protection criterion of the generator, for judging definite time and inverse time protection; and at the same time judging whether the equivalent industrial frequency negative sequence over-current increases continuously, which constitutes the divergence protection criterion. The present invention can detect the multi-mode subsynchronous current in the armature current of the generator, and realize over-current protection and divergence protection of the subsynchronous current.

Description

Generator secondary synchronous current calculation and secondary synchronous overcurrent and divergence protection method

TECHNICAL FIELD The present invention relates to the technical field of generator protection in a power system, and more particularly to subsynchronous oscillation of a turbo generator, and to a secondary synchronous current detection, an overcurrent protection, a divergence protection method, and a corresponding Relay protection device or monitoring device. Background technique

Some large thermal power plants are far away from the load center, and the generator set has large capacity. Long-distance transmission is often used, and power electronic equipment such as series capacitor compensation and controllable series capacitor compensation are arranged in the transmission line, and sometimes DC transmission is adopted. These units pass through the capacitor-compensated line or the unloaded long-distance line. When the parameters are not properly matched, the asynchronous self-excitation phenomenon (referred to as asynchronous self-excitation) may occur, and the armature current in the stator winding of the generator appears slightly lower than the synchronous frequency. The frequency of the subsynchronous frequency of the frequency, the subsynchronous current will generate the pulse vibration torque, causing mechanical resonance and large-axis torsional vibration, which will cause the unit to generate heat and vibration, which will damage the equipment in severe cases. The subsynchronous oscillation frequency is mostly between 10 Hz and 45 Hz.

For example, China's Inner Mongolia Datang Tuoketuo Power Plant has an installed capacity of 8 X 600 MW, which is transported to the Wuyuan Substation via the four-line line of the Token source, and then sent to the Beijing-Tianjin-Tangshan Power Grid via the Yuan'an Double-circuit Line and the Yuanba Double-circuit Line. . Eight sets of fixed capacitor series are installed on the line of the Wuyuan substation. After analysis, there is a risk of subsynchronous resonance. For this purpose, a static blocking filter (SBF) is installed on the high voltage side of the generator main transformer, and shaft torsional vibration protection (TSR) is installed for the unit. SBF is a circuit in which a group of inductors and capacitors are connected in series and in parallel. In April 2008, during the test of SBF input, the Tuoketuo power plant showed obvious asynchronous self-excitation, and the current was diverging, which made the generator set unable to operate normally. After adjusting the design parameters of SBF, 2 years later, SBF was officially put into operation, and asynchronous self-excitation was suppressed. To prevent damage to the device due to similar asynchronous self-excitation, a secondary synchronous overcurrent protection device is required.

When the generator exhibits asynchronous self-oscillation, the three-phase current of the generator stator winding has both the power frequency (50 Hz) component and the secondary synchronization (10 Hz to 45 Hz) component, and sometimes multiple secondary synchronization components may occur. The sub-synchronous component is called the modal component of the corresponding frequency. The three-phase steady-state subsynchronous current flows through the generator armature winding. The positive sequence and the negative sequence current generate a rotating magnetic field, which has relative motion with the rotor. The eddy current of the slip frequency is generated in the rotor, which causes the rotor to generate heat and also causes vibration.

At present, there is very little overcurrent protection for the subsynchronous current. Usually, only one modal subsynchronous current is extracted by one filter, and then the amplitude of the current is compared with a preset value, and a preset is passed. The delay sends an alarm or trip signal. Although this method is simple, it has obvious disadvantages: (1) It is not enough to extract only one modal current. When the actual unit is running, there may be multiple modal secondary synchronous currents, such as Tocto When the generator of the plant is asynchronously self-excited, the secondary synchronous currents of two frequencies near 27Hz and 39Hz appear; (2) The setting of the protection setting is arbitrarily set, lacking theoretical basis; (3) The protection function is single, only provided Time-limited overcurrent protection. SUMMARY OF THE INVENTION The object of the present invention is to provide a method for detecting a secondary synchronous current of a plurality of modes in a generator armature current, and a method for implementing overcurrent protection and divergence protection of a secondary synchronous current. The technical solution adopted by the invention is: detecting a plurality of sub-synchronous modal components in the armature current of the generator, converting into an equivalent power frequency negative sequence current, realizing a method of definite time, inverse time overcurrent protection and divergence protection. The protection device or the monitoring device collects the three-phase current signals _A and _B k of the generator with a fixed sampling frequency: through low-pass filtering and band-pass filtering, separates multiple sets of signals of each modal subsynchronous modal frequency " _m , _A , ' _m , _B , r _m , _C , modal m=l, 2,... _; use the zero-crossing algorithm to obtain the frequency f _{m of the} subsynchronous mode, and use Fourier's calculation to obtain the amplitude sequence of the modal signal and Corresponding phasor Lu i); Find the positive sequence component i _mJ and negative sequence component Im, ₂ corresponding to the three-phase current of each mode _; convert the positive sequence and negative sequence components of each mode into equivalent power frequency The negative sequence current I _eq , ₂ ; then the calculation of the definite time, inverse time over current protection and divergence protection.

(1) Current signal sampling. The protection device or the monitoring device uses a fixed sampling frequency to sample the secondary side current of the generator current transformer CT to obtain a three-phase current, _B ic;

(2) Low-pass filtering, the filter parameters are fixed in the fixed value of the protection device or the monitoring device, and no adjustment is needed. After _A , _B , and a high-order low-pass filter, the result is:

Among them, Hz is a low-pass filter, and the cutoff frequency is in the range of 43Ηζ~48Ηζ.

(3) Bandpass filtering, the filtering parameters are fixed in the settings of the protection device or the monitoring device. According to the requirements of the protected generator, it is determined that m sets of high-order band-pass filters are required, generally 2~4, for example, only concerned with the secondary synchronous currents of the two modal frequencies, then only 2 band-pass filters need to be designed. For example, a bandpass filter covering two frequencies near 27Ηζ, 39Ηζ. The above signal is further filtered to obtain a three-phase secondary synchronous modal current: m = 1,2.

Where H _m W is the mth bandpass filter.

(4) Zero-crossing frequency measurement and Fourier filtering. The signals obtained after the above processing are all sinusoidal waves having a relatively good sine. The zero-crossing algorithm can be used to calculate the frequency of the signals " _m , _A , ' _m , _B , " _m , _c , and the frequency of the three-phase synchronous modal current is averaged as the final modal frequency^. Using the Fourier algorithm to calculate the amplitude sequence of the subsynchronous modal signals r _m , _A , r _m , _B , r _m , _c / _m » and the corresponding phasor α ί), get: -jb _mi (n))

i = A, B, C, m = 1, 2. Equation 3

Roundif f _m ) where / _s is the sampling frequency of the protection device or monitoring device, / _m is the subsynchronous modal frequency, ^ is the data window length corresponding to / ^, /^(«) is the amplitude sequence of the subsynchronous current , is the corresponding phasor. Since the previous two filterings may cause signal attenuation, the gain correction coefficient _gm is added in Equation 3 to perform amplitude correction. The correction factor is a function of the modal frequency, which is determined in advance by the amplitude-frequency characteristics of the filter.

(5) For the three-phase currents of each mode, find their positive and negative sequence components, and obtain: ( ) + a ■ I ( ) + a ² -I _m , cn) 3

n,A(n) + a ² - lm,B(n) + ■ l _m ,c

4 Equation 5

The theoretical basis of this conversion is: a three-phase steady-state subsynchronous current flows through the generator armature winding, and its positive sequence and negative sequence current generate a rotating magnetic field, which has a relative motion with the rotor, and a vortex that generates a slip frequency in the rotor, which generates electricity. The principle that the machine frequency negative sequence current produces 2 times frequency eddy current on the rotor is similar; if the generator is regarded as an ideal model, for the solid rotor of the turbo generator, the eddy current of a certain frequency, its loss and The frequency is proportional to the power of 1.5 and is proportional to the square of the magnitude of the magnetic field, approximately proportional to the square of the effective value of the armature current sequence component that produces the magnetic field. To this end, the subsynchronous current can be converted to an equivalent power frequency negative sequence current. In the above formula 5, k ₁ and k _m , ₂ are correction coefficients, and Λ is the measured power frequency. For the actual turbo generator, the rotor structure is very complicated, the rotor is slotted, and there are ventilation holes and crescent grooves, which will affect the path of the magnetic field and the eddy current; the wedge of the aluminum material or the magnetic material and the rotor end retaining ring It forms a damper circuit, and the damper circuit has eddy currents, which have more complicated effects on the magnetic field. In addition, the eddy current of the rotor also weakens the magnetic field. Magnetic field saturation will increase the equivalent air gap in the magnetic circuit and weaken the magnetic field. Therefore, the correction coefficient k _m , ^B k _m , _{2 is} retained in Equation 5, and the value range is 0.8 to 1.0, which is 0.9 depending on experience. When necessary, the electromagnetic field finite element calculation method is used to accurately calculate the loss generated by the secondary synchronous armature current on the rotor, thereby determining the accurate k ₁ and

(7) Refer to the generator power frequency negative sequence overcurrent protection, and determine the time limit and inverse time limit protection. The criterion is:

K

Definite time criterion: > rel type 6

Oh,

A - Inverse time criterion: And, 2 >; 3⁄4t and > Equation 7 where K _rel is the reliability coefficient, the value range is 1.0~1.2; the return coefficient is the range of 0.8~1.0; t _set is the definite time protection delay When the value ranges from 0 to 20 s, 1 „^ is the inverse action limit minimum action delay, and the value ranges from 0 to 20 s; The constant for the generator to withstand the power frequency negative sequence current capability is provided by the OEM, and the value range is 1~100; ₂ is the equivalent power frequency negative sequence current calculated by the formula 5; 1 _2∞ is the generator long-term allowable negative The sequence current value, I _srt is the definite time limit current setting, 1 „ is the rated current value of the generator, and I _pk is the anti-time limit protection current starting value. These current values are all CT secondary values. Here, the original power frequency is fully borrowed. Negative sequence current overload protection, the protection criterion form is unchanged, and the fixed value is unchanged.

(8) Conducting divergence protection discrimination, the protection criterion is: q) > I _eq (n - T) Equation 8

Cnt > C _set Equation 9 where T is the data comparison interval constant and C _srt is the divergence count setting. When the aforementioned definite time limit and inverse time limit protection are activated, Equation 8 is discriminated. If Equation 8 is satisfied, the corresponding counter Οί is incremented by one; if the protection is not activated or Equation 8 is not satisfied, the counter Οί is decremented by 1 until it is reduced to When 0 is satisfied, it is judged that there is continuous divergence. The protection device alarms or trips. In general, the process of sub-synchronous divergence is not too fast, so the value of the data comparison interval constant Τ can be determined according to its corresponding time in the range of 0.02 s~ls, for example, the sampling frequency is 1200 Hz, T=120. The corresponding time is the time corresponding to sampling 120 points, which is 0.1s. When the power system is disturbed (short circuit, open circuit, reclosing, load shedding, etc.), the amount of sub-synchronous frequency bands will appear in the current signal, but its attenuation is very fast, generally it can be attenuated within 0.2s~0.5s. The divergence count setting can be determined by avoiding this decay time. For example, when the sampling frequency is 1200 Hz and C _set = 1800, the corresponding time is the time corresponding to sampling 1800 points, which is 1.5 s. The aforementioned timing and inverse time protection are only for the relatively stable asynchronous self-excitation phenomenon. Actually, the secondary synchronous current may gradually increase. The divergence protection is a functional supplement. The invention has the beneficial effects that: the high-order low-pass filtering and the band-pass filtering can separate the instantaneous values of the plurality of modal currents; the Fourier algorithm can accurately calculate the amplitude and phasor of each modal signal, and Calculate the positive sequence and negative sequence components for the three-phase currents of each mode; use the principle of generating the eddy current phase on the rotor with the power frequency negative sequence current, and convert the positive and negative sequence components of each modal current into conversion. The equivalent power frequency negative sequence current can be borrowed from the original power frequency negative sequence current protection to realize the time limit and inverse time limit over current protection. The protection criterion can be unchanged and the fixed value can be unchanged. Finally, by discriminating the equivalent Whether the amplitude of the current continues to increase, achieving divergence protection; the invention has a large amount of calculation, Multiple CPU parallel calculations can be used when necessary, but the protection design has a theoretical basis. The criteria and timing settings of the definite time and inverse time overcurrent protection are the same as the original power frequency negative sequence overcurrent protection. DRAWINGS

Figure 1 is a general flow chart of the calculation process of the present invention,

2 is a graph showing the amplitude-frequency characteristics of an 8-stage Chebyshev II low-pass filter designed by the present invention,

3 is a graph showing amplitude-frequency characteristics of two 8-stage Chebyshev II band-pass filters designed by the present invention,

4 is a logic diagram of the definite time and inverse time protection of the present invention,

Figure 5 is a logic diagram of the divergence protection of the present invention.

In Figure 4:

a. Current criterion logic in Equation 6. Output 1 when the condition is satisfied, otherwise 0.

b. The "delay" logic element in Equation 6. When the input changes from 0 to 1, the output is 1 after the delay t _set . When the input is 0 or 0 changes to 0, the output is 0.

c Definite time protection alarm or trip result output.

d. The current in Logic 7 starts the logic element. Output 1 when the condition is satisfied, otherwise 0.

e. The "delay" logic element associated with the current value in Equation 7. The time t is determined by Equation 7, which is a function of the current I _eq , ₂ . When the input changes from 0 to 1, the timer starts counting. When the delay reaches t, the logic outputs 1; when the input is 0 or changes from 1 to 0, the timer starts to decrement until it is 0, then the logic output is 0. .

f. The "delay" logic element of the minimum time in Equation 7. When the input changes from 0 to 1, the delay is output ^1, when the input is 0 or 0 changes to 0, the output is 0.

g. "AND" gate operation logic.

h. Inverse time protection alarm or trip result output. In Figure 5:

b. The current-starting logic element in Equation 7. Output 1 when the condition is satisfied, otherwise 0.

c Equation 8 current criterion logic element. Output 1 when the condition is satisfied, otherwise 0.

d. "OR" gate logic operation.

e. "AND" gate operation logic.

f. Divergence count. When the input is 1, the count is incremented; when the input is 0, the count is decremented until it is 0; The output is the count value Cnt.

g. The count comparison logic of Equation 9. Output 1 when the condition is satisfied, otherwise 0.

h. Diverging protection alarm or trip result output.

DETAILED DESCRIPTION OF THE INVENTION The present invention devises a method for detecting sub-synchronous currents of a plurality of modes in a generator armature current and implementing overcurrent protection and divergence protection of subsynchronous currents. The specific implementation of the method is described below in combination with the specific situation of a 600MW turbine generator in a power plant. The generator set is equipped with a static blocking filter SBF on the high voltage side of the main transformer. SBF is a circuit in which a series of inductors and capacitors are connected in series and in parallel. Field tests show that after SBF is put into operation, the generator has an asynchronous self-excitation phenomenon. The three-phase current of the generator contains these two subsynchronous modal components at frequencies around 27.0 Hz and 39.0 Hz. After adjusting the parameters of SBF, the asynchronous self A certain degree of inhibition is obtained. In order to prevent the asynchronous self-oscillation from damaging the generator set, a torsional vibration protection device TSR and a secondary synchronous overcurrent (asynchronous self-excitation) protection device are installed. In order to facilitate the description of the specific method, it is assumed here that the secondary synchronous currents of the two modal frequencies of 27.0 Hz and 39.0 Hz appear as an example. According to the calculation process shown in Figure 1, the specific steps are as follows:

(1) The protection device or the monitoring device samples at a fixed sampling frequency (for example, 1200 Hz), and obtains the secondary side current of the current transformer of the generator terminal, that is, obtains the three-phase current signals _A , _B , k:.

(2) Design a low-pass filter Hz s) with a cutoff frequency in the range of 43Ηζ~48Ηζ. The designed filter parameters are fixed as fixed values and stored in the protection device or monitoring device. Here is an example, using an 8th-order Chebyshev II low-pass filter, the transfer function of the low-pass filter H (s) is:

The parameters in the filter are: b0~b8 (in scientific notation): 0.01, -1.1910e-14, 3.1583e4, 4.3568e-9 1.5585el0, 2.9148e-3, 2.4612el5, 2.9683e2 , 1.2145e20. The results of a0~a8 (represented by scientific notation) are: 1.0, 1.5282e3, 1.1680e6, 5.8185e8, 2.0770ell, 5.4613el3, 1.0651el6, 1.4105el8, 1.2145e20. The parameters of the low-pass filter are independent of the modal frequency and are fixed values. The amplitude-frequency characteristics of the filter are shown in Figure 2. The type of filter is not limited to the Chebyshev II type. The first filtered values ' _A , i' _B , i' _{c are} calculated according to Equation 1.

(3) Two sets of high-order bandpass filters are designed for the two modal frequencies of 27.0 Hz and 39.0 Hz. The designed filter parameters are fixed as fixed values and stored in the protection device and the monitoring device. Here is an example of an 8th-order Chebyshev II bandpass filter in the form: H _B (s) = , 2 bands

The center frequency of the pass filter is 27.0 Hz and 39.0 Hz, respectively. The specific filter parameters are as follows: Hs.iW parameters: The results of b0~b8 (represented by scientific notation) are: 0.01, -1.4043e-016, 2.0466e3, -3.4929e-011, 1.0471e8, -3.3800e-006, 1.3394el2, - 1.4832e-002, 4.2829el5. The results of a0~a8 (represented by scientific notation) are: 1.0, 152.84, 1.1402e5, 1.2258e7, 4.5378e9, 3.1359el l, 7.4618el3, 2.5588el5, 4.2829el7.

The parameters of H 2 (the results of b0~b8 (represented by scientific notation) are: 0.01, 1.1199e-015, 3.2972e3, 2.5408e-010, 3.2334e8, 6.3763e-006, 1.0656el3, -0.10984, 1.0445el7. The results of a0~a8 (represented by scientific notation) are: 1.0, 152.84, 2.3909e5, 2.6595e7, 2.0733el0 1.5119el2, 7.7268el4, 2.8080el6, 1.0445el9.

The type of filter is not limited to the Chebyshev II type. The amplitude-frequency characteristics of the filter are shown in Figure 3. Two sets of subsynchronous current signals (m=l, 2) are calculated according to Equation 2: " _m , _A , " _m , _B , i" _m , _c .

(4) Perform zero-crossing frequency measurement and Fourier calculation on the obtained sub-synchronous current signal. The signals obtained after the above processing are all sinusoidal waves having a relatively good sine. The zero-crossing algorithm can be used to calculate the modal frequency / _m . Zero-crossing method frequency measurement: Measure the length of time corresponding to the zero-crossing point of the sinusoidal waveform from negative to positive (or negative-positive), and the reciprocal of time is the frequency, thus obtaining r _m , _A , r _m , _B , The frequency of r _m , _c , the three frequencies are averaged as the final modal frequency / _m . For a subsynchronous modal frequency, if one of the three phase currents has a phase current signal that is too small to accurately measure the frequency, the frequency of the two-phase modal current is averaged; similarly, If the two-phase current signal of the three-phase current is too small, the frequency of the larger current of the remaining one is the frequency measurement result; if the three-phase current signal is small, the band-pass filter is taken. The center frequencies (27.0 Hz and 39.0 Hz) are measured as modal frequencies. Let the measured two modal frequencies be ₂ = 27.0 Hz, 39.0 Hz, respectively, and the data window length calculated by Fourier is Ni = round (1200 Hz / 27.0 Hz) = round (44.4) = 44, N ₂ = round (1200Hz / 39.0Hz) = round (30.8) = 31 o using Fourier algorithm (formula 3) to calculate a secondary synchronization signal mode _{_{_{"m, a," m,}}} B, "m, c amplitude sequence / _m »and the corresponding phasor Lu i). Among them, because the signal is attenuated by the two filters, the gain correction coefficient g _{m in} Equation 3 is also determined. According to the result of the measured frequency, the low pass filter and the band pass filter are checked. Corresponding amplitude-frequency characteristics Figure 2 and Figure 3, find the gain of the signal after two filterings g!^m) and g _B ,m( m), then the gain coefficient of the modal frequency & = 27.0Hz is: gi = (l /1.0) X (l/1.0) = 1.0, the gain factor of the modal frequency f2 = 39.0 Hz is: g ₂ = (1/0.8602) X (1/1.0) = 1.1625.

(5) Using Equation 4, calculate the positive sequence components I _eq , _u , I _eq , ₁ , and the negative sequence corresponding to the two modal three-phase currents

-S Ieq,l,2, Ieq,2,2 °

(6) Using Equation 5, convert all positive and negative sequence components into equivalent power frequency negative sequence currents I _eq , ₂ . The correction coefficients k _m , ^B k _m , _{2 in} Equation 5 take 0.9.

(7) Refer to the generator power frequency negative sequence overcurrent protection, and perform the definite time and inverse time protection protection judgment. The conversion I _eq , ₂ will be substituted into the criterion 6 and 7. For the example given here, the fixed value is set according to the original power frequency negative sequence current protection, K _rel =1.05, K _r =0.95, generator rated current I _n = 3.5A (the secondary value of the machine end CT, The CT ratio is 25kA/5A), the generator long-term allowable negative sequence current value I _2∞ = 8 I _n = 0.28A, the definite time current setting I _set = 0.31A, the definite time delay t _set = 5.0s ( Avoiding the negative sequence current protection action delay generated by the two-phase short circuit of the high-voltage side of the main transformer, preventing the pre-existing fault preemptive action), the constant of the generator with the power frequency negative sequence current capability A = 10, the inverse time limit protection current starting value I _pk = 0.33A, inverse time protection minimum action delay _n = 2.0s. These protection settings are exactly the same as the original power frequency negative sequence current protection settings.

(8) Perform the divergence protection discrimination, substitute the conversion I _eq , ₂ into the criterion 8, and combine it with Equation 7. Generally, the process of sub-synchronous divergence is not too fast, so the value of the data comparison interval constant T can be determined according to the corresponding time in the range of 0.02 s~ls, for example, the sampling frequency is 1200 Hz, T=120. , the corresponding time is The time corresponding to sampling 120 points is 0.1s. When the power system is disturbed (short circuit, open circuit, reclosing, load shedding, etc.), the amount of sub-synchronous frequency bands will appear in the current signal, but its attenuation is very fast, generally it can be attenuated within 0.2s~0.5s. The divergence count setting can be determined by avoiding this decay time. For example, when the sampling frequency is 1200 Hz and C _set = 1800, the corresponding time is the time corresponding to sampling 1800 points, which is 1.5 s. By using the above method, the secondary synchronous current of multiple modes in the armature current of the generator can be detected, and the overcurrent protection and divergence protection of the secondary synchronous current are realized.

Claims

Claim

1. The secondary synchronous current calculation and the secondary synchronous overcurrent and divergence protection method are characterized in that: the three-phase current signal of the generator is collected, and the positive sequence and the negative sequence component of each mode secondary synchronous current of the current are passed as follows The formula is converted into an equivalent power frequency negative sequence current:

m = 1,2,· Equation 1

Where: ₂ is the equivalent power-frequency negative sequence current value of the final conversion; Im, i and I _m , ₂ are the positive and negative sequence components of the sub-synchronous currents of each mode; k _m , ^ B k _m , ₂ Is the correction factor; / _n is the measured power frequency; / _m is the measured modal frequency.

2. The method of calculating subsynchronous current and subsynchronous overcurrent and divergence protection of a generator according to claim 1, wherein: collecting a three-phase current signal of the generator, and performing low-pass filtering and band-pass filtering to separate more Grouping the signal of the synchronous modal frequency; calculating the frequency of the subsynchronous mode, the amplitude sequence of the modal signal and the corresponding phasor by calculating the plurality of sets of subsynchronous signals; and then obtaining the three-phase current of each modal Corresponding positive sequence component and negative sequence component: Finally, the positive sequence and negative sequence components of each mode are converted into equivalent power frequency negative sequence current by Equation 1.

3. The generator secondary synchronous current calculation and the secondary synchronous overcurrent and divergence protection method according to claim 2, wherein: the calculated equivalent power frequency negative sequence current is substituted into the original power frequency negative of the generator. The overcurrent protection criterion is used to determine the definite time and inverse time protection. When the definite time protection or inverse time protection is activated, observe whether the equivalent power frequency negative sequence current has a growing trend. If the current power frequency negative sequence current value is greater than In the previous value of the power frequency negative sequence current, the divergence counter is accumulated, otherwise it is decremented. If the counter value exceeds the preset value, it is considered to be a continuous increase and is judged as a divergent protection action.

4. The generator secondary synchronous current calculation and the secondary synchronous overcurrent and divergence protection method according to claim 2 or 3, wherein: the protection device or the monitoring device acquires the three-phase current signal _{A of the} generator with a fixed sampling frequency, B ic low-pass filter and a band-pass filter, the separated plurality of sets of sub-synchronous mode frequency signals _{_{_{r m, a, 'm,}}} B, r m, c, mode m = l, 2, _...; use The zero-crossing algorithm obtains the frequency f _{m of the} subsynchronous mode. The amplitude sequence of the modal signal and the corresponding phasor are obtained by Fourier's calculation to obtain the positive sequence component I _mJ and the negative sequence component corresponding to the three-phase current of each mode. I _m , ₂ ; Convert the positive and negative sequence components of each mode into the equivalent power frequency negative sequence current I _eq , _2: Calculate the amplitude sequence and phasor of each sub-synchronous modal signal. The formula used in the Fourier calculation is:

'^(n) = g _m (a _m (n)-jb _mi (n))

i = A, B, C, m = 1, 2, ---

Where _gm is the gain correction coefficient, / _s is the sampling frequency of the protection device or monitoring device, ^ is the subsynchronous modal frequency, ^ is the data window length corresponding to / ^; the positive sequence and negative sequence of each modal subsynchronous current The formula for the calculation is: n) + a- Im (n) + a ² ■ I n) y3

Equation 3

a = e

Then the result is substituted into Equation 1 to calculate the positive sequence and negative sequence components of each mode to be converted into the equivalent power frequency negative sequence current Ieq,2.

5. The method of calculating subsynchronous current and subsynchronous overcurrent and divergence protection of a generator according to claim 1 or 2, wherein: the power frequency negative sequence overcurrent protection of the generator is performed, and the definite time limit and the inverse time limit protection are determined. The criteria are: Time limit criteria:

K

> and t>t _set type 6

K,

Inverse time criterion

A

Where K _rel is the reliability coefficient; is the return coefficient; t _set is the definite time protection delay; 1 „^ is the inverse time protection minimum action delay; A is the constant of the generator to withstand the power frequency negative sequence current capability, The main engine factory provides; I _eq , ₂ is the equivalent power frequency negative sequence current converted by Equation 1; 1 _{2 ∞} is the long-term allowable negative sequence current value of the generator; I _srt is the definite time limit current setting; I _n is the generator Rated current value; I _pk is the inverse time protection current start value; the divergence protection criterion is: i» i _e m type 8

Cnt > C _set Equation 9 where T is the data comparison interval constant value, C _set is the divergence count setting value; when the definite time limit and inverse time limit protection are started, Equation 8 is discriminated, and if Equation 8 is satisfied, then The corresponding counter C is incremented by one; if the protection is not activated or the formula 8 is not satisfied, the counter C is decremented by 1 until it is reduced to 0; when the formula 9 is satisfied, it is judged that continuous divergence occurs, and the protection device alarms or trips.

6. A generator secondary synchronous current calculation and a secondary synchronous overcurrent and divergence protection method according to claim 1 or 2, characterized in that: the three-phase current signal of the generator is acquired with a fixed sampling frequency.

7. The generator secondary synchronous current calculation and the secondary synchronous overcurrent and divergence protection method according to claim 1 or 2, wherein: the correction coefficient! ^^ and k _m , ₂ use the electromagnetic field finite element method to accurately calculate the loss of the secondary synchronous armature current on the rotor, thus determining the accurate k _m , B k _m , ₂ .

8. The method of calculating subsynchronous current and subsynchronous overcurrent and divergence protection of a generator according to claim 2 or 3, wherein: the process of subsynchronous divergence is not too fast, so the data comparison interval T is taken. The value is determined according to the corresponding time in the range of 0.02s~ls; when the power system is short-circuited, broken, reclosed, overloaded, etc., the amount of sub-synchronous frequency band appears in the current signal, but the attenuation is very Fast, generally can be attenuated within 0.2s~0.5s; the divergence count value is determined by avoiding this decay time.