A differential protection method for stator-rotor different-frequency current of a variable-speed pumped storage unit

Abstract

The present invention discloses a differential protection method for stator-rotor different-frequency currents of a variable-speed pumped storage unit, belonging to the field of relay protection for a motor-generator. The method includes: obtaining the peak values and phase angles of the stator three-phase currents, and calculating the peak values of the rotor three-phase currents according to the current transformation ratio relationship between the stator and the rotor; taking the angular frequencies and phase angles of the rotor three-phase currents as decision variables, and taking the minimum difference between the measured values and the calculated values of the rotor three-phase currents as the objective, establishing an optimization model; solving the optimization model to obtain the angular frequencies and phase angles of the rotor three-phase currents; based on the angular frequencies and phase angles of the rotor three-phase currents, converting the measured values of the rotor three-phase currents to the stator to obtain converted currents; using the converted currents and the measured values of the stator three-phase currents to form a different-frequency differential protection; if the protection operates, it is determined that a short-circuit fault has occurred inside the rotor winding of the variable-speed pumped storage unit. This method has high sensitivity and can reliably implement the protection against short-circuit faults inside the rotor winding.

Description

Technical Field

[0001] The invention belongs to the technical field of generator - motor relay protection, and more specifically, relates to a differential protection method for stator - rotor different - frequency current of a variable - speed pumped - storage unit. Background Art

[0002] Variable - speed pumped - storage units can achieve flexible power regulation in a wide range in both pumping mode and generating mode, and have higher efficiency than traditional pumped - storage power stations. They are a key part of China's new power system with new energy as the main body. At present, China is in the initial stage of research on variable - speed pumped - storage technology, and there is less research on the faults of variable - speed pumped - storage units.

[0003] The rotor winding of a variable - speed pumped - storage unit adopts an AC excitation structure, and the excitation voltage is relatively high. During the high - speed rotation process, the insulation of the rotor winding is prone to degradation, and then internal short - circuit faults occur. This fault will cause serious problems such as local overheating of the winding, abnormal vibration of the unit, and even damage to the rotor. Due to the extremely large capacity of variable - speed pumped - storage power stations and the high cost of their rotor equipment, a targeted protection scheme should be equipped for rotor winding short - circuit faults.

[0004] Existing research only uses stator electrical quantities or only uses rotor electrical quantities for fault identification, with low sensitivity and difficult to meet the actual engineering needs. Summary of the Invention

[0005] In view of the above - mentioned defects of the prior art, the invention provides a differential protection method for stator - rotor different - frequency current of a variable - speed pumped - storage unit, aiming to simultaneously use stator and rotor electrical quantities to form differential protection, so as to sensitively and reliably realize the protection of internal short - circuit faults of the rotor winding of a variable - speed pumped - storage unit.

[0006] To achieve the above object, on the one hand, the invention provides a differential protection method for stator - rotor different - frequency current of a variable - speed pumped - storage unit, including:

[0007] S1. Obtain the peak values and phase angles of the stator three - phase currents, and calculate the peak values of the rotor three - phase currents according to the current transformation ratio relationship between the stator and rotor;

[0008] S2. Take the rotor three - phase current angular frequency and phase angle as decision variables, and take the minimum difference between the measured value and the calculated value of the rotor three - phase currents as the objective to establish an optimization model; use an optimization algorithm to solve the optimization model to obtain the angular frequency and phase angle of the rotor three - phase currents;

[0009] S3. Based on the angular frequency and phase angle of the rotor three - phase currents, convert the measured value of the rotor three - phase currents to the stator to obtain the converted current;

[0010] S4. Use the converted current and the measured values of the stator three-phase currents to form a different-frequency differential protection; if the protection operates, it is determined that a short-circuit fault has occurred inside the rotor winding of the variable-speed pumped storage unit.

[0011] Further, in the S1, obtaining the peak values and phase angles of the stator three-phase currents includes: when the variable-speed pumped storage unit is operating stably and normally, obtaining the measured values of the stator three-phase currents for at least one power frequency cycle and performing Fourier transform to obtain the peak values and phase angles of the stator three-phase currents.

[0012] Further, in the S1, the peak value I of a certain phase current of the rotor r cal is expressed as:

[0013]

[0014] where is the peak value of a certain phase current of the stator, N s and N r are the number of turns in series of each single-branch winding of the stator and rotor respectively; k ws and k wr are the fundamental wave winding coefficients of the stator and rotor respectively.

[0015] Further, in the S2, the optimization model is expressed as:

[0016]

[0017]

[0018] where represents the angular frequency of a certain phase current of the rotor, represents the phase angle of a certain phase current of the rotor, T c is the measurement interval time, N is the number of measurements within a single power frequency cycle, I ar [nT c is the measured value of a certain phase current of the rotor at the nth measurement interval time, is the peak value of a certain phase current of the rotor, ω s is the stator current angular frequency, s ce is the measured value of the slip ratio, and α is the threshold coefficient, and 0.05 ≤ α ≤ 0.1.

[0019] Further, in the S3, a certain phase converted current is expressed as:

[0020]

[0021] where T represents the current moment, I ar (T) is the measured value of a certain phase current of the rotor at the moment T, is the phase angle of the current of a certain phase of the stator.

[0022] Further, in the step S4, a different-frequency differential protection is formed by using the converted current and the measured values of the three-phase stator currents, including: performing Fourier transform on the converted current of a certain phase and the measured current I as (T) of a certain phase of the stator to obtain phasors and and forming a differential protection. The operating current I d and the braking current I z of the differential protection are respectively:

[0023]

[0024] On the other hand, the present invention also provides a different-frequency current differential protection system for the stator and rotor of a variable-speed pumped storage unit, including: a computer-readable storage medium and a processor;

[0025] The computer-readable storage medium is used to store executable instructions;

[0026] The processor is used to read the executable instructions stored in the computer-readable storage medium and execute the above method.

[0027] Generally speaking, compared with the prior art through the above technical solutions conceived by the present invention, the following beneficial effects can be achieved:

[0028] The present invention first proposes to form a different-frequency current differential protection by simultaneously using the stator electrical quantities and the rotor electrical quantities of a variable-speed pumped storage unit, solving the problem of lack of targeted protection for the internal short-circuit fault of the rotor winding due to the inability to install current transformers at the neutral point of the rotor of the variable-speed pumped storage unit. The different-frequency current differential protection method proposed by the present invention has higher sensitivity, which helps to achieve targeted and highly sensitive protection for the internal short-circuit fault of the rotor winding of the variable-speed pumped storage unit, and avoid major economic losses caused by the failure to identify the fault in the first time and resulting in the damage of the unit. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 is the implementation flowchart of the different-frequency current differential protection method for the stator and rotor of the variable-speed pumped storage unit provided by the embodiment of the present invention;

[0030] Figure 2 is the waveform of the instantaneous values of the three-phase stator currents when a short-circuit fault occurs between the 5th turn of the first branch of the rotor A phase and the 1st turn of the second branch of the A phase in the embodiment of the present invention;

[0031] Figure 3 is the waveform of the instantaneous values of the three-phase rotor currents when a short-circuit fault occurs between the 5th turn of the first branch of the rotor A phase and the 1st turn of the second branch of the A phase in the embodiment of the present invention;

[0032] Figure 4 For the case of a short - circuit fault occurring between the 5th turn coil of the first branch of phase A of the rotor and the 1st turn coil of the second branch of phase A in the embodiment of the present invention, it is the result of obtaining the real - time converted current by converting and calculating the current of phase A of the rotor.

[0033] Figure 5 For the case of a short - circuit fault occurring between the 5th turn coil of the first branch of phase A of the rotor and the 1st turn coil of the second branch of phase A in the embodiment of the present invention, it is the change trajectory of the operating current and braking current of the different - frequency differential protection. Detailed implementation manners

[0034] In order to make the objectives, technical solutions and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0035] The present invention provides a differential protection method for the stator - rotor different - frequency current of a variable - speed pumped - storage unit. As Figure 1 shown, the proposed method specifically includes:

[0036] S1. Obtain the peak values and phase angles of the stator three - phase currents, and calculate the peak values of the rotor three - phase currents according to the current transformation ratio relationship between the stator and the rotor.

[0037] Specifically, step S1 specifically includes:

[0038] S11. When the variable - speed pumped - storage unit is operating stably and normally, extract the measured values of the stator and rotor three - phase currents for at least one power - frequency cycle, and the stator and rotor current transformers are synchronously measured and have the same sampling rate. Taking phase a as an example, the measured value sequences of the phase - a currents of the stator and rotor are respectively denoted as: I as [T c ,..., I as [nT c ,..., I as [NT c and I ar [T c ,..., I ar [nT c ,..., I ar [NT c . Wherein, T c is the measurement interval time, N is the number of measurements within a single power - frequency cycle, and 1 ≤ n ≤ N.

[0039] S12. Taking phase a as an example, for the stator phase - a current I as [Tc ,..., I as [nT c ,..., I as [NT c are subjected to Fourier transform to obtain the peak value and phase angle of the stator phase-a current, which are respectively denoted as: and According to the relationship between the stator and rotor current transformation ratios, calculate the peak value of the rotor phase-a current Specifically:

[0040]

[0041] where N s and N r are respectively the number of turns in series of each single-branch winding of the stator and rotor; k ws and k wr are respectively the fundamental winding factors of the stator and rotor.

[0042] S2. Taking the angular frequency and phase angle of the rotor three-phase current as decision variables, and taking the minimum difference between the measured value and the calculated value of the rotor three-phase current as the objective, establish an optimization model; use an optimization algorithm to solve the optimization model to obtain the angular frequency and phase angle of the rotor three-phase current.

[0043] Specifically, step S2 specifically includes:

[0044] S21. Taking phase-a as an example, the corresponding optimization model is expressed as:

[0045]

[0046]

[0047] where s ce is the measured value of the slip ratio, the decision variable represents the angular frequency of the rotor phase-a current, the decision variable represents the phase angle of the rotor phase-a current, α is a threshold coefficient, and 0.05 ≤ α ≤ 0.1.

[0048] S22. Preferably, the optimization algorithm for solving the optimization model selects the particle swarm optimization algorithm.

[0049] S3. Based on the angular frequency and phase angle of the rotor three-phase current, convert the measured value of the rotor three-phase current to the stator to obtain the converted current.

[0050] Specifically, step S3 specifically includes:

[0051] Taking phase-a as an example, the conversion formula for converting the rotor current to the stator is:

[0052]

[0053] where ω s is the angular frequency of the stator current, ω s = 314.15 rad / s. T represents the current moment, and I ar (T) is the real-time measured value of the rotor phase-a current, which is the real-time converted current of phase-a.

[0054] S4. Use the converted current and the measured values of the three-phase stator currents to form a different-frequency differential protection; if the protection operates, it is determined that a short-circuit fault has occurred inside the rotor winding of the variable-speed pumped-storage unit.

[0055] Specifically, step S4 specifically includes:

[0056] S41. Taking phase-a as an example, perform Fourier transform on the real-time converted current and the measured stator phase-a current I as (T) to obtain phasors and and form a differential protection. The operating current and braking current of the differential protection are respectively:

[0057]

[0058] S42. The differential criterion adopts a variable-slope ratio braking criterion, and the protection criterion is:

[0059] I d > K bl × I z + I cdqd I z ≤ kI s

[0060] K bl = K bl1 + K blr × (I z / I s )

[0061] I d > K bl2 × (I z - kI s ) + b + I cdqd I z > kI s

[0062] K blr = (K bl2 - K bl1 ) / (2k)

[0063] b = (K bl1 + K blr × k) × kIs

[0064] Among them, K bl1 and K bl2 are the starting and maximum ratio differential slopes respectively, generally taking 0.1 and 0.7; k is the braking current multiple at the maximum slope, fixed at 6; I cdqd is the differential current starting setting value, taking 0.3 times the rated current; I s is the rated stator current.

[0065] S43. The differential protection criteria can be formed by the a-phase, b-phase, and c-phase currents of the stator and rotor respectively. The export logic of the three-phase different-frequency current differential protection is an "OR" logic.

[0066] The effectiveness of the proposed fault diagnosis method is verified by simulation below.

[0067] Taking a certain actual variable-speed pumped storage unit as an example, the basic parameters of this unit are shown in Table 1.

[0068] Table 1 Basic parameters of a certain actual variable-speed pumped storage unit

[0069] Parameter Stator Rotor Number of slots (Z) 252 294 Winding form Double-layer lap winding Double-layer wave winding Number of pole pairs (P) 7 7 Number of parallel branches 4 2 Number of turns per branch 21 49 <![CDATA[First pitch (y 1 )]]> 15 21 <![CDATA[Second distance (y 2 )]]> 14 21

[0070] Based on the multi-loop method, a simulation calculation model of the unit shown in Table 1 is established:

[0071]

[0072] In the formula, p is the differential operator; U s and U r are the stator and rotor branch voltage matrices respectively; I s and I r are the stator and rotor branch current matrices respectively; R s and R r are the stator and rotor branch resistance matrices respectively; L ss is the self and mutual inductance matrix of each stator branch; L rr is the self and mutual inductance matrix of each rotor branch; L sr and L rs are the mutual inductance matrices between the stator and rotor branches. This formula is a time-varying ordinary differential equation system, and the fourth-order Runge-Kutta algorithm can be used to solve it.

[0073] When the slip rate s is 0.1, it is assumed that a short-circuit fault occurs between the 5th turn of the first branch of the rotor A-phase and the 1st turn of the second branch of the A-phase, and the fault time is 35 s. Through simulation calculation, the instantaneous value waveforms of the stator three-phase currents are obtained as Figure 2 shown, and the instantaneous value waveforms of the rotor three-phase currents are as Figure 3As shown, the angular frequencies of the two currents are different, and the currents in the figure are all in per-unit value form. Taking phase a as an example, using the current conversion method proposed in the present invention, the rotor phase-a current is converted to obtain the real-time converted current. As Figure 4 shown. It can be found that during normal operation, the real-time converted current is exactly equal to the stator phase-a current I as (T), but there are obvious differences after an internal short-circuit fault occurs in the rotor winding.

[0074] Using the real-time converted current and the actually measured stator current I as (T) to form a different-frequency differential protection, the change trajectories of the operating current and the restraining current are as Figure 5 shown. It can be found that after the fault, the different-frequency differential protection gradually enters the operating region from the restraining region, and the protection operates when it intersects with the protection criterion, which can reliably protect the internal short-circuit fault of the rotor winding of the variable-speed pumped-storage unit.

[0075] According to the connection sequence of the rotor windings of the actual unit in Table 1, the number of possible internal short-circuit faults in the rotor windings of this unit is shown in Table 2, with a total of 12,054 faults. Protection verification is carried out for all possible internal short-circuit faults in the rotor windings.

[0076] Table 2 Number of possible short-circuit faults in the rotor windings

[0077] Same phase and same branch Same phase and different branches Different phases Total number Fault in slot 252 42 0 294 End fault 1584 1944 8232 11760 Total number of faults 1836 1986 8232 12054

[0078] When the slip rate s is 0.1, the protection operation situation of the different-frequency differential protection method proposed in the present invention is shown in Table 3, and the protection coverage rate is 93.27%. When the slip rate s is 0.01, the protection operation situation of the different-frequency differential protection method proposed in the present invention is shown in Table 4, and the protection coverage rate is 92.36%. It can be found that the different-frequency differential protection method proposed in the present invention has a high operation rate and can protect most of the internal short-circuit faults of the rotor windings. In addition, when the slip rate changes, the protection operation rate changes very little, and the protection performance of this method is less affected by the change of the rotor speed.

[0079] Table 3 Protection operation situation of the different-frequency differential protection method when s is 0.1

[0080] Same phase and same branch Same phase and different branches Different phases Total number Fault in slot 222 36 0 258 End fault 1266 1523 8196 10985 Total number of faults 1488 1559 8196 11243

[0081] Table 4 Protection operation situation of the different-frequency differential protection method when s is 0.01

[0082] Same phase and same branch Same phase and different branches Different phases Total number Fault in slot 219 36 0 255 End fault 1258 1436 8184 10878 Total number of faults 1477 1472 8184 11133

[0083] Those skilled in the art can easily understand that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A differential protection method for stator-rotor different-frequency currents of a variable-speed pumped storage unit, characterized in that, it includes: S1. Obtain the peak values and phase angles of the three-phase stator currents, and calculate the peak values of the three-phase rotor currents according to the stator-rotor current transformation ratio relationship; S2. Take the angular frequencies and phase angles of the three-phase rotor currents as decision variables, and establish an optimization model with the minimum difference between the measured values and calculated values of the three-phase rotor currents as the objective; use an optimization algorithm to solve the optimization model to obtain the angular frequencies and phase angles of the three-phase rotor currents; S3. Based on the angular frequencies and phase angles of the three-phase rotor currents, convert the measured values of the three-phase rotor currents to the stator to obtain converted currents; S4. Use the converted currents and the measured values of the three-phase stator currents to form a different-frequency differential protection; If the protection operates, it is determined that a short-circuit fault has occurred inside the rotor winding of the variable-speed pumped storage unit; In the said S1, obtaining the peak values and phase angles of the three-phase stator currents includes: when the variable-speed pumped storage unit operates stably and normally, obtain the measured values of the three-phase stator currents for at least one power frequency cycle, and perform Fourier transform to obtain the peak values and phase angles of the three-phase stator currents; In S1, the peak value of the current of a certain phase of the rotor is expressed as: Among them, is the peak value of the current of a certain phase of the stator, and N s and N r are the number of turns in series of each single-branch winding of the stator and rotor phases respectively; k ws and k wr are the fundamental winding factors of the stator and rotor respectively; In the said S2, the optimization model is expressed as: Among them, represents the angular frequency of a certain phase current of the rotor, represents the phase angle of a certain phase current of the rotor, T c is the measurement interval time, N is the number of measurements within a single power frequency cycle, I ar [nT c is the measured value of a certain phase current of the rotor at the nth measurement interval time, is the peak value of a certain phase current of the rotor, ω s is the stator current angular frequency, s ce is the measured value of the slip ratio, α is the threshold coefficient, and 0.05 ≤ α ≤ 0.1; In S3, the converted current of a certain phase is expressed as: where T represents the current moment, and I ar (T) is the measured value of the current of a certain phase of the rotor at moment T, is the phase angle of the current of a certain phase of the stator.

2. According to the differential protection method for stator-rotor different-frequency currents of a variable-speed pumped storage unit described in claim 1, characterized in that, In S4, a different-frequency differential protection is constituted by using the converted current and the measured values of the three-phase stator currents, including: for the converted current of a certain phase and the measured current I as (T) of a certain phase of the stator are subjected to Fourier transform to obtain phasors and and a differential protection is constituted. The operating current I d and the restraining current I z are respectively:

3. A differential protection system for stator-rotor different-frequency currents of a variable-speed pumped storage unit, characterized in that, it includes: a computer-readable storage medium and a processor; the computer-readable storage medium is used to store executable instructions; the processor is used to read the executable instructions stored in the computer-readable storage medium and execute the method described in any one of claims 1-2.

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