CN103293448B - Identification method of single-phase ground fault and virtual grounding based on semi-cycle energy ratio - Google Patents

Abstract

The invention relates to a method for identifying single-phase ground faults and phantom ground faults based on half-cycle energy ratios, and belongs to the technical field of electric power system relay protection. After the distribution network fails, the invention extracts the zero-sequence voltage data of one cycle after the start-up of the starting element, respectively performs wavelet packet decomposition on the two half-cycles before and after, and obtains the highest reconstruction coefficients from the decomposed wavelet packet reconstruction coefficients. The highest frequency energy value of the first half cycle is compared with the highest frequency energy value of the second half cycle, and the identification of the single-phase ground fault and the phantom grounding of the distribution network can be reliably realized by comparing it with the set threshold. The invention has relatively simple principle, obvious effect and high reliability, adopts a finer wavelet packet analysis method, divides frequency bands into multiple levels, has higher time-frequency resolution and precision, and has wider practical value.

Description

Identification method of single-phase ground fault and phantom ground fault based on half-cycle energy ratio

technical field

The invention relates to a method for identifying single-phase ground faults and phantom ground faults based on half-cycle energy ratios, and belongs to the technical field of electric power system relay protection.

Background technique

With the continuous development of the distribution network, there are more and more feeders in the urban distribution network, and the proportion of cable lines is getting higher and higher. The distribution network system is likely to be close to or reach a fully compensated state, resulting in resonant overvoltage, whose voltage value may approach or even exceed the neutral point offset voltage during a single-phase ground fault, resulting in phantom grounding. The increase of the neutral point voltage is caused by a single-phase grounding fault, the arc suppression coil should be adjusted to reduce the detuning degree, and adjusted to the direction of full compensation; if it is a phantom grounding phenomenon, the arc suppression coil should be adjusted to make it disconnect As the harmonic degree increases, adjust to the direction of overcompensation increase, eliminate series resonance, and make the neutral point voltage return to normal as soon as possible.

By comparing the offset voltage of the neutral point, it is difficult to find that there is a significant difference in the steady-state value between the single-phase ground fault and the phantom ground, so it is impossible to identify it only by the steady-state value of the neutral point offset voltage fetched.

Contents of the invention

The technical problem to be solved by the present invention is to overcome the traditional identification method for single-phase grounding fault and phantom grounding, which needs to adjust the gear change of the arc suppressing coil to monitor the neutral point offset voltage caused by it or by monitoring the rising slope of the zero-sequence voltage To identify, and need a long time window, the accuracy is not high, poor reliability and other shortcomings.

The technical solution of the present invention is: a single-phase ground fault and phantom ground fault identification method based on half-cycle energy ratio. The half cycle is decomposed by wavelet packet respectively, and the highest frequency energy value is obtained from the decomposed wavelet packet reconstruction coefficient respectively, and the highest frequency energy value of the first half cycle is compared with the highest frequency energy value of the second half cycle to obtain value, set the threshold according to its The size relationship of the distribution network can reliably realize the identification of single-phase ground fault and phantom ground fault.

The specific steps of the single-phase ground fault and phantom ground identification method based on half-cycle energy ratio are:

(1) After a fault occurs in the distribution network, the fault zero-sequence voltage value can be obtained according to the three-phase voltage measured at the protection installation :

In the formula: , , are the voltages of the three phases of line A, B, and C when a fault occurs;

(2) Extract the zero-sequence voltage data of a cycle after a fault occurs in the distribution network, use the wavelet packet analysis method to decompose the two half-cycles before and after, and calculate the energy value of each highest frequency;

Assuming that the input signal is S , the frequency range of the signal component obtained after wavelet packet decomposition is:

Among them: j is the number of layers of wavelet decomposition, k is the kth joint of wavelet decomposition, f _s is the frequency of the input signal;

Perform three-layer wavelet packet decomposition on the input signal S , and the decomposition has the following relationship:

S=AAA 3 +DAA 3 +ADA 3 +DDA 3 +AAD 3 +DAD 3 +ADD 3 +DDD 3

Among them: A represents low frequency, D represents high frequency, and the number at the end represents the level of wavelet packet decomposition (that is, the number of scales);

Calculate the highest frequency energy value E ₁ of the signal from the wavelet packet reconstruction coefficient obtained after wavelet packet decomposition, namely ;

(3) After a single-phase ground fault or phantom grounding occurs in the distribution network, the highest frequency energy of the first half cycle after wavelet packet decomposition is E ₁₁ , and the highest frequency energy value of the second half cycle is E ₁₂ , and the two are obtained ratio

;

(4) According to the ratio with the set threshold The relationship between distinguishing single-phase ground fault and phantom grounding:

when , it is judged as a single-phase ground fault;

when , it is judged as phantom grounding.

The working principle of the present invention is:

With the continuous development of the distribution network, there are more and more feeders in the urban distribution network, and the proportion of cable lines is getting higher and higher. The distribution network system is likely to be close to or reach a fully compensated state, resulting in a resonant overvoltage, whose voltage value may approach or even exceed the neutral point offset voltage during a single-phase ground fault, resulting in a phantom grounding phenomenon. Since phantom grounding is a power frequency resonant overvoltage, it does not contain high-frequency voltage components in theory, while the neutral point offset voltage caused by single-phase ground faults contains rich high-frequency voltage components. Based on this, the present invention proposes a single-phase ground fault and phantom ground fault identification method based on half-cycle energy ratio.

The present invention starts from the zero-sequence voltage information of the feeder when the single-phase grounding fault and phantom grounding of the distribution network system are carried out, and the wavelet packet decomposition is performed on the two half-cycle signals, and the optimal reconstruction coefficients are respectively obtained from the decomposed wavelet packet reconstruction coefficients. High frequency energy value, compare the highest frequency energy value E ₁₁ of the first half cycle with the highest frequency energy value E ₁₂ of the second half cycle ,get value, according to its relationship with the set threshold The relationship between distinguishing single-phase ground fault and phantom grounding:

when , it is judged as a single-phase ground fault;

when , it is judged as phantom grounding.

The beneficial effects of the present invention are:

(1) The principle of this method is relatively simple. It only needs to collect a zero-sequence voltage signal of one cycle to quickly identify single-phase ground faults and phantom ground faults.

(2) This method studies the difference between single-phase ground fault and phantom ground from the perspective of energy, the effect is more obvious and the reliability is high.

(3) The present invention adopts a more refined wavelet packet analysis method, divides the frequency band into multiple levels, has higher time-frequency resolution and precision, and has wider practical value.

Description of drawings

Figure 1 is a structural diagram of the distribution network system used for simulation in the embodiment: in the figure, L ₁ , L ₃ , L ₅ overhead feeders, L ₄ wire-cable hybrid feeders, L ₂ , L ₆ cable feeders, G is the ideal Infinite capacity power supply, T is the main transformer, T _Z is the zigzag transformer, L is the arc suppression coil, R is the damping resistance of the arc suppression coil;

Fig. 2 is the zero-sequence voltage waveform diagram of feeder _L1 when the fault angle of embodiment 1 is 5 °;

Fig. 3 is the zero-sequence voltage waveform diagram of feeder _L1 when the fault angle of embodiment 2 is 90 °;

Fig. 4 is a zero-sequence voltage waveform diagram of the feeder _L1 in the embodiment when the virtual ground is present.

Detailed ways

Example 1: The simulation model of the 110kV/35kV distribution network system is shown in Figure 1. In the figure, the power supply voltage is 110kV, which is reduced to 35kV through the transformer, and there are 6 feeder lines at the outlet, and the neutral point of the Z-shaped transformer passes through the arc suppression coil series resistor to ground. The feeder adopts three types of lines: overhead line, overhead line-cable hybrid line and cable line. The load uses a constant power load model. Overhead feeder L ₁ =15km, L ₃ =18km, L ₅ =30km; wire-cable hybrid feeder L ₄ =17km, overhead feeder 12km, cable 5km; cable feeder L ₂ =6km, L ₆ =8km. Among them, the overhead feeder is JS1 rod type, LGJ-70 type conductor, span 80m; the cable feeder is YJV23-35/95 type cable. G in the power grid is an ideal infinite capacity power supply; T is the main transformer, the transformation ratio is 110 kV /35kV, and the connection group is YN/d11; T _Z is a zigzag transformer; L is an arc suppression coil; R is an arc suppression coil the damping resistance. A single-phase ground fault occurs 5 km away from the beginning of the feeder _L1 , the ground resistance is 20Ω, the fault angle is 5°, and the sampling frequency is 10kHz.

The steps of the single-phase ground fault and phantom ground identification method based on the half-cycle energy ratio are:

(1) After a fault occurs in the distribution network, the zero-sequence voltage value of the fault can be obtained according to the three-phase voltage measured at the protection installation;

(2) Extract the zero-sequence voltage data of one cycle of the feeder _L1 after the single-phase ground fault of the distribution network, use the wavelet packet analysis method to decompose the two half-cycles before and after, and calculate the energy value of each highest frequency;

(3) Obtain the highest frequency energy E ₁₁ of the first half cycle after the fault after wavelet packet transformation and the highest frequency energy value E ₁₂ of the second half cycle, and make a ratio ;

(4) The resulting ratio with threshold Compare to identify whether the system is a single-phase ground fault or a phantom ground;

The zero-sequence voltage waveform of the feeder L ₁ obtained through the simulation experiment is shown in Figure 2, and the highest frequency energy value E ₁₁ =2376.1 of the first half cycle, the highest frequency energy value E ₁₂ =2333.6 of the second half cycle, and the threshold The setting of is 0.8 according to the reliability of the identification method;

=1.018＞ =0.8, so it is judged that the feeder L ₁ is a single-phase ground fault feeder.

Embodiment 2: The simulation model of the 110kV/35kV distribution network system is the same as that of Embodiment 1. A single-phase ground fault occurs at a distance of 5 km from the beginning of the feeder _L1 , the ground resistance is 20Ω, the fault angle is 90°, and the sampling frequency is 10kHz.

The steps of the single-phase ground fault and phantom ground fault identification method based on the half-cycle energy ratio are the same as those in Example 1.

The zero-sequence voltage waveform of the feeder _L1 obtained through the simulation experiment is shown in Figure 3, and the ratio of the highest frequency energy value of the two half-cycles before and after is obtained =169.6＞ =0.8, so it can be judged that the feeder L ₁ is a single-phase ground fault feeder.

Embodiment 3: The simulation model of the 110kV/35kV distribution network system is the same as in Embodiment 1. After the voltage value exceeds the limit, a cycle of zero-sequence voltage data is obtained.

The steps of the method for identifying single-phase ground faults and phantom ground faults based on the half-cycle energy ratio are the same as those in Embodiment 1.

The zero-sequence voltage waveform of the feeder _L1 obtained through the simulation experiment is shown in Figure 4, and the ratio of the highest frequency energy values of the two half-cycles before and after is obtained =0.439< =0.8, so it can be judged that the phantom grounding phenomenon has occurred in the system.

In the same way, according to the same steps in the above example, the highest frequency energy ratio under different fault initial angles and phantom grounding can be studied. The results are shown in Table 1. This ratio is much greater than 1 in the case of a single-phase ground fault, and close to 1 in the case of a phantom ground fault. From Table 1, it can be seen that the single-phase ground fault identification method based on the half-cycle energy ratio fails when the fault initial angle is near 0°. A certain dead zone.

The specific implementation of the present invention has been described in detail above in conjunction with the accompanying drawings, but the present invention is not limited to the above-mentioned implementation, and within the knowledge of those of ordinary skill in the art, it can also be made Various changes.

Claims (2)

1. the singlephase earth fault based on half cycles energy Ratios and Xuhanting oral solution recognition methods, it is characterized in that: after power distribution network breaks down, extract the residual voltage data that starting element starts a rear cycle, 3 layers of WAVELET PACKET DECOMPOSITION are carried out respectively to former and later two half cycles, its most high-frequency energy value is tried to achieve respectively by the wavelet package reconstruction coefficient after decomposing, the most high-frequency energy value of front half cycles and the most high-frequency energy value of later half cycle are made ratio, obtains value, according to itself and setting threshold value magnitude relationship reliably realize the identification of one-phase earthing failure in electric distribution network and Xuhanting oral solution.

2. the singlephase earth fault based on half cycles energy Ratios according to claim 1 and Xuhanting oral solution recognition methods, is characterized in that: the concrete steps of described one-phase earthing failure in electric distribution network and Xuhanting oral solution recognition methods are as follows:

(1), after power distribution network breaks down, its fault residual voltage value can be obtained according to the three-phase voltage that protection installation place records :

In formula: , , be respectively the voltage of circuit A, B, C three-phase when breaking down;

(2) extract power distribution network to break down the residual voltage data of a rear cycle, utilize analysis method of wavelet packet to decompose former and later two half cycles respectively, and calculate each most high-frequency energy value;

If input signal is s, after WAVELET PACKET DECOMPOSITION, the frequency band range of gained component of signal is:

Wherein: jfor the number of plies of wavelet decomposition, kfor a kth contact of wavelet decomposition, f _sfor the frequency of input signal;

To input signal scarry out 3 layers of WAVELET PACKET DECOMPOSITION, decompose and there is following relation:

S=AAA3 +DAA3 +ADA3 +DDA3 +AAD3 +DAD3 +ADD3 +DDD3

Wherein: arepresent low frequency, drepresent high frequency, the sequence number number at end represents the level of WAVELET PACKET DECOMPOSITION (being also scale parameter);

By the most high-frequency energy value of the wavelet package reconstruction coefficient calculations signal of gained after WAVELET PACKET DECOMPOSITION e ₁, namely ;

(3) the most high-frequency energy that before after setting singlephase earth fault or power distribution network generation Xuhanting oral solution, half cycles obtains after WAVELET PACKET DECOMPOSITION as e ₁₁, the most high-frequency energy value of later half cycle is e ₁₂, ask for the two ratio

；

(4) according to ratio with set threshold value between relationship distinguish singlephase earth fault and Xuhanting oral solution:

When time, be then judged to be singlephase earth fault;

When time, be then judged to be Xuhanting oral solution.