CN112787318B - Setting method for stage type zero sequence overcurrent protection of small resistance grounding system - Google Patents

Abstract

The invention discloses a setting method for staged zero-sequence overcurrent protection of a grounding system via a small resistance, belonging to the field of relay protection of power distribution networks. For the distribution network grounded by small resistance, no matter how large the transition resistance is in the case of a single-phase grounding fault, the zero-sequence current of the faulty line is much larger than the zero-sequence current of the sound line. Optional. The invention provides a stepwise zero-sequence overcurrent protection setting method based on lateral coordination for a grounded system via a small resistance, which mainly includes a starting current setting method, a setting method for the number of protection stages, a setting method for each stage setting, a setting method for the action time limit, and the like. , the existing zero-sequence overcurrent protection can be decomposed into multi-stage definite-time zero-sequence overcurrent protection, in order to reduce the protection starting current setting value of the lowest stage and improve the sensitivity of high-resistance ground fault protection, which has a wide range of practical application prospects.

Description

A Setting Method for Staged Zero Sequence Overcurrent Protection of Small Resistance Grounding System

technical field

The invention relates to a setting method for ground fault protection of a small resistance grounding system, which is suitable for a power distribution system in which a neutral point is grounded through a small resistance, and belongs to the field of relay protection of power distribution networks.

Background technique

The neutral point grounding method with small resistance has the advantages of quickly cutting off the ground fault, low overvoltage level, eliminating resonance overvoltage, using cables and electrical equipment with low insulation level, and convenient operation and maintenance. distribution network in many large cities. Since the zero-sequence current amplitude does not change much when single-phase grounding occurs at different positions on the line, it is generally impossible to achieve selective action through the coordination of zero-sequence current protection settings, and the grounding current does not exceed 1000A, which is less harmful to the system. Only the definite time zero sequence (III stage) overcurrent protection is used, and the cooperation with the downstream branch line and distribution transformer protection is realized through the action time limit.

However, due to the fact that 10kV distribution lines go deep into densely populated areas and are affected by factors such as the natural environment and the low overhead distance of the lines, single-phase high-resistance grounding faults via non-ideal conductors often occur, such as conductors falling on grass, roads, sand, water, etc. Tang and so on. However, the current setting value of zero-sequence overcurrent protection of 10kV small resistance grounded distribution network in my country (referring to 3 times the zero-sequence current setting value, the same below) is generally 40A~60A, and the maximum grounding resistance of about 85Ω~135Ω can only be detected. . Therefore, when the high resistance is grounded, it will refuse to operate, and the system runs with faults for a long time, which may cause the grounding transformer protection action to cut off the power supply or interphase short-circuit fault, and expand the scope and harm of the fault.

The existing low-resistance grounding system high-resistance grounding fault detection algorithms are mainly divided into two categories: one is based on harmonic or distortion information, and adopts analysis tools such as pattern recognition; the other is to use the power frequency zero-sequence current/ Electrical quantities such as voltage constitute grounding protection. Compared with the traditional definite-time zero-sequence overcurrent protection, the sensitivity of these two methods is improved. The former is more suitable for unstable grounding points, while the latter is simpler and more reliable to set and configure, and is easier to implement in the field.

According to the characteristic that the zero-sequence current of the faulty outgoing line is always far greater than the zero-sequence current of the sound outgoing line in the small-resistance grounding system, and combined with the traditional staged zero-sequence overcurrent protection idea, the invention proposes a staged zero-sequence overcurrent suitable for the small-resistance grounding system. The current protection setting method is based on the horizontal cooperation between the outgoing protections. This setting method decomposes the original definite time zero sequence (III stage) overcurrent protection into multi-stage definite time zero sequence overcurrent protection, and reduces the starting current setting of the lowest stage protection. It has a wide range of practical application value to improve the sensitivity of high-resistance ground fault protection.

SUMMARY OF THE INVENTION

Combining the characteristics of traditional staged zero-sequence overcurrent protection, the invention proposes a staged zero-sequence overcurrent protection setting method suitable for small resistance grounding systems, which can effectively improve the sensitivity of high-resistance grounding fault protection.

The technical solution of the present invention is:

A method for setting the stage-type zero-sequence overcurrent protection of a small resistance grounding system, which mainly includes the setting method of the starting current I _S , the setting method of the number of protection stages k, and the setting value of the IIIX/IIIX' segment I _{set.IIIX /I'set.IIIX} ( ⅢX is the protection of each section of the outgoing line, ⅢX' is the protection of each section of the grounding transformer, X=1, 2, 3...k) setting method and action time limit setting method.

Further, the starting current I _S setting method is as follows: the starting current I _S should take the maximum value of the maximum unbalanced zero-sequence current of the system and the minimum precision current of the zero-sequence current transformer.

Further, the setting method of the number of protection segments k is as follows: considering the existence of the zero-sequence TA measurement error, in order to avoid the measurement value of the zero-sequence current of the sound outgoing line and the measured value of the zero-sequence current of the faulty outgoing line being located in the same segment, whether it is outgoing line protection or grounding protection , the difference between the upper and lower limits of the current setting of each stage should not exceed 1/(ωR _N C _0n K _rel ² ) times (ω is the power frequency angular frequency, R _N is the neutral point resistance, C _0n is the maximum zero-sequence capacitance of each outlet line, K _rel is the reliability coefficient, generally 1.2~1.3). Assuming that the fault line and neutral point zero-sequence current does not exceed α ₁ A when the single phase of the distribution network is grounded, the number of protection stages k should be taken to satisfy [1/(ωR _N The smallest integer value of C _0n K _rel ² )] ^k >α ₁ / _IS .

Further, the setting method of IIIX/IIIX' segment setting value I _{set.IIIX /I'set.IIIX} is:

1) Section Ⅲ1/Ⅲ1' is the highest section of the algorithm, it is only necessary to ensure that the current setting of this section avoids the maximum zero-sequence current of the sound line when the single-phase grounding occurs, assuming that the zero-sequence current of the sound line does not exceed α ₂ A, then I _set.Ⅲ1 ≥K _rel ×α ₂ , I' _set.Ⅲ1 ≥K _rel ×I _set.Ⅲ1 ;

2) IIIk/IIIk' segment is the lowest segment of the algorithm, I _set.IIIk can be set as the protection starting current constant value I _S , that is, I set. _IIIk =I _S , considering the existence of the zero-sequence TA measurement error, it is To avoid overstepping action due to the start of the grounding transformer protection, then I' _set.Ⅲk = K _rel ×I _S ;

3) Ⅲ2/Ⅲ2'～Ⅲ(k-1)/Ⅲ(k-1)' is the middle section of the algorithm, because the difference between the upper and lower limits of the current setting of each section should not exceed 1/ωR _N C _0n K _rel ² times, then I _set.Ⅲ2 ~ _{Iset.Ⅲ(k-1)} should satisfy:

_{I'set.III(x-1)} =Krel× _{Iset.III (x-1)} .

Further, the action time limit setting method is as follows: set t _set.Ⅲ1 , t _set.Ⅲ2 , ..., t _set.Ⅲk as the action time limit values corresponding to each section of the outgoing line protection; _t'set.Ⅲ1 , _t'set.Ⅲ2 , ..., t' _set.Ⅲk are the corresponding action time limit values of each stage of the grounding transformer protection. When considering the minimum action, the limit value should cooperate with the grounding protection of the lower branch line or distribution transformer, and should also cooperate with the grounding protection of the circuit breaker on the secondary side of the main transformer at the upper level. It is assumed that the return time of the protection device is Δt, so t _set.Ⅲ1 ≥ 2Δt. At the same time, the time step between each protection section should be above 2Δt, that is, t _set.Ⅲ2 , t _set.Ⅲ3 ... t _set.Ⅲk should satisfy:

The action time limit of the grounding transformer protection shall satisfy: t _set.Ⅲ(x+1) >t' _{set.Ⅲx ≥t set.Ⅲx} +Δt.

Description of drawings

Accompanying drawing 1 is the schematic diagram of small resistance grounding system grounding fault and protection installation position;

Accompanying drawing 2 is the stage type zero-sequence overcurrent protection characteristic;

Figure 3 shows the zero-sequence voltage and current waveforms when grounded through a 100Ω resistor;

Figure 4 shows the zero-sequence voltage and current waveforms when the wire falls to the ground on the wet grass.

Detailed ways

Below in conjunction with accompanying drawing and embodiment, the present invention is further described:

A setting method for staged zero-sequence overcurrent protection of small resistance grounding system, suitable for neutral point grounded distribution network with small resistance, mainly including starting current I _S setting method, protection section number k setting method, IIIX/IIIX' section Fixed value I _{set.ⅢX /I'set.ⅢX} (X=1,2,3…k) setting method and action time limit setting method. The typical topology and protection installation position of the small resistance grounded distribution network (P1 ~ Pn, PR _N are the grounding protection of each outgoing line and the grounding transformer protection respectively) are shown in Figure 1.

The starting current I _S setting method can be calculated as follows in a 10kV grounding system via a small resistance:

(1) When the system is in normal operation, it is required that the protection cannot be malfunctioned. Therefore, the starting current setting value I _S of the protection must be higher than the maximum unbalanced zero-sequence current of the system to prevent the grounding protection from malfunctioning when the system is running normally. It is generally believed that the maximum unbalanced zero-sequence current of overhead lines and cable lines of 10kV low-resistance grounding system is about 0.37A and 0.26A, respectively, and the protection device is generally connected to three times the zero-sequence current signal, so there are constraints: I _S >1.11A;

(2) Secondly, the linear range and measurement error of the zero-sequence TA must be considered. Considering that the minimum precision current of the zero-sequence current transformer is generally 0.5% of the full scale (600A), it is generally necessary to satisfy IS _≥600A ×0.5%=3A ;

(3) Finally, because the fixed value of I _S directly affects the withstand resistance capability of the grounding protection, when the 10kV small resistance grounding system has a grounding fault through a 1.5kΩ resistance, the zero-sequence current of the fault outlet is about 3.8A, regardless of the line impedance. Therefore, in order to make it able to protect the high-resistance ground fault of about 1.5kΩ transition resistance and avoid the maximum unbalanced zero-sequence current on the line, the setting value of the starting current I _S can be set to 3A, that is, I _S = 3A.

The setting method for the number of protection stages k can be calculated as follows in a 10kV grounding system with a small resistance of 10Ω:

(1) When a metallic ground fault occurs at the bus or line exit, the fault current I _f ≈ 3E _A /3R _N ≈ 600A, so it is only necessary to perform segmentation within the current range of 3A to 600A.

(2) In order to avoid that the measured value of the zero-sequence current of the sound outgoing line and the measured value of the zero-sequence current of the faulty outgoing line are located in the same section, whether it is the outgoing line protection or the grounding transformer protection, the difference between the upper and lower limits of the current setting of each section should not exceed 1/ωR _N C _0n K _rel ² times, in a 10kV grounding system with a small resistance of 10Ω, ωR _N C _0n can generally be taken as 0.1, then the number of protection sections k should satisfy (10/K _rel ² ) ^k >600/3, take K _rel =1.25, it can be If k>2, in actual engineering, the more the protection sections are, the more complex and difficult the setting process and the coordination between the upper and lower protections will be. Therefore, k should take the minimum value k = 3, that is, the number of protection sections should be set to three sections, which is recorded as Section III1, Section III2, Section III3.

The setting method of the IIIX/IIIX' segment setting value I _{set.IIIX /I'set.IIIX} (X=1,2,3) can be calculated as follows in a 10kV grounding system with a small resistance of 10Ω:

(1) Section III1/III1' is the highest section of the algorithm, it is only necessary to ensure that in any case, the current value of this section escapes the maximum zero-sequence current of the sound line when the single-phase grounding The zero-sequence current of the line does not exceed 60A), then I _set.Ⅲ1 ≥K _rel ×60, I' _set.Ⅲ1 ≥K _rel ×I _set.Ⅲ1 , take K _rel =1.25, then I _set.Ⅲ1 ≥75A, I ' _set.Ⅲ1 ≥94A;

(2) Section III3/III3' is the lowest section of the algorithm, and _Iset.III3 can be set as the protection starting current constant value I _S , that is, I set. _III3 =I _S =3A, considering the measurement error of zero sequence TA exists, in order to avoid the grounding transformer protection from starting and overstepping action, then I' _set.Ⅲ3 =K _rel ×I _S , taking K _rel =1.25, you can get I' _set.Ⅲ3 =3.75A;

(3) Section III2/III2' is the middle section of the algorithm. Since the difference between the upper and lower limits of the current setting of each section should not exceed 10/K _rel ² times, then I _set.III2 should satisfy:

_I'set.III2 = _{Krel×Iset.III2 .} Taking K _rel =1.25, 12A< _Iset.III2 <19A can be obtained. For grounding transformer protection, there are 15A <I' _set.Ⅲ2 <24A.

The action time limit setting method can be calculated as follows in a 10kV grounding system via a small resistance:

It is generally considered that the return time of the protection is more than 0.3s, that is, Δt≥0.3s. When considering the minimum action, the limit value should be coordinated with the grounding protection of the lower branch line or distribution transformer, and should also be combined with the grounding protection of the circuit breaker on the secondary side of the upper main transformer. Therefore, t _set.Ⅲ1 ≥ 2Δt=0.6s, and the time step between each protection stage should also be above 2Δt, that is, t _set.Ⅲ2 and t _set.Ⅲ3 should satisfy:

Therefore, t _set.Ⅲ2 ≥1.2s, _tset.Ⅲ3 ≥1.8s, the action time limit of the grounding transformer protection should satisfy: t _set.Ⅲx+1 >t' _{set.Ⅲx ≥t set.Ⅲx} +Δt, then 1.2s >t' _set.Ⅲ1 ≥0.9s, 1.8s>_t'set.Ⅲ2 ≥1.5s, _t'set.Ⅲ3 ≥2.1s. The relationship between the setting value of each segment and the action time limit after being set by the present invention is shown in FIG. 2 .

Based on the artificial grounding test results of the busbar of the 110kV ZY substation in a city of Shandong Province, the beneficial effects of the staged zero-sequence overcurrent protection setting method proposed in the present invention are verified.

The neutral point of the experimental line is grounded with a 10Ω resistance through a grounding transformer. There are 12 overhead mixed cables in the system, and the capacitance current to ground is 72A. The artificial simulated faults include stable grounding (transition resistances are 4Ω, 100Ω, 500Ω, and 1000Ω, respectively) and unstable grounding (respectively, 100Ω resistors connected in series with ball gap simulation arc grounding and conductor falling wet grass grounding). Select two sound lines F1 and F2 with the largest capacitance current to ground, one sound line F3 with smaller capacitance current to ground, and the test data of fault line F4 for verification. The method proposed in the present invention is set. The zero-sequence current of each outgoing line, the zero-sequence current of the neutral point and the action conditions during each fault are shown in Table 1. The zero-sequence voltage and zero-sequence current of each outgoing line when grounded through a 100Ω transition resistance and grounded through the grass are shown in Figure 3. Figure 4 shows.

Table 1 RMS value and action of each outgoing zero-sequence current

Note: ↑-start; ↓-return; √-trip; ×-not start; III 1, III 2, III 3 are the corresponding sections of protection

It is easy to know from the data in Table 1 that when the ground is grounded through a 4Ω resistance (fault point grounding body resistance), the grounding protection of the fault line F4, the grounding-to-neutral point and the two lines F1 and F2 with the largest capacitance current to the ground will be activated at the same time. The fault line F4 is the first to act. After the fault is removed, other protections that are activated but not selective will have enough time to return reliably without misoperation. In other types of ground faults, only the fault line F4 and the neutral point transformer protection are activated, and the zero-sequence current of the sound line does not exceed the protection starting current value _IS = 3A, so the sound line protection device will not be activated. , Although the grounding transformer protection starts with the fault line protection, but because of its long action time, it can reliably return after the fault line protection action, and will not malfunction.

When the high resistance is grounded, such as 1000Ω grounding, setting the fixed value according to the method proposed in the present invention can still remove the fault in about 1.8s. For unstable grounding faults, although the zero-sequence voltage and zero-sequence current waveforms are distorted, the protection will still operate correctly as long as the measured zero-sequence current is higher than the current set value of the protection.

The above are only specific embodiments of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art can easily think of changes or substitutions within the technical scope disclosed by the present invention, All should be included within the protection scope of the present invention. Therefore, the protection scope of the present invention should be subject to the protection scope of the claims.

Claims (1)

1. A setting method for staged zero-sequence overcurrent protection of small resistance grounding system, suitable for neutral point grounding system with small resistance, including starting current setting method, protection stage number setting method, each stage setting method and action time limit The tuning method is characterized by: a. The starting current I _S takes the maximum value of the maximum unbalanced zero-sequence current of the system and the minimum precision current of the zero-sequence current transformer; b. The number of protection segments k is satisfied

The minimum integer value of , where ω is the power frequency angular frequency, R _N is the neutral point resistance, C _0n is the maximum zero-sequence capacitance of each outgoing line, K _rel is the reliability coefficient, take 1.2, and α ₁ is the single-phase grounding of the distribution network is the maximum value of the zero-sequence current of the fault line and the neutral point, I _S is the starting current; c. The multi-stage definite time zero-sequence overcurrent protection III1, III2, III3...IIIk of each outgoing line has the fixed values I _set.III1 , _Iset.III2 , _Iset.III3 ... _Iset.IIIk and the multi-stage definite time of the grounding transformer Zero sequence overcurrent protection Ⅲ1', Ⅲ2', Ⅲ3'...Ⅲk' set value I' _set.Ⅲ1 , I' _set.Ⅲ2 , I' _set.Ⅲ3 ...I' _set.Ⅲk meet the set. 1) The segment III1/III1' is the highest segment of the algorithm, and the fixed values I _set.III1 and _I'set.III1 satisfy I _set.III1 ≥K _rel ×α ₂ , and I' _set.III1 ≥K _rel ×I _{set. III1} , _α2 is the maximum value of zero-sequence current of sound line; 2) Section _IIIk / _IIIk ' is the lowest section of the algorithm, and the fixed values _Iset.IIIk and _I'set.IIIk satisfy _Iset.IIIk =Is, _I'set.IIIk = _Krel ×Is respectively; 3) The segment III2/III2'～III(k-1)/III(k-1)' is the middle segment of the algorithm, and the fixed values _Iset.III2 ～ _{Iset.III(k-1)} satisfy the

I' _set.Ⅲ(x-1) =K _rel ×I _set.Ⅲ(x-1) ; d. The action time limit of each section outgoing line protection t _set.Ⅲ1 /t' _set.Ⅲ1 , t _set.Ⅲ2 /t' _set.Ⅲ2 , ..., t _set.Ⅲk /t' _set.Ⅲk satisfies t _{set. Ⅲ1} ≥ 2Δt, Δt is the return time of the protection device, take 0.3s, t _set.Ⅲ2 , t _set.Ⅲ3 ...t _set.Ⅲk satisfy respectively

The action time limit of each stage protection of the grounding transformer satisfies t _set.Ⅲ(x+1) >t' _set.Ⅲx ≥ t _set.Ⅲx +Δt.

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