CN111080475A - Transfer random inspection method suitable for distribution network key material inspection - Google Patents

Abstract

A transfer random inspection method suitable for distribution network key material inspection comprises the following steps: the method comprises the following steps: selecting a form of sampling; step two: assigning different quality grades according to the quality requirements; step three: determining sample substitute characters according to the batch size or the production capacity and the quality grade during production to obtain a specific sampling inspection method; step four: executing a transfer spot check method; the method accords with relevant requirement files of the national network for detecting key materials of the distribution network, and the sampling scheme meets the requirement of four hundred percent of key materials, thereby having performability; the method of counting adjustment is introduced, the sampling quantity can be adjusted according to the feedback of the inspection result, and the problems that the probability of misjudging unqualified batches into qualified batches is high when the batches are large and the like are solved; the current inspection workload of the power grid is increased little after implementation; the same number of samples are extracted for batches within a certain number of intervals, so that the strictness degree consistency of different batches is ensured, and the unscientific performance of adopting a percentage sampling method in the conventional power grid is avoided.

Description

Transfer random inspection method suitable for distribution network key material inspection

Technical Field

The invention belongs to the field of distribution network material detection of a power system, and provides a transfer sampling detection method based on a counting adjustment type sampling scheme aiming at the sampling detection of key materials of a distribution network.

Background

The power grid material detection is the key work of putting good power grid materials into a gateway and ensuring the intrinsic safety of a power grid. With the advance of the upgrading and transforming work of distribution networks and rural networks, the coverage and working requirements of power grid material detection are constantly improved, the detection traffic volume is increasing day by day, the existing material quality detection system has some defects in the aspects of bearing capacity, working efficiency, resource utilization, deep analysis and the like, can not adapt to the practical needs of rapid development of power grids and intensive management of material resources, and needs to be improved in technology and management modes urgently

Taking a national grid as an example, the number of samples extracted in sampling inspection is constantly changed according to the sampling inspection coverage requirement of the national grid, and for a long time, the three-hundred-percent coverage is adopted according to ' all bidding batches, all suppliers and all material categories ', and for key materials such as distribution boxes, distribution transformers, overhead insulated wires, cement poles, power cables, iron accessories and the like, the coverage is guaranteed to be four-hundred-percent ' (covering one-hundred-percent winning batches, covering one-hundred-percent winning suppliers, covering one-hundred-percent material categories and covering one-hundred-percent arriving batches).

From 2017, the national power grid provides a quantitative requirement for spot check, and each material spot check quantitative requirement is different and is roughly divided into three categories, namely coverage according to a proportion of 5%, quarterly coverage, annual coverage and the like. Taking the distribution as an example, the requirement of 5% of the quantity of the goods to be delivered is satisfied besides covering each supplier of the supply; while the cable is intended to cover all quarter arrival suppliers, i.e., if a supplier arrives quarter by quarter, it requires 4 spot checks a year. In 2018, the regulations of national network on the sampling inspection quota are changed, quarterly coverage and annual coverage are not distinguished any more, only coverage is carried out according to a proportion of 5% and the average annual coverage frequency, in short, distribution transformer, switch cabinet, JP cabinet, ring main unit, column switch and the like can meet the sampling inspection proportion of 5% of the goods quantity except all supply suppliers, and most of the equipment is implemented according to the standards of 2 times of sampling inspection per year of equipment suppliers and 3 times of sampling inspection per year of circuit material suppliers.

In 2019, the national network adjusts the coverage requirement, and the quota requirement is also met according to three hundred percent coverage of 'supplier, material category and supply lot'.

Four hundred percent, the scheme adopted is that a sample test is extracted for each arriving batch of each supplier, and if the batch is qualified, the batch is received; if not, a sample is extracted, if the second sample is qualified, the batch is still received, and if not, the batch is rejected.

At present, four 'one hundred percent' schemes have a sampling mode which is more original, and the current spot inspection scheme aiming at key materials in the power grid belongs to an (n | c) scheme. The reception probability thereof can be calculated as follows.

Suppose there is a product to be inspected with a capacity of N, where the number of misapprovals is M. N samples are randomly drawn from the N products, and the number of rejects d in the samples is a random variable subject to a hypergeometric distribution. If the random variable is X, the probability that the number of disqualification is d in n samples is:

the probability of receipt of the batch of product is

The above equation is an expression of the (n | c) scheme sampling characteristic function.

Currently, the (n |0) scheme is implemented in the power grid, and if 1 out of n samples is found, the batch of products is judged to be unqualified, namely c is 0, and the formula (2) is substituted to obtain the following result:

let the fraction defective of the product be p, and the sampling rate be q, i.e. M-N p, N-N q,

substituting equation (3) can obtain:

taking an important material distribution transformer as an example, from historical statistical data, the reject ratio p is in a [ 90%, 95% ] interval, the current power grid is a fixed 5% proportion sampling inspection, that is, q is 5%, and usually, the distribution transformer batch N purchased by the provincial power grid is between 20-200, then the sampling characteristic function is:

calculated, the OC curve is as shown in figure 1

As can be seen from fig. 1, as the number N of distribution transformer product batches increases, the probability of acceptance decreases. In an extreme case, if the historical average value of the failure rate of the distribution transformer is 10%, if the supply quantity of a certain manufacturer reaches 200, the probability that the batch of products passes the sampling inspection is only about 35%. That is to say, the currently used 5% sampling inspection rule has significant drawbacks, that is, the sampling inspection widths of different batches of products are obviously inconsistent, and the larger the batch is, the lower the receiving probability is, and there are the following 2 main disadvantages:

⑴, checking that the width and the severity are inconsistent, namely the width and the severity of the goods spot inspection of different batches are obviously inconsistent, and the larger the batch is, the lower the receiving probability is.

⑵, there is no consideration of feeding back the supplier's historical supplier qualification to the sampling plan for the supplier's next shipment.

Under the strict constraint of various national standards, the detection reject ratio of electric power materials, in particular to key materials such as distribution transformers, insulated wires, distribution boxes, power cables, cement poles, iron accessories and the like is relatively low. But the production process can not completely avoid unqualified products, and from the perspective of a production party, the production party hopes that the unqualified products produced in the production process are not detected during sampling inspection so as to ensure the profit of the production party; from the perspective of a user, the materials received by the power grid are all qualified products, and if the batch quality is unqualified, the materials are detected in sampling detection to prevent the unqualified materials from flowing into the power grid.

On the other hand, in order to avoid the problem that the acceptance probabilities of different batches are not consistent in the existing sampling method, percentage sampling is avoided, the acceptance probability of each batch is consistent or similar, and the scientificity and the rationality of sampling inspection are improved.

Aiming at the above situation, in order to guarantee the rights and interests of users and meet the requirements of power grids on materials, it is the most reliable method to adopt total inspection to inspect all products in the lots. In practical use, however, a great deal of time and labor cost is required, and the total test is obviously impractical. If the amount of detection work required for the designed solution is too large, it is likely that it cannot be actually performed.

Disclosure of Invention

Aiming at the problems, the invention provides a transfer sampling method based on a counting adjustment type according to an MIL-STD-1916 sampling standard based on the sampling requirement of key material detection of a distribution network and the actual condition of the material detection of the distribution network.

The technical scheme adopted by the invention is as follows: a transfer random inspection method suitable for distribution network key material inspection is characterized in that: the method comprises the following steps:

the method comprises the following steps: selecting a form of sampling;

step two: assigning different quality grades according to the quality requirements;

step three: determining sample substitute characters according to the batch size or the production capacity and the quality grade during production to obtain a specific sampling inspection method;

step four: a transfer snapshot method is performed.

Further, the method for assigning different quality grades according to the quality requirement comprises: according to the formula:

(1) calculating the receiving probability of different numbers of batches under the unqualified rate of the conventional sampling inspection method; taking a fixed receiving probability according to the interval to avoid the inconsistency of strict degrees;

(2) and (3) calculating the quality grade and the receiving probability of the sample representative character in the reject ratio of the conventional sampling inspection method, and comparing the receiving probability with the receiving probability in the step (1) to determine the quality grade under the condition of ensuring that a larger receiving probability is adopted.

Further, the sampling mode adopts counting sampling with a small sampling sample amount.

Further, the transfer spot check method in the fourth step specifically comprises the following steps:

the following six cases are divided:

(1) normal test conversion stringency test: the test must be switched from normal to stringent when either of the following conditions occurs:

A. counting value and metering value sampling: 2 of the last 2-5 batches were rejected;

(2) and (3) converting a normal test by a strict test: when the following two conditions are met, the normal test is converted from the strict test:

A. the defect causes of the unqualified parts are corrected;

B. counting value and metering value sampling: and 5 consecutive batches were received;

(3) normal inspection transition relaxation inspection:

when the following four conditions are met, the test is relaxed by normal test conversion:

A. counting value and metering value sampling: 10 consecutive batches were received;

B. the production is in a stable stage;

C. the quality system operates properly and the performance is considered satisfactory by government agencies or customers;

D. government agencies or customers agree to decrement checks;

(4) and (4) converting decrement test into normal test:

when any one of the following conditions occurs, it is necessary to switch from the decrement check to the normal check

A. Counting value and metering value sampling: there were 1 lots rejected;

B. the production situation is not easy and delayed;

C. quality system performance is considered unsatisfactory by government agencies or customers;

D. government agencies or customers believe that normal testing should be returned due to other production conditions;

(5) tight check is changed into pause check

In the scheme, after the strict inspection is started, when the number of batches is not accumulated to 2 batches, the inspection according to the standard is temporarily stopped; when the supplier stopping the inspection is given, effective improvement measures are implemented and the approval of the competent department is given, the recovery inspection can be proposed; recovery of a product lot for inspection typically begins with a rigorous inspection; meanwhile, other penalties can be made for the suppliers according to relevant regulations;

(6) special cases

After the inspection is started, if the problems of unqualified products detected in any batch are serious, the inspection can be directly switched to a pause inspection stage from any stage.

The invention has the advantages and characteristics that:

1. the method accords with relevant requirement files of the national network for detecting key materials of the distribution network, and the sampling scheme meets the requirement of four hundred percent of key materials, thereby having performability;

2. the method for adjusting the counting is introduced, the sampling quantity can be adjusted according to the feedback of the inspection result, and the problems that the probability of misjudging unqualified batches into qualified batches when the batches are large is high and the like are solved.

3. The current inspection workload of the power grid is increased little after implementation.

4. The same number of samples are extracted for batches within a certain number of intervals, so that the strictness degree consistency of different batches is ensured, and the unscientific performance of adopting a percentage sampling method in the conventional power grid is avoided.

Drawings

FIG. 1 is a graph of sampling characteristic functions for different product lot numbers according to a preferred embodiment of the present invention;

Detailed Description

The invention is further illustrated below:

the invention designs a dynamic sampling scheme aiming at the detection of key materials of a power grid distribution network based on a common MIL-STD-1916 sampling standard. The severity of the sampling scheme should vary with the quality of the product, and when the quality of the batch is better than expected, a more relaxed sampling scheme can be used, and when the quality of the product batch is reduced, the more stringent sampling scheme is used to ensure that the unqualified product cannot pass the inspection. For the workload problem of the power grid reaction, a scheme that the sampling inspection sample amount is less in the design process is considered, but the judgment of batch quality is reduced due to the reduction of the sampling inspection sample, unqualified batches are easily judged to be qualified products, and the risk of a user is caused. For this problem, an adjustment transfer method can be used to compensate: when the product quality is reduced, the process can be easily shifted to strict inspection or even stop inspection, and meanwhile, other penalties are carried out on the supplier according to relevant regulations of the power grid, so that the cost which can be paid when the supplier tries to 'mix the unqualified batches to close' is increased. Thus, although the probability that a reject lot is not detected is not small, once detected, the consequences are very serious for the supplier, and this expectation can cause psychological stress to the supplier, which no longer tends to conceal its own rejects, and prompt the supplier to improve its own product quality, thereby securing the rights of the user.

Examination level and sampling scheme determination:

when the MIL-STD-1916 standard is used, the following steps are required:

① selecting the form of the sample (metering, counting, continuous sampling)

② assigning different levels of VL (total of 7 levels) based on quality requirements

③ sample representative CL (code Lett er) is determined based on lot size or throughput during production and VL.

④ execute a conversion routine (normal, tightening, decrement).

And sampling by counting according to the requirement of the power grid.

The quality class is the overall severity of the sampling scheme implemented, and there are a total of 7 quality requirements in MIL-STD-1916, denoted vii, vi, v, iv, iii, ii, i, respectively, where vii is the most stringent and i is the most relaxed.

In the case of past failure rate determination, the severity can be represented by the reception probability, and the higher the reception probability is, the lower the severity is, so that the quality level can be determined by determining the past failure rate of the product and the desired reception probability.

When N >10N is satisfied, the receiving probability has the following calculation formula:

wherein:

l (p) is the probability of receipt of the batch of products;

p is the reject ratio of the product in the past, and is not set to be 5% according to the past experience of the power grid;

n is the total number of the products in the batch;

n is the sampling number of the batch of products;

c is the number of unqualified products allowed to appear, and 0 is taken in the scheme

Specific description of sampling scheme formulation:

the sampling scheme of the present invention is further described in detail below by way of a sampling calculation example for a distribution transformer.

⑴ quality rating determination

Distribution transformer belongs to the most important key material in distribution network, and at present, five percent of fixed proportion sampling is adopted in each batch, according to statistics, the sampling average failure rate is about 5 percent, and the quantity of the arrived goods in each batch is 2-200.

Then the probability of reception for different number of batches using the original scheme, i.e. five percent sampling test, can be calculated according to equation (6):

TABLE 1 fraction defective P₀When the sampling rate is 5%, the receiving probability table of different batches under the 5% sampling method is adopted

Total number of	20	60	100	140	200
						Number of samples	1	3	5	7	10
Receiving rate	95％	85.7％	77.38％	69.83％	59.87％

It can be seen from the table that when the population number is different, the receiving probability of sampling by 5 percent changes with the change of the population number, but the change range is between 59.87 percent and 95 percent, so that when the new sampling scheme is used, a fixed receiving probability can be obtained by referring to the interval, and the error of inconsistent past strictness is avoided.

The quality level (VL) and the reception probability of the sample representative (CL) at a failure rate of 5% can also be calculated according to the above equation (6) as shown in table 2 below:

TABLE 2 count value sampling (P)₀Table of reception ratios (Pc) of 5%, Ac 0, and Re 1)

Referring to table 2, it can be seen that the condition of the receiving probability can be better satisfied when selecting the quality level i, and when selecting the quality level ii or even a stricter quality level, the receiving probability is only 54.03% at most, and the too low receiving rate greatly increases the labor cost of the drawer and the supply difficulty of the provider, which affects the normal operation of the transaction. More importantly, the receiving rate of the sample code A, B under the quality level I is close to the receiving rate of the original 5% sampling inspection scheme with the total quantity of about 100, namely the power grid company can accept the strict degree, so that the quality level I is selected most suitably.

⑵ sample surrogate word determination

After the quality grade is selected, the sample word substitution is determined according to the batch number, and the MIL-STD-1916 is inquired to obtain a sample word-carrying comparison table as follows:

TABLE 3 sample representative word (CL) comparison table

Distribution transformers often have between 2-200 pieces per batch, so the sample code A should be selected between 2-170 and the sample code B should be selected between 170 and 200.

(3) Sampling scheme determination

Selecting the substituted character and looking up the table to obtain the specific sampling inspection method, inquiring the MIL-STD-1916 standard, and obtaining the table as follows:

TABLE 4 count value sampling Schedule

The sampling pattern for quality class i, sample designation A, B, is easily found from the above table and is summarized as follows:

TABLE 5 sampling plan design table

The sampling scheme mainly comprises single sampling (including tightening, normal and decrement), double and multiple sampling are deleted, the sampling takes '0 receiving 1 withdrawing' (ZBA Zero Based Acceptance) as a determination standard, the existence of a defective product is emphasized, and the defective product is determined to be rejected once.

Is determined to be rejected, the supplier must perform the following actions:

① to isolate the rejected pieces and perform the necessary rework or rework, the corrected product supplier must first screen and then re-sample for inspection.

② determine the cause of the failure and perform the appropriate process change.

③ implement a conversion rule of normal, tightening and decrement checks.

④ the corrective action must be informed to the government agency or customer and again re-screened to the government or customer for evaluation.

4. Transfer spot-check method

The transfer method is a set of method for determining which sampling scheme is specifically selected from three levels of normal sampling schemes, relaxation sampling schemes and intensification sampling schemes, and the transfer method of the sampling scheme is specifically as follows:

(1) normal test conversion stringency test: the test must be switched from normal to stringent when either of the following conditions occurs:

A. counting value and metering value sampling: of the last 2-5 batches, 2 were rejected.

(2) And (3) converting a normal test by a strict test: when the following two conditions are met, the normal test is converted from the strict test:

A. the causes of the defects of the unqualified parts are corrected.

B. Counting value and metering value sampling: and 5 consecutive batches were received.

(3) Normal inspection transition relaxation inspection:

when the following four conditions are met, the test is relaxed by normal test conversion:

A. counting value and metering value sampling: 10 consecutive batches were received;

B. the production is in a stable phase.

C. The quality system functions properly and performance is considered satisfactory by government agencies or customers.

D. Government agencies or customers agree to an abatement test.

(4) And (4) converting decrement test into normal test:

when any one of the following conditions occurs, it is necessary to switch from the decrement check to the normal check

A. Counting value and metering value sampling: there were 1 lot rejected.

B. The production situation is not easy and delayed.

C. Quality system performance is considered unsatisfactory by government agencies or customers.

D. Government agencies or customers believe that normal testing should be returned due to other manufacturing conditions.

(5) Tight check is changed into pause check

In the protocol, when the number of lots is not counted up to 2 lots after the stringent test is started, the test according to the standard is temporarily stopped. When a supplier who has stopped the test is given effective improvement measures and has agreed by the competent authorities, a recovery test can be proposed. Recovery of a product lot for inspection typically begins with a rigorous inspection. While other penalties may be made to the provider in accordance with relevant regulations.

(6) Special cases

After the inspection is started, if the unqualified products detected in any batch have serious problems (such as the I-type problems in the classification and classification table of electrical performance supervision problems or serious influence on production activities), the inspection can be directly switched to a suspension inspection stage from any stage according to the approval of relevant responsible departments, and the suppliers are punished according to relevant regulations.

Therefore, the invention has the following advantages: 1. the method accords with relevant requirement files of the national network for detecting key materials of the distribution network, and the sampling scheme meets the requirement of four hundred percent of key materials, thereby having performability; 2. the method for adjusting the counting is introduced, the sampling quantity can be adjusted according to the feedback of the inspection result, and the problems that the probability of misjudging unqualified batches into qualified batches when the batches are large is high and the like are solved. 3. The current inspection workload of the power grid is increased little after implementation. 4. The same number of samples are extracted for batches within a certain number of intervals, so that the strictness degree consistency of different batches is ensured, and the unscientific performance of adopting a percentage sampling method in the conventional power grid is avoided.

The present invention will be described in further detail below with reference to 2 exemplary embodiments.

(1) Example 1: normal turning into tight and normal turning

A supplier is set to continuously supply distribution transformers to the grid, and the batch and spot inspection conditions of each delivery lot are shown in table 1. Detecting 1 unqualified product in batch 6, judging the batch is unqualified and rejecting, and resetting the number of continuous qualified batches; in batch 10, 1 unqualified product is detected again, and the calculation example that the table 6 is normal and tight is judged

Rejecting the batch, detecting 2 rejected batches in no more than 5 batches, transferring from the next batch, namely 11 th batch, to a strict test according to a transfer method, and sampling and testing each batch to obtain the sample amount

From 5 to 12. During the execution of the rigorous inspection, five consecutive batches 11, 12, 13, 14 and 15 are inspected, all qualified batches, and normal inspection is resumed from the next batch, namely 16 th batch according to a transfer method. As shown in table 6.

(2) Example 2: normal width and normal rotation

A supplier was set to continuously supply distribution transformers to the grid, and the batch, spot check conditions for each lot are shown in table 6. The total 10 continuous batches from the 1 st batch to the 10 th batch are qualified by sampling inspection, and the next batch, namely the 11 th batch is transferred to the relaxation inspection according to the transfer method, wherein the sampling inspection is performed each time

The sample size was down-regulated from 5 to 3. After the relaxation test criteria have been executed, defective products are again detected in the 14 th lot of the supplier, and according to the transfer method, the relaxation test should be stopped and the normal test should be resumed in the next lot, i.e., the 15 th lot. As shown in table 7.

TABLE 7 relaxation example of Normal rotation

The invention takes the sampling inspection of the distribution transformer as an example, and gives detailed step descriptions of the formulation of the sampling scheme and the transfer rule. The sampling scheme meets the requirement of four hundred percent of key materials of the distribution network, has consistent wide and strict degree and does not change along with the change of the number of batches; moreover, after the scheme is used, the number of the sampling samples is moderate and is approximate to that of the sampling samples of the current sampling and detecting method; more importantly, the spot check scheme reflects the historical supply levels of the various suppliers. The transfer sampling method can be applied to the actual work of detecting key materials of the power grid distribution network.

The foregoing shows and describes the general principles and broad features of the present invention and advantages thereof. It will be understood by those skilled in the art that the present invention is not limited to the embodiments described above, which are described in the specification and illustrated only for the purpose of illustrating the structural relationship and principles of the present invention, but that various changes and modifications may be made without departing from the spirit and scope of the invention as defined in the appended claims. The scope of the invention is defined by the appended claims and equivalents thereof.

Claims (4)

1. A transfer random inspection method suitable for distribution network key material inspection is characterized in that: the method comprises the following steps:

the method comprises the following steps: selecting a form of sampling;

step two: assigning different quality grades according to the quality requirements;

step three: determining sample substitute characters according to the batch size or the production capacity and the quality grade during production to obtain a specific sampling inspection method;

step four: a transfer snapshot method is performed.

2. The transfer random inspection method suitable for distribution network key material inspection according to claim 1, wherein the method for assigning different grades of quality grades according to quality requirements comprises: according to the formula:

(1) calculating the receiving probability of different numbers of batches under the unqualified rate of the conventional sampling inspection method; taking a fixed receiving probability according to the interval to avoid the inconsistency of strict degrees;

(2) and (3) calculating the quality grade and the receiving probability of the sample representative character in the reject ratio of the conventional sampling inspection method, and comparing the receiving probability with the receiving probability in the step (1) to determine the quality grade under the condition of ensuring that a larger receiving probability is adopted.

3. The transfer random inspection method suitable for distribution network key material inspection according to claim 1, characterized in that: the sampling mode adopts counting sampling with a small sampling sample amount.

4. The transfer random access method suitable for distribution network key material inspection according to claim 1, wherein the transfer random access method in the fourth step is specifically:

the following six cases are divided:

(1) normal test conversion stringency test: the test must be switched from normal to stringent when either of the following conditions occurs:

A. counting value and metering value sampling: 2 of the last 2-5 batches were rejected;

(2) and (3) converting a normal test by a strict test: when the following two conditions are met, the normal test is converted from the strict test:

A. the defect causes of the unqualified parts are corrected;

B. counting value and metering value sampling: and 5 consecutive batches were received;

(3) normal inspection transition relaxation inspection:

when the following four conditions are met, the test is relaxed by normal test conversion:

A. counting value and metering value sampling: 10 consecutive batches were received;

B. the production is in a stable stage;

C. the quality system operates properly and the performance is considered satisfactory by government agencies or customers;

D. government agencies or customers agree to decrement checks;

(4) and (4) converting decrement test into normal test:

when any one of the following conditions occurs, it is necessary to switch from the decrement check to the normal check

A. Counting value and metering value sampling: there were 1 lots rejected;

B. the production situation is not easy and delayed;

C. quality system performance is considered unsatisfactory by government agencies or customers;

D. government agencies or customers believe that normal testing should be returned due to other production conditions;

(5) tight check is changed into pause check

In the scheme, after the strict inspection is started, when the number of batches is not accumulated to 2 batches, the inspection according to the standard is temporarily stopped; when the supplier stopping the inspection is given, effective improvement measures are implemented and the approval of the competent department is given, the recovery inspection can be proposed; recovery of a product lot for inspection typically begins with a rigorous inspection; meanwhile, other penalties can be made for the suppliers according to relevant regulations;

(6) special cases

After the inspection is started, if the problems of unqualified products detected in any batch are serious, the inspection can be directly switched to a pause inspection stage from any stage.

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