A method for evaluating the reliability of medium voltage power supply

Abstract

The present invention belongs to an evaluation method, and specifically relates to a method for evaluating the effect of medium-voltage power supply reliability. It includes: step one: data collection; step two: adjusting grid parameters; step three: voltage and current synthesis; step four: curve fitting; step five: selecting the main curve; step six: curve normalization; step seven: merging the main parameter normalization curves; step eight: finding the threshold range; step nine: evaluation. The significant effect of the present invention is: the effect evaluation method of the present application evaluates the power consumption effect of each device in the power grid, and then evaluates the medium-voltage power reliability from the power consumption effect. During the execution of the method, the main performance parameters of the power-consuming equipment are selected, so that the evaluation results are directly linked to the effects of the power-consuming equipment, and the evaluation results are reliable and accurate. By selecting the main curves, normalization and curve merging, the multi-dimensional evaluation becomes a simple numerical comparison, ensuring that the evaluation results are simple and easy to understand.

Description

Technical Field

[0001] The invention belongs to an evaluation method, and in particular relates to a method for evaluating the reliability effect of a medium voltage power supply. Background Art

[0002] The power grid is an important form of energy supply in modern society. Many devices, both industrial and civilian, are highly dependent on power supply from the grid.

[0003] However, due to systemic reasons, the effect of power supply from the power grid cannot meet the needs of all electrical appliances. First of all, in terms of power supply form, power supply from the power grid is a form of energy supply that generates electricity in a centralized manner and then transmits electricity through the power grid, which is used by end users. In order to ensure power supply efficiency, the power generation devices of the power grid are often centralized, using coal, hydropower, nuclear energy, wind energy, solar energy or other power generation methods. Also based on efficiency considerations, the transmission process uses high or ultra-high voltage transmission according to the transmission power size and the different distances across regions. Therefore, before the end user uses electricity, the most original electric energy has been transformed and processed many times, and the resulting power pollution will affect the end user's power consumption effect. Secondly, in terms of the end user's power demand, different users have different power demand, and different equipment also has different power demand. For example, motors are more sensitive to current and frequency, lighting equipment is often more sensitive to voltage, and high-precision electronic equipment is more sensitive to voltage and frequency.

[0004] The existing technology usually only adds a voltage stabilizer in front of the power equipment to deal with the power pollution. However, this treatment method cannot adjust the power demand of the power equipment in real time, nor can it allow the power generation and power management units to grasp the direction of power adjustment in real time.

[0005] Therefore, a method for evaluating the reliability effect of medium voltage power supply is needed. Summary of the invention

[0006] In view of the defects of the prior art, the present invention provides a method for evaluating the reliability effect of medium voltage power supply.

[0007] The present invention is implemented as follows: 1. A method for evaluating the reliability effect of medium voltage power supply, characterized in that it comprises the following steps:

[0008] Step 1: Data Collection

[0009] Collect data for the power grid and each device in the grid;

[0010] Step 2: Adjust grid parameters

[0011] Adjust the grid voltage, grid current and grid frequency of the grid respectively, so that all electrical equipment in the grid stops working, and record all relevant parameters at the same time;

[0012] Step 3: Voltage and Current Synthesis

[0013] Synthesize the recorded voltage and current;

[0014] Step 4: Curve fitting

[0015] The resultant voltage u t , synthetic current I t , grid frequency f t , the main performance parameters S of the i-th device iat , S ibt , S ict Fit them respectively to obtain the main performance parameter curves of the i-th device;

[0016] Step 5: Select the main curves

[0017] Calculate the derivatives of the three curves of the same performance parameter of the same equipment, take the absolute value of the extreme value, and select the curve corresponding to the maximum absolute value of the extreme value as the main curve;

[0018] Step 6: Curve Normalization

[0019] Normalize the main curves corresponding to each performance parameter of each device;

[0020] Step 7: Merge the normalized curves of main parameters;

[0021] Step 8: Find the threshold range;

[0022] Step 9: Evaluation.

[0023] 2. A method for evaluating the reliability of medium voltage power supply according to claim 1, characterized in that: said step 1 comprises the following contents:

[0024] For the power grid, collect the following parameters: grid voltage u 1t 、u 2t 、u 3t , grid current I 1t ,I 2t ,I 3t , grid frequency f t , where u 1t 、u 2t 、u 3t Represents the three voltages of three-phase alternating current, I 1t ,I 2t ,I 3t represents the three currents of three-phase alternating current, t is the time, that is, u 1t represents the voltage of the first phase at time t, u 2t represents the voltage of the second phase at time t, u 3t Represents the voltage of the third phase at time t; I1t represents the current of the first phase at time t, I 2t represents the current of the second phase at time t, I 3t represents the current of the third phase at time t; f t represents the frequency of the power grid at time t,

[0025] For each device in the power grid, collect the following parameters: S 1at , S 1bt , S 1ct , S 2at , S 2bt , S 2ct , S 3at , S 3bt , S 3ct ,…S iat , S ibt , S ict , where i represents the device serial number. If there are n electrical devices in the power grid, then the value range of i is [1, n]. iat , S ibt , S ict They represent the main performance indicators of the i-th device at time t, where S iat The value of S must be meaningful. ibt and S ict The value of is not necessarily meaningful. When it is meaningless, it is set to -1.

[0026] The main parameters of the equipment are the main parameters when the equipment is operating normally;

[0027] In this step, t is the time, and the interval between t and t+1 is 1 second. When the value of t is less than or equal to 3000, sampling is performed continuously. When the value of t is greater than 3000 and all devices in the power grid continue to work normally, in addition to sampling, subsequent steps are performed in each cycle.

[0028] 3. A method for evaluating the reliability of medium voltage power supply according to claim 2, characterized in that: said step 3 comprises the following contents:

[0029] The composite voltage u is calculated using the following formula t

[0030]

[0031] The synthetic current I is calculated using the following formula t

[0032]

[0033] 4. A method for evaluating the reliability of medium voltage power supply according to claim 3, characterized in that: said step 5 comprises the following contents:

[0034] Step 5 is performed on different performance parameters separately.

[0035] When the same values ​​appear, the main curve is selected according to the priority of voltage>current>frequency.

[0036] 5. A method for evaluating the reliability of medium voltage power supply according to claim 4, characterized in that: said step 6 comprises the following contents:

[0037] Assume g iat (x) is the main parameter S of the device with serial number i iat The main curve of , where x is one of the voltage u, current I, and frequency f. The specific determination method is based on the calculation in step 5.

[0038] Then the normalized curve f iat The calculation method of (x) is as follows:

[0039]

[0040] where min{g iat (x)} is g iat The minimum value of (x), max{g iat (x)} is g iat The maximum value of (x),

[0041] Similarly, the main parameters S of the device with serial number i can be obtained ibt The normalized curve f ibt (x),

[0042] Similarly, normalized curves of all main parameters of all devices can be obtained.

[0043] 6. A method for evaluating the reliability of medium voltage power supply according to claim 5, characterized in that: said step 7 comprises the following contents:

[0044] Use the following formula to calculate the main parameter normalization curve merging function f of device i it (x)

[0045] f it (x) = P1f iat (x)+P2f ibt (x)+P3f ict (x)

[0046] where f iat (x), f ibt (x) and f ict (x) is the normalized curve of the main parameters of device i calculated in step 6. Of course, if device i does not have the corresponding main parameters, the corresponding items are directly omitted.

[0047] P1, P2 and P3 are coefficients, and these three values ​​can be determined by any of the following methods:

[0048] Method 1: External setting

[0049] If the staff can determine the weight relationship of several main parameters, the weight ratio of the three parameters can be directly given externally.

[0050] If the staff cannot determine the weight relationship of several parameters, they can directly use fixed values. The typical value given in this application is P1: P2: P3 = 5: 3: 2

[0051] Method 2: Determine the weight ratio based on the values ​​of several main parameters

[0052] When |max{(f iat (x))″}|+|max{(f ibt (x))″}|+|max{(f ict When (x))″}|≠0, P1, P2 and P3 are calculated using the following formula

[0053]

[0054]

[0055]

[0056] Where (f iat (x))″ represents the function f iat (x) find the second-order derivative, max{(f iat (x))″} means (f iat (x))″, |max{(f iat (x))″}|means max{(f iat (x))″}, similarly (f ibt (x))″ represents the function f ibt (x) find the second-order derivative, max{(f ibt (x))″} means (f ibt (x))″, |max{(f ibt (x))″}|means max{(f ibt (x))″} absolute value; (f ict (x))″ represents the function f ict (x) find the second-order derivative, max{(f ict (x))″} means (f ict (x))″, |max{(f ict(x))″}|means max{(f ict The absolute value of (x))″},

[0057] If |max{(f iat (x))″}|+|max{(f ibt (x))″}|+|max{(f ict (x))″}|=0, then P1=P2=P3=1 / 3,

[0058] Of course, if there are only two main parameters, then P1 = P2 = 1 / 2,

[0059] If there is only one main parameter, then there is no need to merge the main parameter normalization curves in this step.

[0060] 7. A method for evaluating the reliability of medium voltage power supply according to claim 6, characterized in that: said step eight comprises the following contents:

[0061] Normalized curve merging function f for the main parameters of device i it (x) Find the upper and lower limits, which are the boundary thresholds of the piezoelectric quality of device i.

[0062] In the above-mentioned medium voltage power supply reliability effect evaluation method, the step nine includes the following contents:

[0063] When a new voltage, current and frequency appear in the power grid at the next moment, the above fitting formula is used to calculate the main parameter normalization curve merging function f of each device. it (x), if the calculation results are all within the threshold range of step eight, the medium voltage electricity quality is determined to be qualified, otherwise the medium voltage electricity quality is determined to be unqualified.

[0064] The significant effect of the present invention is that the effect evaluation method of the present application evaluates the power consumption effect of each device in the power grid, and then evaluates the piezoelectric reliability from the power consumption effect. During the execution of the method, the main performance parameters of the power-consuming equipment are selected, so that the evaluation results are directly linked to the effects of the power-consuming equipment, and the evaluation results are reliable and accurate. By selecting the main curves, normalizing and merging the curves, the multi-dimensional evaluation becomes a simple numerical comparison, ensuring that the evaluation results are simple and easy to understand. DETAILED DESCRIPTION

[0065] A method for evaluating the reliability of medium voltage power supply comprises the following steps:

[0066] Step 1: Data Collection

[0067] For the power grid, collect the following parameters: grid voltage u 1t 、u 2t 、u3t , grid current I 1t ,I 2t ,I 3t , grid frequency f t , where u 1t 、u 2t 、u 3t Represents the three voltages of three-phase alternating current, I 1t ,I 2t ,I 3t represents the three currents of three-phase alternating current, t is the time, that is, u 1t represents the voltage of the first phase at time t, u 2t represents the voltage of the second phase at time t, u 3t Represents the voltage of the third phase at time t; I 1t represents the current of the first phase at time t, I 2t represents the current of the second phase at time t, I 3t represents the current of the third phase at time t; f t Represents the frequency of the power grid at time t.

[0068] For each device in the power grid, collect the following parameters: S 1at , S 1bt , S 1ct , S 2at , S 2bt , S 2ct , S 3at , S 3bt , S 3ct ,…S iat , S ibt , S ict Where i represents the device serial number. If there are n electrical devices in the power grid, the value range of i is [1, n]. iat , S ibt , S ict They represent the main performance indicators of the i-th device at time t, where S iat The value of S must be meaningful. ibt and S ict The value of is not necessarily meaningful. When it is meaningless, it is -1.

[0069] The main performance indicators are not necessarily the main parameters of the device. The main parameters of the device are the main parameters of the device during normal operation, generally including current, voltage, power, etc.; the main performance indicators refer to the evaluation dimensions of the effect of the device as a whole providing a certain function to the outside world. In different occasions and application scenarios, the main performance indicators of the device may be different. As long as the main performance indicators of the device are determined before carrying out this method, it is sufficient.

[0070] In order to unify calculations and improve efficiency, this application provides each device with a maximum of three main performance indicators. Of course, technicians in this field can set more performance indicator dimensions as needed.

[0071] For example, the main function of an electric fan is to blow air, so its main performance indicator is the speed. 7at It indicates the speed of the electric fan (assuming the device number of the electric fan is 7). Generally speaking, electric fans do not have other main performance indicators, so S 7bt and S 7ct The value of is -1; the main parameters of electric fans may include rated voltage, rated current, rated power and other parameters. These parameters do not affect the most important functions of electric fans, so they are not the main performance indicators.

[0072] For example, the main function of a light is lighting, so its main performance indicator is brightness. 8at It represents the brightness of the lamp (assuming that the equipment number of the lamp is 8), and the rated voltage, rated current, rated power and other parameters of the lamp are not the main performance indicators of the lamp.

[0073] Another example is still the electric light. In some special cases, in addition to the lighting needs, there are also restrictions on the heat release of the electric light. Then the S 9at It represents the brightness of the light, S 9bt represents the heat release of the lamp (assuming the device number of this lamp is 9), S 9ct The value of is -1.

[0074] In this step, t is the time, which can be set arbitrarily according to the accuracy of the equipment in the power grid. Generally, the interval between t and t+1 is 1 second. When the value of t is less than or equal to 3000, sampling is continued; when the value of t is greater than 3000 and all equipment in the power grid continues to work normally, in addition to sampling, each cycle also executes the subsequent steps.

[0075] Step 2: Adjust grid parameters

[0076] Adjust the grid voltage u of the grid respectively 1t 、u 2t 、u 3t , grid current I 1t ,I 2t ,I 3t , grid frequency f t , causing all electrical equipment in the power grid to stop working.

[0077] The purpose of this step is to check the main performance parameters of the electrical equipment under extreme conditions and the corresponding power grid parameters.

[0078] Since most electrical equipment cannot withstand electrical energy exceeding the rated voltage and rated current, when the voltage or current reaches the rated voltage limit or the rated current limit (upper and lower limits respectively), the electrical equipment is actively disconnected through the switch to protect the safety of the electrical equipment. The main performance parameters of the equipment recorded at this time are used as valid data for subsequent calculations.

[0079] Step 3: Voltage and Current Synthesis

[0080] The composite voltage u is calculated using the following formula t

[0081]

[0082] The synthetic current I is calculated using the following formula t

[0083]

[0084] Step 4: Curve fitting

[0085] The resultant voltage u t , synthetic current I t , grid frequency f t , the main performance parameters S of the i-th device iat , S ibt , S ict The main performance parameter curves of the i-th device are fitted respectively.

[0086] The fitting in this step is performed on data whose main performance parameters are not -1, that is, the main performance parameters of the i-th device should be meaningful.

[0087] For example, for the i-th device, the device has two meaningful main performance parameters S iat and S ibt According to the above steps, the S ict The value is always -1, so subsequent calculations are only for S iat and S ibt For the first performance parameter S iat , respectively, and the composite voltage u t , synthetic current I t , grid frequency f t By fitting, three curves can be obtained: voltage performance curve K iau (t), current performance curve K iaI (t), frequency performance curve K iaf (t). Similarly, the second performance parameter S can be obtained ibt Three curves: voltage performance curve K ibu (t), current performance curve KibI (t), frequency performance curve K ibf (t).

[0088] Step 5: Select the main curves

[0089] Derivatives of the three curves of the same performance parameter of the same device are calculated, and the absolute values ​​of the extreme values ​​are taken. The curve corresponding to the maximum absolute value of the extreme value is selected as the main curve.

[0090] The above process is performed for different performance parameters respectively.

[0091] When the same values ​​appear, the main curve is selected according to the priority of voltage>current>frequency.

[0092] For example, for the i-th device, the device has two meaningful main performance parameters S iat and S ibt , according to step 4, we get the corresponding parameter S iat Three curves of voltage performance curve K iau (t), current performance curve K iaI (t), frequency performance curve K iaf (t), and the corresponding parameter S ibt Three curves of voltage performance curve K ibu (t), current performance curve K ibI (t), frequency performance curve K ibf (t). At this time, the voltage performance curve K iau (t), current performance curve K iaI (t), frequency performance curve K iaf (t) Take the derivatives respectively, take the absolute values ​​of the extreme values ​​respectively, and compare the three values. The curve corresponding to the maximum value is taken as the parameter S iat The main curve of K. ibu (t), current performance curve K ibI (t) Take the derivatives respectively, take the absolute values ​​of the extreme values ​​respectively, and compare the three values. The curve corresponding to the maximum value is taken as the parameter S ibt The main curve of .

[0093] Step 6: Curve Normalization

[0094] Normalize the main curves corresponding to each performance parameter of each device. The specific steps are as follows:

[0095] Assume g iat (x) is the main parameter S of the device with serial number i iat The main curve of , where x is one of the voltage u, current I, and frequency f, and the specific determination method is obtained according to the calculation in step five.

[0096] Then the normalized curve f iat The calculation method of (x) is as follows:

[0097]

[0098] where min{g iat (x)} is g iat The minimum value of (x), max{g iat (x)} is g iat The maximum value of (x).

[0099] Similarly, the main parameters S of the device with serial number i can be obtained ibt The normalized curve f ibt (x).

[0100] Similarly, normalized curves of all main parameters of all devices can be obtained.

[0101] Step 7: Merge the normalized curves of the main parameters

[0102] Use the following formula to calculate the main parameter normalization curve merging function f of device i it (x)

[0103] f it (x) = P1f iat (x)+P2f ibt (x)+P3f ict (x)

[0104] where f iat (x), f ibt (x) and f ict (x) is the normalized curve of the main parameters of device i calculated in step 6. Of course, if device i does not have corresponding main parameters, the corresponding items are directly omitted.

[0105] P1, P2 and P3 are coefficients, and these three values ​​can be determined by any of the following methods:

[0106] Method 1: External setting

[0107] If the staff can determine the weight relationship of several main parameters, the weight ratio of the three parameters can be directly given externally.

[0108] If the staff cannot determine the weight relationship of several parameters, they can directly use fixed values. The typical value given in this application is P1: P2: P3 = 5: 3: 2

[0109] Method 2: Determine the weight ratio based on the values ​​of several main parameters

[0110] When |max{(fiat (x))″}|+|max{(f ibt (x))″}|+|max{(f ict When (x))″}|≠0, P1, P2 and P3 are calculated using the following formula

[0111]

[0112]

[0113]

[0114] Where (f iat (x))″ represents the function f iat (x) find the second-order derivative, max{(f iat (x))″} means (f iat (x))″, |max{(f iat (x))″}|means max{(f iat (x))″}, similarly (f ibt (x))″ represents the function f ibt (x) find the second-order derivative, max{(f ibt (x))″} means (f ibt (x))″, |max{(f ibt (x))″}|means max{(f ibt (x))″} absolute value; (f ict (x))″ represents the function f ict (x) find the second-order derivative, max{(f ict (x))″} means (f ict (x))″, |max{(f ict (x))″}|means max{(f ict The absolute value of (x))″}.

[0115] If |max{(f iat (x))″}|+|max{(f ibt (x))″}|+|max{(f ict (x))″}|=0, then P1=P2=P3=1 / 3,

[0116] Of course, if there are only two main parameters, then P1 = P2 = 1 / 2,

[0117] If there is only one main parameter, then there is no need to merge the main parameter normalization curves in this step.

[0118] Step 8: Find the threshold range

[0119] Normalized curve merging function f for the main parameters of device i it (x) Find the upper and lower limits, which are the boundary thresholds of the piezoelectric quality of device i.

[0120] Step 9: Evaluation

[0121] When a new voltage, current and frequency appear in the power grid at the next moment, the above fitting formula is used to calculate the main parameter normalization curve merging function f of each device. it (x), if the calculation results are all within the threshold range of step eight, the medium voltage electricity quality is determined to be qualified, otherwise the medium voltage electricity quality is determined to be unqualified.

Claims

1. A method for evaluating the reliability of medium voltage power supply, characterized in that: The steps include: Step 1: Data Collection Collect data for the power grid and each device in the grid; Step 2: Adjust grid parameters Adjust the grid voltage, grid current and grid frequency of the grid respectively, so that all electrical equipment in the grid stops working, and record all relevant parameters at the same time; Step 3: Voltage and Current Synthesis Synthesize the recorded voltage and current; Step 4: Curve fitting The resultant voltage u t , synthetic current I t , grid frequency f t The main performance parameters S of the i-th device iat , S ibt , S ict Fit them respectively to obtain the main performance parameter curves of the i-th device; Step 5: Select the main curves Calculate the derivative of three main performance parameter curves of the same main performance parameter of the same equipment, take the absolute value of the extreme value, and select the curve corresponding to the maximum absolute value of the extreme value as the main curve; Step 6: Curve Normalization Normalizing the main performance parameter curve corresponding to each main performance parameter of each device to obtain a normalized curve; Step 7: Merge the normalized curves of the main performance parameters; Step 8: Find the threshold range; Step 9: Evaluation; The step 1 includes the following contents: For the power grid, collect the following parameters: grid voltage u 1t 、u 2t 、u 3t , grid current I 1t ,I 2t ,I 3t , grid frequency f t , where u 1t 、u 2t 、u 3t Represents the three voltages of three-phase alternating current, I 1t ,I 2t ,I 3t represents the three currents of three-phase alternating current, t is the time, that is, u 1t represents the voltage of the first phase at time t, u 2t represents the voltage of the second phase at time t, u 3t Represents the voltage of the third phase at time t; I 1t represents the current of the first phase at time t, I 2t represents the current of the second phase at time t, I 3t represents the current of the third phase at time t; f t represents the frequency of the power grid at time t, For each device in the power grid, collect the following parameters: S 1at , S 1bt , S 1ct , S 2at , S 2bt , S 2ct , S 3at , S 3bt , S 3ct ,…S iat , S ibt , S ict , where i represents the device serial number. If there are n electrical devices in the power grid, then the value range of i is [1, n]. iat , S ibt , S ict They represent the main performance indicators of the i-th device at time t, where S iat The value of S must be meaningful. ibt and S ict The value of is not necessarily meaningful. When it is meaningless, it is set to -1. The main performance parameters of the equipment are the main performance parameters of the equipment when it is operating normally; In this step, t is the time, and the interval between t and t+1 is 1 second. When the value of t is less than or equal to 3000, sampling is continued; when the value of t is greater than 3000 and all devices in the power grid continue to work normally, in addition to sampling, subsequent steps are executed in each cycle; The step three includes the following contents: The composite voltage u is calculated using the following formula t The synthetic current I is calculated using the following formula t The step five includes the following contents: Step 5: Calculate the absolute value of the extreme value for different main performance parameters. When the same values ​​appear, the main curve is selected according to the priority of voltage>current>frequency; The step six includes the following contents: Assume g iat (x) is the main performance parameter S of the device with serial number i iat The main curve of , where x is one of the voltage u, current I, and frequency f. The specific determination method is based on the calculation in step 5. Then the normalized curve f iat The calculation method of (x) is as follows: where min{g iat (x)} is g iat The minimum value of (x), max{g iat (x)} is g iat The maximum value of (x), Similarly, the main performance parameters S of the device with serial number i can be obtained ibt The normalized curve f ibt (x), Similarly, normalized curves of all main performance parameters of all devices can be obtained; The step seven includes the following contents: Use the following formula to calculate the normalized curve merging function f of the main performance parameters of equipment i it (x) f it (x)=P1f iat (x)+P2f ibt (x)+P3f ict (x) where f iat (x), f ibt (x) and f ict (x) is the normalized curve of the main performance parameters of device i calculated in step 6. Of course, if device i does not have the corresponding main performance parameters, the corresponding item number is directly omitted. P1, P2 and P3 are coefficients, and these three values ​​can be determined by any of the following methods: Method 1: External setting If the staff can determine the weight relationship of several main performance parameters, the weight ratio of the three parameters can be directly given externally. If the staff cannot determine the weight relationship of several parameters, they can directly use fixed values, which are P1:P2:P3=5:3:

2. Method 2: Determine the weight ratio based on the values ​​of several key performance parameters When |max{(f iat (x))″}|+|max{(f ibt (x))″}|+|max{(f ict When (x))″}|≠0, P1, P2 and P3 are calculated using the following formula Where (f iat (x))″ represents the function f iat (x) find the second-order derivative, max{(f iat (x))″} means (f iat (x))″, |max{(f iat (x))″}|means max{(f iat (x))″}, similarly (f ibt (x))″ represents the function f ibt (x) find the second-order derivative, max{(f ibt (x))″} means (f ibt (x))″, |max{(f ibt (x))″}|means max{(f ibt (x))″} absolute value; (f ict (x))″ represents the function f ict (x) find the second-order derivative, max{(f ict (x))″} means (f ict (x))″, |max{(f ict (x))″}|means max{(f ict The absolute value of (x))″}, If |max{(f iat (x))″}|+|max{(f ibt (x))″}|+|max{(f ict (x))″)|=0, then P1=P2=P3=1 / 3, Of course, if there are only two main performance parameters, then P1 = P2 = 1 / 2, If there is only one main performance parameter, then there is no need to merge the main performance parameter normalization curves in this step; The step eight includes the following contents: The normalized curve merging function f of the main performance parameters of device i it (x) Find the upper and lower limits, which are the boundary thresholds of the piezoelectric quality of device i; The step nine includes the following contents: When a new voltage, current and frequency appear in the power grid at the next moment, the normalized curve merging function f of the main performance parameters of each device is calculated. it (x), if the calculation results are all within the threshold range of step eight, the medium voltage electricity quality is determined to be qualified, otherwise the medium voltage electricity quality is determined to be unqualified.

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