Metering system and method for influence of three-phase load imbalance on loss of low-voltage distribution network

Abstract

The invention discloses a metering system and method for influence of three-phase load imbalance on low-voltage distribution network loss, relates to the technical field of low-voltage distribution network loss metering, and detects three-phase current, neutral current and three-phase phase of a distribution network in real time; judging a three-phase imbalance condition according to the detected three-phase current and three-phase phase, wherein the three-phase imbalance condition specifically comprises three-phase amplitude imbalance, three-phase phase imbalance and three-phase amplitude and three-phase phase imbalance; and selecting a corresponding algorithm according to the judged three-phase imbalance condition to calculate the distribution transformer loss and the line loss of the distribution transformer under the corresponding condition. According to the invention, an algorithm relation of the influence of three-phase load imbalance on the loss of the low-voltage power distribution network is established for three kinds of three-phase imbalance conditions. According to the method, the influence of the three-phase load imbalance on the loss of the low-voltage distribution network is promoted from comprehensive evaluation to accurate calculation. The system has the advantages of being simple in structure, convenient to operate and high in practicability.

Description

Technical Field

[0001] The present invention relates to the technical field of low-voltage distribution network loss measurement, and in particular to a measurement system and method for measuring the influence of three-phase load imbalance on low-voltage distribution network loss. Background Art

[0002] Low-voltage distribution network systems mostly use Dyn11 and Yyn0 wiring operation modes, and use distribution transformers to reduce the 10kV voltage to 380V to supply power to the user end. The three-phase power supply is balanced during power distribution, but the uncertainty and randomness of the load access on the user side leads to an imbalance in the three-phase load of the distribution network. The imbalance of the three-phase load will inevitably lead to an increase in the loss of the distribution network, but at present, the increase in the loss of the distribution network caused by the imbalance of the three-phase load is mostly evaluated and calculated by empirical methods, which cannot be accurately and effectively calculated. In addition, the current relationship between the impact of three-phase imbalance on the loss of the low-voltage distribution network is not clear. The main manifestations are that first, there is no specific discussion on the three-phase imbalance, and secondly, the definition of the imbalance degree of the three-phase load is not unified, so there is no fixed device or method to effectively measure the impact of the three-phase load imbalance on the loss of the low-voltage distribution network.

[0003] Three-phase imbalance can be divided into three situations: three-phase amplitude imbalance, three-phase phase imbalance, and three-phase amplitude and three-phase phase imbalance. Therefore, it is necessary to establish algorithms for the impact of three-phase load imbalance on low-voltage distribution network losses based on the above three three-phase imbalance situations. If the three-phase load of the distribution network is unbalanced, current will flow through the neutral line, resulting in a significant increase in neutral line loss, and will also lead to increased losses in distribution transformers and increased losses in the three-phase lines of the distribution network, resulting in a waste of power quality, causing greater economic losses to power companies, and an increase in CO2 emissions.

[0004] Therefore, a system and method for measuring the impact of three-phase load imbalance on low-voltage distribution network losses has become an issue that needs to be urgently addressed. Summary of the invention

[0005] The technical problem to be solved by the present invention is to provide a metering system and method for the influence of three-phase load imbalance on the loss of low-voltage distribution network, solve the influence of three-phase load imbalance on the loss of distribution transformer and distribution network line, and establish corresponding algorithms for the influence of three-phase load imbalance on the loss of low-voltage distribution network and corresponding operating systems for three three-phase imbalance situations.

[0006] In order to solve the above technical problems, the present invention provides a technical solution: a method for measuring the influence of three-phase load imbalance on low-voltage distribution network loss, comprising the following steps:

[0007] S1, real-time detection of the three-phase current, neutral line current and three-phase phase of the distribution network;

[0008] S2. Determine the three-phase imbalance according to the detected three-phase current and three-phase phase, including three-phase amplitude imbalance, three-phase phase imbalance, and three-phase amplitude and three-phase phase imbalance;

[0009] S3. According to the determined three-phase imbalance situation, a corresponding algorithm is selected to calculate the distribution transformer loss and the line loss of the distribution transformer under the corresponding situation.

[0010] The present invention also provides a metering system for the influence of three-phase load imbalance on the loss of low-voltage distribution network, including a three-phase current detection module, a neutral current detection module, a three-phase phase detection module, an intelligent control module, a liquid crystal display module, a wireless communication module, an alarm indication module, and a local power supply module;

[0011] The three-phase current detection module detects the three-phase current of the distribution network in real time, the neutral line current detection module detects the neutral line current of the distribution network in real time, and the three-phase phase detection module detects the three-phase phase of the distribution network in real time;

[0012] The intelligent control module determines whether the three-phase imbalance situation belongs to the three-phase amplitude imbalance, the three-phase phase imbalance, or the three-phase amplitude and three-phase phase imbalance according to the three-phase current detected by the three-phase current detection module and the three-phase phase detected by the three-phase phase detection module, and determines the algorithm for the influence of the three-phase load imbalance on the loss of the low-voltage distribution network according to the three-phase imbalance of the distribution network, and then calculates the distribution transformer loss and the line loss of the distribution network under the corresponding situation, and displays them through the liquid crystal display module;

[0013] When any of the above functional modules fails, the alarm indication module provides a corresponding alarm indication.

[0014] Furthermore, it also includes a phase loss detection module, a reverse sequence detection module, a three-phase voltage detection module, and a leakage detection module;

[0015] The phase loss detection module detects the phase signal of the distribution network in real time, the reverse sequence detection module detects the phase sequence signal of the distribution network in real time, the three-phase voltage detection module detects the voltage signal of the distribution network in real time, and the leakage detection module detects the leakage signal of the distribution network in real time.

[0016] The advantages of the present invention compared with the prior art are:

[0017] 1. The present invention enumerates the situations of three-phase load imbalance and establishes corresponding three-phase load imbalance and three-phase load balance algorithms.

[0018] 2. The present invention establishes an algorithmic relationship for the impact of three-phase load imbalance on low-voltage distribution network losses for three three-phase imbalance situations.

[0019] 3. The present invention advances the impact of three-phase load imbalance on low-voltage distribution network losses from comprehensive evaluation to precise calculation and visualization.

[0020] 4. The present invention discusses three-phase imbalance for the first time and systematically demonstrates the impact of three-phase imbalance on the copper loss of distribution transformers and the line loss of distribution networks.

[0021] 5. The system of the present invention has the characteristics of simple structure, easy operation and strong practicality. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is a flow chart of a method for measuring the effect of three-phase load imbalance on the losses of low-voltage distribution network.

[0023] Figure 2 It is a system block diagram of a metering system for the effect of three-phase load imbalance on low-voltage distribution network losses.

[0024] Figure 3 This is a schematic diagram of the connection between the metering system used to measure the impact of three-phase load imbalance on low-voltage distribution network losses and the low-voltage distribution network. Figure 1 .

[0025] Figure 4 This is a schematic diagram of the connection between the metering system used to measure the impact of three-phase load imbalance on low-voltage distribution network losses and the low-voltage distribution network. Figure 2 . DETAILED DESCRIPTION

[0026] In order to make the purpose, technical solutions and advantages of this application clearer, the technical solutions in the embodiments of this application will be clearly and completely described below in conjunction with the drawings in the embodiments of this application. Obviously, the described embodiments are only part of the embodiments of this application, not all of them. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this application.

[0027] It should be understood that in the description of the present application, unless the context clearly requires otherwise, words such as "include", "comprises", and the like in the entire specification and claims should be interpreted as inclusive rather than exclusive or exhaustive; that is, the meaning of "including but not limited to".

[0028] It should be noted that the terms "S1", "S2", etc. are only used for the purpose of describing the steps, and do not specifically refer to the order or sequence, nor are they used to limit the present application. They are only for the convenience of describing the method of the present application, and cannot be understood as indicating the order of the steps. In addition, the technical solutions between the various embodiments can be combined with each other, but they must be based on the ability of ordinary technicians in this field to implement them. When the combination of technical solutions is contradictory or cannot be implemented, it should be considered that such a combination of technical solutions does not exist and is not within the scope of protection required by the present application.

[0029] The following is a further detailed description of a system and method for measuring the impact of three-phase load imbalance on low-voltage distribution network losses according to the present invention in conjunction with the accompanying drawings.

[0030] Combined with Figure 1-4 The specific implementation process of the system and method for measuring the impact of three-phase load imbalance on low-voltage distribution network loss of the present invention is as follows:

[0031] A method for measuring the impact of three-phase load imbalance on low-voltage distribution network losses, comprising the following steps:

[0032] S1, real-time detection of the three-phase current, neutral line current and three-phase phase of the distribution network;

[0033] S2. Determine the three-phase imbalance according to the detected three-phase current and three-phase phase, including three-phase amplitude imbalance, three-phase phase imbalance, and three-phase amplitude and three-phase phase imbalance;

[0034] S3. According to the determined three-phase imbalance situation, a corresponding algorithm is selected to calculate the distribution transformer loss and the line loss of the distribution network under the corresponding situation.

[0035] Under three three-phase unbalanced conditions: The calculation method for the impact of three-phase load imbalance on low-voltage distribution network losses is as follows:

[0036] 1.1 Impact of three-phase imbalance on iron loss of distribution transformer

[0037] Since the transformer iron loss is determined by the factory transformer itself, in normal operation, the iron loss can be regarded as an inherent loss, which is approximately equal to the no-load loss, that is, a constant. However, when the three-phase load is unbalanced, additional DC resistance loss and additional eddy current loss caused by increased leakage magnetic field will be generated. According to the definition of iron loss, it can be known that:

[0038] P Fe =P h +P e +P se (1)

[0039] In formula (1): P h , Pe , ΔP se They are hysteresis loss, eddy current loss and additional loss respectively;

[0040] When the three-phase load is unbalanced, there will be a three-phase voltage imbalance at the transformer outlet. The ratio of the actual operating voltage of the transformer to the nominal voltage is defined as the voltage deviation coefficient, and the value is α. At this time, the transformer iron loss is:

[0041] P Fe =(1+α) 2 P N0 (2)

[0042] In formula (2): P Fe is the transformer operating iron loss, P N0 is the rated no-load loss of the transformer.

[0043] 1.2 Impact of three-phase imbalance on copper loss of distribution transformer

[0044] Since the three windings of the three-phase distribution transformer are approximately the same when they leave the factory, the impedances of the three-phase windings are equal. ACU , Z BCU , Z CCU is the impedance of the three-phase winding on the primary side of the distribution transformer, Z acu , Z bcu , Z ccu is the impedance of the three-phase winding on the secondary side of the distribution transformer, then the copper loss of the distribution transformer can be expressed as the sum of the copper loss of the primary winding and the copper loss of the secondary winding. That is:

[0045] P CU =I A 2 Z ACU +I B 2 Z BCU +I C 2 Z CCU +I a 2 Z acu +I b 2 Z bcu +I c 2 Z ccu (3)

[0046] In formula (3): P CU is the copper loss of the distribution transformer, I A ,I B ,I C is the primary current of the distribution transformer, I a ,I b ,Ic is the secondary current of the distribution transformer;

[0047] Assuming the transformation ratio of the distribution transformer is K, we can know:

[0048]

[0049] Substituting formula (4) into formula (3), we can get:

[0050]

[0051] In formula (5): Z CU It is the copper reactance of the single-phase winding of the distribution transformer;

[0052] When the three-phase load is unbalanced, the distribution transformer will generate additional copper loss, namely:

[0053]

[0054] In formula (6): ΔP FCU is the additional copper loss of the distribution transformer, I av is the average three-phase current on the secondary side of the distribution transformer, β a , β b , β c is the current unbalance coefficient of the secondary side of the distribution transformer, and:

[0055]

[0056] By summing up formula (7), we can get:

[0057]

[0058] Since the three-phase load imbalance can be divided into three cases, the additional copper loss of the distribution transformer is mathematically modeled and analyzed according to the three three-phase load imbalance cases:

[0059] 1.2.1 Case 1: Additional copper loss of distribution transformer when three-phase phase is balanced and amplitude is unbalanced

[0060] According to the deduction, we can get:

[0061]

[0062] Substituting formula (9) into formula (6), we can get:

[0063]

[0064] In formula (10): ε i is the three-phase load balance;

[0065] Defining ε ui is the three-phase load imbalance, and the three-phase load imbalance is:

[0066]

[0067] Comparing formula (9) and formula (A1), we can get:

[0068] 1.2.2 Case 2: When the three-phase phase is unbalanced and the amplitude is balanced, the additional copper loss of the distribution transformer is β a =β b =β c =β, substituting into formula (6) we can get:

[0069]

[0070] and:

[0071]

[0072] Substituting formula (12) into formula (11), we can get:

[0073]

[0074] In formula (13):

[0075] is the three-phase load power factor angle of the distribution transformer;

[0076] In this case, the three-phase load imbalance is:

[0077]

[0078] 1.2.3 Case 3: Additional copper loss of distribution transformer when the three-phase phase is unbalanced and the amplitude is also unbalanced

[0079]

[0080] Substituting formula (14) into formula (6) we can get:

[0081]

[0082] In formula (15):

[0083]

[0084]

[0085] At this time, the three-phase load imbalance is:

[0086]

[0087] Combining the analysis of formula (10), formula (13) and formula (15), it can be seen that under condition 1, the additional copper loss of the distribution transformer changes inversely with the three-phase load balance. Under conditions 2 and 3, the additional copper loss of the distribution transformer not only changes inversely with the three-phase load balance, but is also affected by the three-phase load power factor, and the influence relationship can be accurately calculated.

[0088] 2. Impact of three-phase imbalance on distribution network line loss

[0089] Since three-phase load imbalance can be divided into three situations, the impact of three-phase imbalance on distribution network line loss can also be mathematically modeled and analyzed according to the three three-phase load imbalance situations:

[0090] 2.1 Case 1: Impact on distribution network line loss when three-phase phase is balanced and amplitude is unbalanced

[0091] According to the three-phase phase balance of the distribution network, it can be known that the power factor angles of the three-phase loads are equal. Since the three-phase amplitude of the distribution network is unbalanced, we know that β a , β b , β c will not be constant; Assume that the amplitudes of the three-phase currents on the secondary side of the distribution transformer are I a ,I b ,I c The three-phase current vectors are i a 、i b 、i c , then it is expressed as:

[0092]

[0093] Assuming the neutral current amplitude on the secondary side of the distribution transformer is I0, and the neutral current vector is i0, the neutral current at this time can be expressed as:

[0094] i0=i a +i b +i c (17)

[0095] Substitute equation (16) into equation (17) and rotate the three-phase current vector symmetrically Degrees, modulo simplification:

[0096]

[0097] Assume that the impedance per unit length of the secondary phase line of the distribution transformer is Z L The impedance per unit length of the neutral line is Z0, which is usually taken as Z in a three-phase four-wire low-voltage distribution network. L =Z0, assuming that the three-phase line loss per unit length on the secondary side of the distribution transformer is ΔP L , then we can get:

[0098] ΔP L =(I a 2 +I b 2 +I c 2 )Z L (19)

[0099] Substituting formula (7) into formula (19), we get:

[0100]

[0101] Assuming the line loss per unit length of the neutral line on the secondary side of the distribution transformer is ΔP0, then:

[0102] ΔP0=I0 2 Z0 (21)

[0103] Substituting equations (7), (8), (9), and (18) into equation (21) and rearranging them, we obtain:

[0104]

[0105] Taking the three-phase amplitude balance as the reference condition, let ΔP BL is the three-phase balanced line loss per unit length on the secondary side of the distribution transformer, then:

[0106]

[0107] Let ΔP FAL is the additional line loss per unit length on the secondary side of the three-phase amplitude unbalanced distribution transformer, then:

[0108] ΔP FAL =ΔP L -ΔP BL (twenty four)

[0109] Substituting equations (9), (20), and (23) into equation (24) and simplifying them, we can obtain:

[0110]

[0111] Let ΔP ZFAL is the total additional line loss per unit length of the three-phase amplitude unbalance on the secondary side of the distribution transformer, then:

[0112]

[0113] Let ΔP ZAL is the three-phase amplitude unbalanced bus loss per unit length on the secondary side of the distribution transformer, then

[0114]

[0115] From the analysis of formula (22), formula (25) and formula (26), it can be known that under the condition of case 1, the unit length line loss of the neutral line on the secondary side of the distribution transformer, the unit length additional line loss of the three-phase line and the total unit length additional line loss of the three-phase line are inversely proportional to the balance of the three-phase load, and vice versa, they are directly proportional to the imbalance of the three-phase load.

[0116] 2.2 Case 2: Impact on distribution network line loss when the three-phase phase is unbalanced and the amplitude is balanced

[0117] In this case, It will not be constant. At this time, the three-phase current vector of the low-voltage distribution network can be expressed as:

[0118]

[0119] Substituting equation (7) into equation (28) and expanding it according to trigonometric functions, and then substituting it into equation (17) and simplifying it modulo, we can obtain:

[0120]

[0121] Substituting equation (29) into equation (21) and then into equation (12) yields:

[0122]

[0123] Since the three-phase amplitude is balanced, let β a =β b =β c =β, substituting formula (12) into formula (20) and rearranging it, we can get:

[0124]

[0125] Let ΔP FΦL is the additional line loss per unit length on the secondary side of the three-phase unbalanced distribution transformer, which should be equal to the three-phase line loss per unit length minus the three-phase balanced line loss per unit length. Substituting it into equation (23) and equation (31), we can get:

[0126]

[0127] Let ΔP ZFΦL is the total additional line loss per unit length of the three-phase phase imbalance on the secondary side of the distribution transformer, then:

[0128]

[0129] Let ΔP ZΦL is the unbalanced three-phase bus loss per unit length on the secondary side of the distribution transformer, then:

[0130]

[0131] From the analysis of equations (30)-(34), it can be seen that under condition 2, except for the line loss per unit length of the neutral line on the secondary side of the distribution transformer, which changes inversely with the three-phase load balance, other forms of line losses change nonlinearly with the three-phase load balance.

[0132] 2.3 Case 3: Impact on distribution network line losses when the three-phase phase is unbalanced and the amplitude is also unbalanced

[0133] In this case will not be constant, β a , β b , β c The neutral current is:

[0134]

[0135] Substituting equation (35) into equation (21) and then into equation (14), we can obtain the neutral line loss per unit length as follows:

[0136]

[0137] Substituting equation (14) into equation (20), we can get the three-phase line loss per unit length on the secondary side of the distribution transformer at this time:

[0138]

[0139] Let ΔP FL The additional line loss per unit length on the secondary side of the distribution transformer with unbalanced three-phase phase and amplitude should be equal to the three-phase line loss per unit length minus the three-phase balanced line loss per unit length. Substituting it into formula (37) and formula (23), we can get:

[0140]

[0141] Let ΔP ZFL is the total additional line loss per unit length when the phase and amplitude of the three-phase secondary side of the distribution transformer are unbalanced, then:

[0142]

[0143] Let ΔP ZL The phase and amplitude of the three-phase on the secondary side of the distribution transformer are unbalanced, and the total loss per unit length of the distribution network is:

[0144]

[0145] From the analysis of equations (36)-(40), it can be seen that under condition 3, the neutral line loss per unit length on the secondary side of the distribution transformer, the three-phase line loss per unit length, the total additional line loss per unit length of the three-phase, and the total line loss per unit length of the distribution network all change inversely proportional to the three-phase load balance, and are also affected by the three-phase power factor.

[0146] For 1.2.3 Case 3: The impact of three-phase phase imbalance and amplitude imbalance on the loss of low-voltage distribution network is given as a specific application calculation example as follows:

[0147] The general low-voltage three-phase four-wire power distribution system mostly operates in the state of unbalanced three-phase amplitude and phase. Select a 10kV / 0.4kV distribution transformer, set the impedance of the phase line and neutral line in the distribution system to 0.239+j0.256Ω / km, and measure the three-phase currents on the secondary side of the distribution transformer:

[0148]

[0149] Substituting into formula (7) we can get:

[0150] Since the three-phase power supply is symmetrical and 120 degrees apart, it is assumed that the power factor angles of the three-phase loads are Combining formula (B1), we can get:

[0151]

[0152] Substituting equations (B2) and (B3) into equation (15)

[0153]

[0154] We can get:

[0155] Substituting equation (B1)-(B3) into equation (A3) yields:

[0156]

[0157] Then, the three-phase load balance ε i =1 / ε ui =87.97%.

[0158] Substituting the above calculation results into formula (15), the additional copper loss of the distribution transformer can be obtained as follows:

[0159] ΔP FCU =68.8W

[0160] Similarly, we can calculate:

[0161] Additional line loss per unit length on the secondary side of the distribution transformer when the three-phase phase and amplitude are unbalanced ΔP FL :

[0162]

[0163] The total additional line loss per unit length of the secondary side of the distribution transformer when the three-phase phase and amplitude are unbalanced ΔPZFL :

[0164]

[0165] The phase and amplitude of the three-phase secondary side of the distribution transformer are unbalanced. The bus loss per unit length of the distribution network is ΔP ZL :

[0166]

[0167] According to the above parameters, the numerical quantification of the impact of three-phase imbalance on the copper loss of the distribution transformer and the impact of three-phase imbalance on the line loss of the distribution network can be obtained. According to the example and experimental analysis, the additional copper loss of the distribution transformer is only related to whether the three-phase amplitude is balanced and has nothing to do with whether the three-phase phase is balanced. The three-phase imbalance caused by phase imbalance has a more significant impact on the line loss of the distribution network. The above three three-phase load imbalance experimental conditions have an impact on the line loss of the distribution network, and the impact relationship is case 2> case 1> case 3.

[0168] Based on the same idea as the above-mentioned metering method for the impact of three-phase load imbalance on low-voltage distribution network losses, this embodiment also provides a metering system for the impact of three-phase load imbalance on low-voltage distribution network losses, which can be used to execute the above-mentioned metering method for the impact of three-phase load imbalance on low-voltage distribution network losses.

[0169] A metering system for the influence of three-phase load imbalance on the loss of low-voltage distribution network, including a three-phase current detection module, a neutral current detection module, a three-phase phase detection module, an intelligent control module, a liquid crystal display module, a wireless communication module, an alarm indication module, a local power supply module, a phase loss detection module, a reverse sequence detection module, a three-phase voltage detection module, and a leakage detection module;

[0170] The three-phase current detection module detects the three-phase current of the distribution network in real time, the neutral line current detection module detects the neutral line current of the distribution network in real time, and the three-phase phase detection module detects the three-phase phase of the distribution network in real time; the phase loss detection module detects the phase signal of the distribution network in real time, the reverse sequence detection module detects the phase sequence signal of the distribution network in real time, the three-phase voltage detection module detects the voltage signal of the distribution network in real time, and the leakage detection module detects the leakage signal of the distribution network in real time.

[0171] The intelligent control module determines whether the three-phase imbalance belongs to the three-phase amplitude imbalance, the three-phase phase imbalance, or the three-phase amplitude and three-phase phase imbalance according to the three-phase current detected by the three-phase current detection module and the three-phase phase detected by the three-phase phase detection module. The intelligent control module determines the algorithm for the influence of the three-phase load imbalance on the loss of the low-voltage distribution network according to the three-phase imbalance of the distribution network, and then calculates the loss of the distribution transformer and the line loss of the distribution transformer in the corresponding situation, and displays the data through the liquid crystal display module. The relevant data can also be sent to the host computer through the wireless communication module.

[0172] When any of the above functional modules fails, the alarm indication module provides a corresponding alarm indication.

[0173] The present invention and its embodiments are described above, and such description is not restrictive. The drawings show only one embodiment of the present invention, and the actual structure is not limited thereto. In short, if ordinary technicians in the field are inspired by it, without departing from the purpose of the invention, they can design a structure and embodiment similar to the technical solution without creativity, which should belong to the protection scope of the present invention.

Claims

1. A method for measuring the impact of three-phase load imbalance on low-voltage distribution network losses, Features: The following steps are included: S1, real-time detection of the three-phase current, neutral line current and three-phase phase of the distribution network; S2. Determine the three-phase imbalance according to the detected three-phase current and three-phase phase, including three-phase amplitude imbalance, three-phase phase imbalance, and three-phase amplitude and three-phase phase imbalance; S3. According to the determined three-phase imbalance situation, a corresponding algorithm is selected to calculate the distribution transformer loss and the line loss of the distribution network under the corresponding situation.

2. A method for measuring the impact of three-phase load imbalance on low-voltage distribution network losses according to claim 1, Features: The distribution transformer loss includes the distribution transformer iron loss and the distribution transformer copper loss; The influence of three-phase imbalance on the iron loss of distribution transformer is calculated as follows: P Fe =P h +P e +P se (1) In formula (1), P h , P e , P se They are hysteresis loss, eddy current loss and additional loss respectively; When the three-phase load is unbalanced, there will be a three-phase voltage imbalance at the transformer outlet. The ratio of the actual operating voltage of the transformer to the nominal voltage is defined as the voltage deviation coefficient, and the value is α. At this time, the transformer iron loss is: P Fe =(1+α) 2 P N0 (2) In formula (2): P Fe is the transformer operating iron loss, P N0 is the rated no-load loss of the transformer; The influence of three-phase imbalance on the copper loss of distribution transformer is calculated as follows: Let Z ACU , Z BCU , Z CCU is the impedance of the three-phase winding on the primary side of the distribution transformer, Z acu , Z bcu , Z ccu is the impedance of the three-phase winding on the secondary side of the distribution transformer, then the copper loss of the distribution transformer is expressed as the sum of the copper loss of the primary winding and the copper loss of the secondary winding; that is: P CU =I A 2 Z ACU +I B 2 Z BCU +I C 2 Z CCU +I a 2 Z acu +I b 2 Z bcu +I c 2 Z ccu (3) In formula (3): P CU is the copper loss of the distribution transformer, I A ,I B ,I C is the primary current of the distribution transformer, I a ,I b ,I c is the secondary current of the distribution transformer; Assuming the transformation ratio of the distribution transformer is K, we can know: Substituting formula (4) into formula (3), we can get: In formula (5): Z CU It is the copper reactance of the single-phase winding of the distribution transformer; When the three-phase load is unbalanced, the distribution transformer will generate additional copper loss, namely: In formula (6): ΔP FCU is the additional copper loss of the distribution transformer, I av is the average three-phase current on the secondary side of the distribution transformer, β a , β b , β c is the current unbalance coefficient of the secondary side of the distribution transformer, and: By summing up formula (7), we can get:

3. A method for measuring the impact of three-phase load imbalance on low-voltage distribution network losses according to claim 2, Features: The additional copper loss of the distribution transformer when the three-phase phase is balanced and the amplitude is unbalanced is calculated as follows: Substituting formula (9) into formula (6), we get: In formula (10): ε i is the three-phase load balance; Defining ε ui is the three-phase load imbalance, the three-phase load imbalance is: Comparing formula (9) and formula (A1), we can get:

4. A method for measuring the impact of three-phase load imbalance on low-voltage distribution network losses according to claim 3, Features: The additional copper loss of the distribution transformer when the three-phase phase is unbalanced and the amplitude is balanced is calculated as follows: Let β a =β b =β c =β, substituting into formula (6) we can get: and: Substituting formula (12) into formula (11), we get: In formula (13): is the three-phase load power factor angle of the distribution transformer; In this case, the three-phase load imbalance is:

5. A method for measuring the impact of three-phase load imbalance on low-voltage distribution network losses according to claim 4, Features: The additional copper loss of the distribution transformer when the three-phase phase is unbalanced and the amplitude is also unbalanced is calculated as follows: Substituting formula (14) into formula (6) we can get: In formula (15): At this time, the three-phase load imbalance is:

6. A method for measuring the impact of three-phase load imbalance on low-voltage distribution network losses according to claim 5, Features: The influence of three-phase phase balance and amplitude imbalance on the distribution network line loss is calculated as follows: According to the three-phase phase balance of the distribution network, it can be known that the power factor angles of the three-phase loads are equal. Since the three-phase amplitude of the distribution network is unbalanced, we know that β a , β b , β c It will not be constant; set up The three-phase current amplitudes on the secondary side of the distribution transformer are I a ,I b ,I c The three-phase current vectors are i a 、i b 、i c , then it is expressed as: Assume that the neutral line current amplitude of the secondary side of the distribution transformer is I 0 , the neutral current vector is i 0 , then the neutral line current can be expressed as: i 0 =i a +i b +i c (17) Substitute equation (16) into equation (17) and rotate the three-phase current vector symmetrically Degrees, modulo simplification: Assume that the impedance per unit length of the secondary phase line of the distribution transformer is Z L , the impedance per unit length of the neutral line is Z 0 In a three-phase four-wire low-voltage distribution network, Z is usually taken. L =Z 0 , assuming that the three-phase line loss per unit length on the secondary side of the distribution transformer is ΔP L , then we can get: ΔP L =(I a 2 +I b 2 +I c 2 )Z L (19) Substituting formula (7) into formula (19), we get: Assume that the line loss per unit length of the neutral line on the secondary side of the distribution transformer is ΔP 0 ,but: ΔP 0 =I 0 2 Z 0 (21) Substituting equations (7), (8), (9), and (18) into equation (21) and rearranging them, we obtain: Taking the three-phase amplitude balance as the reference condition, let ΔP BL is the three-phase balanced line loss per unit length on the secondary side of the distribution transformer, then: Let ΔP FAL is the additional line loss per unit length on the secondary side of the three-phase amplitude unbalanced distribution transformer, then: ΔP FAL =ΔP L -ΔP BL (24) Substituting equations (9), (20), and (23) into equation (24) and simplifying them, we can obtain: Let ΔP ZFAL is the total additional line loss per unit length of the three-phase amplitude unbalance on the secondary side of the distribution transformer, then: Let ΔP ZAL is the three-phase amplitude unbalanced bus loss per unit length on the secondary side of the distribution transformer, then 7. A method for measuring the impact of three-phase load imbalance on low-voltage distribution network losses according to claim 6, Features: The influence of three-phase phase imbalance and amplitude balance on the distribution network line loss is calculated as follows: In this case, It will not be constant. At this time, the three-phase current vector of the low-voltage distribution network can be expressed as: Substituting equation (7) into equation (28) and expanding it according to trigonometric functions, and then substituting it into equation (17) and simplifying it modulo, we can obtain: Substituting equation (29) into equation (21) and then into equation (12) yields: Since the three-phase amplitude is balanced, let β a =β b =β c =β, substituting formula (12) into formula (20) and rearranging it, we can get: Let ΔP FΦL is the additional line loss per unit length on the secondary side of the three-phase unbalanced distribution transformer, which should be equal to the three-phase line loss per unit length minus the three-phase balanced line loss per unit length. Substituting it into equation (23) and equation (31), we can get: Let ΔP ZFΦL is the total additional line loss per unit length of the three-phase phase imbalance on the secondary side of the distribution transformer, then: Let ΔP ZΦL is the unbalanced three-phase bus loss per unit length on the secondary side of the distribution transformer, then:

8. A method for measuring the impact of three-phase load imbalance on low-voltage distribution network losses according to claim 7, Features: The impact of unbalanced phase and unbalanced amplitude on the distribution network line loss is calculated as follows: will not be constant, β a , β b , β c The neutral current is: Substituting equation (35) into equation (21) and then into equation (14), we can obtain the neutral line loss per unit length as follows: Substituting equation (14) into equation (20), we can get the three-phase line loss per unit length on the secondary side of the distribution transformer at this time: Let ΔP FL The additional line loss per unit length on the secondary side of the distribution transformer with unbalanced three-phase phase and amplitude should be equal to the three-phase line loss per unit length minus the three-phase balanced line loss per unit length. Substituting it into formula (37) and formula (23), we can get: Let ΔP ZFL is the total additional line loss per unit length when the phase and amplitude of the three-phase secondary side of the distribution transformer are unbalanced, then: Let ΔP ZL The phase and amplitude of the three-phase on the secondary side of the distribution transformer are unbalanced, and the total loss per unit length of the distribution network is:

9. A measurement system for the effect of three-phase load imbalance on low-voltage distribution network losses. Features: It includes a three-phase current detection module, a neutral current detection module, a three-phase phase detection module, an intelligent control module, a liquid crystal display module, a wireless communication module, an alarm indication module, and a local power supply module; The three-phase current detection module detects the three-phase current of the distribution network in real time, the neutral line current detection module detects the neutral line current of the distribution network in real time, and the three-phase phase detection module detects the three-phase phase of the distribution network in real time; The intelligent control module determines whether the three-phase imbalance situation belongs to the three-phase amplitude imbalance, the three-phase phase imbalance, or the three-phase amplitude and three-phase phase imbalance according to the three-phase current detected by the three-phase current detection module and the three-phase phase detected by the three-phase phase detection module, and determines the algorithm for the influence of the three-phase load imbalance on the loss of the low-voltage distribution network according to the three-phase imbalance of the distribution network, and then calculates the distribution transformer loss and the line loss of the distribution network under the corresponding situation, and displays them through the liquid crystal display module; When any of the above functional modules fails, the alarm indication module provides a corresponding alarm indication.

10. A metering system for measuring the impact of three-phase load imbalance on low-voltage distribution network losses according to claim 9, Features: It also includes a phase loss detection module, a reverse sequence detection module, a three-phase voltage detection module, and a leakage detection module; The phase loss detection module detects the phase signal of the distribution network in real time, the reverse sequence detection module detects the phase sequence signal of the distribution network in real time, the three-phase voltage detection module detects the voltage signal of the distribution network in real time, and the leakage detection module detects the leakage signal of the distribution network in real time.

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