Brief Analysis on Performance Evaluation Method of Electronic Multi-function Gateway Electric Energy Meter

0 Introduction

Since the 1990s, imported 0.2S-class electronic multifunctional watt-hour meters have dominated the high-end metering of gateways, and the huge price difference of high-end watt-hour meters has caused power companies to face major difficulties in the selection and procurement of watt-hour meters. How to effectively identify the quality differences of high-end watt-hour meters, grasp the selection criteria, and establish a fair watt-hour meter selection mechanism, so that domestic high-end watt-hour meters can enter the dominant position of gateway metering, is an expectation of the power grid metering application department and the meter industry.

Gateway watt-hour metering has the characteristics of large power, wide dynamic load change range, frequent phase changes, severe electromagnetic interference and high reliability requirements. In view of the particularity of gateway metering, not all 0.2S-class multifunctional watt-hour meters can be used for gateway metering. Internationally, gateway watt-hour meters are called GT&D meters (Generation Transmission & Distribution Application) and multifunctional watt-hour meters for billing industrial and commercial users are called C&I meters (Commercial & Industrial Application).

At present, there is no unified international standard for gateway multifunctional meters, and the special requirements for multifunctional electric energy meters proposed by national standards have limitations. How to identify the performance and quality differences of high-end multifunctional electric energy meters is an urgent problem that power companies need to solve.

Since 2007, the State Grid Corporation of China's Electric Energy Metering Equipment Performance Evaluation Laboratory (hereinafter referred to as the laboratory) has carried out research on the performance evaluation method of gateway electric energy meters. On the basis of meter type test methods and statutory metrological verification procedures, it focuses on demonstrating and introducing gateway electric energy meter performance requirements. From a multi-faceted perspective, it introduces new technologies such as physical condition changes and algorithm analysis of electric energy meters, explores gateway electric energy meter performance evaluation methods, and aims to establish a safe, reasonable and fair gateway electric energy meter selection mechanism and a domestic high-quality gateway electric energy meter design, development and testing platform.

This article will briefly describe the performance requirements of gateway electric energy meters, performance evaluation methods, and introduce the preliminary test results of some meter accuracy evaluation examples.

1 Gateway electric energy meter performance requirements

In a broad sense, the "performance" of a meter is an important part of the product quality characteristics, which refers to the sum of the ability characteristics and characteristics of the meter itself to meet explicit or implicit needs.

The energy requirements of gateway electric energy meters are divided into two categories according to their origins:
the first category, basic performance requirements: performance requirements proposed by relevant IEC standards, national standard industry standards and legal metrology verification procedures.
Such requirements are detailed in the relevant standards and will not be repeated in this article.
The second category, special performance requirements: adopt the latest and most stringent technical standards, and extract performance requirements that are higher than the instrument reference standards or not specified in these reference standards from the technical conditions of imported 0.2S-level gateway electric energy meters; performance requirements proposed by the power grid's demand for gateway metering and changes in power load and operating environment; performance requirements for type tests that need to be improved and expanded after analyzing the operating quality problems of domestic 0.2S-level multi-function meters.

1.1 Special performance requirements (Part 1)

indicate that the gateway electric energy meter is adapted to the particularity of gateway metering and needs to have a high level:

(1) Power metering has high accuracy
Active metering is 0.2S level, the actual control error is ±0.1%, and the maximum error difference between the main meter and the auxiliary meter is 4-0.05%;
reactive metering is 0.5S level, the actual control error is ±0.2%;
apparent metering is 1.0S level, the actual control error is 4-0.2%.

(2) The active/reactive metering error curve is flat and reasonably distributed
The load current range is 0.0021～. 1mn (i.e. the dynamic range is 1:2000); the metering linearity error is 1/10 of the meter accuracy grade.

(3) Active/reactive metering is accurate within the range of 0 ~ 360 degrees, and the critical phase angle metering accuracy assessment is added:
active metering 904±0.5., 2704±0.5. Assessment;
reactive metering 04±0.5., 180±0.5. Assessment.

(4) Repeatability and stability of measurement error
The standard deviation of the test from preheating to 1h of working period, and the change of measurement error from preheating to 8h and 24h of working period.

(5) The maximum error difference between batch instruments is ±0.06%.

(6) The reactive power measurement principle must ensure that the reactive power measurement is correct under the actual harmonic load.

(7) The error change within the entire working temperature range must be assessed.

(8) The error change limit caused by other influencing quantities must be significantly compressed compared to the requirements of relevant standards such as IEC; the electromagnetic compatibility test level must be appropriately improved compared to relevant IEC standards; and the measurement accuracy and working stability assessment under electromagnetic interference must be increased.

(9) Tolerance to impact load test of 10 times or more of the high and low differences.

(10) The instrument constant must ensure 0.01/a. The maximum demand measurement during continuous operation is accurate.

(11) Communication requirements
It has multiple communication modes and can work independently at the same time; the communication interfaces on the same bus have no influence on each other’s working status; and it supports commonly used international and domestic communication protocols.

(12) Load curve storage capacity: With a time interval of 1 minute, it can store various types of electric energy and electric parameter data for more than 15 days.

(13) It has remote alarm monitoring function.

1.2 Special performance requirements (Part 2)

indicate the cutting-edge metering functions that gateway electric energy meters need to have to meet modern metering needs:
(1) Power quality monitoring;
(2) Harmonic electric energy metering and harmonic instantaneous power factor and harmonic average power factor calculation;
(3) Accurate maximum demand measurement under dynamic load;
(4) Self-correction capability for voltage, frequency, phase, and temperature changes;
(5) High-speed, real-time measurement and recording.

2 Gateway electric energy meter performance evaluation method

Gateway electric energy meter performance evaluation refers to the use of existing reference standards and explored test methods to comprehensively identify the characteristics and differences of meter performance.
Corresponding to the classification of gateway electric energy meter performance requirements, the performance evaluation items of the meter are also divided into two categories: basic performance evaluation items and special performance evaluation items.

2.1 Basic performance evaluation items
The test is mainly carried out using the methods proposed by IEC related standards, national standards, industry standards and legal metrology verification procedures to assess whether the instrument performance (quality) is qualified. The main problem is that it cannot fully and correctly reflect the characteristics of gateway electric energy meters and the technological development of modern electronic electric energy meters, including:
(1) Focus on active power, ignore reactive power, and lack accuracy testing of apparent electric energy measurement;
(2) The load and power factor change range of the test is small, which is difficult to fully meet the actual application requirements;
(3) There is a lack of new technologies such as non-sinusoidal, dynamic load, and fast measurement;
(4) There is only functional display of communication, power quality, and event recording, and there is no correct evaluation method.

2.2 Special performance evaluation projects
Through the expansion of test projects, adding critical tests, conducting algorithm analysis, increasing test severity, simulating environmental changes, introducing new test technologies and other technical means, the performance characteristics and differences of the meter, as well as its adaptability to changes in physical conditions, are comprehensively evaluated.

There are 47 special performance evaluation projects for gateway electric energy meters, mainly including identification of power measurement principles, accuracy evaluation, electrical performance tests, communication performance tests, and improvements and expansions of type test projects. Through repeated tests and verifications, the laboratory has identified 10 key special performance evaluation projects with strong identification capabilities:

(1) Identification of active and reactive power measurement principles under sinusoidal and non-sinusoidal waveforms;
(2) Drawing of measurement error curves within the full range of load current, voltage, phase, frequency, and integration period based on the definition of electric energy; (3
) 0-360 degree active and reactive metering error consistency test;
(4) Voltage loop critical voltage test;
(5) Identification of active and reactive energy metering differences under harmonic load;
(6) Measurement of active power and reactive power accuracy under extremely low load and low power factor;
(7) Test of the impact of three-phase voltage imbalance on reactive power metering;
(8) Impact load test;
(9) Assessment of communication, data storage, and metering performance under practical and extreme communication conditions;
(10) Verification of pulse constants of electric energy meters under various requirements.

In general, the gateway performance evaluation test technology spans multiple fields, and its characteristics are: using the latest and most stringent technical standards; error curves, critical tests, impact load tests, metrological principle identification, harmonic response capabilities, etc. have become the highlights of the test items; new technologies such as rapid measurement and metrological principle identification have been introduced; due to multiple policies, controversial projects account for a certain proportion; the conclusion given by the performance evaluation is a "graded evaluation result" rather than a qualified or unqualified conclusion

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3.1 Instrument special performance test method

Due to space limitations, the following are three special performance test methods: 0-360 degree active energy metering error consistency test, impact load test, and reactive energy metering difference identification under harmonic load. The test samples are all mainstream 0.2s-level threshold energy meters currently used in domestic power systems, including 5 imported brands and 4 domestic brands.

(1) 0-360 degree active power metering error consistency test This test project was conducted on five imported brands (A-E) of electric energy meters. The test results are shown in Figure 1.

(2) Impact load test
This test project is conducted on 0.2S-level electric energy meters of 5 imported brands (A~E) and 4 domestic brands (F~I). The test principle of the impact load test is shown in Figure 2.

Meters B and C are the same brand of tested electric energy meters, and Meter A is a standard meter. The impact load (with 2 seconds as the impact time interval) function is realized through the program-controlled switch. During the test, a constant current of 5A is applied to Meter A, and by closing and opening the switch, a 2-fold impact load (2.5-5A) is applied to Meter B. The impact load applied to Meter C is 0-2.5A.

The measurement error under impact load is calculated according to the following formula:

Where E2 is the active energy measured by meter A; E2 is the active energy measured by meter B; Ec is the active energy measured by meter C. It should be noted that this error is the combined measurement error of the two tested meters for impact load. After the above test, meter B and meter C can be interchanged and tested again. The average value of the two test results is used as the basis for calculating the impact load measurement error.

The impact load test results are shown in Table 1.

(3) During the reactive energy measurement difference identification test under harmonic load, the harmonic voltage and current are applied according to Table 2. The same harmonic load is applied to all the tested electric energy meters at the same time. The reactive energy measurement difference of electric energy meters of different brands under the same harmonic load is compared through the running test.

This test project was conducted on 0.2S-level threshold electric energy meters of 5 imported brands (A~E) and 4 domestic brands (F~I). The test results are shown in Table 3.

3.2 Evaluation of single test results of instrument special performance

(1) 0～360 degrees, active power measurement error consistency

First, evaluate the metering accuracy performance within the range of 0 to 360 degrees, using the error distribution range as the evaluation basis; second, assess the symmetry of the error distribution of the meter, and the error size under the same load nature (inductive or capacitive) should be close and consistent in positive and negative signs under the positive and negative power flows. According to this principle, the evaluation ranking of gateway meters of different brands on this project can be given as shown in Table 4.

(2) Measurement accuracy under impact load.

According to the test results (Table 1), it can be seen that the electric energy meters of brand F and brand I will undercount the electricity under impact load. Therefore, it can be concluded that the electric energy meters of brand F and brand I are weaker than other brands in terms of impact load measurement accuracy.

(3) Identification of reactive power measurement differences under harmonic loads

It can be seen from the test results that there are certain differences in reactive power measurement under harmonic loads between domestic meters and imported meters, and between different brands of imported meters. At present, there is still controversy internationally about the accurate measurement of reactive power under harmonic loads. In this case, it is recommended to use the consistency principle to evaluate it, that is, for the same gateway metering point, the harmonic reactive power measurement between multiple electricity meters should be consistent.

4 Conclusion

Starting from the particularity of gateway electric energy metering, this paper proposes an outline of gateway electric energy meter performance requirements, and then explores the gateway electric energy meter performance evaluation method. Tests have proved that the multi-faceted evaluation of metering accuracy based on the definition of electric energy is very effective in distinguishing the performance differences of gateway electric energy meters. Due to limited space, other special performance evaluation projects with existing results will not be listed one by one, and the evaluation technology of cutting-edge metering functions is being studied in the future.

The development of modern power grid harmonic measurement, power quality measurement, power grid power and loss balance calculation and other metering application technologies has put forward higher and higher requirements for gateway power meters. The performance requirements and evaluation technologies of meters need to adapt to the actual situation of the constantly changing on-site operating environment. As the first step, it is an urgent issue to introduce new power industry standards for electronic multifunctional gateway power meters.