Single-phase smart meter-reliability enhancement test research

 

 

As the infrastructure of the smart grid information collection system, single-phase smart meters have the characteristics of providing timely and effective services to residents. If the reliability of smart meters is poor, it will not only seriously affect the operation of the smart grid, causing huge meter replacement costs, but also may easily cause electricity disputes. Therefore, it is crucial to evaluate and improve the reliability of single-phase smart meters for the entire smart grid system. In order to study the reliability of single-phase smart meters, we chose the accelerated life testing (ALT) method, which is an effective, efficient and widely used method for life assessment and reliability research of high-tech products. However, before testing, we must determine the safety range of the accelerated life test (ALT) and the most likely failure mode of the smart meter under synthetic stress.

 

Single-phase smart meter is a high-tech product with the characteristics of high reliability and long service life. It is inconvenient and time-consuming to conduct reliability evaluation test under normal working conditions. In contrast, reliability enhancement test, as an environmental stress stimulation test, has been widely studied by scholars at home and abroad since the end of the 20th century. A large number of experimental results show that RET can quickly expose the failure mode of the product, effectively determine the working limit and damage limit of the product, and then provide a safety range for the later accelerated life test according to the working limit. Nowadays, countless RET examples have been successfully expanded to the fields of communications, energy, medical, electronics, instrumentation, etc., improving the reliability of products, shortening the design and production cycle, and achieving significant economic benefits. However, there is no complete system for comprehensive analysis of the reliability of smart meters. Zhejiang Reallin Electron Co., Ltd explores the RET method of single-phase smart meters, with the aim of exploring the failure mode and product limit under the action of comprehensive stress. This is a beneficial attempt at RET for single-phase smart meters, which provides a scientific basis for later research and improvement of the reliability of smart meter products in the industry.

 

Reliability Enhancement Test (RET) is an environmental stress test based on the failure physics theory and taking the failure mode of the product as the research object. The test quickly detects potential defects by gradually increasing the single or combined environmental stress that is much more unfavorable than the normal stress condition, thus shortening the test time and increasing efficiency. Its main purpose is to determine the weak links and failure modes under the combined stress, and to explore the operating limit and failure limit of the product.

 

Figure 1 Structural diagram of a single-phase smart meter

 

Based on the results of historical data analysis and failure mode and effects analysis (FMEA), we found that the function of this single-phase smart meter is easily affected by temperature stress in the natural working environment. Unsuitable temperature can cause a variety of failures, such as measurement errors, indicator light failure, display abnormality, and communication interruption.

 

The study found that the metering unit, LCD display, and 485 communication module are the main weak links. Low temperature mainly affects the LCD display, while high temperature has a greater impact on the whole, especially the electronic components, which may cause physical changes and cumulative damage.

 

In summary, high temperature is identified as a key stress factor affecting the reliability of the meter. Based on these findings, we selected a single-phase smart energy meter for high temperature reliability enhancement test (RET) to further study the impact of temperature on the performance of the meter and provide a scientific basis for improving its reliability.

 

2.2 High-temperature RET of single-phase smart meter

 

The RET on the single-phase smart meter is undergoing a step temperature test, which continuously applies high-temperature step stress to the test instrument. The test will not stop until the stress level reaches the damage limit or the maximum limit of the test instrument. During the test, the key performance parameters of the smart meter are monitored in real time, and its failure mode is recorded. Then, through the analysis of the test data, its working limit and damage limit at high temperature are determined.

 

Parameter Category	Parameter Name	Parameter Value	Description
Electrical Characteristics	Reference voltage	220V	Standard working voltage
	Current specification	5(60)A	Basic current 5A, maximum 60A
	Accuracy level	Level 1	Suitable for active power measurement
	Frequency range	(50±2.5)Hz	Applicable to standard power grid
	Quiescent power consumption	<1.5W,10VA	Low power consumption design
Environmental adaptability	Operating temperature	-25℃~60℃ -40°℃~70°℃	Specification temperature range
	Relative humidity	<95%	Extreme operating temperature
Reliability	MTTF	≥ 10 years	Mean time between failures

 

2.2.1 Overview of high temperature step RET

 

This study only considers the influence of high temperature, and adopts the following scheme: except for the temperature parameters, other parameters are set according to the technical parameter values ​​shown in Table 1. The normal temperature range of the smart meter is -25°C~+60°C, and the working limit range is -40°C~+70°C, so 70°C is selected as the initial temperature S1.

 

Considering that the recommended working limit of the chip inside the meter is usually between 80°C~85°C, the temperature step is uniformly set to 5°C, and the temperature change rate is controlled below 2.5°C/min. In order to fully observe the influence of temperature on the meter, each temperature level is maintained for 30 minutes.

 

Figure 3 shows the process of the step high temperature RET test. During the test, the temperature is continuously increased until all test pieces fail at a certain temperature level (t+1, i≥ 1). Then the temperature is reduced to the previous level (ti). If all test pieces can work normally at level ti, ti+1 is determined as the working limit temperature. After determining the working limit, continue testing to explore the damage limit. The method is to conduct more detailed tests in the temperature range near the working limit. For example, if the test piece cannot resume normal operation after staying at temperature tj for 30 minutes, then tj can be identified as the damage limit temperature. If the test piece can resume normal operation, continue to increase the temperature test until the maximum temperature limit of the test equipment is reached.

 

 

This method can accurately determine the working limit and damage limit of smart meters, providing key data for evaluating their high temperature reliability.

 

2.2.2 Test items

 

Based on the test instrument and test chamber capacity, 16 single-phase smart meters were selected as samples for testing. This RET includes three steps: First, an overall performance evaluation test is conducted before RET to ensure that all test pieces are qualified. Second, the online monitoring items used during RET are used to explore the changing trends of the weak links of single-phase smart meters. Finally, after RET, a comprehensive performance evaluation test is conducted at room temperature, and the test data is recorded in real time. The test items of each step are shown in Figure 4.

3. Test results analysis

(1) Measurement error change: 23°C: 0.151% (maximum error value, benchmark), 70°C: 0.701% (maximum error value, significant increase), 95°C: 1.150% (maximum error value, continuous increase), 120°C: no signal (complete failure).

(2) LCD display performance: 90°C: slightly dimmed, 95°C: characters cannot be displayed, 100°C: black screen.
(3) 485 communication: 85°C: first failure, 110°C: most failures, 120°C: complete failure.
(4) Critical temperature points: 95°C: operating limit temperature (LCD large-scale failure), 120°C: comprehensive functional failure temperature, 150°C: estimated damage limit temperature.
(5) Failure process: 70°C~85°C: individual samples began to fail, 95°C: the number of failures increased significantly, 120°C: all samples completely failed.
(6) Recovery ability: When the temperature drops to 90°C, most functions are restored; when the temperature drops to room temperature, most samples return to normal.

 

According to the system reliability theory, the single-phase smart meter can be regarded as a series system composed of multiple unit modules. Any unit failure may cause the entire system to fail. Based on the "bathtub effect", the unit that fails first during the reliability enhancement test (RET) is the weakest part of the system. The experimental results show that the LCD display unit is the component most susceptible to temperature in the single-phase smart meter.

 

Based on the phenomenon that the LCD liquid crystal display disappears at 95°C and all the tested meters can be identified as failed at this temperature, we can estimate 95°C as the operating temperature limit of the single-phase smart meter. Although the online monitoring function completely fails at 120C, most of the test pieces have recovered when the temperature drops to 90°C. This RET was terminated at 150°C (the upper limit of the high and low temperature alternating damp heat test chamber). After dropping to room temperature, most of the meters passed the comprehensive performance test and resumed normal operation. It can be inferred that the damage limit temperature of the smart meter should be higher than 150°C.

 

The results show that the working limit of the test sample is 25K higher than the design limit, and there is a large margin between the damage limit and the working limit. This shows that the single-phase smart meter used in this RET has high reliability and can maintain stable operation in harsh environments.

 

4 Conclusion

 

Based on the RET principle and the characteristics of single-phase smart meters, a high-temperature reliability enhancement test was designed and conducted. The test verified the results of historical data analysis and FMEA, and believed that the weak links of the meter were the metering unit, LCD display unit and 485 communication unit, among which the LCD display was the weakest; the failure mode under high temperature was measured error out of tolerance, LCD display failure and 485 communication failure; the high-temperature working limit was determined to be 95°C, and the damage limit was higher than 150°C. The successful implementation of this RET not only verified the feasibility of the test plan, but also provided a basis for the parameter selection and safety range determination of the subsequent accelerated life test. At the same time, it laid the foundation for further research on reliability enhancement tests of smart meters