Design and Application of Multi-function Guideway Three-phase Watt-hour Meter Based on SOC Technology

Abstract: Aiming at the practicality and difficulty of development of the three-phase energy meter with guide rail, an implementation method using SOC technology is designed. The design and application of DTSD1352 are introduced from the perspective of hardware and software.

Keywords: SOC; rail type; three-phase electric energy meter

0 Preface

According to statistics, the average annual power consumption per unit area of ​​public buildings in China is huge, accounting for about 22% of the total urban power consumption in the country. In terms of energy efficiency in public buildings, itemized measurement is an effective and feasible method at this stage. As a terminal element, the electric meter in the sub-metering system has strict requirements in terms of function, performance and installation method. At present, the electric energy meters used for charging in China's power supply department are wall-mounted installation methods. This installation method occupies a large space, is not easy to install, and is not suitable for renovation projects where the space is originally small. Moreover, the sub-item energy measurement of large public buildings belongs to internal measurement management. It is not appropriate to use charged meters, but electric meters should be used. The guide rail installation is superior to the traditional wall-mounted and embedded instruments in volume and structure. Its special installation method can be installed in the terminal lighting box as well as in the large switch cabinet and power cabinet without re-opening.

1. Design basis

As a measuring instrument, the electric energy meter is closely related to the economy and people's livelihood. Therefore, its related standards are the most stringent among many industrial standards. The main reference standards for domestic energy meters are:

l    GB / T17215.322-2008 "Special requirements for alternating current measurement equipment Part 22: Static active energy meters (Class 0.2S and 0.5S)"

l    GB / T17215.211-2006 "General Requirements, Tests and Test Conditions for Alternating Current Measurement Equipment Part 11: Measurement Part"

l    DL / T614-2007 "Multi-function Electric Energy Meter"

The above standards specify detailed technical parameters for the performance and function of the energy meter in design, production and application. Among the more important performance indicators are:

1). Power frequency withstand voltage. All current lines and voltage lines and auxiliary lines whose reference voltage exceeds 40V are connected together as one point, and the other point is ground. The test voltage of 4kV is added between these two points, and no flashover, spark discharge or breakdown shall occur.

2). Fast transient burst test. Add 4kV pulse group to the voltage and current lines, the accuracy of the meter should be within the limits specified by the corresponding standards.

3). Electrostatic discharge immunity test. Contact discharge 8kV, air discharge 15kV, the instrument can not have a crash or accuracy is not allowed to appear.

4). Surge immunity test. Connect the reference voltage to the voltage line, open the current end, apply 4kV to the voltage and current lines, 5 times each of the positive and negative poles, the instrument should not be damaged and cannot work.

The above indicators are the highest level of cruelty in their respective test specifications. For traditional wall-mounted energy meters, the volume is large and there is enough design space inside to meet the various requirements in the standard. The rail type three-phase multi-function energy meter has a small volume, and the mold determines that the internal available space is not large. Therefore, the design of the rail-type three-phase energy meter is more difficult than the traditional wall-mounted energy meter. This article takes DTSD1352 three-phase multi-function energy meter as an example to explain a scheme based on SOC technology.

2. Hardware reliability design

2.1 Chip selection

The main chip used by DTSD1352 is 71M6543 of American letter company. This chip contains an 8051 microprocessor core and a 32-bit DSP digital engine (CE) that can realize the measurement function. It also includes some peripherals necessary for the energy meter. Such as LCD driver, compensable RTC, UART, etc. The chip has good electromagnetic compatibility and fully meets the performance standards of the energy meter. Due to the extremely high degree of integration of this chip, it leaves valuable space for the internal structure of the product.

2.2 Overall plan

The overall scheme block diagram is shown in Figure 1. The overall design mainly revolves around 71M6543. The peripheral circuits mainly include: switching power supply, voltage and current sampling circuit, buttons, liquid crystal display, data storage, RS485 communication module, pulse output. The original signal is input to the CE through the resistance voltage divider and the current transformer signal attenuation. CE processes the voltage signal collected from the analog input pin in sequence, calculates the functional amount (Wh) and the non-functional amount (VARh), and A2h and V2h. The MPU then accesses these measured values ​​for further processing and outputs through the peripheral devices of the MPU, such as LCD display, RS485 communication, etc. In addition to advanced measurement functions, the real-time clock (RTC) function allows the 71M6543 to implement multi-rate time-division measurement without plugging in any real-time clock chip. At the same time, with the plug-in ferroelectricity, it can save 12 months of historical data.

figure 1

2.3 Sampling circuit

71M6543 pins IADC0 to IADC7 are used as current sensor inputs. These 8 current sensor inputs can be configured as 8 single-ended inputs, or paired to form 4 differential inputs. For best performance, DTSD1352 configures the current sensor input as a differential input (ie: IADC0-IADC1, IADC2-IADC3, IADC4-IADC5, IADC6-IADC7) in the design. 71M6543 provides two access methods: shunt access and current transformer access. It is under the consideration of electrical safety. The DTSD1352 solution uses the transformer solution. The specific circuit schematic is shown in Figure 2:


Picture
2

In Figure 2, IN_IA + and IN_IA- are connected to the secondary output of the transformer, and IA + and IA- are connected to IADC0 and IADC1. The voltage sampling circuit uses a single-ended input to detect the grid voltage through a resistor divider. The VADC8, VADC9, and VADC10 pins are single-ended and return to the V3P3A pin.

2.4 Input multiplexer and sampling timing

Because there is only one ADC inside the 71M6543, an input multiplexer is used to switch the acquisition of 7 signals (IA, VA, IB, VB, IC, VC, IN). The input multiplexer applies the analog input signal to the ADC input in sequence according to the order determined by the 11 MUXn_SEL [3: 0] control fields. A complete sampling process is called multiplexing frames. Figure 3 is a timing diagram of a complete multiplexed frame:


Picture
3

3. Software modular design

DTSD1352 adopts modular software design. The module design includes: measurement operation, calibration module, human-computer interaction module, communication module, complex rate calculation module, demand calculation module, event recording module, etc. The overall flow chart is shown in Figure 4:

The task scheduling module in the flowchart is actually a combination of multiple interrupts and timers with a certain priority. It dominates the entire program, and the driver works normally. For periodic tasks, such as metering, complex rate calculation, etc., the timer regularly assigns the task execution order. For some non-periodic tasks, such as communication, human-computer interaction, etc., they are triggered by interrupts. Most modules are independent of each other, and code maintenance is simple.


Picture
4

4. DTSD1352 test data

Table 1 is the data of DTSD1352 in each temperature environment test. These data show that the DTSD1352 active energy accuracy level can meet the 0.5S level requirements and meet the high and low temperature performance.

Table 1

test environment

Input signal (3P4L)

cosφ

Energy error ( %) and instrument number

1#

2#

25 ℃

220V

6.0A

1.0

-0.0533

-0.0798

220V

5.0A

1.0

-0.0261

-0.0702

0.5L

0.0271

-0.071

0.8C

-0.0642

-0.111

220V

0.5A

1.0

0.1427

-0.0416

60 ℃

220V

6.0A

1.0

-0.0434

-0.0171

220V

5.0A

1.0

-0.0195

0.0138

0.5L

0.0445

0.0364

0.8C

-0.0695

-0.0614

220V

0.5A

1.0

0.286

0.0425

-20 ℃

220V

6.0A

1.0

0.2912

0.2258

220V

5.0A

1.0

0.2097

0.2382

0.5L

0.2899

0.1896

0.8C

0.1957

0.2117

220V

0.5A

1.0

0.2062

0.2649

5. Application of DTSD1352 in practice

Figure 5 is the system diagram and wiring diagram of DTSD1352 in an office building. The picture on the right is the electrical system diagram. DTSD1352 is used as the terminal element in the system to measure the electrical energy and electrical operating parameters of terminal loads including lighting, air conditioning or sockets. The figure on the left is the external wiring diagram of a single DTSD1352. The three phases A, B, and C in the power grid are connected to the UA, UB, and UC (terminal number 1, 3, 5) terminals of the meter. The current is connected to the corresponding current interface after the transformer. The DTSD1352 also has direct access. The current below 80A can be directly entered into the meter. The wiring method is similar to the access of the transformer. Due to space issues, it will not be repeated here.


Picture
5

5. Conclusion

Rail-mounted multi-function energy meters can play a great role in sub-metering projects, which can effectively reduce the difficulty and cost of construction. The performance index of DTSD1352 meets the technical requirements of the national standard GB / T17215 and the power industry standard DL / T614-2007 for electric energy meters. It is suitable for the sub-metering of electric energy in government agencies and large public buildings. It can also be used for electric energy in enterprises and institutions. Management assessment.

The article comes from: "Electric Times", Issue 11, 2012.

references

[1] "Principle and Application Guide of Electric Power Measuring Instrument" . Ren Zhicheng, Zhou Zhong. China Electric Power Press

[2] "Design and Implementation of AC Sampling" . Chen Xuezhen. Data Acquisition and Monitoring

About the Author:

Yu Jing, female, undergraduate, engineer of Wuhan Ankerui Electric Co., Ltd., the main research direction is intelligent power monitoring and power management system

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