Design of power supply for electronic watt-hour meter

Abstract: This paper analyzes the power supply characteristics of electronic single-phase and three-phase watt-hour meters, proposes design methods and precautions, and provides solutions to users' special requirements for watt-hour meters such as wide input voltage and low power consumption.

Keywords: electronic watt-hour meter; power supply; design

Design of Power Supply for Electronic Watt hour Meter

LI Chang-bo

Abstract:The characteristics of the power supplies for use in single phase and three phase electronic watt hour meter are analyzed.The design methods and points for attention are proposed.The solution for a wide range of input voltage and lower power consume of electronic watt hour meter as consumer′ s especial requests are provided.

Keywords:Electronic watt-hour meter; Power supply; Design

1 Introduction

With the vigorous development of the national power industry and the implementation of the "one household, one meter" system in the power industry, the electromechanical watt-hour meters used in the early days are increasingly unable to meet market requirements. Electronic watt-hour meters have the characteristics of accurate measurement, remote meter reading, time-sharing measurement, and product reliability, so the market is good. It is reported that there is a demand of about 50 million units in China each year. Since watt-hour meters are uninterrupted power measurement products, they have strict requirements on measurement accuracy, reliability, and EMC characteristics, so the power supply part is one of the keys to the design of electronic watt-hour meters.

There are detailed design, production and testing standards for watt-hour meters, such as GB/T 17215-1998 Class 1 and Class 2 static AC active watt-hour meters (the corresponding international standard is IEC1036:1996), GB/T 15284-1994 multi-rate (time-sharing) watt-hour meters, DL/T 698-1999 Technical conditions for centralized meter reading systems for low-voltage power users, DL/T 614-1997 multi-function watt-hour meters, etc. These standards have put forward detailed requirements for the power consumption of the whole machine, input voltage range, EMC testing and other aspects related to the power supply design of the watt-hour meter. The following discusses the power supply design methods and issues that should be paid attention to for simple single-phase pure meter, single-phase multi-function meter, and three-phase watt-hour meter.

2 Power supply design of single-phase electronic watt-hour meter

The power supply requirements for pure meter and multi-function meter in single-phase electronic watt-hour meter are different.

2.1 Power supply design for single-phase pure meter

Single-phase pure meter generally contains only a basic metering unit, an electronic counter, a light-emitting diode and a pulse output port, etc. The power consumption of the whole machine generally does not exceed 50mW. The low-cost series resistor-capacitor step-down circuit can meet the power supply requirements of the entire meter; some metering ICs only need a single +5V power supply, such as ADE7755, as shown in Figure 1. Some metering ICs need ±5V or ±2.5V dual power supplies, such as BL0932, SA9602M, etc., as shown in Figure 2.

Figure 1 Single power supply

Figure 2 Dual power supply

The resistor R1 (or R1, R2) of the input circuit should be a metal film resistor or cement resistor of good quality, with a power of 2 to 5W and a resistance of 100 to 820Ω; the step-down capacitor C2 (or C5) should be a high-quality high-voltage metallized polypropylene capacitor, generally 0.22 to 0.47μF, and the withstand voltage should be ≥AC 250V. Too large a capacitor capacity will cause the apparent power to exceed the standard.

When the metering pulse drives the meter, the stepper motor of the meter will absorb a large current instantly. If the output filter capacitance is insufficient, it will cause voltage fluctuations and affect the accuracy of measurement. Especially when the meter inputs a large current, the frequency of the metering pulse is faster and the power supply voltage is more easily affected. Therefore, a larger output filter capacitor should be selected to ensure that the metering accuracy is not affected when the metering is working at a large current. In general, the main circuit filter capacitor is 1000μF~2200μF, and a long-life (>2000h) electrolytic capacitor should be used to improve reliability. At the same time, it should also be considered that when the light-emitting diode and the pulse output port have metering pulse input, they will also absorb part of the current instantly, which will also affect the stability of the power supply voltage.

2.2 Power supply design of single-phase multi-function watt-hour meter

The basic unit of a single-phase multifunctional watt-hour meter generally includes metering, CPU, display, etc. For the power supply, the main consideration is how many power supplies the watt-hour meter needs, and the voltage and power requirements of each power supply; some meters may also have external control lines or communication lines, and these lead lines should have safety isolation requirements. Since the components and integrated circuits used in this watt-hour meter are far more than those of a single-phase pure meter, the required power is also larger. According to the standard requirements, the static power consumption of the voltage line should be ≤1.5~3W for different types of watt-hour meters, and the apparent power should be ≤10VA. Generally, a low-cost linear transformer can be used for power supply. Some power supply bureaus have put forward more stringent requirements on the power consumption of electronic watt-hour meters, such as static power consumption <1W, and the input voltage fluctuation range is required to be (70%~130%)Un, or even larger. If the meter can output normal working voltage at the minimum input voltage of 70% Rn, according to the characteristics of linear transformer, the output voltage will be higher at the rated input voltage Un. If the output voltage is rectified and filtered and then powered by a general linear regulator, the power consumption of the whole machine will increase; at the highest input voltage, the temperature rise of the transformer will increase, the loss of the linear voltage regulator circuit will increase, and the heat will also increase; for electronic watt-hour meters with large power consumption, the solution is to use silicon steel sheets of better materials, such as Z11 silicon steel sheets; the second is to use a low-cost high-efficiency DC/DC conversion circuit for the low-voltage output voltage regulator circuit, which can significantly reduce the power consumption of the whole machine and work stably and efficiently within a wide input voltage range. These DC/DC circuits can use simple low-cost DC/DC converters such as MC34063 and LM2574. The +5V output DC/DC converter designed with MC34063 is shown in Figure 3.

Figure 3 DC/DC converter

Now, in order to prevent the meter from being damaged due to the wrong connection of the neutral line of the power grid or the damage of the meter by artificial overvoltage power supply, most power supply departments have required that the newly installed single-phase meter can withstand high voltage (such as 380V voltage) for a certain period of time (such as 4h) without damage. If the input circuit of the single-phase meter is not processed, the input high voltage will burn the meter in a short time. As long as a suitable PTC thermistor is connected in series at the input end of the transformer for overcurrent protection, and the external voltage is mainly applied to the PTC thermistor, it can be ensured that the linear transformer will not heat up and be damaged. In order to ensure that no false operation occurs, the non-operating current of the thermistor should be selected with reference to the actual AC input current of the meter, and a 50% margin should be left. The Curie temperature should be ≥80℃; at the highest working temperature, when the maximum load is working, the meter cannot be protected, and at the lowest working temperature, when 380V AC is added to the input end, the PTC in the meter should play a protective role in a very short time to ensure that the meter is not damaged. The circuit diagram is shown in Figure 4.

Figure 4 Single-phase multi-function watt-hour meter power supply

3 Three-phase watt-hour meter power supply design

Three-phase watt-hour meters have three-phase three-wire and three-phase four-wire, and the input voltages are 3×(220～240)V/380～400V, 3×57V/100V, etc. The power supply circuit of the three-phase watt-hour meter can refer to the design method of the single-phase watt-hour meter power supply. For the three-phase four-wire power supply circuit, a linear transformer can be connected between each phase line and the neutral line, for a total of three linear transformers; for the three-phase three-wire, one phase such as phase B can be taken as the common ground line, and two linear transformers are connected between phases A and C and phase B. After the secondary sides of these transformers are rectified separately, the positive and negative poles of the outputs are connected together, and after capacitor filtering, a stable voltage is output through a DC/DC converter to supply power to the watt-hour meter, as shown in Figure 5.

Figure 5 Three-phase watt-hour meter linear power supply

This power supply method is large in size and weight, and its cost is also high. Obviously, when connected according to the above circuit, the neutral wire cannot be missing for three-phase four-wire power supply, and the common ground wire, such as phase B, cannot be missing for three-phase three-wire power supply. In addition, some power supply bureau users require that the three-phase watt-hour meter can work when there is voltage input on any two wires at the input end, so the design of Figure 5 cannot achieve the purpose. When a special switching power supply module for three-phase watt-hour meters is used for power supply, the above problems can be solved, and its internal structure diagram is shown in Figure 6.

Figure 6 Three-phase watt-hour meter dedicated switching power supply module

This three-phase watt-hour meter special switching power supply module has a volume of 32mm×75mm×22mm and a wide input voltage range. Within the range of 60~480V, any two-wire, three-wire, and four-wire input can work. It has input overvoltage and output overcurrent protection functions, and the EMC characteristics of the power supply are also good. In addition, it has the advantages of small size, light weight, high efficiency, and low loss. The self-loss of each phase of power supply is <0.4W. It has been put into batch trial by our company for three years with good results.

4 Conclusion

The power supply design of electronic watt-hour meters should reasonably select components and devices to improve product reliability and reduce costs; the host should choose low-voltage power supply and low-power circuits to reduce the power consumption of the whole machine; in addition, the reasonable arrangement of components, devices and PCB wiring is extremely important to improve the EMC performance of the watt-hour meter.