Method for accurately measuring electric energy meter after power supply line of shunt in electric energy meter is disconnected

This is a special metering design in the meter chip. In the smart meter, the metering part of the circuit usually uses the shunt as the current sampling, but due to transportation or other external forces, the line that supplies the shunt may be accidentally disconnected. A design scheme is provided to solve the problem that the meter can still be accurately metered when the power supply line of the shunt is accidentally disconnected.

1 Introduction

The electric energy meter is a meter for measuring electric energy. The metering principle is: by sampling the current and voltage on the electric wire of the user, and then by multiplying with the time, the electric energy usage of the household is obtained. In current sampling, the use of shunts is the most commonly used sampling method on electronic energy meters, covering almost all domestic household electronic energy meters [1].

Figure 1 shows the partial circuit of the energy meter using the shunt as current sampling [2]. In normal use, the power supply line is connected to the mains, L and N are connected to the transformer, thus powering the entire meter, the metering chip Ground, transformer a, the b of the shunt and the live line are the same potential, that is, the fire ground. The current sampling component is a shunt, and the shunt has three leads, which are a power supply line, a sampling line +, and a sampling line-. The power supply line is connected to the b-end of the shunt, and is connected to the mains hot line L. The sampling line + is connected to the n-end of the shunt, the sampling line is connected to the m end of the shunt, and the m-point and the n-point have a certain resistance, generally 200~ A defined shunt resistance between 300 μΩ. In Figure 1, the two resistors R1 and R2 and their connected capacitors C1 and C2 form an RC filter circuit (the resistance is the same, generally about 1 kΩ).

When the user uses electricity, the current passes through the m and n points of the shunt. Because when the current passes, a certain voltage difference is generated at the m and n points. By sampling the voltage difference between m and n, Find the current through this, this is the method of sampling the current in the energy meter. After sampling the current, the signal is sent to the metering chip, and the metering chip multiplies the sampled voltage value and time to complete the calculation of the electrical energy.

2 Metering problem caused by disconnection of the power supply line of the shunt in the metering circuit

In the actual production and transportation process, due to welding and environmental stress changes, disconnection occurs between two points on the power supply line of the shunt, as shown in Figure 2. Although this situation is not common, once disconnected, the power supply loop of the entire meter will change. The supply current flows from the transformer, the current sampling part and the current sampling input pin, and generates about 1 to several at the current sampling input. Volt common mode voltage (related to AC supply current, R1 resistance, chip internal circuit, etc.), while existing energy metering chips do not have a common mode voltage input suppression circuit, and existing meters generally fly away (domestic, In foreign countries, the existing single-phase electric meters of various brands and models are used to generate large disputes between the power settlement department and the power users, which has a great negative impact on the power settlement department and the meter manufacturing enterprises.

After actual testing, the existing meters of the single-phase metering chip of Ruinengwei, Juquan Optoelectronics, Belling and ADI have generally had this problem. Even if some meters do not fly away under this fault, it is because the meter has detected the abnormally large current in this fault (such as the meter with a range of 60 A measuring more than 100 A), and the software is shielded. The metering output, the result of this treatment is correct only when the meter is unloaded, and the metered electric energy is underestimated when the load is carried.

3 Current sampling circuit scheme with detection function

3.1 Current sampling circuit design with detection function

The design of Figure 3 actually adds fault detection circuitry and fault handling circuitry to Figure 1. The fault detection circuit is composed of a fault detecting resistor R2+, R2-, and a fault voltage measuring circuit that is added inside the metering chip (or externally). R2- is connected between the sampling line - and AGND, and R2+ is connected between the sampling line + and AGND. Generally, the same resistance value is used, and the fault voltage measuring circuit is connected to the c terminal in the above figure.

The resistors R1+, R1- and k1, k2 and their control sections form a fault handling circuit. It does not work when it is normal, and it starts work after a power supply line failure occurs.

One end of the resistor R1- is connected to the sampling line-, the other end is connected to the drain of the MOS switch K1, the source of K1 is connected to AGND, the end of the resistor R1+ is connected to the sampling line +, the other end is connected to the drain of the MOS switch K2, and the source of K2 is connected to AGND. The gates of K1 and K2 are connected together and controlled by the master MCU. R1-, R1+ also generally use the same resistance value, which can reduce the influence of the different pressure drop of the sampling line + and the sampling line - itself.

The values ​​of R2+ and R2- are generally more than 100,000 times the shunt resistor value to avoid shunting the load current through the shunt. Calculated according to the shunt 300μΩ, R2+ and R2- should be above 30Ω.

The apparent power consumption of a single-phase meter during normal operation is generally between 0.6 VA and 10 VA (standard upper limit). According to the power consumption, the current flowing through R2+ and R2- after the fault occurs can be calculated (apparent power consumption/2R2- ), the c-terminal voltage is more than tens of mV, causing the measurement error to exceed several times the standard. In order to ensure that the measurement accuracy meets the standard, it is necessary to start a fault handling circuit that does not work normally.

The R1+, R1- resistance values ​​need to be less than one tenth of the R2+ and R2-resistance values. When K1 and K2 are turned on, it is equivalent to R1- and R2- in parallel, and R1+ is connected in parallel with R2+. Selecting the appropriate R1+ and R1- resistance values ​​can make the c-terminal voltage within the allowable common-mode input voltage range of the metering chip. The measurement accuracy after the fault occurs is less than the 1% error allowed in the primary table.

3.2 Fault detection method

After a fault occurs, when the power supply line is disconnected, it is equivalent to disconnection between a and b. The external input AC power (between L and N) is supplied through the transformer and R2+ and R2-, and the voltage sampling circuit (circuit C) is used to detect R2- The voltage VR2rms, when VR2rms>>threshold Verr (several mV), and the duration is greater than N1 (at least 2) power frequency cycles or more, is judged as a break line fault.

After determining that the fault has occurred, open the MOS switches K1 and K2 through the MCU or the control port of the metering chip, so that the AC supply current is reduced by R1+, R1-, VR2rms, and the common-mode input voltage is within the allowable input range of the metering chip. The influence of the measurement part of the error has dropped significantly. Selecting the appropriate R1+, R1- resistance values ​​allows the measurement error to be within the allowable range (typically less than 1%).

If you need to achieve higher metering accuracy after this fault occurs, you can also do software compensation, that is, write a compensation value to the current channel gain or power bias register of the metering chip.

If the resistance value of R1+ and R1- is small, the load current is shunted, then the current channel gain of the metering chip needs to be adjusted, and the error of the rated voltage and rated current after the fault is adjusted to less than 0.1%; if it is under the rated voltage and rated current The error is 0, and after the fault occurs and the error ERR is -0.4% after starting the fault processing circuit, the compensation coefficient can be calculated as -ERR/(1+ERR)=0.004, the current channel gain is: 0.004×2N, the metering chip For RN8209, N=15 (the different metering chip current channel gain register bits are different, N is different), and it is 0X83 after calculation. If the current channel gain is originally zero, then only 0X83 needs to be written to the current channel gain register of the RN8209 metering chip.

If the R1+ and R1- resistance values ​​are slightly larger, the load current is shunted negligible, and the error is required to compensate for the small load current. The power offset register is generally adjusted. Put the calibrated electric meter in normal working state into the fault state, clear the creeping threshold, read the active power value P0 measured by the measuring unit, and convert P0 into power offset when only the rated voltage is not applied. The register value Pr0 is set, and Pr0 is added to the power offset register Poffset1 in the normal operation condition to obtain Poffset2 after the fault occurs, and is written into the power offset register of the metering chip.

3.3 Failure Recovery Detection

Detects the VR2rms voltage. When the time is longer than 100 ms and the time is less than the fault recovery threshold Vnor (can be set according to the actual situation), it is judged that the fault is recovered, the switch K1 is turned off, the current channel gain is restored, or the power offset register value is restored to the normal value Poffset1. .

3.4 Resistance R1 and R2 resistance selection

According to the power range of the whole table, the current flowing through the resistors R2+, R2-, R1+, and R1- after the power supply line is disconnected can be calculated. According to the sampling input voltage input range, the maximum allowable value of R2+ and R2- can be obtained. The general metering chip allows the input voltage to be 660 mV to 1 V, and the allowable value of R2+ and R2- is about 30-100 Ω. According to the error can not exceed 1%, the value of R1+ and R1- can be obtained as several ohms.

4 Design changes

In the embodiment, the meter metering chip adopts the RN8209 of Ruinengwei. When using the conventional circuit, the AC supply current is about 5 mA when the AC 220 V is powered. When the power supply line is disconnected, the meter is in a fault state. When the load is not applied, the meter measures the load current by more than 100 A, and the meter flies away.

(1) After R2+ and R2- are 100Ω, R1+ and R1- are 2.4Ω, and the voltage measurement circuit and MOS tube and control circuit on R2- are applied to the meter, the measured VR2rms is 250 mV (100×5/2). ) Left and right, it can be determined that the meter is in the state of the power supply line; the meter error is about -4% at 220 V 5 A.

(2) Control the MOS transistor conduction through the master MCU, so that R1+ and R1- are added to the power supply circuit, and VR2rms is measured to be close to 6 mV when not amplified. The meter error is measured with a standard meter. The data is shown in Table 1. The error at each point meets the requirements of the Class 1 table.

(3) Calculate the current channel gain compensation value according to the error of -0.43% at 5A, then adjust the meter chip current channel gain register to re-measure the meter error. The data is shown in Table 2.

5 Conclusion

In this paper, a method for accurate metering of the energy meter after the power supply line of the shunt is disconnected in the electric energy meter is given. The rationality of the design is demonstrated and the improved measurement results are given. The results show that through such design improvement, the idea of ​​troubleshooting when there is a huge error in meter metering is proposed, and the reliability of meter design is enhanced, which is of great practical significance for the design of smart meter.