Voltage loss detection of electric energy meter: Does the meter know if there is a power outage?

Criteria for determining voltage loss in electric energy meters

The matter of stress loss is not too complicated to explain. The standard is already very strict: when the voltage of a certain phase is lower than 78% of the reference voltage, and the load current is greater than the starting current of the meter, and this state continues for a specified delay, it can be determined as a voltage loss event.

Pay attention to two keywords here: 78 reference voltage, 60 second standard judgment time.

This refers to triggering detection when the voltage drops below 78%, rather than being counted as zero V voltage. As long as the voltage drops below 78% of the reference voltage, the voltage triggering threshold is reached; Secondly, the instantaneous voltage drop will not be recorded, and the industry standard judgment delay is 60 seconds. Only some customized monitoring scenarios can set a short delay of 3-5 seconds.

Why do we need 60 seconds? Think about it. Without this time threshold, the power grid would experience instantaneous voltage fluctuations when high-power devices such as air conditioners and motors are switched on and off. If there is no delay filtering, the electricity meter will frequently report voltage loss and generate a large amount of invalid data, which will affect the backend operation and maintenance analysis.

Alright, let's move on from defining to hardware.

How does voltage enter the electricity meter? The 220V/380V power frequency voltage of the power grid cannot be directly connected to the metering chip of the electricity meter (the analog input of the chip is generally 0-5V), so all electricity meters will use a high resistance resistor voltage divider network to complete voltage conversion

Take the RuiNeng Micro RN8302B metrology chip as an example. This chip has three voltage sampling channels, one for phase A, one for phase B, and one for phase C. Each phase voltage is stepped down by an external voltage divider resistor, and an analog signal of 0-5V is output and sent to the on-chip ADC. The chip calculates the effective voltage value (RMS) to complete voltage monitoring.

Circuit structure: L/N input → sampling resistor R1/R2 voltage division → 0~5V conversion → input to ADC → chip processing

There is a small detail when it comes to this circuit board. The accuracy and temperature drift of the voltage divider resistor need to be closely monitored. If the resistor deviates, the collected voltage will also deviate, resulting in an inaccurate threshold of 78%. So I usually use resistors with 1% accuracy and below 25ppm for this route to ensure long-term stability and peace of mind.

Software judgment logic

After the hardware extracts the voltage, there is a threshold issue on the software——

Current voltage<78% x reference voltage, and duration>60 seconds → Determine voltage loss and record the event and report to the system backend.

Yes, it's that simple.

Similar to a smoke alarm. The smoke concentration exceeds the threshold and will continue for a while before triggering the alarm. If it weren't for this duration, the cooking fumes would have rendered the alarm useless long ago.

What are the troubles of three-phase meters compared to single-phase meters

However, three-phase meters are much more complicated than single-phase meters.

Single phase: just one voltage line, remember when it's low, simple logic

O • Three phase four wire: common for residential users, with each tube of ABC three-phase. Record the voltage loss event of the phase where the voltage is low, without interfering with each other

Three phase three wire: There are many industrial users, and any phase loss must be noted. At the same time, it is necessary to determine whether all three phases have been lost - this situation is quite serious, and it is likely that there is a problem with the higher-level circuit

Distinguish between true power outage and remote power off

During actual debugging, there is a situation that I find quite interesting - how to distinguish between "true power outage" and "remote power off".

The user owed the fee, and the power supply company remotely issued an instruction to turn off the switch. The electricity meter voluntarily cut off the voltage output. What kind of depressurization is this? Is this a normal operation. But the question is, can the electricity meter distinguish clearly? If you can't distinguish clearly, every remote trip will be recorded as a voltage loss event, and the backend data will be messy.

The method of differentiation is very simple, just look at the current.

The voltage is below the threshold, and the current is also zero - normal tripping or power failure does not count as voltage loss. The voltage is below the threshold and the current is not zero - the load is still running, but the electricity is gone. This is true voltage loss, remember.

This logic is clearly defined in DL/T 614-2007 "Multifunctional Energy Meters".

What data was recorded during the decompression event

After the voltage loss is determined by the electricity meter, the event data will be fully stored according to the DL/T 645-2007 communication protocol, and the power supply backend can read it in batches through carrier, 485, and other methods. The regular record content includes:

O • Occurrence time: Which year, month, day, hour, minute, and second, accurate to the second

End time: the moment when the voltage is restored

Duration: How long did it last in total

O • Pressure loss phase: A phase, B phase, or C phase

Accumulated times: The electrical energy indication at the time of occurrence/recovery of voltage loss.

These data can be read out through the DL/T 645-2007 communication protocol. The backend of the power supply company serves as the core basis for troubleshooting, electricity billing, and anti electricity theft inspections.

Electricity billing and anti theft application in voltage loss scenarios

How to calculate the electricity bill if it loses pressure - this is probably the most concerning issue for most people.

1. Allocation of responsibility for electricity bills.

The pressure loss record can clearly define the fault responsibility:

Power supply side faults (line maintenance, substation faults): borne by the power supply company, no electricity charges will be charged to users during the corresponding period, and voltage loss records are valid evidence;

User side faults (indoor circuit aging, short circuit, unauthorized wiring changes): The responsibility lies with the user, and normal metering and billing will be carried out.

2. Anti electricity theft inspection.

Stealing electricity is often achieved by artificially creating voltage loss and underestimating the amount of electricity: the electricity meter frequently reports voltage loss, but there are still load currents and abnormally low meter readings during the same period. By cross comparing voltage, current, and voltage loss records, suspicion of electricity theft can be quickly identified, and the accuracy of inspection is extremely high.