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Wrong Phase Sequence

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The three phases of an AC source are depending on country named as

  • A - B - C
  • L1 - L2 -L3
  • R - Y - B
  • U - V - W (typically for transformers)
  • a - b - c
  • R - S - T

For this post we are using A - B -C with the phase colours red, yellow, blue.
Most utilities are operating with a A - B - C sequence, some are operating with A - C - B sequence. It's only a matter of power grid definition and has historical background.

So we can't say that the phase sequence is wrong, we can only check whether the phase sequence is matching with the country/utility regulations. IEC and IEEE standards are referring to A -B -C as the right phase sequence. So we are using this also for our animation.

What is the impact of a wrong phase sequence?

  • Direct to the AC connected three-phase motors will turn in the opposite direction
  • Certain types of protection relays will trip
  • Electromechanical meters are measuring wrong
  • Connecting a additional power source will lead to equipment damage

How to detect?

For verification of transformer- and metering installation we recommend to use a portable device, such as our standard RS350. For permanent grid monitoring the best choice is a CLOU three-phase energy meter.

electronic three-phase meter

The measurement circuit of CLOU three-phase energy meters constantly monitors transition through zero on all phase voltages. When the zero- crossings do not follow correct order (A, B, C), the measurement circuit detects an incorrect phase sequence. This information is sampled every 200 ms and averaged over the time period given in the OBIS code below.

If measurement circuit detects incorrect phase sequence for longer than one time period, the meter records wrong phase sequence condition as event. The same algorithm also applies to detection of correct phase sequence, which means that correct phase sequence must also be present for longer then defined period in order to detect it.

DLMS has defined a OBIS code for storing the waiting time until the event is recorded.

Wrong phase sequence, time threshold128.7.90.255

A reversed phase sequence is also indicated on the energy meter display. If the meter is connected to an AMI-system, the event can be monitored by the supervisor or operator.


Welcome to the Year of the Rat 2020

The Year of the Pig is coming to an End and we are welcoming the Year of the Rat. As usual our company has rented the Shenzhen sports stadium for the annual Spring Festival Gala and Outstanding Staff Award Ceremony.
Colleagues from CLOU subsidiaries all over China have been present for this event. The audience was more than 1,100 people. This year we've provided a live stream for all those who had not been able to attend.

CLOU New Year Gala 2020

Some impressions

All show acts have been performed by CLOU staff. The preparations started around three month ago. The colleagues have spend a lot of their free-time to create this amazing event.


Thanks for watching and Happy New Year. We'll be back after the break 🙂
If you like you can leave some Lunar New Year greetings in the comment box below.

transient voltage

What are Transient Voltages?

This post deals only with the aspects related to energy metering.

Transient voltages are short time surges in the a.c. power supply. They typically last a few milliseconds and the voltage can reach several thousand volt. A transient is by definition shorter than half cycle.

transient voltage
voltage transient

Causes for transients

There are two major groups of transient voltages.

Power system switching transients

Power system switching transients can be separated into transients associated with:

  • major power system switching disturbances, like capacitor bank switching
  • minor local switching activity or load changes in the power distribution system
  • resonating thyristors circuits
  • short circuits
  • arcing faults to the grounding system

Lightning transients

  • direct lightning stroke
  • induction due to an indirect lightning stroke
  • nearby lightning-to-earth discharge

Energy meters can not detect a voltage transient. The event is too short. Most power quality analyzers are designed to capture surges in voltage.
The energy meters are protected to surge effects according to IEC61000-4-5.
For certain regions additional external arresters can be added during meter installation.