Unbalanced loading in three phase system

Thread Starter

TheKapilesh

Joined Aug 27, 2017
1
1. If a three phase system is loaded in unbalanced condition and if it is fed through RCCB, will the RCCB trip? How does the electrical power company ensure all three phases are loaded equally (with some tolerance)?
2. In a three phase star connected system the unbalanced current will flow through the neutral. if neutral is earthed then will this current trigger the earth fault relay?
3. What will happen to the unbalanced delta connected system if the load is unbalanced? where will the unbalanced current flow?

Pls help out
 

MaxHeadRoom

Joined Jul 18, 2013
19,198
1/ Why would it? The RCCB requires current in the Earth conductor to trip.
The power company cannot ensure that the resultant customer loads are equal.
The RCCB montors the relative neutral current to the total 3 phase current.
2/NO, again current has to flow in the earth conductor itself.
3/Taken phase-to-phase, each pair of phases will have unequal currents.

Max.
 

Attachments

Last edited:

MaxHeadRoom

Joined Jul 18, 2013
19,198
Per Group Schneider:

ELCB’s contain a core-balance transformer and work on the following simple principal: If the current which goes out on the phase does not return on the neutral, or on another phase, in the event of a three phase type, it will be detected as an earth fault through an imbalance in the core balance transformer and this imbalance will activate a tripping coil so that the circuit will be disconnected. Therefore, if one comes in contact with a live appliance, the current, which flows through this person to return to earth, is sufficient to trip a 30mA RCD instantly.
Max.
 

MrAl

Joined Jun 17, 2014
6,646
1. If a three phase system is loaded in unbalanced condition and if it is fed through RCCB, will the RCCB trip? How does the electrical power company ensure all three phases are loaded equally (with some tolerance)?
2. In a three phase star connected system the unbalanced current will flow through the neutral. if neutral is earthed then will this current trigger the earth fault relay?
3. What will happen to the unbalanced delta connected system if the load is unbalanced? where will the unbalanced current flow?

Pls help out
Hi,

Some of these questions could be answered by yourself using circuit analysis ideas, or even in a simulator. You could simulate a three phase system and change the load to whatever you wanted it to be, then observe the current in the different phases and if there is a neutral in that line too. You'll get some interesting results.

Since you are asking about only differences in balanced and unbalanced systems, it could be that resistive loads are good enough to use to answer your questions. These kinds of systems are easier to analyze than when we have capacitors and inductors included. We could even probably work up a few simple equations that would show what happens when the system goes from balanced to unbalanced.
 

Janis59

Joined Aug 21, 2017
819
May tell the sad real life situation with unbalancing. My mother`s home had been electrified by 127 V `delta` at beginning of 1900ies, and then in 50ies shifted to Y of 220 just digging parallel to main cable one naked (non-insulated) earth wire, then in 70ies system was shifted to standard 380/220 until at the middle of 80ies that nude wire corroded one night and became unexisting. Now the 8-flat house became self-balancing. We was happy, mum in that moment was ironing something, thus we got maybe 50 Volts instead of 220. Our neighbours had not so much a luck, they watched a TV, therefore 300 W against 2200 W won we, they received maybe 310 Volts and if to remember that TV in those years cost about 3 or 4 monthly salaries.... oh my, how they was angry to electric company. Later I measured in the null wire against real good earth digged in flowerbed - it was variating between 50 and 300 Volts according who was switching on what.
 
Top