Hi folks.
There are some areas in our branch where, if we apply basic concepts, we end up with mined blowing results. Let us talk about one of such concepts.So here comes one of such concepts.
Q. there is a delta-star connected power transformer. A L-G fault occurs on the secondary of the transformer. What will be the fault sensed on the primary side?
Answer: For sure it’s not L-G. Let us see what it is.
As we all know, L-G fault is associated with 3 symmetric components which are positive, negative and zero. These 3 components pass through star winding of the transformer and enter delta. In delta winding, the zero sequence components circulates and only positive and negative get chance to pass. So the device connected on primary will sense a fault that relates positive and negative components that is L-L fault. So L-G fault on the primary of the transformer will be sensed as L-L fault on the secondary of the transformer.
The Basics: Why did the zero sequence components pass through star winding but were blocked by delta winding?
Straight forward answer is, they are in the same phase (phase difference is zero). If you carefully observe the 3 phase wave form, when 1 phase is at its maximum, the rest are at half of the negative maximum. It means one phase in incoming, and rest 2 combining acting as a return path. For zero sequence, as there is no phase difference, all the maxima will be happening at the same time instance. So they fail to flow through star connections (and line conductors). Yet in delta connection, they get a closed path in the phases (draw diagram of star and delta, you will get it). So they circulate within the phases and cause serious overheating issues.
There are some cases where zero sequence currents escape delta. Imagine a 3-phase arc furnace. You can’t assure that arc will be taking place between 3 electrodes all the time. If arc takes place between only 2 electrodes, there will be supply imbalance enabling zero sequence currents to come out of delta.
Thank you for your time and please feel free to comment on your views. You can mail me your doubts and suggestions on dnachiketa1010@gmail.com
Q. there is a delta-star connected power transformer. A L-G fault occurs on the secondary of the transformer. What will be the fault sensed on the primary side?
Answer: For sure it’s not L-G. Let us see what it is.
As we all know, L-G fault is associated with 3 symmetric components which are positive, negative and zero. These 3 components pass through star winding of the transformer and enter delta. In delta winding, the zero sequence components circulates and only positive and negative get chance to pass. So the device connected on primary will sense a fault that relates positive and negative components that is L-L fault. So L-G fault on the primary of the transformer will be sensed as L-L fault on the secondary of the transformer.
The Basics: Why did the zero sequence components pass through star winding but were blocked by delta winding?
Straight forward answer is, they are in the same phase (phase difference is zero). If you carefully observe the 3 phase wave form, when 1 phase is at its maximum, the rest are at half of the negative maximum. It means one phase in incoming, and rest 2 combining acting as a return path. For zero sequence, as there is no phase difference, all the maxima will be happening at the same time instance. So they fail to flow through star connections (and line conductors). Yet in delta connection, they get a closed path in the phases (draw diagram of star and delta, you will get it). So they circulate within the phases and cause serious overheating issues.
There are some cases where zero sequence currents escape delta. Imagine a 3-phase arc furnace. You can’t assure that arc will be taking place between 3 electrodes all the time. If arc takes place between only 2 electrodes, there will be supply imbalance enabling zero sequence currents to come out of delta.
Thank you for your time and please feel free to comment on your views. You can mail me your doubts and suggestions on dnachiketa1010@gmail.com
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