Part 2: Electrical Engineering Queries Asked by Our Valuable Students

Q1. How will we calculate short circuit current in a specific area instead of converting all given circuit in an equivalent circuit and finally applying Ohms Law on it?

Just remove all the circuits after the point at which you want to calculate the short circuit current and just attach Zload to it. Then solve it using a per-unit system and apply Ohms Law in the end.

Q2. If in LV (Low Voltage) side any one phase grounded, so what will be the voltage & current in HV (High Voltage) & LV side in delta star 11 transformer.

LV Side:
When a line to ground fault occurs on one line on the LV side then all current will flow from the faulty line. Other two lines will have no current passing through them. Similarly, the line which is faulted will have zero volts on it while having rated voltage on other two lines.

HV Side:
Transformer will feed its maximum fault current which will eventually saturate the transformer with a slight decrease in the rated HV voltage.

Q3. In the window type current transformer, how does the primary conductor passing through CT, make it ‘one turn’.

In literal terms, the conductor passing through the window type CT does not have any ‘turns’. However, the term ‘turns’ is extensively used in electrical engineering literature to refer to the number of times the current must travel through a similarly shaped part of the circuit. See the image in the link below which shows a solenoid, where there are 6 ‘turns’ — or say ‘identical portions’ of the circuit, so in a circuit where there are no turns, there will only be one ‘identical portion’ for the current to pass through.

https://circuitdigest.com/sites/default/files/field/image/What-is-Solenoid.png

The term ‘turns’ has been adopted for use in several equations, one of the most famous being the transformation ratio of a transformer. To keep consistency with the existing literature, a single straight conductor is called having ‘one turn’.

Q4. How to bank one 167kva 3phase 280V transformer in 4wires?

A single 3-phase transformer can be connected in Wye configuration to get a 4-wire system, 3 of the wires will be for each of the 3 phases, while the 4th wire will be used for a neutral connection.

Q5. If a burden of a CT is reduced, then what happens on primary current Ip after some time?

A Burden is an estimation of load connected on the secondary side of CT (measured in VA). Ideally, a CT should reflect the current on secondary side as accurately as possible from the current flowing in the primary side.
However, this does not happen so because of the presence of magnetization current and other factors that generates an error in the reflection of current on secondary side. Nonetheless, an effort is made to make this error minimal. CT measurement is close to maximum accuracy when the actual CT burden is equal to the rated CT burden.
The CT burden influences the accuracy of the CT measured value on the secondary side. Higher burden selection against the required burden results in CT measurement inaccuracy. For instance, if a CT designed for 15 VA burden is operated for 5 VA, there will be a higher error in its measurement even when the CT is operating at its rated current.
As to answer your question, the primary current of the CT is independent of the CT burden on the secondary side. Since the primary current depends on the load current flowing through the line.

Q6. Can power flow in a delta connected power line if two phases are lost (Open)?

It depends on which two HV Lines are lost. You will have to trace through the transformer connections to see if there are any HV windings that are energized. If there is any HV energized winding, then you can expect power to flow into the LV (Low Voltage) winding.

Q7. How choose the type of curve of relays?

To balance the right amount of overcurrent protection against optimal machine operation, we need different trip curves. Selecting a circuit breaker with a trip curve that trips too soon can result in nuisance tripping. Choosing a circuit breaker that trips too late can result in catastrophic damage to machine and cables.

The X axis represents a multiple of the operating current of the circuit breaker and The Y axis represents the tripping time.

The three major components of the Trip Curve are:

1. Thermal Trip Curve. This is the trip curve for the bi-metallic strip, which is designed for slower overcurrent to allow for in rush/startup, as described above.
2. Magnetic Trip Curve. This is the trip curve for the coil or solenoid. It is designed to react quickly to large overcurrent, such as a short circuit condition.
3. The Ideal Trip Curve. This curve shows what the desired trip curve for the bi-metallic strip is. Because of the organic nature of the bi-metallic strip, and changing ambient conditions, it is difficult to precisely predict the exact tripping point.

Trip curves represent the predicted behavior of a circuit breaker in ambient room temperature. This means when the bimetallic strip is within the specified ambient operating temperature for the breaker. If the breaker has experienced a recent thermal trip and has not cooled down to the ambient temperature, it may trip sooner.

Trip Curves predict the behavior of circuit protection devices in both slower, smaller overcurrent conditions, and larger, faster over current conditions. Choosing the correct trip curve for your application provides reliable circuit protection, while limiting nuisance or false trips.

Q8. In a 220/132kV Three phase combined winding (Single Equipment) trafo, one of the LV sides fails so what will be the effect to the lv current of remaining phases? Also, will the trafo can be operated with 2phases in LV side?

Perspective, it receives abnormal voltages and currents from two phases but little (or even zero) from the phase that was lost on the HV side.

Q9. What is Zs in a system where fault level calculations are done. Input value, Zs = V/Ifault.

We can define Zs as the maximum impedance due to a single line to ground Earth Fault (also called Earth Fault Loop Impedance). For the maximum fault current, I(fault) and nominal system voltage V, Zs can be calculated by the formula Zs = V/Ifault. Zs is calculated and used in Fault Analysis in power systems. Also, it can be calculated from the time-current curve of the protective device during protection coordination.

Q10. At (D/Y) transformer, If short circuit in one phase ia (earth fault) at secondary of transformer about 2000 amp, what the effect on primary (D)?

Typically for any an earth fault that would occur on the LV (Wye) side of the transformer, the impedance reduces and the current on the secondary side will increase causing the current in the primary side (Delta) of the transformer to be increased due to the cascaded effect and the current feeding through the upstream devices will also increase. The overcurrent protection devices will come into play to mitigate the effect of the fault.

In case you require a more specific response you can choose to specify the size in KVA and %Z of the transformer as well.