CT saturation

CT saturation is one of the phenomena which is important for differential protections. During saturation, core of CT gets saturated due to higher level of flux density than designed value. After this no more change of flux occurs even the primary current is still changing. 

Due to no change in flux secondary current vanishes. We may get secondary current having distorted waveform during CT saturation.

For differential protections, this may lead to differential current for out of Zone faults and relay may give unwanted trip command. Most of the manufacturers compensate CT saturation with different techniques.

 Let us see two cases with and without CT stauration

For case-1 External fault without CT saturation:

Differential current = 1 + 1 + 2 – 4 = 0 kA
Bias current = 1 + 1 + 2 + 4 = 8 kA
Idiff / IBias = 0/8 = 0.0

For case-2 External fault with CT saturation, CT for fourth feeder is staurated and measuring only 1kA current instead of actual 4kA current: 

Differential current = 1 + 1 + 2 – 1 = 3 kA
Bias current = 1 + 1 + 2 + 1 = 5 kA
Idiff / IBias = 3/5 = 0.6

For case-2 we can avoid unwanted relay operation if we keep differential slope setting higher than 0.6

* For simplicity current is considered without any DC component.

Variation in Synchronization check parameters

Introduction: In earlier post, we have learnt about various parameters related to synchronization checks in electrical power syatem. 

Click here for earlier post regarding synchronization check.

Now, we will see voltage waveforms if these parameters are varied. In all waveforms, Red and Blue are two voltages to be compared for check synchronization. Black is vector difference of Red and Blue.

1. Phase angle variation: Typical setting for phase angle difference is 15-30 deg. Two cases for 30deg and 60deg are shown below. The vector difference will be Sine of angle difference between two voltages. Therefore, for 30 deg it will be 0.5 and for 60 deg it will be 0.866

2. Voltage variation: Typical setting for voltage difference is 7.5 to 15%. Two cases for 10% and 20% are shown below. The vector difference will be same as difference between two voltages. 

3. Frequency variation: Typical setting for voltage difference is +0.2 to +0.5%. Two cases for 5% and 10% are shown below. Case for higher frequency difference are choosen for proper visualization. Showing diagram for 0.5% frequency difference will require longer time scale and it would not be clear to visualize the diagram.. 

In the image below, the vector difference is gardually inreasing and decreasing at a frequency equal to frequency difference between two voltages. The CB has to be closed when voltage difference is at lower level. Therefore, if frequency difference is low, we have more time to close the CB. In the image below (for 5%), voltage is low for ~6 cycles, which would be ~120ms for 50Hz system. It will be impossible for operator to close CB in this short duration. In actual case the check sync setting will 10 times lower i.e. 0.5%, this will give operator a time of 1200ms (1.2 sec), which is sufficient for closing of CB.