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.

Auto-Recloser in feeders

Introduction: Electricity is transferred on higher voltage for long distances. Transmission / distribution feeders pass through forests, hills, fields etc befor reaching destination. 90% of the faults in feeders are transient in nature. These are caused by lightning, birds, falling trees, Forest growth, swinging of conductor, High velocity winds etc. Transient faults disappear after a short interval of time. In this manner, If line is tripped, 90% probability is that line will charge immediatly without any rectification required. For reducing the outage time, auto-reclose is used. Which means automatic closing of circuit breaker (CB) after small gap of time, called dead time. Minimum dead time is selected such that ionized air due to arc is de-ionized within dead time. It is generally between 0.5 s to 1.0 s.

Different approaches in auto recose: Auto reclose selection has four options:

1. No Auto reclose: Auto reclose disabled, no auto reclose will be attempted.
2. 1 phase auto reclose: 1-ph auto reclose will be attempted for 1-ph faults. If the fault is 2-ph or 3-ph, 3-ph trip will be initiated and no auto reclose will be attempted. It is used without synchronism check as remaining two phases will be connected from both ends. Typical time for auto-reclose attempt (Dead time) is 1 sec.
3. 1+3ph auto reclose: 1-ph auto reclosed will be attempted for 1-ph faults and 3-ph auto reclose will be attmpted for 2-ph or 3-ph faults. 
4. 3 ph auto reclose: 3-ph auto reclose will be attmpted for 1-ph, 2-ph and 3-ph faults 

Multi-cycle 3-ph auto reclose: In some cases multiple attampt may be required for auto reclose. First cycle is fast one having dead time less than 1 minute. remaing cycles can have dead time upto 1 hour. It is mostly used on radial type of feeders.

Circuit Breaker requirements: CB is key equipment during auto-recloser. Its operating mechanism should have sufficient energy for simultaneous Open- Close -Open operation to take care of persistant fault during auto-reclose. When CB is ready for this operation, CB ready signal is provided to Auto-recloser relay. AR will happen only in case of CB ready signal.

Another requirement is arc quenching capability of interrupting chamber. In case of SF6 CBs, SF6 gas retains its dielectric properties after ~25 sec. Therefore, after autoreclose operation, next auto reclose can happen only after 25 sec. This delay is called reclaim time. Any fault during reclaim time will lead to 3-ph trip and no AR attampt will be taken.