Current Transformer switching (CT swithing)

CT switching is used in different protection schemes where protection boundaries can change with the operation of isolators (dis-connectors). 

In protection schemes, having differential protections, protection boundaries are defined by the locations of CTs. Let us look into the Double Bus Scheme, which is used widely upto 400kV levels. 

Every Feeder is having three isolators, one for Bus-1, second for Bus-2 and third for Feeder. In this case any feeder can be connected to either Bus-1 or Bus-2 by changing the position of isolators.

In the diagram above, Feeder-1 and Feeder-3 are connected to Bus-1. Feeder-2 is connected to Bus-2. Now, let us come to bus bar protection. Bus bar protection is differential protection, which calculates the vector sum of all the feeders terminating on a bus. In above figure differential current for Bus-1 shall be vector sum of current measured by CT1 + CT3 + CT4. Differential current for Bus-2 shall be vector sum of CT2 + CT4.

In the image above all the feeders are connected to Bus-1, bus coupler is kept out. Now in above figure differential current for Bus-1 shall be vector sum of current measured by CT1 + CT2 + CT3. Differential current for Bus-2 shall be zero as no feeder is connected to it.

In this way we can see that with the change in isolator positions of primary circuits, secondary circuits of CTs are required to be changed, so that the bus bar relay could calculate the differential current correctly. For this we need CT switching relay, which would change (switch) the secondary CT circuits depending on primary CT circuits.

Understanding disturbance record files (COMTRADE files)

The Disturbace Record (DR) files are used to analyse faults in power systems. DR continuosly record Analogue Signals (voltages, currents etc,) and Digital signals (Status of CB, protection operation etc.). As the memory of disturbance recorders is limited, the recorded data is flushed out continuosly on First-in-First-Out basis, so that the data for last few seconds (typically 1-5 sec) is always in the memory of DR. When they any fault occurs the Disturbance Recorder receives command to save the data in its memory. DR  will record pre-fault and post-fault data as per settings already done in DR. Typical values for feeder DR may be Total fault duration = 2.0 sec, Pre-fault time = 0.5 sec, Post-fault time = 1.5 sec.

The disturbance record can be extracted sebsequently from PC for further analysing. The standard file format for saving this data is COMTRADE file format which has following files with same name and different extensions:

1. Configuration file:  *.cfg extension 
2. Data file: *.dat extension 
3. Header file:  *.hdr extension
4. Information file: *.inf extension

Header and Information files are optional and disturbance record extracted from some device does not contain one or both of these files. Only CFG and DAT files are required for evaluation of record in evaluation software.

Sample screenshot of DR file in WAVEWIN (one of the DR evaluation software) :-

In the abobe image, A-phase to ground fault can be seen with VA (A-phase voltage) going down and increase in IA (A-phase current). This has caused Distance trip of A-phase and opening of A-phase CB, which can be seen in digital channels in bottom of image. The A-phase would have been closed by auto reclose relay after dead time (typically 1.0 sec), but after ~240ms there is fault in B-phase to ground with VB going down and increase in IB. Now remaining two phases are tripped, because of fault within dead time. (Click here for more about Auto reclose)

The same data can also be viewed in vector form :-

 

Normal condition

Faulty condition (A - phase to Ground fault)

CFG file can be opened in any DR evaluation software like WAVEWIN, SIGRA etc. All the screenshots here are given for Wavewin software as it is provided alongwith ABB and GE make relay softwares.

In the above image A-phase voltage has been taken as reference signal. Any analogue signal can be set as reference by single clicking on it. After single clicking it is selected and shown in red colour.

Wavewin has many advance features like 

1. Fault location calculation: Click here for detailed procedure
2. Creating software analogue channels from one or more available analogue channels by putting some calculations like vector sum / difference of some channels. Mutiply / divide any channel by some constant etc.
3. Merging two or more DR files manually or automatically by time stamps.