Why potential transformers are installed before current transformer in a structure of switching yard?

Reason for installing potential transformers at the very first instrument of a switching yard? Why shouldn't we place a current transformer before potential transformers? Do they effect the measurement?|||No they dont affect the measurement, the placement does not matter. In every arrangement Ive seen, it is the opposite, the first device is an air disconnector, then a circuit breaker, which usually has the CTs built in. The voltage transformers Ive seen are nearly always on the bus bar after this.



Sometimes, control signals are modulated onto the transmission line at audio or RF frequencies, to remotely control equipment. often, the device to recieve/detect this signal is built into the potential transformer. If our CTs and circuit breaker were together, we'd want to make sure that even when the breaker was open we could still recieve these signals! But Like I said in all cases Ive seen, the voltage transformer is connected to the bus, and then in/out going conenctions to transmission lines to the bus have CT on them.



current transformers dont produce any significant voltage drop in this case.|||In electrical engineering, a current transformer (CT) is used for measurement of electric currents. Current transformers, together with voltage transformers (VT) (potential transformers (PT)), are known as instrument transformers. When current in a circuit is too high to directly apply to measuring instruments, a current transformer produces a reduced current accurately proportional to the current in the circuit, which can be conveniently connected to measuring and recording instruments. A current transformer also isolates the measuring instruments from what may be very high voltage in the monitored circuit. Current transformers are commonly used in metering and protective relays in the electrical power industry. Current transformers induce voltage drops and measurements of voltage (potential) should be taken before them.|||VT placement in a switchyard is primarily a matter of economics.





Voltage is the same at all points in a switchyard, so a single VT can be used to provide all of the voltages that are needed for both measurement and protection. Placing the VT on the bus means that one set of VTs is sufficient for the entire installation.





By contrast, CTs measure the current in a circuit, and so it is necessary to place a CT in each phase of each circuit.





That said - - - there are exceptions. The most obvious is where the electrical configuration of the switchyard can be segmented and operated as separate systems - a ring bus for example. In that case, it may be necessary to have multiple sets of VTs to assure that there is always a set of VTs in each active portion of the system. Another situation is where the physical construction makes installation of bus-connected VTs either inconvenient or expensive. This might be the case in a gas-insulated substation.





There are also exceptions where the location of the VT does affect the measurement. A classic example of this is a ring bus where lines are connected directly to each bus segment and where VTs are usually connected to the bus segment. The problem is with a protective relay looking into a line. The relay may be unable to 'see' a close-in fault on the line at the instant that a ring-bus breaker closes to energize the bus segment and line because the voltage that it receives from the VTs on the bus segment will be zero. On the other hand, if the relay were to be powered from a VT that is located 'behind' the relay, it will receive a finite voltage until the instant that the breaker closes, at which point the voltage will collapse to zero.