Potential Transformer: Industrial Usage & Connection

[rain2359]

hi,

May I know does anyone knows the industrial usage of the potential transformer and probably how it is connected?

Thanks.

 

[dlgoff]

At a power substation, they are used in metering applications. They are essentially a step-down transfromer where the secondary will be at, here in the US, 115volts when the high-side line is at is designed operation voltage. On a three phase system, lots of times one may only need to measure one phase.

 

[rain2359]

Hi, may I know how are they actually used in metering? Are the potential transformer used to measure the voltage carried across the transmission line? Do you know where I can find any circuit diagram to show how the potential transformer is connected in the substation?

Thank you very much.

 

[dlgoff]

"Are the potential transformer used to measure the voltage carried across the transmission line?"

Yes. And current transformers measure the current carried across the transmission lines.

My experience in substation metering is in the area of remote monitoring or SCADA. Potential transformers (PT) and current transformers (CT) secondaries are wired to transducers that provide a 4-20mA dc output that can be monitored by a remote unit which transmitts the data back to the central control office. They can also be wired to Watt and Var tranducers to measure real and reactive power.

"Do you know where I can find any circuit diagram to show how the potential transformer is connected in the substation? "

No. But if you wanted to measure the line to ground voltage of one phase, then the primary would be wired between the line and ground. With 3 PTs you could measure each line voltage.

 

[PnCTech]

They are also used with the Protective Relaying to detect faults on the power system. They are essential your eye into the system. On systems 230kV and lower, PTs are used as described by dlgoff but on systems higher than 230kV CVT's are typically used which are PTs with the use of capacitors to bring the system voltage down to a more workable level for the PT.

 

[rain2359]

Hi, may I know

1. how will the fault in the power system be detected using PT of higher than 230kV?

2. if there is a fault occurred or any disturbance that cause transient, then would the PT ( less than 230kV) be affected? if yes, how is it affected?

3. I am currently trying to run a simulation of the PT, so I am thinking of what are the industrial usage of the PT so that I can clearly define the objective of my simulation, and probably run a mini experiment on the PT. I wonder if anyone have any suggestion to give me.

Lastly, thank you very much.

 

[PnCTech]

My understanding is both the PT and CVT should represent the fault both accurately and quickly.

 

[rain2359]

how would the PT and the CVT know that there is fault? What will be shown in the PT that indicate that fault had occured.

The PT is connected in parallel with the transmission line right? Then say if at the customer end there is a fault that cause transient to the upper stream of the network, then how would it affect the PT?

Thank you.

 

[PnCTech]

The PT is used in conjunction with the CTs to measure the magnitude of voltage and current and their phase relationship with each other. The fault is detected by the inter-relationship of these three quantities. For example line protection detects faults by measuring the line impedance magnitude and angle. Typical load conditions (normal operation) will have a phase angle of about 30degrees with a high impedance value where as a fault (abnormal conditions) will have a phase angle equal to the line impedance often around 75degrees and a low impedance value.

 

[rain2359]

oh, that is how it works. Then may I know if let say the PT is used for metering purpose, but not fault detection, would the measurement of the PT be affected if there is a transient?

Does it have to be at the substation level of 230kV for fault detection? Say for the PT of 22kV/110V, could that be used as fault detection device too?

Is it possible to connect the PT of 22kv/110V together with an ammeter(acting as a CT) in the labortary and probably connect some load at the end and let the load undergo switching to create transient, so that I can try to find out if the reading would be affected by the transient?

 

[PnCTech]

The PT just steps down or up the primary voltage specified by the PTs ratio. Thats it, no more or no less. Nothing fancy about it. No different if you are detecting faults with it or metering for revenue metering or scada but maybe accuracy. Its not a complicated device to model. A more complicated model would be the fault itself or a S.C.I.M. (squirel cage induction motor). Many PhDs done on those topics. By the way what grade are you in or what University/College program are you performing this model for. Seems like a big waste of time for a PT.

 

[PnCTech]

Now a better but closely related topic would be the mechanical and electrical effects of a fault on a Power Transformer (similar to a PT but used for power transfer applications). There many configurations of Power Transformers such as: single phase bank, three phase bank, three single phase banks, step up, step down, and auto. They are all affected differently by the fault. Now that is an excellent modelling project, even for a PhD thesis. Its also a topic I would love to see the results of the models. Especially for the scenario of a fault on the secondary of a step-down distrubution transformer that has a wind generator on the secondary. Also many other configurations could be modeled such as the windgen on the primary and fault on secondary or windgen on primary fault on primary. You see these windfarms are being connected to the secondary of distribution stepdown transformers causing a reverse power flow on those connected feeders, however only auto transformers are designed for bi-directional power flow. Anyone with any more detailed information on this topic I would greatly appreciate. Thanks.

 

[rain2359]

Hi PnCTech,

Actually the objective of this project is to understand if the PT is working perfectly even under frequency other than the norminal frequency. I am just trying to find out more about the PT so that I can create a simulation such that I can see if under any transient(which would create many high frequency components) the PT will still give an accurate voltage when measuring the voltage across the transmission line. This is not a PhD project.

I do appreaciate your help, because you gave me pretty much information that I need. Thank you very much.

 

[PnCTech]

Which education level is this project for? It is a topic I would like to further my own knowledge in as well.

 

[rain2359]

It is for undergraduate final year project, B.Eng course. Basically, my school does not provide specific transformer course, however, you might want to visit the school's website for more information,