Distribution Substation

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Main Question or Discussion Point

I'm an undergraduate studying physics & I live in a place that is about 15m away from a 33kva distribution substation,i already know the transformer work based on the principle of faraday law of EM induction,the feeder pillar distributes electricity in units,the fuses in the feeder pillar protects against too much power,the arrester protect against lightning by grounding charge,what about the fuse that connects the HV cable to the transformer?i was told by a technician that it is called "J&P"fuse,is it true?what is the function of these fuse with respect to the substation?i want to know the functions and ratings of every electrical equipment associated with a typical distribution substation,I'm just curious.thanks
 

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  • #2
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It's backup protection in case the feeder protection fails. It is also the protection for the transformer (or other equipment behind the feeder fuses) in case a problem occurs with it: snake on the transformer conductors, internal fault, loss of insulating oil, lightning arrestor failure (shorts the power line to ground), potential transformer failure, etc.

The fuses on the high side are designed so that they "operate" more slowly than the fuses on the feeders (This is also called coordinating the fuse time / current curves). The power company wants to make sure that the minimum number of customers are out of power if a problem occurs, so they would much prefer one feeder be de-energized than the whole transformer.

Hope this helps! If you have any other questions, or I wasn't clear enough just let me know....my job is substation protection and controls, so I'm familiar with the subject. :-)
 
  • #3
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Thanks alot,it helps,but you didn't mention the name of the fuse or is it truly 'j&p fuse' as mentioned by the technician?.In addition to that,what's the difference between star connection and delta connection?,where does the neutral phase originate from in a transformer and what role does it play?
 
  • #4
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We use many different fuse brands in our company, so "J&P" may be one, but it's not a slang term that I've heard for a fuse. More modern substations don't use fuses, but instead have PCBs (Power Circuit Breakers), circuit switchers, and transrupters. They can operate without having to be replaced like a fuse would be.

As far as Delta and Wye ( I'm assuming that's what you meant by star) connected transformers, there are many different reasons for using one or the other...and there is probably a thread on here where someone more qualified than me can explain the exact reasons why you choose one or the other.

As far as the neutral goes, our distribution transformers are Delta high side and Wye-grounded low side. The neutral (common of the 3 windings) of the Wye is connected to the copper ground field buried underneath the substation. The effect of this is that if there is a fault on the distribution line (say a tree falls on the line), the current from the line will flow through the tree and return to the source (complete the circuit). This path is through the ground back to the grounded neutral of the transformer.

Because of this current flowing up the neutral of the transformer, there has to be an equal current flowing on the high side to match it. Because of a property of delta connections, this current is "trapped" inside the delta and simply circulates without traveling back onto the transmission line that feeds the substation. The official term is that a delta will trap zero sequence current, which is what the current flowing up the neutral is. You can get a better understanding of that by studying sequence networks.

Hope that helps a little!
 
  • #5
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We use many different fuse brands in our company, so "J&P" may be one, but it's not a slang term that I've heard for a fuse. More modern substations don't use fuses, but instead have PCBs (Power Circuit Breakers), circuit switchers, and transrupters. They can operate without having to be replaced like a fuse would be.

As far as Delta and Wye ( I'm assuming that's what you meant by star) connected transformers, there are many different reasons for using one or the other...and there is probably a thread on here where someone more qualified than me can explain the exact reasons why you choose one or the other.

As far as the neutral goes, our distribution transformers are Delta high side and Wye-grounded low side. The neutral (common of the 3 windings) of the Wye is connected to the copper ground field buried underneath the substation. The effect of this is that if there is a fault on the distribution line (say a tree falls on the line), the current from the line will flow through the tree and return to the source (complete the circuit). This path is through the ground back to the grounded neutral of the transformer.

Because of this current flowing up the neutral of the transformer, there has to be an equal current flowing on the high side to match it. Because of a property of delta connections, this current is "trapped" inside the delta and simply circulates without traveling back onto the transmission line that feeds the substation. The official term is that a delta will trap zero sequence current, which is what the current flowing up the neutral is. You can get a better understanding of that by studying sequence networks.

Hope that helps a little!
it did,thanks
 
  • #6
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Using a Clamp-on meter, what is the importance of the recorded currents from the upriser cable?is there a normal current that you compare with the recorded current?what current level triggers load shedding?
 
  • #7
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I'm not sure what you mean by the upriser cable, but there are current measurements being taken all over the substation.

The substation has current transformers (CTs) and potential transformers (PTs) to monitor power flow in and out of the substation for metering purposes as well as for line protection. They want to know how many amps are on each feeder leaving, and they should add up to equal the current entering the substation. If there is more current entering than leaving, then there is protection that will activate. There is also protection for if there is too much current on the feeders, which is set based on the available fault current for that substation ( there are calculations to figure that out).

As far as load shedding goes, it depends on the substation and what is causing the problem. If the grid wants more vars than the generators and capacitor banks can supply, then the voltage will drop and the system operators or protective equipment may drop load to reduce the var requirement.
This can be a problem in the summer when air conditioners are heavily used, as electric motors draw vars as part of their operation. If a lot of air conditioners and other motors turn on at the same time, it can drop the area voltage and cause problems.

There is also load shedding for frequency. If the grid frequency drops, utilities can have plans to drop specific load to keep the frequency at 50/60Hz. An example would dropping 40% of non-essential load if the frequency drops to 59.5Hz, and it increases if the frequency is lower. I don't know the specific numbers for my utility, and they probably wouldn't want me to post them online even if I did know.

Hope this helps.

-Aaron
 
  • #9
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Aaron, thanks for the 1st class explanations. And Bobbywhy, your link is fantastic.
 
  • #10
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substation maintanane & installation

hi guys
i am an electrical engineer ... need to know about the substation installation and maintenance. i want to know the process . now i got a new job in substation . can you please help me to know about the substation , its components , and is process. need to know about top to bottom
 
  • #11
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help me to know about substation ...
 
  • #12
davenn
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hi guys
i am an electrical engineer ... need to know about the substation installation and maintenance. i want to know the process . now i got a new job in substation . can you please help me to know about the substation , its components , and is process. need to know about top to bottom
help me to know about substation ...
since you are new on the job.... you will get lots of on the job and classroom training and that should teach you more than what most people on here would know.
once you have finished your training you may be able to teach all of us how it works :)

Dave
 
  • #13
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thanks dave
but i am not sure about the classroom training . can you please teach me the basics of substation
 
  • #14
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Hello Tyson - in this forum there is no way we can "teach me the basics of substation". If you are an EE - graduated from 4 Yr University, than I would look to work with a NETA (http://www.netaworld.org/ ) - contractor. They should FIRST teach you some safety - and then you will learn by doing. You will learn about every part of a substation - should only take 3-5 years to get all of the names right, 5-10 to be able work on your own, then hopefully after about 10 years they will be asking you for advice. However - after 10+ Years - you will be VERY employable across a wide spectrum of the industry. From Engineering/PLanning- Manufacturing, Application Engineering, Field Installation and Service. Everyone in the Utility respects 10+ Years of field work.
 
  • #15
psparky
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thanks dave
but i am not sure about the classroom training . can you please teach me the basics of substation
I can brief you about a factory substation example:

The substation will have main switch, a transformer (maybe 12,000 volts down to 480 volts), a main breaker (1600 amps for example), then typically bus bar that goes to large breakers, maybe 15 breakers rated between 400 to 800 amps. These breakers will then typically feed 400 amp 480 volt three phase panels, motor control centers, large HVAC chillers, 250 HP motors...just to name a few. They come in cubes, this substation would have roughly 6 cubes involved, stacked left to right. All these parts are "shockingly" expensive.

The primary substation in a factory is basically the same except even more expensive because of the super high voltage. In this case, maybe you have 138KV coming off the power lines, down to 12KV thru its transformer. Then it has maybe the 15 breaker compartments again that feed the factory substation above at 12KV. Actually feeds all the factory substations. Some of these factories are acres and acres....so a 12 KV feeder can be ran a couple thousand feet (also relatively small and cheap) with very little voltage drop...then to the factory substation (down to 480 volt)...then to loads.

The 138KV lines coming into the factory (primary substation) could be as small as 2 AWG or 35mm2. The cable coming off the breakers at the factory substation are typically huge at 500 MCM and often multiple 500 MCMs. Thats the beauty of high to low voltage. Small (cheap) wires along long runs with very little voltage drop.....then low voltage and big amps for small runs (expensive cables, but relatively short runs).

Working around 480 volts requires a certain certification, 2,400 another class higher, 12 KV another class higher...etc....give or take country to country. Electrical arcs off a 12 volt battery are actually kind of scary. An electrical arc off of 2,400 volts is enough to burn ALL the skin off your body and blow you thru a wall. So if you are not certified, you should not be within 50 feet of anything high voltage to be on the safe side. Do not go near this stuff especially when an electrician asks you to give him a hand......that's when people get badly burned or dead. When an electrician is opening a breaker panel....also be no where near this area....as far away as possible. A faulty high voltage breaker can make a huge explosion (arc) burning and killing everything near by. Big arc explosions also shoot projectiles like bullets. Serious stuff.
 
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  • #16
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thanks

what is the significance of using CT/PT?
 
  • #17
psparky
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thanks

what is the significance of using CT/PT?
Direct quote from mtb856 above:


"The substation has current transformers (CTs) and potential transformers (PTs) to monitor power flow in and out of the substation for metering purposes as well as for line protection. They want to know how many amps are on each feeder leaving, and they should add up to equal the current entering the substation. If there is more current entering than leaving, then there is protection that will activate. There is also protection for if there is too much current on the feeders, which is set based on the available fault current for that substation ( there are calculations to figure that out). "
 
  • #18
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thanks a lot psparky
 
  • #19
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what is ring main unit (RMU) ... what is its purpose and uses ?
 
  • #20
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Hello Again Tyson

- actually the CT/PT are used for measurement and protection - but their Purpose is to allow the instruments to operate at low voltage, isolated from the primary conductors, as well as to scale the voltages and currents to a proportional lower level. A 2000A Feeder is difficult to measure - but by using a 2000:5 CT - now 5A in the secondary represents 2000A in the Primary.

THE RMU - is the main connection of a RING circuit to the utility feed - the benefit of a ring is reliability, each point in a circuit has 2 available supplies, so that a failure at some point in the ring all of the other points still have a supply. So the RMU usually has one Feed from the utility and then 2 outgoing feeders.

Most singular terms like this can be researched on line -
 
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