Questions tagged [power-engineering]

Power electrical engineering. Includes industrial electrical engineering (i.e. motors, switchboards), transmission and distribution (poles, wires, transformers, grids) and generation.

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34
votes
5answers
26k views

Why don't we use neutral wire for to ground devices and earth wire for closing the circuit?

I know that the neutral point of a distribution transformer is connected to the grounding (earthing) wire, so I think they are the same. Can I use the neutral wire for protecting metal-cased electric ...
13
votes
14answers
25k views

Why are the power transmission/distribution systems AC and not DC?

Is there a good reason why we are not in the process of completely converting our electrical transmission system to DC? The main reason for using AC on the grid (no offense Tesla, I love you man) was ...
12
votes
4answers
60k views

Why does a delta/wye transformer make 30 degrees phase shift ?

I'v heard that a delta/delta or Wye/Wye transformers do not make any phase shift. In delta/wye or wye/delta transformers, There is a 30 degrees phase shift between the primary and secondary coils. So, ...
71
votes
8answers
13k views

Can a bird, previously at earth potential, get electrocuted by landing on a powerline at high-enough voltage due to the initial “equalization charge”?

At the voltage levels of typical overhead transmission lines in the US, a bird can land on one and be just fine (as long as it doesn't do something like spread its wings and touch a tree or something ...
38
votes
8answers
10k views

Why do utility companies supply at a frequency of either 50/60 Hz?

It just struck me that Utility Companies world-wide supply domestic users at either 50/60Hz. It could have been selected to be 10 Hz, or 100 Hz ... whatever Was 50/60 chosen at random? Is there some ...
46
votes
11answers
23k views

Why three-phase power? Why not a higher number of phases?

Is there a reason, beyond historical reasons, that three phases has become the dominant number of phases? I am aware of the advantages against one phase and two phase, namely the reduced amount of ...
12
votes
7answers
2k views

Power grids : AC vs DC

We know that we have now 50/60Hz in our walls due to mainly historical reasons - back 100 years ago there were no ways to up/down scale DC voltage. These days we just have problems due to that - ...
8
votes
3answers
16k views

How to synchronize a generator on the electrical grid?

I've read many articles about paralleling two portable generators or synchronizing a power generator on the electrical grid. I have the theoretical background about that topic and I know the four ...
31
votes
3answers
11k views

Why 3 phase and not 1 phase power transmission?

Why does power transmission use three lines with three different phases? Why not three lines all in the same phase? Does it have to do with the alternators used for generating the power, or is there ...
8
votes
3answers
2k views

Why is the earth a viable return path for power distribution?

I'm having trouble understanding why the earth is a viable fault path for electricity brought into my home. Isn't there really high resistance? I mean, it's dirt? Also, let's say I connect the hot ...
5
votes
1answer
5k views

What is the percent impedance of a typical residential power line transformer?

Power transformers are rated by percent impedance. A 5% transformer, when loaded to full spec secondary current, will show a ~5% deviation from nominal output voltage. What is the percent impedance ...
1
vote
1answer
1k views

Why are the ground and neutral bonded in my panel?

I took a look inside my panel today and saw a big green wire connected to the neutral bus bar. Why are these two bonded together? Why not just send the neutral back where it came from? Where does the ...
22
votes
8answers
61k views

What happens to excess energy fed into the power grid?

The closest question to this is Linear useage of excess power generation. I'm no engineer so I may not be able to phrase this correctly and would appreciate an answer which assumes minimal ...
10
votes
3answers
14k views

How is transition from three-phase distribution without neutral to three-phase consumption with neutral achieved?

A typical distribution grid supplies 6 or 10 kilovolts AC to a substation near the consumers. This is typically done with a three-phase line without neutral - just three wires going in parallel. Then ...
36
votes
4answers
17k views

How do transposition towers in transmission lines work?

There're such things as transposition towers in power distribution powerlines. The idea is that for example you have three conductors running in parallel at the same height and the leftmost of them is ...
8
votes
1answer
4k views

What are these coils on transmission lines?

What are these coils? This was asked by doetoe in this physics.stackexchange Question Where I saw them (maybe six or eight pairs) they came in pairs, always on the same two lines, and they were at ...
9
votes
2answers
513 views

mains frequency accuracy worldwide

I like to build a device that needs long term time stability. In germany, one of the best simple clocks use the mains power frequency to obtain a time measure. While the frequency is allowed to change ...
4
votes
2answers
3k views

Why does frequency splitting happen when two resonant circuits are magnetically coupled in wireless power transfer?

When two circuits are tuned to a self resonant frequency and are then magnetically coupled, the resonance happens at multiple frequencies if they are near. How and why does this happen?
3
votes
1answer
9k views

What is the zero-sequence equivalent of a YNy0 transformer?

The zero sequence equivalent network of a YNy0 transformer with grounded primary is shown below (source). It looks like there is no connection between the primary and secondary side of the ...
2
votes
3answers
562 views

Isolation provided by transformers

I have a few questions about transformers and isolation. I will describe my understanding (maybe wrong) and questions about them. Transformers provide safe isolation by disconnecting two circuits ...
11
votes
3answers
16k views

What is the purpose of the thin wire at the top of power transmission lines?

I know a power transmission line carries usually two sets of 3 phase conductors. But I have noticed a relatively thin wire at the center-top of the power transmission line. What is the purpose of this ...
10
votes
3answers
10k views

Why would a power cable need copper wire shielding?

Today I witnessed burying 10 kilovolt power cable in the ground. I noted the cable mark and Googled it. The description is rather interesting. It's a single conductor cable - it takes three such ...
5
votes
2answers
189 views

Transmitting VCC and GND signals for use with RS-485

The specific question I want to ask is whether I can transmit VCC(5VDC) and GND power signals for powering the destination circuit (in an RS-485 arrangement) or not. The power lines will be grouped ...
5
votes
4answers
5k views

How does DC transmission require several times less wire than AC?

In this YouTube video (scrolled to 5:55) there's a claim that replacing an AC high voltage transmission line with a high voltage DC transmission line allows for having seven times fewer powerlines (...
2
votes
3answers
681 views

Electrical System Harmonics

I'm a mechanical engineer so I don't know much about electric circuits and power generation to begin with. Can somebody explain what a harmonic is in the power grid and what is the reason why it is ...
0
votes
1answer
4k views

Synchronising Power & Synchronising Coefficient

I am really confused with the concept of synchronising power and synchronising coefficient. I was taught 2 simple formulae in class. P = 3VEsin(del)/X Sp = dP/d(del) = 3VEcos(del)/X Now, I am ...
0
votes
2answers
132 views

Harmonics in power system [closed]

I have seen in some text books that power converters produce harmonics in power systems. How do power converters inject harmonics in a power system?
18
votes
3answers
45k views

Why does reactive power affect voltage?

Why does reactive power influence the voltage? Suppose you have a (weak) power system with a large reactive load. If you suddenly disconnect the load, you would experience a peak in the voltage. Is ...
31
votes
6answers
13k views

What is the speed of “electricity”?

In a distribution grid, what is the speed of energy signal Is it strictly the same? And how is synchronisation achieved? Is an electric grid essentially an orchestra where everyone is the chief ...
35
votes
4answers
1k views

If we could start our electrical grid from scratch with today's technology, which would be the most efficient choice? AC or DC?

Lately, I’ve been reading about the many advantages of HVDC transmission systems for long distance transmission, undersea links, and others. The historical reason of why AC was picked over DC was ...
20
votes
6answers
13k views

How does earthing work when earth is dry?

Earthing is meant to provide reliable contact of an electric appliance to earth so that if there's an insulation fault current goes into earth instead of through a person's body. This requires ...
22
votes
3answers
3k views

What is this device attached to a high-voltage power line?

I noticed this white, cylindrical device on a power pole near Seattle that facilitates a 90-degree turn of the transmission line. It looks like the lightning conductor comes down the pole into a sort ...
9
votes
3answers
12k views

How would an electric company detect wireless theft of electricity?

As legend goes it is maybe possible to steal electricity from power lines in a wireless way (also see this and this related questions). The legend continues that once someone does that he gets caught ...
3
votes
3answers
4k views

Understanding ground current

I am trying to understand ground current in a power distribution system. The basics, of course, are simple: in the power grid, the "hot" wires carry high voltage, while the return path (the neutral ...
17
votes
5answers
3k views

Why do generators have to rotate at a slower frequency if demand outpaces the supply?

EU residents might have noticed that some of the clocks in their house have been running out of sync with other clocks. Apparently, the cause for this is that power suppliers had to reduce the ...
8
votes
1answer
8k views

Comparison between AC and DC power flow analyses

Consider a power flow problem. Injected real and reactive power at a bus are functions of voltage magnitudes and voltage angles and are given by \begin{align*} P_i = \sum_{k=1}^N|V_i||V_k|(G_{ik}\cos(...
5
votes
5answers
3k views

On a transmission line, why are voltage and current waves reflected at a short circuit?

I do not intuitively understand this. A short circuit simply means there's no impedance. If there's much less impedance than the transmission line, the waves should simply move through the short-...
4
votes
2answers
611 views

How does a residential grid-tied solar system work exactly?

Ok.. so I know how solar panels are connected to a combiner box & tied into a charge controller/inverter. I also know that the AC from the inverter must be in phase with the line/mains power. ...
6
votes
3answers
533 views

Producers consumers balance in the grid

In a large distribution grid, it is said that consumers and producers must be in balance; what that means physically, in correct (not simplistic) mathematical formulation, is not clear: perfect ...
4
votes
1answer
718 views

How is solar power consumed before grid power

This one has been on my mind for a while now... I recently installed rooftop solar panels with micro-inverters. While I was informed I would be using the solar power first, and any remaining needs ...
3
votes
4answers
534 views

Is there a minimum amount of electricity that can be fed back into the grid? [closed]

This is not fully related to electrical engineering, but I have been reading lately about Feed-in-Tariffs and solar energy production and I cannot find any information on the amount of energy that can ...
3
votes
2answers
417 views

What is the effect of heavy loads on the electrical grid?

When our neighbors turns on a motor, a bulb in our home turns off for a very very short time and then it turns on again. It is something like a quick flash but a man eye can notice it. Would you tell ...
2
votes
0answers
2k views

How to calculate the earth fault current in a transmission system with no ground wire?

Background information, and my thought on the matter are provided in the end of the question. I need to calculate the fault current in case a fault occur for instance 10 km away from a transformer ...
2
votes
2answers
275 views

A confusion about radial schemes in power distribution

My lecture notes about distribution systems/radial schemes say the following: •In this scheme, feeders radiate from a substation in all directions and feed distributors at one end only. • SA is the ...
11
votes
5answers
51k views

Guessing power line voltage from the number of disc insulator

Is there a good way to guess the power line voltage based on the number of insulator discs that attach the line to the transmission tower? Wikipedia seems to suggest that there is a "Typical number ...
8
votes
3answers
9k views

How are multiple power sources synchronized in a grid that uses a distribution ring?

A large distribution grid can work like this. There're several power station each outputting 50 Hz AC. Each power station feeds energy into a substation next to it which raises the voltage and then ...
4
votes
0answers
2k views

Why DC is not used for long distance voltage transmission? (Tesla vs Edison 2015) [duplicate]

It seems this question has been marked duplicate but I believe the answers given here is much more in detail. Why we are still using AC voltage for long distance voltage transmission. As we know with ...
2
votes
1answer
730 views

Voltage drop-out after lightning strike

A powerful thunderstorm rolled through my area yesterday and something happened that I had never noticed before. Many times before when powerful thunderstorms with lightning rolled through, the AC ...
1
vote
5answers
130 views

Transformers Construction

Seeing all the transformers around me looks like a huge piece of iron. I was wondering why transformers have such a huge mass. Consider if we need 11V/ 440V voltage transformation. Why don't we have ...
1
vote
1answer
645 views

Phasor Measurement Unit (PMU) vs Synchro-check Relay

What is the difference between a Phasor Measurement Unit (PMU) and a Synchro-check relay? I need to know if Synchro-check relay can be used as replacement for PMU in a Synchrophasor data system. We ...