USA converting back to Direct Current

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Energy-Miser

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dereckbc said:
Don, High voltage direct current (HVDC) is used to transmit large amounts of power over long or short distances or for interconnections between asynchronous grids like TX. DC is way to expensive to regulate and change voltages which would require very expensive electronic converters, invertiers, and rectifiers. The conversion is also be very inefficient, and prone to failures.

Look at it another way from the POCO plant. The 1200 or 2400 VAC out of the generator would have to stepped up via transformer, rectified, transmitted to distribution, then at distribution converted back to AC, stepped down in voltage, rectified again to dc arriving at you interconnection, be converted to AC again, and stepped down to a usable lower voltage. Keeping it AC all the way is simple and cheap with transformers.
I don't think you need to go through all the conversions between AC and DC anymore. Certainly you will not be able to use magnetic core transformers with DC, but you can step DC up or down, with power electronics. I will be more expensive, however the tradeoff is that tansmission in HVDC is much less lossy compared to AC. e/m.
 

dereckbc

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Energy-Miser said:
I don't think you need to go through all the conversions between AC and DC anymore.
You most certainly do to step up in voltage. Only way to step up DC is convert to AC, step up in voltage via transformer, then rectify back to DC. One trick you can use is convert to very high frequency AC just like a switch mode rectifier does so you can use much smaller transformers in the converter.

Now you can step down DC voltage with linear solid state voltage regulator, but what a huge loss in power that is. For example let?s say the distribution voltage is 16 KV and user is say 400 volts. Now with a solid state voltage regulator to step down means you have the same current on the input as the output. Now do a little math. 16,000 volts - 400 volts = 15,600 volts dropped across your regulator with say 100-amps of current flowing. Care to guess how much power is being burned up by that solid state regulator is and how big that thing is? Try 1.56 MW. Also keep in mind the input and output current is the same with DC. So here you are drawing 100 amps current and your neighbor shares the same line and he draws 100 amps. Does not take many customers before that line melts down from all that current.

Energy-Miser said:
Certainly you will not be able to use magnetic core transformers with DC, but you can step DC up or down, with power electronics. I will be more expensive, however the tradeoff is that tansmission in HVDC is much less lossy compared to AC. e/m.
The only place where DC has economic advantage is between transmission and distribution sub-stations. It is not practical, economical, or useful for distribution, for the reasons I stated above
 

George Stolz

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I thought that, historically speaking, the whole reason AC won out over DC in the days of Westinghouse and Edison was because AC could travel long distances without a problem unlike DC.

Edison had a power plant to cover extremely small areas, unlike today's plants that cover millions of square miles apiece...?
 

Energy-Miser

Senior Member
georgestolz said:
I thought that, historically speaking, the whole reason AC won out over DC in the days of Westinghouse and Edison was because AC could travel long distances without a problem unlike DC.

Edison had a power plant to cover extremely small areas, unlike today's plants that cover millions of square miles apiece...?
I think the reason AC won over DC back then was, mainly two things: one is that power generation via mechanical motion (rotation), is going to naturally be sinusoidal in waveform, and you can leave it like that and not worry about things like rectifiers or commutators, etc., making it a simpler and cheaper technology. The second big reason is the ease with which sinusoidal AC (or any other waveform of high enough frequency for that matter) can be stepped up / down using a transformer. Of course high voltage is necessary for long distance transmission efficiency, and this gave AC an advantage, because again, it was easy to boost it up, transmit it and then bring it back down at the distribution end. I think the inefficiency of DC back in those days may have stemmed from the simple fact that it was not easy to attain very high votage DC, so transmission over long distances remained a problem. e/m
 
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No Show

Member
I think some of you may have missed the perspective of my question, I was not really asking about HVDC or even distribution although I learned alot from your imput and it will be apart of what happens in the future. I was asking more along the lines of 600V DC and down. Maybe less than 100V. Starting with more like a home owner who gets a renewable energy system on his home or smal buisness, (which is how I see this stating out)and he has solar, wind, thermal,and a few other ways of generting DC current under 600v his home is 5 star rated with high efficiency appliances so that he is self efficiant and the entire system is on or around his home or small buisness so that there is no reason to ever step any thing up or convert it he would just use low voltage (600v-0v DC) for everything. No transformers, rectifiers, converters just generate DC and use DC.
 

dereckbc

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No Show said:
I was asking more along the lines of 600V DC and down. Maybe less than 100V. Starting with more like a home owner who gets a renewable energy system on his home or smal buisness, (which is how I see this stating out)and he has solar, wind, thermal,and a few other ways of generting DC current under 600v his home is 5 star rated with high efficiency appliances so that he is self efficiant and the entire system is on or around his home or small buisness so that there is no reason to ever step any thing up or convert it he would just use low voltage (600v-0v DC) for everything. No transformers, rectifiers, converters just generate DC and use DC.
I think maybe you are missing something, the renewables you mentioned are not all DC. Wind is AC, no way around that, it can be converted at the turbine with diodes however. What you might also be overlooking even if you do have something like Solar PV, you still have to be connected to the grid. Solar PV systems cannot power high-wattage items like dryers, HVAC, ovens, your blow dryer, etc. Well I guess you could if you are foolish enough to spend 6 or 7 figures on the system and had a acre or two to devote to it.
 

tom baker

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A question came up on why we don't use DC for traffic signal cabinets, as most signal heads are DC leds. The answer was there is a significant corrosion issue with DC, and there is a voltage drop issue that you don't have with AC.
 

dereckbc

Moderator
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tom baker said:
A question came up on why we don't use DC for traffic signal cabinets, as most signal heads are DC leds. The answer was there is a significant corrosion issue with DC, and there is a voltage drop issue that you don't have with AC.
Bingo Tom, I was just going to bring up the corrosion issue. Would be a nightmare for homeowners. Imagine sleeping in bed and hearing a hissing sound. Upon investgation you discover a water pipe leak in your foundation or walls. Or throw a switch and it burst into flames.:cool:

Tom however I will say the VD problem is more pronounced with AC because of the added reactance in addition to resistance. Could it be the lower voltage natue of traffic light bulb reuirements? I can then see VD as a problem as with the Telecom industry using 48 and 24 VDC systems. We have to upsize conductors to compensate for VD.
 
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No Show

Member
dereckbc said:
I think maybe you are missing something, the renewables you mentioned are not all DC. Wind is AC, no way around that, it can be converted at the turbine with diodes however. What you might also be overlooking even if you do have something like Solar PV, you still have to be connected to the grid. Solar PV systems cannot power high-wattage items like dryers, HVAC, ovens, your blow dryer, etc. Well I guess you could if you are foolish enough to spend 6 or 7 figures on the system and had a acre or two to devote to it.
You do not "have to be connected to the grid" you could have a battery back up system.(ups)With the advances in battery technology it may not have to be all that big either. And remember I am talking about a house or buisness that is 5 star rated, you do not have the demand on the heatig or cooling or the rest of your appliances like your traditional home would and as for the turbines, do your home work look at the ones that tie in with the pv systems.
 

Energy-Miser

Senior Member
dereckbc said:
Y... Now with a solid state voltage regulator to step down means you have the same current on the input as the output. ...
I don't think that this is an accurate statement. e/m.
 

Energy-Miser

Senior Member
tom baker said:
A question came up on why we don't use DC for traffic signal cabinets, as most signal heads are DC leds. The answer was there is a significant corrosion issue with DC, and there is a voltage drop issue that you don't have with AC.
Can somone elaborate on this please (the corrosion issue)? I have never heard of it. The closest thing that I have seen is that they actually put a small DC potential on long distnace metal gas pipes to prevent or slow down corrosion. The voltage drop is probably a function of low operating voltage of these lights and not AC/DC issue. Anytime you tranmit through conductors at a low voltage, you incure higher voltage drops due to higher required current. e/m.
 
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Energy-Miser

Senior Member
dereckbc said:
... I will say the VD problem is more pronounced with AC because of the added reactance in addition to resistance. Could it be the lower voltage natue of traffic light bulb reuirements? I can then see VD as a problem as with the Telecom industry using 48 and 24 VDC systems. We have to upsize conductors to compensate for VD.
Other than the added reactance, AC faces slightly higher ohmic resistance as well due to the skin effect. This is more relevant for higher frequency signals, but even at 60 hz it still produces measurable change in the effective resistance of the conductor. The reason for higher votage drop at 48 and 24 volt DC is that at low voltage you need to pump higher currents through to transmit the needed power (compared to when operating at a higher voltge). This higher current is the reason for the higher voltage drop (IR), and not the fact that you are using DC. e/m
 

dereckbc

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Energy-Miser said:
I don't think that this is an accurate statement. e/m.
It is 100% accurate. A series voltage regulator is nothing more than a electronic controled varible series resistor.
 

dereckbc

Moderator
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Energy-Miser said:
Can somone elaborate on this please (the corrosion issue)? I have never heard of it. The closest thing that I have seen is that they actually put a small DC potential on long distnace metal gas pipes to prevent or slow down corrosion. The voltage drop is probably a function of low operating voltage of these lights and not AC/DC issue. Anytime you tranmit through conductors at a low voltage, you incure higher voltage drops due to higher required current. e/m.
It is called electrolysis, open the hood of your car and look at the battery terminals for an example.

What you are referring too in pipe lines is called Cathodic Protection system. Cathodic protection (CP) is a technique to control the corrosion of a metal surface by making that surface the cathode of an electrochemical cell.
It is a method used to protect metal structures from corrosion. Cathodic protection systems are most commonly used to protect steel, water/fuel pipelines and storage tanks; steel pier piles, ships, offshore oil platforms and onshore oil well casings.
Reverse the polarity and the pipes or whatever is being protected will be gone in short order. Telephone companies used the same thing when the decided to reference the positive polarity to ground. The reason was the lead sheathed cables leaving the central office. If they grounded the negative polarity the lead sheath would be gone within a year. With a positive reference the cable actually grow more lead sheath.
It is the same principle as electro-plating. You take a carbon or copper cathode and a gold anode, submerge it, run a DC current and you get a copper clad gold cathode.
 

Energy-Miser

Senior Member
dereckbc said:
It is called electrolysis, open the hood of your car and look at the battery terminals for an example.

What you are referring too in pipe lines is called Cathodic Protection system. Cathodic protection (CP) is a technique to control the corrosion of a metal surface by making that surface the cathode of an electrochemical cell.
It is a method used to protect metal structures from corrosion. Cathodic protection systems are most commonly used to protect steel, water/fuel pipelines and storage tanks; steel pier piles, ships, offshore oil platforms and onshore oil well casings.
Reverse the polarity and the pipes or whatever is being protected will be gone in short order. Telephone companies used the same thing when the decided to reference the positive polarity to ground. The reason was the lead sheathed cables leaving the central office. If they grounded the negative polarity the lead sheath would be gone within a year. With a positive reference the cable actually grow more lead sheath.
It is the same principle as electro-plating. You take a carbon or copper cathode and a gold anode, submerge it, run a DC current and you get a copper clad gold cathode.
Thanks yes cathodic protection, I could not think of it for the life of me. Very intersting use of electricity. Thanks, e/m.
 

Energy-Miser

Senior Member
dereckbc said:
It is 100% accurate. A series voltage regulator is nothing more than a electronic controled varible series resistor.
I was referring to the statement that input and output currents have to have the same mangnitude. They may be in a specific case, but do not have to, depending on the load. e/m.
 

dereckbc

Moderator
Staff member
Energy-Miser said:
I was referring to the statement that input and output currents have to have the same mangnitude. They may be in a specific case, but do not have to, depending on the load. e/m.
That is exactly what I am referring too. So let me clarify what we are talking about; A DC-DC converter stepping down in voltage using solid-state electronics. All those devices are serial or series circuits meaning Current In = Current Out, well actually the input current is a little higher than output to power the control electronics plus source the load current. There is just no way around the physics.

This is exactly why switch mode rectifiers and converters were conceived to get around the inefficiency of linear DC power supplies and rectifiers or DC>AC>DC. Switch mode also has some other benefits such as size and weight, but efficiency is the main driving force. Therein lays the magic of AC because it is very easy, economical, and efficient to step-up or step-down voltages using a very simple transformer.
 

George Stolz

Moderator
Staff member
dereckbc said:
What you might also be overlooking even if you do have something like Solar PV, you still have to be connected to the grid. Solar PV systems cannot power high-wattage items like dryers, HVAC, ovens, your blow dryer, etc. Well I guess you could if you are foolish enough to spend 6 or 7 figures on the system and had a acre or two to devote to it.
Well, yes and no, IMO. Provided a decent battery storage and a strong inverter, and efficient appliances, the everyday comforts can be attained without a grid connection.

I wired a house up in the mountains, miles from grid power, where they had no other choice with their desired site. I don't know if the system reached six figures, but I know it wasn't cheap.

Despite more and more appliances available for 12VDC for RV use, the designer of the system (the homeowner) opted to stay with a 120/240VAC system throughout the house, with a scant three 12VDC receptacles for certain specific appliances.

I felt the system would have been more sound if they had left the lighting DC, circumventing the inverter - but in retrospect, I am glad I didn't have to think about the voltage drop that would have been present with only 12 volts to begin with. There were enough headaches in that house without adding to it (it was also a SIP wall home).

When it came to running the larger motor loads, they "made hay when the sun was shining", literally: The water well pump only ran when he went to the basement and manually turned it on, to pump into the cistern, on sunny days. He also relies on a propane generator that the inverter can activate or he can manually as well.

I remember clucking my tongue at the dryer, but I don't remember what it was. It was not particularly efficient, IIRC, it was more of a nod to the extravagance of the other appliances of the home.
 

dereckbc

Moderator
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georgestolz said:
I wired a house up in the mountains, miles from grid power, where they had no other choice with their desired site. I don't know if the system reached six figures, but I know it wasn't cheap.
George that is kind of my whole point about Solar PV, it is a niche application where either grid power is not available or prohibitively expensive to bring in.

I have designed and installed about 6 of the systems in remote cell radio sites in TX and NM. These systems only powered a 250 watt load, (radio operating continuously 24x7x365). There had to be enough panels to be able to supply the load and charge the batteries in 3-hours to be able to carry through the remaining 21 hours. The system has to be designed for worst case which happens to be short winter days. All the systems ran around $90K, and there was no inverter involved because the radio operates on 24 VDC. The batteries alone are $30K and that does not include the installation. Also a LP generator had to be used for those cloudy days.

So for this small application it took the same size or number of panels being touted for a full size home of 3 KW in panel output during peak sun.
 
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