dbNet Transmission

Transmission and Distribution

(06 Aug 2010) Summary

Partial de-regulation has had significant effect on the grid, described in "The US Electrical Grid: Will it be Our Undoing?" by Gail E. Tverberg at The Oil Drum.
- Declining investment. Responsibilities among participants are not clear. Each participant minimizes investment in the shared grid, to maximize its own profitability.
- Overuse of lines between systems. Widespread trading of electricity among partners has led to use of lines between utilities beyond design of the lines.
- More rapid deterioration. As electricity is traded among utilities, plants are cycled on and off resulting in repeated heating and cooling and consequent accelerated deterioration of metal parts.
- Un-planned additions to the grid. Wind, solar, and merchant natural gas plants are added to the grid without sufficient grid capacity.
- Difficulty in assigning costs. Addition to the grid must ultimately be paid for by consumers. Consumer rates are regulated, resulting in difficulty recovering costs.
- Increased line congestion. Difficulty in placement of new lines due to NIMBY, resulting in increasing congestion on existing lines.
- No overall plan. Many players, difficulty in recovering cost has resulted in little action.
- Little incentive to add generating capacity. Ability to trade electricity has resulted in lowered incentive to add capacity. This is economically efficient but results in periodic technical failure.
Transmission line capacity is a bottleneck to development and use of increased electrical energy. Power generation is frequently located far from consumption. Transmission facility age may be a problem however.
Transmission facilities are often not available to sites advantageous to renewable-source power generation. For example, whereas the Pickens Plan has withdrawn its plan for a large wind farm in Texas, the TANC project in California has planned a several-hundred mile transmission facility to a site with no current power generation.

Bing Search "Power Line Transmission Loss"

AC and DC Transmission

Most power distribution is done over an AC grid. Most electricity is generated from primary energy sources using steam turbine generators, and an AC grid couples efficiently and cheaply with AC power sources. Furthermore, AC power can be manipulated cheaply and efficiently to minimize transmission losses.

Power loss in a conductor is P = I² * R where R is the resistance of the conductor. Power loss is proportional to the square of current I, so lower current will be more efficient. P = I² * R = I * V, so to transmit power P, current I can be minimized by increasing voltage V.

AC voltage is cheaply and efficiently amplified by a transformer. So the combination of AC power generation with cheap and efficient voltage conversion for transmission, favors distribution of AC power. However, many load devices run on DC so conversion to DC at the load adds back some inefficiency.

Solar PV sources generate DC, and an additional inversion step from DC to AC is needed when used with an AC transmission system, adding incremental inefficiency of 15% - 25% for high-power conversion.

Article on AC and DC transmission systems.

DC Distribution

High-voltage DC systems are also in use for transmission. Voltage amplification for these systems is more expensive than transformers used for AC, but the tradeoff between source and load conversion efficiency vs. transmission efficiency gives them advantages depending on configuration. Comparisons are shown in the analysis below.

Equipment inside telcos runs of 48v DC, and standards and equipment exist today to support this. Telco equipment standards, and limited economies of scale result in higher costs than consumer equipment but breadth of use could mitigate this. Here's a thread discussing some history and tradeoffs.
Intel research on DC Datacenter Power Distribution shows elimination of multiple power conversions, resulting in lower power loss.
Micro-grids used to transmit PV-generated DC power.
- US Department of Energy
- Varentec Smart utility "DC Transformer" (North Andover, MA, USA)
- DC Power Systems
- Go Solar Company
- Solatron Technologies
- RenewableEnergyFocus
- The Solar Guide
- Micro Grid Bing search

Photovoltaic power, DC Grid, Electric Vehicles?

Since PV solar cells generate DC, and electric vehicles run on DC and much load equipment runs on DC, would it be possible to distribute and use DC service to increase power efficiency with EVs and other DC equipment?

Proposal including DC Grid, by EPRI and DC Interconnect, Inc.
...and here's a short article.

AC Scheme

DC Scheme

Power Generation

AC Primary Steam Generation: Coal, Natural Gas, Nuclear, Solar Thermal, Solar Tower. Steam conversion of coal and natural gas results in about 66% loss of BTU between commodity and electricity generated.

AC Direct Generation: Hydroelectric, Wind turbine.

Power Generation

DC Direct Generation: Solar Photovoltaic, Concentrated.

Availability limited to daytime hours, and variable availability due to weather and season.

AC Grid Distribution

Voltage is amplified to reduce current. Current is reduced in order to minimize line IR losses.

AC generation sources use passive transformers.

DC generation sources require a DC-AC inversion and step-up (about 10% loss).

About 7% Transmission Loss

DC Grid Distribution

Voltage is amplified to reduce current. Current is reduced in order to minimize line IR losses.

AC grid distribution system is already ubiquitous so grid distribution from AC sources on a DC grid is not considered here.

DC generation sources require voltage step-up (about 10% loss)

About 7% Transmission Loss

AC Local Distribution

AC local distribution from AC grid requires transformer voltage stepdown to local (120, 220v AC etc.).

AC local distribution is already ubiquitous so local distribution of AC from a DC grid is not considered here.

DC Local Distribution

DC local distribution from DC grid requires voltage stepdown (likely 48v DC). (about 10% loss)

DC local distribution from AC grid requires AC-DC conversion to local (likely 48v DC). (about 10% loss)

AC Local Usage

AC is used directly for motors, pumps etc. Motor efficiency is not considered here beyond power distribution.

DC Local Usage

Home/office/industrial electronic equipment, computers/servers, televisions, lighting etc. need regulation to operating voltage (about 10% loss). If local distribution is AC, then AC-DC conversion is also required.

Electric Vehicle battery requires charger regulation (about 10% loss). If local distribution is AC, then AC-DC conversion is also required. (As noted elsewhere on this site, generation capacity, grid capacity, and grid scheduling and management all need to increase proportionately with EV deployment.) Motor efficiency is not considered here beyond power distribution.

Estimated comparison of source-to-use loss shows how conversions build up loss. High-power inverter and step-up/down competitive with transformers would make a key difference. Efficiency of load device is not incorporated. Click figure for spreadsheet.

Util = Utility transformer step-up/down. HPR = High-power Inverter/regulator, step-up/down. Trans = Transmission Line Loss. LPR = Low-Power inverter/regulator, step-down. AC/DC Use = power input terminals to usage device. Min/Max Loss = accumulation of range of losses through the delivery chain.