Transmission And Distribution Engineering

[Leroy Turcios]

TheTransmission & Distribution Engineering Committee (TDEC) is a group of volunteer engineers, operational and materials managers from electric cooperatives, NRECA and engineering consultants who work with electric co-ops.

TDEC assists RUS on the development, analysis and updating of federal technical standards, guidelines and specifications. TDEC also monitors and reviews other engineering and operational standards, code changes and new designs involving the transmission-and-distribution and supply-chain-management fields, so as to maintain the competitiveness of electric cooperatives in today's changing environment.

At Electric Power Engineers, LLC (EPE), we are passionate about delivering cutting- edge solutions for integrated transmission and distribution (T&D) planning and operations. With a steadfast commitment to innovation, reliability, and sustainability, we empower utilities to navigate the evolving energy landscape and seize the opportunities presented by distributed energy resources (DERs).

SEAMLESS INTEGRATION OF DERs

Our solutions enable seamless integration of

DERs, promoting renewable energy adoption,

reducing dependency on traditional sources,

and enhancing sustainability. In parallel,

our transmission planning expertise ensures

the efficient transmission of power, supporting

the integration of DERs at the utility scale.

IMPROVED OUTAGE MANAGEMENT

With enhanced visibility, predictive analytics,

and DER capabilities, you gain greater

control over outages, minimizing their impact

and accelerating restoration efforts. Our

transmission planning services help identify

potential bottlenecks and enhance system

reliability, reducing the likelihood of outages.

EPE provided engineering support with the interconnection, registration, testing, and commissioning of the 230 MW Horse Creek Wind project with the Electric Reliability Council of Texas (ERCOT). EPE also performed a Sub-Synchronous Oscillation (SSO) analysis to determine if the proposed 230 MW Horse Creek Wind project could become radially connected to a series compensated transmission line under specific contingency conditions and in turn experience SSO disturbances causing risk to the project or to the ERCOT grid.

Leidos has extensive experience in the consulting, project management, engineering, design, procurement, construction management, and commissioning of supervisory control and data acquisition (SCADA) systems for electrical investor-owned and municipal transmission, distribution, and generation utilities and industrial customers.

Leidos applies our in-depth knowledge and experience with equipment in generation, transmission, substations, and distribution to help our clients design and implement protection and controls schemes for more reliable systems. We assist our clients in understanding the variables in developing protection system designs and relay settings within the constraints of their systems.

Power systems are prime targets for planned and coordinated physical and cyber attacks, and utilities need to be prepared to protect the grid. The best protection comes from an appropriate blend of risk management analysis, grid design, security technology implementation, and facility hardening. Leidos offers robust experience in programs and procedures that achieve full compliance while anticipating and deterring evolving threats.

We provide lifecycle services that span consulting, outage sequencing and planning through environmental and siting programs to engineering, procurement and construction. In addition, we provide engineering services to meet the demands of modern delivery systems, including state-of-the-art, extra high-voltage, alternating current and direct current technologies.

Our technical personnel have extensive backgrounds in system planning, transmission, distribution and substation engineering, with specific experience in feasibility studies, conceptual and detailed design, rehabilitation of existing systems, and the evaluation of advanced concepts in design and utilization. Our construction and project management teams have overcome challenges to successfully build transmission lines, substations, and SCADA systems in locations ranging from cramped urban environments to the mountains of the desert southwest.

Our transmission and distribution team is active in realizing a 21st century energy grid economy that is founded on comprehensive utilization of renewable energy, distributed generation, and demand-side participation. We help fulfill the promise of a sustainable future through lowering the carbon emissions of the power sector.

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 to enable transformation to higher voltages in order to drop line losses (\$P=IE=I^2R\$) and if the conductor size remains the same, when \$E\$ is increased in the equation \$E=IR\$ then \$I\$ must necessarily decrease, in turn decreasing losses as the square of \$I\$). But now we have the ability to transform AC (at all thermal, hydro and wind generators) and DC (at solar generators) to any level of DC we desire and transmit, usually to residential or commercial loads which tend to use DC anyway. If need be it can be converted back to AC at industrial loads (motors usually).

There are several reasons. One: power loss in a wire is I^2 * R. Therefore it is better to transmit power at very high voltage and low current. AC is much more easily boosted to high voltage (no electronics are needed). To boost industrial loads using silicon electronics is not practical.

Another is ease of switching under load. If you turn off a load connected to DC, the arcing at the switch due to wire inductance and load inductance becomes problematic. This forces DC switches to be more robust.

The 60 Hz noise created by transformers is much less than the switching noise that would be created by all the electronics required to buck and boost DC and then convert it to AC at point of load as you propose.

HVDC is used: List of HVDC project. The two dominant technologies used for HVDC (thyristors and IGBT's) weren't invented until 1950 and 1968 respectively. In the mean time, countries were building AC transmission equipment.Why replace something that works when you've already spent a lot of money building a grid? Just wait until the existing system is no longer workable, and upgrade then.

The data appears to justify this: China is building a large number of HVDC transmission lines because they have money, and don't really have any existing network to interact/compete against. Similarly, there are projects in Europe and the Americas, but these appear to be more limited to areas where HVDC really shine (underwater systems) because there are existing networks so the cost of upgrading isn't justified yet.

Mkeith has answered the question as asked, i.e. what are the main disadvantages of HVDC distribution. A "counter-answer" to that by helloworld922 (the next most-voted answer here currently) points in the direction of a bunch of cases where HVDC is/was used. All these engineers couldn't have been crazy, so I think it's important to actually explain here when HVDC makes sense. (That would have been a better question than what the OP asked, by the way.)

To start, there are some cases where AC would be almost infeasible. This includes connecting power AC grids that operate asynchronously with respect to each other, such as connecting 50 and 60 Hz systems; it happens in Japan for example: Eastern Japan uses 50Hz and Western Japan uses 60Hz. There are actually a few more niche applications where HVDC is the only reasonable choice, but they're not easy to explain to neophytes in a few words. If you want a more detailed list (with real-world examples), Delea and Casazza's Understanding Electrical Power System has a longer list.

Leaving aside such niche cases, I think it's important to emphasize that there is a total cost optimization that can (and in fact should) be performed when deciding whether AC or DC should be the transmission method for a power line. The two main factors are the cost of the line itself (cables, towers if applicable, e.g. not undersea) and the cost of the terminals. Generally, the DC transmission cables cost less than those of equivalent power for tri-phase AC. This happens for a reason that is easy to explain: you need fewer wires for DC than three-phase AC, but the insulation for the AC wires (and this may be just the air gap, but that translates into tower costs) needs to withstand the peak AC value, while you're only benefitting from transmitting "RMS power" (more correctly, average power corresponding to the RMS voltage) at AC. On the other hand, the terminating power electronics cost more for HVDC than the AC transformers, but why this happens isn't easy to summarize, none the least because the two terminating technologies are different.

This total cost optimization actually gives you the main application of HVDC today: transmitting large amounts of power over long distances (and by that meaning with no tapping/interruption). The typical values where HVDC is more economical than AC is transmitting more than 500MW over more than 500km (according to Delea and Casazza). Many (if not most) of the examples from the Wikipedia list (linked in helloworld922's answer) are of this kind. It shouldn't be a surprise than such examples are from China, Canada or Australia. In Europe, most of the medium/large HVDC transmission lines are undersea cables.

Below is what a synthetic (meaning textbook-level rather than real-world) optimization example looks like for a pre-determined power level, thus in which only the cost vs. transmission distance is plotted; it is excerpted from Kim et al. HVDC Transmission, the first chapter of which is freely available.

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