Understanding interaction between different utility transmission systems

[rockman7892]

Can anyone help explain or give a high level overview of how different utility owned transmission systems typically interact with each other and buy/sell power from each other. For example in a given area where you have utility Company A with a 230kV transmission system which they own and operate and nearby there is utility company B with a 230kV transmission system which they own and operate and then likely a common connection point between the two. In reality I believe there are many common connection points between multiple utilities or transmission owners.

My question is really how these various independently owned transmission networks interact with each other for sharing of power. For example

1) What is typically the common approach for connecting two utility owned transmission points? In a transmission switching station or is it also done at substations?

2) Where is the buying and selling of power between two utilities typically metered and monitored? At the switching station or substation?

3) Are connection points between utilities typically at the transmission level or are there sometimes connection points at the distribution level?

I live in FL so have a high level understanding that there are many utilities that operate in the state (FP&L, Duke Energy, TECO, etc...)but never quite understood how they all interact together at transmission level. Any explanations, examples, or references would be much appreciated.

 

[cranky108]

In your mind, what is the difference between a switching station, and a substation?
If we own half a line, and another company own the other half, where would the metering logically be?
Rarely are distribution systems tied.

 

[Palletjack]

http://www.psc.state.fl.us/Files/PDF/Publications/Reports/Electricgas/transmissionreport2007.pdf

https://www.eia.gov/state/maps.php

Here are a couple of links to start with.

Also go to the FRCC website. Florida is alone .

 

[davidbeach]

At the Ohm's Law/Kirchhoff's Law level there is just The System. Ownership is just a human construct that the electrons don't understand.

At a commercial level power flow proceeds as directed by "Tags" or some other accounting trick. Interchange metering occurs at the substation/switching station closest to the point of interchange. There's a whole settlement process that probably differs from place to place and market to market.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[crshears]

It depends . . .

If the two companies A and B are the only two companies on the grid, they'd negotiate ways and means to operate, generally on a split savings principle; after all, since neither wants to get screwed over by the other, the only way to proceed is on an even win-win situation. The contracts are typically negotiated with business, legal, and technical reps from both sides weighing in on the content and phraseology during normal business hours, with government/AHJ ["authority having jurisdiction"] types ensuring compliance with any pertinent laws and regulations. Once any/all documents have been signed and have come into full force and effect, operations staff are the ones who end up applying the terms in real-time on a 24/7/365 basis, with operational experience leading to tweaks and adjustments "as the way opens".

Having multiple companies involved complicates this process exponentially, and almost always involves the creation and activation of an Independent Electrical System Operator [IESO] of one stripe or another.

Now try setting things up to govern the Eastern Interconnection, which involves many, many companies, two countries and their Federal governments, dozens of American states and a handful of Canadian provinces, each with their own legislative assemblies and laws, as well as the independent-minded province of Québec and the "semi-autonomous" [at least to this Canadian's understanding] state of Texas, and things can get really interesting.

Hope this helps.




CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[rockman7892]

Thanks for the responses above. Those look like some good references to start reading up on.

@cranky108 - To answer the questions you presented.

In my mind a Substation is defined as one in which a transmission line comes into a yard and is transformed down to a lower voltage for multiple distribution circuits at distribution voltage levels. A Switching station on the other hand is on in which multiple transmission lines come into a yard with different switching configurations to connect and route transmission lines only (no distribution level). I may be off in my understanding here and appreciate any corrections and clarifications to my understanding.

In example with (2) utilities owning half a line the logical metering point would be in the middle of the line. To me in order to have metering equipment (PT's, CT's, etc..) this would have to take place in either a switching station or substation which gives me the understanding that this typically happens in a switching station where multiple utility lines connect to a common node and can be metered accordingly? Are there cases where two different utiltieis own half of a line that is between two different stations?

 

[crshears]

Metering at the half-way point? Um . . . no; each entity will measure at their end, and substantial agreement [minus losses] must be achieved. If it is not, difficult questions will be asked . . .

Four tie lines exist between Ontario, Canada and Michigan, USA; mid-point of both line and ownership for all four is above the St. Clair River. It is my understanding that the costs of any repairs in that single span only are split evenly between the two owners [Hydro One and the International Transmission Company].

OP raises good points about terminology; I am in substantive agreement with that definition of a substation, but my utility does not generally use the term.

To us, a switching station interconnects only circuits that all operate at the same voltage level [ OP did not stipulate absence of trafo's ].

Any station with transformers we call a transformer station, whether it contains only autotransformers to tie transmission circuits [ > 50 kV ] of different voltage levels together, only step-down transformers to supply distribution load [ <50 kV ], or one or more of each.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[thermionic1]

I'm on a project currently, where one utility (Gen & Dist) is connected to a neighbor utility (Gen & Dist) via a separate transmission utility line. A single revenue meter, with revenue class CT's at one end talks to the Transco RTU, Utility A & Utility B through separate com ports of the same meter to independent RTU's for each entity. At the remote end there are no revenue meters.

I've seen in the IPP world at least 2 revenue meters at the IPP end with extended range, Fiber Optic CT's at a 250MW wind farm. At a 1000MW combined cycle plant, there were 2 revenue meters per generator (2x Gas & 1x HRSG) + 2 revenue meters for the tie line. I guess the level of trust is a bit different.

 

[cranky108]

Actually there is not much difference between a switching station, and a substation, and it is common to intermix the two terms. A metering station might be what you are referring to, but again, not much of a difference. I don't know on many pure metering stations, except where one company has had bad relations with another, and that is not typically large companies. More likely interconnects with Genco's or REA's, where the general manager is a .....
Even for transformer interties, the metering will be at a convenient place, and the meret may compensate for the impedance to where the metering is desired.
Actually for our interchange we have an agreement with one company we call our BA, and they have agreements with other BA's for metering interchange. We follow the rules of our BA for boundary metering, and metering with other members of the BA.

 

[bacon4life]

My region is a bit unusual in that there are lots of small publicly owned utility districts. Utility ties exist at the distribution level, but the ties are run normally-open. Metering at these backup ties varies from nothing to full revenue grade with real-time communication.

At the transmission level, it common for point of demarcation to be a random point in the middle of a transmission line.

 

[cranky108]

Do you meter at the point of demarcation? Or at a substation?
Here, the break point is some random point, that looks good on a map. A county line, is a common point.

 

[bacon4life]

For transmission the metering is always at a substation that has CTs and PTs. Ownership changes can be any random location. I'll also note that simply specifying span of wire where ownership changes only narrows it down to the nearest 1000 feet. Sometimes the agreements go into great detail as to ownership of the tower versus the insulator versus the clamp versus the conductor. I suspect that during actual maintenance activities, crews from both utilities will show up with differing expectations. However, the crews will likely agree that the documented demarcation line was stupid.

On a distribution interconnection there may be pole mounted 15 kV instrument transformers. The distribution interconnects often look similar to a typical primary metered customer. Often there will be two separate air break disconnects so that each utility can lockout their own visible clearance point.

 

[crshears]

BA = Balancing Authority

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[crshears]

At the Ohm's Law/Kirchhoff's Law level there is just The System. Ownership is just a human construct that the electrons don't understand.

At a commercial level power flow proceeds as directed by "Tags" or some other accounting trick. Interchange metering occurs at the substation/switching station closest to the point of interchange. There's a whole settlement process that probably differs from place to place and market to market.

To elaborate a bit on the above:

It is patently evident to IESOs and Transmission System Operators that power flows within the Eastern Interconnection behave in accordance with Ohm's Law/Kirchhoff's Law, and the means by which these flows are balanced is by devices like phase shifting transformers / phase angle regulators[PARs] / quadrature boosters that alter the quantities being plugged into the formulas by which Ohm's/Kirchhoff's values are derived.

The choice of which generators in which geographic locations to place in service and/or how heavily to load them also has profound effects on power flows and line loading.

Both of the above have direct commercial implications: in the Eastern Interconnection, the rule "You can only charge for what you can control" very much comes into play. More specifically, in past years some of the aforementioned ties between Ontario and Michigan did not have PARs in them, and only partial re-balancing of the flows on these tie lines could be achieved.

This became problematic when power producing entities in New York State negotiated "tagged" sales of energy to Illinois; the overall impedance of the grid running through the states south of the Great Lakes being higher than that of the grid spine in the province of Ontario between Buffalo/Niagara Falls and Windsor/Detroit, these sales took the path of least resistance through southern Ontario, loading up the transmission circuits there. Not only did this sometimes constrain the transport of electricity within Ontario, but Ontario could not charge for the "wheeling" of power on its circuits since it could not control that flow.

Ontario Hydro [and, ultimately, one of its successor companies, namely Hydro One], therefore ended up spending what amounted to just into an eight-figure sum in Canadian dollars to install PARS in the Ontario-Michigan tie circuits that didn't have them. Having PARs at the Ontario-Michigan interface and the Ontario/New York interface at Cornwall/Massena also enabled the Niagara interface to be controlled, and reportedly the revenues that suddenly became collectable provided a most handsome and prompt ROI.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]