Equitable Cost Distribution for Losses on a Shared Transmission Line

Introduction

The rapid development of renewable energy sources has created complications for the substations and transmission lines that deliver the energy to the end users. Grid owners and operators are working feverishly to enable the interconnections that are being requested. One concerns of those multiple interconnections is the proper accounting of losses on the shared lines and substations.

The Issue

When considering the revenue metering design for a new generation site, losses must be factored into the site’s revenue projections. Because losses are non-linear, the losses a site accrues grow exponentially in relation to how much power is generated. This is expressed as P = I2R, where P is the “Power Lost.”

Losses are fairly easily computed within a revenue meter when it is a single generator on a line, but what happens when another generator shares the same line? Two generators make the “I” in that equation larger, thus making the losses larger! Since each resource owner is not able to prevent another generator from being connected to the line, it is imperative to make sure that each resource is accounting for their fair share of lost revenue.

The Engineering

Ensuring the proper sharing of losses is not as easy as it looks. If Generator 1 is adding 5A to the line and Generator 2 is adding 20A to the line, should Gen 1 take 20% of the losses because it is generating 20% of the total? Let’s look at the math:

                Generator 1 = 5A (I1)
                Generator 2 = 20A (I2)
                Assume the line resistance = 1Ω (R)
                Total Current = Gen 1 + Gen 2 = 25A (Total I)
                Power Lost on the Line = (25A)2*1 Ω= 625W

However, because the losses increase exponentially, each Generator is actually responsible for:

Generator 1 = 5A
                                5A2 = 25
Generator 2 = 20A
                                20A2 = 400
Weight attributed to Generator 1 =                 25          
Weight attributed to Generator 2 =                 400
Total                                                                       425

The Dilemma

Reviewing these loss results, it is clear that Generator 1 accounts for 25/425 of the losses, or 5.88% and Generator 2 accounts for 400/425 of the losses, or 94.12% of the losses. Therefore, an equitable allocation of the losses would be:

                Generator 1 Apportioned Loss = 5.88% of 625W = 36.75W
                Generator 2 Apportioned Loss = 94.12% of 625W = 588.25W

This example shows the proper allocation for a fixed production, but does not consider that generation is constantly variable! How do we manage that reality? Since Generator 1 and Generator 2 are constantly changing, their share of losses will constantly be changing as well. While two Independent Power Producers may make any agreement they choose, their regulatory authority who is making settlements must have a logical allocation of losses in order to distribute payment appropriately. Most agreements state the gross generation has to be adjusted for system losses to the Point of Delivery settlements.

The Solution

One solution to addressing the constantly fluctuating power production is a peer-to-peer scheme. This solution requires all meters on a line to constantly send their generation data to each other. Each calculates their portion of losses and adjusts their own point of metering kW for those losses and then records the result as revenue data.

Another method is to have all meters on a line send data to a calculating device and receive their calculated losses from a device known as a dynamic loss compensator.

Trimark has implemented both of these methods and many variations using a Trimark Data Gateway or entirely with peer-to-peer meters.

 

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