HVDC Technology: When 2,000 MW is 2,000-Plus-1,200 MW

by Pterra Consulting

In this Blog, we discuss the amazing story of how a lowly 2,000 MW HVDC line was able to support a transmission capacity increase of 3,200 MW. Read on …
In a previous Blog, (HVDC Technology: DC Overlay on an AC System, by R. Austria, K. Dartawan, M. Elfayoumy, M. Gutierrez, R. Tapia, Nov 2, 2009), we posed the question: “Will a 2,000 MW HVDC line transfer 2,000 MW?€ Using a proposed HVDC line in New York State as the case study, the conclusions were:

“So, the proposed 2000 MW line, increases transfer capacity in the vicinity of the sending area by an average of 2340 MW but delivers only an average of 1527 MW at the receiving end of the line. The reasons for the variations: elimination of certain limiting contingencies, and need for must-run generation at the load pocket.”

In this Blog, we consider a 2nd case study involving a similar 2000 MW HVDC line, this time a bipole with +/- 400 kV line voltage.

Background

The State of Texas embarked a few years ago to identify geographic areas throughout the state in which renewable energy resources and suitable land areas were sufficient to develop generating capacity from renewable energy technologies. These areas were designated as Competitive Renewable Energy Zones or CREZ. One of the CREZ was located in Central Texas, and it was proposed that a 200-mile HVDC line rated 2000 MW be overlaid on the existing 345 and 138 kV transmission system to the large load pocket of Dallas-Fort Worth (DFW).

Case Study 2

The amount of new wind capacity that can be supported by the existing Texas grid between Central Texas and DFW with and without transmission upgrades is summarized below:

Transmission Option

Additional Wind Capacity in The Central CREZ (MW nominal)[1]

Limiting Element[2]

No transmission additions

1100

Wolf Hollow-Rocky Creek 345 kV line

New 345 kV AC Line

1600

Wolf Hollow-Rocky Creek 345 kV line

New 2000 MW HVDC Bipole

4300

Alliance-Eagle Mountain 345 kV line

New 345 kV AC Line and New 2000 MW HVDC Bipole

4600

Alliance-Eagle Mountain 345 kV line

[1] Rounded to the nearest 100 MW.
[2] Transmission component that is at thermal capacity at level of additional wind in The Central CREZ, as shown in column to the left.

The results say that:

  • Adding a 345 kV AC line supports an additional 500 MW of wind capacity from a base capacity of 1100 MW
  • Adding an HVDC line supports an additional 3200 MW
  • Adding both the AC and the DC lines supports an additional 3500

This is a case where a 2000 MW HVDC line gives 3200 MW additional capacity. How did this happen? Where did the extra capacity come from?

Explanation

HVDC Line

2,000 HVDC Line


Reviewing the third column of the table above, note that the limiting element changes when the HVDC line is in service. The limiting element is the transmission line that hits its thermal capacity following loss of another line in the system. This is the well-known concept of the first contingency, or n-1, criteria. In essence, what this criteria says is that the transmission capacity is not the sum that can be supported by all the transmission circuits, but rather, the capacity when any one circuit or transformer is out of service. The rationale behind this criteria is that the transmission system should be able to withstand the loss of any single element without immediate operator intervention. Another way to think of this is that the system has a built in redundancy that improves its reliability. However, n-1 redundancy also has another effect — it reduces the utilization of the AC system. Whereas, without the n-1 criteria, we might load a line to 100% of capacity, with the n-1 condition, the thermal load on the line may be limited to, say, 50%.
Going back to the case study, the change in limiting element is significant. As shown in the diagram, the location of congestion shifts from the southern part of the DFW load pocket to the northern part. Furthermore, the congestion is closer to the load. The addition of the HVDC line shifts power flows in the underlying AC system in a manner that relieves the pre-existing congestion. Unlike Case Study 1 (the New York HVDC line), where must-run generation reduced the amount of injection into the load pocket, there is no such constraint in Case Study 2. Another illustrative fact is that if we average the thermal loading on the AC transmission system and compare the value before and after the HVDC line is added, we see that the average thermal load increases with the HVDC line in service. The HVDC line thus improves the utilization of the transmission system.
Thus, it may seem that more power is supported by the DC line even though it still carries the nominal 2,000 MW. In fact, the HVDC line, in Case Study 2, improves utilization of the underlying AC system so that it can support more thermal load.
It should be emphasized here that the condition that determines whether the additional transmission capacity on the underlying AC system will be greater or less than an installed HVDC is primarily the location of the DC terminals. Thus, it is mightily important, to avoid costly re-location assessment, to conduct a thorough planning assessment for any proposed HVDC line that is intended to parallel an existing transmission system.

Conclusion

For Case Study 2, we make the following startling conclusion: the addition of a 2,000 HVDC line supports an additional 3,200 MW of new wind capacity for this particular study. It does so by improving the utilization of the underlying AC transmission system.
Considering both Cases 1 and 2, overlaying a new 2000 MW HVDC line on an existing AC transmission system does not necessarily result in an additional 2,000 MW of transmission capacity. Under specific conditions, the additional capacity can be much less than 2,000 MW, or alternatively, can be much more. Careful study of the underlying AC system and selection of terminals for the HVDC line is needed to confirm if you can get more or less than what you installed.