N-1 Reliability Criteria for HVDC - Symmetrical Monopole VSC

[cuky2000]

We are investigating if an HVDC - Symmetrical Monopole VSC can meet the N-1reliability criterion. The concern is triggered by the issue that a fault on any of the (+) or (-) DC poles will get the system totally out. Similarly, a single AC output from the converter station does not have redundancy in case of any fault.
Any thought will be helpful

 

[davidbeach]

The one I’m semi familiar with runs each pole independently. One side can trip and the other remain in service. When one pole trips it relies on ground return.

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

 

[cuky2000]

At a difference of a bipolar configuration, a monopolar arrangement, including the symmetrical monopole one, does not have redundancy and a fault on any of the HVDC poles will shoot down entirely the system as indicated in the enclosed article.

There are several planned offshore wind power project on the East coast as large as 1,200 MW. Many of those projects are planning stages intended to replace retiring fossil generating plants that generally meet the N-1 and reliability redundancy planning criteria. However, I had a hard time harmonizing the use of a large HVDC symmetrical monopole considering potential vulnerability for a single point of failure.

We anticipate major challenges for power engineers in the next decade in the USA with the large penetration of renewable generation. One of the concerns of getting approval for organizations such as NERC, NPCC, and NYSRC. Noted that conventional generating plants can trip individual generators and HVDC tripping suddenly 1,200 MW somehow could disturb any bulk power system including subsynchronous resonance, potential system stability, among other issues.

[highlight #FCE94F]Any thought regarding potential single point of failure for symmetrical monopole application will appreciate it[/highlight]

 

[FacEngrPE]

This is a bit out of my skillset, but I have a few thoughts.
PJM (

https://www.pjm.com)

publishes its procedural documents several of which go to a fair depth into the method they you to simulate their system to determine if they are N-1 safe, and also the behavior for N-1-1. The document leads me to think that any new major facility or power link will be investigated for the pass fail criteria response (voltage collapse, exceeding thermal limits, exceeding stability limits, possibly some more) to both N-1 and N-1-1. N-1 failures must be resolved. N-1-1 failures are calculated, and used for system planning. Most of these are also resolved.
The important results of these studies wind up published in the form of mitigation projects.
PJM has several DC Links to midwest sources under study, and one I am aware of one DC link in operation (

https://neptunerts.com/

660 MW to Long island).

I think getting NERC approval - at least in PJM's zone is not really an issue as NERC approval would be unlikely to be requested by PJM until after system simulation tests for N-1 and N-1-1 are passed.

Mileage in other regions will be different.

Reharding OP's question
While I could not find a specific reference to modeling A DC link the documentation seems to indicate that all failure modes of a link would be simulated in system stability modeling (in the PJM zone). So a bipolar link able to operate as a monipole link would have possible path states of 1, .5, or 0, while a bipoler link not able to operate in monopole only has 1 or 0.

The Symmetrical Monopole VSC is supposed to have a mode where it can continue to operate (with no derating) with one leg temporarily shorted to ground as might occur during a lightning strike when the MOV operates. Longer events with one leg earthed require the link to be taken off line as the insulation requirements on the unearthed pole double. My opinion is that this impacts the likelyhood or time between link outage (statistical measure), but by itself this feature can not change the impact of a link outage. Ability to meet N-1 or N-1-1 is then a feature of the system(s) the link is connected to.

 

[stevenal]

It seems rather odd to me to be speaking of reliability with respect to intermittant resources. Unless you have batteries on those offshore platforms you're at N-N when the wind dies down.

 

[FacEngrPE]

The intermittent nature of generation is one of many reasons why "System Reliability Impact" needs to be considered. The N+1 and N+1+1 criteria should identify potential common mode failures. Mitigation can include things that improve distribution system integrity under stress or injection of power into the system at critical locations. The mitigation power storage could be at the generation point, but as long as the system model is satisfied, there is no reason why that is the only location acceptable.

Bath County Pumped Storage Station (

https://www.dominionenergy.com/proj...projects/bath-county-pumped-storage-station).

This station was built to take off peak power from conventional and nuclear plants and time shift it to peaking power.
[ul]
[li]maximum generation capacity of 3,003 MW[/li]
[li]an average of 2,772 MW,[/li]
[li]total storage capacity of 24,000 MWh (11.5 hours drawdown time)[/li]
[/ul]
I could not find the time to reverse power flow (pumping to generating), but it is known to be quite fast. Likely limited by the ability of the dynamic breaking system to absorb the generating system inertia.

PJM seems to treat the ability of a battery to smooth the power supply curve out as one of the aspects of the ancillary services market. The ability to respond serious system disturbances by making a fast start in respond to major loss of generation is another aspect of the ancillary market.

This paper "Technical Analysis of Pumped Storage and Integration with Wind Power in the Pacific Northwest, USACE Hydro Design Center Aug 2009 discusses some of the interactions between pumped hydro storage and integration of wind energy generation into a power system.

https://www.hydro.org/wp-content/up...ation-Final-Report-without-Exhibits-MWH-3.pdf

This is an interesting article describing the implementation of 100% Stator Ground Fault (SGF) protection (ANSI # 64S) at the Bath Pumped Hydro Station.

https://new.siemens.com/us/en/produ...nions-bath-county-pumped-storage-station.html

(I think this might be of interest due to the considerable amount of discussion related to generator protection that has occurred on this forum. ANSI 64S is relatively expensive but for a high value asset it becomes cheep insurance.)

My comments here are from the standpoint of professional engineer who is a power customer. It is not part of a field of engineering where I am likely to practice. In other-words I am a spectator to the technically challenging subject of integrating renuables into the North American Grid.

Fred

 

[bacon4life]

Is this specifically for NERC TPL defined contingencies? I had assumed a symmetrical monopole system would be treated the same as an AC line of the comparable capacity for TPL studies. If the cables or converters have significant lead-times to repair, it could certainly make sense to model an extended outage. There are a number of conventional power plants with multiple generators rated more than 1000 MW each, so loosing a single DC cable to a 1200 MW wind farm seems not unreasonable to me.

 

[stevenal]

Other than the duration of the outage, I don't see how loss of the DC cable to the wind farm differs from a loss of wind. If planning allows for windless days, loss of the cable is already covered reliability wise. Of course NERC and reliability don't always align.

 

[cuky2000]

Hi Stevenal.

It is recognized that windmills have daily and seasonal generation variability but the range window is typically several hours. The operator is getting better at forecasting the power to be dispatched from wind and other generation resources for a daily power cycle.

DC cable outages do not happen often, but went it does the repair time is fairly long for several days. The data below despite an indicative time estimated from various sources for illustration purposes. There is a major consequence because of long outages, impact on the system performance, overstresses, overvoltage, lack of revenue, cost of repair, operational confidence, etc.

To assess the system risk a typical repair time for a submarine cable is several days and the longer registered repair time was around 85 days. An average indicative time is as follow:

1) Specialized ship to reach fault position:........3-5 days.
2) Cable splice...............................................1/2-1 day
3) Unforeseen* conditions.............................. Unknown (Estimated for several days).
*[sub]NOTE:[/sub] [sub](bad weather, mobilization, delays & operational difficulties)[/sub]

A typical repair time of an AC line is a lot smaller and almost all OH lines have the reclosing capability with a high success rate for SLG fault.

A force long unplanned outage of a week or so appears to be a long time even for an intermittent power source such as an offshore wind generation.

 

[stevenal]

Whether hours or months, it is still capacity that must be provided from somewhere else, either from a non-intermittent resource or storage. I'm not sure the majority of the greenies realize this little detail. If storage is depended on (batteries or pumped hydro for example), I can see where the the advantage of the bipolar configuration might be desirable.

I always check the weather before turning on the toaster, doesn't everyone?

 

[FacEngrPE]

I found some information which might create some discussion. Dominion Energy is currently building a windfarm east of virginia beach

Dominion's project information site

https://www.dominionenergy.com/proj...s-and-projects/coastal-virginia-offshore-wind

First of 180 wind turbines is installed, build-out is 2.6 GW

https://news.dominionenergy.com/202...t-Offshore-Wind-Project-in-U-S-Federal-Waters

Bureau Ocean Energy Management project information site

https://www.boem.gov/renewable-energy/state-activities/coastal-virginia-offshore-wind-project-cvow

2010 report showing PJM bulk power grid ties needing reinforcement to allow wind project to land power.

https://offshorewindhub.org/sites/default/files/resources/vcrec_4-20-2010_offshorewindstudies_0.pdf

Interconnection of Offshore Wind via Generator and Merchant Transmission Interconnection Processes, Testimony of Kenneth Seiler Vice President – Planning PJM before the FERC, October 27, 2020

https://www.pjm.com/-/media/library...e-offshore-wind-integration-in-rtos-isos.ashx

The information in these documents is not conclusive, I was not able to find the exact type of DC power system planned between the wind farm and shore.
At some point the limiting factor to redundancy will be on land, as there is only one connection to the PJM bulk power grid at Fentress Substation currently. The PJM Planning Process, has done a considerable amount of work on strengthening the bulk power system for other reasons, and will also improve the system's ability to handle renewable energy. The planning process is being adjusted to include offshore wind specific requirements.

 

[cswilson]

A few years ago, I got a great tour of the southern end of the Pacific Intertie HVDC line in Los Angeles, which typically brings power from the big dams on the Columbia River south to the big population centers.

The thing I found most interesting was how they handle the loss of one of a pair on lines -- they use the Pacific Ocean as the return. You can see this neutral return line on transmission towers leading away from the facility towards the ocean.

 

[cuky2000]

Hi cwilson,

Thanks for letting us know about your experience with the ±500 Pacific DC Intertie (PDCI). The PDCI may no be the best example to compare the VSC since this is a long overhead link with a bipolar configuration with ground return using the classical HVDC current source converter (CSC) technology with a ground return providing redundancy to operate at 50% capacity in the event of a fault in any of the positive or negative pole until the fault is repaired.

To pass the economic test with very expensive submarine cable, most of the new offshore wind generation (OSWG) projects prefers using a voltage source converter (VSC) technology because of many operation advantages over the LCC (see note below). The vast majority of new OSWG projects are developed using symmetrical monopole using XLPE cable currently up to ±320 kV bundle submarine cable without ground return.

For a system operating without redundancy in the event of long lead repair time between failure appears to be a disadvantage for this technology and difficult to meet the required system reliability.

VSC Fault Scenario: for a single pole-to-ground fault for the ungrounded VSC symmetrical monopole configuration.
1) The dc line is cleared by the ac circuit breakers at the VSC converters at each end of the two-terminal transmission.
2) Power flows and pole-to-pole voltage stays the same, during the fault.
3) Overvoltage of 2PU on of the un-faulted pole apply excessive stress to a cable
4) No surge of fault current flows through the VSC converters.

[sub]NOTES regarding advantages of the HVDC VSC technology over the LCC HVDC classical:
Some of the advantages of the VSC technology are the cost, compactness, black-start capability, able to connect to weak AC networks, it can be used ordinary transformer, small footprint, independent control of active and reactive, and has a good dynamic ability of reactive power support. Moreover, if used in wind power transmission, VSC-HVDC decouples the wind farm and the AC grid, which effectively enhances the fault ride-through (FRT) capability of the wind power system.[/sub]