hvdc facts sub-committee minutes of meeting of jan in lv r · dennis woodford. segmentation ......
TRANSCRIPT
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Segmentation
Harrison K. ClarkPresented by:
Dennis Woodford
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Segmentation
Use dc to segment large ac grids into smaller synchronous ac segments
Precedent: – Texas– Hydro Quebec– WECC-Eastern Interconnection– Norway– India– Australia (Tasmania)
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Segmentation
Propose ac islands of (roughly) 10GW to 60GW– Convert existing longer regional ac ties to
HVDC• Bipole• Tripole
– Insert back-to-back dc converters in remaining ac ties
• Conventional dc• Voltage Source Converter, or• Variable Frequency Transformers
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Example
~2000 miles
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Boundary Selection Criteria:
Boundaries formed according to:– Locations that require least back-to-back
MVA
– Locations that break up stability limited paths
– Congested paths that need more transfer capability
– Where longer DC lines can be formed from existing ac lines
– Political boundaries where practical
An optimization problem!
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Select segment boundaries
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Form synchronous ac segments (islands)
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Concatenate some ac circuits to make longer dc lines
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Future – add long haul HVDC lines
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Why Segmentation?
Difficulties with Existing Large ac Grids:
– Disturbances can propagate across ac grids
– Vulnerability to simple sabotage strategies
– Transfer capability limited by:
• Angular stability, voltage stability
• Loop flows
• N-1 and N-2 contingencies
– Limited ROW power densities (relative to dc)
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Benefits of Segmentation
Reduced risk of widespread blackoutsIntra-segment blackout risk is lowerCascading/collapse is limited to one segmentReduced operational complexity & uncertaintyInter-area power flows where it is directed– Schedule for lower ac losses– Schedule for best use of ac line
capability
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Benefits of Segmentation
Substantial increases in transfer capability– Conversion of ac to dc
• Bipole
• Tripole
– Back-to-back converters control flow on upstream and downstream ac system
• Back-to-back tie and dc line power can be stepped down to cover upstream and downstream ac contingencies
– By operator for N-1 (prepare for next N-1)
– By SPS for N-2 (automatic)
– Allows higher loadings of adjacent ac circuits
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Benefits of Segmentation
Facilitates grid planning & investment decisions– More predictable capability of upgrades
– More predictable impacts & usability of upgrades
– Stability is no longer an issue (some intra-segment )
Market benefits– Enhanced commodity values and trading
opportunities
– Simpler and more easily applied and policed market rules
– Less uncertainty over rights to transmission upgrade capacity
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Benefits of Segmentation
Makes WAMS easier and more effective
Intelligent/self-healing grid becomes feasible
State Estimators perform much better– Finite segments
– Simple neighboring system model (boundary flows)
– SE based applications far more useful & reliable
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Existing Interconnection Benefits Remain
Traditional benefits:– Generation sharing– Reserve sharing– Emergency response– Firm transfers– Economy transfers
Most are increased or improved
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Power Can Be More Precisely Scheduled:
For lowest losses
Where intra-segment line capacity is available
Where total transfer can be maximized
To accommodate intra-segment maintenance
Power can be routed via multiple segments
For market management
Involves multiple segments … requires coordination
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How it Works A
B
C
“Governors” on dc lines and back-to-back converters replace Kirchoff’s laws, synchronizing power, power-angle, etc.
– Loss of generation in segment A
– Frequency drops in segment A, generators in A respond, ties to adjoining segments ramp up
– Segment B frequency decays, generators in B respond, ties to adjoining segments ramp up
– Segment C frequency decays, generators in C respond, ties to adjoining segments ramp up, etc.
– UFLS may occur in segment A; might also be allowed in segment B for a problem in A
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How it Works
More and larger frequency excursions
– A bad thing?
– No, a good thing!
– Just ask the folks in Texas
– When system frequency excursions occur …..
• Generator governors and firing systems are exercised
• Generator response problems are identified and fixed
• Governing and boiler equipment is adjusted for best performance
– Generator response is more predictable and reliable
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How it Works
dc tie governors limit assistance to neighbors;
– Within spinning reserve and/or UFLS tolerance
– Within thermal/voltage capability of ties and associated ac circuits
– Different limits for each direction
Bottom line -- Provide as much assistance as you can without jeopardizing your own system
Neighboring segment allowed to collapse without taking neighboring segments with it
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How it Works
Automatic central control?– Not essential, but interesting possibilities– Local controls must backup central controls
• Pre-set tie governor dp/df and maximum power
Many central control possibilities– For steady state optimization
• Minimize losses• Maximize total flow capability (normal or
emergency assistance)• Accommodate intra-segment constraint(s)
– For emergency response (fast, automatic)• Reroutes power upon local problem• Responds to requests from neighboring segments
– More or less transfer (before frequency decays)– Different route (to accommodate grid problems in
neighboring segment)
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Cost
Huge cost, huge benefits
Who pays?– Private party investment for incremental
transfer capability• a business opportunity
– Public investment for increased reliability• Resistance to malevolent attack (Federal)
• Social benefits of fewer and lesser blackouts
– Market benefits?
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More to Think About ... Is grid reliability dropping?
Every line and transformer addition or upgrade loads upstream, downstream, and parallel lines and transformers more heavily;– Average line loading is thus increasing over time, and
this– Increases stress on the grid, making N-1 events more
likely
The same additions/upgrades also lead to uniformly heavily loaded lines thus;– Leading to multiple limiting N-1 contingencies– And multiple “most heavily impacted” elements
This combination leads to;– The system is “tested” more frequently– Each test has higher risk of compounding events and
cascading
Beyond the above, an increasing share of N-2 events are troublesome
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More to Think About . . . .
Heavily loaded systems• Huge reactive supply and losses
• Increasingly difficult reactive balance
• Greater increase in I2X losses upon N-1 events
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More to Think About . . . .
Criteria/Standards are not being adjusted to accommodate these effects
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Can we halt the reliability decay?
Is Segmentation the answer or part of the answer?
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Questions?