Download Impact of Wind Generation on ERCOT Operations

Voltage and Transient
Stability limits
Bill Blevins
Objectives
• Discuss computing the West – North Stability Limits
• Discuss computing the Houston area Voltage limits
• Discuss adding the Stability and Houston area Voltage Stability
limits to the list of items in setting the CSC flow limits.
ERCOT Objective
Keep flows under limits,
Or ELSE
• Equipment may be damaged
• System may become unstable
Even when the flow on the constraint
is brought down below the limit by
redispatch, we are in a congestion
situation
Congestion must be
managed, and it costs $$$!
ERCOT System Map
West-to-North Stability Interface
• Monitoring Elements:
– Selected machines in West Texas and South Texas were
monitored for speed deviations
• West to North Interface:
– Bowman to Jacksboro SS,
– Bowman to Graham,
– Tonkawas to Graham,
CSC
– Sweetwater to Graham,
– Mesquite to Graham,
– Red Creek to Comanche,
ERCOT West-to-North Stability Limit
• “Why” do we need to have West-North transfer stability limit?
– Potential inter-area oscillation may cause units in the West and
South have large oscillations and trip during disturbances which
degrade system reliability and lead to firm load shed or islanding.
• “Why” do we need to concern system oscillation?
– Many static exciters in system are designed to help voltage
regulation, but also induce “undesired” negative damping, which
may cause oscillation during system disturbance.
• “When” do we need to consider system oscillation?
– Weak transmission links and heavy power transfers.
• “How” do we prevent system from undesired oscillation?
– Have PSS in service.
– Limit/maintain the power transfer on the weak link.
• The WN stability limit is updated periodically or if there is a
significant change in the system topology.
Example of good stability during disturbance
Example of poor stability during a disturbance
Unacceptable stability during disturbance
W-N Transient Stability Limit
•
Set the Stability limit based on the annual stability study. This includes about
500 scenarios which are most credible for analysis.
•
This limit is then supplied to operations and a spreadsheet continuously
monitors the Real-Time conditions for generation combinations to inform
operations which limit is currently effective.
•
Operations sets the CSC transfer limit based on the 6 monitored line flow.
•
The proposal is to use Zonal congestion deployments of BES to control CSC
flow as a proxy limit when the Transient stability limit is reached. This will
instruct Resources in both West and North to move similar to when the CSC is
Thermally limited and therefore be the quickest and most effective method for
maintaining the West to North flow below the transient stability limit.
Building a Zonal Constraint
“Actual Flow” here is
actually a SPD calculated
flow, not a real time flow
value.
Constraint Limit (OC1) is
number set by the
operator which SPD uses
to control the “Actual
Flow” (SPD Calculated
Flow)
Calculated vs. Real-Time Flow
1200
Calculated Flow
OC1 Limit
Redispatch
1000
Operator
and
System
Actions
800
600
400
200
0
1900
1915
1930
1945
2000
2015
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2100
2115
2130
2145
2200
2215
2230
2245
2300
RT Flow
RT Limit
RT + Redispatch
1200
1000
800
Physical
System
Impact
600
400
200
0
1900
1915
1930
1945
2000
2015
2030
2045
2100
2115
2130
2145
2200
2215
2230
2245
2300
Voltage stability limit
• Some CSC limits may also require VSAT studies as well.
• Two voltage stability SOLs will be calculated daily: NorthHouston and South-Houston.
• If the VSAT study comes up with a limit that is less than the
Thermal Limit for that CSC then we will use the VSAT limit
provided limit to establish a proxy CSC limit.
ERCOT North-to-Houston Stability Limit
• ERCOT runs the voltage stability analysis in real-time for the North to
Houston transfer once every hour.
• ERCOT studies 3 transfer scenarios to analyze the voltage stability for
the Houston region
– Increase generation outside Houston - Increase load in Houston
– Increase generation in North – Increase load in Houston
– Increase generation in North – Decrease generation in Houston
The lowest of the 3 limits calculated for the different transfers will be used
to constrain the North to Houston transfer.
The interface for the North to Houston includes the four 345 KV lines
- Jewett to TH Wharton
- Jewett to Tomball
- Gibbons Creek to O’Brien
- Roans Prairie to Kuykendahl
VSAT Results in EMS
•
•
•
•
Below is a snap shot of the N-H transfer limit from the ERCOT EMS. The information available
for the operator from the display is shown below.
The current (operating point) flow on the N-H interface is 2569.5 MWs
VSAT study is showing that when the interface flow reaches 3789.9 MWs, under the Dckt
contingency DJEWTHW5, we will experience a voltage collapse in the Houston region.
The operator monitors the Extra interface limit (1220.4MW) and N-H transfer is constrained
when the Extra interface limit approaches zero.
This is the max MWs
transfer allowed on the
interface
This is the available transfer capability on the
interface before the contingency DJEWTHW5
causes a voltage collapse in the Houston area
This is the current flow on the
North-Houston Interface
Building a Zonal Constraint
“Actual Flow” here is
actually a SPD calculated
flow, not a real time flow
value.
Constraint Limit (OC1) is
number set by the
operator which SPD uses
to control the “Actual
Flow” (SPD Calculated
Flow)
Two Types of Congestion
Zonal
•
•
•
•
•
Congestion on CSCs or CREs that can be resolved by zonal deployment of
Balancing Energy Services (BES)
Controlled on a QSE portfolio basis
ERCOT software determines the optimal inter-zonal portfolio balancing
instructions to relieve CSC congestion
Cost of solution is assigned directly to the QSEs whose schedules impact the
CSCs
BES instructions used to solve zonal congestion and/or energy imbalance are
Category 1 instructions
Two Types of Congestion
Local
•
•
•
•
Not defined by a CSC or CRE
Controlled with resource-specific deployment or OOM instructions
ERCOT software (SPD) determines resource-specific instructions that best
accomplish congestion relief based on constraint limits and resource-specific
premiums
Resource-specific instructions may be:
– Category 2: At or below a MW level
– Category 3: At or above MW level
– Category 4: Go to a specific MW level
– Will also come with an offsetting Category 1 to make them “instructed”