Impact of Distributed Series Reactances on
Power Networks
From Power Line to Pipe Line
Deepak Divan
Harjeet Johal
School of Electrical Engineering
Georgia Institute of Technology, Atlanta, GA, USA
777 Atlantic Drive
Atlanta, GA
deepak.divan@ece.gatech.edu
US Power Grid: Problems (Opportunities)
•
Grid is seeing increasing congestion
and degraded reliability
•
Uncontrolled power flow is a major
issue for transmission & distribution
•
Building new transmission/distribution
lines is no longer a simple process
•
First line to reach thermal rating limits
system transfer capacity, even as
neighboring lines are under-utilized
•
Real situation is worse as reliability has
to be ensured with (N-X) contingencies
•
Lack of visibility and control leads to
conservative operation resulting in
significant under-utilization of assets
•
Possible cascading failures under
contingency conditions
•
Reliability, load growth, RPS standards
and ‘Smart Grid’ initiatives under EISA
2007 will be major drivers for new
investments
138kV
∠0°
j1 6 Ω
675A
138kV
∠ 7 .7 5 °
j2 4 Ω
450A
Power flow path from Wisconsin to TVA*
*Courtesy: Tom Overbye, UIUC
Confidential
Unarticulated Market Need – Controlling Power Flows
•
Reliability of meshed networks is substantially higher than for radial systems.
However, utilization is poorer.
•
Controlling network power flows along existing lines has not been considered feasible
– either technically or economically.
•
Benefits of controlling power flows includes:
•
–
Enhancement in system utilization and capacity without building new lines
–
Tool for managing load growth, congestion and contingency issues
–
Self-healing grid which responds to contingencies, improving system reliability
–
Routing of power along desired pathways, from ‘power-line’ to ‘pipe-line’. ‘Green’
electrons can be verifiably sent from renewable sources to specific loads.
–
Effective management of congestion and uncontrolled loop flows
–
Reduced level of generation reserves required to ensure system reliability
Traditional approach has been through the use of phase-shifting transformers (too
slow) or FACTS devices (too expensive).
Confidential
Distributed FACTS for Power Flow Control
•
Distributed FACTS suggested by Divan – Smart Wires
– Provide the functionality of FACTS at lower cost and high reliability
P12 =
–
–
–
–
–
–
V1 V2
sin (δ )
Xs
Series VAR injection controls effective line impedance & real power flow
Large number of modules float electrically and mechanically on the line
Can be incrementally deployed to provide controllable power flow
Standard low-cost mass-manufactured modules
Redundancy gives high reliability and availability
Phase I supported by TVA, Con Ed, DOE and others
D-FACTS
Confidential
Active Smart Wires
•
Distributed Static Series Compensator (DSSC)
– Active solution employing a synchronous voltage source inverter
– Each module rated for 5 KVA (capable of injecting ± 4.6 V @ 1000 A)
– Communication interface is required to realize the bi-directional control
– Can be made larger for distribution applications (one per line)
Main transformer
Current feedback
Line Current
Power
supply
V
Filter
PWM
Inverter
Controls
Communication
Module
DC Capacitor
Confidential
Distributed Series Reactance – Passive Smart Wires
Power Line
N Xm
Transformer
XM
Power
Supply
SM
S1
Ithermal
I0
Control
•
•
•
•
•
Line Current (ILine)
Simplest implementation of DSI, with inductive impedance injection (Current
Limiting Conductor or CLiC) – functions as a current limiting system
As current in a line approaches the thermal limit, CLiC modules incrementally
turn on, diverting current to other under-utilized lines
Increase in line impedance can be realized by injecting a pre-tuned value of
magnetizing inductance of the STT
Each module is triggered at a predefined set point to reflect a gradual
increase in line impedance
No communication required and the devices operate autonomously
Confidential
Increase in System Capacity With DSR Modules
Simplified Four Bus System
180
160
C Line 2, Line 4 Overload
140 B
Overload
100
Line 2, Line 5
A
Profile of Line 2 Current
1
Overload
80
Line 2 Overload
750 A
0.75
60
40
Line 5 Overload
20
C'
Line 1, Line 2,
Line 5 Overload
A'
20
40
60
80
100
Load2 (MW)
120
B'
140
160
Line Current (KA)
Load 1 (MW)
Line 1, Line 2, Line 4, Line 5
Line 2 Overload
120
624.2A
0.5
548.7 A
Generator
Taken Off
0.25
System Capacity with DSI/DSR Modules
Blue: Normal, Red: DSR, Green:DSI
0
0
0.5
1
DSR Active
1.5
2
2.5
Time (sec)
Contingency Condition: Generator Outage
Confidential
Increase in Network Utilization
IEEE 39 Bus System
Increase in utilization from 59% to 93.3%
for 14 lines
G8
G10
37
12.86 MVAR
26
25
28
29
30
2
38
Network Performance With CLiC
27
G6
8
9
10
G2
36
Power Lines
34
32
G5
G4
G7
G3
• Increase in Transfer Capacity from 1904 MWs to 2542 MWs (congested
corridors and the required MVARs are shown by red lines)
• With (N-1) contingency, capacity is increased from 1469 MW to 2300 MW
without building additional lines
• Would require 9 additional lines to realize capacity increase
Confidential
Line29_28
20
3.52 MVAR
Line29_26
13
11
23
Line26_27
31
7
19
Line25_26
12.64 MVAR
0
22.76 MVAR
9.15 MVAR
12
Line23_24
14.75 MVAR 6
Line currents with CLiC
Line currents without CLiC
Line22_21
14
5
20
Line19_16
4
40
Line2_3
39
35
22
Line13_14
21
60
Line12_11
15
80
Line10_13
19.52 MVAR
3
G1
12.04 MVAR
16
Line9_39
18.77 MVAR
100
Line6_7
24
G9
Line6_5
17
Line Currents
(%Thermal Limit)
18
1
DSR Prototype
Complete module with the casing
•
Electrical
– Operating Level : 161 KV, 1,000 A
– ACSR Conductor: Drake (795 Kcmil)
– Injection: 10 kVA, 750 A
•
Mechanical
– Target weight per module: 120 lb
– Packaging to avoid corona discharge,
and other mechanical, thermal and
environmental issues
• Simple low-cost design suitable for mass
manufacturing
• Suitable for distribution and transmission
applications
Confidential
Validation at High Voltage and Current
Corona inception: 125 kV
Operation Under Fault Currents
Extinction: 123 kV
Photographs correspond to 166 kV
Confidential
Investment Cost for IEEE 39 Bus System
•
Cost of laying additional lines: $500,000/mile
•
Target cost of DSR unit: $100/KVA, cost of
redeployment: $30/KVA
•
ATC of the system is limited to 1904 MWs
•
DSR modules can relieve network congestion at a
lower cost for the first 400 MWs
•
DSR continues to be attractive for next 300 MWs
7
2.5
Investm ent Cost w ith
only DSI technology
– Building additional lines can be suspended for
the first 700 MWs
A combination of the two schemes can provide a
much lower investment cost
•
Allows incremental investment decisions with rapid
implementation.
2
Investment Cost ($)
•
x 10
A'
Investm ent Cost w ith a
com bination of DSI technology
and building additional lines
1.5
B'
Line 19-16
Line 6-5
Line29-28
X
1
Line 2-3
0.5
Line 22-21
0
1400
Confidential
1600
A
1800
2000
2200
2400
Load (MWs)
2600
2800
Solution Cost Under Contingencies
7
•
•
•
•
Network congestion occurs at a load of 1470
MW with one contingency, as opposed to 1900
MW with no contingencies
DSR modules are attractive until 2000 MW, i.e.
for the first 530 MW of load growth.
DSR modules can maintain optimal congestion
levels from a pricing perspective
If 2.5% load growth is assumed, the investment
in the new line can be deferred for 13-15 years.
Reduces uncertainty and risk, improves ROI.
Interest on deferred cost of new line may pay
for DSR modules!
Confidential
x 10
2
Investment Cost ($)
•
2.5
Line 6-5
Line 29-28
1.5
Line 2-3
1
Line 22-21
Line 21-16
0.5
Line 22-21
0
1400
1600
1800
2000
2200
2400
Load (MWs)
2600
2800
3000
Drivers for Transmission Capacity Investments
•
In a free market, price differentials should provide incentives for investments to
increase transmission capacity
–
–
–
Lower cost generator’s opportunity for windfall profits should incent him to invest in
incremental transmission capacity
Congestion costs provide the opportunity of arbitrage
Price differentials help to finance investments
LMPA ($/MW)
With Congestion
25
With Complete Congestion Relief
25
LMPB ($/MW)
50
49
Consumers costs in region A ($)
3750
3750
Consumers costs in region B ($)
7500
7350
Gen. A revenue ($)
6250
11100
Gen, B revenue ($)
2500
0
0
4850
Available revenue for transmission Inv. ($)
•
In a regulated market, increase in societal welfare is supposed to form the
basis of any policy initiative
–
•
Reliability improvements are easily supported by PSC’s and can support capacity
increases
Increased societal benefits can be realized from congestion relief, but it is not
clear how such investments may be funded.
Confidential
Impact of Incremental Transmission Capacity
•Incremental increase in transmission capacity affects
market participants differently
Participants
Change in Welfare
Congestion costs
(LMPB-LMPA) Δx
Producers
- (LMPB-LMPA) Δx
Consumers
(LMPB-LMPA) Δx
•The congestion revenue collected by ISO increases
•Gen. B loses revenue, while Gen. A gains revenue. Net
change in generator revenue is negative.
Generator Dispatch Curves
30
•Consumer welfare increases; ISO’s allocate congestion
costs back to market participants (FTR)
•Market operates under the assumption that
infrastructure changes happen too slowly, and so
congestion costs are redistributed to participants.
Confidential
LMPB = 27
Gen. B Dispatch
Under Congestion
26
Price/MW
•Consumers may not benefit uniformly with increase in
transmission capacity (marginal costs can cause
consumers in Region A to pay more)
28
24
LMPA = 24.18
LMPA = 23
Gen. A
DispatchUnder
No-Congestion
22
Gen. A Dispatch
20
Tranm ission Capacity
18
XA = 500 MW
0
100
200
300
400
500
MW
600
700
800
900
1000
Business Models for Smart Wires
Reliability
•Works well with the traditional utility approach of maintaining system reliability
•In line with EISA, “Creation of smart grid to improve reliability and performance.”
Economics/Congestion
•Investor model: A profit sharing mechanism between investor and consumer
Congestion relief through outside investment can provide break-even in 2 years
Congestion
Costs
Congested
Hours/year
Increase
in welfare
DSR Cost
PI=8
Profit-Sharing Formula
$25/
MWHr
500 hours
$12,500/
year/MW
$13000 /MW
(cost for 13
modules)
50% consumers, 50%
investors till desired ROI
is reached
Time
Investment
Costs
Investor
profit
Consumer
welfare
12 months
$ 13000
- $ 6250
$ 6250
24 months
-
- $ 500
$ 12500
36 months
-
$ 6250
$ 18750
Directed Power Flow
• ‘Green’ electrons can be directed along a designated network path (pipeline)
• Carbon cap & trade may generate opportunities for investment
Confidential
Title XIII of the Energy Independence & Security Act of 2007
Places great emphasis on Smart Grid – Electricity delivery network modernization using
latest technologies to meet key defining functions:
• Optimizing asset utilization and operating efficiently
• Self healing and resilient operation
• Greater control of the grid to enable new business models and functionality
Financial Incentives
• $100M/year for 2008-12, 50% cost share for utilities on demonstration projects, 20%
reimbursement for expenditures on Smart Grid
• Rate recovery and accelerated depreciation allowed
Renewable Portfolio Standards and Real-Time Pricing.
• Results in significant level of dynamics on the grid, with significant challenges
• Increased adoption of Advanced Meters to serve as pricing gateways will result in
greater volatility of loads
Key Technologies Targeted:
•
Controls to improve reliability, dynamic optimization of grid operations, smart
appliances, to facilitate integration of advanced technologies in electric networks to
improve performance, power flow control and reliability.
Confidential
Smart Wires in a Smart Grid
• It is proposed that the use of distributed solutions based on low-power power
electronics can allow utilities to move towards dynamically controllable meshed grids,
significantly enhancing grid reliability, capacity and utilization. This can enable:
• Improved reliability without having to build new lines
• Improved dynamic coordination between regions
• Reduction in dynamic capacity reserve for generators
• Possibility of moving power along a predetermined contract path
• Can be applied at the transmission, sub-transmission and distribution levels.
• Can be layered incrementally onto the existing infrastructure as desired, and will not
degrade the inherent reliability of the existing system.
• Makes the grid self-healing, automatically maintaining safe operating levels even in
the face of contingencies.
• Funding such investments on the basis of congestion relief is problematic in
regulated environments, new mechanisms may have to be found.
• Project is supported by IPIC, TVA, ConEd, Southern, NRECA, and others. Target
full-scale pilot demonstrations in 18-24 months.
Confidential