Principal Research Results

Development of the Evaluation Program for Transmission Congestion Cost− Study of Cost Reduction Effects by Transmission Line Expansions −Background
Transmission expansion planning with open access to the electric network has become a critical issue for electrical utilities
in recent years. More reliable network structure may be needed with consideration of uncertainties such as change of generation
planning and occurrence of unexpected transmission congestions to establish a successful nondiscriminatory and competitive environment of electricity trading, because the electric network provides the foundation for the wholesale electricity market.

Objectives
Our study proposes an evaluation program for transmission congestion cost using economic and reliability indices withconsideration of unplanned outages of generators and transmission lines for evaluating transmission network expansion plans

Principal Results
1. Development of an evaluation program for transmission congestion cost
We developed an evaluation program for transmission congestion cost including generator re-dispatch and transmission
congestion management by TLR with consideration of unplanned outages of generators and transmission lines (Fig.1). Using ourevaluation program, the expected transmission congestion cost can be calculated. This congestion cost is consistent with increment
generation cost occurred by reduction of transmission congestion and expected outage cost calculated through multiplying outage unit
cost by expected unserved energy (EUE). This EUE is a total of each demand reduction to satisfy security constraints with consideration of unplanned outages of generator and transmission lines.
2. Study of congestion cost reduction effects by transmission network exposition
In our study, the effect of transmission network expansion plans is evaluated by the change of transmission congestion cost.
From numerical simulation studies using a simple system model ＊ 1, we obtained the following fundamental results;(1) As for the effects of transmission network extension, the depression effect of the expected EUE caused by transmission constraint
is bigger than the reduction effects of the increment generating cost by adjustment between generators to reduce transmission
congestions.
(2) There are cases where transmission congestion cost reduction effects cannot be obtained by extending power transmission lines
unless transmission congestion is reduced (Expansion at Transmission line L8 in Fig.2). Furthermore, it is possible to increase
congestion costs by aggravating transmission congestion and EUE through inappropriate power transmission line extension(Expansion at Transmission line L3 in Fig.2).
Main Researcher: Kenji Okada, Dr.,
Senior Research Engineer, Socio-Economic Research Center
Reference
K. Okada and K. Furusawa, 2008,“Development of the evaluation program of transmission congestion cost - Study of cost reduction
effects by transmission line reinforcement -” CRIEPI Report, Y070318 (in Japanese)＊1：Test system model: 8 nodes, 11 branches, 8 generators, 5 customers. It is assumed that all demands increase by 1.5%/year.

64

4. Power Delivery

Transmission congestion
management by TLR : Transmission
congestion management based on
TLR (Transmission Loading Relief)
proposed by National Electric
Reliability Corporation. When

output, locational demand is shed

Step 3: Transmission line
congestion management undereach generator’s unplanned
outage and transmission line N-1
criterion

canceled by adjustment of generation

distribution factor.

Yes

Occurrence ofUnserved Energy

Load shedding Is distributed to each demandin the proportion of demand volumeTransmission congestion
under transmission lineoutageNotransmission congestions are notaccording to the power transfer

Step 2: supply-demandbalance under each
unplanned generator
outage withouttransmission constraints

Step1: Assumption of simulation scenario and relied simulation data(such as generation, transmission data)

Volume of Loadshedding by
proportion of
supply baseTransmission
congestion relief
by TLR base load
shedding＋No

Determination of generator output underminimizing total fuel cost

Yes

Transmission congestion
under transmission lineoutage

Transmission congestion relief by
re-dispatching generators’ output

Yes

Yes

Unplanned
outage ofgenerator

Transmission congestion
under transmission lineoutage

Volume of Load shedding by TLR baseNo

Total Fuel cost
under notransmission
constraints
No＋
Total fuel cost undertransmission
constraints−−
Unsaved energy by transmission constrains

Outageunit cost

Expected
Outage Cost

Expected
Congestion
Cost

increment generating cost

Expected increment
generating cost

Probability of System condition with consideration of unplanned generator’s outage

Fig.1 Overview of the evaluation program for transmission congestion cost

4Expansion at Transmission line L880

Reduction rate oftransmission congestion cost [%]Cost
Decrease

has a small transmission congestion

60

cost reduction effect, because this40

expiation doesn’t reduce transmission
20

congestion.

0-20

When an expanded line at L3 is-40

operated after 20 years, transmission-60

cost can be improved by demand

-80

Cost
Increase

Present

After 5 years After 10 years After 15 years After 20 years

Operative start period of the extension power transmission line
Expansion at Transmission line L1（Increase of Transmission Capacity 140MW)
Expansion at Transmission line L8（Increase of Transmission Capacity 190MW)
Expansion at Transmission line L3（Increase of Transmission Capacity 170MW)

increase and change of generation
dispatch’s pattern by the secularvariation of the fuel cost.

Fig.2 Effect of congestion cost in each transmission expansion case＊Outage unit cost：700ＪＰＹ/kＷｈ. Each annual reduction of transmission cost obtained by transmission expansion is converted
to net present value by discount rate (3%). It is assumed that there is not new power development in these transmission
expansions＊Reduction rate of transmission congestion cost (%)＝（「Transmission congestion cost without transmission expansion」−「Transmission congestion cost with transmission expansion」）「/ Transmission congestion cost without transmission
expansion」
×１００65