technical considerations for power grid interconnection in

Workshop on Power Grid Interconnection in Northeast AsiaMay 14-16, 2001, Beijing, China

Felix WuFelix WuThe University of Hong Kong andThe University of Hong Kong andUniversity of California, BerkeleyUniversity of California, Berkeley

Technical Considerations for Technical Considerations for Power Grid Interconnection in Northeast AsiaPower Grid Interconnection in Northeast Asia

Outline

Cost-benefit analysis» Probabilistic production

costingOperation and control» Frequency control» Protection» Reliability

Interconnection alternatives» HVAC, HVDC, Hybrid

Reinforcement of existing networks» FACTS devices» EMS

Simulations and analysis» Power flows» Stability

NEA interconnection» Technical assessments

Potential Benefits of Interconnection

Economical» Utilization of most favorable energy resources» Use of larger and more economical power plants» Flexibility of building new power plants at favorable

locations» Sharing generation as a result of non-coincidental load

demand» Cost reduction through joint economic dispatch.» Sharing spinning reserve

Potential Benefits of Interconnection

Environmental» Coordinated use of environmentally friendly resources» Replacing resources with adverse environmental effect.» Generating electricity away from population.

Reliability» Providing emergency support.» Strengthening network connection

Others» Stimulating economic cooperation and growth.» Technology transfer

Benefit Analysis

SystemA

SystemB

InterconnectionSystem

ASystem

B

InterconnectionInterconnection

Comparative study» For each system, compare the costs of production

of alternative plans (interconnection included).Tool» Probabilistic production costing

Production Simulation

Load» Forecast load-B

Generation» Existing generation B» Import from the

interconnection.» Merit order

Simulate power system operationTo calculate production cost

From each generator and for the whole systemEnergy, fuel, pollution, etc.

Load» Forecast load-A.

Generation» Existing generation A» Export to B» Merit order

Production Costing Simulation

0

20

40

60

80

100

120

140

0 4 8 12 16 20 24 28 32 36 40 44 48 52

ExpensiveThermal

Hydro

CheapThermal

Load (GW) Annual Load Curve

Production Costing Simulation

0

20

40

60

80

100

120

140

0 4 8 12 16 20 24 28 32 36 40 44 48 52

ExpensiveThermal

CheapImport

Hydro

CheapThermal

Load (GW) Annual Load Curve

Probabilistic Production CostingTaking into account of uncertainties in generator scheduling, forced outages, hydro availability, etc.Multi-year breaking down to a series of monthly or weekly simulationsOutput provides detailed information of » Fuel usage and cost» Pollution generated» Loss of load probability» Long-run marginal cost

Can be used to assess benefits (economical and environmental) of interconnection.Data requirements

Transmission Cost

Capital costsOperational costs» Losses» Maintenance, etc.

Transmission Costs

0

0.5

1

1.5

2

2.550

0kV

DC

2 50

0kV

AC

2 73

5kV

AC

650k

V D

C

3 50

0kV

AC

2 73

5kV

AC

2 10

50kV

AC

750k

V D

C

3 75

0kV

AC

2 10

50kV

AC

USc

/kW

h

Loss costCapital cost

Based on 1200km

0

0.5

1

1.5

2

2.550

0kV

DC

2 50

0kV

AC

2 73

5kV

AC

650k

V D

C

3 50

0kV

AC

2 73

5kV

AC

2 10

50kV

AC

750k

V D

C

3 75

0kV

AC

2 10

50kV

AC

USc

/kW

h

Loss costCapital cost

Based on 1200km

Factors Determining Cost/Benefit

Benefit» Transfer capability

Cost» Interconnection site» Interconnection method

How much

Where

What

Operation and Control

Where, what, and how much to interconnect affects operation and control of both systems.Existing systems are AC.Will look at AC system operation and control requirements.

AC Networks

Power flow control» Power flows and voltage distribution in an AC power

network are determined by Kirchhoff and Ohm laws. » Individual element has very little control over the whole

system.System dynamic performance» Quick and wide-spread response to local disturbance.» Disturbance may cause cascading outages.» Mitigating measures are designed and built into the

system.

Operation and Control of AC Network

Frequency control» Frequency is an indication of power balance

between generation and load.» Frequency deviation from normal (50Hz or

60Hz) requires immediate action to balance supply/demand.

» Limitation on generator response.

Operation and Control of AC Network

Fault protection» Disturbances or faults such as short-circuit

caused by lightening, tree falling, etc. happen frequently on power system.

» Protective relays are used to sense a fault.Circuit breakers are used to isolate the fault.

» Fast clearing of faults is essential to prevent the effect to spread and propagate.

Operation and Control of AC Network

Reliability operation» System should be able to withstand credible

disturbances, called n-1 criteria, in the west.» Steady-state: no transmission line overload and

no abnormal voltage immediate after a disturbance.

» Transient: no stability problem.

Stability

0 2 4 6-2 0 0

0

2 0 0

4 0 0

6 0 0

8 0 0

1 0 0 0

1 2 0 0

s e c o n d s

ang

le in

de

gre

es

m a c h in e a n g le s

0 2 4 6 8 1 00

1 0

2 0

3 0

4 0

5 0

6 0

7 0

8 0

9 0m a c h ine a ng le s

s e c o nd s

ang

le in

de

gre

es

0 2 4 6-2 0 0

0

2 0 0

4 0 0

6 0 0

8 0 0

1 0 0 0

1 2 0 0

s e c o n d s

ang

le in

de

gre

es

m a c h in e a n g le s

0 2 4 6 8 1 00

1 0

2 0

3 0

4 0

5 0

6 0

7 0

8 0

9 0m a c h ine a ng le s

s e c o nd s

ang

le in

de

gre

es

Interconnection Alternatives

System A System BHVAC

System A System BHVDC

System A System BHybrid

System A System BHVAC System A System BSystem A System BHVAC

System A System BHVDC System A System BSystem A System BSystem A System BHVDC

System A System BHybrid

System A System BSystem A System BHybrid

HVAC Interconnection Pre-conditions

Adjustment of frequency control» Adding interchange schedule into frequency control» Area control error = (interchange schedule) + B x δ f» Enough generators under control to respond» Joint responsibility

Protection coordinationCommon reliability criteria

Stability of Weak Interconnection

HVDC Interconnection

Technical advantage» No need for common frequency control.» Stable operation at low power interconnection.» Back-to-back HVDC connection is used.

Technical disadvantage» Generating harmonics» Need reactive power source at converter station» Might bring about subsynchronous oscillation

Bipolar HVDC Transmission

Hybrid AC/DC Interconnection

HVDC can damp oscillations

Example: Pacific intertie

Reinforcement of Regional Networks

Reinforcement may become necessary after interconnection» Stability and other reliability considerations

Flexible AC transmission system (FACTS) devicesComputer-controlled energy management systems (EMS)

FACTS Devices

Power electronics devicesProviding reactive power/voltage support» Static var compensator (SVC)

Improving transfer limit» Static synchronous copensator (STATCOM)

Improving stability limit» Thyristor controlled series compensator (TCSC)

Power flow control (both real and reactive)» Unified power flow controller (UPFC)

Damping power oscillations» STATCOM, UPFC

UPFC

Energy Management System

Generator

Substation

Large load

Tie line

RTU

Communication Operator’sconsoleComputer

Application programs

Interconnected System Performance: Simulation and Analysis

Steady-state performance» Power flow analysis

System dynamic performance» Synchronization after a fault: Transient stability, long-

term dynamics» Low frequency oscillations: small-signal stability» Voltage collapse: voltage stability» Subsynchronous oscillations

Summary

Technical issues arising from large system interconnection are very complex» Power flow and voltage» Dynamics

Cost-benefit analysis and technical issues and solutions need to work out in parallel.

Northeast Asia Interconnection

Technical Factors Considered

Long-distance DC links/ Short-distance AC linksPotential benefits» Balancing surplus and deficiency» Utilizing more desirable resources » Leveling summer and winter demands

Technical difficulty with » Reducing reserve» Joint dispatch» Savings (economical and environmental) from

exchanges over multiple regional networks.

Technical Assessment of NEA Interconnection

Difference of regional networks in» Strength» Planning & operational criteria» Technical standards

Coordination and AgreementResults of technical analysis depends on» Models» Data» Assumptions

Objectivity and Consensus