TỐI ƯU HÓA VẬN HÀNH HỆ THỐNG ĐIỆN

Course Outline Concept and definition Introduction to optimization Economic dispatch Unit commitment Optimal power flow Hydrothermal coordination Smart grid optimization Project presentationCourse Outline Concept and definition Introduction to optimization Economic dispatch Unit commitment Optimal power flow Hydrothermal coordination Smart grid optimization Project presentation

• In 1831, Michael Faraday’s many years of efforts rewarded when he

discovered electromagnetic induction

• Later, he invented the first generator

• Today, electric energy technologies have a central role in social and

economic development at all scales

• Energy is closely linked to environmental pollution and degradation, to

economic development and quality of life

• Today, we are mostly dependent on nonrenewable fossil fuels that have

been and will continue to be a major cause of pollution and climate

change

• Finding sustainable alternatives is becoming increasingly urgent

• Operation and control of power system is an extremely complex task

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• Electric Capacity is a term that defines the rated continuous

load-carrying ability, expressed in megawatts (MW) or megavolt-amperes

(MVA) of generation, transmission, or other electrical equipment

• Electric Energy is the term that defines the generation or use of

electric power by a device over a period of time It is expressed in

kilowatt-hours (kWh), megawatt-hours (MWh), or gigawatt-hours

(GWh)

• In context of electric circuits, the term ‘load’ refers to any device in

which power is being dissipated (i.e consumed)

• In larger context of the power system, loads are usually modeled in an

aggregated way rather than an individual appliance Load may refer to

an entire household, a city block or all the customers within a certain

region

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Type of loads

• Resistive loads (25%): Heating and lighting equipments

e.g.Toaster, iron, electric blankets, Incandescent lamps

• Motors (70%): Compressors (air conditioner, refrigerator)

Pumps (well, pool), Fans Household appliances (washer, mixer, vacuum cleaner)

Large commercial 3-phase motors (grocery store chiller)

Power tools (hand drill, lawn mower) Electric street cars

Basically ‘anything’ that moves!

• Electronic devices (5%): Power supplies for computers etc.

Transformers (adapter, battery charger)

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Type of loads

• From system’s point of view, there are 5 broad category of loads: Domestic,

Commercial, Industrial, Agriculture and others

Domestic:

lights, fans, domestic appliances like heaters, refrigerators, air conditioners, mixers,

ovens, small motors etc.

Demand factor = 0.7 to 1.0; Diversity factor = 1.2 to 1.3; Load factor = 0.1 to 0.15

Commercial:

Lightings for shops, advertising hoardings, fans, AC etc.

Demand factor = 0.9 to 1.0; Diversity factor = 1.1 to 1.2; Load factor = 0.25 to 0.3

Industrial:

Small scale industries: 0-20kW

Medium scale industries: 20-100kW

Large scale industries: above 100kW

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Type of loads ……Cont’d

Industrial loads need power over a longer period which remains fairly

uniform throughout the day

For heavy industries:

Demand factor = 0.85 to 0.9; Load factor = 0.7 to 0.8

Agriculture:

Supplying water for irrigation using pumps driven by motors

Demand factor = 0.9 to 1; Diversity factor = 1.0 to 1.5; Load factor = 0.15

to 0.25

Other Loads:

Bulk supplies, street lights, traction, government loads which have their

own peculiar characteristics

“Load” is an externally given quantity, a variable beyond control, in a completely

unselfconscious manner.

Load Connected Total

Demand Maximum Actual

=

Factor

Demand

system the of Peak Actual

demands maximum

individual of

period given time a

over Load Average

=

r

Load Facto

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Electric Power System Operation.

• Operational objectives of a power system have been to provide a

continuous quality service with minimum cost to the user These

objectives are:

• The term “continuous service” can be translated to mean “secure and reliable

service”

First Objective: Supplying the energy user with quality service, i.e., at

acceptable voltage and frequency

Second Objective: Meeting the first objective with acceptable

impact upon the environment.

continuously, i.e., with adequate security and reliability.

Fourth Objective: Meeting the first, second, and third objectives

with optimum economy, i.e., minimum cost to the energy user.

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Operation Control

• The primary functions of operations control are satisfying the

instantaneous load on a second-to-second and minute-to-minute basis.

Some of these functions are:

Load Frequency Control

On-Line Load Flow

Economic Dispatch Calculation (EDC)

Operating Reserve Calculation (ORC)

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Operation Control……Cont’d.

• Load Frequency Control (LFC) This function is also referred to as

governor response As the load demand of the power system increases, the

speed of generators will decrease and this will reduce the system

frequency Similarly, as system load demand decreases, the speed of the

system generators would increase and this will increase the system

frequency The power system frequency control must be maintained for the

power system grid to remain stable.

• Online Load Flow (OLF): This function generally utilizes the output of

network topology, i.e the real time network model, and the bus injections

from state estimation for purpose of security monitoring, security analysis

and penalty factor calculations This function performs “if then condition”

to determine the possible system states (voltages) in face of system

outages such as loss of a line due to weather condition or sudden loss of a

generator.

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Operation Control……Cont’d.

calculation of a power system determines the loading of

each generator on a minute-by-minute basis so as to

minimize the operating costs

operating reserve calculation is to calculate the actual

reserve carried by each unit and to check whether or not

there is a sufficient reserve in a system The operating

reserve consists of spinning reserve (synchronized),

non-spinning reserve (non-synchronized), and interruptible load

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Operation Philosophy

Important Terms

-• Stability:

- Continuance of intact operation following a disturbance It depends on the

operating condition and the nature of the physical disturbance.

• Security:

- Degree of risk in power system ability to survive imminent disturbances

(contingencies) without interruption of customer service It relates to

robustness of the system to imminent disturbances and, hence, depends on the

system operating condition as well as the contingent probability of

disturbances.

• Reliability:

- Probability of power system satisfactory operation over the long run It

denotes the ability to supply adequate electric service on a nearly continuous

basis, with few interruptions over an extended time period.

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- is inability of a power system to maintain steady frequency

within the operating limits

- it is in its nature rather a tracking than truly a stability control

problem

• Voltage instability

- the inability of a power system to maintain steady acceptable

voltages at all buses

- system enters a state of voltage instability when a disturbance,

increase in load demand, or change in system conditions causes

a progressive and uncontrollable drop in voltage

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Operation Philosophy

Threats of Power Systems Security

-• Transient angular instability

- inability of the power system to maintain synchronism when

subjected to a severe transient disturbance

• Small-signal angular instability

- inability of the power system to maintain synchronism under

Operation States

Normal – no equipment overloaded The system can withstand any contingency without violating any of constraints.

- Alert – no equipment overloaded yet The system is weakened - a contingency may cause an overloading of equipment, resulting in emergency state.

- Emergency – Some equipment overloaded If no control action executed, system progresses into In Extremis.

- In Extremis – Cascading spreading of system components outages resulting in partial or system-wide blackout.

- Restoration – Energizing of the system or its parts and reconnection and resynchronization of system parts.

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Operation Philosophy

Operation States

-16

of the system to imminent disturbances and, hence, depends on the system operating condition as well as the contingent probability of disturbances.”

- Normal state is secure

- All other states are insecure

• The transition/border between Normal and Alert state is

expressed by N – 1 criterion:

- Outage of a single component can not lead to violation

of operation limits of any other component.

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Operation Philosophy

Preventive Control

-• Preventive Control:

- to keep the system in Normal state

- to bring the system back into Normal state

- Hierarchical automatic control:

• Frequency control

• Voltage control

- Centralized manual control:

• Decision support tools

• Operator judgment

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Preventive Control

-• Preventive control measures :

- Generation redispatch (change of active power production of generators)

- Change of reference points of controllable devices (e.g FACTS, phase-shifting transformers)

- Start-up of generation units

- Change of voltage reference points of generators and voltage control devices (e.g Static Var Compensator)

- Switching of shunt elements (e.g reactors, capacitors)

- Change of substation configuration (e.g busbars splitting)

- to bring the system back into Alert state

- Protection based systems

• Under frequency load shedding (UFLS) schemes

• Under voltage load shedding (UVLS) schemes

• System Protection Schemes (SPS)

- Damping control

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- Controlled islanding of local system intoseparate areas with generation-load balance

- Blocking of tap changer of transformers

- Insertion of a breaking resistor

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Day-ahead Planning

Time Scale Decomposition

-22

Day Ahead Operation Planning

-• Construction of the base case plan (i.e loading and

generation) for the coming day (0:00 – 24:00, basic unit is 1

hour):

- Expected loads’ values

- Scheduled generators’ production

- Limitations of the transmission system are not considered yet !

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Day-ahead Planning

Day Ahead Operation Planning

-• Security considerations and possible adjustments of the

base case plan

- Scheduled outages (i.e expected topology)

- Security Assessment of the base case plan, i.e compliance with

- Generation dispatch (including basic generation, AGC

participation and reserves allocation)

• Security Considerations

- Security Assessment of the base case plan, i.e compliance with

N-1 Criterion

- If the base plan violates security constraints => Security

constrainedOPF (Optimal Power Flow)

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Day-ahead Planning

Deregulated markets => TSO/ISO

-• Construction of the Base Case Plan

- Collection of long-term bilateral agreements

- Clearance of day-ahead,AGC and balancing markets

- Scheduling of AGC areas interchanges

- Allocation of transmission capacity between systems

• Security Considerations

- Security Assessment of the

base case plan, i.e compliance

Online Operation

-• On-line system state differs from the day-ahead forecast

because:

- Day-ahead plan unit is one hour

- Load values vary

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Preparation of Operation Procedures

• Emergency Scenarios:

- Recognition signs of a dangerous situation

- Employment of necessary controls

- NTC (Net Transfer capacity) – the maximum exchange program

between two areas compatible with security standards applicable in both

areas and taking into account technical uncertainties on future network

conditions

• NTC = TTC – TRM

- AAC (Already Allocated Capacity) – total amount of allocated

transmission rights, whether they are capacity or exchange programs

depending on the allocation method

- ATC (Available Transmission Capacity) – the part of NTC that remains

available, after each phase of the allocation procedure, for further

commercial activity

• ATC = NTC - AAC

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Trans Capacity

• Important remarks:

- There are two phases of activities related to trans capacities:

• planning phase – computation of NTC etc

• allocation phase – market mechanism

- NTC does not consider transient stability dependent on

clearing time !

- NTC considers all other stability and components overloads

limits under N-1 criteria assumption

1

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Dr Vo Ngoc Dieu Department of Power Systems

Ho Chi Minh City University of Technology

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0 X

x

f

x x f x

H and ) xˆ f(

0, ) xˆ f(

that necessary

ly continuous

∆x

x f 2

1

∆x

x

f f(x)

1 j ij j i T

2 2

1

n

2

n 1 2

2 1

⋅

∂

∂ +

M O M

M O

M

L

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0 8x x 1 x 4x 1 x f x f )

2

1 2

1

2 2 1 2 2 1 2

− + +

⋅

∇ +

=

+

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necessary

Thus,

minimum) not is (This

) xˆ , xˆ f(

1 0 x

f ) xˆ , xˆ f(

f Say

∆x

∆x ) x , f(x ) xˆ , xˆ f(

of

on

Minimizati

2 1 1 2 1 2

2

1

1 1

2 1 2 1 2 1 2

≈ +

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0 X

x

f

x x f x

x

f x

H and ) xˆ f(

that sufficient

0, ) xˆ f(

that necessary

ly continuous

∆x

x f 2

1

∆x

x

f f(x)

1 j ij j T

2 2

1

n

2

n 1 2

2 1 2 2

2 2

2 2 2

⋅

∂∂+

M O M

M O

M

L

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0 8x x 1 x 4x 1 x f x f )

∆X

2 1

∆X

)

Xˆf(

2

1 2

1

2 2 1 2 2 1 2

− + +

⋅

∇ +

=

+

0 a a a AX X (1,1) X

0 a AX X (0,1) X

2 he Consider t IR X nonzero all for 0 AX X if definite positive

22 22 11 T T

22 T T 12

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11 T T 22

21

12

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2 T

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Dr Vo Ngoc Dieu Department of Power Systems

Ho Chi Minh City University of Technology

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Lagrange Relaxation - Dual Variables

Another way to solve an optimization problem is to use a technique that

solves for the Lagrange variables directly and then solves for the problem

variables themselves.

) x , (fix x ) q(

Max ) ( q

find to is problem"

dual

"

Then the

) (fix ) , x , L(x Min ) q(

) q(

function dual

λ(5

x 0.25x ) ,

x 0.25x ) x , f(x Min

Dual

2 1 0

*

2 1 x , x

2 1 2

2 2 1 2

1

2 1 2 1

2 2 2 1 2

1

2 1

λ λ

λ λ

λ

λ λ

λ

=

=

+ +

5

2 2 5 x x 5 0

or 2 2 5 4

2 2 5 2 ) 2 ( 25

x

L

2 x x

Max ) , x , L(x Min

2 2 2

2

2 2

2 2

2

1 1

1

* 0

2 1 x

λ

λ λ λ

λ λ λ λ

λ λ λ λ λ λ

λ λ

λ λ

λ λ λ

λ

λ

negative is d ) dq(

d ) dq(

,

in function explicit

Max )

(

q

on optimize

first

We

functions.

complex be

may functions objective

the

since

variables problem

the eliminate cannot

one problems,

on optimizati

some

In

1 x

4 x

5 )

λ

λ α

α λ

λ λ

λ

λ

λ λ

λ λ

2 2 1 x , x

*

q q J gap duality

will

value

This

) x - x -

λ(5

x 0.25x Min L Min

J

on optimizati primal

of value

between

gap"

"

relative the is

on optimizait dual

in the solution final

−

=

+ +

=

=

5 - d

0 5 4

5 - ) ( q

0 5 x x - 5 ) x , h(x

0 2 x

0 2 x

0

(1) (1) 2

(1) (1) 1

(1)

= +

λ λ λ

λ λ λ

non and

with problem

convex

For

0666 0 4.6875 4.6875 - 5 q q J

6875 4 ) 5 2 ( 5 ) 5 2 ( 4

5 - ) ( q

25 1 ) x , h(x

25 1 x

5 x

2.5 ) 5 0 ( 5 0 d

dq

λ

λ

α λ λ λ