Bài giảng Power system operation and control - Chapter 1: Concept and definition - Võ Ngọc Điều

main 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 ControlCont’d.
• Economic Dispatch Calculation: Economic dispatch 
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: The objective of 
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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Operation Philosophy
- Threats of Power Systems Security -
• Frequency instability
- 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 
small disturbances
- modes:
• local
• Inter-area
• Cascading spreading of components overloads and outages
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Quiz
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• Question: How many states are there in power system 
operation? What is the relationship among these states?
- Students establish groups of 5-6 people
- Each group will discuss and give solution (5 minutes)
- Each group will present their answer (3 minutes)
- Other groups can make questions
Operation Philosophy
- 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 -
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Operation Philosophy
- Security -
• 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.”
- 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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Operation Philosophy
- 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)
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Operation Philosophy
- Emergency Control -
• Emergency control:
- to stop the further system degradation and 
failure propagation
- 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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Operation Philosophy
- Emergency Control -
• Emergency control measures:
- Tripping of generators
- Fast generation reduction through fast-valving or 
water diversion
- Fast HVDC power transfer control
- Load shedding
- Controlled opening of interconnection to 
neighboring systems to prevent spreading of 
frequency problems
- Controlled islanding of local system into 
separate 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 -
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Day-ahead Planning 
- 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
N-1
- Modifications of the base case plan, if needed:
• Generation redispatch
• Topology changes (including maintenance disapprovals)
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Day-ahead Planning 
- Regulated markets => vertically 
integrated utility -
• Construction of the Base Case Plan
- Load forecast
- 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 
constrained OPF (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 
with N-1 Criterion
- If the base plan violates 
security constraints 
=> Congestion Management
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Operation 
- On-line Operation -
• On-line system state differs from the day-ahead forecast 
because:
- Day-ahead plan unit is one hour
- Load values vary
- Contingencies:
• Transmission components
• Generators
• Operator observes:
- if the day-ahead plan is followed
- System security:
• Operator‘s judgment
• On-line security assessment (either regular time intervals or 
‘on demand‘)
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Computations and Analysis
• Activities:
- Load forecast
- Adequacy assessment
- Security assessment
- Short-circuit studies
- Operation limits computation
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Preparation of Operation Procedures
• Emergency Scenarios:
- Recognition signs of a dangerous situation
- Employment of necessary controls
• System Restoration
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Transmission System Operation Rules –
Trans. Capacity
• Definitions by ETSO:
- 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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Transmission System Operation Rules –
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
- NTC is time dependent
- NTC refers to an interface between two systems (i.e. may be 
several tie-lines)
- NTC is theoretical value (parallel flows etc.)
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End class questions
• Please write out your questions for the lecture today and 
they will be explained in the next class.
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Thank You.
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