Module 7 - Frequency control Flashcards
What is frequency?
- rate at which current or voltage changes direction per second
- for an AC system, generators can be interconnected in parallel if they are of the same frequency
Early Power frequency
- Before interconnection in late 19th century, people used many different frequencies (16Hz to 133 Hz)
- choice of frequency influenced many factors like lighting, motor transformers, generators, transmission lines etc.
Advantages and Limitations of interconnected Power Systems
Advantages:
- increased reliability of supply because of continuity of supply if breakdown of bad station
- increase in load diversity factor as different power systems have different load curves
- can draw upon another power systems generator when needed
- economical operation
- efficient plants can provide power during peak hours
- seasonal diversity for example hydro vs thermal plants
Limitations:
- disturbances not defined to one utility
Interconnected Power System Operation
- Real power generated has to adjust to meet changes in load requirements
- large interconnected power systems are divided into interconnections
- to be safe and reliable, each interconnection must have several control areas to monitor and control power flow
- each area carries its own load under normal operating conditions
- each control area will regulate and control the power system
What must a power system that is connected to other power systems ensure?
- Power generation should be equal to load demand
- maintain reserves
- assist during an emergency
- support frequency
Why frequency control?
- Must control active power to stabalize frequency
- load demands changing constantly
- deviations of frequency will impact the quality of the supply
what does excess generation mean?
generator speeds up and frequency rises
what does excess demand mean?
generator slows down and frequency drops
South African Grid Code (SAGC)
- each country has a grid code to specifcy design and grid requirements
- for hydro alternators: if f>54Hz for t>1s, trip
- system f<46Hz for t>1s, trip can occur to protect unit
- For renewable power plants, different specs. if system f>51.5Hz for t>4s, RPP disconnected. ALso is system f<47 Hz for t<200ms, RPP may be disconnected.
Single and Multiple contingencies
- Single contingency is single event that is sudden and unplanned. Typically restored within 5 mins
- Multiple contingency is more than one unplanend or sudden event. Can significantly impact power system
Frequency boundaries
- typically +-1Hz is deviation allowed internationally
- not necessary to make drastic measures to restore f rapidly through load shedding because this f is acceptable to all.
What is primary frequency control?
- Governer control (within a few seconds
- proportional controller designed to increase generator output (opening governer valve) in response to frequency drop
- located at individual generators ( distributed)
- controller changes power output in proportion to changes in f
Objectives of primary frequency control?
- maintain balance between generation and demand
- proportional controller
- must have real time reliability by stabilizing the system f
- must be adequate and this depends on availability of generator reserves.
causes of frequency deviation?
- imbalance between generation and demand
- PL>PG means falling frequency
- PL<PG means rising frequency
What is regulation/droop characteristic?
- ratio of change in f verses change in active power (in percentage)
- participation of generator to correct f deviation
- smaller droop means LARGER participation
- IMPORTANT TO NOTE (Primary control doesnt bring it back to nominal, it brings it to equilibrium.
what does the load do when there are frequency deviations
- motor loads are the ones sensitive to frequency
- load of asynchronous machine varies with frequency deviations
What is automatic generation control (ACG)?
- Secondary frequency control
- second line of defense against frequency deviation
what are the objectives of AGC
- maintain frequency at scheduled frequency
- maintain total power exchange at contractual value
- ensure optimal value for generation
Main features of AGC?
- centralized control
- proportional-integral controller to ensure than change in f returns to zero
what are the three modes of operation of AGC?
- flat frequency control
- flat tie-line control
- tie-line bias control
Flat frequency and flat tie-line control
- isolated systems inherently operate in flat frequency mode cause f is only quantity that is affected when load changes
- when systems interconnected, must operate at same speed and speed change in one system
- when one system of interconnected system senses f changes, it cant control the flow of an interconnected line, only its own system
- flat tie-line control occurs when tie-line flow changes occur and not when frequency changes.
tie-line bias control?
- each control area should provide enough power to carry its expected load at scheduled f
- each control area should operate so that it does not impose regulating burden on controlling areas
- following load change, each area should assist in returning steady-state f error to zero.
Main reasons for frequency control? **
1.) reduce mechanical and electrical stresses on generators. To comply with acceptable standard of operation.
2.) To balance load and generation
3.) To reduce damage to consumer devices connected to the grid.
