The problems that arise in system control and management are solved on models, using programming. A model is an intermediate process which is used for analysis of the prototype process. In economics, a model is a set of equations based on certain assumptions that describe the national economy as a whole or a part thereof. Among the problems facing the control and management of large power systems, the most important is the search for a better method of power system optimisation:
Tasks in control and management
The tasks involved in control and management of an interconnected power system are divided into three groups:
Group I – (from seconds to hours)
This group includes the direct operational control of the power system. The control engineer does this as a matter of daily routine. The tasks include involving normal steady-state, normal transient state, abnormal state, post-fault or restorative (transient and steady) state.
Group II- Short-term management cycle (from days to weeks to months)
The tasks in this group involve short-term scheduling, including the near future.
Group III- Long-term management cycle (from one to several years)
This group involves long-term cycles, that is, long–term planning and forecasting. This group may include planning the level of operation for a day, or forecasting loads for the next month. However, in most cases forecasting refers to longer and planning to shorter periods of time.
The details of tasks involved in the three groups are given below:
Tasks involved in Group 1 – (from seconds to hours)
Normal steady state:
- Optimal system regulation on hourly basis, including active and reactive power dispatch at power stations and substations equipped with reactive power sources
- Frequency and power interchange control over tie lines (including limits of interchange)
Normal transient state:
- Routine system switching operations
- Frequency and active power interchange control over tie lines (including limits of interchange)
- Optimum system scheduling on hourly basis
- Optimum economic and var dispatch at stations and substations with reactive power sources
- Voltage control at system centres
- Load limiting and load shedding (scheduled outages)- order to lower control levels
- Integrated operation of automatic failure clearing means, including:
– switching in networks (system sectionalizing and area isolation)
– automatically connecting to inter-system and trunk lines
– Emergency load shedding
– Control of mechanical and electrical braking for generators
– Emergency regulation of generator excitation and voltage at system centres
– Limiting active power at stations.
- Regulation of frequency and active power flow over transmission line in emergency.
Post-fault or restorative (transient and steady state)
- Integrated operation of automatic state-restoration means including:
– Switching in network, re-synchronisation of isolated system parts, restoration of disrupted connections (including loads)
– change-over to hot and cold stand-by sources
- Frequency and active power control
- System dispatch including
– Economic and var dispatch for stations and substations with reactive power sources
– Voltage regulation at system centres.
Tasks involved in Group 2 (from days and weeks to months)
Short-term management cycle
- Forecasts on active and reactive loads at system centres (on daily and monthly basis)
- Forecasts on river runoff
- Calculation of static operating states of the system
- Calculation of short-circuit currents
- Calculation of protective relay settings, and creating a selectivity chart
- Selection of gains of regulators for excitation, speed, transformers etc.
- Selection of failure-preventive control settings
- Analysis of steady-state stability, and determination of maximum power interchange over power transmission line
- Transient analysis and determination of maximum power interchange over transmission lines under dynamic stability
- System reserve dispatch (monthly, weekly, daily), including:
– Selection of plant mix
– Selection of plant for cold standby
– Maintenance scheduling on monthly basis.
- Determination of interoperation logic for automatic controls and regulators in case of emergency
- Determination of logic for restorative conditions
- Inflow, usage and spillage for hydro reservoirs and cascaded stations (on weekly and monthly basis)
- Outage scheduling
- Monthly power balance and power generation for system areas.
Tasks involved in Group 3 (from one to several years)
Long-term management cycle
- Forecasts on active and reactive loads at system centres
- Forecasts on reserve run-off
- Forecasts on steady-state load characteristics for system centres
- Processing of emergency and failure statistics
- Calculation of static operating states for the system
- Calculation of short circuit currents
- Selection of gains for regulators(excitation speed ,transformers etc.)
- Selection of settings for automatic failure-clearing means
- Steady-state stability analysis and determination of maximum power flow over transmission line under steady-state conditions
- Transient analysis and determination of maximum power flow under dynamic stability conditions
- Reserve dispatch ( on yearly basis) including:
– Selection of plant mix in operation
– Selection of plant for cold-standby
– Scheduling of mail plant.
- Determination of interoperation logic for automatic controllers and regulators in emergency conditions
- Determination of logic for post-fault (restorative ) system state
- Calculation of automatic frequency regulation on semi-annual basis
- Calculation of over-voltages (internal and atmospheric)
- Inflow, usage and spillage scheduling for hydro reservoirs and cascaded stations on annual and quarterly basis
- Calculation of maximum and optimal levels of operation
- Scheduled load outage
- Annual power balance and power generation for system areas.
Among the problems that face the control and management of large power systems, the most important is the search for better and novel methods of power system optimization. It is necessary to work out newer mathematical models that could reflect the actual hierarchical structure of very large systems. It is necessary to look for reliable methods for building such models.
It is also necessary to set up a system which would gather, process and present data, evaluate the effects of data corruption, and advance both theoretical and experimental work on data transmission.
The problems that need solution before a reliable management information system can be set up for the power industry are interrelationships between large power systems and the national economy and the biosphere, which is the environment, in the broad sense of the word.
It is essential to study the principal directions in which the power system in the country may develop in the next few years, and evaluate the prospects of using more renewable energy sources and the converting means in the existing power system.