In electrical grid all synchronous generators operate in synchronism and deliver the power to the load. When load on the generator increases, the rotor of synchronous generator slows down resulting in reduction in the frequency of the grid. However, the governors associated with the generators will act and tries to bring back the frequency to the normal value. The control of frequency by the action of governors associated with turbine-generators is called “primary frequency control“.Governors
control the frequency by providing more steam to the turbine or more water to the turbine. However, at times the frequency control by governors alone is not adequate and secondary frequency control is required. In “secondary frequency control” the loading on different plants is changed according to the instructions given by the load dispatcher.[/ezcol_2third_end]
Load frequency characteristics of rotating machines:
The frequency control is influenced by the favorable characteristics of the large rotating machines connected in the network.
- Reduction in the supply frequency to induction motors cause reduction in the speed of the induction motors, thereby causing the reduction in the power requirement and demand
- Inertias of rotating machines have flywheel effect. Energy is released when the frequency falls below and energy will be absorbed by the rotor when the supply frequency increases
The effective load connected to the network is therefore depends on the supply frequency and voltage. A dip observed in the supply frequency and voltage results in the reduction in effective load. This in turn leads to reduction in the frequency drop i.e, the rate of dropping of frequency (df/dt) will be flatter. Load frequency control in power systems are classified into two type: Primary frequency control and secondary frequency control. They are explained below:
Primary Load Frequency Control:
The frequency control by the action of governors in the power plants for a varying load is termed as primary frequency control. Electrical energy cannot be stored in large quantities. Therefore the generator output should match the load demand. When the demand in the grid suddenly falls down due to tripping of tie line associated with bulk load, the generator output should also be reduced to cater the load demand. On the other hand, when the load on the grid increases, generator power output should also be increased. This can be possible by allowing more steam to the turbine in case of thermal power plant or by allowing more water to turbine in case of hydel plant by the governor in the power plant. Thereby the electrical output of the generator is matched with the load demand. How much load the plant must should be instructed by grid control authority. To avoid hunting after the transient, the governors are designed to remain stable at a speed corresponds to new steady state level which may differ from earlier speed. Hence primary frequency control by governor action alone may not return the frequency to required 50 or 60 Hz.
Secondary Load Frequency Control:
The frequency of the generating stations is brought back to the required value (50 or 60 Hz) by appropriate transfer of load between different areas or zones. This type of frequency control is termed as secondary frequency control. This type of control will be in addition to the primary frequency control. In primary frequency control, the amount of power to be shared by generator is determined by the turbine control system settings. In secondary frequency control, the amount of load shared by the generating station or group of generating stations in an area is determined by load dispatch center. The secondary load frequency control takes into account of economical operations of the complete system having several interconnected generating stations.
Load Frequency Control of Electrical Grid:
Electrical grid consists of several interconnected generating stations and load centers. The entire interconnection system is termed as electrical grid. The electrical grid network has following advantages or merits compared to an isolated power system.
Advantages of Interconnected Power System:
- Transfer of power among different zones helps to provide power supply
- Improving compensation of load fluctuations Multi-assistance in the event of fault Electrical grid is divided into number of grid control zones.
Exchange of power between the different zones in the power system is usually governed by certain fixed set of programs so that for a given period of time, a constant amount of power is exchanged between the two interconnected systems. If there is a frequency drop in particular zone during operation this may be due to tripping of one of the generating stations in the zone or tripping of tie line. That zone is instructed to increase the amount of power import or reduce the amount of power export . Such a control is based on Load frequency bias. The tie line control is a secondary action following the primary governor action. The grid control load centers covers all areas under the secondary frequency control.