Interconnected Power Stations: In order to supply loads from two or more power stations, they are interconnected such that there is no overloading and loads are shared equally. In this section, we will discuss the interconnected power stations, the need for interconnection of power stations, illustration of how the power stations are interconnected through an interconnector, load sharing in interconnected power stations, and power limit of the interconnections used in interconnected power stations.
Needs of Interconnected Power Stations
The need of interconnecting the power stations in a power system network are as follows:
- The design allows a generation station’s peak load to be exchanged. In the case of individual generating station operation, if peak load exceeds the generation station’s capacity, we must impose partial load shedding on the system. However, by interconnecting the power stations in a power system network we can eliminate the partial load shedding by exchanging the loads.
- During the event of fault or breakdown, the power can be supplied from another interconnected station in the case of an interconnected power station thereby the reliability of the power system network can be increased.
- Different generating stations operation can be made cost-effective by interconnecting them. Such as the operation of hydroelectric and thermal power plants is made more cost-effective through interconnection. The hydroelectric power station is used as a baseload station when water is plentiful, such as during the rainy season, while the thermal power station is used as a peak load station since the hydroelectric plant’s operating costs are cheap. When water is in short supply, such as during the summer, the thermal power plant takes the baseload and the hydroelectric power plant takes peak load.
- By interconnecting the power stations in a power system network, the reserve capacity of the system is increased, along with the load factor, and the efficiency of operation is increased.
Illustration of Interconnected Power Stations
Let us consider a scenario where a load is connected through two power stations. The S1 and S2 are the power stations which is supplying the load through transmission lines 1 and 2 and the respective currents on the line are I1 and I2 respectively.
The power stations S1 and S2 are interconnected by an interconnector through which Ii flows.
As we are connecting loads to the power stations, the voltage at their respective bus bar has to be maintained at a constant value.
Delivering Equal Power in Interconnected Power Stations
In order that both the transmission lines 1 & 2 to deliver an equal amount of power at the same terminal voltage, the active component of the current I1 and I2 should be equal.
To maintain the total current flowing through transmission lines 1 & 2 at a minimum value the reactive components of the current must be equal.
For achieving this in practical life, we need a regulating device. This regulating device needs to be installed at the sending end of each of the transmission lines and in the interconnector of interconnected power stations.
The regulating device which we will connect at the sending end of each transmission line and in the interconnector will help in compensating the voltage drops in a line and the interconnector.
For stable operations of the interconnected power stations where alternators are operating in parallel, we would require a reactor that is to be interconnected between two stations.
During the operating conditions, it may be required for the power to flow from one station to the other. This is achieved by the incorporation of interconnectors that introduces displacement between the two stations and power flow from one station to another station occurs.
Load Sharing in Interconnected Power Stations
Here, we will observe two power stations operating in parallel. A and B are interconnected power stations. They are connected with each other through interconnectors.
VA and VB be the operating voltages at the station A and B. Both of these voltages are equal in magnitude but are displaced from each other by an angle Φ.
VAB is the resultant voltage of VA and VB acting from B to A and it causes a flow of current I through the interconnector.
The I lags the voltage VAB across the interconnector by an angle 90o which is shown in the phasor diagram below:
Unless there exists an angular displacement between the voltages of the power station the power flow does not take place.
In the above case, VB leads VA, so power flow occurs from station B to station A.
Inphase Voltage Boost
Here, the voltage of station B is boosted to V’B such that the voltage across the interconnector is V’AB, current I lags VAB by 90o and I is in phase with VA.
As the current is perpendicular to V’B and in phase with VA, no active power flow occurs.
Quadrature Voltage Boost
Here, the voltage of station B is made to V’B, and the voltage of VA is made equal to VB. Such that the current I is in phase with V’B.
Here, active power flow occurs from station B to station A.
Power Limit of Interconnectors in Interconnected Power Stations
The power which is transmitted through the interconnects depends upon the phase displacement between the voltage of the interconnected power stations.
The power transfer is maximum when the phase displacement between the voltages is 90o.
Under the mentioned condition for maximum power transfer.
Maximum power transfer is also known as the synchronous capacity of the interconnector.
The amount of power transmitted (active power) per radian of displacement between two voltages of the power stations is called synchronous capacity.