Interconnected Power System:
In some parts of the country, there are resources for generating power but their requirements are low where as in other parts power requirement is large as compared to the available resources of power.
These above difficulties can be reduced by interconnecting different power stations in parallel.
The connection of numbers of power generating stations in parallel is known as Interconnected Power System.
The interconnected power system is basically an interconnection of the buses of more than one generating station, so as to utilize the unused generated power capacity of connected generators during peak demand.
It is also simply defined as "A distribution system with multiple available power sources that can loop throughout the network." If one source goes down, a different source can be activated to maintain service.
Fig. shows the Inner connected Power system having two Power Generating Station G1 and G2 supplying current I1 and I2 to the load through the transmission lines 1 and 2 in addition to their local load.
The Power Generating Stations are also interconnected by the Transmission line, which is carrying the current I1.
The transfer of power is taking place as shown by the direction of arrows. In order that both transmission lines 1 and 2 deliver equal power and the system operates at the same terminal voltage, it is necessary that the active components of the line currents I1 and I2 are equal.
Types of Interconnected Power Systems:
There are mainly three types of interconnection.
1. AC Interconnection
2. DC Interconnection
3. Hybrid AC/DC Interconnection
AC Interconnection :
Fig. shows the AC interconnection scheme, two systems are directly connected.
DC Interconnection :
Fig. shows the DC interconnected system. Both systems A and B have no converting station to change the voltage level and control the voltage.
Transmission of large power blocks over long distances is however utilized by HVDC at the same time can strengthen the interconnection to avoid dynamic problems.
Hybrid AC/DC Interconnection :
Fig. shows the hybrid interconnection system. Both AC and DC power is transmitted over a long distance. The hybrid interconnection could therefore offer the best possibility for large system interconnections in the case of continental networks, consisting of a number of small systems.
Problem Associated with Interconnected Power System:
The Problems are associated with the satisfactory working of an interconnected Power System in India are :
1. Voltage Control :
It is Necessary that a constant voltage is maintained in the interconnected power system.
The problem of low voltage is mainly serving in the summer season due to heavy inductive loads of Air conditioners, fans, water pumps, etc.
These problems are overcome by meeting reactive power requirements at the load centers. It can be overcome by connecting a capacitor in the shunt of the line to provide reactive power to the system.
2. Frequency Control :
It is known that for the proper system operation, the system frequency should maintain at least between 48.5 to 50.5 Hz.
In some cases during a peak load condition, frequency tends to be very low 48.5 Hz and during peak off condition, frequency goes up to about 51 Hz.
Under this condition, we use under frequency relay for automatic load shedding and connected new generating units in parallel with the system.
3. Load Dispatch and Communication :
The present load dispatch and communication facilities are inadequate.
The present power line carrier communication (PLCC) system cannot be expanded due to congestion in the available frequency spectrum difficulty in cascade random operation etc.
It is proposed to add a multiple channel digital microwave system and fiber optic communication system to provide better load dispatch and communication facilities.
4. Shortage of Spinning Reserve :
During the peak load time, the demand is more than the power generation, for better load management the additional generation facilities are needed.
5. Lack of Operational Discipline among Constituents:
In an interconnected power system, each constituent in a region maintains a balance between its own generation and demand.
However, due to the overall shortage in installed capacity, the constituents tends to overdraw share from the central station. Due to this disturbance created in system and system stability problem occurs.
6. Metering and Instrumentation :
Adequate metering and instrumentation are needed at different levels.
It is necessary to install sequential recorders, disturbance recorders, time of day meters, etc.
Advantages of Interconnected Power Systems:
Interconnected power systems have the following advantages :
1. Exchange of Peak Loads :
An important advantage of an interconnected system is that the peak load of the power station can be exchanged.
If the load curve of a power station shows a peak demand that is greater than the rated capacity of the plant, then the excess load can be shared by other stations interconnected with it.
2. Use of Older Plants :
The interconnected system makes it possible to use the older and less efficient plants to carry peak loads of short durations.
Although such plants may be inadequate when used alone, yet they have sufficient capacity to carry short peaks of loads when interconnected with other modern plants.
Therefore, an interconnected system gives a direct key to the use of obsolete plants.
3. Ensures Economical Operation :
The interconnected system makes the operation of concerned power stations quite economical.
It is because sharing of load among the stations is arranged in such a way that more efficient stations work continuously throughout the year at a high load factor and the less efficient plants work for peak load hours only.
4. Increases Diversity Factor :
The load curves of different interconnected stations are generally different.
The result is that the maximum demand on the system is much reduced as compared to the sum of individual maximum demands on different stations,
In other words, the diversity factor of the system is improved, thereby increasing the effective capacity of the system.
5. Reduces Plant Reserve Capacity:
Every power station is required to have a standby unit for emergencies.
However, when several power stations are connected in parallel, the reserve capacity of the system is much reduced. This increases the efficiency of the system
6. Increases Reliability of Supply:
The interconnected system increases the reliability of supply. If a major breakdown occurs in one station, continuity of supply can be maintained by other healthy stations.
Disadvantages of Interconnected Power Systems:
The disadvantages of the interconnected Power system are given below.
1. Synchronizing Problem:
The generators of all the interconnected generating stations must operate at the same frequency and in a synchronized manner.
During the heavy load condition, some generators go out of step due to this synchronization breakup.
And finally, the complete system that goes out of step is also called the blackout condition.
2. Expensive Tie Lines :
In the interconnected power system to control the system reactive power, tie lines are used.
The expense of tie lines for the construction of interconnecting transmission line between generating stations is very high.
3. Expensive Circuit Breaker :
An interconnected system use a circuit breaker to isolate the faulty part from the healthy part during the fault condition.
For high voltage power systems, a higher rating circuit breaker is used.
During the large flow of current on transmission line under the faulty condition with a consequent increase in capacity of circuit breakers, therefore, expensive circuit breakers are required with the interconnected power system. Hence cost of circuit breakers increases.
4. Metering and Instrumentation :
Adequate metering and instrumentation are needed at different levels.
It is necessary to install sequential recorders, disturbance recorders, time of day meters, etc.
Reserve Capacity of Interconnected Power Stations:
Every Power Station has a certain amount of reserve capacity to facilitate periodic maintenance and repair of its generating unit and to meet the demand of power when a unit has a forced outage (due to fault).
A reserve generating capacity is also required to meet minute-to-minute variation in load and load forecasting errors.
The reserve generating capacity may be classified into two categories are:
1. Cold Reserve
2. Operating Reserve
1. Cold Reserve :
The cold reserve is the reserve generating capacity that is available for the service but not in operation.
It is used when a generating unit is to be taken out for maintenance or гераіr.
2. Operating Reserve :
The operating reserve is the reserve capacity that is required to provide for a minute-to-minute variation in load, load forecasting error, and forced out of the unit.
It is of two types
(a) Hot Reserve :
It is that operating reserve capacity that is in operating but not in service.
It is provided to meet the load demand under forced outage conditions.
(b) Spinning Reserve:
It is that operating reserve generating capacity that is connected to the system bus and ready to take the load.
It is provided to meet minute-to-minute load variations and any extra load due to load forecasting errors.
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