• Isolated  and  interconnected power system


    Isolated power systems and interconnected power systems exhibit different features. Isolated systems are much smaller than interconnected systems. In addition, they can not count with the support of the neighbor systems. That is, the size and the lack of external support make isolated systems more vulnerable than interconnected systems. It is especially true in case of disturbances. Hence, the system stability is at risk.


    Frequency stability is at risk in isolated power systems because of the fact that the frequency rate of change in case of generator tripping is bigger than in an interconnected power system. The inertia or the kinetic energy of the rotating masses of an interconnected system is much bigger than the inertia of the rotating masses of an isolated system. In addition of, the magnitude of the generation that can be tripped compared to the total rotating generation is much bigger in an isolated system than in an interconnected one.


    Following are the basic operating principles of an interconnection of power systems.

    1) Under normal operating conditions, each control area should strive to carry its own load, except such scheduled portions of the other members’ loads as have been mutually agreed upon.
    2) Each control area must agree upon adopting regulating and control strategies and equipment that are mutually beneficial under both normal and abnormal situations.

    A worldwide trend in the development of power systems is to build interconnections with the goal to achieve economical benefits. Such large interconnected systems can cover many countries or even wide continental areas. Interconnections of power systems may offer significant technical, economical and environmental advantages, such as pooling of large power generation stations, sharing of spinning reserve and use of most economic energy resources taking into account also ecological constraints: nuclear power stations at special locations, hydro energy from remote areas, solar energy from desert areas and connection of large off-shore wind farms.


    The liberalization in the power industry also supports more interconnections to enable the exchange of power among the regions or countries and to transport cheaper energy over long distances to the load centers. Examples for such interconnections are systems in Russia, North America, Europe and Asia. However, there are technical and economical limitations in the interconnections if the energy has to be transmitted over extremely long distances. In future, the situation can, however, change if ecological and political terms change or the present cost conditions alternate.


    The interconnections are mostly realized by synchronous links where such solutions are technically feasible and economically justified. On the other hand HVDC links often offer technically better and more economical solutions. A large number of examples worldwide shows, that HVDC is a quite suitable solution. However, in many situations, hybrid solutions for interconnection are more advantageous: a synchronous high voltage AC link, supported by an additional HVDC link. In cases where the synchronous interconnection is technically at the limit, HVDC can support the operation of the interconnected systems and thus makes the synchronous AC link more reliable.

    The connection of several generating stations in parallel is known as interconnected grid system.

    Several Advantages

    (i) Exchange of peak loads :

    An important advantage of 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.


    (ii) 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, interconnected system gives a direct key to the use of obsolete plants.


    (iii) 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 throughouts the year at a high load factor and the less efficient plants work for peak load hours only.


    (iv) 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.


    (v) 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.


    (vi) 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.


    1. Enhanced reliability is assured for the most critical loads in the system

    2. Backup power remains available in the event of one unit’s failure

    3. Large single engine gensets are inefficient when not operated near or at full load. The flexibility of paralleling allows multiple configuration options for various types of loads.

    4. The multiple unit configurations permits operation levels at partial load to be handled only by the generators needed, while the others remain offline.

    5. Limited operation is possible based upon kilowatt demand levels.

    6. The flexibility to combine two, three, four, or five gensets of the same or different kilowatt outputs (400, 500 or 600 kW) to more precisely match load requirements from 800 to 2400 kilowatts

    7. Paralleling allows commonality of equipment with a lower cost structure and ease of upgrade to add additional units.

    8. Greater power availability and redundancy as units back up each other, which also provides coverage during maintenance.

    9. Serviceable by diesel technicians, unlike larger single engine units requiring more specialized and costly service.

    10 . Replacement parts less expensive and more commonly available than for larger single engine units.

    11. Ease of expansion is another reason for paralleling. If electrical demand is expected to grow substantially over time, the initial investment can be reduced by installing one smaller genset, then adding additional units in parallel as the load increases. Power capacity often can be added with minimal disruption.

    12. Costs are lower for the breakers and other system components associated with smaller sets.

    13. Paralleling also permits closer matching of the power produced to the actual loads. For example, it may be possible to operate a single genset when loads are light. When loads increase, other gensets in the paralleled system can be added. This saves fuel and wear and tear on the gensets since they run only when needed. This type of operation, known as loaddemand mode, is often used in prime power situations or during long power outages.