Studies for the Design of Substation
Load flow studies:
The purpose of a substation is to provide a path for reliable delivery of power to system loads.
Load flow studies establish the current carrying requirements of a new substation, when all lines are in and when selected lines are out for maintenance.
After studying a number of load flow cases, the continuous and emergency ratings required for various equipment can be determined. The load center also can be finalized by load flow studies.
Short circuit studies:
In addition to the continuous current ratings, the substation equipment must have short time ratings.
These must be adequate to permit the equipment to sustain, without damage the severe thermal and mechanical stresses of short circuit currents.
In order to provide adequate interrupting capability in the breakers, strength in post insulators and appropriate setting for protective relays, which sense the fault current.
Transient stability studies:
Under normal conditions, the mechanical input to a generator will be equal to the electrical output plus generator losses. So long as this continues, the system generators rotate at 50Hz.
If this balance is destroyed by upsetting the mechanical or the electrical flow, the generator speed deviates from 50Hz, and begins to oscillate about a new equilibrium point.
The factors which affect the stability are
- the severity of the fault,
- the speed with which the fault is cleared and
- ties between the machine and the system after the fault is cleared.
The aspects of transient stability that are important in substation design are
- the type and speed, of the line and bus protection,
- the interrupting time of the breaker and
- the bus configuration after the fault has been cleared.
Transient overvoltage studies:
transient overvoltage may be due to lightning strokes or circuit switching. The most reliable means to establish switching overvoltage is through the use of a transient network analyser (TNA) study.
- System security: the ideal substation is one where each circuit is controlled by a separate breaker with facilities for replacement of bus bar or breaker in the event of a fault or during maintenance. System security may be specified, based on whether complete reliance on the integrity of the substation for a percentage of outage due to periodic faults or maintenance is permissible.
- Operational flexibility: for the efficient loading of the generators it is necessary to control the MVA and MVAR loading under all conditions of circuit connections. The grouping of load circuits requires to capable of being arranged giving the best control under normal and emergency conditions.
- Simplicity of protection arrangements: if more than one circuit is to be controlled from one CB or greater number of CBs is to be tripped during fault condition, the protection arrangements are complex. The most advantageous arrangement is single bus bar with no sectionalising.
- Ability to limit short circuit levels: any arrangement which incorporates means of providing a substation in to two separate sections either completely or through reactor coupling, is suitable for limiting short circuit levels.
- Maintenance facilities: during the operation of the substation, maintenance will have to be carried out, either planned or emergency. The performance of the substation during maintenance is also dependent on the protection arrangements
- Ease of extension: the substation arrangement shall be such that extension of bays for new feeders are possible as the system expands, there shall be space and expansion facilities
- Site considerations: the availability of site plays an important role in planning the substation. The substations which are simple in diagram and use least number of breakers occupy the least site.
- Economy: a better switching arrangement on technical requirements can be constructed, if the economics are reasonable