TRANSMISSION LINES SYSTEM PLANNING BASICS AND TUTORIALS

NERC specifies transmission systems planning standards that cover the types of contingencies that must be examined for conditions for all facilities in service and with facilities out-of-service for maintenance while delivering generator output to projected customer demands and providing contracted firm (non-recallable reserved) transmission services, at all demand levels.

These contingencies can result in the loss of single or multiple components. For each of the contingencies, the system must be stable and applicable thermal and voltage limits must be observed.

For the loss of multiple components, the controlled interruption of customer demand, the planned removal of generators, or the curtailment of firm (non-recallable reserved) power transfers may be
necessary.10

The standards also require evaluation of the risks and consequences of a number of extreme contingencies such as the loss of all circuits on a R-O-W, all generators at a generating station, or failure of circuit breakers to clear a fault. Individual Regions may develop their own regional planning criteria to reflect circumstances applicable to their own situation.

These Regional Criteria are evaluated by NERC to ensure consistency with NERC’s planning standard.
NERC also covers in its Planning Standards:
• Reliability assessment;
• Facility connection requirements;
• Voltage support and reactive power;
• Transfer capability;
• Disturbance monitoring.

The present complexity of the NERC Standards reflects the changing state of the electric utility industry. When NERC and the Regional Councils were first formed, their membership was almost entirely utilities and the rules, standards, best practices that were produced relied on voluntary observance by the members.

As the industry has moved to its present structure with many more participants, NERC has been working to make the planning and operating rules for the utility industry clear, universal, and well documented. To do this has meant that the volume of the associated material has grown to a point that no one text could hope to cover it all.

As the footnotes indicate, individuals wishing more detail on these matters can find them at NERC’s Web site: www.nerc.com.

As a generation expansion pattern was being developed, transmission planners would address the transmission expansion needed to accommodate the generation and the forecast load growth. Development of a transmission plan has been described as part science and part art.There are three situations that confront the transmission planner:

1. Connect a new generator or generating station to the grid.
2. Connect a new substation to the grid.
3. Reinforce the existing grid.

The obvious first step for connecting a new generator or a new distribution substation is to build one or more lines to the nearest bulk power substation. However, this may not be sufficient or adequate.An examination is needed to see if the capability of the existing grid is sufficient to accommodate either.

This examination has to consider a wide range of operating conditions including different load levels, different power transfer patterns on the grid, and various maintenance outages. The analysis should evaluate a number of years into the future including additional generation and distribution substation requirements.

It may well be that because of future developments, larger more robust facilities should be installed initially or, even, that future expansion may mitigate the need for facilities now. For example, if generation is presently being sited outside a generation deficient load area, an initial reaction might
be to build a large-scale transmission development into that area.

What if, however, subsequent generation additions are within the generation deficient area? The result could be that the transmission additions could be lightly loaded and do not carry enough power to pay for their costs.

An important consideration is that the transmission additions may not always be near the new generation. The restriction on the capacity of a section of the grid can be far removed from the new generator addition.

Other instances have been seen where new lines are added to increase stability margins although they carry little if any power themselves. After examining the need over a sufficiently long time span, decisions are needed on the voltage level of the new line(s), their thermal capacity, their terminal locations, and the circuit breaker arrangements at these locations.

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