RATE UNBUNDLING: ARE WE THERE YET?
FEBRUARY 15, 1996
The three Ps of power system reliability are planning, protection and performance—and they’re interrelated. Transmission planners are responsible for planning the transmission system and studying future transmission system conditions to ensure that it’s capable of handling anticipated needs. Future load growth, generation additions, transmission expansion, reconfiguration, and changes in protection schemes are some of the key elements affecting grid reliability, and therefore a comprehensive reliability evaluation is needed on a periodic basis.
The North American Electric Reliability Corp. (NERC) transmission planning (TPL) standards are guiding principles for transmission planners for conducting such evaluations. The standards require assessment of system conditions in near-term and longer-term planning horizons under various contingency conditions to ensure the transmission system can survive all credible single-element disturbances without loss of any firm demand. The purpose of planning studies is to catch any potential reliability issues well in advance, so that proper mitigation measures can be implemented before an outage situation or crisis arises. Protection engineers are responsible for designing and studying protection systems. As the job title suggests, the goal of a protection engineer is to design a safety net to protect the power grid. Design of a power grid protection system is more of an art where engineers ensure that different protective elements in the power grid work in harmony to isolate network components in the event of a disturbance and avoid the cascading impacts and damage to equipment.
Coordination among the protective devices in a networked transmission system is critical. Proper coordination ensures that each device operates under specific triggering events for the intended purpose. In a simplistic way, protection engineers design a complex safety net working mainly with two categories of components — sensing devices (relays) and isolating devices (breakers, switches). The sensing device acts like the brain of a protection scheme, which sends signals to isolating devices to take action based on predefined logic. The challenge for protection engineers is to design a scheme that protects the power system and its components from possible fault events and at the same time avoids conflicts to ensure that operation or mis-operation of multiple devices doesn’t result in unintended consequences. Planning and protection functions greatly affect the performance of the power grid. Coordination between the two functions with mutual understanding of the importance of each could play a vital role in limiting widespread power system outages.
Learning from Experience
According to NERC, there were three major system disturbances between 2004 and 2008, caused by a failure of a single component in protection system. A joint report from the Federal Energy Regulatory Commission (FERC) and NERC staff on the Arizona-Southern California outage on Sept. 8, 2011 identifies that protection system elements had major contributions to the outage. The report also cites the interaction of various protection systems and unintended consequences of remedial action schemes (or special protection schemes).
The PacifiCorp outage in 2008 gave rise to a new debate in the transmission planning community. The question was if a Category B disturbance, as defined in the TPL-002 standard, is limited to faults with normal clearing where the protection