What are some of the advantages and disadvantages of using Remedial Action Schemes or Special Protection Schemes to reject generation or load following contingencies under certain system conditions instead of building more transmission and generation for a more robust system in the first place?

As you state RAS/SPS are in general a remedial action that eliminates or defers the construction or upgrade of new facilities. In many cases the justification is cost, they are exponentially cheaper to design and install than say building a new 345 kV transmission line or adding a large scale SVC. In the bigger picture sense, they are band-aids used to triage a system problem. They introduce their own set of inherent risks. Some of these systems are extremely complicated, taking multiple inputs across a system, possessing the state of the system then potentially applying logic to determine an appropriate response. Example on a critical interface they may: monitor flow and voltage, if flow exceeds a certain threshold and voltage either drops below a set point or decays at a predetermined rate, trip unit A, if unit A is offline trip unit B, etc, etc. Many are not straight forward. With the complexity of the RAS/SPS one is then at the mercy of metering, relaying, communications and programmable logic systems. One hiccup in any of them and the system will either not work when needed or potentially just as bad work when not intended. In the end either end state may cause a wide spread cascading event, an operator’s worst nightmare. It all boils down to risk management. Personally I’d rather hang more wire and add one of these.

John, I agree that the cure from a poorly designed RAS/SPS could be worse than the disease of a contingency overload. However, in the WECC region (Western Interconnection), all RAS/SPS that can impact a large area must be redundant with no single point of failure. Even local RAS/SPS should be redundant (if they are not redundant, then contingencies need to assume failure of the RAS/SPS, which kind of forces everything to be redundant). I also agree that some RAS/SPS can be quite complicated and that great care needs to be taken to make sure there are no unintended consequences from RAS/SPS operation under all design conditions and even under conditions where the RAS/SPS operates when not intended.

However, I don't agree that more wire in the air is always the best solution. As you noted, RAS/SPS is typically less expensive than new transmission. In addition, there are significant environmental impacts from construction of new lines. In many cases RAS/SPS is the overall best solution.

Pete,
Couple of thoughts. NERC is hot on the trail of relay misoperations. It is an increasing problem in the industry. Both FERC and the new PRC standards are addressing this. RAS/SPS inherently increase that risk especially in the more complex systems. I concur anyone installing a RAS/SPS, especially if it protects against an IROL, had better be making it fully redundant. We’ll have to see what falls out of the current NERC SPS standards drafting effort to see it if becomes mandatory.

While RAS/SPS is less expensive than hanging new wire my concern is the future. If the forecast in topology changes, load forecast and future generation mix is slanted in one direction, the RAS/SPS may be a temporary fix and eventually will be retired with no additional remedial action needed. Well that’s a legitimate application of RAS/SPS. If the changes over time will eventually require transmission expansion anyway and all the RAS/SPS does is delay the inevitable, then I personally feel putting up wire now rather than later is worth the effort especially based on rapidly escalating construction costs.

John, I agree that if the analysis shows that the present value of a transmission line installed now is less than the present value of a RAS/SPS installed now and a transmission line installed later, then you should install the transmission line now.

However, in some cases, the RAS/SPS is installed to allow higher economy transfers. The savings from these transfers can justify the RAS/SPS, but not a new transmission line. In this case, it is better to install the RAS/SPS rather than to lose out on the savings from the economy transfers.