Question

What is/are the benefit(s) and advantage(s) of both the spinning reserve and calculating the interchange power between regions?

What is/are the disadvantage(s) of both the spinning reserve and the calculating the interchange power between regions?

Answer 1

A variety of reliability demands apply to the electric utilities. Amongst others, and perhaps the most significant is that a utility or a group of utilities must be able to accommodate the loss of the largest generator in the system with limited power flow and frequency variation. This generally means that all generators on the system must have a few percent of immediate reserve capacity associated with their rotational inertia and their primary energy sources such as steam or hydro. Besides the fact that each generator must run below its rated value, additional fuel is used or water is wasted.
Thus, excess CO2 is emitted and the overall efficiency of the power grid is reduced.

Papers as early as 1973 suggested that a large SMES system designed for diurnal use could accommodate some of a utilities’ spinning reserve requirements by installing additional power capacity into its four-quadrant ac-dc power converter. That is, the rated value of the converter would be some 20 % greater than the normal power rating of the system. The additional capacity would always be held in reserve (i.e., not used for arbitrage) and used only for in the case of loss of generation on the grid. Though the larger converter requires greater capital expenditure, because the operating temperature is somewhat reduced, it would not only provide this security, it bout be more efficient during normal operation than a device with a lower rating. At the time, no other electricity storage concept used a controllable converter and further, there was no way for the owner to accrue monetary benefits from the service.

Today, many electricity storage systems including batteries, capacitors, and flywheels interact with the grid via an electronic power controller and thus have this capability. In addition, some power markets assign high value to this functionality, allowing the owner of a facility with this characteristic to accrue monetary benefits.

The utilities generally have several reserves.

Spinning Reserve – Generation capacity that is on-line but unloaded and that can respond within 10 minutes to compensate for generation or transmission outages. “Frequency-responsive” spinning reserve responds within 10 seconds to maintain system frequency. Spinning reserves are the first type used when shortfalls occur.

Supplemental Reserve – Generation capacity that may be off-line or that is comprised of a block of “curtailable” and/or “interruptible” load and that can be available within 10 minutes. Unlike spinning reserve capacity, supplemental reserve capacity is not “synchronized” with the grid (frequency). Supplemental reserves are used after all spinning reserves are on-line.

Backup Supply – Generation that can pick up load within an hour. Its role is, essentially, a backup for reserves. Backup supply may also be used as back up for commercial energy sales.

Load Following
Load Following is required during the so-called “shoulder hours” during the daily electric demand cycle:

While electric demand increases in the morning as people get begin their day and get ready for work and school and other normal daily activities, and
As electric demand diminishes in the evening as work and home activities diminish.
As shown in Figure 1, as electric demand increases, generation output increases to provide load following up and as demand decreases generation output is reduced to provide load following down.

Many utilities are experiencing a rapid development of PV generation on the distribution system. This movement is particularly prominent in islands, with the Hawaiian islands at the forefront, soon to be followed by the Caribbean. The impact of distributed PV on islands has been accelerated in two ways: the generally high price of conventional generation makes distributed PV generation cost-effective immediately, while the distributed generation has a more significant impact on a small, closed transmission system (versus the large interconnected systems on the US mainland, for example). A major consideration for the utility in terms of reliability and efficiency is the required available reserve in the presence of high PV penetrations.

Available reserves are comprised typically of two parts: spinning reserve and energy storage. The main requirement of a these reserves is that they are available to be dispatched immediately. Spinning reserves normally refers to the excess capacity of generators which are online but not operating at full capacity. Most conventional generators operate at peak efficiency at rated output. So, when the utility requires an increase in the level of spinning reserve, there is a consequential increase in the cost of meeting customer demand.

Adding a lot of distributed PV to the system increases the requirement for reserves in three key ways:

Reserves are normally required to step in when the frequency drops rapidly (such as due to the outage of a generator). Another thing that happens when the frequency drops is that distributed generation will likely trip offline due to the prevailing anti-islanding schemes on inverters. This exacerbates the drop in frequency, so the utility must take measures (such as increasing the available reserve) to maintain customer reliability.
PV systems can exhibit large drops in output due to a transient event such as a cloud passing. This can result in a large drop in output in a short space of time – a 50% drop can occur in 30 seconds. The speed of this drop in output precludes the startup of another conventional unit, so the utility must plan for this scenario. Where large amounts of solar power are installed in a small area, this may have an effect on reserve requirements.
Distributed generation can impact the effectiveness of load shedding schemes. Load shedding is employed to help arrest a drop in frequency, as the load is reduced to help balance the available generation. This is deployed by opening a circuit breaker and disconnecting a specific part of the distribution system. However, if there is distributed generation on this part of the system, the net load dropped is less than may have been designed, reducing the effectiveness of the load shedding scheme.
While distributed PV can be part of this problem for utilities, they can also be part of the solution, along with some other distributed energy sources.

Energy Storage: Energy storage, installed with PV systems or elsewhere, increases the available reserves on the system without decreasing the efficiency of conventional generators. Distributed energy storage can also be used towards this purpose, provided that a minimum discharge level is set so that some energy is always available for reserves.

Demand Response: Demand response schemes, where the load can be controlled by the utility, offer the advantages of a load shedding scheme without the disadvantages of dropping the distributed generation. They can also be useful in increasing the amount of spinning reserve in situations where demand is low by allowing the utility to dispatch more generation against the load.

Smart Inverters: Smart inverters enable the utility to randomize inverter trip points. This prevents the situation where multiple (or all) distributed PV systems trip offline at the same frequency point, which can result in a significant drop in generation and make a small outage into a system-wide blackout. Utilities can set an acceptable range of time delays for inverters to trip due to under-frequency, which results in a gradual drop in generation (and the possibility of some generation not being dropped at all). Smart inverters also offer the possibility of ceasing to energize the system, rather than disconnecting completely. This enables the inverters to re-energize the system when requested by the utility, providing that the necessary communication protocols are in place, and this can further help the system to recover.

The chart below shows an example of the system frequency following a large generator trip on systems with high penetration of distributed PV in the following cases:

No mitigation;
Energy Storage (ES);
Energy Storage with Randomized Generator Trip (ES + RGT); and
Energy Storage with Randomized Generator Trip and Demand Response (ES + RGT + DR)