On July 2, 2010, the California Independent System Operator (CAISO) proposed changes to its large generator interconnection agreement (LGIA) that will affect all asynchronous generators (predominantly wind and solar facilities) that are larger than 20 MW and interconnecting with the CAISO transmission system. The proposed changes will require asynchronous generators to comply with the same technical operational characteristics as conventional generators, including low-voltage and frequency ride-through capabilities, power factor design and reactive power capabilities, voltage regulation, and generator power management. The proposed changes diverge from the Federal Energy Regulatory Commission's (FERC) requirements established in Order No. 661-A, and were brought about by the high penetration levels of variable energy resources in the CAISO interconnection queue. The changes will apply to interconnection customers who are tendered an LGIA for execution on or after July 3, 2010. Thus, asynchronous generators with existing interconnections will not be required to retrofit their facilities in response to CAISO's proposed changes, in contrast to retrofits that were recently required by the Electric Reliability Council of Texas.
Power Factor Design and Reactive Power Capabilities
Reactive power is supplied to the transmission system from a variety of sources, mainly conventional generators, and it is essential for transmitting power and maintaining voltage stability on the transmission system. CAISO's current LGIA requires wind generators to operate at a power factor within a 0.95 leading to 0.95 lagging if the operational requirement was shown by interconnection studies to be necessary to maintain safety and reliability. The proposed changes will require all asynchronous generators to achieve a power factor range of 0.95 leading and 0.95 lagging without exceeding the ratings of any equipment. Facilities may meet this requirement by using equipment designed to supply reactive power, fixed or switched capacitors, or a combination of the two. CAISO has also reserved the ability to curtail asynchronous generators to the extent needed to achieve the designated power factor range. Historically, the requirement to supply reactive power had been seen by FERC as a significant added cost to wind generators but not for conventional generators. However, CAISO's expert testimony claims that the cost of compliance will likely be in the range of 0.25% to 1% of the total plant cost.
Voltage Regulation and Reactive Control Requirements
CAISO's current LGIA requires each generating unit to maintain a voltage schedule by operating the generator to produce or absorb reactive power within design limitations. CAISO proposes to require asynchronous generators to use an automatic system to control reactive power capability that has both a voltage regulation and net power factor regulation operation mode. Voltage deviations outside of a voltage band (+/- 0.02 per unit of the nominal voltage schedule set by the Transmission Owner or CAISO) may not exceed five minutes duration per incident, except where there is insufficient reactive capacity to maintain a voltage schedule within the band. Furthermore, interconnection customers will not be allowed to disable voltage regulation controls, without specific permission from CAISO, when a facility is operating at a level greater than 20% of its capability.
Low-Voltage and Frequency Ride-Through
Low-voltage and frequency ride-through defines a generating facility's ability to stay interconnected to and synchronized with the transmission system during disturbances. The frequency of the transmission system is adversely affected when generators trip offline. Wind generators are already required to have ride-through capabilities but, according to CAISO, asynchronous generators have been allowed or encouraged to trip offline in response to a grid disturbance. CAISO's proposal clarifies that the ride-through requirements will apply to all asynchronous generating facilities, and clarifies that such facilities will be required to remain in continuous connection with the transmission grid (although not necessarily injecting current) through voltage recovery after single-phase faults with delayed clearing. In addition, asynchronous generators will be required to ride through all types of normally cleared faults, including severe three-phased faults. CAISO did clarify, however, that the ride-through requirements do not apply to multiple-fault events. For solar facilities, the main technical issue will be whether their balance of plant systems, e.g., cooling systems, will not trip off or can restart following a ride-through event. CAISO believes these requirements will provide better protection against the possibility of losing generation during a transmission system disruption, and that the costs of compliance does not concern generation developers (even those with power purchase agreements already in place).
Generator Power Management Requirements
CAISO proposes to revise its power management requirements in order to ensure that the transmission system remains reliable as conventional generators are replaced by variable energy resources. First, asynchronous generators must have Supervisory Control and Data Acquisitions (SCADA) communications links with CAISO, and have the ability to limit active power output to between minimum and maximum operating limits (in increments of 5 MW or less) in response to a dispatch instruction or operating order. Eligible intermittent resources will only be held to a maximum output limit; they will not be required to operate below levels that cannot be supported by the available energy source. Asynchronous generators must also have the ability to respond to automated dispatch system instructions.
Second, asynchronous generators will also be required to have the installed capability to limit power change ramps, with the exception of downward ramps due to a loss of energy resource for eligible intermittent resources. Asynchronous generators must be capable of limiting their ramp rates to 5%, 10%, and 20% of the facility's maximum capacity per minute. Generators may implement the ramping limit by using stepped increments with individual increments being 5 MW or less. CAISO does not intend to enforce ramp rate limitations continuously; rather, ramp rate limitations will apply when a resource's natural ramp rate could threaten transmission grid reliability.
Third, CAISO proposes to require all asynchronous generators to have the installed capability to reduce output in response to an over-frequency event. When the frequency response control is enabled by CAISO, the control must continuously monitor system frequency and reduce output in response to system conditions. Asynchronous generators will not, however, be required to adjust output in response to an under-frequency condition, as that would require facilities that cannot control their fuel supply to increase output upon demand. CAISO proposes that its power management requirements be made effective as of January 1, 2012 in order to give asynchronous generators a reasonable time to comply.
Exemptions from New Requirements
CAISO is proposing exceptions to the proposed operational requirements in order to minimize the impacts on financial commitments made by interconnection customers.
- Interconnection customers that can demonstrate a binding commitment made as of May 18, 2010 to purchase inverters for 30% or more of the facility's maximum generating capacity will be exempt from the new low-voltage ride-through requirements, if the inverters are incapable of complying with the requirements.
- For customers that, as of May 18, 2010, purchased equipment that is not compliant with the new power management criteria, CAISO will develop operational standards that are consistent with the capabilities of the purchased control equipment and submit such standards to FERC for approval in a non-conforming LGIA.