Author: Sandia National Laboratories
Existing requirements have for the most part been centered on conventional utility-scale generation plants such as thermal (coal, natural gas, nuclear) and hydro units. Given the growing number of renewable energy generation facilities, a current challenge for the industry is to determine what expectation should be placed on these plants to reliably operate the grid.
Existing and proposed performance requirements potentially applicable to large-scale PV plants are contained in the following key documents:
- FERC LGIA – Standard Large Generator Interconnection Agreement. Article 9 deals with Operational Requirements. Explicitly there is no mention of PV in the FERC LGIA; however, a term commonly used in FERC documentation is “best utility practice,” referencing PV plants to the LGIA requirements is a reflection of that statement.
- FERC Order 661A – Specifies the technical standards applicable to a wind generating plant greater than 20 MW. Requirements refer to Low-Voltage Ride-Through, Power Factor Control, and SCADA systems. Although specifically only mentioning wind farms, many utilities are referring to FERC Order 661A for PV requirements as well..
- NERC PRC-024-1 – This proposed NERC standard provides further guidance on voltage and frequency tolerance for all generator technologies. If approved, this standard may be applied in lieu of FERC Order 661A as well as off-nominal frequency (ONF) ride-through requirements.
- Regional Criteria – Performance requirements specific to some Reliability Entities are sometimes applied to interconnection of solar generators. For example, Section 4.2 of WECC’s Off-Nominal Frequency (ONF) Load Shedding Plan defines frequency tolerance requirements for bulk generators.
Under NERC’s Planning and Operating Committees, the Integration of Variable Generation Task Force (IVGTF) was formed to evaluate the barriers to integrating variable generation as well as providing recommendations. Within the IVGTF the interconnection subgroup’s objective is that procedures and standards should be enhanced to address voltage and frequency ride-through, reactive and real power control, frequency and inertial response, and must be applied in a consistent manner to all generation technologies. In 2009 the IVGTF released a report covering the characteristics of variable generation and its planning, technical, and operational impacts along with its recommendations for integrating variable resources into the bulk power system. NERC recently released for comment a set of IVGTF recommendations on interconnection standards covering reactive power, voltage and frequency ride-through, etc.
- 1 Voltage and Frequency Tolerance
- 2 Frequency Tolerance (Under/Over-Frequency)
- 3 Reactive Power Capability and Volt/VAr Control
- 4 SCADA Integration Requirements
- 5 Station Configuration and Protection
- 6 Current Efforts to Update Interconnection Procedures and Standards
- 7 References
Voltage and Frequency Tolerance
Low Voltage Ride-Through (LVRT), also referred to as Fault Ride-Through (FRT), requires that the generator remain connected to the grid following a voltage disturbance. The basis for the requirement is that during a fault on the system, the immediate disconnection of a large facility would be counterproductive. LVRT requirements are a relatively new to the PV industry. PV generation initially became prominent as a form of distributed generation (DG), for which the applicable interconnection requirements are defined in the IEEE 1547 standard. According to IEEE 1547, distributed generators are required to disconnect from the grid within a certain period of time following a disturbance. The emphasis is on disconnecting from the grid to avoid interfering with protection schemes and prevent unintentional islanding.
In the United States, LVRT requirements for wind plants were first standardized in FERC Order 661A. This requirement is often applied to transmission-connected PV plants even though the standard states that it applies only to wind plants. FERC’s LVRT requirement mandates that a generator shall withstand zero voltage at the POI (typically the primary side of the station transformer) for up to 0.15 seconds (9 cycles) and the ensuing voltage recovery period. The FERC requirement is not specific about the requirement for ride-through during the voltage recovery period.
NERC’s proposed PRC-024-1 standard addresses voltage tolerance for all generators. If approved, NERC’s voltage ride-through (VRT) standard will have to be reconciled with FERC Order 661A and other LVRT regional standards that may exist.
Frequency Tolerance (Under/Over-Frequency)
Where voltage deviations tend to be more localized, frequency deviations will effect an entire interconnection. Generator frequency tolerance is typically coordinated with under-frequency load shedding (UFLS) schemes.
The FERC LGIA states that proposed generators must meet Off-Nominal Frequency (ONF) tolerance requirements of the applicable reliability council. For example, large-scale PV plants connected in the WECC footprint may need to comply with the existing WECC ONF requirement. The proposed NERC PRC-024-1 requirement also addresses generator frequency tolerance. If the PRC standard is approved, discrepancies with regional ONF requirements would need to be reconciled.
Reactive Power Capability and Volt/VAr Control
According to the FERC LGIA, the generally accepted power factor requirement for large generators is +/- 0.95. The ‘+/-’ refers to leading or lagging power factor. In a conventional power plant, the reactive power range is dynamic, which means that the generator can adjust continuously within this range. Reactive power requirements for PV plants are not well defined. Sometimes, the FERC provisions in Order 661A are applied even though the document states that it applies to wind generation only. FERC Order 661A requires that wind plants have a power factor range of +/- 0.95 measured at the POI, and provide sufficient dynamic voltage support “if the Transmission Provider’s System Impact Study shows that such a requirement is necessary to ensure safety or reliability.” Wind power plants are normally designed to meet the +/- 0.95 power factor range by default. For some types of wind power plants, the requirement to provide dynamic voltage support requires additional reactive power support equipment as part of the plant. For the PV industry, provision of reactive power is a departure from the PV application in distribution systems. By default, PV inverters designed for distribution interconnection are designed to operate at unity power factor, and are unable to supply reactive power when operating at rated kW output. To maintain a +/-0.95 power factor range at nominal voltage and at rated kW output, the inverter would need to have a kVA rating at least 5.2% higher than the kW rating. Considering that inverter cost is related to the kVA (current) rating, the power factor range requirement comes at a higher cost compared to PV existing industry practice.
SCADA Integration Requirements
FERC Order 661A also contains Supervisory Control and Data Acquisition (SCADA) requirements for wind plants. SCADA requirements contained in FERC Order 661A are sometimes applied to large-scale PV plants. The purpose of the requirement was for the plant owner to be able to transmit data and receive instructions from the transmission provider in order to protect system reliability. SCADA data to be shared are based on needs for real-time operations (line switching, generation dispatch, etc.), state estimation (to determine real-time stability), remedial action schemes (planned response to contingencies), and safety issues (confirming energized/de-energized components). Further details on SCADA for power system applications can be found in IEEE Standard 1547.3 (IEEE Guide for Monitoring, Information Exchange, and Control of Distributed Resources Interconnected with Electric Power Systems), IEC 61850 (Standard for the Design of Electrical Substation Automation), IEC 61400-25 (Communications for Monitoring and Control of Wind Power Plants), and the RUS Design Guide for Rural Substations (Chapter 14: Substation Automation).
Station Configuration and Protection
During the Feasibility Study different options for station configuration and POI for the PV generator facility are evaluated. A number of options may be feasible; however, one configuration is selected based on factors such as cost, permitting options, construction time, and system reliability. Below figure shows some examples of possible interconnection facility options. Other configurations are possible. In all the examples shown, it is assumed that there is an existing transmission line between Station A and Station B. Existing equipment is drawn in black, and new construction is shown in blue. For simplicity, line terminations into Stations A and B are represented by a single breaker. In reality, transmission switching stations have a more complex configuration such as a ring or breaker-and-a-half scheme.
- In Option 1 the existing transmission line is broken in order to build a new switching station with a three-breaker ring bus. The proposed PV may require a new interconnection transmission line to this new station depending on the location of the PV site. This option may allow the PV plant to operate even if the line either to Station A or to Station B is out of service. This allows for flexibility for maintenance. The tradeoff is the relatively higher cost.
- In Option 2 the existing line is extended so that the new switchyard can be built next to the PV site. This configuration is similar to Option 1, but could result in higher cost overall.
- Option 3 represents a direct connection to an existing switching station. The existing Station B would have to be upgraded with additional circuit breakers and a new transmission line would be built to the PV site. One advantage of this configuration is it does not require a new transmission switching station to be developed.
- Option 4 represents a very simple and potentially low-cost interconnection method. However, this option is often unacceptable to a utility because of reliability and operations considerations. Even if this option were considered a possibility, a more complex protection scheme would have to be implemented.
Some utilities require the installation of a synchronizing breaker for generators to avoid the use of breakers at the transmission switching stations for protection of the interconnection customer’s transformer or for disconnection and reconnection of the generator.
Current Efforts to Update Interconnection Procedures and Standards
It should be stressed that the procedures and requirements for interconnecting large-scale PV plants greater than 20 MW per the FERC LGIP are continuously evolving, and some of the discussion and proposed changes specifically pertain to variable generation (PV and wind). For example, California Independent System Operator Corporation (CAISO) proposed revisions to its tariff relating to interconnection requirements applicable to large asynchronous generators, predominantly wind and solar photovoltaic resources. CAISO’s proposed revisions were in four specific areas: (1) power factor design and operations criteria; (2) voltage regulation and reactive power control requirements; (3) frequency and LVRT requirements; and (4) generator power management. FERC rejected the proposed changes related to reactive power design criteria, voltage regulation, and generator power management. However, FERC accepted proposed changes related to frequency and voltage ride-through, including clarification that the revised voltage and frequency ride-through standards apply to all asynchronous facilities (including PV). Specifically, CAISO’s proposed revisions were summarized by the commission as follows:
- Separate the requirements for ride-through of single-phase faults with delayed clearing from those applicable to all normally cleared faults, in order to make clear that asynchronous generators must ride through the recovery phase of single-phase faults.
- Clarify that the LVRT provisions apply to all types of normally cleared faults, not merely three-phase (i.e., two-phase or single-phase faults).
- Establish criteria to define which breaker clearing time sets the “normal” clearing time for purposes of the ride-through requirements. Specifically, the CAISO proposes that the “normal” clearing time be defined as the lesser of the maximum normal clearing time for any three-phase fault that causes the voltage at the POI to drop to or below 0.2 per unit of nominal.
- Clarify that remaining on line does not require injection of power, but requires remaining physically connected.
- Clarify that the ride-through requirement applies to the facility, but does not necessarily require each individual unit to remain connected.
- Clarify that the ride-through requirements are not applicable to multiple-fault events.
With regard to frequency ride-through, CAISO sought clarification that asynchronous generators must comply with the ONF requirements in the WECC Load Shedding Guide.
NERC is working on revisions to interconnection requirements and performance standards for variable generators that will eventually need to be reconciled with FERC’s ongoing proceedings. For this reason, it is recommended that stakeholders remain current with FERC and NERC proceedings.
- Sandia NL, Utility-Scale Photovoltaic Procedures and Interconnection Requirements (SAND2012-2090), February 2012, [Online]. Available: http://energy.sandia.gov/wp/wp-content/gallery/uploads/PV_Interconnection-SAND2012-2090.pdf. [Accessed February 2013].
- Large Generator Interconnection Agreement (FERC LGIA), http://www.ferc.gov/industries/electric/indus-act/gi/stnd-gen.asp. [Accessed May 2013]
- FERC Order 661A, http://www.ferc.gov/EventCalendar/Files/20051212171744-RM05-4-001.pdf. [Accessed May 2013]
- NERC PRC-024-1, http://www.nerc.com/_layouts/PrintStandard.aspx?standardnumber=PRC-024-1&title=Generator%20Frequency%20and%20Voltage%20Protective%20Relay%20Settings&jurisdiction=United%20States, [Accessed June 2015]
- WECC’s ONF Load Shedding Plan, https://www.wecc.biz/Reliability/Off-Nominal%20Frequency%20Load%20Shedding%20Plan.pdf, [Accessed June 2015]
- NERC Special Report, Accommodating High Levels of Variable Generation, http://www.nerc.com/docs/pc/ivgtf/IVGTF_Report_041609.pdf. [Accessed May 2013]
- FERC Docket ER10-1706-000, http://www.ferc.gov/EventCalendar/Files/20100831182129-ER10-1706-000.pdf. [Accessed May 2013]