How is the standard related to NERCWECC Standard III.C.S3? Has WECC complied with S3? The title says low voltage ride through, but voltage excursions is used in the text which implies high voltage ride through also.
Generator Low Voltage Ride Through Capability (Posted June 2, 2004) Comments Carson Taylor Bonneville Power Administration cwtaylor@bpa.gov How is the standard related to NERC/WECC Standard III.C.S3? Has WECC complied with S3? The title says "low voltage ride through," but "voltage excursions" is used in the text -which implies high voltage ride through also E-mail sent 7/19/04 Abraham Ellis Transmission Development and Contracts Public Service Company of New Mexico E-mail: aellis@pnm.com Phone: (505) 241-4595 PNM Comments on WECC Proposed Standard “WECC Generator Low Voltage Ride Through Capability” July 26, 2004 On June 2, 2004, WECC posted the proposed standard “WECC Generator Low Voltage Ride Through Capability” (Herein referred to as “Standard”) and solicited comments from WECC members by August 2, 2004 The Standard was prepared by WECC’s Reliability Subcommittee with input from various stakeholders This Standard is needed primarily to address the propensity of wind farms to trip off line for transmission system faults, especially in light of the large wind development activity projected for the near future Although the wind industry is rapidly moving to develop low voltage ridethrough capability, this feature has not yet become a standard offering PNM supports WECC’s efforts to develop and ultimately adopt a standard addressing low voltage ride through (or more generally, fault tolerance) by new generators As currently written, the Standard conveys the main idea, but has shortcomings in several areas PNM’s comments on the Standard are discussed below G:\DEPT\SEC\Standards Development\COMMENTS-LINDA\Comments_WECC Gen_Low_Voltage_Ride_Through_Capability_10-18-04.doc 10/19/04 Scope: As written, the Standard would apply to all generators in the WECC, regardless of the size, type or voltage level at which they are integrated to the system PNM believes the scope is unnecessarily wide, and offers the following suggestions: a) The Standard should not apply to generators connected to the transmission system (69 kV and above) that are below a certain size An exemption on the basis of size is consistent with WECC policy in other areas such as generator power system stabilizers (PSS) PNM proposes that generators with net nameplate capacity not exceeding 10 MVA be exempt from the Standard In the vast majority of the cases, the loss of such small generators as a result of a contingency does not materially compromise system reliability In the case of facilities consisting of multiple generators, such as wind farms, the Standard should apply if the total net nameplate capacity exceeds 10 MVA To account for the higher risk exposure due to multiple units operating under diverse operating conditions, the Standard should allow a small portion of the clustered generators (e.g., 10%) to trip as a result of a system fault event b) The Standard should not apply to generators integrated to distribution systems Generators interconnected at the distribution system level are small by definition, and should be excluded based on the discussion above In addition, the application of the Standard to generators at the distribution system level will conflict with other applicable standards, including IEEE 1547 (“Standard for Interconnecting Distributed Resources with Power Systems”) PNM proposes that the Standard specifically exclude generators interconnected at 60 kV or below, which is the same voltage threshold used in the WECC PSS policy c) As written, the Standard addresses tolerance to both voltage and frequency excursions, stating the more general goal of fault tolerance, not just low voltage ride through capability Said another way, sympathetic generator tripping due to common electrical faults on the transmission system should not be acceptable PNM believes that this is the right approach and thus proposes that the title of the Standard be changed from “WECC Generator Low Voltage Ride Through Capability” to “WECC Generator Electrical Grid Fault Ride Through Capability.” d) WECC must be aware that this Standard must coexist with other applicable standards In particular, if FERC adopts a rule addressing ride through capability, this Standard must be revisited Part of the Standard: Part of the Standard states that “Generators are required to remain in service for three phase faults with normal clearing and single line to ground faults with delayed clearing, G:\DEPT\SEC\Standards Development\COMMENTS-LINDA\Comments_WECC Gen_Low_Voltage_Ride_Through_Capability_10-18-04.doc 10/19/04 unless the generator is radially connected to the fault.” PNM agrees with the definition of performance, and offers the following comments: a) The use of the terms “normal clearing” and “delayed clearing” is appropriate in this context, and should not be further specified This language clearly expresses the expected performance in a manner that is consistent with NERC/WECC planning criteria When applied to a specific situation, the Standard conveys the voltage level and duration that the generator is expected to tolerate Clearing times vary considerably depending on the voltage class and characteristics of the existing protection equipment Terminal voltage during the fault is also a function of local system conditions and type of generator being considered These differences should be acknowledged and reconciled on a case-by-case basis (see discussion under Open Issues) PNM believes that the approach used in the Standard to define the performance criterion is superior to the approach used in other existing and proposed standards, which express the expected performance in terms of a specific voltage level for a specific duration at the point of interconnection Such definitions are inflexible with respect to local system conditions, are often in conflict with each other, and may not have the desired effect A fixed voltage and duration may be more stringent than necessary in some cases, and may fail to prevent sympathetic tripping in other cases b) PNM believes that the exception intended by the phrase “… unless the generator is radially connected to the fault” is appropriate However, the term “radially connected” could lead to confusion depending on the system topology To better capture the intent of the Standard, the exception should read “… unless clearing the fault effectively disconnects the generator from the system.” This should provide a simpler test to determine the applicability of the Standard Part of the Standard: a) Part of the Standard states that “During the transient period, generators are required to remain in-service for the voltage and frequency excursions specified in WECC Table W-1.” PNM agrees with the need to require generators to remain online for expected conditions after a fault is cleared As discussed under the Scope heading (paragraph c), the notion of frequency tolerance should be retained b) PNM believes that making a reference to WECC Table W-1 is appropriate, but the resulting ambiguities should be removed Voltage and frequency excursions specified in Table W-1 are different for each NERC/WECC category Voltage deviations for load buses and non-load busses also differ In addition, the term “transient period” should be replaced with the term “transient period after the fault is cleared.” G:\DEPT\SEC\Standards Development\COMMENTS-LINDA\Comments_WECC Gen_Low_Voltage_Ride_Through_Capability_10-18-04.doc 10/19/04 Part of the Standard: PNM agrees with the proposed language subject to the observations made in the previous paragraph Transitional Issues: The cost of retrofitting an existing generator to comply with this Standard could be very significant Requiring compliance after a “major refurbishment” could discourage generator improvements in other critical areas due to cost implications PNM therefore supports the idea of “grandfathering” existing generators for the purposes of this Standard, regardless of any refurbishment that take place in the future Accordingly, the last two sentences in the Transitional Issues section should be removed Open Issues: Defining fault locations for the purposes of this Standard is a complex issue To minimize the risk of sympathetic tripping due to faults, generators should tolerate faults up to the point of interconnection, subject to the exceptions discussed above In most cases, tolerance to faults at the point of interconnection is technically achievable through a combination of reinforcements, including control modifications, power supply for critical auxiliaries and additional dynamic reactive power support (e.g., SVC or STARCOM devices) The cost implications could be significant, depending on the system conditions In some cases, generator tripping may be desired to maintain system stability, or necessary to prevent equipment damage Under these circumstances, the need for fault tolerance could be significantly tempered PNM believes that the best approach is to evaluate the level reinforcements necessary on a case-by-case basis E-mail sent 7/26/04 Ed Riley California Independent System Operator ERiley@caiso.com CISO Comments on WECC Proposed Standard “WECC Generator Low Voltage Ride Through Capability” Proposed New Standard Generators are required to remain in-service during system faults (three phase faults with normal clearing and single line to ground faults with delayed clearing) unless the generator is radially connected to the fault These requirements should not damage the generator and/or the generating equipment G:\DEPT\SEC\Standards Development\COMMENTS-LINDA\Comments_WECC Gen_Low_Voltage_Ride_Through_Capability_10-18-04.doc 10/19/04 21 During the transient period, generators are required capable to remain in-service for the voltage and frequency excursions specified in WECC Table W-1 without any damage to the generating equipment and other related equipment The generator will not trip if the voltage and frequency excursions are within the limits specified in WECC Table W-1 32 Generators may be tripped after, or during, the fault period if this action is intended as part of a special protection system (SPS) However the SPS should insure that after the SPS has operated, the frequency and voltage excursions are still within the limits specified in WECC Table W-1 or if the generator trips on over frequency or under frequency conditions that exceed those specified in WECC Table W-1 Transition Issues Those eExisting generators that cannot presently meet this standard will be can request exemption from meeting this standard for the remaining life of the existing generating equipment The utility can evaluate the impact of this generator tripping during a fault and request an exemption if there is no adverse impact to the system Existing generators that go through major refurbishments or replacement, where the then current upgrade technology is available that would enable the generator to meet this standard, shall meet this standard The intent of this standard is for existing generating equipment (generators that have been ordered from a manufacturer or installed prior to the adoption of this standard) to ultimately comply with this standard Open Issues Should the standard apply to individual generators or generator sites with multiple generators or both? It should apply to all Do we need the standard to be more specific about how far the fault location can be from the generator before it is unacceptable for the generator to trip or is “radially connected" sufficient? The CAISO believes that “radially connected” is sufficient Do we need to define “major refurbishment”? Yes Should we allow a small portion of an existing generation site to upgrade its equipment without meeting this standard? No If there are other options, the generator should discuss with the utility where the generator is connected The Attachment Excerpt from NERC/WECC Planning Standards should be taken out Email sent 8/2/04 G:\DEPT\SEC\Standards Development\COMMENTS-LINDA\Comments_WECC Gen_Low_Voltage_Ride_Through_Capability_10-18-04.doc 10/19/04 COMMENTS RECEIVED FOLLOWING AUGUST 2, 2004 COMMENT CLOSING DATE: Shirley A McKean sxmckean@srpnet.com WECC Generator Low Voltage Ride Through Capability Salt River Project Comments Terri Kuehneman Gary Nolan Jon Sykes Overview • It would be helpful to include a definition for “Reasonable Voltage.” • The following sentence could be deleted since the Standard applies to all generators: “This was identified as a concern primarily with wind turbines.” Proposed New Standard • A clearer cross-reference is needed to the table referenced in #2 Transition Issues We suggest that the following be included: • Specify equipment that will enable compliance • List grandfathered items that are the references for compliance • Entities explain why they can’t comply • Add teeth for compliance Open Issues • Standard should apply to individual generators not just sites since new generators are being added to old generation sites and should not be exempt due to favorable locations G:\DEPT\SEC\Standards Development\COMMENTS-LINDA\Comments_WECC Gen_Low_Voltage_Ride_Through_Capability_10-18-04.doc 10/19/04 • WECC should at least provide some guidance of a distance, even if it is only a ballpark figure If there is no point of reference, everyone can claim they are close enough to be considered radially connected • Yes, define “major refurbishment.” What part(s) of the generator being refurbished qualify? • The fault location should not be specified in the standard E-mail sent 8/4/04 Karl W Schneider Electrical Engineer – Network Planning Bonneville Power Administration 8100 NE Parkway, TOP/PP02-2 Vancouver, WA 98662 V: (360) 619-6665 Fx: (360) 619-6945 kwschneider@bpa.gov Department of Energy Adm Chron File Bonneville Power Administration P.O Box 61409 Vancouver, WA 98666-1409 TRANSMISSION BUSINESS LINE August 4, 2004 In reply refer to: Comments: WECC Standard for Wind-Generator Low Voltage Ride-Through DISCUSSION: The draft proposed standard "WECC Generator Low Voltage Ride Through Capability", was released for comment on June, 2004 In part, it adopts WECC's existing transientstability minimum-performance standard for voltage and frequency on network buses, as a basis to define conditions under which generating units should generally not trip offline With certain exceptions, the proposed standard would require that, "During the transient period, generators remain in-service for the voltage and frequency excursions specified in Table W-1." This requirement may have been based on a document titled, "PacifiCorp Windfarm Interconnection Proposal", released in draft form in 2003 The proposed standard is aimed at wind-turbine generator (WTG) installations, which often are comprised by groups of conventional induction-machine (CIM) units It is G:\DEPT\SEC\Standards Development\COMMENTS-LINDA\Comments_WECC Gen_Low_Voltage_Ride_Through_Capability_10-18-04.doc 10/19/04 important to acknowledge that CIM's "riding through" a low-voltage disturbance, instead of tripping off-line due to undervoltage, may actually contribute to causing a violation of these criteria Fault-induced decay of operating fields and higher "slip speeds" in a plant's wind turbines will raise demand for reactive power during recovery of depressed voltages However, some newer technologies can offer better capability to ride through lowvoltage disturbances without stressing the grid For example, General Electric Wind Energy (GEWE) claims that its 1.5-MW "double-fed" WTG can withstand a 0.30 pu terminal voltage for 100 ms, and will support the network during recovery The fundamental question becomes: Would the network be better off if CIM's continued to produce power at the expense of reduced voltage stability? Or, should voltage security be maintained, even if that entails tripping of the machines by their own undervoltage relaying, and then subsequently restarting them? The answer to this question may also depend on the relative importance of wind-powered generation in the affected area Here is a different approach: Specify compulsory voltage ride-through characteristics for IM-based generating installations, applicable at the Point Of Interconnection (POI) On a plot of time-duration vs voltage magnitude at the POI, regions would be defined for which the CIM would be required to remain in service; allowed to remain in service or trip; and required to trip Such a graph, originally prepared by the German organization abbreviated "E.ON Netz" was included in a draft white paper prepared by the WECC TSS, dated February 28, 2003, titled "The Need for Voltage Ride-Through Performance Standards for Wind Turbines" A similar graph specifying even stricter requirements was provided as Figure of the American Wind Energy Association's (AWEA) comments on FERC Order 2003A (Integration of Large Generation) received by FERC on May 20, 2004 These graphs defined only two regions that might be termed "required to remain I/S" and "allowed to trip" It was acknowledged in the white paper that most if not all WTG's based on present technologies would not be able to meet the standard by riding through all disturbances outside the "allowed to trip" region The AWEA Figure graph described a very stringent performance requirement that even synchronous machines probably could not fulfill These undervoltage relay settings of gen-terminal voltage for tripping actual operational CIM's would not permit low-voltage ride-through operation in most of the region defined by AWEA's Figure graph: G:\DEPT\SEC\Standards Development\COMMENTS-LINDA\Comments_WECC Gen_Low_Voltage_Ride_Through_Capability_10-18-04.doc 10/19/04 ≤ 0.80 pu V for > 12 cycles (Bonus 1.3 MW @ Nine Canyon wind farm in WA) ≤ 0.85 pu V for > 25 cycles (Vestas V80 @ Uinta County wind farm in WY, from PacifiCorp) ≤ 0.75 pu V for 5-25 cycles (ditto) SUGGESTED ADDITIONS AND CLARIFICATIONS: Does this apply to distribution-connected generation? It probably should not as the intent of this standard should be to protect the grid A fault in the area or on one feeder with generation connected to another feeder should be excluded Should there be some exclusion level? Small generation is defined by FERC as < 20 MW total capacity, which could serve as a threshold for exemption from this standard Should there be some criteria in addition to Figure W1 to allow transmission planners to consider the amount of generation likely to trip vs the connected generation in the control area and the spinning reserve level? This proposed standard should serve to protect the system, and it seems that there should be some engineering judgment applied to the use of Figure W1 Dropping a 100 MW in a small control area may have a destabilizing effect, whereas in a large generation area it may have no effect The following could be incorporated to provide a “performance check”: “If interconnected operation of a new generating plant is found to measurably degrade the quality of service to previously connected customers (based upon criteria specified by ANSI, IEEE, and WECC), then said plant will cease or curtail operation until a suitable mitigation plan is conceived and installed, unless all parties, so impacted, agree to continued operation.” This stipulation applies to all generating units and projects, regardless of generating capacity.” COMMENTS ABOUT PRESENT CONTENT: "Overview" The last sentence would be more meaningful if restated as follows: "Generating units that meet this standard will remain on-line during low-voltage excursions of routine and moderate severity." "Proposed New Standard" " unless the generator is radially connected to the fault" should be replaced by, " unless clearing of the fault isolates the generator from the network for any period of time." G:\DEPT\SEC\Standards Development\COMMENTS-LINDA\Comments_WECC Gen_Low_Voltage_Ride_Through_Capability_10-18-04.doc 10/19/04 "Transition Issues" The entire paragraph needs substantial editing to improve clarity "Open Issues" We believe that a voltage ride-through standard should apply to all generating units within plants that meet the criteria for applicability (≥ 20 MW aggregate capacity) Specifying required voltage ride-through characteristics at the point of interconnection of a power plant will cover the issues of fault distance and severity E-mail sent 8/4/04 Craig Quist, PE Principal Engineer, Transmission Planning PacifiCorp 1407 W North Temple, Suite 275 Salt Lake City, Utah 84116 TEL: (801) 220-4264 FAX: (801) 220-2842 Email: craig.quist@pacificorp.com For the new standard, those drafting the standard appear to not have taken into account the limitations of the current wind turbine technology The American Wind Energy Association (AWEA) is in the process of addressing low voltage ride-through for wind plants and have made a proposal to the FERC In the proposal, which is patterned after the German E-ON standard, any voltage below 15 p.u for ~.6 seconds will result in turbine tripping This standard applies to the point of interconnection, not individual turbines Assuming that WECC proceeds with the requirement that "Generators are required to remain in-service during system faults (including three phase)", there will not be a wind turbine in the current or future fleet of wind turbines in the world that can meet this requirement As with past standards, new standards should take into account machine physics, rather than a desire to have all technology meet a "one size fits all" standard We strongly recommend that WECC research the approach that AWEA is taking on this topic and revise the standard to be in-sync with the capability that is going to be proposed to the FERC, and in practice in Europe E-mail sent 8/9/04 G:\DEPT\SEC\Standards Development\COMMENTS-LINDA\Comments_WECC Gen_Low_Voltage_Ride_Through_Capability_10-18-04.doc 10/19/04 Mike Jacobs American Wind Energy Association Eastern Representative 122 C Street, NW, Suite 380 Washington, DC 20001 Phone: 202-383-2517 Fax: 202-383-2505 e-mail: mjacobs@awea.org The American Wind Energy Association is a Class member of WECC and would like to comment on the proposed Generator Low Voltage Ride Through standard The proposed standard has been described in submitted comments from other parties as being too strict for conventional generation using synchronous machines We have seen no comments countering that opinion If this process is to create a meaningful and useful standard, the result should be a reasonable standard From the comments regarding the treatment of existing generators, it appears that this standard requires performance that currently connected generation does not meet Regarding wind technology, the proposed requirement appears to be beyond the standard of any standard in place or contemplated by any organization in the world The wind industry is not opposed to the creation of all new standards for LVRT, only ones that are impossible to meet with wind turbine technology We are concerned that the proposed standard does not reflect the physical reality of wind technology, or existing synchronous generators used widely in the region The wind industry, through the American Wind Energy Association, has proposed the US adopt the low voltage ride-through for wind plants proposed by German utility E.On The concentration of wind capacity in Germany is over 10% of peak load already, prior to the adoption of a LVRT standard Now, the utility is adopting a threshold requiring wind generation to remain on line unless the voltage is below 15 p.u for ~.6 seconds at the point of interconnection This standard is "Best Available Control Technology" for wind, and with efforts, can be met by wind turbine manufacturers WECC's proposed standard is not achievable without using additional technology that is more expensive than the generators themselves This is not reasonable We believe that the proposed WECC standard is insufficiently quantitative to govern equipment design or practice This concern applies not just to wind generation, but to all generating resources The specific concern is that duration of these events (type B and C) are not specified We think that the WECC standard should state clearing times consistent with good utility practice For example, grid codes in some other systems give limits along the lines of maximum fault duration vs transmission voltage class Placing a specific maximum duration of faults for which generation shall remain in operation, holds both the transmission system and generating resource owners to a good standard of performance G:\DEPT\SEC\Standards Development\COMMENTS-LINDA\Comments_WECC Gen_Low_Voltage_Ride_Through_Capability_10-18-04.doc 10/19/04 In the comments already submitted to WECC by PacifiCorp on August 9, 2004, the statement is made Assuming that WECC proceeds with the requirement that "Generators are required to remain in-service during system faults (including three phase)", there will not be a wind turbine in the current or future fleet of wind turbines in the world that can meet this requirement As with past standards, new standards should take into account machine physics, rather than a desire to have all technology meet a "one size fits all" standard We ask that this comment guide the committee Further work is required before a LVRT proposal is ready for adoption E-mail sent 10/8/04 G:\DEPT\SEC\Standards Development\COMMENTS-LINDA\Comments_WECC Gen_Low_Voltage_Ride_Through_Capability_10-18-04.doc 10/19/04