A Tutorial on the Application and Setting of Collector Feeder Overcurrent Relays at Wind Electric Power Plants By: Stephanie Mercer

Objective This presentation will cover the application and setting of overcurrent relays for collector feeders at wind plants and examples of typical calculations and coordination plots.   This paper is intended to introduce novice engineers to the physical and electrical characteristics of collector feeders at wind electric plants and to provide a general method for setting calculations and doing the coordination for these collector feeders, and show some examples Large wind generating plants have become prevalent as “green energy” sources Denmark's Monster Wind Turbine Just Smashed the 24-Hour Record For Energy Production-215 MWh over a 24 hour period, february of last year (8MW model)

Wind Energy Plants A wind plant may have 60 – 100 wind turbine generators (WTGs), each rated at 1.5 – 3.5 MW. Power from the WTGs is collected at medium voltage and supplied to the main substation through the main collector feeder breakers. Collector feeders are typically underground circuits connecting multiple WTGs over a large area. (3.5MW on land, could be up to 8MW offshore—underwater cable is self-healing) Each WTG connected to medium voltage distribution lines called “collector feeders” Medium voltage is typically 34.5 kV – at the secondary output of the step up TX

Collector Feeder Circuit Protection The relays applied to protect the collector feeder circuits must: Coordinate with the protective devices at each WTG transformer. Not limit the ability of the WTGs to provide power and effective voltage control for the system. 2 main things we want to accomplish In other words, they have to be able to trip for faults but carry the intended load

Wind Farm Collector Feeder Characteristics Collector systems of large wind power plants normally have a radial configuration where the WTGs are connected together in series, moving from the main feeder breaker at the collector substation to the farthest turbine. The collector feeders may have multiple branch circuits connected at underground junction boxes. -connected in a sort of daisy-chain style -Collector feeders can therefore be thought of as radial distribution lines when all the generators are off-line and as multi-terminal sub-transmission lines when one or more generators are on line. -(The protection will typically be set to the impedance characteristics of the farthest turbine, but worst case fault could be the farthest or closest WTG, depending on the fault conditions)

Example of a Collector Feeder One-Line Diagram Operates as a radial system when WTG’s are offline and drawing only station service power Operates as a looped system when WTG’s are online and supplying power to the Collector Substation Relay protection schemes that are commonly applied for either radial or network distribution circuits may be considered. we can see here that individual wind turbine generators are connected to the collector through a generator step-up transformer. -we can see the collector relay at the main bus

Common Collector Feeder Protection Schemes Non-Directional Phase and Ground Overcurrent Protection Applicable for protecting most collector feeder circuits. Trip current pickup setting is above normal load currents in both forward and reverse directions. Provides complete collector feeder protection but limited backup protection for individual WTG installations on the collector feeder. Directional Phase and Ground Overcurrent Protection Applicable when greater current sensitivity is required for coordination. Relay trip current level can be set below combined WTG output current level. Provides complete collector feeder protection and expanded backup protection for individual WTG installations on the collector feeder. Can mis-operate under certain system load conditions. -Non-directional phase time overcurrent elements are set above the combined WTG load, will accommodate any change in generator loading conditions, and will generally provide excellent protection over the length of the collector feeder. -DOC is set more sensitive -losing voltage Input (it calculates direction with it, losing it would turn it into a ND and subsequently operate under load -Most of the fault comes from the collector subs, a much smaller amt comes from each WTG down from the fault location

Non-Directional Instantaneous Phase & Ground Overcurrent Settings Set phase instantaneous overcurrent 50P low enough to operate for a 3-phase fault at the end of the collector feeder circuit but high enough to avoid operation for the WTG transformer inrush current. Typical trip current settings are 75-80% of the maximum current for a line-end fault. Set residual ground or neutral instantaneous overcurrent 50G low enough to operate for a single phase to ground fault at the end of the collector feeder circuit but high enough to avoid operation for any unbalanced WTG transformer inrush current. Typical trip current settings are 75-80% of the maximum current for a line-end fault. The inrush current from the GSU transformers is 5382 A.  This value is below the 8400 A 50P setting (appx 75-80% of line end fault). Each WTG typically produces about 40 A at full output.

Non-Directional Instantaneous Phase & Ground Overcurrent Settings “Delayed” Instantaneous: If the WTG inrush current is greater than the 50P (50G) setting for a line-end fault, then set a second 50P2 (50G2) overcurrent element to operate for a line-end fault and apply a definite time delay to “ride out” the WTG inrush current but avoid thermal damage to the cables installed at the last WTG on the feeder. Then set a first 50P1(50G1) element to operate for a main line fault up to the first WTG on the collector feeder and apply an instantaneous trip time.   Overcurrent protection schemes for collector feeder circuits typically includes time delay and instantaneous phase and ground overcurrent elements The inrush current is 5382 A.  This value is below the 8400 A 50P setting.

Non-Directional Phase & Ground Time Overcurrent Settings Phase Time Overcurrent Setting: Set phase time overcurrent 51P pickup at some multiple of the maximum collective WTG output for the collector feeder circuit. Ground Time Overcurrent Setting: Set the ground time overcurrent 51G pickup at 10-30% of the 51P pickup setting, but above the minimum operating current of the WTG transformer high-side fuse. Phase Time Overcurrent Setting: In North America, NERC Standard PRC-025 requires the pickup to be greater than or equal to 1.3 times the collective WTG output rating. Select a curve and time dial setting that will coordinate with the WTG transformer high-side fuse. Ground Time Overcurrent Setting: Select a curve and time dial setting that will coordinate with the WTG transformer high-side fuse.  

Non-Directional Phase Overcurrent Coordination Advantages: Simple to apply and set. Immune to changes in generator power and VAR outputs. Coordinates with WTG expulsion fuse and protects the smallest conductor at the end of the feeder. Disadvantage: Provides marginal backup overcurrent protection for individual WTG transformers. The high trip current setting provides little protection for individual wind turbine generator transformers. This is because the transformer thermal damage curve ends approximately where the 51P curve starts to operate.

Non-Directional Ground Overcurrent Coordination Must coordinate with the expulsion fuse. Does not need to coordinate with the current limiting fuse at lower currents. Provides good backup ground overcurrent protection for individual WTG transformers. Protects smallest conductor at the end of the collector feeder. Explain why no need to coordinate with the CLF at High current (because EXP is going to sense fault 1st)

Directional Overcurrent Setting Criteria for WTG Collector Feeders Instantaneous Directional Overcurrent Phase: Set directional phase instantaneous element using the same criteria as for non-directional elements (75-80% of the maximum current for a line-end fault, but high enough to avoid operation for the WTG transformer inrush current). Ground: Set residual ground or neutral instantaneous overcurrent element using the same criteria as for non-directional elements (75-80% of the maximum current for a line-end fault but high enough to avoid operation for any unbalanced WTG transformer inrush current). Use the same “delayed” instantaneous setting criteria as for non-directional elements when WTG inrush is greater than the desired setting for a line-end fault. The protection of wind farm collector circuits is complicated by long runs of cable, high inrush current, etc. Now, non-directional overcurrent protection often does not provide the necessary sensitivity for back-up protection for wind turbine generators (this issue is further complicated when you are trying to mitigate arc flash incident energies). Directional OC protection elements are necessary to set OC pickup below maximum generation levels. Although they have to be carefully set, and can be further secured by a load encroachment function that will block OC elements from operating for load.

Directional Overcurrent Setting Criteria for WTG Collector Feeders Directional Time Overcurrent Phase: Set above combined WTG station service load or above the minimum operating current of a single WTG transformer high-side fuse, whichever is higher. Select curve and time dial to coordinate with the WTG transformer high-side fuse. Ground: Set 10-30% of phase TOC pickup or above the minimum operating current of a single WTG transformer high-side, whichever is higher. Here is the setting criteria for directional OC relays applied to a collector feeder. Note that the IOC setting criteria is no different from that of the non-directional relays. The directional phase TOC trip current setting can be set well below the combined WTG load because its tripping direction is exactly opposite from the direction of the generator output current. The only WTG load current flowing in the trip direction is station service load, so as long as the directional phase TOC trip current setting is above station service load and high enough to coordinate with the EXP fuse at the WTG GSU, the setting will be valid. The ground TOC trip current setting should be low enough to provide good sensitivity but high enough to coordinate with the EXP fuse at the GTW (GSU)Generator Step Up.

Directional Phase Overcurrent Coordination Advantages: Provides excellent coordination with WTG expulsion fuse and protects the smallest conductor at the end of the feeder. Phase coordination with the expulsion fuse is as good as the non-directional ground time overcurrent coordination. Disadvantages: May require backup non-directional elements to provide protection for the loss of polarizing voltage. Directional supervision settings must be selected carefully to avoid mis-operating for changes in generator VAR outputs. For this reason, it’s important to manage the protection system’s response to an inadvertent loss of voltage on one or more phases and on occasions when the WTGs are required for system voltage support. -Back-up non-directional element could be the way to go -polarization voltage clarification, LOSS OF POTENTIAL DETECTION CKT WILL BLOCK OC RELAY FUNCTION

Directional Element Operation Relay line impedance angle setting may need to be reduced from the actual line impedance angle to ensure that load current remains outside of the relay trip zone when the WTGs are supplying leading VARs to the system. Feeder faults will most likely occur in the 4th quadrant for inductive line faults. Describe reverse and forward trip -DISAD.:LOSS OF V, AND HOW YOU SET DIRECTION Additional considerations are required in the selection of directional phase time overcurrent settings to ensure that the relay can accommodate any special generator loading conditions mandated by system voltage support requirements.

Phase Element Operation with Directional and Load Encroachment Control Load encroachment control may be employed with normal directional control to limit the relay trip zone exclusively to the area in which collector feeder faults are most likely to occur. Regarding Directional Overcurrent protection with load encroachment: -A traditional directional overcurrent element application on a wind farm collector is susceptible to incorrect tripping for the desirable generator output (I.E.: WHEN GENERATOR SUPPLYING LEAD vars). Adding load encroachment supervision provides security over the entire range of generating current angles without compromising detection of faults on the collection collector

Conclusions Non-directional phase time overcurrent elements may provide only limited backup OC protection for individual WTGs Directional phase time overcurrent elements can be set below the combined WTG load to provide excellent collector feeder protection and backup overcurrent protection for individual WTGs. Both directional and non-directional phase and ground instantaneous overcurrent elements are generally set low enough to detect a fault at the end of the collector feeder but high enough to avoid mis-operation for WTG transformer inrush. Both directional and non-directional OC schemes can be successfully set to provide primary collector feeder protection and backup WTG transformer protection. Non-directional phase time overcurrent elements may provide only limited backup overcurrent protection for individual WTG step-up transformers because the pickup setting can be as much as 8 – 10 times higher than the rating of one WTG on the circuit. Both directional and non-directional ground time overcurrent elements can be set at a third or less of the combined WTG load current but higher than the minimum operating current of WTG transformer high-side expulsion fuses. with the advance in wind turbine generator and protective relay technology, it is important for us as protection engineers to design protection systems that are optimized to effectively address the specific protection requirements at hand. This is the starting point

Questions ?