Modelling Parameters Affect on DER Impact Study Results
Agenda Distributed Energy Resources (DER) Impact Studies DER Challenge Study Steps Lessons Learned Modeling Reverse Power Transformer Configuration Regulator Current Transformer (CT) Voltage Flicker DER Interconnection Transformer
The Challenge Distributed energy resources (DER) are sources of electric power that are not directly connected to a bulk power transmission system The electric power system (EPS) was not designed to accommodate generation at the distribution voltage level IEEE 1547 Standard for Interconnecting Distributed Resources with Electric Power Systems DER interconnection impact studies determine potential issues and identify mitigation strategies that allow for successful integration of the proposed DER
Background DER can be thermal generation, renewable generation, and energy storage Point of Interconnection (POI) Point of Common Coupling (PCC)
DER Interconnection Impact Study Steps Screenings Reverse Power Evaluation Steady State (Voltage Drop): Thermal, Voltage, Regulation Voltage Flicker Risk of Islanding Evaluation Short Circuit Protection http://energy.leidos.com/blog/why-impact-studies-are-vital-for-successful-der-interconnections
LESSONS LEARNED
Model Scope Steady State Scenarios Minimum Load without Proposed DER Minimum Load with Proposed DER Peak Load without Proposed DER Peak Load with Proposed DER Chose Load Normal Load Solar Daytime Load: 8am-6pm, 10am-4pm Include DER Impact is Cumulative Existing Approved, including Mitigation Proposed Scope Substation or Feeder Contingencies Distributed Automation Substation Autotransfer Connection Direct Behind the Meter
Reverse Power Desktop Review Worst case analysis Substation Transformer Load (kw) Daytime Minimum Load 5,516 Approved DER Projects -3,666 Proposed DER Under Study -4,450 Total Forecasted Load -2,600 Reverse power is expected to be an issue Possible substation upgrades: regulator (LTC) controls, bidirectional meter May indicate need for transmission study or revised protection
Reverse Power Detailed Review Detailed Analysis: Hourly Substation Load and Forecasted Hourly Generation One Approved Solar Not Reach Nameplate Seven Hours Reverse Power at 1,266 kw Worst Case Proposed Solar At Nameplate 4 Hours https://pvwatts.nrel.gov/index.php
Transformer Parameters Our Model 69 kv Source 13.2 kv Feeder De-energized Tap Changer (DETC) Unknown Different Client 69 kv Source 12.47 kv Feeder 67 kv Transformer 68.8 kv DETC Tap 70.4 kv DETC Tap
Transformer Parameters Correct setup Model shows low voltage Transformer LTC: -5,-5,-5 (4) 13.2 kv Caps: One 1200 kvar
Transformer Parameters Incorrect setup Undervoltage not detected Transformer LTC: 2, 2, 2 1200 kvar : 1311 kvar
Transformer Parameters Assume a 67 13.8 kv transformer Compare breaker fault current center tap versus DETC Voltage Settings for Sources and Transformers with Off-Nominal Taps, Greg Shirek P.E., Milsoft, 2011
Transformer Parameters kvar impacted by applied voltage Standard C37.010 substation transformer X/R ratio kvar = kv 2 * 1000 X c Nominal, nameplate, and tap voltages affect load flow and fault current results
Regulator Current Transformer (CT) CT primary rating (not the ratio) controls regulator output https://www.beckwithelectric.com/docs/app-notes/ appn-17.pdf V bus = V line + I load (R + jx) CT rating
Regulator CT ~ 100 A at 0.9 PF ~ 90 A real current Correct CT Rating 250 A V = 121 V + 90A/250A * 6 = 123.1 V Incorrect CT rating 100A V = 121 V + 90A/100A * 6 = 126.4 V
Regulator CT Substation LTC: CT on Nameplate Regulator: Typical CT Ratings http://www.cooperindustries.com/content/dam/public/ powersystems/resources/library/225_voltageregulators/ TD225011EN.pdf
Voltage Flicker A change in electric light source intensity due to variation in input voltage of the power supply GE Flicker Curve Maximum allowable percent voltage change at 2%, 2.7%, or 2.8% Once per day changes above 2.8% would be noticeable. Voltage change is more irritable when there are more dips per second.
Voltage Flicker Methodology utility dependent Generation changes - On: 100%, Off: 0%, 5%, or 20% Reduce generation on the whole feeder or just on part of the feeder near the POI All generation or just solar Study flicker whole feeder or just POI and substation Quasi time series analysis IEEE Std 1453 supplanted IEEE 519 and calls for short-term (Pst) and long-term (Plt) flicker indices
Voltage Flicker Voltage drop generation on Record all overhead and underground voltages Lock regulators and capacitors, and generation off Second voltage drop Record second set of voltages Compare percent voltage change using Excel Voltage violations? Set up for first voltage drop Set up for second voltage drop
Voltage Flicker Utility has Rooftop DER information available Proposed DER under study Lesson learned: smaller DER impacting results Model had 1-phase PV (3 to 20 kw) Added 20 kw PV in low voltage area POI checked for flicker 100% generation to 0% Criteria is 2%
Voltage Flicker Flicker 20 kw POI: Before (120V Base) Primary POI 20 kw After (120V Base) Percent Change Example 20 kw Assuming the flicker criteria is 2%, the approved and proposed projects are exacerbating a pre-existing flicker problem Check flicker at all locations Question rooftop nameplate generation Other Small DG PV Status Approved 2000 kw Proposed 1980 kw 115.9 113.9 1.8% On to Off On Off Off 115.9 112.6 2.8% On to Off On to Off Off Off 115.7 111.1 4.0% On to Off On to Off On to Off Off 116.0 110.1 5.1% On to Off On to Off On to Off On to Off
Interconnection Transformer Winding Interconnection transformer winding configuration affects POI fault current Available fault for unit under study ~ 108 A 13.2 kv Fault Model Actual Results POI Grounded Wye Grounded Wye Incorrect Results POI Grounded Wye Delta
Summing It All Up DER interconnection impact studies identify issues and mitigation to allow successful interconnection Studies range from simple screening to multiple steps Chose the model scope based on situation Studies depend on correctly choosing model parameters and reviewing lessons learned
QUESTIONS
Impacts of Interconnecting DER on a Utility Distribution System What are the issues associated with interconnecting DER to a distribution system? Reverse power flow Thermal loading Voltage regulation Voltage flicker Protection Risk of unintentional islanding DER interconnection impact studies determine potential issues and identify mitigation strategies that allow for successful integration of the proposed DER
Transformer Parameters Incorrect transformer secondary LTC not correct Transformer LTC: 2, 2, 2 (4) 13.2 kv Caps: One 1200 kvar
Transformer Parameters System short circuit limits (8kA, 10kA) 65k fuse rated 7.1 ka first fuse on OH Results by transformer setup
Voltage Flicker Results at Proposed POI Before (120V Base) Proposed POI 1980 kw After (120V Base) Percent Change PV Status 121.3 119.3 1.6% Proposed Off, All other DG 100% to 0% 122.4 118.4 3.3% Proposed and all other DG 100% to 0% Annual hourly PV forecast https://pvwatts.nrel.gov/index.php AC Size: 20 kw DC/AC Ratio: 1.15 Tilt: 30 o (7 in 12) Azimuth: 225 o Check flicker at all locations Monitor impact of smaller DER