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2014 PV Distribution System Modeling Workshop: Modeling Effective Grounding for Grid Tied Inverters: Taylor Hollis, Schneider Electric
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2014 PV Distribution System Modeling Workshop: Modeling Effective Grounding for Grid Tied Inverters: Taylor Hollis, Schneider Electric

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2014 PV Distribution System Modeling Workshop: Modeling Effective Grounding for Grid Tied Inverters: Taylor Hollis, Schneider Electric

2014 PV Distribution System Modeling Workshop: Modeling Effective Grounding for Grid Tied Inverters: Taylor Hollis, Schneider Electric


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  • 1. Modeling Effective Grounding of Grid Tied Inverters 5/6/14 EPRI Modeling Workshop Taylor Hollis Interconnections Requirements Analyst Solar Business Unit Schneider Electric
  • 2. 2 Contents 1 Background 2 Compliance Issues for Inverters 3 Potential Solutions 4 Modeling Considerations 5 Conclusions
  • 3. 304/26/14 In North America, more than 70% of circuit miles below 69kV are three phase, four-wire multi-grounded distribution circuits. In the event of a single phase-to-ground fault, circuit breakers at the substation open. If the DR is of sufficient size to support the loads, an unintentional island could be created. Based upon the method of symmetrical components, the phase-to-ground voltages on the unfaulted phases could increase up to 173% of the pre-fault voltage level. This can damage single phase customers or utility surge arresters. Background Source: Johnson et al.
  • 4. 405/6/14 Background IEEE C62.92.1: a system is effectively grounded if the Coefficient of Grounding is less than 0.8 where the COG is the ratio of max phase to ground voltage divided by phase-to-phase voltage. IEEE I42: “…for all points on the system the ratio of zero-sequence reactance to the positive-sequence reactance is less than 3 and the ratio of zero sequence resistance to positive sequence reactance is less than 1 for any condition of operation and for any amount of connected generator capacity.” The X0/X1 and R0/X1 ratios are conditions that usually provide compliance with the actual definition. The Industrial Application Society of IEEE took the sequence ratios as the definition when they made their "color books”, therefore many use the 142 definition, but it is a logical consequence, not an a priori definition.
  • 5. A Quick Word on Symmetrical Components The methodology introduced in 1913 by Charles Fortescue, who demonstrated that any set of unbalanced three-phase quantities could be expressed as the sum of three symmetrical sets of balanced phasors. 5 Background
  • 6. 604/26/14 In this diagram, representing a SLG fault, the generator is represented by a balanced positive sequence voltage phasor. This is not accurate for an inverter! If a current source is used, it is of vital importance to also consider the topology of the load. Source: Johnson et al. Background
  • 7. Traditionally, generators are modeled as a voltage behind a sub- transient, transient and steady state reactances. These are physical values that can be verified with proper test procedures. In their grid- tied behavior the inverters modulate the switching of their transistor bridges to inject current from their DC bus based upon the AC output current. That is emphatically not a voltage source. 7 Compliance Issues for Inverters The inverter does have an AC line inductor, but the impedance to sequence currents vary with time as the switches open and close. A PWM Switching PatternInverter H-Bridge
  • 8. Transformer Based Solutions For DER plants which install a new transformer, a Delta/Wye(g) [gen/grid] transformer presents a grounded source to the utility and is generally accepted as effectively grounded. This, may, however, desensitize the ground fault relays (51N) at the substation. For customers with an existing transformer Wye(g)/Delta or Wye(g)/ Wye, a grounding bank must be installed, which adds cost, project delays, desensitizes the substation relay, and ignores other mechanisms for TOV which may be more relevant to a given application. 8 Potential Solutions
  • 9. Controls Based Approaches Based upon the IEEE C62.92.1 definition, if a generator limits over voltages to less than 140% L-N it is effective grounded. Therefore, any mechanism that achieves this should be valid. Inverters have fast microprocessors onboard. Fast gate blocking an extra high-voltage fast-trip setting is included by most manufacturers for self-protection purposes. Spain and Australia require TOV tests that determine the reaction to a loss of load. They resemble the CBEMA/ITIC curve. The inverter shuts down upon seeing a high terminal voltage. Typically, 1.4 p.u., in 50 ms. Note: these are single phase tests and do not account for different transformer configurations. 9 Potential Solutions
  • 10. Problem: There are many ways to accomplish the aforementioned behavior. Therefore, it is challenging to generalize a given inverter’s behavior. “Rotating machines are 90% physics and 10% engineering design. Inverters are the exact opposite.”- Reigh Walling This is the main issue in predicting the behavior of an inverter during a TOV event. 10 Modeling Considerations
  • 11. 11 Modeling Considerations 1 2 3 4 5
  • 12. Modeling Considerations Variables: 1. Current Sense: RMS or instantaneous in control loop, sequence measurements, phasor average or independent control loops, sampling frequency. 2. Voltage Sensing: 1 or 3 phase, L-N or L-L, phasor average or independent control loops, RMS or instantaneous, sampling rates. 3. Current Control Loop: independent phase control or 3 phase space vector, saturated controls, different inverter bridge topologies. 4. Phase Lock Loop: αβ transform phase angle error. 5. Power Control: reactive power support, Volt/Var functions, power limiting. 12
  • 13. There is an open debate whether defining the symmetrical component impedances of an inverter has merit. But it is generally accepted that a current source model is more accurate. However, this approach has its limitations as well. • Abnormal voltages producing imbalanced currents - non idealized behavior. • Constant power regulation is more common than constant current. Power is regulated in the steady state by an outer loop, typically with slower dynamics than the inner loop which regulates current at high bandwidth. Power regulation also will tend to have limits so that excess current is not ordered during undervoltage conditions. • New functions like LVRT change everything in a snap of the fingers. 100% real power to 100%, VARS in 10ms! How is that accounted for? • Furthermore, most inverters do not inject zero sequence current. Even if the switch controls permitted it, the unit is typically a 3 wire connection without a neutral return. 13 Modeling Considerations
  • 14. Model validation is of course necessary Proposed UL 1741 SLG TOV Test Setup 14 Modeling Considerations Keep in mind: There is more than one mechanism of TOV at play here.
  • 15. Modeling can be a potential powerful tool for assessing the impact of inverters on distribution systems. However, to date, most software packages do not have an accurate model for the inverter. Inverter manufacturers and simulations vendors need to define a library of models that manufacturers can certify their products to. Otherwise there is way too much variability for a generic model to be accurate. Transformers and loads must be considered when simulating an inverter and EPS. The dynamics are determined partly by Ohm’s Law, because of the current-source inverter pushing current into the loads; and partly by the controls of the inverter itself, because its ability to source current does depend on its terminal voltage. In any case, it’s all about the interaction between the inverter and the loads, and not about an inherent GFO mechanism in the inverter, because it doesn’t have one. 15 Conclusions
  • 16. Thank you for your attention! With Special Thanks to Michael Ropp, Northern Plains Power Technologies and Reigh Walling, Walling Energy Systems Consulting 16 Questions
  • 17. 1704/26/14 Effective Grounding Slide References 1. Protection for Unexpected Delta Sources Ken Behrendt Schweitzer Engineering Laboratories, Inc. New Berlin, WI USA 2. A Review of System Grounding Methods and Zero Sequence Current Sources Gerald Johnson (Basler Electric), Mark Schroeder (Dominion VA Power) and Gerald Dalke (Power System Relay Services). 3. Advanced Grid Planning and Operations Mark McGranaghan, Thomas Ortmeyer, David Crudele, Thomas Key, Jeff Smith, Phil Barker. 4. Alternate Energy Customer Interconnection Requirements PECO Solar Conference. Barry Hornberger. June 1, 2011. 5. PECO Grey Book 17