1. Use of Relay-to-Relay Communications to Improve the Security and Reliability of DG Interconnection Protection Employing Reverse Underpower Protection Wayne G. Hartmann Member, IEEE & IEEE Power System Relay Committee Clemson University Power Systems 2005 Conference Distributed Generation, Advanced Metering & Communication March 10-12, 2005 Madren Center, Clemson University, Clemson, SC, USA

2. 2 Clemson Power Systems Conference - 2005 Introduction This paper explores the use of relay-to-relay communications to aid in the prevention of nuisance tripping DG employing reverse underpower protection (32R-U) in DG interconnection protection schemes 32R-U is applied to detect loss of utility supply –This protection is sometimes referred to as “low import power protection” It is impossible to export if you are tripped for not importing Applied in non-exporting DGs (peak shaving)

3. 3 Clemson Power Systems Conference - 2005 Introduction: Use of the 32R-U Element 32R-U is a cost effective means of determining loss of utility and subsequent islanded operation The 32R-U element may be insecure or may have to be set with wide margin causing unnecessary economic loses This is due to the interaction of the DG’s on-site power (OSG) control response (governor system) and the fluctuation of load, intentional or unplanned, in the DG facility. Upset between the OSG’s output and the facility’s energy consumption may cause inadvertent export of power to the utility.

4. 4 Clemson Power Systems Conference - 2005 Introduction: What we’ll cover Address how non-exporting DG facilities operate The interaction of that operation and the areas of exposure for insecure 32R-U protection Present adaptive protection techniques that are applied to improve security New methods employing relay-to-relay communications (RRC) and adaptive protection to improve the security and reliability of the 32R-U protection to new levels

5. 5 Clemson Power Systems Conference - 2005 Islanding and 32R-U “Islanding”: when a utility grid is divided into different asynchronous areas In the context of DG, it’s when the DG facility supplies power to load on a feeder or area of the utility distribution system that has been disconnected from the utility source Use of 32R-U has been accepted to detect loss of utility supply Works great if the DG facility is not expected to export power to the utility grid This often mitigates the need for costly transfer trip protection initiated by the utility’s substation breaker some distance away from the facility with the DG.

6. 6 Clemson Power Systems Conference - 2005 32F vs. 32R-U 32R-U is more reliable than 32F to detect loss of utility supply 32F sometimes called “inadvertent export” 32R is more reliable as there is no chance that an unintentional island on the utility’s system can be created and maintained by the DG facility Absolutely no power export from the DG facility is allowed.

7. 7 Clemson Power Systems Conference - 2005 32R-U: How Applied

8. 8 Clemson Power Systems Conference - 2005 Non-Exporting DG Facility Operation DG facilities that may employ the 32R-U element as a means of utility loss of supply are typically non- exporting, and fall into three general categories: Peak Shaving: employ the grid interconnected on-site generation (OSG) to offset peak power consumption. Grid interconnected OSG is operated during the intervals of high demand where the fuel cost of the on-site generation is offset by the utility high peak demand charges.

9. 9 Clemson Power Systems Conference - 2005 Non-Exporting DG Facility Operation Load Following: employ the grid interconnected OSG for long time intervals to offset high utility base load power cost If the fuel cost to operate the grid interconnected OSG is less expensive than the utility base rate, the grid interconnected OSG output is typically adjusted to assume as much of the facility’s load as possible (without exporting) to minimize the power import from the utility. Intermittent Grid Interconnected: employ the OSG to operate critical loads During unplanned utility outages During planned outages in isolation from the utility when weather or other conditions would make the utility supply less reliable

10. 10 Clemson Power Systems Conference - 2005 Peak Shaving vs. Load Following

11. 11 Clemson Power Systems Conference - 2005 Typical Intermittent Grid Interconnected Facility for Critical Load

12. 12 Clemson Power Systems Conference - 2005 OSG not in operation prior to an unplanned utility outage Failure of the utility supply Critical load is isolated from the utility OSG is started and the critical load supported by the OSG Utility restores power to the DG facility OSG is connected (paralleled) to the rest of the facility, and therefore the utility OSG power output is backed off and disconnected leaving all load served by the utility

13. 13 Clemson Power Systems Conference - 2005 OSG is in operation prior to an unplanned utility outage Failure of the utility supply Critical load and OSG is isolated from the utility Critical load supported by the OSG Utility restores power to the DG facility Critical load supported by the OSG is connected (paralleled) to the rest of the facility, and therefore the utility OSG power output is backed off and disconnected leaving all load served by the utility.

14. 14 Clemson Power Systems Conference - 2005 OSG is operated during a planned separation of load from the utility OSG is started and paralleled to the facility OSG output is adjusted to assume the power requirements of the critical load Once the power output of the DG is equal the critical load, the critical load is separated from the balance of the facility –Load fully assumed by the OSG as an off-grid island To place the critical load back on utility supply, the critical load and OSG is connected (paralleled) to the rest of the facility and the utility OSG power output is backed off and the OSG disconnected leaving all load served by the utility.

15. 15 Clemson Power Systems Conference - 2005 Facility Operations and Impact on 32R-U Protection Security Certain DG interconnection guidelines, such as California’s Rule 21, have recommended settings for the 32R-U protection A typical value is at least 5% of the aggregated nameplate rating of the DG Implies if there is more than one generator in the DG line up, to add up all the nameplate ratings to arrive at the aggregated power value

16. 16 Clemson Power Systems Conference - 2005 Power Setting Up = Security Down The term “at least” modifying the 5% aggregated power value in terms of protection security is very significant: The higher value of import power used in the 32R-U protection, the larger the value of the margin that must be used in load following applications. This can have undesired economic impact to the facility The higher the value of import power used in the 32R-U protection, the more prone the facility is to trip for losses of load within the facility In most cases, the lower the value of power used when setting the 32R-U protection, the more secure the protection will be.

17. 17 Clemson Power Systems Conference - 2005 Large Loss of Facility Load Large loss of facility load when the facility’s OSG is used in a load following mode with little margin This margin is the difference between the facility’s load and OSG’s power output A sudden loss of load in the facility can lead to a power inflow from the utility less than the setting of the 32R-U protection This would be a transient condition that would rectify itself after the OSG’s governor system backed down on the power output to maintain an import margin.

18. 18 Clemson Power Systems Conference - 2005 Large Loss of Facility Load

19. 19 Clemson Power Systems Conference - 2005 Reconnection of Self-Supporting Facility Island to the Grid When a facility operating as an island to the utility (off grid) is reconnected to the utility, the net power flow across the point of common coupling (PCC) is zero –Facility’s OSG was supplying all of the power required by the off-grid load This would be a transient condition that would rectify itself after the DG’s governor system backed down on the power output to maintain an import margin.

20. 20 Clemson Power Systems Conference - 2005 Reconnection of Self-Supporting Facility Island to the Grid

21. 21 Clemson Power Systems Conference - 2005 Traditional Approaches to Increase 32R-U Security Construct controlling logic in protection schemes to increase security –Use status of circuit breakers at the PCC, the facility’s DG (single or multiple generators) and large blocks of load These schemes employ hard wiring between relay locations and would detect the status of the switchgear –Temporarily block the 32R-U element –Switch settings groups where a value for the 32R-U time delay would be increased so the transient operating condition could be mitigated by controlling the DG output.

22. 22 Clemson Power Systems Conference - 2005 Tripping of a Large Block of Load The normal setting of the 32R-U element is 5% of the aggregate rated power of the OSG; time delay of 60 cycles If the facility was operating with the OSG assuming a portion of the load, and a large block of load was tripped, the power import at the PCC import could momentarily be lower than 5% setting –OSG’s governor does not adjust instantaneously Exposure to this unwanted situation would increase as the portion of the facility’s load that is assumed by the OSG is increased

23. 23 Clemson Power Systems Conference - 2005 Tripping of a Large Block of Load Possible mitigation techniques are to monitor large blocks of load that if tripped while the OSG was operational –Lower the 32R-U setting (move to zero power) –Convert the 32R-U into a forward power element (inadvertent export protection) –For both cases, increase the timer setting to allow the governor action to decrease the output of the OSG to a lower value so required normal power import from the utility could be reestablished

24. 24 Clemson Power Systems Conference - 2005 Tripping of a Large Block of Load: Adaptive Protection Based on CB Status of Load

25. 25 Clemson Power Systems Conference - 2005 Returning Critical Load to Utility Power from On-Site Power The normal setting of the 32R-U element is 5% of the aggregate rated power of the OSG; time delay of 60 cycles If the facility was operating with the OSG assuming the critical load, and the facility is reconnected to the utility at the PCC, the power import at the PCC import could momentarily be lower than 5% setting –Other loads are not immediately picked up

26. 26 Clemson Power Systems Conference - 2005 Returning Critical Load to Utility Power from On-Site Power By monitoring the status of the PCC breaker and the OSG, you could –Increase the 32R-U time delay –Convert the 32R-U into a forward power element (inadvertent export protection) –For both cases, increase the timer setting to allow the governor action to decrease the output of the OSG to a lower value so required normal power import from the utility could be reestablished

27. 27 Clemson Power Systems Conference - 2005 Returning Critical Load to Utility Power from On-Site Power: Adaptive Protection Based on CB Status of Critical Load and OSG

28. 28 Clemson Power Systems Conference - 2005 Adaptive Scheme: Hardwired

29. 29 Clemson Power Systems Conference - 2005 Adaptive Scheme: Hardwired These schemes may require long wiring runs for inter-relay connection May be subject to step potential rise during ground faults in the facility if the ground mats at the distinct areas (PCC, OSG location, large load switchgear location) are not closely connected with very low impedance. May not be self-diagnostic to wiring failure –Failed relay contact output –Open/shorted wiring –Wetting power supply failure –Control status input failure

30. 30 Clemson Power Systems Conference - 2005 Adaptive Scheme: Hardwired May not adopt a failsafe protection characteristic if a compromise in the circuit is detected These schemes also make use of timers to account for the time delay of the OSG’s governors in controlling their power output –Depending on the amount of OSG’s applied in aggregated service, the response time may be variable.

31. 31 Clemson Power Systems Conference - 2005 Adaptive Scheme: Fiber using RRC

32. 32 Clemson Power Systems Conference - 2005 Adaptive Scheme: Fiber using RRC Digital communication employing fiberoptic links between relays at different physical locations can mitigate shortcomings of the hard-wired schemes in use today Schemes based on use of the IEC 61850 standard for RRC can offer the following over hard wired schemes: Fiberoptic cable is inherently an insulator, therefore the step potential concerns are eliminated. The speed of the inter-relay communications can be very fast, typically less than 8mS. –There are no delays caused by output relays and control/status input filtering.

33. 33 Clemson Power Systems Conference - 2005 Adaptive Scheme: Fiber using RRC RRC can assure that the communication link is viable –Immediately self-diagnostic to any failure –Modify protection action accordingly Redundant communication paths can be employed to boost reliability and security of the system. Proper action for a failure in the communications depends on if redundant communication is employed.

34. 34 Clemson Power Systems Conference - 2005 Adaptive Scheme: Fiber using RRC Single communication path: –Failure is detected in the system –All relays involved would know and could take action –Revert back to fixed protection setpoint for the 32R-U element –When hard wired schemes are applied, it is difficult to determine the viability of the output relays and control/status inputs. Redundant (two) communication paths: –One of the communication paths fails –an alarm is asserted –adaptive protection is still maintained by the remaining functioning communication path.

35. 35 Clemson Power Systems Conference - 2005 RRC: More Diagnostics When using RRC, logic may be applied within a relay to determine agreement between CB auxiliary contacts and the absence or presence of current. If all is in agreement, a signal is then passed via RRC to the remote relay for action. If the current level and CB auxiliary current status do not show agreement, then a scheme error message would be broadcast. Agreement of the CB auxiliary contract status and current level can be defined as: –OSG Off = Current < 0.05 of OSG rated and CB auxiliary contacts indicate CB opened –OSG On = Current 0.05 of OSG rated and CB auxiliary contacts indicate CB closed

36. 36 Clemson Power Systems Conference - 2005 Looking to the Future: RRC using Analog Data, Math and Logic Mission: Improve the Security of 32R-U Protection As IEC 61850 is further defined, it will be possible to transmit analog data, such as power levels, across the communication link. It will also be possible to implement high speed math computation and logic in protective relays. –32R-U element could actually have its setpoint (pick up and/or time) reset to adapt to changing power balance conditions across the PCC brought about by changes in the load/OSG balance within the DG facility.

37. 37 Clemson Power Systems Conference - 2005 Looking to the Future: RRC using Analog Data, Math and Logic Fault induced power flow changes would not be made in the 32R-U reset calculation, making the system sensitive and high speed for utility caused power flow changes. In the same manner, data and math comparisons could be used to convert the 32R-U into a 32F with the power level set to match the upset within the facility. –There would be a feedback loop established to adaptively set the 32R-U, and perhaps employ a 32F for short time periods. –Protective elements would never be blocked or desensitized any more than necessary to ride through the transient load/OSG unbalance conditions within the DG facility.

38. 38 Clemson Power Systems Conference - 2005 Adaptive 32R-U Protection Using RRC

39. 39 Clemson Power Systems Conference - 2005 Conclusions RRC, when applied in DG interconnection protection employing reverse underpower protection (32R-U), offers: Increased self diagnostic abilities to verify adaptive protection inputs are viable Increased security and reliability as any failures in the system can be alarmed for rapid corrective action to Immunity to noise and step potential issues in facilities where OSG and PCC switchgear are on different ground planes Redundant communications offers the ability to maintain the adaptive scheme and alarm a communication system failure, increasing reliability and security