Use of Relay-to-Relay Communications to Use of Relay-to-Relay Communications to Improve the Security and Reliability of Improve the Security and Reliability of
DG Interconnection Protection DG Interconnection Protection Employing Reverse Underpower ProtectionEmploying Reverse Underpower Protection
Wayne G. HartmannWayne G. HartmannMember, IEEE & IEEE Power System Relay CommitteeMember, IEEE & IEEE Power System Relay Committee
Clemson UniversityClemson UniversityPower Systems 2005 ConferencePower Systems 2005 Conference
Distributed Generation, Advanced Metering & CommunicationDistributed Generation, Advanced Metering & Communication
March 10-12, 2005March 10-12, 2005Madren Center, Clemson University,Madren Center, Clemson University,
Clemson, SC, USAClemson, SC, USA
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IntroductionIntroduction
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)
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Introduction:Introduction:Use of the 32R-U ElementUse 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.
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Introduction:Introduction:What we’ll coverWhat 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
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Islanding and 32R-UIslanding 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.
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32F vs. 32R-U32F 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.
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32R-U:32R-U:How AppliedHow Applied
GRID
ReversePower
ForwardPower
REVERSE UNDERPOWER (32R-U)
NOTRIP
ReversePower
(Import)
ForwardPower
(Export)
TRIP
Pick up
DG
FacilityLoads
32R-U*
52I
52DG
52L
Trip
Zero Power
* = plus other DG interconnection protection
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Non-ExportingNon-Exporting DG Facility Operation 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.
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Non-ExportingNon-Exporting DG Facility Operation 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
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Peak Shaving vs.Peak Shaving vs.Load FollowingLoad Following
Peak Shaving
Morning Noon Night
Load
Generation
Morning Noon Night
Load
Generation
Load Following
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Typical Intermittent Grid Typical Intermittent Grid Interconnected Facility Interconnected Facility
for Critical Loadfor Critical Load
Utility
DG
FacilityLoads
32R-U*
52I
52DG
52L2
Trip
CriticalFacilityLoads
52L1
Trip
* = plus other DG interconnection protection
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OSG not in operationOSG not in operation prior to an prior to an unplanned utility outageunplanned 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
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OSG is in operationOSG is in operation prior to an prior to an unplanned utility outageunplanned 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.
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OSG is operated during a OSG is operated during a planned separationplanned separation of load from of load from
the utilitythe 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.
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Facility Operations and Impact Facility Operations and Impact on 32R-U Protection Securityon 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
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Power Setting Up = Security Power Setting Up = Security DownDown
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.
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Large Loss of Facility Load 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.
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Large Loss of Facility LoadLarge Loss of Facility Load
DG
Utility
Sudden lose of 50 kVAmotor load
Momentary poweroutflow develops at PCC
Circuit breaker closed
380 kVA
Feeder Loads
LocalLoad
360 kVA
Low Import Pow er (32R-U)set at 20kVA
(5% of 400kVA OSG) momentaryexport
<= 50 kVA
Gen = Load - Bias
380 = 360 - 3050 kVA
Motor breaker tripped
M
Circuit breaker closed
380 = 330
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Reconnection of Self-Supporting Reconnection of Self-Supporting Facility Island to the GridFacility 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.
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Reconnection of Self-Supporting Reconnection of Self-Supporting Facility Island to the GridFacility Island to the Grid
DG
Utility
Islanded load = OSG No import of power when
PCC CB closes
Circuit breaker closed
350 kVA
Feeder Loads
LocalLoad
300 kVA
Low Import Pow er (32R-U)set at 20kVA
(5% of 400kVA OSG)
No import at theinstant the CB
closes
Gen = Load - Bias
350 = 350 - 050 kVA
Circuit breaker tripped
M
Circuit breaker closed
380 = 350
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Traditional Approaches Traditional Approaches to Increase 32R-U Securityto 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.
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Tripping of a Large Block of Tripping of a Large Block of LoadLoad
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
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Tripping of a Large Block of Tripping of a Large Block of LoadLoad
• 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
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Tripping of a Large Block of Load:Tripping of a Large Block of Load:Adaptive Protection Based on CB Status of LoadAdaptive Protection Based on CB Status of Load
DG
Utility
Sudden lose of50 kVA motorload
Change 32R-Utime delay, orchange toshort time 32F
Circuit breaker closed
380 kVA
Feeder Loads
LocalLoad
360 kVA
Low Import Pow er (32R-U)set at 20kVA
(5% of 400kVA OSG) momentaryexport
<= 50 kVA
Gen = Load - Bias
380 = 360 - 3050 kVA
Motor breaker tripped
M
Circuit breaker closed
380 = 330
Monitor CBstatus
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Returning Critical Load toReturning Critical Load to Utility Power from On-Site Utility Power from On-Site
PowerPower
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
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Returning Critical Load toReturning Critical Load to Utility Power from On-Site Utility Power from On-Site
PowerPower
• 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
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Returning Critical Load toReturning Critical Load to Utility Power from On-Site Power: Utility Power from On-Site Power:
Adaptive Protection Based Adaptive Protection Based on CB Status of Critical Load and OSGon CB Status of Critical Load and OSG
Utility
DG
FacilityLoads
32R-UCriticalFacilityLoads
Low Import Power (32R-U)set at 20kVA
(5% of 400kVA OSG)
Monitor CB status
Close PCC CBwith no import
Change 32R-Utime delay
Circuitbreakerclosed
Circuit breakertripped
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Adaptive Scheme: HardwiredAdaptive Scheme: Hardwired
52
OSGCB
or
(+)
(-)
32R-UTrip PCC CB
CSI
OSGSWITCHGEAR
PCCSWITCHGEAR
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Adaptive Scheme: HardwiredAdaptive 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
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Adaptive Scheme: HardwiredAdaptive 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.
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Adaptive Scheme: Adaptive Scheme: Fiber using RRCFiber using RRC
or
(+)
(-)
32R-UTrip
PCC CB
OSGSWITCHGEAR
PCCSWITCHGEAR
OSGCB
CSI ETHERNET
ETHERNET
OSG
Comparators,Timers &Logic
Protective Relay Protective Relay
DG Facility& Grid
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Adaptive Scheme: Adaptive Scheme: Fiber using RRCFiber 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.
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Adaptive Scheme: Adaptive Scheme: Fiber using RRCFiber 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.
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Adaptive Scheme: Adaptive Scheme: Fiber using RRCFiber 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.
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RRC: More DiagnosticsRRC: 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 (permitted for 5 seconds only, then scheme error alarm), or, Current >0.05 of OSG rated and CB auxiliary contacts indicate CB closed
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Looking to the Future:Looking to the Future:RRC using Analog Data, Math and RRC using Analog Data, Math and
LogicLogicMission: 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.
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Looking to the Future:Looking to the Future:RRC using Analog Data, Math and RRC using Analog Data, Math and
LogicLogic• 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.
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Adaptive 32R-U ProtectionAdaptive 32R-U Protection Using RRC Using RRC
IEC 61850
Fiberoptic CableFiberoptic Cable
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ConclusionsConclusions
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
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