Use of Relay-to-Relay Communications to Improve the Security and Reliability of DG Interconnection...

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

2 Clemson Power Systems Conference - 2005

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)


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.


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


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.


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.


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


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.


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


Peak Shaving vs.Peak Shaving vs.Load FollowingLoad Following

Peak Shaving

Morning Noon Night

Load

Generation

Morning Noon Night

Load

Generation

Load Following


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


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


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.


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.


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


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.


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.


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


380 = 330


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.


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


350 kVA

Feeder Loads

LocalLoad

300 kVA


(5% of 400kVA OSG)

No import at theinstant the CB

closes

Gen = Load - Bias

350 = 350 - 050 kVA

Circuit breaker tripped

M


380 = 350


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.


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


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


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


380 kVA

Feeder Loads

LocalLoad

360 kVA


(5% of 400kVA OSG) momentaryexport

<= 50 kVA

Gen = Load - Bias

380 = 360 - 3050 kVA

Motor breaker tripped

M


380 = 330

Monitor CBstatus


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


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


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


Adaptive Scheme: HardwiredAdaptive Scheme: Hardwired

52

OSGCB

or

(+)

(-)

32R-UTrip PCC CB

CSI

OSGSWITCHGEAR

PCCSWITCHGEAR



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



• 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.


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



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.



• 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.



• 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.


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


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.


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.


Adaptive 32R-U ProtectionAdaptive 32R-U Protection Using RRC Using RRC

IEC 61850

Fiberoptic CableFiberoptic Cable


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

Use of Relay-to-Relay Communications to Improve the Security and Reliability of DG Interconnection...

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