distributed generation protection eugeniusz roso protection ...• fault detection and fault removal...

Distributed Generation Protection

Eugeniusz Rosołowski

Protection and Controlof Distributed Energy Resources

Chapter 3

[email protected]

1. Introduction 3. Distributed Generation Protection

General Considerations

DG uses many forms of energy, such as wind, solar,biomass, hydroelectric, and fossil fuel energy.

During major system disturbances and blackouts, DGcan provide reliable on‐site generation that keepscritical loads in service.

Utilities typically prevent DG sources from feeding adisconnected section of the utility network afterutility system outage what results in islandingcondition.

2. Islanding Condition 3. Distributed Generation Protection

Islanding Condition


Islanding Condition• When the utility breaker trips, the

DG source may continue supportingfault current and may also causeovervoltage in the network.

• Without utility network working inparallel, the DG source may notsatisfy quality standards and coulddamage customer equipment.

• Utility and DG equipment may sufferdamage if an asynchronous breakerreclosure occurs.


Islanding Condition

• The continuity of service may be assured even duringislanding condition. As DG source control systemimprove, islanding operation will increasingly supportcontinuity of service.

• Successful islanded operation requires adaptive loadshedding to match DG to islanded load.

• Load shedding ‐ cutting off the electric load on certainlines/areas when the demand becomes greater thanthe supply.

3. Interconnection System 3. Distributed Generation Protection

Interconnection System• The interconnection system performs the functions

necessary to maintain the safety, power quality, andreliability of connected EPSs and DGs.

• System complexity depends on the level of interactionrequired between the DG and the EPS.

• Interconnection is not a simple issue.• Interconnection requirements are far from standard.

GUtility Grid

Coupling Point

DGInterconnection System

3. Interconnection System 3. Distributed Generation Protection

General Requirements• Protection must be provided for DG installations to

protect personnel, utility facilities, and equipment fromthe abnormal voltage and frequency that may occur whenthe DG is operating as an island.

• This protection must operate for short circuits, overloads,low or high voltage, and low or high frequency, and mustprevent out‐of‐phase reclosing and energization of a deadsupply line by the generator.

• The generator owner is responsible for the protection ofhis equipment, which is usually specified by his consultantor equipment manufacturer.

4. Intertie Protection 3. Distributed Generation Protection

Intertie Protection• Recent interest in distributed generator installation into

low voltage buses near electrical consumers, hascreated some new challenges for protection engineersthat are different from traditional radially basedprotection methodologies.

• Therefore, typical protection configurations need to bere-thought such as re-closures, out-of-step monitoring,impedance relay protection zones with the detection ofunplanned islanding of distributed generator systems.


Intertie Protection

Intertie protection requirements are influenced bythe size and characteristics of the DG, along with thecharacteristics of the associated utility supply system.

Large generators with utility transmission tie points• 10MVA or greater generators,• Usually dispatched directly with utility load dispatcher,• Relay protection and control as if it were part of the utility

system connected to transmission grid.

Generators with utility distribution tie points.


Large Generator – Transmission Tie

G64KV and

Above

Utility Grid

Other Plant Loads

Point ofCommon Coupling

(PCC)

DG


Small Generator – Distribution Tie

DG

64KV andAbove

Utility Grid

Tapped toRadial Distribution

Circuit

Transmission Distribution

13KV and Below

Radial Lines

PCC

• Transmission and Distribution kV levels vary


Typical Utility Operation Requirements

• Maintain voltage ±10% approximately.• Maintain frequency ±0.1Hz approximately.• Power quality – Limits on voltage dips, sags, swells,

flicker and harmonics.• Minimize interference with communications

equipment. Outage times should be minimized.


Typical Utility Operation

• Fault detection and fault removal promptly tominimize damage to equipment and voltagedistortions.

• Reclosing substation feeder breaker, as fast as 1/3second, to minimize outage times.

• Provide safe, de‐energized condition when dead linework is done or line is on the ground.


Overcurrent Protection3-CT’s

3-PT’s

51N

1-PT

51

DG

PCC

51V

46UtilityBus

67

50

• Phase and ground overcurrent devices isolate DGfor faults on DG facility or on distribution system.• 67/67N may be used for anti-islanding in place of 32.• 51V may be optional to prevent nuisance 27 tripping.


Voltage Protection

• UV and OV relays operate when voltage outside predetermines levelsand within predetermined times.

• UV relays have time delay to prevent unnecessary trips for externalfaults (may include inst 27R for very low levels)...

3-CT’s

3-PT’s

27

81O25

1-PT

47

DG

PCC

59TUtilityBus

27R59I81U


Voltage Protection

• UV and OV relays operate when voltage outsidepredetermines levels and within predetermined times.

• UV relays have time delay to prevent unnecessary trips forexternal faults (may include inst 27R for very low levels).

• OV relays may include inst 59I for ferroresonance.• UF and OF relay levels operate when frequency outside

predetermined levels and within predetermined times.• 67/67N may be used for anti-islanding in place of 32.• 51V may be optional to prevent nuisance 27 tripping.


Synchronizm

• Not required for induction generators or line commutatedinverters.

• Utility cannot synch to DG.• DG must not energize utility when utility is deenergized.

3-CT’s

3-PT’s

25

1-PT

DG

PCC

UtilityBus


Intertie Protection

• Manufacturers of Protective and Control Relays. This isthe most conventional and experienced source for thecontrol of DG protection. The protective relay industryhas supported the protection of transmission,distribution, and generation of electricity for many years.

• The relays are the main controlling elements ofinterconnection.

• For large DG systems connected at the transmissionlevel, the relaying design for interconnection is likelyintegrated into and coordinated with the existing designof transmission protection.


Intertie Protection

Conventional Independent Relaying Packages for Utility Feeder and Non-Utility Generators

UtilityBus DG

Distrib. Relays

EPS

End-use Load

Generator Relays

Interconnection equipment


Intertie Protection

• The distribution-feeder relay package and the generator-protection relay package can be provided by thetraditional protective relay industry.

• Paralleling control and transfer from utility to generatorsource may also be offered in this integrated package.

• This can add functionality while simplifying installationand reducing the overall cost of interconnection.


Intertie Protection

Integrated Distribution Feeder and Generator Protection in One Package

UtilityBus DG

Distribution Relays

EPS

End-use Load


Intertie Protection – DG with Electronic Inverter

• In the case of smaller distributed generators that areinterconnected via an electronic inverter, the controlcapability of the inverter will play a role in making theconnection. In this case, the customer-side protectivefunctions can be completely addressed in the inverter.

• Depending on inverter size and design, a transformer maynot be required between the generator inverter and thelocal facility power system. If the inverter connection canbe made without a transformer and without additionalsynchronization and relaying protection, significantsavings in the interconnection costs can be realized.


Intertie Protection

Interconnection Functions Provided by Electronic Inverter and Transformer

UtilityBus DGEPS

End-use Load

In-verter



Intertie Protection – DG with Electronic Inverter

• If the inverter-connected DG is large compared to thelocally connected load, additional transformer isolationand additional relaying will likely be required.

• The protection is a hybrid between integrated inverter andseparate conventional relaying.

• In this case, a step-up isolation transformer is likely to berequired for both the load and the DG.


Intertie Protection

Protection for a Large DG Inverter May Require for both Built-In and Conventional Relay Functions

UtilityBus DG

Distribution Relays

EPS

End-use Load

In-verter



DG - EPS Grid Interconnection Options

• No Interconnection - Complete Isolated Operation.• Isolated DG - Automatic Transfer to EPS.• Parallel DG Operation - No Power Export.• Parallel DG Operation with Power Export.

5. Loss of mains detection 3. Distributed Generation Protection

Anti-islanding Problem

The condition of islanding, defined as when a section of the non utility generation system

is isolated from the main utility system, is often considered undesirable because of:

• the potential damage to existing equipment,• utility liability concerns, • reduction of power reliability and• power quality.



• If the total feeder load is approximately equal to DG output, then DG willtry to supply feeder load if utility feeder breaker trips.

• DG and Feeder are now islanded (loss of mains problem).• Can't close utility breaker without synch check (25).

LoadTotal

Utility Bus

Net Flow Zero

Utility Feeder

DG

3MVA

3MVA



• If islanding did occur, to close thefeeder breaker would requiremonitoring the phase angleacross the breaker.

• The autosynchronizer (25A)would need to control voltageand frequency of the DG or theentire utility system. Notpractical.

• Islanding is usually avoided.

UtilityBus

Utility Feeder

DG

25A

Open



Typical distributed system with DG’s


1. Active Islanding Detection Methods

a) Communication based scheme: transfer trip scheme



Power Line signaling scheme



b) Method of impedance measurement


1. Active Islanding Detection Impact

Degradation of power quality and system stability as DGpenetration becomes higher.Currently the local islanding detection methods virtuallyguarantee that the DG will be unable to provide gridsupport or improve grid stability when the grid is stressedanti-islanding protection disconnects the DG when itdetects voltage and frequency excursions on the grid.Because of the reclosing practice, anti-islandingtechniques must trip DG’s within about 200 millisecondsbefore the breaker is reclosed. Failure to do so will lead toout-of-phase re-energization of the DG.


2. Non-active (Passive) Islanding Detection Methods

a) Over/Under Voltage Amplitude (O/U VA)Islanding is detected when the amplitude of VMP is out ofpre specified interval of voltage amplitude.

UtilityBus

Utility Feeder

DG

Open

Measurement point, VMP

IDG


2. Non-active (Passive) Islanding Detection Methodsb) Over/Under Voltage Frequency (O/U VF) and Rate of

Change of Frequency (ROCOF).Islanding is detected when the amplitude of VMP is out of

pre-specified interval of voltage amplitude. A df/dt detectionmethod (ROCOF) might decrease the detection time andNon Detective Zone. The ROCOF relay is generallyaccepted as the standard method for islanding protectionof the distributed system. These relays monitor the voltagewaveform, and operate when the rate-of-change offrequency exceeds a setting and duration exceeds a timedelay setting.


2. Non-active (Passive) Islanding Detection Methodsc) Voltage Phase Jump (VPJ).The phase difference between VMP and IDG is monitored. VMPwill jump in phase if IUtility ≠ 0 at the instant of islandingoccurs.d) Impedance measurement.Change of positive or negative sequence impedance isapplied for islanding detection.e) Mixture of some sellected methods is applied

(multicriteria method).Multicriteria decision making algorithm is needed forselection the final signal.


Anti-islanding Protection - example

DG

PCC

UtilityBus

TransferTrip

TransmitTT

Rec.

Trip

52b

3-PT’s

81O81U32

3-CT’s

27 59

• OF and UF used with load shedding for anti-islanding.• OV and UV trip for faults on distribution feeder.• 32 trips for export of power (may be specific level).• DG must consider utility reclosing to ensure clearing prior

to reclose.

6. Protection Issues 3. Distributed Generation Protection

DG Impact on Protection and Reclosing

DG must be disconnected here

6. Protection Issues 3. Distributed Generation Protection

DG Impact on Protection and Reclosing

The total fault current is composed of the followingcontributions from both the source IB and the distributedgeneration current IDB. The Relay B measures only current IBwhat reduces protected zone (relay under-reach).

5. Generator Protection 3. Distributed Generation Protection

Type of Generation

•Rotating machine - speed controlled to matchutility 50/60 Hz

- Synchronous - capable of supplying load as anindependent source with voltage and frequencycontrol.- Induction - cannot operate alone since it gets itsexcitation from the ac system to which it isconnected.- May be single phase or three phase


Type of Generation

Permanent Magnet Synchronous Generator


Type of Generation

Dobly Feed Induction Generator


Type of Generation

• Power inverters change DC to ACelectronically. The DC source could bebatteries, fuel cells or photovoltaic panels.

-Line commutated inverters need an externalsource of reactive power to operate and wouldnormally shut down when the utility tie opens.- Self commutated inverters can continue to supplypower independently and do not require a sourcefrom the utility. These inverters should be trippedwhen the utility tie opens.


Type of Generation

Power inverters change DC to ACelectronically. The DC source could bebatteries, fuel cells or change non-50Hzgeneration to 50Hz power. The AC sourcecould be micro-turbines generating 400Hz.

-These Static Power Converters can synchronize tothe 50Hz utility system faster than a synchronousgenerator where the excitation must be controlledto match voltage and frequency of the utility.- These static power converters are also known asisochronous generators.


Converters connected Generation

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