Interconnecting Industrial Interconnecting Industrial DG to the Main GridDG to the Main Grid

Dr. Mithulananthan Nadarajah

Energy Field of Study

Asian Institute of Technology

Thursday, 07 September 2006

Presentation OutlinesPresentation Outlines

Introduction of DG

Benefits

Interconnection Issues

Technical Issues

Commercial and Regulatory Issues

Other Issues

Conclusion

Distributed Generation (DG)Distributed Generation (DG)Distributed generation is not a new concept. In fact, the history of power generation started with the distributed generation.

Early power plant only supplied electricity to customers in its close neighbourhood.

In the last decade, technological innovations, changing economy and environment concern have resulted in a renewed interest in distributed generation.

The ‘economy of scale’ in power industry has shrunk in the recent years and utility generation pattern is shifting from ‘economy of scale’ to ‘economy of mass production’

Recent studies suggest that the difference in cost of electricity production (cost per kWh) between large and small scale generation has reduced to 30% in 2000 from 60% in 1960.

Due to these and various other reasons including environmental concern, distributed generation are gaining popularity around the globe.

Definition of DGDefinition of DG

Distributed generation in simple term can be defined as a small scale generation. It is active power generating unit that is connected at distribution level.

IEEE defines the generation of electricity by facilities sufficiently smaller than central plants, usually 10 MW or less, so as to allow interconnection at nearly any point in the power system, as Distributed Resources.

Electric Power Research Institute (EPRI) defines distributed generation as generation from a few kilowatts up to 50 MW.

International Energy Agency (IEA) defines DG as “Power generation equipment and system used generally at distribution levels and where the power is mainly used locally on site”.

The International Council on Large Electricity Systems (CIGRE)defines DG as generation that is not centrally planned, centrally dispatched at present, usually connected to the distribution network, and smaller than 50-100 MW.

Present and Future Distribution Present and Future Distribution SystemSystem

Fig 1: Changing Distribution Structure (Source: Distributed Utility Associates)

DG TechnologyDG Technology

Reciprocating Engines

Photovoltaic

Wind Power

Fuel Cells

Micro-Turbines

Reciprocating Engines

Bagasse Fired Plants

Combined Heat and Power Plants (CHP)

Driving Forces of DGDriving Forces of DG

IEA (2002) has listed five major reasons why we should choose distributed generation:

Developments in distributed generation technologiesConstraints on the construction of new transmission linesIncreased customer demand for highly reliable electricityElectricity market liberalizationConcerns about climate change

Other driving forces are:Reduction of overall cost to electricity consumersNeed for increased generation capacityNeed for improved reliabilityNeed for increased securityNeed to respond to demand peaks

DG in Sugar IndustryDG in Sugar Industry

Fig 2: Schematic diagram of a Bagasse-fired plant for combined heat and power production (Source: Williams and Larson, 1993)

Benefits of DG to IndustriesBenefits of DG to IndustriesIncreased reliability: Installation of DG in the industry’s backward will

improve the reliability of the power supply. Industry can have “Uninterrupted Service” by installing a DG where it can be used as the backup to the main grid.

Power quality: Many types of electrical equipments are very sensitive to voltage and frequency. With DG, industry can have a micro-grid operation to prevent its equipment from voltage sag and swells as well as deviation of frequency.

Energy cost savings: Cost of energy is increasing with increasing fuel price and industries have to pay heavy demand charges. Industry can save the energy cost by running the DG at peak load. Depending on the technology and fuel cost, sometime generating energy from DG is less costly than buying from grid.

Benefits of DG to the Industry (2)Benefits of DG to the Industry (2)Increased energy efficiency: Industry can make use of waste heat to

generate power by having a Combined Heat and Power (CHP) generation. This will improve the overall efficiency of energy use in the industry.

Reduce exposure to price volatility: In case of the deregulated environment, DG can help the industry to reduce the exposure to the price volatility.

Environmental concern: Environment is major concern today, through out the world. People are looking for green power. Even in Thailand, there are rules are regulations to make sure that certain portion of total power generation comes from renewable sources. DG, especially those based on renewable energy, helps the environment and it is eligible to many financial benefits.

Interconnecting DG to GridInterconnecting DG to Grid

Even though DG is a viable option to meet the power demand of the industry, there are varieties of technical, commercial, and regulatory issues that have to be considered before the interconnection of DG plant with main grid.

Interconnection is basically meant for back wheeling the excess power produced by a DG plant to the grid.

The interconnection issues have potential enough to prevent distributed generation projects from being developed. Interconnection issues are broadly classified as:

Technical issues

Commercial and regulatory issues

Other issues

Interconnecting DG to Grid (2)Interconnecting DG to Grid (2)

Fig 3: Connecting DG to the grid(Source: General Electric Corporate Research and Development)

Technical barriers Technical barriers for Interconnecting DGfor Interconnecting DG

Most significant technical barriers are:Protection

Power quality

Stability

Islanding issues

Standards

ProtectionTradition distribution systems were not designed to have active power generating units in them.

Power is supplied by the transmission system and power flow is mainly unidirectional. But with the DG in the system, power flow can be bi-directional

Impact of DG on Protection System Impact of DG on Protection System CoordinationCoordination

Fig 4: Typical feeder with recloser and fault at EOL

IUS is the current from the utility sourceFault current seen by the recloser is IR

The fault current at the fault location is IF.Without DG

IUS = IR = IF

With the DG connected

IF = IUS + IDG and IR = IF

However, IR ≠IUS

The condition indicated is not seen in the typical radial distribution system.

IUS without the DG does not equal IUS with the DG. With the DG connected the fault current seen by the recloser(IR) will be greater than without the DG connected.

Impact of DG on Protection System Impact of DG on Protection System Coordination (2)Coordination (2)

Furthermore, in short-circuit faults, DG generates fault current that depends strongly on both the generator type and the network configuration. Synchronous generators are able to feed rather large sustained fault current while inverter based systems may be controlled so that their output is limited to the rated current.

This would normally not cause a problem with the recloser size as long as the new greater IR does not exceed the recloser maximum interrupting rating.

However, it is very likely that coordination between the recloser and any down-line fuses will be lost.

Because both the recloser and fuses operate faster at higher fault currents, the required margins between the recloser fast curve and the fuse minimum melt curve could be reduced enough to lose coordination.

Other Protection IssuesOther Protection Issues

Depending on characteristics of the network and DG, various other protection problems can arise. They are namely:

False tripping of feeders (sympathetic hipping)

Nuisance tripping of production units

Blinding of protection

Increased or decreased fault levels

Unwanted islanding

Prohibition of automatic reclosing

Unsynchronized reclosing

Power Quality and DG Power Quality and DG A major issue related to interconnection of distributed resources onto the power grid is the potential impacts on the quality of power provided to other customers connected to the grid.

Voltage Regulation

Over-voltages due to reverse power flow: If the downstream DG output exceeds the downstream feeder load, there is an increase in feeder voltage with increasing distance. If the substation end voltage is held to near the maximum allowable value, voltages downstream onthe feeder can exceed the acceptable range.

Interaction with load tap changers (LTC) and static voltage regulators (SVR) controls: The presence of DG can cause localized changes in flow patterns which are not reflective of the general trend on the feeder. As a result, LTC or SVR can be set such that a good voltage profile may not be obtained.

Power Quality and DG (2)Power Quality and DG (2)

Fig 5: Effect of DG penetration on global voltage profile(Source: General Electric Corporate Research and Development)

DG can cause high voltage at light load for significant penetrations

Power Quality and DG (3)Power Quality and DG (3)DG Grounding Issue

A grid-connected DG, whether directly or through a transformer, should provide an effective ground to prevent un-faulted phases from over-voltage during a single-phase to ground fault.

Harmonic DistortionAll power electronic equipments create current distortion that can impact neighbouring equipment. DG like PV, fuel cells are likely to introduce harmonics problem in the system.

FlickerSome energy source (e.g., wind turbine or fuel cell) has some mechanical (or chemical) fluctuations in power output and some electrical equipment (e.g., the dc bus and inverter) does not have sufficient energy storage to smooth out these fluctuations. This will result in fluctuations in the power delivered by a DG and can cause flicker in the power system in a fashion very similar to that caused by load fluctuations.

Power Quality and DG (4)Power Quality and DG (4)

DC Current InjectionWhen DG power converters are directly connected (without isolation transformers) to the utility grid, there is the potential to inject DC current. This can impact transformers and other magnetic elements causing saturation and can cause torque ripple in adjacent machine loads.

Unbalanced GridConnection to the unbalanced grid will have an impact on rotating machines as well as Phase Width Modulation (PWM) inverter-based DG. Since the DG is controlled as a balanced (positive-sequence) current source, the product of positive-sequence current and the negative-sequence voltage will cause two times fundamental frequency power ripple. Furthermore, the unbalanced grid will also cause harmonics in the inverter output current.

Stability and DynamicsStability and DynamicsFew number of DG is considered as harmless to overall stability and dynamics of the distribution system. But with increasing penetration of DG, it is believed to effect distribution system's stability.

In fact, its impact is no longer restricted to the distribution network but starts to influence the whole system, including transmission system.

The small synchronous generators have simple exciter and governor control schemes compared to large central power generators and induction generators are not able to control the reactive power at all. DG units connected via electronic power converters do not have large capabilities to control.

These special characteristics of the DG units and their low inertia can produce many technical and operating challenges regarding the stability of power systems.

Transient Stability and DGTransient Stability and DGTransient Stability

The power generation from DG leads to a reduction of power generation from central power units, the number of online generators and spinning reserve. This results in a larger uncertainty in terms of power system stability when a major disturbance occurs.With a high level of DG penetration, its connection strength may influence the transmission system transient stability in case of a fault.

Voltage StabilityThe tendency of induction motors to restore their power after disturbances by adjusting their operating slips would increase the reactive power consumption causing further voltage reduction. If the required power consumption by loads is beyond the capability of the generators or transmission systems, a run-down situation causing voltage instability takes place.

Oscillatory Stability In the electromechanical mode, the damping is slightly improved and the corresponding frequency is increased with the insertion of more DG units

Oscillatory StabilityOscillatory StabilityIncreasing penetration of DG causes lower damping and higher frequency

With small numbers of DG units near some of the load nodes, the DG controllers have only local action and the global damping of the controller mode is worsened.

The use of a large number of DG units, which are uniformly dispersed in the low voltage area, extends the controller action to cover most of the load nodes.

Hence, the performance of this mode is slightly improved with the high penetration levels of the DG units.

Fig 6: Critical Eigen values with different penetration of DG

(Source: Azmy and Erlich)

Islanding IssuesIslanding Issues

If an island should occur, it should persist for only a very brief period, unless the aggregate real and reactive output of all the DG supporting the island is close to the load demand. Otherwise, island voltage and frequency will change rapidly and all the DG has to be shut down to prevent this.In case the DG in the distribution system is capable to meet the load demand, DG can be operated in the island mode and continue to energise the distribution system. But the major issues with this type of inadvertent islanding are:

Line worker safety can be threatened by DG sources feeding a system after primary sources have been opened and tagged out.Public safety can be compromised as the utility does not have the capability of de-energizing downed lines. The voltage and frequency provided to other customers connected to the island are out of the utility’s control, yet the utility remains responsible to those customers.Protection systems on the island are likely to be uncoordinated, due to the drastic change in short-circuit current availability.

Islanding Issues (2)Islanding Issues (2)The islanded system may be inadequately grounded by the DG interconnection.Utility breakers or circuit reclosers are likely to reconnect the island to the greater utility system when out of phase.

Other problems of islanding include:Out-of-step reclosing: Many utilities use an “instantaneous” reclosingpractice, where breakers and circuit reclosers reenergize the protected circuit without any intentional delay and this could result in out of phase reclosing of the distribution system. As a result of out of phase reclosing :

Large mechanical torques and currents are created, which can damage the generator or the prime mover. Transients are created which are potentially damaging to utility and other customer equipment.Out-of-phase reclosing, if it occurs at a voltage peak, will generate avery severe capacitive switching transient. In a lightly dampedsystem, the crest over-voltage can approach three times rated voltage.

DG Interconnection StandardsDG Interconnection Standards

Current interconnection processes, worldwide, are lengthy and make it relatively expensive for a small power producer to apply for connection to the grid in both money and time delay costs.

This is a lack of uniform national interconnection standards and tests for interconnection, operation, and certification as well as the lack of uniform national building, electrical, and safety codes for DG interconnection.

SCC21, established by the IEEE Standards Association (IEEE-SA), develops IEEE 1547 series standard for interconnecting Distribution Resources with electric power systems.

IEEE 1547 offers power engineers and other stakeholders a broad-based tool to interconnect DG to the distribution system to meet our electricity needs.

Major Points of IEEE DG Major Points of IEEE DG Interconnection StandardInterconnection Standard

Fundamental HypothesisThe primary function of a protective relay and control scheme for a proposed DG unit operating in parallel with a network grid is to separate the power source from the network within 3 cycles (50 milliseconds) of the occurrence of any anomaly in the network voltage, frequency or power flow direction.

Reverse Power Flow CriteriaThe Interconnection System (IS) shall prevent the occurrence of reverse power flow through the network protector(s) under all normal conditions and absolutely minimize reverse power flow under adverse conditions such that network protector capability is never impaired. There shall be at least two independent subsystems employed in the IS to accomplish this requirement.

Fault Current Contribution CriteriaThe IS will respond to any indication of a fault on the network by disconnecting the DG unit from the network within 3 cycles (50 milliseconds) so that the fault current contribution from the DG unit is either zero or minimized to the point of being inconsequential.

Area Network Load Flow Analysis and ConclusionThe utility customer or the customer’s agent will work with the utility to analyze network load flow parameters at the proposed DG site to determine whether the proposed type of DG unit can be accommodated for operation. If the conclusion is negative, alternative measures such as reconfiguration for a radial interconnection shall be evaluated.

Commercial and Regulatory IssuesCommercial and Regulatory IssuesNet Metering

The use of a single meter to measure the total consumption of anelectricity consumer against their total onsite generator production over a fixed period of time.Net metering is required as DG relative to consumption cannot justify the additional administrative and metering cost of typical customer power sales arrangements. The main issue is what should be the policies or consensus towards payment to DG plant owners on account of back wheeling of power.

Interconnection fee and service charges for activities related to interconnection is another issue with DG. Application and interconnection fee were frequently viewed as arbitrary and found disproportionate for the smaller projects in the US.One of the global issues with DG interconnection is that the judicial or regulatory processes are often highly expensive for relatively small-scale distributed generation projects.

Commercial and Regulatory Issues (2)Commercial and Regulatory Issues (2)Other Economical Issues

High costs compared to alternatives and the solutions to technical issues may add to costs affecting economics of DGFinancing hurdleCustomer does not capture system (e.g., distribution reliability) and non-monetary (e.g., environmental) benefits (externalities) of DG tooffset own (internal) costs

Siting, Certification and Permitting issuesRegulated variables when siting a power plant of any size include air quality, fuel supply, noise and safety. These same issues affect each DG installation. Two major siting, certification and permitting concerns that have been raised are a) how to deal with the requirements of multiple local jurisdictions and different permitting agencies, and b) what might be the aggregate effects of locating multiple units in a common geographical area.

Other IssuesOther Issues

Lack of utility experience in dealing with the contractual and procedural interconnection issues is one of the most widespread and significant issues.

Absence of a mechanism for giving appropriate credit for the contributions made by DG proponents in meeting power demand, reducing transmission losses, or improving environmental quality.

ConclusionConclusion

Fig 7: DG interconnection issue (Source: The Alpha Group)

ConclusionConclusion

Distributed generation is a potentially important, new, cost-effective power supply alternative that can redefine the competitive energy market.

Furthermore, substantial technical and commercial issues must beresolved before DG can be successfully integrated with grid operations.

Policy makers therefore need to be more conscious about the cumulative impact of these issues posed by DG with other public policy objectives.

The strongest objective of the DG regulatory policy is to encourage competition and economic efficiency, followed by the need to ensure safety and grid reliability.