Power Quality Impacts of Distributed Generation Roger C. Dugan Sr. Consultant Electrotek Concepts, Inc. Tennessee Tech. February 9, 2004

Electrotek Concepts, Inc2 Introduction Renewed interest in Grid-connected DG –Utility Perspective T & D Capacity Relief Hedge against high market prices –End User Perspective CHP and Improved Reliability Incentives from Utility –Commercial Power Producer Sell kWh or “ancillary services”

Electrotek Concepts, Inc3 Emphasis of This Presentation Units connected to distribution (MV) –Typically < 10 MW –Commonly 1-2 MW –Frequently owned by end user Impact of these units on Power Quality –Operating conflicts –We will look at a few of the many …

OverCurrent Protection Conflicts

Electrotek Concepts, Inc5 Typical Overcurrent Protection

Electrotek Concepts, Inc6 Only One Device Has to Operate Operating Device Fault Current

Electrotek Concepts, Inc7 Transmission Fault Protection Multiple Sources Two Devices Must Operate to Clear Fault

Electrotek Concepts, Inc8 LV Network Systems

Electrotek Concepts, Inc9 Utility Fault Clearing Due to cost, distribution systems are designed expecting only one source of fault current –Only one device has to operate DG must disconnect to allow utility fault clearing process to continue This is the source of most operating conflicts

Electrotek Concepts, Inc10 Reduction of Reach Utility relays and DG relays may not know anything is wrong.

Electrotek Concepts, Inc11 Reduction of Reach More of a problem at peak load –When peaking units are likely to be connected !! More of a problem for high resistance faults Longer fault detection times

Instantaneous Reclosing

Electrotek Concepts, Inc13 Reclosing Most faults are temporary Reclosing is prevalent in North America Reclosing Interval: 0.2 – 5 sec –“Instantaneous” < 30 cycles

Electrotek Concepts, Inc14 Reclosing, cont’d If DG does not get off, two bad things could happen: –DG is damaged by the reclose Shafts Winding damage –Fault is not cleared Reduced reliability Damage to utility system

Electrotek Concepts, Inc15 Reclosing DG Must Disconnect Here

Electrotek Concepts, Inc16 Instantaneous Reclosing Utilities use this for PQ concerns Increases the probability that DG will not disconnect in time –Makes it almost a certainty that some won’t If 1-2 sec, most operations will be easily detected by DG protection Some utilities use 5 sec reclose on feeders with DG

Unintended Consequences...

Electrotek Concepts, Inc18 Voltage Drop After Fault

Electrotek Concepts, Inc19 Low Voltage After Interruption A problem when there is a fault at peak loading –When peaking DG is likely to be on !! This is often the most limiting condition for how much DG can be accommodated without change –Particularly on rural feeders –500 – 1000 kW

Ferroresonance Sometimes Conflicting Interconnect Requirements Collide …

Electrotek Concepts, Inc21 Ferroresonance G 1) Fuse blows or switch opens 2) DG breaker detects negative sequence and trips leaving transformer isolated on cable capacitance with no load/generation 3) Ferroresonance develops DG Required to have separate Transformer and disconnect at first sign of trouble

Electrotek Concepts, Inc22 Ferroresonance

Electrotek Concepts, Inc23 Ferroresonance Waveform

Reduced Insulation

Electrotek Concepts, Inc25 Arrester Duty for SLG

Varying Generation

Electrotek Concepts, Inc27 Varying Generation Example 336 MCM ACSR (Untransposed) ft ft 1000 kW 500 kW Regulator Wind Gen 8 MW 600 kvar 115 kV12.47 kV Unbalanced impedances Rapidly-varying “load”

Electrotek Concepts, Inc28 Varying Generation Regulator Operations

Transformer Connection and Ground Faults

Electrotek Concepts, Inc30 Grd Wye-Delta Connection Some claim this is the best way to interconnect DG –Same way as nearly all central station generation is connected –However, most utilities don’t allow this connection on their system.

Electrotek Concepts, Inc31 Wye-Delta Connection

Electrotek Concepts, Inc32 Problems with Wye-Delta Increases ground fault current Interferes with coordination Faulted Circuit Indicators get fooled Transformer subject to failure –Special design and/or neutral reactor –Utilities often discourage “Specials”

Electrotek Concepts, Inc33 On Wye-Wye Most common U.S. connection Generally well behaved Does not block Triplen Harmonics –Leads to “Harmonic Surprise” when many machines paralleled with utility system Also occurs with Delta-Wye –2/3 pitch machines lessens the impact

Electrotek Concepts, Inc34 Conclusions Interconnected DG likely to increase DG must adapt to Distribution protection system Too much infrastructure already in place to overhaul the system to better accommodate DG Who would pay for it? Non-participating ratepayers?

Electrotek Concepts, Inc35 Conclusions DG impact on Power Quality –Often good for individual end users Largest gainers: Those with high-value product that suffer sustained interruptions But, should not expect power quality improvement in all areas –Maybe slightly negative for utility Contributes to operating conflicts Can we see it in the Average indices?

Electrotek Concepts, Inc36 Conclusions, cont’d Revisit Overcurrent Protection –Utilities may have to change some practices –Instantaneous reclosing ? –Use of line fuses Tradeoffs for Transformer Connections –WYE-Delta good for DG but not for ground fault coordination Special engineering req’d for some DG application.

Electrotek Concepts, Inc37 Despite These Conflicts … DG can almost always be successfully engineered –Larger conflict may be over who pays –Some long-standing utility practices may have to be changed

Electrotek Concepts, Inc38 How Much DG ? What is the “No Changes” “Penetration Limit” ? –Typical: 15% of feeder capacity –5% - 10% if Rural