Lessons Learned from DG Integration

Robert F. Arritt, PE

1

Introduction

• Insight to Distributive Generation (DG) interconnection issues

– Perform Interconnection studies

– Multiple Power Quality (PQ), Distributed Resources (DR), and Distribution hotline calls (EPRI Program 1, 174, and 180)

– Industry survey EPRI 3002001277 – “EPRI Survey on Distribution Protection with Emphasis on Distributed Generation Integration Practices”

2

Overvoltage Concerns • 75% of participants selected overvoltages and islanding as

biggest concern

• Over 25% of participants have experienced overvoltage issues and 28% were not sure.

69/34.5

+

34.5/.66

+

V_Source

69kV/_0

Substation DG

DG_Gnd

+

DG

660V

5 MW

Sub_Gnd

+

DTT

Single-Line

To Ground

Fault

Va

Vc Vb

1.73 pu

1.73 pu

Events resulted in sustained overvoltages on the primary side of

the substation transformer

Interconnection Transformer

• Survey Results

4

Overvoltage Concerns • Majority of participants selected system primary

overvoltages as the biggest concern for transformer selection

• Over 40% recommend grounded wye-wye for interface transformer.

Yg/Yg with simple dq0 inverter model

Utility breaker opens in response to SLG Fault

Yg/Δ with simple dq0 inverter model

High Voltage on a Secondary

DG Customer A Customer B

126 v 127 v

127.75 v

128.25 v Distribution Transformer

Primary

Note: voltages shown based on 120 V base voltage

Protection Supervision Concerns • Multiple events of inverters not turning on or cycling off due to

overvoltages associated with high penetration.

• Reported problems of DG operators changing initial protection settings resulting in high voltages at PCC

– Resulted in complaints from other customers of electronic equipment problems/failures

Substation End of Feeder

ANSI Range A Lower Limit

Distance

Voltage

ANSI Range A Upper Limit

After DG

Before DG

Additional Issues Identified • Open-Phase protection

– Open-phase concerns identified in commissioning tests

• Reclosing practices – Some have different reclosing practices for inverter based

system compared to rotating DG systems

• Limitations placed on substation switching • Transformer Inrush, Ferroresonance, etc.

Ferroresonance Problem

• The solar array was not able to come on-line due to the PV system’s protection detecting an overvoltage on Phase A.

9

300kVA

Wye Grounded

5-Legged Core

300ft 15 kV

#4/0 AWG

Phase A = Open

Phase B = Closed

Phase C = Closed

Van

125%

-125%

Ferroresonance Problem

• Concluding this investigation it was discovered that a single-phase fuse was blown but the door failed to fall open.

• The door failing to open did not make it obvious that the A phase fuse was blown.

• The fuse was replaced and the voltages where then checked and verified to be in specification.

10

Harmonic Impact • Used harmonic spectrum of 66% loaded inverter.

• Examined four scenarios of capacitor switching.

0.1

1

10

100

1000

1.1 5.3 9.4 13.6 17.8 21.9 26.1 30.3 34.4

|Z|

oh

ms

Harmonic Number

Driving Input Impedance from DG HSB

No Capacitors

6000kvar

12000kvar

18000kvar

Under certain conditions i.e. loading, capacitor state, and PV output. Voltage distortion on the feeder (MV) can exceed 3% at the 5th and THD can exceed 5%.

Analysis assumes no background distortion.

Inverter Harmonic

Spectrum

Conclusions

• Identify new requirements and challenges associated with protecting the feeder with DG.

• Researching utility practices & technologies and identifying gaps.

• Goal is to assist protection engineers can learn about leading practices for protection schemes and philosophies.

• Industry survey EPRI 3002001277 – “EPRI Survey on Distribution Protection with Emphasis on Distributed Generation Integration Practices”