Slide 1

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Lessons Learned from DG Integration Robert F. Arritt, PE

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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”

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. DTT Single-Line To Ground Fault 69/34.5

Substation

DG 34.5/.66

+

69kV/_0

Sub_Gnd

V_Source

Va

+

+

+

DG_Gnd DG

1.73 pu

Vc

1.73 pu

Vb

Events resulted in sustained overvoltages on the primary side of the substation transformer

660V 5 MW

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Interconnection Transformer • Survey Results

Overvoltage Concerns • Majority of participants selected system primary overvoltages as the biggest concern for transformer selection Yg/Yg with simple dq0 Yg/Δ with simple dq0 inverter model inverterfor model • Over 40% recommend grounded wye-wye interface transformer.

Utility breaker opens in response to SLG Fault

High Voltage on a Secondary Primary

127 v

126 v Distribution Transformer

Customer A

127.75 v 128.25 v

DG

Customer B

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. After DG

Voltage

ANSI Range A Upper Limit

ANSI Range A Lower Limit

Substation

Before DG

Distance

End of Feeder

• 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

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.

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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. 125%

Van

-125%

300kVA Wye Grounded 5-Legged Core Phase A = Open Phase B = Closed Phase C = Closed

300ft 15 kV #4/0 AWG

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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.

Harmonic Impact • Used harmonic spectrum of 66% loaded inverter. • Examined four scenarios of capacitor switching. Driving Input Impedance from DG HSB Analysis assumes no background distortion.

1000

|Z| ohms

100

No Capacitors

10

6000kvar 12000kvar 18000kvar

1

Inverter Harmonic Spectrum

0.1 1.1

5.3

9.4

13.6

17.8

21.9

26.1

30.3

34.4

Harmonic Number

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%.

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”