Integration Case Studies

Michael CoddingtonNREL

2

Details of the PV System

Located at Colorado State University 5.2 MWDC PV System 6.6 Miles from Substation Installed in 2 Phases (2 MW + 3.2 MW) Capacity Penetration of Approximately 57%• (Instantaneous Penetration can be Much Higher)

2

3

Details of the Utility Distribution Circuit

3

• Interconnection with PSCo

• Distribution Voltage 13.2 kV

• Peak Load on Feeder 9.1 MW

4

Xcel Energy Priority: Maintain Voltage Levels

• Large Conductors serving CSU campus

• Two sets of voltage regulators on circuit and LTC at substation

• Voltage at CSU campus maintained to approximately ± 1Volt (@ 120V)

• The following steps are to be followed, sequentially, if there are voltage problems;

4

5

Possible Steps to Maintain Voltage Limits

1. Adjust the voltage regulators to stabilize the voltage levels, and if necessary;

2. The inverters (10 total) will be configured to absorb 100 kVAR each, and if necessary;

3. The inverter configurations will be changed so they will absorb 150 kVAR each, and if necessary;

4. Request that PV system operator to disconnect part or all of the PV system and install a power factor controller or dynamic VAR compensator.

5

6

Load Profile of CSU West Campus

6

Exported Power Below Line

Phase 1 – 2 MW PV Phase 2 – +3.2 MW PV

7

Profile of Feeder Load and PV Generation

7

-

1,000

2,000

3,000

4,000

5,000

6,000

7,000

8,000

9,000

2/9/2011 2/9/2011 2/10/2011 2/10/2011 2/11/2011

kVA

Substation Demand PV System Output Total Power Delivered to Load

8

Most Common PV Penetration Definition

• Capacity Penetration* ≈ Total PV System MW

Peak Load MW

• Energy Penetration ≈ Total DG kWh

Total kWh

• Penetration Screen Fails at 100% MDL or 15% peak load (depending on rules)

Note: Most people define “ PV Penetration” as how many MW of PV are on a circuit, regardless of the load on that circuit

* This is how “Penetration” is calculated in Technical Screens

8

9

10 MW PV System Case StudyCarlsbad, NM – Xcel Energy

Located near Carlsbad NM

10 MWDC PV System 3/4 Mile from Substation

Capacity Penetration of Approximately 100% based on peak annual load

Daytime load on substation bus approximately 3.87 MVA

(Capacity Penetration ~300%)

9

10

Details of the Utility Distribution Circuit

10

• Interconnection with Xcel Energy

• Distribution voltage 12.47 kV

• Substation transformer bank size 16.8 MVA

• Minimum Daytime Load on substation 3.87

MVA, and less on this feeder

• Circuit conductor ¾ mile 336 ACSR from PV

system to substation

• Other customers served from this feeder

• Impact study focused on Protection,

Voltage, Power Flow using SynerGEE

11

Mitigation Measures Required

1. Less than 2 miles (3/4 mile) from substation and using 336 ACSR feeder conductors installed (low impedance cables)

2. Additional line protection added to address “desensitized relaying”

3. Voltage supervised reclosing was added to the substation breakers to address potential problems with PV system size

4. PV developer was required to regulate their output power factor to a fixed value to address possible voltage issues.

5. Additionally the PV owner was required to energize the inverters incrementally in order to avoid large voltage steps.

11

12

Design Observations

Detailed impact studies were required

This PV system exports power back to transmission system

Large conductors, short distance to substation, modifications to substation equipment

Most of the design specifics fall under the utilities guidelines for large (<10MW) PV

13

Secondary Network PV

13

And a quick review of types of distribution systems

• Radial distribution systems (99% of systems)

• Secondary Network Distribution Systems

– Spot Networks

– Area Networks (aka Grid Networks or Street Networks)

14

Radial Distribution Feeder Design

15

Secondary Network PV Interconnection

Spot Network Area Network

16

• Facility Name: Moscone Center

• Owner: City of San Francisco

• Secondary Network Type: Spot (4 kV)

• PV System Size: 676 kWp DC

• Type of Interconnection –

– Maintain Minimum Load

– Minimum Import Relay

– >50% NP Closed Monitor (custom)

• Utility Company: PG&E

San Francisco, Secondary Network PV

17

Moscone Center

18

• Facility Name: Colorado Convention Center

• Owner: City and County of Denver

• Secondary Network Type: Spot

• PV System Size: 300 kWp DC

• Type of Interconnection –– Minimum Facility Load greater than PV System

– Reverse Power Relay

– Dynamically Controlled Inverters

• Utility Company: Xcel Energy (PSCo)

Denver, Colorado Secondary Network PV

19

Colorado Convention Center

20

Dynamically-Controlled Inverter Results

21

Network Grid Interconnection - Strategies

• Maintain minimum load (MML)

• Reverse power relays (RPR)

• Minimum import relays (MIR)

• Dynamically controlled inverters (DCI)

(update: Advanced Inverters coupled with utility grade controls and relays can meet these relay functions, DCI, and more!)

22

NREL/Advanced Energy Concept

TCP/IP to MODBUS

23

Results & Next Steps

Results

• Initial testing had outstanding results

• Use of commercial off-the-shelf parts realized

• SEL (and other) relays are inexpensive, reliable, and well liked by utilities!

Next Steps

• Further testing and programming of relays in MW level laboratory and using PHIL

• Build a “Laboratory Secondary Network” and test speed, reliability, safety, and fault currents

• Develop SEL Application Guide(s) for network engineers, PV system designers

24

Case Study – Residential 20 kW

24

25

Traditional Distribution Voltage Drop

26

Residential Case Study – 20 kW PV System

26

Voltage Rises with minimal local load

The ANSI C84.1 (Range A) upper limit is violated at the PV system, and worst violation at the micro-inverters farthest from the service disconnect

This facility experienced 127.5 Volts at the utility meter and 131 Volts at the PV array (ANSI Range A upper limit 126 Volts)

If there was more load than generation, there would not be a high voltage problem in this case

Thus, there are very good reasons to have load near PV generation sources (despite negative comments about capacity penetration rationale)

2727

Voltage Rises Toward PV Generation

28

Observations for this 20kW PV PV System

Steady-state voltage can be out of acceptable ANSI C84.1 range A (or B) with even with a small residential PV system

This PV system, if only one of a few small PV systems on this feeder, means capacity penetration was likely <1%, yet there are localized voltage problems

No other utility customers were negatively impacted in this example

Should all PV systems <25kW be given a quick approval? Why, or Why not? What would you look for if you were the reviewer?