Capabilities of Advanced Inverters

John Berdner

Director of Worldwide Standards

Overview of Presentation

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Nomenclature for Advanced Inverters and Functions

Ride through Functions

Voltage Ride Through, Frequency Ride Through

Real Power Control Functions

Frequency/Watt, Volt/Watt

Reactive Power Control Functions

Volt/VAr

Communications

Commanded Operation, Autonomous Profiles

Communication issues

Nomenclature

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Need to be careful about

using anthropomorphic

terms…

- What comes after

“advanced” inverters?

- What comes after “smart”

Inverters ?

Suggestion:

Inverters with real and

reactive power control

functions

EPRI “Common Functions for Smart Inverters, Version 3”

An excellent overview of functions / capabilities / issues

Ride Through Functions

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Historically inverters were programmed to get offline

quickly in response to grid voltage or frequency

excursions

- Existing regulatory structures in the US did not

adequately address ride through requirements.

- Requirements were written as “must disconnect” not

as “must remain connected”

- As levels of Distributed Energy Resources (DER)

increase this strategy becomes problematic.

- Loss of large amounts of DER in advance of load

shedding can lead to cascade failure.

L/H Voltage Ride Through

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EPRI Common Functions For Smart Inverters V3, page 14-2

Voltage Ride Through Issues

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Inverters can provide controlled full current down to near zero voltage for extended periods of time but… there are some issues.

- Very low voltages are normally seen in response to feeder faults and additional fault current may be undesirable.

- At very low voltages, frequency measurement signal to noise ratio deteriorates

- Very low voltage operation of inverters may increase power supply costs and complexity

Focus of requirements to date has been on what happens at beginning of event. Of equal import is what happens when / how the DER returns to service.

L/H Voltage Ride Through

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0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

1.10

1.20

1.30

0.01 0.1 1 10 100 1000

Voltage (

per

Unit)

time (seconds)

Operating Regions - Must Remain Connected

OV_MSC UV_MSC

Normal Operation Region High (NORH)

Undervoltage Region 2MSC - UV2MSC

Undervoltage Region 1 - UV1MSC

Overvoltage Region 1 - (OV1MSC)

Frequency Ride Through

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EPRI Common Functions For Smart Inverters V3, page 15-2

Frequency Ride Through Issues

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Inverters can provide controlled full current down to

very low frequencies for extended periods.

- Frequency excursion is normally in response to

system level events.

- Low frequency behavior needs to be coordinated

with local load shedding schemes.

- Inverters are less sensitive to damage from low

frequency operation than rotating machines.

During high frequency excursions ride through and

Frequency / Watt functions are normally used together.

- Real power may be programmed to be at zero before

reaching the ride through limits.

Real Power Control Functions

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Real power control functions modulate real power

output in response to changes in Voltage or frequency

Typical Real Power Control functions include:

- Volt / Watt

- Reduces real power output with rising voltage

- May be possible to increase power with lower voltages

if storage is present in the system

- Can be used with ramp rate control on initial startup.

- Frequency / Watt

- Reduces real power output with rising frequency

- May be possible to increase power with lower voltages

if storage is present in the system

- Can be used with HF ride through for more stability

Example: Real Power Control Function

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Example Volt Watt Diagram Parameters

- Dead band (Vnom – V2) = no power reduction

- Response Gradient %P/Volt = (P3-P2)/(V3/V2)

- Other considerations – Response time, averaging

intervals, intentional delay

Reactive Power Control Functions

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Reactive power control functions modulate reactive

power output in response to changes in Voltage, or real

power output.

Typical Reactive Power Control functions include:

- Volt / VAr with Watt priority (available VAr or VV11).

- Provides VAr support with no reduction in real power

- VAr capacity varies inversely with solar resource

- Volt / VAr with Var priority (available VAr or VV12).

- Provides VAr support and may reduce real power

- VAr capacity is independent of solar resource

- May require specific Tarriffs or Rates

- Watt /Var (not common in the North America)

Example: Reactive Power Control Function

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Example Volt VAr Diagram Parameters

- Dead band (V3 – V2) = no power reduction

- Response Gradient %Qa/Volt = (Q4-Q3)/(V4/V4)

- Other considerations – Response time, averaging

intervals, intentional delay

Communication – Commanded Operation

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Inverters respond in “near real time” to commands from

the utility.

Example commanded modes:

On/Off (P&Q = 0)

Fixed Real Power (may require storage)

Maximum Real Power

Fixed Power Factor

Mode Change – Profile A or Profile B

Communication – Autonomous Profiles

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High speed SCADA communications is not cost

effective for high numbers of smaller systems.

Inverters can be pre-programmed with one or more

autonomous operating profiles.

- Inverters would then continue to operate according

to this profile until commanded to do otherwise.

- Provides high speed response without need for

high speed communications

Updates to profiles or changes in operating modes can

occur over public IP networks

- Customer Network or Cellular Modem (M2M)

Mode changes might also be possible over AMI system.

Communication – Issues 1

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Cyber Security of communication to DER systems

No global standard communication protocols

- DNP 3 is one common protocol used by utilities.

- Not well suited to control of large numbers of inverters

- Not used by all utilities

- IEC 61850 Data model may be a common language

- Sunspec Mapping to IEC 61850 for inverters

- DNP 3 to IEC 61850 for Utilities

Response time will be dependent on communication

system used

Not all systems will receive / respond to commands

(Stochastic planning needed)

Communication – Issues 2

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Undefined demarcation points for communication

protocols

- Where are various protocols used ?

- Who is responsible for protocol translations ?

- DNP 3 to IEC 61850 based protocol

- IEC 61850 protocol to inverter specific protocol

Undefined commercial issues

- Who is responsible for processing commands ?

- Inverter manufacturers, system aggregators,

system owners

- How are ongoing “back end” costs covered ?

Questions

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Thank you for your attention

John Berdner

jberdner@enphaseenergy.com