Rasool Aghatehrani

Opportunities and Challenges in Using Advanced Inverter Functionality

PV Grid Integration into Distribution Workshop May 2016

Rationale oAdvanced inverter functionalities (AIF) o Implementation of AIF in PV plants o Active power controls

• Active power limit • Frequency-Watt

o Reactive power controls • Power factor control • Volt-Var

o PV plant design considerations • Adjusting power factor to decrease voltage variations • Evaluating PV plant reactive power capability

o Conclusion

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Advance inverter functionalities

o Active power • Active power limit

• Soft start

• Frequency-watt

• Volt-watt

o Reactive power • Constant power factor

• Constant reactive power

• Volt-var

• Voltage regulation

o Protection/grid support • Over/under voltage ride

through

• Over/under frequency ride through

• Anti islanding

3

Advanced inverter functions allow for more elaborate monitoring and communication of the grid status, the ability to receive operation instructions from a centralized location, and the capability to make autonomous decisions to improve grid stability, support power quality, and provide ancillary services. *

* Reference: NREL Advanced inverter functions to support high levels of distributed solar, Nov. 2014.

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Implementation of advanced inverter functionalities Advanced functions can be implemented at the inverters or at the point of interconnection (POI):

o Point of interconnection: • Active power limit

• Power factor

• Volt-var

• Voltage regulation

• Constant reactive power

• Under/over frequency ride through (relay)

• Under/over voltage ride through (relay)

o Smart inverter: • Volt-var

• Frequency-watt

• Soft start

• Under/over frequency ride through (inverters)

• Under/over voltage ride through (inverters)

System operator

Active power controls

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oActive power limit: • Establishes an upper limit on the real

power output of the plant at the point of interconnection.

• Compensates the power loss associated with power transformers, cables and auxiliary instruments.

o Frequency-Watt: • Decreases the active power if the

frequency is above a certain threshold.

• Is not generally provided for under-frequency conditions.

Reactive power controls

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oPower factor control: • Adjusts the reactive power

generated/consumed by the inverters to set the power factor of the plant to a fixed value.

• Compensates the reactive power loss associated with power transformers, cables and auxiliary instruments.

o Volt-Var: • Injects/consumes reactive power if

the voltage is outside of a certain range.

PV power factor and voltage variations

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−+=

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=

=

)sin(...

)cos(...

1

1

knknknn

N

nkk

knknknn

N

nkk

YVVQ

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δδθ

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QP

SSSS

V VQVP

QP

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vsvs

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Reference: R. Aghatehrani and T Golnas, “Reactive power control of photovoltaic systems based on the voltage sensitivity analysis,” IEEE PES GM 2012.

IEEE 123-bus distribution system Bus 28 voltage UPF (red), Leading PF(blue)

Adjusting the plant power factor can effectively decrease the voltage fluctuations caused by the output variability.

Power factor control and plant power flow

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V : 100%

PF: 0.95cap V : 103%

PF: 0.93cap

o At rated output power, the inverters should generate reactive power to maintain the target power factor at the point of interconnection and to compensate the reactive power impact of power transformers.

o Based on the transformer BIL, IEEE/ANCI C57.12.10 recommends 5.5% to 6.5% as impedance values for transformers.

P

Reactive power and inverter capacity

o Extra inverter capacity can be considered for the reactive power support.

o Inverter may have different active power (kW) and apparent power (KVA) capacities.

o Example:

• Pmax=100kW

• Smax=100kW+37kVar=108kVA

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Inverter capacity limit

108kVA inverter

100kW inverter

37 kVar compensator

Reactive power and inverter DC voltage

10

450

700

250 400

Min

. DC

Vol

tage

(V)

Nominal AC Voltage (V)

Vdc EAC

Current-controlled voltage source inverter

Vac

Xs

Vac

Eac

Iac.jXs

Iac ϕ

Lagging power factor

PV inverters without a DC-DC stage need a minimum DC voltage to create the internal AC voltage (Eac). The minimum DC voltage could be higher than the PV module maximum power point voltage (Vmpp). Operating inverters in such a condition may result in significant real power loss.

For a PV plant with 0.95PF requirement (POI) connected to the distribution system.

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Reactive power and inverter DC voltage (cont.)

How to assess the inverter reactive power capability? o Power flow analysis:

• Simulate system losses (Sloss)

• Size the inverters:

Sinv=Sloss+Spoi

• Calculate AC voltages (Vac_inv) and power factor (PFinv) for inverters.

o Inverter specifications: • determine the minimum DC voltage (Vmmp_min)

for Vac_inv and PFinv at the rated capacity.

o PVsyst/NREL SAM simulations: • Estimate DC voltages (Vmmp) and active powers

(Pac) for each of the 8760 hours in one year.

• Compare simulated (Vmmp) with Vmmp_min.

680 Vdc

560 Vdc

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Conclusion

oAdvanced inverter functions can help address the grid stability problems posed by high levels of variable distributed generation.

oActive power limit, frequency-watt control, over/under frequency and over/under voltage ride through can support the system stability during contingencies.

oPower factor and volt-var controls can significantly decrease the voltage

fluctuations caused by output variability of PV plants and potentially increase the hosting capacity of the distribution system.

oPV plant active and reactive power capabilities should be diligently evaluated during the plant design phase.