opportunities and challenges in using advanced inverter functionality

1. Rasool Aghatehrani
Opportunities and Challenges in Using
Advanced Inverter Functionality
PV Grid Integration into Distribution Workshop
May 2016

2. Rationale
o Advanced 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
2

3. 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.

4. 4
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

5. Active power controls
5
o Active 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.

6. Reactive power controls
6
o Power 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.

7. PV power factor and voltage variations
7
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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.

8. Power factor control and plant power flow
8
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

9. 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
9
Inverter capacity limit
108kVA
inverter
100kW
inverter
37 kVar
compensator

10. Reactive power and inverter DC voltage
10
450
700
250 400
Min.DCVoltage(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.

11. 11
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

12. 12
Conclusion
o Advanced inverter functions can help address the grid stability problems posed
by high levels of variable distributed generation.
o Active power limit, frequency-watt control, over/under frequency and
over/under voltage ride through can support the system stability during
contingencies.
o Power 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.
o PV plant active and reactive power capabilities should be diligently evaluated
during the plant design phase.