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An Adaptive P-Q Management Technique For Grid�Voltage Regulation in PV Systems

Anubrata Das1, Efstratios Batzelis2, Sandeep Anand3, and Soumya Ranjan Sahoo1

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1- Indian Institute of Technology, Kanpur, India

2- Imperial College of London, England

3- Indian Institute of Technology, Bombay, India

21st National Power Systems Conference (NPSC 2020), December 17-19, 2020, IIT Gandhinagar

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Motivation

Reasons for poor voltage regulation?

  • Embedded Solar and EVs bring stochastic power flows to the network
  • Changes on minute-basis (e.g. clouds passing over PVs) 🡪 conventional voltage regulation devices (OLTCs, switched caps, volt regulators) cannot act so frequently
  • Need for inverter-based resources to support voltage locally

Why adjusting both reactive (Q) and active (P) power?

  • Voltage is coupled with P in resistive networks (R/X > 1)

Open questions:

    • What is the ideal amount of P and Q (P/Q ratio)?
    • What is the minimum P adjustment (power curtailment)?

21st National Power Systems Conference (NPSC 2020), December 17-19, 2020, IIT Gandhinagar

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How PV node voltage changes with P and Q?

  • The voltage drop (ΔV) across the line is:

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  • For ΔV to be minimum, the inverter should operate at nominal capacity Snom

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  • Therefore,

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  • Differentiating ΔV w.r.t Ppv and equating to zero

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Single node grid connected PV Inverter system

Condition for Maximum PV Node Voltage

21st National Power Systems Conference (NPSC 2020), December 17-19, 2020, IIT Gandhinagar

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Voltage-P/Q characteristic: Non-monotonic!

  • Only Q injection can improve voltage up to some point

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  • Does not guarantee the operation at maximum voltage point (Vmax).

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  • Conventional Active Power Curtailment allows reaching Vmax.

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  • But there is a risk to “overtake” Vmax point under heavy overloading condition…

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  • The optimal P/Q ratio changes with the loading conditions!

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  • Question: How to identify the optimal P/Q ratio in real time?

PV node voltage for different R/X ratio

21st National Power Systems Conference (NPSC 2020), December 17-19, 2020, IIT Gandhinagar

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Adaptive PQ Adjustment

Flowchart for adaptive PQ adjustment

Our approach: Adaptive adjustment of P and Q

  • Aim at Vref if feasible, otherwise locate Vmax
  • Make fixed-step changes in Q. If Q exceeds inverter capacity, curtail P
  • Search and locate the voltage-peak!
  • Identifies the “just-right” amount of P curtailment
  • Adaptive: tracks Vmax while it changes
  • Similar to a P&O based search algorithm

21st National Power Systems Conference (NPSC 2020), December 17-19, 2020, IIT Gandhinagar

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Overall Control Structure

Grid Connected PV Inverter

Detailed Control Diagram

21st National Power Systems Conference (NPSC 2020), December 17-19, 2020, IIT Gandhinagar

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Simulation Results

System Under Consideration

  • Loosely based on UKGDS 95 bus system

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  • Thea actual UKGDS system consists of two PV inverter systems

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  • To reduce the simulation time a part of UKGDS system is considered.

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  • Only one PV system is considered, and rest of the network is simplified to aggerated loads and lines.

21st National Power Systems Conference (NPSC 2020), December 17-19, 2020, IIT Gandhinagar

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Simulation Results

Voltage Profile for PV Operating at upf

Duration

Vref

(pu)

Final Value

(pu)

t1

0.96

0.946

t2

0.96

0.923

t3

0.96

0.932

t4

0.96

0.954

PV Node Voltage

21st National Power Systems Conference (NPSC 2020), December 17-19, 2020, IIT Gandhinagar

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Simulation Results

Duration

Vref

Vmax

Final Value

pu

pu

a

b

c

t1

0.96

0.971

0.96

0.96

0.96

t2

0.96

0.949

0.945

0.893

0.949

t3

0.96

0.958

0.951

0.91

0.958

t4

0.96

0.98

0.96

0.927

0.96

Performance of Different Voltage Regulation Methods

Conventional Q

  • Works fine, but yields sub-optimal voltage…

Conventional PQ

  • Can reach the maximum voltage, but if the peak is overtaken it goes to instability
  • May result in massive P curtailment, low voltage
  • Algorithm remains trapped for long durations

Proposed Adaptive PQ

  • Most favorable performance!
  • Curtails P only if needed and at the proper amount
  • Always stable and reliant

a- conventional Q injection technique

b- conventional active power curtailment

c- proposed technique

21st National Power Systems Conference (NPSC 2020), December 17-19, 2020, IIT Gandhinagar

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Conclusions

    • Investigation of the Voltage – P/Q ratio relation

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    • Adaptive PQ adjustment method to identify the optimal point

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    • Validation of the proposed technique through simulation in Simulink

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    • Better voltage regulation performance compared other existing techniques

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21st National Power Systems Conference (NPSC 2020), December 17-19, 2020, IIT Gandhinagar