FREQUENCY SUPPORT AMONG ASYNCHRONOUS AC GRIDS THROUGH MULTITERMINAL DC GRID

Dr. Nilanjan Ray ChaudhuriAssistant Professor

School of Electrical Engineering and Computer Science

Pennsylvania State University, University Park, PA

SUstainable Power & Energy Research Group(SUPER Group)

Thrust I: HVDC and MTDC

Thrust II: PMU Data Anomaly Detection & Correction

Thrust III: Wide-Area Damping Control

Thrust IV: Coupled Cascading Failure

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Amirthagunaraj YogarathinamPh.D. Student

Jagdeep KaurPh.D. Student

Kaveri MahapatraPh.D. Student

Kaustav ChatterjeePh.D. Student

Sai Gopal VennelagantiPh.D. Student

SUPER Group Team

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Asynchronous AC Grids

(a) (b) (c)

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Tres-Amigas Project

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Plan to capture rich offshore wind resources of North Sea, solar resources in sub-Saharan Africa

Plan to interconnect the major generation and load centers of UK, Scandinavia, and continental Europe forming a 'Super Grid'

Motivation for MTDC

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System Configuration

Schematic of the bipolar MTDC grid with metallic return (single-line diagram) connecting 4 asynchronous AC areas

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Controller Structure [J2]

Distress signal carries basic information as to which area is seeking help

With this new inertial and frequency droop control, the power of converter in itharea is given by,

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Nth-order Model

where, �𝑘𝑘𝑣𝑣𝑣𝑣 represents the normalized voltage droop coefficient (i.e., they add upto one)

The unique structure of matrices 𝐻𝐻𝑁𝑁 and 𝐾𝐾𝑁𝑁, enables application of certain mathematical tools

With certain assumptions, the Nth-order model for an equivalent monopolar model of the system can be derived as,

where,

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Performance Constraints

Satisfying two constraints individually, ensures ratios are met at 𝑡𝑡 = 0 or ∞ Satisfying two constraints together, ensures ratios are met at all time, post-disturbance

In general, there are multiple solutions to each of these performance constraints

The requirement is that the frequency of participating areas be in a certain prescribed ratio,

Initial-slopes constraint: Steady-state constraint:

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Summary and Parameter Design [J2]

Voltage-droop Control Constant Power Control(i) Nominal Mode

(ii) Participating Area

(iii) Non-participating Area

Power and Voltage Referencesby System Operators

(i) or (ii) or (iii) based on distress signal

Inner Current Control Loop

Current References

Power and Voltage References

Inertial, Frequency and Voltage Droop Control

Modulation Index

Inverter Switching Control

Market Mechanism or Grid Code

Ratios andParticipation of Areas

for disturbance in Area#i

Solve for Droop Coefficients

Small Signal Stability Analysison Full-order Model

choose aset of droop coefficients

Unstable Stable

Repeat till Stable

End

Based on

System Operators Provide Ratios

Satisfying Performace Constraints

Controller summary: Design of droop coefficients:

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Response in Nth-order Model (Ideal)

A 20% reduction in generation of G7 (Area#4) : (a) frequency dynamics and (b) ratios among frequency deviations

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Response in Full-order Model (Non-ideal)

A 20% reduction in generation of G7 (Area#4) : (a) frequency dynamics and (b) ratios among frequency deviations of G7, G5 and G1 and (c) positive-pole DC voltage

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System Configuration with OWF

Schematic of the bipolar MTDC grid with metallic return (single-line diagram) connecting 3 asynchronous AC areas and offshore wind farm (OWF) [J2]

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De-loaded Control of OWF [J2]

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Simulation Results

A 10% increase in load at bus #19 simulated in full-order model with OWF: (a) frequency response along with emulated frequency and (b) positive-pole DC voltage [J2]

A 10% increase in load at bus #19 simulated in full-order model with OWF: (a) WF power output and (b) rotor speed [J2]

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Conclusion

A reduced-order model, i.e., Nth-order model of N-asynchronous-area MTDC system was derived

A ratio-based selective power routing scheme based on minimal communication was proposed for the provision of primary frequency support through MTDC grids was proposed

The method is extended to MTDC with OWF, wherein we can quantify and controlfrequency support provided by OWF

For converter outage, power references were modified to minimize the AC-side frequencies while respecting appropriate constraints

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References• J1. S. G. Vennelaganti, and N. R. Chaudhuri, “Selective Power Routing in MTDC

Grids for Inertial and Primary Frequency Support,” IEEE Transactions on Power

Systems, vol. 33, no. 6, pp. 7020-7030, Nov. 2018.

• J2. S. G. Vennelaganti, and N. R. Chaudhuri, “Ratio-based Selective Inertial and

Primary Frequency Support through MTDC Grids with Offshore Wind Farms,” IEEE

Transactions on Power Systems, vol. 33, no. 6, pp. 7277-7287, Nov. 2018

• C1. S. G. Vennelaganti, and N. R. Chaudhuri, “Controlled Primary Frequency Support

for Asynchronous AC Areas through an MTDC Grid,” in proceedings of IEEE PES

General Meeting, Portland, OR,2018.

• C2. S. G. Vennelaganti, and N. R. Chaudhuri, “Inertial Support from Offshore Wind

Farms Interfaced through MTDC Grids,” in proceedings of ICRERA Conference, Paris,

France, pp. 1-5, 2018.

SUstainable Power & Energy Research Group(SUPER Group)

Funding from NSF grant award ECCS 1656983 is gratefully acknowledged.

Thank You!