Local Dynamic Reactive Power for Correction of System Voltage Problems RDSI Peer Review John Kueck, Tom Rizy, Fran Li Yan Xu, Huijuan Li, Sarina Adhikari, Philip Irminger Power & Energy Systems Group Energy and Transportation Science Division November, 2008

Voltage Problems in Power Systems •

This project addresses the specific problem of local (or micro) voltage collapse.

•

Conventional voltage regulation is inadequate in some cases.

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Problem providing/transmitting reactive power from generators – doesn’t travel well

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Areas experiencing voltage problems – lack of adequate reactive reserves and growing dynamic loads that pose problems

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Stalling of induction motors (compressors) in highefficiency air conditioners 2

OAK RIDGE NATIONAL LABORATORY U. S. DEPARTMENT OF ENERGY

Voltage Effect on Motor Load and Power System Response • This project is a 1st step in a research plan to control local voltage – a plan to address market barriers, technical feasibility, and to identify risks. • Team with PG&E and CAISO to develop a reactive tariff. • Effect of voltage sag on air conditioner compressor (A/C) motor torque and stalling • Possible effect on efficiency – higher voltage means less power usage • Existing Tools and Models to Study the Problem and Determine Solution are inadequate thus customization of tools • Maintaining System Reliability by providing Local Dynamic Reactive Power is key OAK RIDGE NATIONAL LABORATORY U. S. DEPARTMENT OF ENERGY

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Dynamic behavior of A/C compressor load with temperature • Dynamic motor behavior with temperature and load. • Growing air conditioning load leading to 20 to 70% of total system load • Need for better dynamic load models

Increase in running power of motor with increasing temperature for single-phase (residential) compressor motor operating at steady-state. OAK RIDGE NATIONAL LABORATORY U. S. DEPARTMENT OF ENERGY

• Reduction in Margin to Voltage Collapse with local dynamic regulation 4

Energy efficient A/C compressors motors present new power system issues • Unique features of air conditioning motors • Conventional Voltage Ranges Required by Standards • Regulating Local Voltage To Improve Efficiency • Low-inertia of motors makes them more susceptible to stall OAK RIDGE NATIONAL LABORATORY U. S. DEPARTMENT OF ENERGY

Voltage zones (normal, stall and drop-out) for air conditioner (A/C) operation. Under fault conditions, we can drop into the 50 to 70% voltage range.

Rapid stall time of air conditioner motor when voltage sags due to a fault

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OAK RIDGE NATIONAL LABORATORY U. S. DEPARTMENT OF ENERGY

Often suggested voltage solutions and their drawbacks • More Definitive Voltage Standard – Not Coming Soon • Under-voltage tripping of Air Conditioners – Expensive and Maybe not be Fast Enough since A/C motors can stall in 3 cycles • Local Voltage Regulation – Requires a Lot of DER, or short-time overload capacity • Fast Tripping of Air Conditioner Motors – still will result in delayed voltage recovery (just shorter) 7

OAK RIDGE NATIONAL LABORATORY U. S. DEPARTMENT OF ENERGY

Local Voltage Regulation from Distributed Energy (DE) Resources

Types of DE • Micro-turbine • Industrial gas turbine • Fuel cell • Photovoltaic • Wind-turbines

Characteristics of DE • Power-electronics-based interface is required for most DE • Generator or inverter-based DE can be sized and controlled to provide both real and reactive power dynamically

• DE location near load 8 • Recip Generators centers offers best solution OAK RIDGE NATIONAL LABORATORY for local voltage regulation U. S. DEPARTMENT OF ENERGY

DECC Lab – Unique in that DE at the Lab are interfaced to an Actual Distribution System

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OAK RIDGE NATIONAL LABORATORY U. S. DEPARTMENT OF ENERGY

DE with inverter interface and controller interconnected to Distribution System

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OAK RIDGE NATIONAL LABORATORY U. S. DEPARTMENT OF ENERGY

Feedback Control vs. Adaptive Feedback Control for DE real-time controller

Controlled system: local power system Controller: perform the feedback control Control variable: PCC voltage Reference: desired value of the PCC voltage Error: the difference between the reference and measured PCC voltage Objective: reduce error by varying reactive power output 11 OAK RIDGE NATIONAL LABORATORY U. S. DEPARTMENT OF ENERGY

Adaptive Control Conventional Feedback control limitations: •

Proportional (P) and Integral (I) controller

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Gains (Kp, Ki) are fixed and are chosen by trial and error

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Incorrect gains result in under-performance, oscillation or instability

ORNL’s Method - Adaptive feedback control, offers: •

PI controller but with automatic gains adjustment

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Gains (Kp, Ki) are set initially conservatively, but then are adjusted in real-time to achieve desired system response time

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Faster response achieved and system stability is ensured

OAK RIDGE NATIONAL LABORATORY U. S. DEPARTMENT OF ENERGY

Test of Voltage Regulation with Conventional Feedback Control (Fixed Gains)

Load current (rms) during load change

PCC voltage (rms) without regulation

Response time is 2.42s. PCC voltage (rms) with non-adaptive voltage regulation

Inverter current (Arms) during load change

Test of Voltage Regulation with Adaptive Feedback Control How adaptive gain reduces error faster.

Load current (rms) during load change

PCC voltage (Vrms) with adaptive voltage regulation

Inverter delta voltage (ΔV) compared to ideal exponential response curve.

Response time is now 0.44s; transparent to system level regulation.

ramped up current response

Inverter current (Arms) during load change.

Adaptive Control Validity – Actual Test Results Response Time to Restore Voltage after Load Change Inverter

Synchronous condenser

Setting (s)

Actual (s)

Setting (s)

Actual (s)

0.1

0.176

0.5

0.663

0.2

0.332

1.0

1.006

0.5

0.64

2.0

2.417

1.0

0.927

5.0

5.214

2.0

2.408

10.0

7.151

5.0

2.82

15.0

9.158

10.0

4.212

25.0

12.007 15

OAK RIDGE NATIONAL LABORATORY U. S. DEPARTMENT OF ENERGY

Air-Conditioner Stall Modeling System configuration with 10 MVA loads. Transmission system

Primary feeder

500kv

13.8kv/208v

115kv/13.8kv

500kv/115kv

DER at

Line R fault

lateral 3

Voltage profile of Bus1 with DER at bus3 1.4

1.2

1.2

1

1 Voltage in pu

Voltage in pu

Voltage profile of Bus 1 (Without DER) 1.4

0.8

Voltage Profile without DER

0.6

0.4

Voltage Profile with DER

0.6

0.4

0.2

0

0.8

0.2

0

0.1

0.2

0.3

0.4 0.5 0.6 Time in secs

0.7

0.8

0.9

OAK RIDGE NATIONAL LABORATORY U. S. DEPARTMENT OF ENERGY

1

0

0

0.1

0.2

0.3

0.4 0.5 0.6 16 Time in secs

0.7

0.8

0.9

1

Partnerships and Next Steps • SCE has performed voltage tests on air conditioners and provided test results and models. • PG&E has teamed with us to prepare a study and paper on a tariff for reactive power. • We have established a team with SCE and One Cycle Control to install a high penetration of DER and implement our controls on Catalina Island. • Testing of high-efficiency air conditioning at DECC with the connection of adaptively controlled DER, all grid connected. • System modeling to evaluate needed dynamic reactive capacity levels, overload capability and system reliability. OAK RIDGE NATIONAL LABORATORY U. S. DEPARTMENT OF ENERGY