Voltage Control with Distributed Generators to Enhance Voltage Stability

Presenter: Fangxing (Fran) Li, Ph.D.Assistant Professor, University of Tennessee

Advanced Electricity Infrastructure Workshop

Global Climate & Energy Project

STANFORD UNIVERSITY, Nov. 1~2, 2007

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Assistant Professor, University of Tennessee

Other contributors: Tom Rizy, John Kueck, and Yan X u (ORNL)

Outline• Introduction to voltage collapse

– What is it and what causes the problem– Prevention of voltage collapse using reactive power

compensation

• Current research, development and demonstration (RD&D) effort at ORNL/UT

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demonstration (RD&D) effort at ORNL/UT – Using DG to provide dynamic reactive power– System and control models– Testing & simulation results– Future Plans

• Summary & Challenges

Voltage Collapse

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Voltage Collapse: what and why?

• Most of recent blackouts are related to voltage collapse.• A severe voltage depression without the system’s ability

to recover.• The key cause is inadequate reactive power (VAr or Q)

supplies.• The following factors are usually involved:

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• The following factors are usually involved:– High impedance– High load current– Insufficient generation of Q– Line outage– Line congestion

Voltage Collapse: Technical (1)Sending

EndReceiving

EndE V XIQ

RIP

loss

loss

2

2

=∆

=∆

• R

Real power and reactive power

0. 5

1

1. 5

2

PS

6

- 1

- 0. 5

0

0. 5

Q

Transmitting Reactive Power

Reactive power cannot be effectively

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transmitted across long distances or through power transformers due to high

I2X losses.

It is an uphill battle!

Source: Max Chau, Chairman, EEI Transmission Planning & Operations Working Group, 2004

Reactive power should be addressed locally.

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Source: Max Chau, Chairman, EEI Transmission Planning & Operations Working Group, 2004

RVθ∠LNZ

I φ∠LDZRR jQP +

E

Voltage Collapse: Technical (2)

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Voltage Collapse: Technical (3)

• A single-line system

P+jQ

EVR+jX

Nose Points

P-V Curves

Var compensation

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P+jQQc

Voltage Collapse: Vulnerability (1)

• Power systems are operated under much more stress than in the past– Generation pattern changed under competitive

markets

– Continuous growth of consumption in heavy load

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– Continuous growth of consumption in heavy load areas

– Transmission expansion does not keep pace with Gen and Loads

– Future concerns

Voltage Collapse: Vulnerability (2)

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Transmission capacity relative to load has been declining in everyNERC region since 1982

Common Solutions

• Load shedding (only for urgent needs)

• Reactive power compensation• switched capacitor banks (ability drops quickly

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• switched capacitor banks (when needed most)

• power electronics such as SVC• Additional local generation like DG (dynamic

Var injection; responses very fast !)

Voltage collapse prevention

• A single-line system

P+jQ

EVR+jX

Qc

Nose Points

P-V Curves

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Qc=V2/Xc for capacitors.

Ability drops quickly when V is low.

Blue: no compensation (Q c=0)

Green: capacitor compensation

Red: constant Q c injection

Dynamic Performance: voltage control Vo

ltage

With fast response

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Volta

ge

Time

With slow response

Disturbance occurs

DG to support voltage• DOE has goal of more penetration of DG

• 20% by 2020

• Modern DGs are interconnected to the grid with power electronic interface

• With the right controls, PE can provide flexible, dynamic reactive power (Var) to support voltage

• We are exploring the possibility of using DG’s dyna mic Var capability

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• We are exploring the possibility of using DG’s dyna mic Var capability to help improve voltage stability.

• PV, Wind, fuel cells, microturbines

• Rotating machine interface• Synchronous condenser• Reciprocating-engine synchronous generator

Current RD&D (Research, Development, and

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Development, and Demonstration) at ORNL/UT

• Objective– evaluate the impact and benefits of DG on voltage collapse

• Approach– develop the model of ORNL’s X-10 power distribution system to

perform voltage collapse studies– develop dynamic models of different DG sources– explore impact of DG placement location

Objective and Approach

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– explore impact of DG placement location– determine the degree of voltage collapse protection provided by

DG– identify general engineering guidelines for the design and

operation to use DG to prevent voltage collapse

• Facility– Distributed Energy Communications and Controls (DECC)

Laboratory at ORNL serves as the test-bed since it is actually interfaced to the ORNL Campus distribution system.

DECC Lab is Interfaced with the ORNL Distribution System

Circuit #4Other Circuit

3000 Substation C

ircuit #2 Other Circuit

ORNL Substation

161kV/13.8kV

13.8kV/2.4kV

TVA TransmissionSystem

Other Substations

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2.4kV/480V750kVATransformer

2.4kV/480V750kVATransformer

2.4kV/480V300kVATransformer

Loads

300kVar SCC

ircuit #4Other Circuit

Loads

Loads

Circuit #2 Other Circuit

Loads

Panel PP2

150A/88kVar Inverter

30kW Capstone Micro-turbine

Panel PP3Panel PP1

Induction Motor

DECC Lab

DECC Lab LayoutPanel PP2

Transformer 4-4 Load Banks

Panel PP3 Transformer 2-3

Disconnect

Bldg. 3114

1000A Service

600A Service

door

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Inverter Test Area

Synchronous Condenser Test Area

PP2

PP3

Bay Area

N

door

DG with Power Electronics Interface

• DG with an inverter (compensator) interface is connected in parallel at PCC (point of common coupling).

• PCC voltage is regulated by the inverter.

Load

ic vdc

vtis il

ic

vt il Cf

Ls Rsvs

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• By generating or consuming reactive power, the inverter regulates the PCC voltage.

Controller

switchingsignals

Lc

ic

vcvdcDE

Compensator

Synchronous Condenser (SC)

• SC only generates or consumes reactive power.

• System voltage is controlled by increasing or

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increasing or decreasing the SC current.

• SC current is controlled by the SC excitation voltage.

Control Diagrams for each DG TypeInverter• Inverter reference voltage vc*

is always in phase with vt, so that the inverter only provides reactive power.

• The magnitude of vc* is controlled according to the error of PCC voltage.

Inverter control diagram

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error of PCC voltage.Synchronous Condenser (SC)• 85 V** is the base excitation

voltage for the SC used in the system.

• Excitation voltage is controlled according to the error of the system voltage.

SC control diagram

**The level at which the SC (an overexcited synchronous motor) neither absorbs nor generates reactive power.

Experimental Results (RL Load)

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Local Voltage Regulation on May 1, 2007

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Synchronous Motor Start

Regulation with balanced sags Aug. 28, 2007

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Regulation with unbalanced sags 08/30/2007

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Motor Start Current (Ph a) Inverter Current

Multiple DGs on the Same Circuit

• SC and inverter are installed in a system at ORNL.– Currently they are on

different circuit for testing

– Simulated on the

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– Simulated on the same circuit

• They control the same voltage.

• Inverter response time is faster than SC, therefore an integrated control is required.

Integrated Control DiagramInverter• Faster response, but is

current is limited.• Responsible for the fast

changing voltage ripples.SC• Picks up what the

inverter is unable to.

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inverter is unable to.Integrated System• The overall power rating

of the combined Inverter/SC system is increased.

• Faster response for the integrated system.

• No competing between the inverter & SC.

Simulation Results

Regulated voltage Inverter current SC current

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• Voltage is controlled at the desired level.• Inverter current is limited to 100A peak (70Arms).• SC and inverter share the reactive power required for

the voltage regulation.• The total power rating of the voltage regulation

devices is improved (greater than the inverter and SC individually)

Regulated voltage Inverter current SC current

Micro-Turbine Available for FY08• 100kW Micro-turbine• Donated by SCE• Plan to wire up in FY08• Capable of limited voltage

& power factor correction– ±2% voltage regulation

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• Plan to connect to circuit #4– Panel PP2 has an available

350A CB

Elliott TA-100 Micro-Turbine

New DG Technologies to be Added

• Photovoltaic (PV) Array– assess opportunity for

non-active power compensation

– 50kW PV Array to be added by end of the year

– Storage could be added

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– Storage could be added later to provide output when it is cloudy/rainy/night

Summary• The key cause of voltage collapse is inadequate

reactive power supplies.

• DG, equipped with Var-capable power electronic device is an effective approach to provide dynamic reactive power.

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reactive power.

• ORNL/UT is carrying out research work on dispatching DG for reactive power compensation to better regulate voltage and enhance system voltage stability.

Challenges

• Many utilities are still reluctant to DG interconnection because they are worried that DG may mess up their network and make them lose control of the grid. – IEEE Standard 1547 is restrictive in DG interconnection– Technical issues (e.g. protection) related to

interconnection need to be addressed.

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interconnection need to be addressed.

• Cost of dynamic Var from DG power electronic interface– Has been improved and still needs further improvement

Questions and Answers

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Questions and Answers