Design and Control of an AntiDesign and Control of an Anti--Resonance Resonance Hybrid DeltaHybrid Delta--Connected Capacitor Bank for Connected Capacitor Bank for

LowLow--Voltage Industrial Power SystemsVoltage Industrial Power SystemsPichai JintakosonwitPichai Jintakosonwit

((SirindhornSirindhorn International Institute of Technology, International Institute of Technology, ThammasatThammasat University)University)

SuntSunt SrianthumrongSrianthumrong

(National Electronics and Computer Technology Center, NECTEC)(National Electronics and Computer Technology Center, NECTEC)

PichitPichit JintakosonwitJintakosonwit

(Metropolitan Electricity Authority, MEA)(Metropolitan Electricity Authority, MEA)

Capacitor Banks for Power Factor CorrectionCapacitor Banks for Power Factor Correction

AdvantagesAdvantages

• low cost (compared with SVC or active filter)low cost (compared with SVC or active filter)

• simple structure (Y and △ connections)

• ease of installation

DisadvantagesDisadvantagesharmonic resonanceharmonic resonance

between line inductance and capacitance of capacitor banks

significant amplification of voltage and current harmonics

Capacitor Banks with Resonance ProtectionCapacitor Banks with Resonance Protection

Series capacitor/reactor combination (tuned Series capacitor/reactor combination (tuned LCLC filter)filter)

• capacitor C is used to improve the power factor above 0.95.

• tuned frequency is set to the most dominant harmonic frequency. (typically the 5th-harmonic frequency)

Research Objectives and Proposed SystemResearch Objectives and Proposed System

In real power systemsIn real power systems

uncertain system parameters due to system configurations

and loads

Objectives :Objectives :

AntiAnti--Resonance Hybrid DeltaResonance Hybrid Delta--Connected Connected Capacitor BankCapacitor Bank

compensating for reactive power without harmonic compensating for reactive power without harmonic resonance in lowresonance in low--voltage industrial power systemsvoltage industrial power systems

existence of harmonic resonanceexistence of harmonic resonanceeven if series capacitor/reactor even if series capacitor/reactor

combination is installedcombination is installed

Hybrid DeltaHybrid Delta--Connected Capacitor BankConnected Capacitor Bank

Advantages : low cost, low switching losses, Advantages : low cost, low switching losses, no resonance, no matching transformerno resonance, no matching transformer

Drawbacks : complex control, series protection circuitDrawbacks : complex control, series protection circuit

Aim of Capacitor: To compensate for reactive power Aim of Capacitor: To compensate for reactive power

Aim of Inverter: To improve characteristic of capacitorAim of Inverter: To improve characteristic of capacitor

System ConfigurationSystem Configuration

Principle Operation of Hybrid Capacitor BankPrinciple Operation of Hybrid Capacitor BankPrinciple Operation of Hybrid Capacitor Bank

•• An infinite impedance for dominant harmonic frequenciesAn infinite impedance for dominant harmonic frequencies

Control of InverterControl of Inverter

•• A zero impedance for the fundamental frequencyA zero impedance for the fundamental frequency

equivalent circuitequivalent circuit

vC = an inverter voltage [V]K = a control gain [Ω]iCh = a harmonic current in iC [A]

(Note: CY = 3C = 60 μF)

Harmonic Detection and DC Voltage ControlHarmonic Detection and DC Voltage ControlHarmonic Detection and DC Voltage Control

•• the PQ theory for harmonic detectionthe PQ theory for harmonic detection

•• the PI control for dc voltage regulation withoutthe PI control for dc voltage regulation withoutany external dc power supplyany external dc power supply

Control Block Diagram of InverterControl Block Diagram of InverterControl Block Diagram of Inverter

time delay T = 50 μs

G(s)neglecting the effect of

ripple capacitor Cf

ω1 = fundamental frequencyωc = cutoff frequency (13 Hz)

the impedance looking into the PCC

where

Control Gain SelectionControl Gain SelectionControl Gain Selection

high impedance

high harmonic voltage

suitable gain

KK = 15 = 15 ΩΩ

250 Hz

261 Hz

560 Ω (55 dB)

180 Hz

250 Ω

12.6 Ω (22 dB)

Harmonic Resonance and DampingHarmonic Resonance and DampingHarmonic Resonance and Damping

Simulation System under load conditionsSimulation System under load conditionsSimulation System under load conditions

• The control gain: K = 15 Ω• The PI gain for dc voltage control: Kp= 0.5 and Ki = 20 s-1

Transient ResponseTransient ResponseTransient Response

25 V

terminal voltage

source current

capacitor current

dc-bus voltage

Simulation Results Simulation Results Simulation Results THD = 3.3% THD = 16.4% THD = 2.5%

4.6%

4.6%

27.8%

49.8%

4.6%

31.1%

0 82.9%

6.0%

4.9%

28.5%

7.8%

10ms

without C only C hybrid bank

PF = 0.74PF = 0.74 PF = 0.85PF = 0.85 PF = 0.97PF = 0.97

IC = 5.3A IC = 4.1A

Simulation System under no-load conditionsSimulation System under noSimulation System under no--load conditionsload conditions

5th harmonic voltage : 1%7th harmonic voltage : 1%

source harmonic voltages

only C

hybrid bank

THD = 53%

THD = 267%

VT = 260 V

IS = 12.3 A

THD = 1.4%

THD = 4.3%

VT = 229 V

IS = 4.4 A

ConclusionConclusionConclusion

Anti-Resonance Hybrid Delta-Connected Capacitor Bank• △-connected capacitors in series with three single-phase inverters

• No matching transformer and external dc supply

• Low VA rating, low dc bus voltage and low switching losses

effectiveness of reactive power compensation withouteffectiveness of reactive power compensation without

harmonic resonance, irrespective of system conditionsharmonic resonance, irrespective of system conditions

Advanced capacitors:Advanced capacitors:

existing △-connected capacitors

replacing the reactors with the smallreplacing the reactors with the small--rating invertersrating inverters

anti-resonance hybrid capacitors

Protection SystemProtection SystemProtection System

Power Electronics Lab.Power Electronics Lab.

SirindhornSirindhorn International Institute of TechnologyInternational Institute of Technology

During standby condition:two lower-arm MOSFETs in each inverter are turned on.

To prevent overvoltageacross dc capacitor

Power Electronics Lab.Power Electronics Lab.

SirindhornSirindhorn International Institute of TechnologyInternational Institute of Technology

Reactive Power CompensationReactive Power CompensationReactive Power Compensation

1. Shunt Capacitor Bank1. Shunt Capacitor BankAdvantages : low cost, simple structure Advantages : low cost, simple structure Drawbacks : fixedDrawbacks : fixed--harmonic compensation, resonanceharmonic compensation, resonance

2. Static 2. Static VarVar Compensator (SVC)Compensator (SVC)Advantages : unity power factor, no resonanceAdvantages : unity power factor, no resonanceDrawbacks : high cost, switching losses, complex controlDrawbacks : high cost, switching losses, complex control

3. Synchronous Condenser3. Synchronous CondenserAdvantages : leading or lagging power factor, no resonanceAdvantages : leading or lagging power factor, no resonanceDrawbacks : starting problem, cooling systemDrawbacks : starting problem, cooling system

Power Electronics Lab.Power Electronics Lab.

SirindhornSirindhorn International Institute of TechnologyInternational Institute of Technology

Harmonic PollutionHarmonic PollutionHarmonic Pollutionsubstation

motor control equipment

crane control equipment

television

computer

elevator

capacitor bankor

transformer

sinusoidal voltage

distorted voltage

high distorted voltage

Inverter DesignInverter DesignInverter DesignIEEE std. 519-1992 : individual harmonic voltage ≤ 3%, THD ≤ 5%

DC voltage

Let ma= 0.8. The required dc voltage is 25 V.

Inverter current

The minimum current rating of inverter is 4.2 A.

Inverter current == the fundamental current flowing into the capacitor for reactive compensation