Control of Power Electronic Converters in Distributed Electrical Energy Systems Vinod John Power Electronics Group EE Department , IISc Bangalore March 2009
Organization • Need for Distributed Generation (DG) of power • Power electronics in DG / renewable energy • State of the art control of power converter – – – –
Pulse width modulation Phase locked loops for unit vector generation Current control in power converters Reference calculation
• Emerging power converter control requirements • Conclusions Mar 2009
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Why Distributed Generation?
Source: Wikipedia
Source: International Energy Agency - World Energy Outlook, Nov. 2008
Renewable energy and distributed generation • Concerns of pollution and climate change • Concerns of resource depletion • Use of local resources to meet local needs • Technical efficiency, cost and other considerations Mar 2009
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Distributed Electric Power Generation Generation / Transmission system CPP1
CPP2
CPP
Load2 Load1 CPPn
DG
DG1
Load1
DG2
...
... Loadn
Load1
...
Loadn
...
DGn
Distribution / Consumption / DG
Grid connected DG
DG1
Load1
DG2
Load1
...
Loadn
...
DGn
Distribution / Consumption / DG
Stand alone DG
• Central power plant and distributed power generation • DG systems that are grid connected or stand alone • Renewable energy systems are often DG systems Mar 2009
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DG Interconnection Grid Power Conditioning
Prime Source
Electrical Loads
Thermal
Prime Source
Power Conditioning Output
• Combustion engine (Diesel, natural gas, biomass,…)
• Synchronous machine
• Electric grid
• Induction machine
• Loads
• Hydro (stream, river, tidal, ocean current,…) • Wind (residential, wind farms, offshore,…)
– Electrical
– Doubly fed
–Thermal
• Inverters
• Solar (photovoltaic, solar thermal,…) • Chemical (fuel cell, secondary cell,…)
– Squirrel cage
– DC-DC converters & inverter – machine, rectifier & inverter
• Many other varieties Mar 2009
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Power Electronic Converters
250kVA power converter
Mar 2009
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Power Converter Specification High level specifications • Power level – Real and reactive – 3 or 1phase, 3 or 4 wire
• Grid voltage – Amplitude, frequency, tolerance range
• Environmental conditions – – – –
Indoor/outdoor Min/Max ambient temperature Enclosure, humidity, elevation Other mechanical requirements
• Control Specifications Mar 2009
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Shunt Grid Side Power Converter Grid Interconnection Power Converter
Unconditioned electric power from prime source
Lf Vci
Vdc
Vbi
Lg
vc
Conditioned electric power
Cf
Vai
Vag
Vbg
Vcg
Filter + Switchgear
Sensing and Controls
• Large number of applications of shunt VSI – Active front end for loads – Grid interactive UPS – Power quality devices • Statcom, Active filters, …
– Distributed Generation front ends Mar 2009
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VSI Topology • Topology for grid interconnection • 1phase, 3 phase – 1 to 4 legs
Prime Source
Vdc Prime Source
Filter Vdc
Prime Source
Grid Filter
Vdc Prime Source
Grid Filter
Grid
Vdc Prime Source
Filter
Grid
Vdc
Filter
Grid
N N N N N
Mar 2009
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Grid Side Converter Configurations Vg1
Vg1
Vg3
Grid
Vg2
Load
Vg2
Load
Grid
Vg3
i1
i2
i3
PWM Pulses
Shunt type
PWM Pulses
Grid
Grid
PWM Pulses
Series type
PWM Pulses
Combination of series and shunt type Mar 2009
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Control of Grid Interactive Power Converters
Control Structure Power*
Voltage Control Loop
Current Control Loop
Modulation
Switching Converter + Filter
Reactive Power* Command Calculation
Unit Vector Generation
Nested grid connected power converter control • Inner control loops: modulation, current control,… 1. Faster in response 2. Responsible for quality of injected current and protection
•
Outer control loops: voltage, command calculation,… 1. Slower in response 2. DC link voltage control, ac reactive power-voltage droop 3. Produces parts of reference current
Mar 2009
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Simplified Power Converter System Vg
ig
iload
PCC
Grid Side L
i
R
Gate Pulses Vi
Inverter Side ig / iload
Feedback Current Vdc
i Modulation Strategy
m
Current Controller
i*
Reference Current Calculation Cosθ Sinθ
Mar 2009
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Sensed Grid Voltages
Unit Vector Generation
13
Pulse Width Modulation
Pulse Width Modulation isp
idcp Vp Sap
Cdcp
Prime Source
io
Vmp Vdc
Va
240 2 sin(2 50t)
San Cdcn Vn ref
• Phase leg equivalent to SPDT switch • System responds to average phase leg voltage – High switching frequency – System time constants are longer – Ripple in io and Vdc is small Mar 2009
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Duty Cycle - Switching Function 1
isp
idcp
da
Vp tri
Prime Source
Sap
Cdcp
io
Vmp
0 Tsw
t
Vdc
Va
240 2 sin(2 50t)
hap
1
San Cdcn Vn ref
0 Tsw
t
Va_ref = hap Vp_ref + han Vn_ref han = 1 - hap Va_ref = hap (Vp_n) + Vn_ref
idcp = io da
idcp= io hap Mar 2009
d [ 0,1] Va_n = da Vdc
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Space Vector B S A N C
d vs = dt
vs = vaej0 + vbej120 + vcej240
• Representation of 3 phase voltage as space vector • 3 phase grid voltage as an equivalent machine • Combination of switch positions in 3 phase converter equivalent to machine voltage space vector Mar 2009
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Space Vector PWM sin(60T1 = Vref Tsw sin(60) sin( T2 = Vref Tsw sin(60) Tz = Tsw - T1 - T2
• Vectors based on a combination of Sap, Sbn, Scn • Output voltage (Vs) generated using adjacent voltage vectors Mar 2009
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Space Vector PWM
Vdc dc
Space Vector PWM
Sine triangle PWM
• 14% increased voltage in linear range of space vector compared to sine triangle PWM • Modulation methods with third harmonic in the neutral cannot be used in 4 wire systems Mar 2009
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Summary – Modulation • Objective of PWM is to obtain an ideal amplifier • Some DG applications can limit the type of space vector modulation methods
1.
V. T. Ranganathan, “Space Vector Modulation – A status review”, Indian Academy of Science, Sadhana Vol. 22, Part 6, December 1997, pp. 675–688.
Mar 2009
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Unit Vector Generation Using Phase Locked Loops
Simplified Power Converter System Vg
ig
iload
PCC
Grid Side L
i
R
Gate Pulses Vi
Inverter Side ig / iload
Feedback Current Vdc
i Modulation Strategy
m
Current Controller
i*
Reference Current Calculation Cosθ Sinθ
Mar 2009
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Sensed Grid Voltages
Unit Vector Generation
22
Unit Vector Generation Unit vector reference based on positive sequence fundamental voltage
Phase Locked Loops (PLL) in power systems • Measurement of frequency, phase and amplitude • Provides a reference sine/cosine signals synchronized to grid waveform
Methods for implementation of PLL • Zero crossing detection • Stationary reference frame • Synchronous reference frame Mar 2009
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Need for PLL
• Generation of unit sine and cosine for scaling of feed back and modulating signals • Grid monitoring 1. Information on frequency, phase and voltage magnitude. 2. Synchronization between two sources -- e.g. A power converter to grid 3. Information on negative sequence and unbalance Mar 2009
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Unit Vector Generation Techniques •
Zero crossing detection Look up table Input signal Sampling Interval Ts
θ
Reset by ZCD Zero crossing detection
•
Mar 2009
Limitation 1. Frequency variation 2. Multiple zero crossing
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Unit Vector Generation Techniques •
Stationary reference frame based method Sinθ
Vα = Vm cos(ωt)
Va Vb Vc
Magnitude Vm extraction
3 phase to 2 phase Transformation
Vβ = Vm sin(ωt)
•
Cosθ
Limitation 1. Unbalance phase voltage 2. Voltage sag or swell 3. Frequency variation
Sivaprasad J.S., T. Bhavasar, R. Ghosh and G. Narayanan, “Vector control of three-phase AC/DC front-end converter”, Indian Academy of Science, Sadhana Vol. 33, Part 5, October 2008, pp. 591–613. Mar 2009
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Unit Vector Generation Techniques •
Structure of the Synchronous Reference Frame (SRF) PLL
Input signal
•
Phase Comparator
Loop Filter
VCO
Synchronized output
Output signal is synchronized to input
Mar 2009
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Synchronous Reference Frame PLL
• SRF PLL structure – Projection of grid voltage vector on D-axis is regulated to zero using PI controller – Grid voltage vector is aligned to Q-axis Mar 2009
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Phase Comparator β b
a
α
c
Xα = Xa – (Xb + Xc)/2 X = 3 (Xb – Xc)/2
Xd = Xα Cos(θe) + Xβ Sin(θe) Xq = Xβ Cos(θe) – Xα Sin(θe) Xα + jXβ = (Xd + jXq)ej
Mar 2009
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e
29
Error Dynamics of SRF PLL
Va = Vm Cos(ωt) Vb = Vm Cos(ωt - 2π/3) Vc = Vm Cos(ωt + 2π/3) θ = ωt – π/2
Assuming θ is close to θe and neglecting negative sequence, the above equation can be simplified to Mar 2009
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Approximate Block Diagram
Controller parameters obtained from desired bandwidth and damping. Drawbacks of SRF PLL 1. Due to unbalance a 100 Hz ripple appearing in Vd and Vq may produce distortions in θe. 2. Presence of harmonic voltage can create similar problems 3. Reduced PI controller corner frequency to reduce problems due to unbalance and harmonic makes the system response slow. V. Kaura and V. Blasko, “Operation of a phase locked loop system under distorted utility conditions,” in IEEE Transactions on Industry Applications, Volume 33, Issue 1, Jan.-Feb. 1997 Page(s):58 - 63 Mar 2009
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Abnormal Grid Conditions • • • • • • •
Unbalance Frequency variation Presence of harmonics Voltage sag and swell Faults Phase jump Line notching
Requirement : Phase tracking of positive sequence fundamental voltage under abnormal grid conditions
Mar 2009
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Improved Unit Vector Generation Methods for improving performance of SRF PLL • Phase tracking by extracting both positive and negative sequence -needs two reference frame transformations
• Removal of ripple in Vd due to unbalance and grid harmonics by filtering 1. Makes the response slow 2. Instability may occur due to reduced phase margin
• Proposed method using Moving Average Filter (MAF)
Mar 2009
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Filter Characteristics Requirements • The unbalance and harmonics from the grid are at 2nf o (100Hz, 200Hz, ….) • The information for tracking phase required by the loop filter is a low frequency signal • Suitable low pass or notch filtering approaches • MAF based on finite impulse response (FIR) characteristics provides – Low pass filtering and – Notches at the harmonics frequencies expected from abnormal grid conditions
Mar 2009
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Frequency Response of MAF •
Transfer function of the selected moving average filter in z domain is given by 9
H ( z) 0.1
z- p
p 0
No. of samples = 10 Sampling frequency = 1kHz
• •
The first order filter for comparison has corner frequency at 10Hz A notch filter having a notch frequency at 100Hz can also be used -- Compared to a notch filter, a MAF provides higher attenuation at harmonic frequencies
Mar 2009
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Simplification For Controller Design ωff
Vd* = 0
θ
+-
3 - Vm Vd Filter 2
-+
++
•
θ'
Time constant of the approximated 1st order system is Tf 10ms
GH
Mar 2009
ω' ∫
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3 1 Vm 2 1 sTf
Kp
1 s s
1 s
36
Symmetric Optimum Background
• •
Integrator and time constant plant model PI gains for desired open loop transfer function characteristics
kp =
1 2 ko
J.W. Umland and M.Safiuddin, “Magnitude and symmetric optimum criterion for the design of linear control systems: What is it and how does it compare with others?” in IEEE Transactions on Industry Applications, Volume 26, Issue 3, Jan.-Feb. 1990 Page(s):489 – 497. Mar 2009
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Controller Parameters Selection •
Loop gain of the system is GH
•
•
3 1 Vm 2 1 sTf
Kp
1 s s
1 s
Using symmetrical optimum method Kp and were designed Kp = 0.101 and = 0.04s bandwidth = 53.7 rad/sec Phase margin = 51 deg A fine tuning of the controller parameter gave higher bandwidth. Kp = 0.3 and = 0.08s Bandwidth = 135 rad/sec Phase margin = 49 deg Mar 2009
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Implementation Details • The proposed PLL has been implemented in a Cyclone FPGA board • Clock frequency = 20 MHz • Integration step size = 50 μs • Moving Average Filter ( MAF ) specification No. of samples = 10 Sampling frequency = 1 kHz • Implementation of MAF needs: 10 latches ( storage ) 2 adder/subtract operations 1 multiplier Mar 2009
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Results
Tracking of reference voltage
Estimation of frequency with SRF PLL Mar 2009
D-axis voltage under unbalanced condition
Estimation of frequency with proposed PLL PEG IISc Bangalore
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Results
Estimation of θ during unbalanced condition
Estimation of frequency with step change in frequency
Phase jump of 10 deg
Estimation of frequency during phase jump
Mar 2009
Estimation of d and q axis voltages during three phase voltage sag
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Summary - Unit Vector Generation • Accurate and fast estimation of magnitude, frequency and phase of grid voltage can be used for control and protection of power converter
• Three phase SRF PLL has limitations under abnormal grid conditions such as unbalanced phase voltage • A MAF based SRF PLL is capable of giving improved results • The PI controller parameter design and experimental verification of the proposed PLL have been validate the proposed approach Mar 2009
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References •
PLL
1.
V. Kaura and V. Blasko, “Operation of a phase locked loop system under distorted utility conditions,” in IEEE Transactions on Industry Applications, Volume 33, Issue 1, Jan.-Feb. 1997 Page(s):58 - 63
2.
Se-Kyo Chung, “A phase tracking system for three phase utility interface inverters,” in IEEE Transactions on Power Electronics Volume 15, Issue 3, May 2000 Page(s):431 - 438
3.
J.W. Umland and M.Safiuddin, “Magnitude and symmetric optimum criterion for the design of linear control systems: What is it and how does it compare with others?” in IEEE Transactions on Industry Applications, Volume 26, Issue 3, Jan.-Feb. 1990 Page(s):489 – 497.
Mar 2009
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Converter Current Control
Power Converter Current Control Objectives of current control • • • • • •
Ideal tracking over a wide frequency range High dynamic response Constant switching frequency Minimal harmonic interactions with grid Utilization of DC link voltage Capability of working under grid disturbance
Mar 2009
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Current Control Techniques • Control of output current is a basic requirement in grid connected power converter operation – – – –
Mar 2009
Hysteresis current control Predictive current control Synchronous reference frame current control Proportional-resonant current control
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Hysteresis Current Control Vg1 Vg2
Load
Grid
Vg3
i1 i1*
+
i2
i3
-
i1 i2*
+
-
i2 i3*
+
-
i3
Cosθ Sinθ
Vg1 PLL
Vg ω
Mar 2009
Vg2 Vg3
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Hysteresis Current Control Advantages 1. Fast dynamic response 2. Independent of parameter variation
Disadvantages 1. 2. 3. 4. 5.
Variable switching frequency Higher stress on devices Difficulty in input filter design Interaction between phases Difficulty in digital implementation
D. M. Brod and D. W. Novotny, “Current control of VSI-PWM inverters,” IEEE Trans. Industry Applications, vol. IA-21, pp. 562–570, July/Aug.1985. Mar 2009
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Hysteresis Current Control Improvements • Variable hysteresis band to make switching frequency constant • Compensation of interaction between phases • Reduction in switching frequency by introducing zero voltage vector states
Concerns • Possible inter-harmonics • Delay in digital detection of band crossing
Mar 2009
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Control in Stationary Co-ordinates Vg1
Load
Vg2
Grid
Vg3
i1 +
-
i1 i2*
+
-
i3
Controller
Controller
Modulator
i1*
i2
PWM Pulses
i2 i3*
+
-
i3
Controller Cosθ Sinθ
Vg1 PLL
Vg ω
Vg2 Vg3
Current reference and measurement are sinusoids at the fundamental frequency Mar 2009
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Control in Stationary Co-ordinates Advantages 1. Co-ordinate transformations are not needed 2. Separate negative sequence controller is not needed in case of unbalance 3. Independent control of each phase can be done
Disadvantages 1. Steady state error in phase and amplitude would be present if PI controller is used
Improvements 1. Steady state error can be made almost zero by • Predictive controller • Proportional-Resonant (PR) controller Mar 2009
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Predictive Current Control R
L i
Vi
Vi
Inverter Side
Ri
di L dt
Vg
Vg
Grid Side
For a given i* to follow
Vi
i* i Ri L Vg T
i* is the predicted value of current at the next switching cycle. i* can be predicted by interpolation. Mar 2009
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Block Diagram of Predictive Control Vg1
Load
Vg2
Grid
Vg3
R i1
+
+
iβ*
-
-
iβ
Vα
Lˆ T
+ +
Lˆ T
+ +
αβ
Vβ
Vg1 αβ abc
Vg2 Vg3
Mar 2009
abc
+ +
PWM Pulses
i1
Cosθ Sinθ
Vβ
Rˆ Vα
++
Modulator
iα*
i3 L
Rˆ
iα
i2
iα iβ
αβ abc
i2 i3
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Vg1 PLL
Vg ω
Vg2 Vg3
53
Predictive Current Control • Advantages – Separate negative sequence controller is not needed in case of unbalance – Fixed switching frequency
• Disadvantages – Steady state error correction is needed – Parameter variation may affect controller performance
• Improvements – Improved prediction algorithm – Online estimation of parameters – Including zero sequence current controller for a three phase four wire systems Marian P.Kazmierkowski, “Current control techniques for Three phase voltage source PWM converters: A survey,” IEEE Trans. Industrial Electronics Vol.45, no.5 PP.691-703, Oct. 1998. Mar 2009
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Control in Synchronous Co-ordinate Vg1
Load
Vg2
Grid
Vg3
i1
id*
+
-
+
-
+
-
iq
αβ abc
iα
i1
dq
Vg iq*
i3
ωLiq
Modulator
id
i2
αβ
+ + +
ωLid
Cosθ
id
PWM Pulses
Sinθ
αβ abc
dq αβ
iq
iβ
Cosθ
Cosθ Sinθ PLL
i2 i3
Vg1
Vg ω
Vg2 Vg3
Sinθ
C.D. Schauder and R. Caddy, “Current Control of Voltage-Source Inverters for Fast Four-Quadrant Drive Performance” IEEE Trans. Industry Applications, vol. IA-18, No. 2, pp. 163–171, Mar./Apr.1982. Mar 2009
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Control in Synchronous Co-ordinate Synchronous reference frame (SRF) control Advantages 1. Zero steady state fundamental error (Under balanced condition) 2. Constant switching frequency
Disadvantages 1. Slower response compared to hysteresis 2. Poor harmonic response 3. Not compatible with 4 wire distribution systems Mar 2009
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Results SRF Control
Experimental result with mains voltage (125V/div) and line current (50A/div) and 25kVA reactive power
Sivaprasad J.S., T. Bhavasar, R. Ghosh and G. Narayanan, “Vector control of three-phase AC/DC front-end converter”, Indian Academy of Science, Sadhana Vol. 33, Part 5, October 2008, pp. 591– 613. Mar 2009
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Improvements for Current Control • All the controllers work well under ideal grid situation – Need for evaluation under abnormal grid conditions
• Under sustained unbalanced condition a negative sequence controller is needed
• For a three phase four wire system zero sequence current controller is required. • Harmonic compensation capability needs be improved
Mar 2009
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Proportional-Resonant Controller • Synchronous reference frame control – Shift the current signals from 50Hz to DC and – PI to accurately control the transformed DC
• Proportional Resonant (PR) control – Keeps the current signal at 50Hz – Shifts the high gain of the PI controller from DC to 50Hz Mar 2009
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Hpi(s) K p
Hpr(s) K p
Ki s
Ki s s
2
2
59
PR Controller Block diagram and transfer function of a resonant integrator
s
Hr(s)
s
2
2
Magnitude Response Kp=0.1 Ki =10 ω=314 rad/sec
Gain (dB)
10 0
|Hpi|
-10
|Hpr|
-20 -30 -40 -50 1
10
100
1000
10000
Frequency (Hz) Mar 2009
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PR Controller for Harmonics i*
+
-
i
Kp Fundamental current controller
+ + +
V*
Ks i 2 s ω2
K s i5 s 2 (5ω) 2
+ +
5th harmonic current controller K s i7 s 2 (7ω) 2
7th harmonic current controller Zmood, D.N.; Holmes, D.G.; Bode, G.H. “Frequency-domain analysis of three-phase linear current regulators”, IEEE Transactions on Industry Applications, Volume 37, Issue 2, March-April 2001 Page(s):601-610 Mar 2009
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PR Controller Characteristics • Advantages – Dynamic response similar to SRF controller – High gain only near the resonant frequency • Suitable for harmonic compensation along with • Fundamental frequency current controller
• Disadvantages – Less well understood compared to PI controller – Complexity with increasing number of harmonics for be compensation – Complexity in implementing control limits
Mar 2009
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Summary – Current Control • Merits and demerits of different current controllers have been discussed • Fixed switching frequency full digital implementation is desirable
• Independent control of each phase current of a grid connected power converter for unbalanced 4-wire distribution systems • Solution to a given application demands appropriate current control strategy to be chosen
Mar 2009
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References •
Current Control Techniques
1.
C.D. Schauder and R. Caddy, “Current Control of Voltage-Source Inverters for Fast FourQuadrant Drive Performance” IEEE Trans. Industry Applications, vol. IA-18, No. 2, pp. 163– 171, Mar./Apr.1982.
2.
D. M. Brod and D. W. Novotny, “Current control of VSI-PWM inverters,” IEEE Trans. Industry Applications, vol. IA-21, pp. 562–570, July/Aug.1985.
3.
Zmood, D.N.; Holmes, D.G.; Bode, G.H. “Frequency-domain analysis of three-phase linear current regulators”, IndustryApplications, IEEE Transactions on Volume 37, Issue 2, MarchApril 2001Page(s):601-610
4.
Marian P.Kazmierkowski, “Current control techniques for Three phase voltage source PWM converters: A survey,” IEEE Trans. Indust Electrons Vol.45, no.5 PP.691-703, OCTOBER 1998.
5.
F. Blaabjerg, R. Teodorescu, M. Liserre, and A. Timbus, “Overview of control and grid synchronization for distributed power generation systems,” IEEE Trans. Ind. Electron., vol. 53, no. 5, pp. 1398–1409, Oct. 2006.
Mar 2009
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Reference Calculation In Grid Interactive Power Converter Control
Simplified Power Converter System Vg
ig
iload
PCC
Grid Side L
i
R
Gate Pulses Vi
Inverter Side ig / iload
Feedback Current Vdc
i Modulation Strategy
m
Current Controller
i*
Reference Current Calculation Cosθ Sinθ
Mar 2009
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Sensed Grid Voltages
Unit Vector Generation
66
Reference Calculation Prime Source
Prime Source
Power Conditioning
Grid
Power Conditioning
Electrical Loads
Electrical Loads
Reference calculations based on operation requirements • Grid parallel operation • Stand alone operating – Power level change is relatively slow – Non dispatchable system – Small DC bus energy storage Mar 2009
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– Power level of load can change in step – Dispatchable prime sources – Energy storage requirement 67
Calculation of id* and iq* Vg1
Load
Vg2
Grid
Vg3
i1
id*
+
-
+
-
+
-
iq
iα
i1
αβ
+ + +
ωLid
Cosθ
id
PWM Pulses
Sinθ
αβ abc
dq αβ
iq
iβ
Cosθ
Mar 2009
αβ abc
dq
Vg iq*
i3
ωLiq
Modulator
id
i2
Cosθ Sinθ PLL
i2 i3
Vg1
Vg ω
Vg2 Vg3
Sinθ
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Reference Current Calculation • Reference calculation for current controlled PWM VSI has application specific requirements Grid connected DG Stand alone DG inverter inverter
Real current command Reactive current command
• DC bus control#
• fixed var injection • pf correction • voltage droop Harmonic current • active filtering command requirement
• AC voltage control* • AC voltage control* • embedded in Idq command
#Additional
devices may also provide DC bus control *Stand alone inverter DG may not use current loop Mar 2009
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DC Bus Control Voltage controller Vdc* +
-
Vdc
Current controller iq* +
Vdc
-
Iinv
Prime Source
3
Grid
iq
• Grid interactive DC bus control – Primary source injects DC bus current proportional to operating power level – Real current command linked to error in DC bus voltage
• Unbalance ac side cause ripple in DC bus voltage – Filtering of 100Hz Vdc can reduce distortion of the iq*
• Stand alone control of the converter – Dispatchable prime sources or energy storage element maintains the DC bus voltage Mar 2009
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Reactive Power Command Open loop id* iload_d Vac*
Current controller id* +
+
-
Prime Source
-
Vdc
Iinv
Vac Grid
3
id
Iload
Vac Load
• Reactive current reference generation options – – – –
Mar 2009
Reactive current injection Compensation (power factor correction) Voltage vs. reactive power droop Above options can be extended from balanced 3 on to 1 basis PEG IISc Bangalore
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Other Reference Options
• Harmonic current filtering – Load harmonic current extraction using synchronous reference frame methods – Unbalance negative sequence extracted separately
• Droop based power converter control – Active power vs. operating frequency – Voltage vs. reactive power Mar 2009
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Reference Command Requirements • DC bus control is critical for safer operation of the converter during the fault duration • Reference calculation under grid stress based on control strategy – During fault duration and – After fault clearance
• Power converter ratings insufficient for providing fault clearing current magnitude • Damping of the system subsequent to fault clearance • Factors such as harmonics are related to steady state operation – Methods for transition from from post fault operation to steady state command generation
Mar 2009
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Summary • Reference generation in grid connected power converter is application specific • Real current command linked to maintaining DC bus voltage even under disturbed grid conditions • Priority based methods required to handle real and reactive power, and harmonics
Mar 2009
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Emerging Requirements In Grid Interactive Power Converter Control
Emerging Control Requirements Generation / Transmission system CPP1
Distribution feeder with DG
CPP2
X
Load2
Load1
A
CPPn
...
B
... Loadn
X
DG1
Load1
DG2
Load1
...
Loadn
...
DGn
Distribution / Consumption / DG
DG
• Reliably operation of power converter under grid disturbances – Ride through grid disturbances – Disconnect from sustained faults Mar 2009
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Load
Simulation Schematic
PCC Load
SM
PWM Pulses
Mar 2009
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Load
Simulation Schematic
PCC Load
SM
PWM Pulses
Mar 2009
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Load
Simulation Schematic
BKR Opens
Load
PCC
SM
PWM Pulses
Mar 2009
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PCC Voltages
•
Line to ground fault on phase R
Mar 2009
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Phase Currents - SRF Control
• Current control in synchronous coordinates • Fault current add up in the neutral for 4-wire distribution systems Mar 2009
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Phase Currents - PR Control
• Current control in stationary abc coordinates • Neutral current is limited even for ground fault in 4-wire systems Mar 2009
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SLG Fault Ride Through Capability Type of CC
Peak phase current after occurrence of fault
Peak neutral current
Peak phase voltage After clearance of fault
SRF current control (dq frame)
3*peak rated current
6*peak rated current
360 V in the faulty phase
Predictive current control without zero sequence control (αβ frame)
2.3*peak rated current
5.5 *peak rated current
360 V in the faulty phase
Predictive current control with zero sequence control (αβ frame)
Normal value
Negligible
370 V in the faulty phase
Resonant current Normal value control (abc frame)
Negligible
375 V in the faulty phase
Peak rated current = 19.67 A ; Peak rated voltage = 338.85 V ( per phase ) Inverter rating = 10 kVA Operating mode = Reactive power compensation; Vdc = 800 V( constant ) Mar 2009
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Conclusions • Emerging control requirements for power converters requires modifications to make it robust to grid side disturbance and required ride through capability
• Methods for reference generation and current control has been proposed to handle abnormal grid conditions – Unit vector generation based on SRF PLL and MAF filters – Independent control of each phase of a grid connected power converter using proportional resonant controllers
• Preliminary simulation studies indicate ride through can be achieved for a wide range of faults even for 4-wire feeders Mar 2009
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84
Summary • What is DG and why we need it? • Role of power electronic converters in DG
• Control requirements for power converters DG – State of the art in unit vector generation and current control – Basics of PWM and reference generation
• Emerging requirements for grid connected DG power converters – Preliminary experimental and simulation results Mar 2009
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85
Thank you
Space Vector PWM
• Determination of sector location – Sign of the line to line voltage – Vs voltage vector
• Equivalent duty cycle calculations Mar 2009
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T1 = Tsw V –
T2 = Tsw V
V 3 2 3
Tz = Tsw - T1 - T2 87
Frequency Response
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88
Unbalanced phase voltages
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Estimated θ with SRF PLL
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90
Under single phase LG fault condition
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91
Harmonic compensation with unbalanced load
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PEG @ IISc • Students interested in any of these issues should consider doing a Ph.D. at IISc – Wide range of research areas in Power Electronics • • • • • •
Motor drives Control and modulation Power supplies and electromagnetic design Renewable energy and distributed generation … www.ee.iisc.ernet.in/new/people/faculty/vjohn
– Other research areas in the EE dept. • Power systems, high voltage engineering, signal processing, computer vision, …
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Thank you
