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M. Monfared, PhD
Renewable energy sources
hydropower and wind energy
photovoltaic (PV) and fuel cell (FC) technology
low efficiency
 poor controllability of the distributed
power generation systems (DPGSs) based on wind
and sun
Source: Renewables 2011 - GLOBAL STATUS REPORT
DPGS Control
Source-side controller -extract the maximum power
from the input source
Grid-side controller
control of active power generated to the grid
control of reactive power transfer between the
DPGS and the grid
control of dc-link voltage
ensure high quality of the injected power
grid synchronization
Topologies of DPGS
Photovoltaics and Fuel Cells – similar topology
Wind Turbines – topology dependent on generator
Wind turbines
WT Systems without Power Electronics
Wind turbines
WT Systems with Power Electronics
Increased complexity
Higher cost
Better control of power input and grid interaction
Partial power solution
WT with full-scale power converter
Different modes of operation
 Grid-Connected
acts as a current source
delivering power to the grid and
local loads
 Island
acts as a voltage source
Control Structures for Grid-Connected
Two cascaded loops (Multiloop PI/PR Control)
 fast
internal current loop, regulates the grid current
 an external voltage loop, controls the dc-link voltage or
the active power transferred to the grid
Good performance under linear loads
Reference Frames
X  xa  axb  a 2 xc
a  e j 2
    d
dq –Control (Park transformation)
synchronous (rotating) reference frame
PI controller
dq –Control example
proportional–integral (PI) controllers
to improve the performance, cross-coupling terms and
voltage feedforward are used
ab-Control (Clarke transformation)
stationary reference frame
PR (proportional –resonant) controller
ab-Control example
PR controllers -very high gain around the resonance
Natural Frame Control (abc control)
PI controller
PR controller
nonlinear controllers
PI and PR controller in abc Frame
Power Quality control
Harmonics Compensation Using PI Controllers
Harmonics Compensation using PR
Harmonic compensation by cascading
several generalized integrators tuned to resonate at
the desired frequency
Harmonics Compensation using PR
Control under Grid Faults
Instability of the power system
Stringent exigencies for interconnecting the DPGS
1) Symmetrical fault (no phase shifting) - rare
2) Unsymmetrical fault
Control Strategies under Faults
Unity Power Factor Control Strategy
Positive-Sequence Control Strategy
Constant Active Power Control Strategy
Constant Reactive Power Control Strategy
Grid Synchronization Methods
Zero-Crossing Method
 simplest
 Poor performance (harmonics or impulse disturbances)
Grid Synchronization Methods
Filtering of the grid voltages in different reference
frames dq or αβ and using arctangent
difficulty to extract the phase angle (grid variations or
PLL Technique
state-of-the-art method to extract the phase angle
of the grid voltages
Better rejection of grid harmonics and any other
kind of disturbances
Problem to overcome grid unbalance
IEEE 1547
IEEE 1547 Standard for Interconnecting Distributed
Resources with Electric Power Systems
This document provides a uniform standard for
interconnection of distributed resources with EPSs
[Electric Power Systems]. It provides requirements
relevant to the performance, operation, testing,
safety, and maintenance of the interconnection.
LVRT -Different Grid Codes !