Download Control and Grid Synchronization for Distributed Power Generation

CONTROL AND GRID
INTRGRATION OF
DISTRIBUTED POWER
GENERATION SYSTEMS
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
Causes
DPGS Control


1.
2.
3.
4.
5.
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


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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


Mode
acts as a current source
delivering power to the grid and
local loads
 Island

Mode
acts as a voltage source
Control Structures for Grid-Connected
DPGS

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

3


t
    d
0

dq –Control (Park transformation)

synchronous (rotating) reference frame

PI controller
dq –Control example
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
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
frequency
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
Controllers


Harmonic compensation by cascading
several generalized integrators tuned to resonate at
the desired frequency
Harmonics Compensation using PR
Controllers
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


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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
implementation
 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
faults)
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 !