Control and Grid Synchronization forDistributed Power

Generation Systems

Z.Leonowicz, PhD

F. Blaabjerg, R. Teodorescu, M. Liserre, and A. V. Timbus: Overview of Control and Grid Synchronization for Distributed Power Generation Systems, IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 53, NO. 5, OCTOBER 2006

Renewable energy sources

• hydropower and wind energy• photovoltaic (PV) technology

• low efficiency• poor controllability of the distributed power generation systems (DPGSs)

based on wind and sun

Overview

1. Main DPGS structures,2. PV and fuel cell (FC) system3. Classification of wind turbine (WT)

systems with regard to the use of power electronics

4. Control structures for grid-side converter 5. Characteristics of control strategies

under grid fault conditions6. Grid synchronization methods

Causes

DPGS Control

• Input-side controller -extract the maximum power from the input source

• Grid-side controller1.control of active power generated to the

grid2.control of reactive power transfer

between the DPGS and the grid3.control of dc-link voltage4.ensure high quality of the injected power5.grid synchronization

Topologies of DGPS

• 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 Solution

WT with full-scale power converter

Control Structures for Grid-Connected DGPS

• Two cascaded loops– Fast internal current loop, regulates the

grid current– an external voltage loop, controls the dc-

link voltage

Reference Frames

• reference frame transformation module, e.g., abc → dq

• PI -controller

dq -Control

• proportional–integral (PI) controllers• controlled current - in phase with the

grid voltage

-Control (Clarke transformation)

• stationary reference frame

• PR proportional –resonant controller

-Control example

• very high gain around the resonance frequency

Natural Frame Control (abc control)• PI Controller• PR Controller

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

• Nonlinear 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

• Unity Power Factor Control Strategy

• the negative sequence component gives rise to oscillations (2nd harmonic)

Positive-Sequence Control Strategy• follow the positive sequence of the

grid voltages• PLL necessary (Synchronous reference

frame PLL)• dc-link capacitor should be rated to

overcome the second-harmonic ripple• grid currents remain sinusoidal and

balanced during the fault

Constant Active Power Control Strategy

• injecting an amount of negative sequence in the current reference, the compensation for the double harmonic can be obtained

Constant Reactive Power Control Strategy

• Reactive power to cancel the double-frequency oscillations

• Current vector orthogonal to the grid voltage vector can be found

Grid Synchronization Methods

• Zero-Crossing Method• simplest implementation• Poor performance (harmonics or

impulse disturbances

• Filtering of the grid voltages in different reference frames: dq or αβ

• 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

Conclusions

• Hardware = Full-scale converter• DGPS control = PR controllers• Faults = strategies• Synchronization = PLL