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SMART GRID

Shahram Javadi Assistant Professor

Electrical Eng. Department

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10- Power Electronics in the

Smart Grid

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Introduction

Smart Grid there will be increasing connection to the distribution network of renewable energy sources electric vehicles heat pumps More flexible loads

For sensitive loads such as computers and high value manufacturing plants, the quality of supply will be important

Three essential features throughout the future power system:

visibility, controllability, and flexibility

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power electronics playing a key role

The power output of some of renewable energy sources is always DC (for example, photovoltaic systems) and an inverter is needed to interface them to the AC grid.

Renewable energy sources using an AC generator (for example, wind turbines) can be connected directly to the grid, often some form of AC to DC and then DC to AC conversion is used.

Some power system operating conditions demand rapid independent control of the active and reactive power output of the renewable energy generators. These control actions can only be achieved conveniently using a power electronic interface.

With the connection of a large number of distributed generators, including micro-generators, traditional methods of active power/frequency control and reactive power/voltage control will no longer be effective.

In these circumstances, a fast acting reactive power source such as a distribution STATCOM is useful, particularly for inductive circuits.

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۶ U.S. Electricity Generation from Various Renewable Sources, 2009

Renewable energy generation

Renewable energy generation

Renewable energy sources are being developed in many countries to reduce CO2 emissions and provide sustainable electrical power.

The balance of particular technologies and their scale changes from country to country.

hydro, wind, biomass (solid biomass, bioliquids and biogas), tidal stream, and photovoltaic (PV) are common choices.

Variable speed turbines are used for wind, small hydro and tidal power generation. These generally use AC–DC–AC power conversion where the turbine is arranged to rotate at optimum speed to extract the maximum power from the fluid flow or minimise mechanical loads on the turbine. ۷

Renewable energy generation

The power electronic interface between a renewable energy source and the grid can be used to control reactive power output network voltage curtailing real power output

and so enable the generator to respond to the requirements of the grid.`

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Renewable energy generation

Photovoltaic systems Wind Hydro Tidal energy systems

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Photovoltaic systems

The PV module contains a number of photovoltaic cells connected in series and in parallel.

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Typical current/voltage and power/voltage characteristics of a PV module for irradiance of 1000 W/m2 and 500 W/m2

The maximum power output of the module is obtained near the knee of its voltage/current characteristic.

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Different configurations of DC–DC converters are used: boost, push–pull, full bridge, flyback converter

The DC voltage on the inverter side of the DC–DC converter is

normally maintained to be constant by the inverter control.

The MPPT algorithm is used to find continually a PV array DC voltage which extracts the most power from the PV array while the cell temperatures and operating conditions of the module change. In this method, the terminal voltage of the PV array is perturbed in one direction and if the power from the PV array increases, then the operating voltage is further perturbed in the same direction.

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Photovoltaic systems

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IRAN Photovoltaic Solar Resource

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U.S. Photovoltaic Solar Resource

2.2 - Wind, hydro and tidal energy systems

Wind, hydro and tidal generation systems all involve converting the potential and/or kinetic energy in water or air into electrical energy.

In recent years there has been a dramatic increase in power generation

from the wind with the capacity of wind turbines that have been installed across the globe now approaching 200 GW.

Hydropower is a mature technology with units varying in size from a few kW to hundreds of MW.

Tidal stream generation is a more recent innovation and the subject of considerable research and development effort.

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Wind Energy in Europe

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Wind Energy in Europe

Wind farms are now being developed both Onshore (on the ground) and offshore (in the sea)

Placing a wind turbine in the sea is more challenging and expensive

but offshore wind farms enjoy a stronger and more consistent wind resource and reduced environmental impact

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Wind, hydro and tidal energy systems

Power electronic converters Control of wind turbines Control of hydro turbines Control of tidal stream turbines

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Power electronic converters

For variable speed operation of wind, hydro and tidal stream turbines, Doubly Fed Induction Generators (DFIG) or Full Power Converter (FPC) based generators may be used.

The DFIG has a wound rotor induction machine where the rotor is connected to back-to-back power electronic converters.

The four quadrant converters control both active and reactive power flow to and from the rotor circuit.

The rotor speed can be changed by absorbing or injecting active power by the rotor side converter.

In the FPC generator, a diode bridge or a four-quadrant converter is connected to the stator terminals of the generator.

The FPC configuration also allows operation without a gearbox.

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Different variable speed generator configurations

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Control of wind turbines

A variable speed wind turbine uses back-to-back Voltage Source Converters (VSC) to control the rotor speed so that it can maintain maximum aerodynamic efficiency at varying wind speeds.

Variable speed wind turbines can be controlled to support the grid

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Variable speed wind turbine operating regions Note: Power base is 2 MW and speed base is 1800 rev/min

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Control of hydro turbines

The design of a hydro turbine is optimised for a defined rotational speed, hydraulic head and discharge.

As the hydraulic conditions change, the conversion efficiency of the turbine also changes.

Variable speed operation changes the turbine speed so as to maximise its efficiency over a range of different hydraulic conditions.

The hill chart gives the efficiency curve of the turbine for different flow rates as the rotor speed changes.

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Control of tidal stream turbines

In tidal stream devices, in order to extract maximum power, the torque presented by the generator to the prime mover varies with the tidal flow conditions.

One possible control approach is to use a hill climbing technique, as described for PV systems.

Alternatively, a control concept similar to that used for wind turbines may be employed for tidal energy converters where the power extracted is determined by off-line calculations of the rotor angular velocity relative to the tidal stream flow and the creation of a look-up table.

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3- Fault current limiting

There will also be a large number of distributed generators that are connected through power electronic converters, whose fault current contribution is limited by the rating of the electronic switches they employ and their control system.

The connection of different types of distribution generation will introduce issues such as:

1. In some circuits, the synchronous generators will increase the fault current through switchgear (which has a limited interruption capability), thus requiring replacement of the switchgear or other measures to limit the short circuit current.

2. In some other circuits, which are rich in power electronic connected distributed generators, the fault current contribution may not be adequate to ensure detection of the fault by existing over-current protection, thus demanding different protection methods.

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4 - Shunt compensation

shunt compensation devices based on voltage source inverters such as STATCOMs, active filters and Voltage Source Converters with Energy Storage (VSC-ES) have begun to be used in the power system.

STATCOMs are used to provide reactive power compensation in both transmission and distribution circuits in order to manage network voltages, reduce losses and overcome possible instabilities.

Mitigating these voltage fluctuations can be effected with shunt compensation devices such as a STATCOM or a VSC-ES that can vary Q or P and Q with the change of voltage.

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Shunt compensation

4.1 D-STATCOM 4.2 Active filtering 4.3 Shunt compensator with energy storage

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Operation of a D-STATCOM

• The STATCOM is the power electronic counterpart of the traditional rotating synchronous condenser.

• A STATCOM connected to the distribution circuits is normally called a

D-STATCOM.

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Two main control approaches for STATCOM control can be found in the literature.

phase shifting control

control approach using PWM

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phase shifting control

When VSTATCOM slightly leads Vterminal, net real power flows from the D-STATCOM to the AC system.

This in turn decreases the DC capacitor voltage and thus VSTATCOM.

Then reactive power is absorbed by the STATCOM.

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control approach using PWM

• This regulates the modulation index (ma) and phase angle (ϕ) of the inverter.

• The d-axis and q-axis vector components of the injected current

(ISTATCOM) are calculated by taking the d-axis aligned with the terminal voltage vector (Vterminal).

• The current component Id_STATCOM controls the DC-link capacitor voltage

and the regulation of Iq_STATCOM provides the reactive power that should be injected or absorbed by the D-STATCOM.

۳۱ A decoupled current control method for D-STATCOM

4.1- D-STATCOM 4.1.1- Load compensation 4.1.2 - Voltage control

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4.1.1- Load compensation

Two compensation techniques are discussed here:

1. Power factor correction of a balanced three-phase load 2. Power factor correction and balancing of an unbalanced three-phase load

۳۳ Load compensation

4.1.2- Voltage control

• A control system that is used with thyristor-based static Var compensators may be used with a D-STATCOM for voltage control.

• In a case where the voltage VT changes due to a variability of renewable energy source connected to that busbar, the D-STATCOM could supply or absorb reactive power to minimise fluctuations in the voltage .

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Figure 18 Closed loop control for voltage regulation

4.2- Active filtering

A shunt active filter consists of a PWM-controlled current or voltage source converter.

It injects the harmonic currents absorbed by the non-linear load such as an arc furnace, a rectifier-fed load (motor, heater) or a thyristor control motor drive.

۳۵ Shunt active filter

• The harmonic current drawn by a thyristor-controlled DC motor drive load

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Load and compensating current for a thyristor control DC motor drive. Note: All the values in the y axis are in pu

4.3- Shunt compensator with energy storage

• With recent advances in energy storage technology, the application of a VSC-ES has now become a feasible option for steady state voltage control and elimination of power system disturbances.

• The VSC-ES can be controlled to exchange both real and reactive power with the AC system.

• As with a D-STATCOM, the reactive power generation or absorption

capability of the VSCES can be used for load compensation or steady state and transient voltage control.

• The active power generation or absorption capability of the VSC-ES can be

used to enhance steady state and transient voltage control, to eliminate voltage sags and to damp power oscillations.

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The applications of VSC-ES include: 1. Load compensation: In many applications, the VSC-ES is operated as a

load compensator where it fully supplies the load reactive power requirement in the steady state, thus maintaining the load power factor near unity.

2. Steady state voltage control: The VSC-ES can be used for steady state voltage control plus mitigation of other system disturbances.

3. Sag mitigation: The application of a shunt device for mitigation of voltage

sags has added advantages when compared to that of a series device, as the shunt devices can simultaneously be used for steady state voltage control, power oscillation damping and as a back-up power source.

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۳۹ Experimental result showing sag minimisation of using a VSC-ES. Note: For clarity only the positive half of the voltage is shown

5- Series compensation

• he connection of a DVR to a distribution feeder is shown in Figure 24.

• If the incoming feeder voltage fluctuates beyond the voltages that a sensitive load could operate, then the DVR adds a voltage in series to compensate for voltage fluctuation.

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Figure 24 Application of a DVR to sensitive load

• Two commonly used compensation techniques are ‘in-phase compensation’ and ‘freeze PLL compensation’

• As shown in Figure 25a, the ‘in-phase compensation’ technique keeps the load voltage phasor always in-phase with the supply voltage.

• In the ‘freeze PLL compensation’ technique the DVR maintains the load voltage magnitude as the same as the pre-sag condition by injecting the difference between the pre-sag supply voltage and the sagged supply voltage

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Thank you

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