Rezolvare Subiecte Mari

8/13/2019 Rezolvare Subiecte Mari

1/30

1

1. Distributed generation (DG) and renewable energy sources (RES).1

2. Integration and interconnection of DG in classical networks2

3. Wind energy conversion systems..3

4. Wind turbine concepts (Type A-Type D) .4

5. Wind turbine power limitation..5

6. Solarphotovoltaic (PV) systems6

7. PV grid-connected systems...7

8. PV stand-alone systems.8-9

9. Small-scale hydroelectric power generation.9

10. Micro-hydro plants for autonomous operation and for grid connected

operation.10 -12

11. Combined heat and power (CHP) systems13

12. The concept of microgrid.14-15

13. Basic types of power quality disturbances...15-17

14. Basics of instantaneous power theory..18

15. Rotating dq reference frame19-20

16. Synchronous reference frame (dq) control of a grid-tied inverter...20-21

17. Voltage control in microgrids..22-23

18. Frequency control in microgrids..24-25

19. Power-frequency control on the electrical side of the generators25-26

20. The concept of smaft grid.27-28

21. Protection of a Microgrid..29


2/30

2

1. Distributed generation (DG) and renewable energy sources (RES)Distributed generation (DG) refers to non-conventional/renewable energy sources

like: natural gas, biogas, wind power, solar photovoltaic cells, fuel cells, combined

heat and power systems, microturbines etc.The term renewable energy sources (RES) refer to everlasting natural energy

sources such as the sun and the wind.RES include:

Hydro power (large and small); Biomass (solids, biofuels, landfill gas, sewage treatment plant gas and biogas);

Wind;

Solar (photovoltaic, thermal electric); Geothermal;

Wave and tidal energy;

Biodegradable waste.The main cost items are the initial investments, fuel costs, energy prices (electricityand heat) and the cost of connecting to the grid. The viability of DG and RES

depends largely on regulations and stimulation measures which are a matter of EU

and national political decisions.General attributes of DG

- Not centrally planned and mostly operated by independent power producers or

consumers;- Not centrally dispatched (although development of virtual power plants, where

many decentralized DG units are operated as one single unit, infringes on this

definition)- Smaller than 50 MW (although some sources consider certain systems up to 300

MW to be classed as DG);- Connected to the electricity distribution network which, although it may vary by

country, generally refers to the part of the network that has an operating voltage

from 230/400 V up to 110 kV.

Advantages and disadvantages of DG and RES-reduction of environmental pollution and global warming acts as a key factor in

preferring RES over fossil fuels

-the efficient use of the heat that is always generated when electricity is generated-disadvantages of DG are the costs of connection, metering and power balancing

-the major drawback is the initial investment, which is larger than for non-RES

systems-other disadvantages of RES are the specific requirements of the site and the

unpredictability of the power generated


3/30

3

2. Integration and interconnection of DG in classical networksIntegrating DG into an existing infrastructure (grid) involves first an analysis of its

impact on the supply line and interaction with consumers. The DG success relies

on a large manner on control and communication systems.The classical model of energy production contains a well defined chain for energy

production, transport and distribution. Nowadays this model is changing from one-directional central delivered power generation to bi-directional DG network.

Smaller units can be directly connected to the low voltage network, while larger

units require a transformer.

There are 3 different DG types: synchronous generator, asynchronous (induction)generator, and inverter. The first two types represent traditional technology based

on rotating electrical machines.

The last type refers to modern power electronic converters. From theinterconnection point of view, these three types have different impacts on thedistribution network.

Impacts of a distribution system with a large amount of DG:-Voltage profiles change along the network, depending on the power produced on

the consumption levels, leading to a behavior different from the typical one;

-Voltage transients will appear as a result of connection and disconnection ofgenerators or even as a result of their operation;

-Short circuit levels increase;

-Losses changes as a function of the production and load levels;-Power quality and reliability may be affected;

-Utility protection need to be coordinated with the ones installed in the generator'sside

The power systems migration towards DG brings some advantages, in terms of

power autonomy and security for the consumers located in the operating area ofthese generating units.


4/30

4

3. Wind energy conversion systems (WECS)WECS convert wind energy into electrical energy:Wind energy -> mechanical rotational energy -> electrical energy

The principal component of WECS is the wind turbine (WT). WT rotor is coupled

to the generator through a multiple-ratio gearbox or, gearless in small powerapplications.Usually induction generators, (squirrel-cage (SCIG) or doubly-fed (DFIG)), or

permanent magnet synchronous generators (PMSG) are used in WECS.

A wind turbine has three major components: the tower, the rotor and the nacelle.Generally, rotor may have two or three blades.

For MW-range wind turbines, the rotational speed is typically 10-15 rpm,

increasing with the decrease of power, up to 400rpm for kW range.The usual wind speed domain is 4 to 14 m/s corresponding to minimum and rated

output power. They can operate up to a wind speed of 25m/s, after that they willstop operating.

The main parts of a WECS include: Wind power rotor (two-, three-blade);

Gearbox (optional - gearless); Generator (SCIG, DFIG, WRIG, PMSG);

Power converter (optional);

Power transformer;

All the active components are placed in the nacelle.


5/30

5

4. Wind turbine conceptsFixed Speed Wind Turbines (Type A)

This concept needs a reactive power compensator to reduce (almost eliminate) the

reactive power demand from the turbine generators to the grid.Smoother grid connection occurs by incorporating a soft-starter, based on

thyristors.In a fixed speed wind turbine, the wind fluctuations are converted into mechanical

fluctuations and further into electrical power fluctuations.The main drawbacks are: does not support any speed control, requires a stiff grid

and its mechanical construction must be able to support high mechanical stress

caused by wind gusts.

Partial Variable Speed Wind Turbine with Variable Rotor Resistance (Type B)

The rotor winding is connected in series with a controlled resistance, whose sizedefines the range of the variable speed(typically 0-10% above synchronous speed).The energy coming from the external power conversion unit is dumped as heat

loss, this being a major disadvantage.

Variable Speed WT with partial-scale frequency converter (Type C)

This configuration, known as the doubly-fed induction generator (DFIG) concept,

corresponds to the variable speed controlled wind turbine with a wound rotorinduction generator (WRIG) and partialscale frequency converter (approx. 30% of

nominal generator power);

Speed range: typically 30% around synchronous speedThe converter performs the reactive power compensation and a smooth grid

connection;The smaller frequency converter makes this concept attractive from an economical

point of view.

Its main drawbacks are the use of slip-rings and the protection schemes in the case

of grid faults.

Variable Speed Wind Turbine with Full-scale Power Converter (Type D)

This configuration corresponds to the full variable speed controlled wind turbine,with the generator connected to the grid through a full scale frequency converter;

The frequency converter performs the reactive power compensation and a smooth

grid connection for the entire speed range.The generator can be SCIG or PMSG; Some variable speed wind turbines systems

are gearless. In these cases, a direct driven multi-pole generator is used.


6/30

6

5. Wind turbine power limitationThe power limitation may be done by:

stall control (the blade position is fixed but stall of the wind appears along the

blade at higher wind speed)turbulent wind flow;active stall (the blade angle is adjusted in order to create stall along the blades);

pitch control (the blades are turned out of the wind at higher wind speed)

Turning the rotor into the wind (Yawing)For maximum power extraction from the wind, the rotor has to be aligned with thewind stream direction. Turning the rotor into the wind is calledyawing.

WT up to 10m diameter may be yawed into the wind passively by using tail vanes.

For larger wind turbines this methods is no longer feasible as the tail vane wouldbe too large. Instead, electronic or hydraulic motors are used to turn the rotor

(nacelle) into the windactive yaw. These are called yaw drives.

A wind vane attached on the back of the nacelle is used to check the wind directionand the WT controller acts the yaw mechanism.Turning the rotor out of the winds (Furling)

After a certain wind velocity, (25 m/s) the wind turbine is turned off;

Small wind turbines (kW range) can still operate at maximum power, up to 40m/swind speed, but require some mechanical control systems to reduce their output

power and the rotational speed.

Changing the angle of the oncoming air stream by turning the nacelle out of thewind is known asfurling.

kW range WT use passive furling methods to turn the blades out of the wind either

horizontally or vertically. They use spring-based mechanisms that at a certain windspeed triggers and deviates the rotor; Large wind turbines use complex

mechanisms to shut down in storm condition using the yaw drive and brakes.

Typical power curve

of a 1500kW pitch

regulated windturbine with a cut-

out speed of 25 m/s


7/30

7

6. Solarphotovoltaic (PV) systemsSolar PV generation involves the generation of electricity from free andinexhaustible solar energy.

The major advantages of a PV systems are:

-sustainable nature of solar energy as fuel;-minimum environmental impact;

-drastic reduction in customers electricity bills due to free availabilityof sunlight;-long functional lifetime of over 30 years with minimum maintenance;

-silent operationno sound pollution (no moving parts)

The major disadvantages of PV systems are:

-Initial cost. According to the latest forecast, in the next years the costs of PV

panels will reach to 1EUR/kWthe other components of a PV plant (inverter,infrastructure) are not included.

-Solar cells produce DC which must be converted to AC (using a grid-tie inverter)

when used in currently existing distribution grids => an energy loss of 4-12%.-The energy conversion efficiency is up to 22% (the latest technology), but usuallyunder 15%;

-Limited power density: approx. 1000W/m2it strongly depends of the location.

-Solar electricity is not available at night and is less available in cloudy weatherconditions. Therefore, a storage or complementary power system is required.

-Solar electricity is almost always more expensive than electricity generated by

other sources.A PV system consists in:

PV panels that convert the solar power into DC electrical power

power converter that transforms the DC power into AC power.A single PV panel is made of multiple cells connected in series and parallel on a

solid frame. Generally, one PV module has a rated power of 100200W.The modules are connected in series and parallel to obtain a certain output voltage

and power. PV panel orientation can be fixed at an optimal angle according to the

location (most used), or variable using a sun trackers (electric or hydraulic).

Basics of PV energy conversionPV cell converts sunlight directly into electricity.

It is made of semi-conducting material in two layers: P and N

When radiation from the sun hits the photovoltaic cell, the boundary between Pand N acts as a diode: electrons can move from N to P, but not the other way

around. Photons with sufficient energy hitting the cell cause electrons to move

from the P layer into the N layer.An excess of electrons builds up in the N layer while the P layer builds up a deficit.

The difference in the amount of electrons is the voltage difference, which can beused as a power source.


8/30

8

7. PV grid-connected systemsThe two main components of a PV system connected to the grid are: PV panels andDC-AC converter (inverter).

PV converter classification:

A.with DC-DC converter with isolation on the low-frequency side (type 1);

on the high-frequency side (type 2); without isolation - transformerless (type 3);

B.without DC-DC converter with isolation (type 4)

without isolation - transformerless (type 5)

Type 1

Type 2

Type 3

Type 4

Type 5


9/30

9

8. PV stand-alone systemsIn islanded mode PV power plants feed local consumers with electrical energy.Due to the solar energy intermittent nature, storage devices have to be used in

conjunction with PV plants in order to achieve continuous supply of the loads.

Moreover, most times PV is part of hybrid power systems, where several energysources are used. Wind and solar with energy storage is the most spread

configuration because of the sources complementarily (sun in the day time and inthe summer, wind in the night time and in the winter).

The first configurationis the simplest, but with the lowest performance.

The PV and battery are connected

on a common DC-bus that suppliesDC loads and the inverter.

A power management system

ensures proper charging/dischargingconditions for the battery, byswitching on/off the PV/inverter/DC

loads in case of over-charging and

over-discharging.The main disadvantage is that the

maximum PV power cannot be

extracted, because it is directlyconnected on the battery thus, the

battery imposes the PV voltage.

The second

configurationincludesin addition a DC-DC

converter in series with

the PV, which acts as

battery chargecontroller and MPPT,

thus extracting

maximum power fromthe PV. The system

performance are

improved, but with thecost of an additional

power converter.


10/30

10

In the thirdconfigurationboth thebattery and the PV are

connected in the system

trough DC-AC convertersand the power exchange

is done in the AC bus.It is the most flexible

configuration and issuitable for higher power

range (multi-kW), but it

is more complex.

9. Small-scale hydroelectric power generationClassification in Romania:-SHP (small-scale hydropower plants) installed capacity from 200 up to 3600 kW;-MHP (micro hydropower plants) with installed capacity from 20 up to 200kW;

-AHP (artizanal hydropower plants) with installed capacity below 20kW

The mechanical power extracted from a hydro turbine is :

Pm=nT **g*Q*H ,where:

Pmmechanical output power of the turbine [W];nT- hydraulic efficiency of the turbine;

- water density, 1000 kg/m3; gacceleration of gravity,g=9.81 m/s2;

Qwater flow [m3/s]; Hhead [m] (effective pressure of water flowing into theturbine).

The equation indicates that the power output can be increased by increasing botheffective head and water flow rate. The electrical power produced by the generator

is: Pe=nG*Pm ,where:

Peelectrical output power of the generator [W];

nG - efficiency of the generator.The efficiency of large hydropower units reaches the level of 80 - 90%. The

efficiency of smaller hydro units (


11/30

11

10.Micro-hydro plants for autonomous operation and for grid connectedoperation

In autonomous (island) mode MHP ensures the control of voltage ( 10%) and

frequency ( 1%).

Synchronous generator (SG)controls its output voltage through the field windingside. Induction generator (IG)controls cannot control directly its output voltage;

it requires an external reactive power source, like capacitor banks (fixed orvariable) or active compensator (based on power converters).

The frequency can be controlled by two methods:-using a mechanical speed controller that acts over the gate of the hydro turbine to

regulate the mechanical power;

-using an electronic load controller (dump load) on electrical side, while theturbine operate at full power.

Speed control on mechanical side


12/30

12

Speed control on electrical side

Grid connected MHP with SG

The SG requires a precise synchronization process, when connected to the grid.

The SG RMS voltage (V), frequency (f) and phase () have to match the gridparameters with

a tight error.

The PF is

modified by

adjusting thefield windingcurrent.


13/30

13

Grid connected MHP with IG

The IG is much easier to connect to a grid, the only condition that has to befulfilled is the speed of the IG shaft to be equal, with a certain tolerance, to the

synchronous speed. The PF is modified by using a switched-capacitor bank.


14/30

14

11.Combined heat and power (CHP) systemsCogeneration, or CHP, is the simultaneous production of power and heat, with a

view to the practical application of both products.

In this category are included thermal power plants that recover the heat, inevitableproduced in the energy conversion process from fuel to electricity.

A cogeneration unit always consists of the following basic components:-A primary driver in which fuel is converted into motion and heat

-A generator to transform the motion into electricity-A heat recovery system to collect the produced heat

The most important benefits of CHP systems are:

-If all the heat produced can be used on the production site, cogeneration is thecheapest way to produce electricity;

-The use of cogeneration leads to lower emissions to the environment (ex. CO2);

-Local production of electricity can improve the local security of the electricitysupply;-Process by-products (e.g. organic waste) can be used as fuel.

The use of cogeneration leads to an energy efficiency improvement of 15 to 25%.

All cogeneration schemes will always include an electricity generator and a systemto recover the heat.

Cogeneration schemes can have different sizes, ranging from an electrical capacity

of less than 5 kWe (e.g., small engines for a single dwelling) to 500 MWe (e.g.,district heating systems or industrial cogeneration).

The following technologies are currently in widespread use:

-Steam turbines-Gas turbines

-Combined Cycle (gas and steam turbines)-Diesel and Otto Engines.

Three other technologies have recently appeared on the market, and have a great

potential in the developing of future smart grids:

-Micro-turbines-Fuel cells

-Stirling engines.


15/30

15

12.The concept of microgridMicroGrid (MG) concept assumes an aggregation of loads and microsources

operating as a single system providing power and heat. It defines all the

equipments and infrastructure required to operate a small-scale power system.The development of MGs implies the use of hardware (power converters, electrical

machines,storage devices, protection, etc.) and control systems (methods andalgorithms) based on recent technology.

The key feature that makes the MicroGrid possible is the power electronics,intelligent control, and communications capabilities that permit a MicroGrid to

function as a semiautonomous power system.

The power electronics are the critical distinguishing feature of the MicroGrid.The generators or microsources employed in a Microgrid are usually renewable

(non-conventional).

Microgrids can operate independently as autonomous islands or in synchronismwith the main grid.The key differences between a Microgrid and a conventional power system:

-Microsources are of much smaller capacity with respect to the large generators in

conventional power plants.-Power generated at distribution voltage can be directly fed to the utility

distribution network.

-Microsources are normally installed close to the consumers so that theelectrical/heat loads can be efficiently supplied with satisfactory voltage and

frequency profile and negligible line losses.

-From grid point of view, the main advantage of a MG is that it is treated as acontrolled entity within the power system. It can be operated as a single load.

-From customers point of view, MG are beneficial for locallymeeting theirelectrical/heat requirements. They can supply uninterruptible power, improve local

reliability, reduce feeder losses and provide local voltage support.

-From environmental point of view, Microgrids reduce environmental pollution

and global warming through utilization of low-carbon technology.Key issues that are part of the MG structure include the interface, control and

protection requirements for each microsource as well as MicroGrid voltage and

frequency control, power flow control, load sharing during islanding, protection,and stability.

The Microgrid is operated in two modes: (1) grid-connected and (2) standalone

(autonomous). The operation and management of Microgrid in different modes iscontrolled and coordinated through local microsource controllers (MCs) and the

central controller (CC).


16/30

16

The main function of MC is to control the power flow and voltage profile (not

always) of the microsource in response to any disturbance and load changes. MCalso participates in economic generation scheduling, load tracking/management

and demand side management by controlling the storage devices. The CC executes

the overall control of Microgrid operation and protection through the MCs. The CCalso performs protection coordination and provides the power dispatch and voltage

set points for all the MCs.

CC functions in grid-connected mode-Monitoring system diagnostics by collecting information from the microsourcesand loads.

-Economic generation scheduling, active and reactive power control of the

microsources and demand side management functions by using collectedinformation.

-Ensuring synchronized operation with the main grid maintaining the power

exchange at the required level.CC functions in stand-alone mode-Performing active and reactive power control of the microsources in order to

maintain stable voltage and frequency at load ends.

-Adopting load interruption/load shedding strategies using demand sidemanagement with storage device support for maintaining power balance and bus

voltage.

-Switching over the Microgrid to grid-connected mode after main grid supply isrestored without affecting the stability of either grid.

13.Basic types of power quality disturbancesA.Voltage sags and swells

Voltage sags and swells are defined by variations in the root mean square (RMS)

voltage magnitude from around a half cycle to several seconds.

Sags refer to drops in the voltage while swells refer to voltage rises.

A voltage swell is usually caused by single line-to-ground faults on the systemresulting in a temporary voltage rise on the healthy phases, removal of bulk loads,

switching on a large capacitor bank, etc.

B.Under-voltagesVoltage sags and swells lasting more than 2 minutes are classified as under- and

over-voltage conditions, respectively.

Under-voltage conditions may be caused by sudden loss of lines or transformers,loss of adequate generation or loading a line beyond its capacity leading to low

voltage at the consumers terminals.


17/30

17

Under-voltage conditions may cause overheating in constant speed motors and it

may lead the malfunctioning of electronic equipment.

C.Over-voltages

Over-voltages, on the contrary, may occur due to problems with voltage regulation

capacitors or transmission and distribution transformers.The problems are magnified when the over-voltage protection devices do not

respond fast enough to completely protect all equipment downstream.Over-voltage problems are usually eliminated by installing voltage regulator

devices at key distribution sites within the customers premises.

D.Outage

Outage or voltage interruption refers to the complete loss of voltage over a certain

period of time. Outages may be short term (less than 2 minutes) or long term.These are normally caused by the protection devices (circuit breaker) or by a

physical break in the line.

Critical loads have to be protected against outage by installing UPS systems, or inthe case of autonomous MGs, by placing them near energy storage devices andwith sectional circuit breaker.

E.Harmonic distortion

Harmonic distortion arises when the shape of voltage or current waveform deviatesfrom the standard sinusoid.

Harmonic distortion implies that apart from standard power frequency component,

higher-frequency components are also present in the power flow. The main sourceof current harmonics in a MG operating at low voltage are the nonlinear loads. The

current harmonics produce in weak grids high voltage harmonics.

These components can degrade equipment performance and may even causedamage to it. Some possible problems caused by harmonics are overheating of

distribution transformers, disrupting normal operation of electronic equipment andsystem resonance with power factor correction banks.

F. Electrical noise

Electrical noise is defined as a form of electromagnetic interference (EMI) caused

by high-frequency, low-voltage signals superimposed on the standard signal in aline. Frequencies of these signals may vary from the range of kilohertz to

megahertz while magnitudes may be up to 20 V.

It arises from a variety of natural and artificial sources like lightning, staticelectricity, presence of power frequency transmission lines in the vicinity,

automobile ignition, high frequency switching in power electronics devices and

fluorescent lamps.Equipment sensitive to noises are computers, industrial process controls, electronic

test equipment, biomedical instruments, communications media, etc.


18/30

18

The impact of noise may be reduced by installing radio frequency line filters,

capacitors or inductors at the equipment level.

G. Transients

Transients are sub-cycle voltage disturbances in the form of very fast voltage

change. Transients are caused by the injection of energy due to lightning,electrostatic discharge, load switching, line switching, energizing of a capacitor

bank or interruption of an inductive load.Transients can have magnitudes of several thousand volts and so can cause serious

damage to both the installation and the equipment connected to it.Transients generated from direct lightning strokes have the greatest potential for

damaging the utility- or customer-end equipment. Transients arising from witching

of power factor correction capacitors or from bulk load transfer switching mayconsiderably hamper normal system operation.

Transients may be eliminated by installing lightning arrestor (suppressor) systems.

H. Frequency variationsFrequency variations are specific mainly for autonomous MGs. They are caused bythe active power unbalance between generation and consumption.

Large frequency variations can cause abnormal operation of induction motors, and

classical generators installed in the microgrid, or even blackouts of the system.Power electronic based microsources are much more immune to large frequency

variations.

The MG PF controllers are responsible of eliminating the frequency variations asfast as possible. Energy storage devices have a great role in this process.

I.Voltage notching

Line notches typically occur in the waveform during SCR (silicon-controlledrectifiers) commutation. This appears as a notch in the voltage waveform.

The most severe and damaging form of notch is the one that touches the voltagezero axes. They are more acute in the weak grids, with high lines impedance.

The types of equipment that frequently use SCR control schemes and experience

notching include DC motor speed controls and induction heating equipment.

J. Flicker

Flicker is defined as a modulation of the voltage waveform at frequencies below 25

Hz, detected by the human eye as variation in light output from standard bulbs.

Voltage flicker is normally caused by arcing on the power system from weldingmachines or electric arc furnaces.

Flicker problems can be eliminated by installing filters, static VAR compensators.

In MGs with high penetration of wind-turbines, flicker can be caused byfluctuating wind power generation (wind gusts) and because of the tower shadow

effect. The effect is more acute in the case of fixed-speed wind turbines, which areusually equipped with induction generators directly connected to the grid.


19/30

19

14.Basics of instantaneous power theoryThis theory known as Nabae and Akagip-q theory, provides theoretical basis forcontrol algorithms of switching converters, but also became the method of

describing power properties of three-phase circuits (e.g. microgrid).

Thep-q theory is based on a set of instantaneous powers defined in the timedomain.

The description of power properties of electric circuits, using instantaneous voltageand current values, in time domain without the use of Fourier series, explains the

interest in this concept. Therefore, this concept can be easily used forimplementing numerical algorithms (used in DSPs) for power converters control.

The instantaneous powers of p-q theory

Three instantaneous powers are defined from the instantaneous voltages andcurrents in the 0 :

- the instantaneous real power: p = u*i+ u*i;

- the instantaneous imaginary power: q = u*iu*i- the instantaneous zero-sequence power: p0= u0*i0In matrix form:

The three powercomponents can be

split into average ( - )

and oscillating (~)components:

For a sinusoidal system the relation between the conventional and the

instantaneous concepts of active and reactive powers are:

P = 3UI cos=p(average)

Q = 3UI sin= q(average)In the presence of harmonics, besidesP and Q, the average powersp , q may

include other components produced by the harmonics.

It can be seen that the total energy flow per unit time, that is, the three-phaseinstantaneous active power is always equal to the sum of the real power and the

zero-sequence power ( p3f=p+p0) and may contain average and oscillating parts.

The imaginary power q, represents the energy quantity that is being exchangedbetween the phases of the system. This means that the imaginary power does not

contribute do energy transfer between the source and the load at any time.


20/30

20

15.Rotating dq reference frameIf we consider a rotating

orthogonal reference frame, dq:

the three-phase voltages (ua, ub,

uc) and currents (ia, ib, ic) can

be transformed into orthogonal

rotating dq coordinates (ud, uq)and (id,iq), by using the

following expressions (for

systems without zerocomponent):


21/30

21

For sinusoidal and balanced three-phase systems, the dq components of three-

phase voltage and/or currents are constant, if the rotating reference frame has thepulsation equal to the pulsation of the sinusoidal signals(=s):

16.Synchronous reference frame (dq) control of a grid-tied inverterThe synchronous reference frame control method, known as dq control, uses a

transformation between the natural abc reference frame to a artificial referenceframe that rotates synchronously with the grid voltage.

In this way, the control variables become DC values and filtering and controllingcan be easily achieved by using classical PI controllers.

The d-axis and q-axis current components are extracted through an abc to dqtransformation, then compared with the corresponding reference signals that are

specified by the external power or voltage control loops.The error signals are applied to a dq current control block to determine the d and q-

component of the reference voltage signals Ud and Uq.

Finally, through a transformation from dq to abc, the three-phase reference signalsfor the PWM signal generator are determined.Details of the internal and external control blocks can vary based on the control

modes and the type of primary source.


22/30

22

The synchronization with the grid voltage (aligning the grid voltage vector withone of the d or q axis) is accomplished with a Phase Locked Loop (PLL), which

tracks the grid voltage phase. The PLL performances affect the entire control loopdynamic behavior and the quality of the injected power into the MG.

The current control scheme includes two PI (proportional-integral) controllers for

each of the two axis (d, q), the voltage feed-forward terms, and the cross-couplingelimination terms.

The PI controllers ensure zero error betweenId, Iq andIdref, Iqrefby the integralaction. The reference currents come from the active and reactive power references.

Thus,Idref is proportional with the output active power, whileIqref is proportional

with the output reactive power:

In the case of grid-feeding inverter, usual the reactive power is set to 0 (Qref= 0).The way the active power reference (Pref) is generated depends of the primary

source type and the DC-side converter topology.

In the case of RES (wind turbines, PV) the objective is to maximize of the

extracted power. The maximum power point tracker (MPPT) does this task.

The outputs Ud and Uq are transformed from dq reference frame to abc reference

frame using the reference angle q provided by the PLL. After transformation thesignals are fed into a PWM signal generator that provides the PWM pulses for the

transistors.

One of the main features of this current control strategy is its inherent

capability to limit the output current during a fault in the MG, and thus

providing overcurrent protection.


23/30

23

17.Voltage control in microgridsVoltage control in a power system is closely related with the reactive power flow.

Basically, the injection of reactive power (capacitive) will lead to the increase of

voltage, while the absorption of reactive power (inductive) will decrease thevoltage.

For maintaining a stable voltage in the grid, the reactive power in the system has tosatisfy the condition: Qsource(Un)=Qloads(Un).

The reactive power transmission over long lines will lead to high RI2 and XI2losses. Therefore, the reactive power has to be produced near the place of

consumption, by using special devices dispersed throughout the system.

In MGs, the generators usually operate at the same voltage level as the loads andthe production of reactive power may be done both by generators and by special

static compensators.

In MGs the voltage stability is much more difficult to achieve than in the classicalpower grids, because of the inherently weakness of the grid.Form the point of view of reactive power, the generators inside the MG can be of

the following types:

-consumers (e.g. induction generators);-controllable consumers/generators (synchronous generators and some grid-

supporting inverters);

-zero reactive poweroperating at unity PF (grid feeding inverters).Wind turbines equipped with directly connected induction generators (DFIG,

SCIG) requires an external reactive power source.

Synchronous generatorsSGs can generate or absorb reactive power depending on the excitation.

When overexcited they supply reactive power (capacitive), while whenunderexcited they absorb reactive power (inductive).

The capability to continuously supply or absorb reactive power is, however,

limited by the field current, armature current, and heating limits.

SGs are normally equipped with automatic voltage regulators (AVR) thatcontinuously adjust the excitation so as to control the armature voltage.

AVRs are based on small DC generators, or on power electronics.

Different types of excitation models are defined in IEEERecommended Practicefor Excitation System Models for Power System Stability Studies.

Static load-voltage characteristicsInduction generators and the loads (which are generally inductive in nature)

require the supply of reactive power. Unbalanced reactive power operation resultsinto voltage variations.


24/30

24

Generally,static load voltage characteristics are in the form of:

,where:PL and QLload active and reactive powers;

PLn and QLnnominal values of the load active and reactive powers;Uload voltage;

Unnominal value of the load voltage;nP and nQexponent in active and reactive load-voltage characteristic.

The values of nP and nQ depend upon the nature of load and can vary between 0and 3 for nP and between 0 and 4 for nQ.

Inverters

Grid supporting inverters or grid-forming inverters are another type of generatorsthat are frequently found in MGs. They are able to control both the active and

reactive power at their output terminals.The control of reactive power flow is much more complex than in the case of a

classical synchronous generator, but the analogy between the two can be of a great

help in the analysis of operation in this regime.

Besides the control of reactive power in steady states, there are situations when thesystem demands short pulses of reactive current required by some dynamic loads

in transitory regimes (e.g. start-up of an induction motor/generator).

These regimes can lead to voltage sags and swells, which can compromise thenormal operation of some equipments.

The capability of the installed reactive power generators in the MG to regulate the

voltage in these situations is of great importance also.SGs are able to supply overload currents but with relatively low dynamics because

of the high field winding inductance.

Contrary power electronics based generators cannot supply more than the

rated current, unless they are intentionally oversized, but they exhibit very

good dynamics. The costs of oversizing and the additional losses has to be

evaluated. The remaining reserve of reactive power control of an inverter will be

determined firstly by its loading with active power.

Reactive power compensators

They are based mainly on power electronic and help to mitigate the voltagefluctuations for short- (sags ,swells) and long-time (overvoltages, undervoltages).


25/30

25

18.Frequency control in microgridsFor satisfactory operation of a power system, the frequency should remain nearly

constant.

Small variations of frequency do not have major impact on the system, but largefrequency deviations can cause malfunctioning of certain equipments, and even

faults.Relatively close control of frequency ensures constancy of speed of induction and

synchronous motors.In a network, considerable drop in frequency could result in high magnetizing

currents in induction motors and transformers.

The frequency of a system is dependent on active power balance.

Unlike the voltage, which is a local indicator (in a grid the voltage can vary from

one point to another), the frequency is a common factor throughout the system. A

change in active power demand at one point is reflected throughout the system by achange in frequency.Each generator that participate at frequency control in the system, is equipped with

a speed governor to provide the primary speed control function. Because there are

many generators supplying power into the system, they have to be coordinated by acentral control unit, which imposes the loading of the generators.

The control of generation and frequency is commonly referred to as load-frequency

control (LFC).The generators inside a power system can contribute to the frequency control or

they can be passive, without participation to the frequency control.

The generators that contribute to the frequency control need a certain powerreserve. Generally, the rated powers of the generators establish the two types.

Organization of frequency controlIn the classical power

systems, the frequency

control is organized on

three hierarchical levels:


26/30

26

The primary frequency control has to reestablish the frequency within 5-30

seconds after disturbance. The generators measure the frequency and immediatelychange the output power (increasing or decreasing it) through the speed governors

of each unit.

The secondary control is much slower, usually within 15 minutes, and it iscoordinated form a dispatcher. Its aim is to reestablish the power setpoints of the

generating units to the prescribed values. The power reserve of the generating unitsis reestablished, they being ready for a new disturbance.

The tertiary frequency control varies from 15 to 60 minutes, depending on thecountry. It reestablishes the power loading of the generators in the best possible

way, in terms of economic considerations. In this stage, connection of power

stations can be accomplished in order to reestablish the secondary control reserve.

19.Power-frequency control on the electrical side of the generatorsThe classical approach of power-frequency control consists in controlling themechanical power of the generating units in order to match the demand (loads

power);

In autonomous microgrids, where the energy comes mainly from renewablesources, another method is used: the frequency control is accomplished on the

electrical-side of the generators, while the prime movers supplies nominal power;

This means that the active power balance has to be accomplished using dedicateddevices that absorb the excessive power, when the generated power is higher than

the loads power, and supply power in the other case.

The main methods that are used for power-frequency control on the electrical-sideof the generators are:

-energy storage systems (generator/load);-dump loads (load);

-load shedding (under-frequency protection);

The first two systems are called also, electronic load controllers.

Combination of both energy storage and dump loads can be used.

Energy storage systems

Energy storage systems consist in the energy storage device (chemical batteries,flywheel) and a power electronic converter. Its main purpose is to transfer active

power in both directions, acting as load or as source.

The frequency controller provides the amount of energy to be transferred to/fromthe microgrid.

The control method can be either isochronous or droop-mode.


27/30

27

Dump loads

Dump loads are used in the frequency control system of autonomous MGs fordissipating the excess of energy produced by the RES generators.

The thermal energy can be used for water or space heating.

It consists in a power electronic converter and one or several dumping resistances.The power electronic converter distinguishes the different types of dump loads.

The amount of power to be dissipated is given by the frequency controller,according to the frequency deviation.

The control method can be either isochronous or droop-mode.

Load shedding

Load shedding is a method to protect the microgrid in the underfrequencyconditions.

It is not a method to control the frequency in normal operation conditions.

Under-frequency or frequency decay appears when the loads active power demandexceeds the capacity of the running generators and storage devices, at a certainmoment.

Heavy loads connection and loss of generation are the main factors that lead to

these conditions.Load shedding is accomplished in a hierarchical way. The common loads are the

first to be disconnected, while critical loads are on the top of hierarchical load

structure.


28/30

28

20.The concept of smart gridThe existing electricity grid is unidirectional in nature. It converts only one-third of

the fuel energy into electricity, without recovering the waste heat.

Almost 8% of its output is lost along its transmission lines, while 20% of itsgeneration capacity exists to meet peak demand (it is in use only 5% of the time).

The next-generation electricity grid, known as the smart grid orintelligentgrid, is expected to address the major shortcomings ofthe existing grid.

The smart grid is required to be self-healing and flexible to system anomalies.

Communication and data management play an important role in the

development of smart grids.

The existing grid

The basic topology of

the existing electricalpower system ispractically unchanged.

Since its inception,

the power industry hasoperated with clear

demarcations between

the generation,transmission and

distribution

subsystems.The existing electricity grid is a strictly hierarchical system in which power plants

at the top of the chain ensure power delivery to costumers loads at the bottom ofthe chain.

The source has no real-time information about the service parameters of the

termination points.

The grid has to withstand maximum anticipated peak demand and since the peakdemand is an infrequent occurrence, the system is inherently inefficient.

Moreover, an unprecedented rise in demand for electrical power has decreased

system stability. With the safe margins exhausted, any unexpected surge in demandcan trigger catastrophic blackouts.

To facilitate troubleshooting and upkeep of the expensive upstream assets, the

utility companies have introduced various levels of command-and-controlfunctions like supervisory control and data acquisition (SCADA).

Although such systems give utility companies limited control over their upstreamfunctions, the distribution network remains outside their real-time control.


29/30

29

Smart grid evolution

Given the fact that nearly 90% of all power outages and disturbances have theirroots in the distribution network, the move towards the smart grid has to start at the

bottom of the chain, in the distribution system.

The metering side of the distribution system has been the focus of most recentinfrastructure investments. The automated meter reading (AMR) systems let

utilities read the consumption records, alarms, and status from costumersremotely.

However, AMR does not address the major issue the utility companies need tosolve: demand-side management. Due to its one-way communication system,

AMRs capability isrestricted to reading meter data. It does not let utilities take

corrective actions based on the information received from the meters.Therefore, AMR systems do not allow the transition to the smart grid, where the

control at all levels is a basic premise.

The advanced metering infrastructure (AMI) provides utilities with a two-waycommunication system to the meter, as well as the ability to modify costumersservice-level parameters.

Through AMI, utilities can meet their basic targets for load management.

Besides the capability of getting instantaneous information about individualdemand, AMI can also interact with special smart device controllers. AMI, will

provide consumers with the ability to use electricity more efficiently and provide

utilities with the ability to detect problems on their systems and operate them moreefficientlyultimately improving reliability and saving money for consumers.

Smart appliances are expected to be developed in the following years, which are

able to interact with the smart grid.


30/30

21.Protection of a MicrogridThe basic principles of the electrical protection of a network are:

-Reliability, the ability of the protection to operate correctly. Under the occurrence

of a fault or abnormal condition, the protection must detect the problem quickly inorder to isolate the affected section. The rest of the system should continue in

service and limit the possibility of damage to other equipment.-Selectivity, maintaining continuity of supply by disconnecting the minimum

section of the network necessary to isolate the fault.-Speed, minimum-operating time to clear a fault in order to avoid damage to

equipment and maintain stability.

-Cost, maximum protection at the lowest cost possible.The MG consists mainly of several microsources, energy storage and loads and it

can operate in both grid-connected mode and island mode.

The microsources are usually made of many new technologies: PV, WECS,microturbines, battery energy storage systems or flywheel energy storage systems.They can be interfaced with the MG through classical machines (synchronous or

induction) or through power electronic interfaces (inverters).

The use of power electronic interfaces leads to a series of challenges in the designand operation of the MG, one of them being the protection system.

The ideal protection systems of the MG should possess the following features:

a. Must respond both to distribution system and MG faults;b. For a fault on the main grid, isolate the MG as quickly as possible;

c. For a fault within the MG, isolate the smallest possible section of the radial

feeder carrying the fault to get rid of the fault;d. Effective operation of customers protection.

Point c indicates that if a fault occurs within the MicroGrid, the protection shouldonly isolate the faulted feeder from the MicroGrid;

Point d requires coordination of protection of the MicroGrid with the customers

protection.

The protection of the MicroGrid should follow the same principles as the electricalprotection in the conventional network.

If a fault is on the main network, the desired response may be to isolate the MGfrom the main network as rapidly as necessary to protect the MG loads. If a fault is

within the MG the protection system may only isolate the smallest possible faulted

section of the MG to eliminate the fault.

Documents

Rezolvare Subiecte Mari