O que é uma rede elétrica?

Prévia do material em texto

08/10/2014
1
What is an Electricity Grid?
Kevin Davis
© KDavis CIT 2014 1
References
1. Renewable Energy in Power Systems by Leon 
Freris & David Infield (Wiley)
2. Electrical Machines, Drives and Power 
Systems by Theodore Wildi, 6th
ed.(Pearson\Prentice Hall)
3. Energy System and Sustainability by Boyle, 
Everett, Ramage Pub: Oxford
4. Integration of Renewable Course Notes by Dr. 
Murray Thomson (Loughborough University Crest 
Centre)
5. WWW.EIRGRID.COM
© KDavis CIT 2014 2
08/10/2014
2
© KDavis CIT 2014 3
Every Household generating their own 
electricity
F = N*P/120
Stand Alone Power Generation
***
What needs to be controlled?
Reference 4
© KDavis CIT 2014 4
Automatic voltage regulator (AVR) 
When a synchronous generator is the only generator 
on a system, or when it is a dominant generator, 
adjusting its excitation current can provide control of 
its main output voltage. 
Normally, a constant output voltage is desirable and 
an automatic voltage regulator (AVR) is installed to 
control the excitation current appropriately. 
V= BUL B=Φ/A I α Φ
Generated 
voltage is 
determined 
by the the
flux density
Flux density 
is just the 
flux per unit 
cross 
sectional 
area
Generated flux 
is proportional 
to the dc 
current in the 
field winding
Reference 4
08/10/2014
3
© KDavis CIT 2014 5
Typical Household electricity demand
Rating of the Generator?
**
Reference 4
Average output Power 
Level
What is an Electricity Grid?
a power system comprises all the apparatus used 
in the generation, transmission and distribution of 
electric energy
An electricity grid is an interconnected system of cables 
and protection equipment which “pools” the POWER 
output of all the operating generating stations and the 
customers use the POWER from this “Pool”.
Each electricity generating station is an independent POWER 
provider.
Each customer is an independent POWER consumer.
© KDavis CIT 2014 6
08/10/2014
4
T1
R2R1
T3T2
R3
G1 G3G2
Tie lines
Circuit 
Breakers
consumers
Power
Plants
© KDavis CIT 2014 7
Power 
In
Power 
Out
Advantages of an interconnected 
System
1.Stability: more reserved power. Can withstand a 
large disturbance
2.Continuity of Service: Allows power flow to 
customers even if one or more power plants has 
a fault or is shut down for maintenance.
3.Economy: The most economical stations can be 
selected to produce the majority of the demand 
and the expensive power plants only used to 
supply peak demand.
© KDavis CIT 2014 8
08/10/2014
5
Running the Grid –(EirGrid)
Transmission System Operator (TSO)
Electricity is essentially a commodity that has to be made 
on demand. Neither the frequency nor the voltage can 
be allowed to stray outside quite strictly defined limits.
There are four tasks for the TSO
• A technical control problem of keeping the grid voltage 
and frequency within tight specified limits
• A more complex problem of keeping supply and 
demand matched at all times
• A need to “keep the lights on “ and to carry out a “black 
start” in the event of a grid failure
• An overall one of optimising resources in the supply of 
electricity
© KDavis CIT 2014 9
Effect of unregulated Voltage 
and Frequency on equipment
Heating and lighting loads 
• insensitive to frequency variations , very sensitive to voltage changes. 
power they consume is proportional to the square of the voltage. 
(overvoltage can lead to overheating .(damage or shortening of life if the 
voltage exceeds the rated value.: Bulb life reduced by 50% for 5% over-
voltage
Induction motors and transformers
• Under-frequency only: causes high currents and over-heating. 
• Under-voltage only : In Motors causes high currents and over-heating
Irish electricity supply range: 230V +/-10% (207 Volts to 253 Volts). 
European Standard EN50160
Frequency range is rarely specified : Operating outside the range of 50Hz (+/-
2Hz) would cause protection equipment to operate.
© KDavis CIT 2014 10Reference 4
08/10/2014
6
T1
R2R1
T3T2
R3
G1 G3G2
Tie lines
Circuit 
Breakers
consumers
Generating
Plants
© KDavis CIT 2014 11
Transmission 
and 
Distribution
Supply
Generation
© KDavis CIT 2014 12
www.eirgrid.com
Transmission 
System
08/10/2014
7
© KDavis CIT 2014 13
www.eirgrid.com
System Voltage Levels
The HV Transmission System comprises 110kV 
/220kV /275kV/400kV lines and stations which 
are used to transport power from the various 
electricity generators to locations where it is 
needed around the country. (think of the 
transmission system as the motorway)
The distribution voltages level are 38,000 Volts 
(38kV), 20,000 Volts (20kV) and 10,000Volts 
(10kV)
http://www.esb.ie/esbnetworks/en/about-us/our_networks/index.jsp
© KDavis CIT 2014 14
08/10/2014
8
© KDavis CIT 2014 15
© KDavis CIT 2014 16
08/10/2014
9
SYSTEM VOLTAGE LEVELS IN THE UK
© KDavis CIT 2014 17
400kV
275kV
132kV
33kV
11kV
Reference 3
Location of Generating Stations
Location based on economics. This can 
mean
1. Near the fuel source (coal mine, gas 
pipe, major river, Wind resource)
2. Near the customer (reduces 
requirement for transmission lines)
© KDavis CIT 2014 18
08/10/2014
10
Type of Generating Stations
Base-Power Stations: 
Deliver full power at all times. (coal fired) Can take 
long time to start up. (Nuclear Power) Or these 
are the most efficient plants
Intermediate Power Stations: 
Respond relatively quickly to changes in demand 
by adding one or more generation units 
(Hydropower)
Peak Generating Stations:
Deliver Power only for brief intervals. Very quickly 
put into service (a few minutes). (Pumped 
storage generation)
© KDavis CIT 2014 19Reference 1
© KDavis CIT 2014 20Reference 2
08/10/2014
11
Controlling the Power Balance
Instantaneous Balance:
Electricity consumed must be immediately 
supplied by generators as there is no storage.
Consider a single power station (Hydro-station). 
The rate of water flowing into the turbine is 
controlled by wicket gates. This effectively 
controls the output mechanical force of the 
turbine, PT. 
© KDavis CIT 2014 21
Water in the Dam
Hydro 
Turbine
Consumers
G
Wicket
gates
Synchronous 
generator
PT
PL
Transmission \Distribution system
Controlling the 
Power Balance-
A Hydropower Generating 
Station
© KDavis CIT 2014 22
08/10/2014
12
Controlling the Power Balance
The electrical load drawn from the generator, PL, 
depends on the consumer demand.
When PT = PL, the generator is said to be in 
dynamic equilibrium and the speed (output 
frequency) remains constant. (Stable System)
If PL increases, PT<PL so the generator starts to 
slow down (frequency decreases)
© KDavis CIT 2014 23
Controlling the Power Balance
If PL decreases, PT>PL so the generator starts to 
speed up (frequency increases).
Using speed regulators (Governors), the speed of 
the generator is monitored and small changes 
(0.02%) lead to an adjustment of the flow rate by 
opening or closing the wicket gates.
System Frequency is an excellent indicator of 
system stability.
© KDavis CIT 2014 24
08/10/2014
13
© KDavis CIT 2014 25Reference 2
Controlling the Power Balance
Longer Term Balance:
Electricity Generating Plants are rated from 4MW 
to 918MW in Ireland. The efficiency of all of 
these plants improves as it operates near it’s 
rated output
The total rated output of all of these plants 
combined is more than twice what is currently 
needed to meet consumer demand.
Need to decidewhich plants are operating and 
which plants are turned off – To do this we need 
to predict consumer demand at any time of the 
day © KDavis CIT 2014 26
08/10/2014
14
Daily Load Variation
Base Station Load
© KDavis CIT 2014 27
MW Night 
Valley
Daily Load Variation
Base Station Load
Intermediate Station Load
© KDavis CIT 2014 28
MW
08/10/2014
15
Daily Load Variation
Peak Station 
Load
© KDavis CIT 2014 29
MW
Intermediate Station Load
Base Station Load
Daily Load Variation –Thursday April 2010
© KDavis CIT 2014 30
4650MW
3990MW
2600MW
MW
08/10/2014
16
Daily Load Variation –Thursday March 2011
© KDavis CIT 2014 31
4650MW
3700MW
3990MW
2600MW
3450MW
2200MW
MW
Power Levels 
in April 2010
Daily Load Variation-Saturday
© KDavis CIT 2014 32
4250MW
3700MW
2550MW
MW
08/10/2014
17
Daily Load Variation- Sunday
© KDavis CIT 2014 33
3950MW
2450MW
3500MW
MW
Wind Energy Generation-Thursday
© KDavis CIT 2014 34
900MW
MW
Blue = actual
Red = forecast
08/10/2014
18
Wind Energy Generation-Friday
© KDavis CIT 2014 35
540MW
MW
Blue = actual
Red = forecast
160MW
Wind Energy Generation-Saturday
© KDavis CIT 2014 36
260MW
MW
20MW
08/10/2014
19
Wind Energy Generation-Sunday
© KDavis CIT 2014 37
680MW
MW
260MW
Daily Load Variation-Christmas Day
© KDavis CIT 2014 38
3700MW
3200MW
2300MW
MW
08/10/2014
20
© KDavis CIT 2014 39
1500
2000
2500
3000
3500
4000
4500
0 5 10 15 20
Sy
st
em
 
De
m
an
d 
[M
W
]
Over a 24 hour Period starting at Midnight
2012 System Demand
Max
Min
Average
MW
© KDavis CIT 2014 40
Reference 4
Minimum Number 
of Generator 
operating to meet 
the Base Load
Number of Generator 
on standby if a Base 
Load Generator fails
Intermediate 
Load 
Generators 
Peak Load 
Generators & 
standby for 
intermediate 
load fails 
08/10/2014
21
Terminology Used for Grid Systems
• Aggregation of Generation
• Scheduling and Dispatch
• Penetration
• Demand Side Management
© KDavis CIT 2014 41
In four year, terminology is extremely important 
when answering exam questions and in 
interviews!!!
© KDavis CIT 2014 42
Aggregation of Demand- Generator Rating
Demand for a single Household
Reference 4
Very short demand peaks 
unlikely to happen at same 
time in all households
08/10/2014
22
Aggregation of Generation- Predictability
“predict and provide”
• Define Aggregation: collection: several things 
grouped together or considered as a whole
• Aggregation of Generation of Renewables ; 
looking at the combined output from 
renewables across the entire grid system
Examples: Wind Farms across the country
• less variable and more predicable than that of a 
single windfarm 
http://www.ewea.org/fileadmin/ewea_documents/document
s/publications/grid/051215_Grid_report.pdf 
© KDavis CIT 2014 43
Reserve, Scheduling & Dispatch
• Efficiency of generators is related to their output 
(maximum efficiency = max load)
• Max efficiency (lowest fuel costs\usage) - most 
generators need to be at full load.
• Reserve: Generators running at light loads, 
capable of increasing their output rapidly if system 
demand increases or generator fails. (security of supply).
• Scheduling: process of deciding and instructing 
which generators to run when and at what 
operating points (aka Unit Commitment)
• Dispatching:Instructing an individual generator 
© KDavis CIT 2014 44
08/10/2014
23
© KDavis CIT 2014 45
January 2011 
see Quarterly report on www.cer.ie
Peak Demand = 75% of Dispatchable Plant
Penetration
• Defined as “The proportion of electricity being 
supplied from renewable sources”
© KDavis CIT 2014 46Reference 4
08/10/2014
24
Penetration
• For low penetration rates of renewables, 
(renewable generation = fossil fuel savings)
• Based on the existing system reserve being able 
to cope with variable demand and variable 
generation.
• As penetration rates increase
(renewable generation > fossil fuel savings)
as system reserve needs to increase (% increase of 
reserve depends on conventional generation technologies 
used)
http://www.ewea.org/fileadmin/ewea_documents/document
s/publications/grid/051215_Grid_report.pdf 
© KDavis CIT 2014 47
Demand Side Management (DSM)
• Involves the management of the quantity of 
electricity required from a grid system at any 
instant. (variable generation from Renewables)
• This is only being tested in small zones around 
the world now.
• It is a departure from the previous philosophy of 
“Predict and Provide”
© KDavis CIT 2014 48
08/10/2014
25
Demand Side Management (DSM)
Components of DSM
– Energy Reduction (using more energy efficient 
devices to achieve the same function; examples 
lighting, “A rated” appliances, variable speed drives)
– Modified Customer behaviour (examples; using off 
peak electricity, smart metering, etc) 
– Deferred Loads (controllable demand that can be 
turned on\off as capacity changes (exampled; 
pumped storage hydro systems)
© KDavis CIT 2014 49
Revision Questions
1. What are the advantages of an interconnected 
power system and what are the tasks of the 
Transmission System Operator?
2. Explain the three types of generating stations 
connected to national grid and how each type is 
used to meet a typical system demand load.
3. What are the main issues when using SEPS as 
generating stations on the national Grid?
4. Using a hydro generation plant, explain what is 
meant by “controlling the power balance”?
© KDavis CIT 2014 50
08/10/2014
26
Revision Questions
5. Outline the difficulty in meeting the daily 
demand for electricity in Ireland from SEPS. 
Include a brief description of Irelands’ electricity 
grid network and a diagram to show the typical 
daily power demand, in your answer.
6. Define what is meant by “Sustainable Energy 
Power Sources” (SEPS). Give examples to 
support your answer. Comment on whether the 
power output from each source can be 
“dispatched” or not.
© KDavis CIT 2014 51
Revision Questions
7. Explain each of the following terms as applied 
to large electricity grids , Aggregation of 
generation from SEPS, Scheduling and 
Dispatch, Penetration , Demand Side 
Management.
8. Explain the selection of sites for electricity 
generation plants
9. What are the effects of unregulated grid voltage 
and frequency on loads connected to a grid 
system
© KDavis CIT 2014 52