Power Quality Theory and Application. To explain the power quality in the electrical system.
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In a powers system network there are many problems related to power quality. So to improve power quality of a system we use different devices such as active power filters. Active power filters are classified into two types that is Shunt Active Power
MS thesis
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Occurrence of a fault in a power system causes transients. To stabilize the system, Power System Stabilizer PSS and Automatic Voltage Regulator AVR are used. Load flow analysis is done to analyze the transients introduced in the system due to the occ
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In this paper a new member of FACTS family Distributed Power Flow Controller has been discussed. DPFC originates from unified power flow controller UPFC . DPFC can be treated as UPFC as both have shunt and series controller beside the main difference
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Recently, power quality has become an important subject and area of research because of its increasing awareness and impacts on the consumers, manufacturers, and utilities. There are a number of economic and reliability issues for satisfactory operat
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Nowadays, usage of non linear loads in power system is increasing. For example, UPS, inverters, converters, etc. These loads make the supply current non sinusoidal and distorted form, which is called harmonics. At this time Active power filters have
Joint MSc in Electrical Engineering (JMEE) Program
6878- Power Quality Quality and and Standards Standards for for Microgr Microgrids ids
Voltage Sags Characterization-
Dr.. Fouad Za Dr Zaro ro
Electric Power System Engineering Palestine Polytechnic University
Sag Magnitude in Non-Radial Systems Local Generat Generators ors •
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Connection of a local l ocal generator generator to a distribution bus. mitigates voltage sags due to faults on the mitigates distribution feeders.
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Sag Magnitude in Non-Radial Systems Equivalent circuit for system with local generation.
Without the local generator the voltage at the equipment terminals would be equal to the voltage at the pcc.
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When a local generat generator or is present, the voltage voltage at the equipment terminals during the sag equals the voltage on the generator generat or bus.
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Where Z1 the source impedance at the pcc; Z2 the impedance between the fault and the pcc; Z3 the impedance between the generator bus and the pcc. Z4 is the impedance of the local generator during the fault (typically the transient impedance); • •
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The voltage on generator bus is related to the voltage at the pcc accor according ding to the follow following ing equation: equation:
The voltage drop at the generat generator or bus is 4 4 + 3 times the voltage drop at the pcc, The voltage drop becomes smaller for –
larger impedance to to the pcc (weake (weakerr connection)
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smaller generation impedance (larger generator) 4
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if the generator delivers delivers 50% of the fault current, a sag down to to 40% at the the pcc (60% voltag voltage e drop) drop) will be reduced to a sag down to 70% (30% voltage drop) drop) at the equipment terminals.
Even a fault Even fault at the pcc pcc will no longer longer cause a sag down to zero voltage voltage but a sag of magnitude mag nitude
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Example •
The industrial system is fed from a 66 kV, 1700 MVA substation via two 66/11 kV transformers in parallel. The fault level at the 11kV bus is 720 MVA, which includes the contribution of two 20 MVA onsite generators with a transient reactance of 17%. The actual industrial load is fed from the 11 kV bus, for which we will calculate the sag magnitude due to faults at 66 kV. The feeder impedance at 66 kV is 0.3 Ω/km.
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Example Cont ’d
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The following impedance values for this system (referred to 66kV): •
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= = 3 =
.3
4 = %
=
7
= 2.56Ω
∗ =
7
= 6.05Ω
=
.7
= 18.5Ω
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Example Cont ’d
There are two methods to further improve the supply: One can increase the number or size of the generators, which corresponds to decreasing Z4 . Alternatively one can increase Z3, which leads to a lower fault level at the 11kV bus. •
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Example
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the transmission system containing the substations PAD-400 and EGG·400, plus 30 km of overhead 400kV line in between them. The impedances have the following values (in % at a 100 MVA base), with L the distance between EGG-400 and the fault:
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Example Cont ’d Sag magnitude as a function of the distance to the fault, for transmissio transmission n systems. For distances up to 30 km the sag magnitude changes with distance like in a radial system; for larger distances the magnitude increases faster faster.. Thus, the sag is less severe than for a fault at the same distance in a radial system. 10
VOLTAQE SAG DURATION Fault-Clearing Time •
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The duration of a sag is mainly determined by the faultclearing time, but it may be longer than the fault-clearing time. Generally speaking faults in transmission systems systems are cleared faster fast er than faults in distribution systems. In transmission systems the critical fault-clearing time is rather small. Thus, fast protection and fast circuit breakers are essential. Also transmission transmission and subtransmission subtransmission sys systems tems are are normally operated as a grid, requiring distance protection (Z=V/I) or differential protection (KCL:Iin=Iout ), both of which are rather fast. 11
The fault-clearing time of various protective devices:
current-limiting fuses: less than one cycle
expulsion fuses: 10-1000 ms
distance relay with fast breaker: 50-100 ms
distance relay in zone 1: 100-200 ms
distance relay in zone 2: 200-500 ms
differential relay: 100-300 ms
overcurrent relay: 200-2000 ms 12
Some typical fault-clearing times at various voltage levels for a U.S.
Voltage Level
Best Case
Typical Typical
Worse Case
525 kV
33 ms
50 ms
83 ms
345 kV
50 ms
67 ms
100 ms
230 kV
50 ms
83 ms
133 ms
115 kV
83 ms
83 ms
167 ms
69 kV
50 ms
83 ms
167 ms
34.5 kV
100 ms
2 sec
3 sec
12.47 kV
100 ms
2 sec
3 sec 13
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the sag duration will be longer when a sag originates at a lower voltage level.
Many utilities operate their distribution feeders in such a way that most faults faults are cleared within a few cycles.
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Magnitude-Duration Plots •
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Knowing the magnitude and duration of a voltage sag, it can be presented by a point in a magnitude-duration plane. This way of sag characteriz characterization ation has been shown to be extremely useful for various types of studies. Used to describe both equipment and a nd system system performance
1. Transmission system faults 2. Remote distribution system faults 3. Local distribution system faults 4. Starting of large motors 5. Short interruptions 6. Fuses 15
General structure of power system, with distribution and transmission networks.
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A short-circuit fault in the local distribution network will typically lead to a rather deep sag. This is due to the limited length of distribution feeders. feeders. When the fault occurs in a remote distribution distribution network, the sag will be much more shallow due to the transf t ransformer ormer impedance between the fault and the pcc. For a fault in any distribution network, the sag duration may be up to a few seconds. Transmission system faults are typically cleared within 50 to 100ms, thus leading to short-duration sags. Current-limiting fuses lead to sag Current-limiting durations of one cycle or less, and rather deep sags if the fault is in the local distribution or low-volt low-voltage age network. 16