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Some of the primary
reasons for short circuits in the power system are insulation failures,
contamination of the insulation or mechanical damages to the equipment.
Insulation failures occur primarily due to over voltages in the system caused
by lightning strokes and switching operations. Some other reasons are pollution
(Salt or fog) in the environment and natural ageing process which occurs over
the life span of insulation. The amount of short circuit current is determined
by short circuit power of external grids, internal voltages of synchronous
generators and the system impedances between the fault location and the power sources.
So, the basic step for short circuit calculations is to determine the network
impedance matrices details of which will be discussed in the later sections of
this chapter.The magnitude of these
currents is several times larger than the normal operating currents of the
system and therefore, their presence for even a small span of time is
detrimental to the equipment involved and should be cut off as soon as
possible. The damages caused otherwise are, thermal damages to the equipment,
mechanical damages to the windings and busbars due to high magnetic forces and
injuries to the personnel. For EHV and HV networks, fault clearing time is
around 3 cycles or 50 ms. Medium voltages networks are more flexible with fault
clearing time between 5 to 20 cycles. Since the goal remains not to compromise
the protection of equipment under any circumstances, the calculation of maximum
short circuit currents which the system could confront are of vital importance
and play the ultimate role in designing the protection scheme for the equipment.

1.1        
Dynamic Voltage Support

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Short circuit faults
in the power system cause sagging in the voltage level. A short circuit at the
medium voltage level causes heavy drops in the voltage levels in the
neighborhood but this drop is not prominent at high voltage level especially if
they are connected to a strong grid but a fault at HV or EHV level causes
massive drop of voltages not only high or extra high voltage level but also in
the medium voltage level. Following the fault and voltage dip, distributed
generators have two options:1.    Immediately disconnect
themselves from the network2.    Remain connected with the
network and support the voltage according to national or international norms.Immediate
disconnection of these distributed generators may cause additional stability
problems in an already un-stabilized network and hence the integrity of the
whole network might be at stake. The following flow chart 3 shows the
possible behavior of distributed generators in case of short circuit.

behavior of full

sized converter
interfaced wind power plants in case of fault

remain connected for

fault the

ride

through (FRT)

disconnect themselves

current infeed

no current infeed

adaption of reactive current control

5

8

unknown behavior

influence on short
circuit currents and

voltage recovery

no influence

Figure 3.1 Possible scenarios for converter interfaced
wind power plants in case of short circuit. Although the term DG
is used for all types of distributed generation sources, the emphasize lies
mainly on the wind power plants as we are investigating the effects of these power
plants on short circuit current calculations.As the penetration of
these DG is increasing and following the trend, will only grow in future, it is
no longer possible to disconnect them from the network unless required by the
fault ride through procedure. So, the ancillary services of these DG are as
much important as the conventional power plants. With dynamic network support,
these DG have to remain connected with the network and supply the reactive
power to the network according to fault ride through curves to support the
voltage dip and try to stabilize the system. With this reactive power feed in,
the impact of faults can be limited and improved voltage recovery can be
achieved.Since type 4 wind
turbine generators are equipped with full-size converters, they have the
capability to inject the reactive current in the network. Their first job is to
remain connected with the network during fault, also called fault ride-through
(FRT), injecting the reactive power in the next phase. The time span of their
connection with the grid depends upon the duration and depth of sagging of
voltage. Since the bi-directional flow of power is possible with these DG, they
can also absorb the reactive power in case the voltages in the system rise
above the nominal voltage resembling the under-excited operation of synchronous
generators. So, the power in these DG can have a bi-directional flow.

1.1.1        
Fault Ride
Through for Type 1 and Type 2 Generators

As the primary goal of
generating sources is to stay connected with the grid and support the voltage,
they are tested to their stability limits in periods of faults or violent
voltage dips in the network. A short circuit a HV or EHV level, in case these
generating sources don’t participate in the voltage support process, may cause
greater loss of generation and put the overall stability of the network at
greater risk. Both Type 1 and Type 2 units participate in dynamic grid support
process. Generating units don’t have to adjust their active or reactive power
according to their terminals, the reference point for voltage support process
is rather the point of grid connection or point of common coupling (PCC). This
dynamic support has to be provided for both symmetrical and unsymmetrical fault
types. The figure below 8 shows the Fault ride through curves for type 1
power plants.

Figure 3.2 Fault ride through curves at network
connection point for type 1 power plantsThe red line in the
figure represents the FRT curve for 3 phase symmetrical fault while green line
shows the voltage profile limit for 2 phase faults. UNAP is the effective value of the voltage at
network connection point. As evident from the figure, if the effective value of
the voltage at network connection point falls below 30 percent, type 1
generation units have the possibility to disconnect themselves after 150 ms in
case of three phase balanced faults and after 220 ms in case of line-to-line
faults. These limits of FRT curves are not applicable if the stability of any
individual generation unit or plant is threatened in which case they can be
disconnected from the network without time constraints. A second attempt to
connect to the network is possible after the disconnection according to the standard
procedure. These FRT curves can be shifted provided the connection concepts of
generating plants allow to do so or the requirements by network operator change
at any point in time 7. The FRT curves for type 2 power plants are shown in
the following figure 8.

Figure 3.3 Fault
ride through curves at network connection point for type 2 power plantsIn comparison with the
FRT curves of type 1 generation units, FRT curves of type 2 generating units
are more flexible right after the inception of short circuit as they remain
connected to the network even if the effective value of the voltage at PCC has
dropped to as low as 15 percent, explaining on one hand better stability
performance of these generating units and more range covering for the entire
voltage support period on the other. Type 1 generating units remain connected
with the grid even at those times when type 1 generating units would have
disconnected themselves due to stability issues. The time constraints to
disconnect from the grid, in case the voltage drops below the specified level
or doesn’t recover in the above mentioned period to required level, remain the
same for both types of generation sources and for both types of faults i.e. 150
and 220 ms.

1.1.2        
Fault
Current Contribution of Inverter-Interfaced Distributed Generators

The idea that the
DG’s should remain connected to the network and contribute the reactive power
in case of network fault is clear but how much current they should contribute
will be discussed in this section. In principle, they should contribute pre-fault
current plus an additional priority based reactive current to support the voltage.
There exists a certain degree of freedom when it comes to injecting reactive current
like1.    Whether the residual voltages and corresponding
reactive currents will be continuously updated or just one-time measurement of the
residual voltage is enough.2.    Whether the currents will be injected only in the
positive sequence or both in positive and negative sequences 8.

3.    The limit to how much current will be injected in
the worst-case scenario and whether there exists a priority among DG’s in terms
network support.

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