Electricity: electrical systems and devices – Safety and protection of systems and devices – Feeder protection in distribution networks
Reexamination Certificate
1999-09-08
2002-09-17
Leja, Ronald W. (Department: 2836)
Electricity: electrical systems and devices
Safety and protection of systems and devices
Feeder protection in distribution networks
C361S047000, C307S018000, C307S051000
Reexamination Certificate
active
06452769
ABSTRACT:
BACKGROUND OF THE INVENTION
a) Field of the Invention
The present invention relates to an electrical power distribution installation and, more particularly, to apparatus and method for grounding and protecting the electrical power distribution installation such as a power station, sub-station, and/or a power generation installation in normal service.
b) Description of the Related Art
FIG. 1
shows a skeleton diagram representing a previously proposed electric power distribution installation.
As shown in
FIG. 1
, two main transformers MT
1
and MT
2
are used to step down respectively corresponding power supply voltages from respective feeders connected to receive powers L
1
and L
2
into main common bus bars BUS-A and BUS-B, each of 11 kV class.
Respective loads LOADs receive the stepped down power supply voltages from the respectively connected main common bus bars BUS-A and BUS-B.
An interconnecting circuit breaker
52
AB which is normally closed serves to interconnect one of the main common bus bars BUS-A with the other main common bus bar BUS-B. Each main common bus bar BUS-A and BUS-B supplies the electric power to a corresponding load connected to a low-voltage bus bar via a corresponding transformer T
11
and T
21
.
The other main bus bar BUS-B is connected directly to a generator common bus bar BUS-G via an interconnection feeder without passing through a step-down or step-up transformer.
The generator common bus bar BUS-G is connected to four generators G
1
, G
2
, G
3
, and G
4
. Each generator G
1
through G
4
can be disconnected from the generator common bus bar BUS-G by means of a corresponding circuit breaker
52
G
1
,
52
G
2
,
52
G
3
, and
52
G
4
. Each main transformer MT
1
and MT
2
can be disconnected from the corresponding main common bus bars BUS-A and BUS-B by means of corresponding circuit breakers
52
S
1
and
52
S
2
. It is noted that each load can be disconnected from each corresponding main common bus bar BUS-A and BUS-B by means of corresponding circuit breakers as shown in FIG.
1
.
A grounding device is installed in such a power distribution installation as shown in FIG.
1
.
That is to say, the grounding device includes: a neutral grounding resistors NGR-
1
and NGR-
2
, each grounding a neutral point of a star connection of a secondary winding of the corresponding main transformer MT
1
or MT
2
; and a neutral grounding resistor grounding each neutral point of the generators G
1
through G
4
via a corresponding (vacuum) switch VS
1
, VS
2
, VS
3
, and VS
4
.
SUMMARY OF THE INVENTION
In the previously proposed grounding system described in the BACKGROUND OF THE INVENTION, suppose that a rating of each grounding resistor NGR-
1
, NGR-
2
, and NGR-G is 300 A-10 s.
If all of the grounding resistors NGR-
1
, NGR-
2
, and NGR-G are operated, a 900 A resistance grounding system can be constituted in the power system of FIG.
1
.
However, if a failure in the feeder connected to the main transformer MT
1
occurs so that the current breaker
52
S
1
interconnected to the secondary winding of the main transformer MT
1
trips, the grounding device, i.e., the neutral grounding resistor NGR-
1
is disconnected from the power system. Consequently, the grounding system is changed to a 600 A resistance grounding system in the power system.
In addition, if the failure occurs in the main transformer MT
1
and, during a repair or exchange (replacement) of the transformer MT
1
, a failure in the generator common bus bar BUS-G occurs, the grounding device, i.e., the neutral grounding resistor NGR-G is also disconnected (separated) from the power system so that the grounding system is changed to a 300 A resistance grounding system.
If an electrical service interruption in a primary side of the main transformers MT
1
and MT
2
, only the generators G
1
through G
4
need to continue to operate the power system through the 300 A resistance grounding system.
In the way described above, the change of the resistance grounding system due to the separation of each or any of the neutral grounding resistors from the power system causes changes in a ground fault detection sensitivity and a ground fault detection time in a protective relay system and often allows a coordinate protection in the power system not to be maintained.
It is therefore an object of the present invention to provide an electrical power distribution installation having an improved resistance grounding system and a protection system which are stable against a failure occurrence in an internal power system with no influence or small influence of a coordinate protection.
According to one aspect of the present invention, there is provided with an electric power distribution installation comprising: at least one main common bus bar; a plurality of main transformers, each main transformer being configured to step down a received power supply voltage and to supply the stepped down power supply voltage to a load via the main common bus bar; a plurality of power generators, each power generator being connected to the main common bus bar; a first grounding device including at least one grounding transformer connected to the main common bus bar, a neutral point of the grounding transformer being grounded in a form of a predetermined low impedance grounding; and a second grounding device connected to each neutral point of the power generators to always ground each neutral point directly in a form of a predetermined high resistance grounding.
According to another aspect of the present invention, there is provided with a method applicable to an electric power distribution installation, the electrical power distribution installation comprising: at least one main common bus bar; a plurality of main transformers, each main transformer being configured to step down a received power supply voltage and to supply the stepped down power supply voltage to a load via the main common bus bar; and a plurality of power generators, each power generator being connected to a generator common bus bar and the generator common bus bar being connected to the main common bus bar, and the method comprising: providing a first grounding device including at least one grounding transformer connected to the main common bus bar, a neutral point of the grounding transformer being grounded in a form of a predetermined low impedance grounding; and providing a second grounding device connected to each neutral point of the power generators to always ground each neutral point directly in a form of a predetermined high resistance grounding.
In the previously proposed electrical power distribution installation shown in
FIG. 1
, the four generators G
1
through G
4
are enabled to be a parallel operation and their one neutral point is grounded via their corresponding circuit breaker (switch) VS
1
through VS
4
by means of the single neutral grounding resistor NGR-G.
Reasons for adopting such a grounding method as described above and problems occurring therein will be described below.
(a) When each neutral point of the four generators G
1
through G
4
is short-circuited without an impedance, a zero-phase voltage component of a generated voltage of each generator G
1
through G
4
generates a zero-phase current at a corresponding neutral point so that an overheat of the respective generators G
1
through G
4
occurs.
To eliminate such a deficiency as described above, with any one of the vacuum switches VS
1
through VS
4
turned on in the normal service, only one of the four generators G
1
through G
4
which is connected to the turned-on vacuum switch is connected to the grounding resistor NGR-G.
(b) Each generator G
1
through G
4
is required for a regular inspection. If one of the four generators G
1
through G
4
which is connected to the grounding resistor NGR-G undergoes the inspection, it is necessary to switch the connection of the grounding resistor to another of the generators with no interruption of the power supply. At this time, any two of the four vacuum switches VS
1
through VS
4
are temporarily turned on in parallel to each othe
Sakurai Takashi
Sohde Toshio
Foley & Lardner
Kabushiki Kaisha Meidensha
Leja Ronald W.
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