The present invention relates to the field of electrical power generation. It relates to a method for connecting an inductive load, in particular a winding through the stator bore of a generator, to a predetermined alternating medium voltage. It also relates to a connecting circuit for carrying out the method.
For carrying out high-flux tests on the stator of a generator, a medium voltage of 6-10 kV must be mechanically and electrically connected to a coil or winding consisting of 5-12 windings of a medium-voltage cable which is wound through the stator bore. Up to now, the medium voltage has been connected by closing a conventional medium-voltage breaker from an associated switchboard. The arrangement for this method is shown in principle in
In doing so, extremely high inrush currents can occur due to a transient direct current component in the switching current and the high magnetic remanence of the stator core. This gives rise to serious problems in keeping the circuit breaker closed when the inrush currents exceed the limiting values of the overcurrent trips in the incoming medium-voltage switchboard.
The present disclosure is directed to a method for connecting an inductive load of a generator to a predetermined alternating medium voltage. The method includes connecting the inductive load to the medium voltage by a breaker and timing the connection to come into effect when the medium voltage has a predetermined phase, to reduce an inrush current.
The present disclosure is also directed to a connecting circuit for carrying out a method for connecting an inductive load of a generator to a predetermined alternating medium voltage. The method includes connecting the inductive load to the medium voltage by a breaker and timing the connection to come into effect when the medium voltage has a predetermined phase, to reduce an inrush current. The connecting circuit includes medium-voltage connections for connecting the medium voltage; and winding connections for connecting the inductive load which are connected to one another via a breaker. The circuit also includes a first voltage transformer arranged between the medium voltage connections and the breaker. An output of the first voltage transformer is connected to an input of a zero-crossing detector; the zero-crossing detector controls the breaker via a downstream delay circuit.
The invention is explained in more detail below with reference to exemplary embodiments in conjunction with the drawing. In the drawing
The object of the invention is therefore to create a method for connecting such an inductive load to a medium voltage which avoids the disadvantages of known methods and is distinguished by the occurrence of minimal inrush currents, and also to specify a connecting circuit for carrying out the method.
The object is achieved by the appended claims. A preferable feature of the invention is that, to reduce the inrush current, the connection is timed to come into effect when the medium voltage has a predetermined phase.
In an embodiment of the method according to the invention, the connection is timed to come into effect when the medium voltage passes through its phase maximum.
In another embodiment of the method according to the invention, the characteristic of the medium voltage with respect to time is sampled, that it is established when the medium voltage assumes a representative value which is reached a fixed time period before passing through the predetermined phase, and that connection takes place on expiry of the fixed time period.
In particular, the representative value of the medium voltage is a zero-crossing.
Another embodiment is distinguished in that the breaker has its own delay time, and that the fixed time period is chosen to be longer than the delay time of the circuit breaker.
In another embodiment of the method according to the invention, the breaker of a medium-voltage switchboard is used as the breaker.
In another embodiment of the method according to the invention, a medium voltage of 6-10 kV is used.
The connecting circuit according to the invention for carrying out the method has medium-voltage connections for connecting the medium voltage and winding connections for connecting the inductive load which are connected to one another via a breaker, wherein a first voltage transformer is arranged between the medium voltage connections and the breaker, the output of the first voltage transformer is connected to the input of a zero-crossing detector, and the zero-crossing detector controls the breaker via a downstream delay circuit.
In an embodiment of the connecting circuit according to the invention, the delay time of the delay circuit is adjustable.
In another embodiment of the connecting circuit, a control panel is provided, by means of which the zero-crossing detector and the delay circuit can be put into a state of readiness.
The idea on which the invention is based deals with minimizing the inrush currents, which occur while high-flux tests are being carried out, to a first approximation in that the associated breaker which connects the associated winding to the medium-voltage source is closed at the right point of time. In particular, the phase maximum of the alternating voltage of the medium-voltage source is taken to be the right point of time.
The internal structure of a corresponding connecting circuit 11, which is particularly suitable for carrying out the method according to the invention, is reproduced in
A breaker 17, which controls the actual switch-on process, is inserted between the isolator 19 and the medium-voltage connections 15a and 15b. The breaker 17 is controlled according to the characteristics with respect to time of the medium voltage present on the medium-voltage connections 15a and 15b. This alternating voltage is tapped off via a voltage transformer 16 and the output signal of the voltage transformer 16 is fed to a zero-crossing detector 22 which detects the zero-crossings of the alternating voltage and passes on appropriate signals to a delay circuit 23. The time-delayed detector signals are then used to control the breaker 17. In order for one of the time-delayed detector signals to be able to close the breaker 17, the zero-crossing detector 22 and the delay circuit 23 must first be put in a state of readiness by appropriate signals (enable command) from a control panel 24. When this has happened, the next detector signal from the zero-crossing detector 22 is used to switch on the breaker 17 after an appropriate delay in the delay circuit 23.
A further voltage transformer 18 arranged between the breaker 17 and the isolator 19 can be used to monitor the behavior of the output voltage during switch-on. In addition, a current transformer 20 can be used to check the current flowing during the switch-on process.
During switch-on, the voltages VT1 and VT2 picked off with the two voltage transformers 16 and 18 have the characteristics with respect to time shown in
In a set-up mode, the breaker 17 and the isolator 19 are initially open. The breaker 17 is then closed and, in the process, voltage characteristics of the transformer voltages VT1 and VT2 are simultaneously recorded on an oscilloscope (see
When the delay time has been set, the connecting circuit 11 can be used to carry out the high-flux testing of the generator. For this purpose, the isolator 19 is permanently closed, and at the start of the test (time t1 in
The corresponding flow diagram of this process is shown in
10 testing arrangement
11 connecting circuit
12 generator
13 winding (of a medium voltage cable through the stator bore of a generator)
14 medium voltage switchboard
15
a,b medium voltage connection
16,18 voltage transformer
17,19 power switch, breaker
20 current transformer
21
a,b winding connection
22 zero-crossing detector
23 delay circuit
24 control panel
VT1, VT2 transformer voltage
t1,t2,t3 point-of-time
T1,2 delay time
FC1-FC5 flow chart section
K1,2 command
Number | Date | Country | Kind |
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102010044600.9 | Sep 2010 | DE | national |