TECH ICAL FIELD

The present invention relates to an apparatus and method for performing instantaneous short-circuit tests on polyphase synchronous generators .

BACKGROUND ART

As is known, the so-called, "instantaneous short- circuit" test is a measurement method which allows to determine some characteristic parameters of polyphase ( in particular, three-phase) synchronous generators, and is unanimously recognized as highly reliable. The method is encoded by internat ional standards ( IEC60034 and IEEE115) and allows to determine the following circuit parameters :

Xd' - direct axis transient reactance

Xd" - direct axis sub-transient reactance

Td' - direct axis transient time constant

Td" = direct axis sub- transient time constant

Ta = direct axis armature time constant

These parameters , obtained experimentally, are essential to verify the correct modeling of a synchronous generator and to refine the calculation codes used, at the design stage .

The three-phase instantaneous short-circuit test requires the rotor to be set to nominal rotation speed in the presence of an excitation current , so that a triad of symmetrical three-phase voltages is present at the terminals of the generator . When the test conditions are achieved, an instantaneous short-circuit is caused between the terminals of the genera tor by closing a dedicated switch. During the transient following the switch closure , the stator short-circuit currents , the rotor excitation current and the temperature of the rotor and stator windings are measured .

In order to obtain the short circuit , a medium voltage switch is generally used, which is dimensioned so as to withstand the strong currents and the electrodynamic stresses during the transient. The switch has first terminals rigidly connected to respective stator terminals of the generator and second terminals connected to one another in short -circuit throug respective low inductive component shunts , which allow the transient of the short- circui stator currents to be measured . The circuit parameters searched can be extrapolated from the short- circuit stator currents .

The generator must be brought to the nominal rotation speed by a prime move . However, the prime mover cannot be powered during the short-circuit , in order to avoid torsional moments which could easily damage both the mover and the generator . A descending speed transient is thus carried out . In prac ice , the prime mover takes the rotor to a rotation speed higher than the nominal speed, then the mover power supply is removed and the rotor is allowed to slow down by inertia .

The short-circuit is carried out at the rotation speed as close as possible to the nominal speed.

The operation of closing the switch to per orm the short - circuit is currently performed manually and is therefore rather inaccurate . In particular, the rotation speed, when closing the switch is only approximately near the nominal rotation speed and there is no control over the sinusoidal voltages of the generator.

DISCLOSURE OF INVENTION

Therefore , it is an object of the present invention to provide an apparatus and. method for performing instantaneous short-circuit tests on polyphase synchronous generators which allow to overcome or at least mitigate the limitations described .

According to the present invention, an apparatus and. a method for performing instantaneous short-circuit tests on polyphase synchronous generators are provided as defined in claims 1 and 9, respectively.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention wi11 now be described wi h reference to the accompanying drawings , which show a non- limiting example thereof , in which:

figure 1 is a simplified block diagram of an apparatus for performing instantaneous short-circuit tests on rotors of polyphase synchronous generators in accordance with an embodiment of the present invention;

- figure 2 is a flow diagram relating to a method of performing three-phase instantaneous short-circuit tests on rotors of polyphase synchronous generator in accordance with an embodiment of the present invention; and

- figure 3 is a graph showing quantities used in the apparatus in figure 1.

BEST MODE FOR CARRYING OUT THE INVENTION

For simplicity , the invention will be hereinafter described with reference to the case of a three -phase electric generator. However, this should not be considered as limiting since the invention can be advantageously applied to electric generators with any number of phases .

With reference to figure 1, an apparatus for performing three-pha.se instantaneous short-circuit tests on rotors of polyphase synchronous generators is indicated as a whole with reference numeral 1. Apparatus 1 is connected to an electric generator 2, in particular a three-phase synchronous generator, the rotor 4 of which is mechanically coupled to a prime mover 3. The electric generator 2 has three stator terminals 2a , 2b, 2c connected to a switch 5 and three stator terminals 2d, 2e, 2f connected to one another and forming the star center of the electric generator 2.

Apparatus 1 comprises the switch 5, a control unit 7 , a voltage measurement stage 8, a current measurement stage 10 , an amplifier 11. and a recording device 12.

Switch 5 is a normally open , electromechanical medium voltage switch and includes first terminals 5a, 5b, 5c, second terminals 5d, 5e , 5f, and an actuator 15 which may be operated by the control unit 7.

The first terminals 5a, 5b, 5c are connected to respective stator terminals 2a, 2b, 2c of the electric generator 2. The connection may advantageously be carried out in a rigid manner, for example by copper bars (not shown) . The second terminals 5d, 5e , 5f are connected to one another through the current measurement stage 10. More in detail , the current measurement stage 10 comprises three shunts 10a, 10b, 10c , having first conduction terminals connected to respective second terminals 5d, 5e , 5f of switch 5 and second conduction terminals shorted- circuited and connected to a common reference 1 ine 16, such as a land line. In practice , the current measurement stage 10 comprises a respective shunt for each phase of the electric generator 2. Measurement terminals of shun s 10a, 10b, 10c are connected to respective inputs of amplifier 11 , the outputs of which are connected to the recording device 12.

Actuator 15 is controlled by the control unit 7 as shown hereafter to selectively open and close electric contacts between pairs of first terminals 5a, 5b, 5c and second terminals 5d, 5e, 5f by respective closing members 17a, 17b, 17c.

The control unit comprises an acquisition module 18 , a processing module 20 and a control module 21.

The acquisition modul e 18 is coupled to the stator terminals 2a, 2b, 2c of the electric generator 2 through transformer 8 in order to receive electric signals V _R ' , V _s ' , V _T ' representative of output voltages V _R , V _s , V _T present on t e stator terminals 2a, 2b, 2c, respectively .

The values of the output voltages V _R , V _s , V _T acquired are supplied to the processing module 20 and used to monitor the test conditions and perform the short-circuit of the stator terminals 2a, 2b, 2c.

In particular , the processing module 20 is configured to determine the rotation speed of the electric generator 2 from a monitoring voltage , for example the linked voltage V _R -V _S and, when the current rotation speed, is equal to a reference speed ( for example , the nominal rotation speed of the rotor 4 of the electric genera or 2) , to cause switch 5 to close in a synchronized manner with the monitoring voltage or one of the output voltages V _R , V _s , V _T ,

The processing module 20 is further provided with an input for receiving a start test control from a manual control device 22 and an enable control from a manual enable device 23. For example , the manual control device 22 and the manual enable device 23 may include each a button operated by an operator or more buttons to be operated simultaneously, for safety reasons. A configuration interface 24 is connected to further inputs of the processing module 20 and allows test parameters (e.g. type of switch 5 , nominal rate of the electric generator 2 ) to be set.

The closing and opening of switch 5 are obtained through a control signal S _c supplied by the processing module 20 to the control device 21, which in turn acts on actuator 15. In one embodiment, the control device 21 is a relay which closes in response to the control signal S _Cf thus connecting the terminals of actuator 15 to a power- source 25. When actuator 15 is powered, the. closing members 17a , 17b, 17c close the electric contacts between the first terminals 5a, 5b, 5c and the second terminals 5d, 5e , 5f causing the short-circuit of the stator terminals 2a, 2b, 2c of the electric generator 2; when the control device 21 removes the power from the terminals of actuator 15 , switch 5 opens and the short-circuit between the stator terminals 2a, 2b, 2c of the electric generator 2 is interrupted .

Apparatus 1 operates as described below with reference to figure 2 .

The electric generator 2 is preliminarily brought to a current rotation speed Q _c greater than the reference rotation speed Ω _ΕΕΙ .· by the prime mover 3 (block 10 0 ) . The prime mover 3 is then stopped and the three-phase instantaneous short-circuit test procedure is performed during the deceleration transient of the rotor 4 of the electric generator 2 (block 10 5 ) . In this step , the rotor 4 of the electric generator 2 receives excitation currents

Ie.

The test procedure begins with the activation of the manual control device 22 by an operator (block 110) .

The control module 2 0 starts receiving the values of the output voltages V _R) V _s , V _T acquired by the acquisition module 1 8 and determines a test voltage ¥ _TEST , such as the linked voltage V _R -V _S (block 1 15 ) .

Until the manual enable device 2 3 remains inactive (block 12 0 , NO), the processing module 2 0 continues to acquire values of t e test voltage V _TES T and waits .

When the manual enable device 2 3 is activated to provide the enable command (block 12 0 , YES ) , the processing module 2 0 calculates the current; oscillation frequency f _c of the test voltage V _TEST (block 12 5 ) and compares it with a reference frequency f _REF , e.g. 50 Hz or 60 Hz (block 130) . Thereby, in practice , the processing module 20 checks whether the current rotation speed Ω ₀ of rotor 4 of the electric generator 2 corresponds to t e reference speed

Until the current frequency f _c of the test, voltage V _T E _S T remains higher than the reference frequency f _REF (block 130, NO) , the processing unit continues to acquire new values of the test voltage V _TEST (block 135) and updates the value of its current frequency f _c (block 125) .

When the current frequency f _c equals frequency f _REF (block 130 , YES) , the processing module 20 checks whether the test voltage V _TEST is null (block 140) and continues to acquire new values of the test voltage _EST (block 140, NO; block 145 ) until the first zero crossing of the test voltage V _TEST is detected (block 140 , YES) .

At this point (block 150) , the processing module. 20 generates the control signal S _c synchronized so that the closure of switch 5 coincides with the first (positive or negative) peak of the test, voltage V _TEST , as shown in figure 3. In practice , the control signal SC is generated with an advance τ compared to the first peak of the test voltage V _TEST . The advance τ takes into account the delay of the control chain (i.e. the response time of the processing module 20 , the implementation time of the control device 21 and the closure time of switch 5 ) and may be determined experimentally . Instant t* in which the control for closing switch 5 must be given through, the control sign l S _c may be calculated as

where t ₀ is the first instant in which the test voltage V _TEST is null after the processing module 20 has recognized condition f _c = f _REF . Note that the first peak of the test vol age V _TEST can be either positive or negative .

Switch 5 is then accurately closed when the current rota ion speed Q _c of the rotor: 4 of the electric generator 2 is equal to the reference rotation speed O _REF and one of the output voltages V _R , V _s , V _T has a positive or negative peak .

Meanwhile, the recording device 12 records the transient short-circuit currents I _R , I _S , I _T which may be measured by shunts 10a, 10b , 10c (block 155 ) .

The control signal S _c has a duration At, after which switch 5 is opened to remove the short-circuit condition (block 160) .

The data collected by the recording device 12 are then processed to determine the circuit parameters of the electric generator 2 (block 165) .

The device described advan ageously allows to pe form sudden short-circuit tests under optimal and repeatable conditions , both as regards the rotation speed and the synchronization of the switch closure with the test voltage . The inherent variability when manually performing the short-circuit is thus overcome and therefore , the measurement of the circuit parameters appears overall more accurate and reliable .

Finally, it is apparent that changes and variations may be made to the device and method described without departing from the scope of the present invention, as defined in the appended claims .

As already mentioned above , in particular, the invention may be advantageously applied to electric generators with any number of phases , for example , but not exclusively, four or five. In these cases, the switch has as many first terminals and second terminals as the phases of the electric generator. Each first terminal is connected to a respective s a or terminal of the electric generator. The stator terminals not used by the switch (i.e. half of the total) are connected to one another to form the star center of the polyphase electric generator.

Likewise, the current measurement module comprises a respective shunt for each phase of the electric generator . The shun s have first conduction terminals connected to respective second terminals of the switch and second conduction terminals short-circuited . The measurement terminals of the shunts are connected to respective inputs of the amplifier for recording .

Thereby, the closure of the switch causes a short- circuit between the stator terminals of the electric generator, and the transient currents generated are measured accordingly through the current measurement module , exactly as in the case of the three-phase electric generator described above in detail .