WO2021213854A1

WO2021213854A1 - Improved fault correction behavior of network-coupled inverter systems on generator drives with variable rotational speeds

Info

Publication number: WO2021213854A1
Application number: PCT/EP2021/059621
Authority: WO
Inventors: Detlef Haje
Original assignee: Siemens Aktiengesellschaft
Priority date: 2020-04-23
Filing date: 2021-04-14
Publication date: 2021-10-28
Also published as: DE102020205132A1

Abstract

The invention relates to a method for reconnecting network-coupled inverter systems on generator drives with variable rotational speeds according to a fault correction of a network disturbance. The inverter Inv undergoes a reconnection to the network N, said reconnection being matched to the generator section St over time such that the connection time is determined in accordance with the vibrational state of the generator section St, said vibrational state being triggered by the network disturbance at the point in time t_o. With this method, referred to as a soft ride through, the torsional vibration due to the occurrence of the network disturbance and the torsional vibration due to the reconnection are matched such that the torsional increase is minimized, advantageously producing a mechanically gentle fault correction behavior.

Description

description

Improved error explanation behavior of grid-connected converter systems on variable-speed generator drives

The invention relates to a method for reconnecting grid-connected converter systems to variable-speed generator drives after a grid fault has been cleared.

In the case of generator strings that are connected to the grid, there are prescribed, nationally different requirements for the tolerability of electrical grid disruptions. Single-phase or multi-phase short circuits must be mastered, as well as disconnection from the mains with voltage recovery after a specified error clearance time. These network disturbances mean a sometimes considerable torsional excitation of the generator string, since the torsionally loaded string is suddenly relieved with the power torque and sometimes loaded again. Depending on the type of disturbance and the design of the line, alternating torques can result which are a multiple of the nominal torque. This is one of the design criteria for high-speed lines and leads to a reinforced design of certain line components, such as shaft sections, clutches, gears. Grid-connected converter systems must also master these requirements, for example when connecting wind turbines that have a variable-speed generator drive. Here, too, the control of (temporary) short circuits or network faults, which is also referred to as "fault ride through" in specialist circles, is an essential design criterion.

The invention is based on the problem of finding a solution which has a mechanically gentle error explanation behavior of grid-connected converter systems on variable-speed generator drives. In the case of an object outlined by the features of the preamble, the problem is solved by the features of the characterizing part of claim 1.

Advantageous further developments of the invention are given in the subclaims.

For variable-speed generator trains or turbo sets with grid-connected converter systems, a mechanically gentle error explanation behavior, which may also be referred to as soft ride through, is achieved by the converter reconnecting the generator train in a timed manner.

In this way, the torsional oscillation caused by the occurrence of the network disturbance (the point in time of which cannot be predicted and thus can be arbitrary) and the torsional oscillation can be adjusted so that the excessive torsion is limited, reduced or minimized.

There are two solution principles that can also be combined with one another, 1. and 2.

1. By reconnecting at a suitable time, an oscillation maximum from the network disturbance can, for example, be superimposed with an oscillation minimum from the reconnection. The amplitudes are thus partially canceled and the stress is clearly limited. For this purpose, the connection time is determined as a function of a measured or computationally determined vibration state. If the voltage returns before this point in time, the connection is delayed. Time windows can also be determined in which it can be switched on again immediately when the voltage is restored.

The switch-on time can be determined, for example, by measuring the rotor position (angle of rotation), the torsional stresses, the tooth forces or other variables. It can also be a arithmetic determination take place. This can be based on an online simulation of the vibration condition, in which the original load condition of the machine is taken into account as well as the natural vibration behavior of the string.

In a special embodiment, the determination can be made directly from stored characteristic values of the strand. A connection can take place, for example, within a selection from all even half-waves of a torsional oscillation of the strand. This embodiment advantageously involves little effort.

2. The torsional vibrations from separation and connection discussed above can be thought of as a system response to a jump in the torsional moment T. The time course of the separation cannot be significantly influenced either, since, for example, short circuits mean an immediate collapse of the generator power. On the other hand, the reconnection can also be stretched over time (in the form of a linearly increasing ramp, for example) in order to limit the torsional oscillation resulting therefrom. A ramp duration of more than one half-wave of the torsional oscillation should be aimed for here. As an alternative to a ramp, other time courses are also suitable, such as a sinusoidal increase (quarter wave) or a sin ^ 2 increase.

1st and 2nd: A further improvement can be achieved by combining both principles. If the voltage returns within a time window suitable for immediate connection, this is also carried out. In the marginal areas or outside the time window, a time-extended connection can initially take place until the time window is fully reached, or the connection is extended over a short period of time (shorter than a half-wave of the torsional oscillation). The torsional stress on the strand can be limited by the choice of the time window and the execution of the temporal stretching. If a connection is carried out at a phase position of 360 °, the oscillation is canceled out (system temperature damping not taken into account). The amplitude of the maximum torsional alternation torque corresponds to the nominal torque T (in the time to of the mains failure). If a connection “close to 360 °” is tolerated, a higher short-term torsional stress and also an initially remaining alternating torque will occur, the level of which depends on the exact phase position. With a given permissible torsional stress it can thus be determined within which phase position can be switched on.

The combination with a time extension allows the reconnection to begin immediately, even in the case of an unfavorable phase position. This means that the grid is fed into the grid again as early as possible (at least in part). Power is also taken from the line again as early as possible so that a start-up (which occurs when the power is disconnected from the load) can be limited.

Another major advantage that should be mentioned is that the permissible error clearance time, which is typically only a few 100 ms for rotating machines (for example 250 ms), can also be significantly extended by the method described here (depending on the Execution of the power electronics).

The invention is explained in more detail below as an exemplary embodiment to the extent necessary for understanding with reference to figures. Show:

1 shows the time profile t of a torsional moment T in the event of a network fault at time to;

In the figures, the same designations denote the same elements. FIG. 1 shows the course of the torsional moment T of the generator train over time t. At time t ₀ , a network fault occurs, as a result of which the generator string executes a natural oscillation, the amplitude of which decays over time. According to the invention, the reconnection takes place when the torsional oscillation reaches a maximum close to the torsional moment before the network disturbance, that is to say at even half-waves of the torsional oscillation.

FIG. 2 shows the course of the torsional oscillation of the generator train when a connection takes place within a window of +/- x ° relative to the point in time of the oscillation maximum. A time window is defined by the period +/- x °. If, as shown in the example, the vibration maximum is switched on within the time window, the torque of the torsional vibration may increase. By specifying the maximum permissible amplitude of the torsional vibration, the permissible width of the time window is specified.

In the arrangement shown in FIG. 3, a generator G is connected via a generator train St to a drive At, which can be provided, for example, by a steam turbine or the rotor of a wind power plant. In the exemplary embodiment, the drive is provided by a steam turbine, which is supplied with working steam via a steam valve V. The electrical energy generated by the generator is supplied in three phases with a voltage U _G , a current intensity I _G and a variable frequency f _{G to} a converter Inv, in particular a frequency converter. The inverter output side predetermined to an electrical network voltage U N _N and a predetermined frequency f _N, for example, 50 _Hz, respectively. A control unit C continuously detects voltage U _G , current I _G and frequency f _{G of} the generator as well as voltage U _N , current I _N and frequency f _{N of} the electrical network N. The control unit sends a signal S for the precise reconnection of the converter to the Mains to the converter. In the event of a monotonically increasing reconnection, the carried power, possibly over several periods of the drive _{frequency f G} or the network frequency f _N , ramped up.

For purposes of illustration, the present invention has been explained in detail on the basis of specific exemplary embodiments. Elements of the individual exemplary embodiments can also be combined with one another. The invention is therefore not intended to be restricted to individual exemplary embodiments, but rather only to be restricted by the appended claims.

Reference character

T - torsion moment t - temporal progression to - time of the network failure U - electrical voltage I - electrical current f - frequency of the alternating current G - generator C - control unit, controller

Inv - converter, frequency converter

At - drive, turbine, rotor of a wind power plant St - generator train V - steam valve N - electrical network

S - signal for time-coordinated reconnection

Claims

1. Procedure for reconnecting grid-connected converter systems to variable-speed generator drives after an error has been cleared from a grid fault, as a result of which the converter (Inv) reconnects to the grid (N) in accordance with the timing of the generator string (St) in such a way that the connection time is in accordance with the the grid disturbance at the time (t ₀ ) triggered oscillation state of the generator string (St) is determined.

2. The method according to claim 1, characterized in that the reconnection takes place at a time t = t ₀ + (n * 360 °) with n = 1,2,3 .. of the oscillation state.

3. The method according to claim 1, characterized in that the reconnection in a time window t = t ₀ + (n *

360 °) +/- x ° with n = 1,2,3 ... takes place.

4. The method according to any one of the preceding claims, characterized in that the reconnection takes place in accordance with a computationally determined oscillation state.

5. The method according to any one of the preceding claims, characterized in that the reconnection takes place in accordance with a vibration state stored in a data memory.

6. The method according to any one of the preceding claims, characterized in that the reconnection takes place in accordance with a currently computationally determined vibration state.

7. The method according to any one of the preceding claims characterized in that the reconnection takes place in accordance with a measured oscillation state.

8. The method according to any one of the preceding claims, characterized in that the reconnection takes place by feeding power from the generator line into the network.

9. The method according to any one of the preceding claims, characterized in that the reconnection is carried out by feeding power from the generator line into the network by regulating the converter upward in the form of a monotonically rising curve (curve).

10. The method according to any one of the preceding claims, characterized in that the reconnection takes place over a quarter wave of the torsional oscillation of the oscillation state, increasing sinusoidally over time.

11. The method according to any one of the preceding claims, characterized in that the reconnection is stretched over more than one half-wave of the torsional oscillation of the oscillation state in an increasing manner over time.

12. The method according to any one of the preceding claims, characterized in that the reconnection takes place in the form of a linearly increasing curve.