METHOD AND ARRANGEMENT FOR DETECTING AND DAMPING TRANSIENT OSCILLATIONS AT OR NEAR A NATURAL RESONANT FREQUENCY IN A POWER TRANSMISSION SYSTEM

Abstract: An oscillation in a power transmission system is detected by multiplying voltage or current f(t) by a signal from an oscillator oscillating at a frequency at which oscillations are expected. The result is sampled with a frequency which exceeds the oscillator's, and the latest samplings which correspond to one or more entire periods of the frequency of the oscillator are successively summated. The successive summations are multiplied by the oscillator signal, whereby a regulating signal is obtained. By analysis of the said summations, a diagnosis is made as to whether damping correction is needed, and if this is the case, said regulating signal is connected to a regulating device to disconnect this when it is no longer required. The invention can be used both to eliminate oscillations in cables for high voltage direct current transmission and to eliminate subsynchronous oscillations in alternating current networks provided with gas and steam turbines.

Description: TITLE: Method and arrangement for detecting and damping transceent oscillations at or near a natural resonant frequency in a power transmission system. The present invention relates to a method for detecting and damping transient oscillations in a power transmission system, and an arrangement for carrying out the method. BACKGROUND OF THE INVENTION: In a plant for transmission of high voltage direct current, an HVDC-plant, transient oscillations can arise due to various reasons. They can occur, for example, due to earth faults, short circuits and misfiring valves, as well as during certain control operations during normal operation, such as ramps during starting, stopping, restarting after faults or stabilizing operations. The natural resonance of transient oscillations at certain frequencies is particu¬ larly large, whereby the natural resonant frequencies are well defined depending on the configuration of the system and the frequency of the alternating current side of the circuit. At resonance, the amplitude of the oscillations can quickly grow to dangerous levels which considerably exceed the rated quantity of the plant. This causes the energy transmission to be interrupted by safety systems. In addition, lasting oscillations can affect other regulating systems in the alternating current parts of the total network. Furthermore, larger alternating current components with fundamental frequency in the direct current transmiss¬ ion can result in the connected alternating current system obtaining an undesired direct current component due to saturation effects in transformers. It can be very difficult to design normal regulators in power transmission systems so that they fulfil both the requirements which are made during normal operating conditions, whilst at the same time offering effective damping of critical resonant frequencies which exist in the networ . Similar resonance problems are present in alternating current networks for transmission when turbine generators are used. Because of the construction of such turbines, with their rotating masses affixed to necessarily torsion- elastic shafts, mechanical natural oscillations at subsyn- cronous frequencies (so-called negative damping) and similar oscillations can thus be fed to the network via the generators and also transmitted to other turbine generators in the network so that connected oscillations arise there¬ between. Similarly, interaction can occur with the network itself which often has large series condensers which, in cooperation with other reactances, can cause the network to oscillate. The subsyncronous oscillations (SSR) can, in certain cases, cause the turbine generators to break down whilst, in other cases, they are eventually damaged due to fatigue. From US-A-4 434 376 it is known to detect subsyncronous oscillations in alternating current systems, whereby the time separation between zero crossings of the voltage is detected and it is determined whether this differs from that which the system frequency should give. Such detected oscillations are thus damped by means of a regulating action which connects in one or more damping resistors. It is also known to dampen SSR by modulation of current/active power in a connected HVDC-station, whereby narrow band conventional filter circuits are used for diagnosis. In the cases where the thyristor circuits of the HVDC-system are sufficiently quick, the regulation itself can be performed without particular problems. In order to avoid interruption, it is desirable to detect and dampen the transient oscillations at an early stage before the amplitude grows to critical values, or if the duration becomes too long. In order that this may be performed in a satisfactory manner, it is important to be able to differentiate between the operationally stipulated transient building-up processes and other processes which can result in resonant oscillations with subsequent disconnection or damage to other parts of the system. In certain underwater cables for HVDC, and also in other cases, combinations of cable capacitance and smoothing inductances in the system can give rise to natural resonant frequencies which can create oscillations which are not self dampening and can grow such that safety devices which are present activate undesired interruption. In accordance with the invention, a temporarily compensating measure is introduced which quickly dampens the possible oscillation. SUMMARY OF THE PRESENT INVENTION: The main object of the present invention is to provide a method and an arrangement for damping transient oscilla¬ tions in power transmission systems which act quickly and correctly during oscillation build-up of critical frequen¬ cy. The method and the arrangement should be able to distinguish undesired transient oscillations from transient oscillations during other regulating actions, such as ramps during starting up after faults or stabilizing actions, without interfering with these control regulations. Another object of the invention is to protect cable and other expensive equipment in HVDC-systems against extreme overvoltage and subsequent damage. It is a further object to dampen subsyncronous resonances in power transmission systems which comprise connected turbine generators. These objects are achieved according to the invention by a method and an arrangement of the type which is given in the characterizing portion claim 1 and 12 respectively. According to the invention, a regulating action is to be performed when it is established that an oscillation is present within the critical frequency range. An instigated action should be carried out for a certain time, with the object to achieve a total damping of the oscillation. It is preferred that it be investigated whether the phase position for the detected oscillation does not vary too quickly with respect to the reference oscillator when there is an oscillation of a certain frequency. If it is determi¬ ned that the phase position varies with an acceptably small speed, the regulating action should in such a case stay on the entire time. These desirable properties can be achieved in several various ways with various logistic conditions. In accordance with preferred embodiments, a disturbance to the amplitude and phase at a natural resonant frequency is determined, whereby a low amplitude rules out any regula¬ ting action, whilst an amplitude which exceeds a limit value implies that a compensating regulating action can be proposed. However, normal regulating can imply that current or voltage in the cable temporarily displays a frequency content where the resonant frequency or a neighbouring frequency arises with an amplitude sufficient to propose regulating action. Typical for such occurrences are rapid crossings and alternating frequencies. It is thus suitably a requirement that a condition for actuation of regulating action is a presence for, for example 10-20 ms, preferably about 15 ms. Total regulating action should take place within 20 ms. In addition, the regulating action should quickly terminate when it is no longer required, especially when the cause is transient. Thus, one should have discon- nection with an absence of exceeded values for up to 2-10 ms, for example, preferably 4 ms, in combination with an absence of exceeded values for 15-30 ms, preferably 20 ms, indicating that the transient oscillation is gone. All of the time intervals relate to the case in which the system frequency is 50 Hz and the oscillation which is to be counteracted lies at or near the system frequency. If the frequency of interest instead is f Hz, the above mentioned times should thus be multiplied by 50/f. The measurement of amplitude and phase for the natural resonant frequency can be carried out in different ways. At present it is preferred to measure these two variables in the form of sine and cosine components in relation to a fixed oscillator frequency. The criteria are triggered when the absolute values of these components to the sum exceed their limit values during the above-given time interval. In this manner, those cases in which the frequency varies can also be eliminated. It is also possible to use other variable pairs, such as for example the amplitude of oscillation calculated by squaring and summating the sine and cosine components and the phase angle which is also derivable from the same components. The proposed regulating action is suitably carried out for disturbances in HVDC plants by means of regulating the included valves. For disturbances in alternating current transmission networks with turbine generators, regulating action can be performed so that the oscillation is damped if an HVDC plant is connected to the network. The voltage regulator of the turbine can also be influenced. A third possibility when subsyncronous frequencies are detected is to allow the regulating signal to control a regulating arrangement which connects damping circuits which are known per se in the system. The method and arrangement are intended to rectify oscilla¬ tions at or near a certain frequency. In those cases in which more frequencies are susceptible to oscillation, it is suitable to use several separate arrangements according to the invention, which each working with an oscillator frequency. This is normally the case for subsynchronous resonance, the source of which comes from mechanical oscillations in steam and gas turbines, though which can also arise in high voltage direct current transmission where the number of oscillation frequencies is normally low. In certain cases it can be suitable to permit the amplifi¬ cation in the regulating loop to increase when the size of the oscillation exceeds a value which is higher, for example three times higher, than the value which can lead to the connection of the regulating signal. Although the invention is exemplified below with specific circuits such as gates and shift registers, it is obvious for the skilled person that it is possible to implement the same with a computer. DESCRIPTION OF THE DRAWINGS In order to simplify understanding of the present in¬ vention, it will be described in the following by way of example only and with reference to the attached drawings in which Fig. 1 shows a schematic representation of the principal configuration in an HVDC-plant. Fig. 2 shows an operating signal in which a 50 Hz resonant oscillation arises because of a misfiring rec- tifier. Fig. 3 shows a flow diagram for the method according to the present invention, which flow diagram at the same time can serve as a block diagram for an arrangement according to the present invention. Fig. 4 shows a circuit diagram for an embodiment according to the invention. Fig. 5 shows a vector diagram for current or voltage in the HVDC-plant. Fig. 6 shows how the invention can be used in an elec- trical network in which one or more steam or gas turbines are present. Fig. 7 shows an additional arrangement for the circuit shown in Fig. 4. DESCRIPTION OF EMBODIMENTS The present invention is intended to monitor and react to the appearance of oscillations with a certain frequency in a plant according to Fig. 1 with regard to the configura¬ tion of capacitances and inductances in the system. In Fig. 1, reference numeral 10 denotes a rectifier, 20 an in¬ verter, 30 and 40 smoothing reactances, 50 a line, 60 a cable, 70 a distributed capacity in the cable, 80 a regulating system, Reg a regulating system from an arrange¬ ment according to the invention and i(t) and u(t) time- dependent current and voltages respectively. It is present¬ ly prefeπed to use i(t) as the measured variable. At certain frequencies, the natural resonance in the system is particularly large and can arise at the system frequency or other frequency. In the same system there may be several such natural resonant frequencies, whereby a device according to the invention is suitably arranged for each such frequency. In Fig. 2, a resonant oscillation at 50 Hz in a 500 kV cable in an operating signal is shown which is caused by a misfiring rectifier, safety equipment which is present has been triggered and the oscillation has slowly been damped out. With a method according to the flow diagram in Fig. 3 and an arrangement according to a block diagram which is totally analogous with the flow diagram, oscillations of the above-mentioned type can be detected within a time period and, by implementing a regulating action based on calculation of the regulating signal according to the present invention, an oscillation can quickly be damped before safety equipment is able to be activated. With reference to Figs. 4 and 5, the invention will now be described with the help of a schematically shown embodi¬ ment. An oscillation Osc adjusted to the resonant frequency in question emits the reference signals sin ω 0 t and cos ω 0 t respectively, each of which is multiplied in its own multiplication circuit PI, P2 respectively by an analogue operational signal f(t) (for example u(t) or i(t) in Fig. 1) corresponding to the present operating condition of the HVDC-plant. The products f(t)*sin ω 0 t and f(t)*cos ω 0 t respectively are sampled at discreet time intervals which are determined by the clock-pulse signal Trig in a shift register Ml, M2 respectively for each product, and each shift register having a series input and a parallel output. The latest sampling values to a number which at least corresponds to the period of the reference signal are stored in the shift registers Ml and M2. The n latest sampling values Al..An, Bl..Bn respectively which corre¬ spond to one or more complete cycles of the reference signal are summated for each clock-pulse in summation circuits SI, S2 respectively and thereafter multiplied in multiplication circuits P3, P4 respectively with a normali¬ zing constant c so that two values A and B are obtained. The values A and B represent a condition of the operating signal f(t) according to the vector diagram in Fig. 5 which corresponds to phase and amplitude of a superimposed oscillation in the direct current system. A and B are tested in a diagnostic and determination circuit which, in the circuit diagram, is only shown for a maximum permitted value of A but is totally analogous to the minimum value of A and the minimum and maximum values of B respectively. In the embodiment, the selected numeri¬ cal limit values are -0,1<A<0,1, -0,1<B<0,1 respectively corresponding to 0,1 times the rated current or rated voltage. For values of A which, according to the threshold circuit Dl, exceed 0,1, the presence of the transgression (the signal level) in time circuits Tl, T2, T3 respectively is tested so that if A>0,1 for more than 15 ms according to Tl, it is considered that an oscillation is present and an RS-flip-flop RSI is set. If A is then greater than 0,1 for 4 ms according to T2 and, at the same time, absent for 20 ms according to T3, the flip-flop is reset via the AND-gate Gl. As long as the RS-flip-flop for A>0,1 or corresponding flip-flop for any one of the other conditions (A<-0,1, B>0,1, B<-0,1) are set, or any other criteria (Other Criteria) are attained, a circuit closer F is operated to apply regulating action via the OR-gates G2 and G3. Sustained oscillations of lower amplitude can also be monitored by, for example, selecting a level 0,02 times the rated value and a time delay of 40 ms. Oscillation within the interval 0,75-1,15 times the adjusted frequency can then be detected. This information can thereafter, possibly after an additional operational delay in order to eliminate interference, be included as "Other Criteria" according to Fig. 4. It has also been shown to be suitable for serious faults with valves which misfire to increase the amplification in the regulating circuit by a factor of approximately 3 in relation to the normal case. This can occur for example as shown in Fig. 7 which is to be compared with Fig. 4. In such an example, a larger limiting value is used, such as 0,3, the exceeding of which during some certain time such as 10-40 ms, preferably 15 ms (for frequency 50 Hz other¬ wise correspondingly corrected) implies that the amplifica- tion is increased by changing the constant k so that the output signal from the multiplication circuit P7 is correspondingly amplified. With reference to Fig. 4, at each sampling interval, A and B are transmitted to an arrangement for generating a regulating signal. In two calculating circuits Bl, B2 respectively (A*cos a - B*sin a) and (A*sin a + B*cos a) are calculated where the angle a is adjustable by means of an adjustment knob Ia. In addition, in multiplying circuits P5, P6 respectively (A*cos a - B*sin a) is multiplied by sin ω 0 t and (A*sin a + B*cos a) is multiplied by cos ω 0 t from the oscillator and thereafter summarized in a summari¬ zing circuit S3 so as to be finally multiplied in a multiplying circuit P7 with yet another normalization constant k so that the regulating signal Reg=k*(A*sin(ω 0 t + a) + B*cos(ω 0 t + a)) is obtained. The delay angle a is chosen for the best damping with regard to delay due to the sampling and in the present embodiment can be selected to be 145°. The regulating signal Reg is, as mentioned earlier, connected to the diagnostic and determi¬ nation circuit via a switch G for operating the regulating action. The regulating signal Reg thus has the same frequency as the resonance frequency, an amplitude determi¬ ned by the disturbance's amplitude and a phase determined by the disturbance's phase, modified by the time delay of the circuit and the angle a. The criteria for amplitude and phase angle can be explained with help of a vector diagram according to Fig. 5. A and B denote the sin and cos component respectively of the vector, and the surrounding rectangle indicates limits for unconditionally allowable values of A and B respectively. If the amplitude of the operating signal exceeds permitted values, the vector leaves the shown rectangles and regulat¬ ing action may possibly be instigated. An observed ex¬ ceeding of the said limiting values does not however have to imply that a dangerous resonant oscillation has arisen, but instead it may be that a normal course of events can have been recognized as coincidental amplitudes at a resonant frequency. Oscillation at a frequency which is somewhat different from the resonant frequency will appear in the indicator diagram as a vector which rotates with a speed determined by the difference in frequency between the oscillator frequency (i.e. the selected resonant frequency) and the oscillation in question. Due to the said condi¬ tions, such an oscillation will not initiate any regulating action. Should, however, the vector remain stationary or rotate slowly, an oscillation with the same or similar frequency as the reference signal is present and resonance must be seen to be present and consequently a regulating action will be initiated. The preferably temporary regulating action (which according to the above in certain cases can be permanent) according to the invention can be initiated in the normal regulating of the HVDC-system which regulates the firing moments for included controlled rectifiers. The normal control equip¬ ment adjusts the firing moments to a certain firing angle α, calculated in each rectifier part from the lead cur¬ rent's positive zero crossings. It is suitable to let the regulating signal according to the invention, with by-pass connection of the normal regulating, directly influence the firing angle α, whereby necessarily rapid damping is advantageously obtained. The described principle for detection and damping of transient oscillations can also be used for those oscilla¬ tions which arise in alternating current networks and have low frequencies in relation to the system frequency. An example of this is shown schematically in Fig. 6 and in which some of the same reference numerals as in Fig. 1 are used. Two turbine plants 100 with generators 110 are connected to a three-phase network which is also connected to an HVDC-network. In such a case, the regulator circuit R in the alternating current network is examined in order to determine in a corresponding manner as given above possible resonant oscillations at subsyncronous frequen¬ cies. Current voltage in a phase conductor is suitably used as a variable and the regulating action REG is transmitted to the rectifier arrangement 10. Alternatively, and as shown in dashed lines, the regulating signal can be transmitted to a control circuit of the generator G. Since in this case a number of possible oscillating frequencies (inherent values) can arise, each of these should be monitored. A particular advantage with the invention is that its action need only be as temporary as the disturbances which it is intended to eliminate, thereby implying that the regular control is disturbed as little as possible. A further particular advantage is that with detection and damping of oscillation carried out according to the invention, oscillations which do not totally lie at the selected frequency of the reference signal can also be effectively detected and dampened.

Claims: Claims 1. Method for damping oscillations at or near a natural resonant frequency in a power transmission system, c h a r a c t e r i z e d b y the steps of obtaining a reference signal (R) from an oscillator (Osc) oscillating at the natural resonant frequency; multiplying the reference signal by an analogue operating signal (f(t)) corresponding to the prevailing operating condition at some point in the power transmission system; sampling the analogue signal resulting from the multiplication with a sampling frequency (ωs_) which is at least twice as high as the frequency of the reference signal; storing the digital sampling values successively in a memory to a number which at least corresponds to the period of the reference signal; thereafter calculating at each sampling instance a sum of a number (n) of the latest sampling values which correspond to one or more entire periods of the reference signal, whereby the sum represents the phase angle and amplitude of an oscillation in the operating signal; testing the sum in at least one diagnostic and determination circuit (Diag) for detection of oscillations according to predetermined criteria and determination of activation of regulating action; calculating a regulating signal (Reg) for each sampling instance dependent on the characteristic sum for the oscillation and dependent on the reference signal R, and transmitting the regulating signal (Reg) to a regulat- ing arrangement in the power transmission system when the oscillation meets any one of the predetermined criteria so that the oscillation is dampened to predetermined toler¬ ances. 2. Method according to claim 1, c h a r a c t e r ¬ i z e d i n that said reference signal consists of two components corresponding to Rl=sin(ω 0 t) and R2=cos(ω 0 t) , and in that the reference signals Rl and R2 are each multiplied by the operating signal f(t) and result in signals a(t) and b(t) respectively . 3. Method according to claims 1 and 2, c h a r a c ¬ t e r i z e d i n that the signals a(t) and b(t) are sampled with said sampling frequency (ω B ) and in that the sampling values (a, b respectively) are stored in a memory. 4. Method according to claims 1-3, c h a r a c ¬ t e r i z e d i n that two set of sampling values (AL.An, Bl..Bn respectively) of the said number (n) are each summated to its resulting value (A, B respectively) , which together represent a condition in said operating signal (f(t)) at each sampling instance. 5. Method according to claims 1-4, c h a r a c ¬ t e r i z e d i n that the summations (A and B) are tested in at least one diagnostic and determination circuit (Diag) for detection of oscillations with various charac¬ teristics, for example oscillations at one or more critical frequencies, oscillations with relatively high frequency and amplitude and/or lasting oscillations with relatively low frequency and amplitude, or other predetermined criteri . 6. Method according to claim 5, c h a r a c t e r ¬ i z e d i n that if said summations (A, B respectively) permanently exceed or fall below predetermined values corresponding to the size of the amplitude of one oscilla- tion, said regulating signal (Reg) is sent to said regulat¬ ing arrangement. 7. Method according to any one of claims 5-6, c h a- r a c t e r i z e d in that said regulating signal (Reg) is generated with the help of the said summations (A, B respectively) and is dependent on the oscillator signal and sin a, cos a respectively, where a is a suitable value for the regulating action, whereby signals corresponding to A*cos a - B*sin a and A*sin a + B*cos a respectively are generated, said above-mentioned signals corresponding to A*cos a - B*sin a and A*sin a + B*cos a respectively are each multiplied by the components Rl=sin(ω 0 t) and R2=cos(ω 0 t) respectively of the first-mentioned oscillation (Osc), the products sin(ω 0 t)*(A*cos a - B*sin a) and cos(ω 0 t)*(A*sin a + B*cos a) respectively are summated so that the regulating signal (Reg) corresponds to A*sin(ω 0 + a) + B*cos(ω 0 + a) . 8. Method according to claim 7, c h a r a c t e r ¬ i z e d i n that said above-mentioned angle a is adjus¬ table. 9. Method according to any one of the preceding claims, c h a r a c t e r i z e d i n that the regulating signal is sent to a regulating arrangement in a static converter in an HVDC-system 10. Method according to any one of claims 1-8, c h a r a c t e r i z e d i n that the regulating signal is sent to a generator control circuit for a turbine generator. 11. Method according to any of the preceding claims, c h a r a c t e r i z e d i n that the degree of amplifi- cation of the regulation is adjustable and in that it is adjusted depending on the size of the detected oscillation. 12. Arrangement for damping oscillations at or near a natural resonant frequency in a power transmission system, c h a r a c t e r i z e d b y an oscillator (Osc) arranged to emit a reference signal (R) oscillating at the natural resonant frequency; a multiplication circuit (II) arranged to multiply an operating signal (f(t)) corresponding to the prevailing operating condition of the HVDC-system with the reference signal (R); a memory (M) and a clock-pulse generator (CPG) arranged to serially sample the output signal from the multiplication circuit by means of clock-pulses (CP), in that the memory (M) has the capacity to store sampling values up to a number which at least corresponds to the period of the reference signal (R); a summation circuit (Σ) which, via a parallel output from the memory (M) , is arranged to summate a number (N) of the latest sampling values which corresponds to one or more whole periods of the reference signal (R); a diagnostic and determination circuit (Diag) arranged to be provided with the output signal from the summation circuit (Σ), in which circuit (Diag) the size, sign, variability and presence of an output signal from the summation circuit (Σ) is tested according to predetermined criteria; an arrangement for generating a regulating signal (Reg) is arranged to be provided with an output signal from the summation circuit (Σ), and by a circuit closer arrangement (F) arranged to be activated by the output signal from the diagnostic and determination circuit (Diag) when a criterion for oscilla- tion which is to be damped is fulfilled, which arrangement is placed between the output of the arrangement for generating a regulating signal (Reg) and a regulating device in the power transmission system. 13. Arrangement according to claim 12, c h a r a c - t e r i z e d i n that said memory (M) is a shift register. 14. Arrangement according to claim 12 or 13, c h a r a c t e r i z e d b y means (P7) for regulating the amplification of the regulating signal depending on the size of the tested signal.

Inventors: JUHLIN LARS-ERIK, JUHLIN, LARS-ERIK
Applicants: ASEA BROWN BOVERI, JUHLIN LARS ERIK, ASEA BROWN BOVERI AB
Application year: 1993
Main classification: H02J3
IPC codes: H02J3