the present invention will be described in detail in connection with a preferred embodiment with reference to attached drawings . fig1 shows , in a block form , an example of the configuration of an apparatus according to the present invention . in this example , the generator is assumed to be the five - mass model , viz ., the model comprising high , medium and two low pressure turbines , and the generator itself . in the example of this model , there appear four shaft torsional frequencies ( f m ) of 8 . 33 , 16 . 6 , 21 . 5 and 22 . 5 hz . in the following discussion , these frequencies will be handled with their being classified into three groups , i . e ., about 10 hz , 10 to 20 hz and more than 20 hz . initially , in the case that the shaft torsional frequency f m is approximately 10 hz , a signal derived from the super synchronous component is used as the stabilization signal . three phase currents i a , i b and i c on the primary side of a converter transformer 4 are employed as an input of the apparatus . by passing these three phase currents through a bandpass filter ( which will be referred to as &# 34 ; bpf &# 34 ; hereinafter ) 15 of the resonant frequency ( f 1 + f m ), super synchronous components δi a , δi b and δi c are obtained wherein f 1 denotes a fundamental frequency . for applying d , q axis conversion to these three phase variables , sin and cos of the phase angle θ g of the generator is multiplied . thus , δi q - sp and δi d - sp are obtained from the super synchronous component . it is to be noted that multiplication of cos can be omitted as described later . ## equ3 ## for obtaining θ g , application of a phase output θ op of a pll 13 of the synchronizing circuit is considered . since there is a phase difference φ p therebetween in general , this phasse difference is taken into account . by inputting θ g to a function generator 14 , sin function is generated . the sin function thus obtained is multiplied by the previously described δi a , δi b and δi c at a multiplier 16 . by summing outputs from the multiplier 16 , - δi q - sp corresponding to the equation ( 8 ) is obtained . by passing - δi q - sp through a bpf 20a of the resonant frequency f m , a stabilization signal δe st is prepared . the stabilization signal δe st thus obtained is subjected to summing operation on the output side of the constant current control regulator 11 . in regard to q factor of bpf , as an example , q factors of the input and output bpfs 15 and 20 are 5 and 1 , respectively . in the case of 10 hz & lt ; f m & lt ; 20 hz , a synchronous phase error δθ ep of the pll 13 is used as the stabilization signal . this δθ ep is subjected to subtracting operation on the output side of the regulator 11 through a phase - leading circuit 21 and the bpf 20b of the resonant frequency f m . in the case of f m ≧ 20 hz , a quantity of - δi q - sp derived from the sub synchronous component is used as the stabilization signal . in this case , the previously described super synchronous component is solely replaced with the sub synchronous component , but the configuration is just the same as the previous case . the configuration in this case includes an input bpf 17 of a resonant frequency ( f 1 - f m ), a multiplier 18 , an adder 19b , and an output bpf 20c of the resonant frequency f m . it is to be noted that the polarity of the adder 12 is minus . in this embodiment , the practically unacceptable frequency range of the shaft torsional oscillation mode frequency is assumed to be 8 to 25 hz . as a result of the emtp calculation , it has been seen that it is suitable for attainment of attenuation effect over the entire mode to divide the above - mentioned frequency range into high , medium and low frequency ranges to apply the stabilization signal thereto . in the application of the stabilization signal , care must be taken in the following problems . ( 1 ) commonly using the stabilization signal in two modes is not preferable because respective attenuation effects are reduced . this is due to the fact that the gain of the stabilization signal for each mode is required to be reduced to one half in view of suppression of higher harmonic ripples . ( 2 ) since the stabilization effect of the sub / super synchronous component has strong frequency dependency , suitable selection is required . ( 3 ) there is an interference between modes of the stabilization signal . particularly , when frequencies in the medium and high modes are relatively close to each other , there is a tendency that sucn an interference occurs . the stabilization is confirmed on the basis of the d e calculation in connection with respective stabilization signals and the detailed explanation will be made as follows . there is a tendency that according as this signal is stabilized , an angle that this signal and the regulator output δe r form becomes small . by inverting the polarity , a vector to cancel δe r is obtained . thus , δθ ep is easily taken out . since pll has the ac bus voltage as synchronous input , this value is assumed to be delayed a little as compared to δθ ep calculated from δω of the generator . the phase - leading circuit 21 corrects this delay . the d e calculation including this stabilization signal is performed . thus , the synchronizing phase error δθ ep ( δe syn ) is stabilized . the result of d e & gt ; 0 is obtained . when the fundamental frequency and the shaft torsional frequency are represented by f 1 and f m , respectively , there occur in the generator current , a so called sub synchronous component having a frequency ( f 1 - f m ) and a super synchronous component having a frequency ( f 1 + f m ) in addition to the fundamental wave having the frequency f 1 . since these current components are naturally included in the converter current in the isolated dc - transmission , they are calculated by fourier analysis . thus , such components are intended to be used as the stabilization signal . for shortening one time period of the fourier analysis , f m is selected so that f 1 / f m makes a suitable ratio of integer . for example , in the system where the fundamental wave has a frequency f 1 of 50 hz , it is sufficient that four cycles of the fundamental wave form one time period of the fourier analysis when f m = 12 . 5 hz . from the analysis of the three phase current of the converter , ( f 1 ± f m ) can be regarded as positive sequence components , respectively . in addition , since ( f 1 ± f m ) components can be regarded as changes with respect to the fundamental f 1 component , they are subjected to d , q axis conversion , whereby δi d and δi q are obtained . in accordance with the conversion formula from well known three phase currents i a , i b and i c to i d and i q , the conversion formula is performed by multiplying sin and cos of the phase angle θ g of the generator by the above three phase currents . when changes of i d and i q are subjected to linearization , the following relational equations ( 10 ) and ( 11 ) are obtained . ## equ4 ## it is to be noted that when three phase currents are represented by i ao + δi a , i bo + δi b and i co + δi c , the steady state current of phase a and its change are expressed as follows : ## equ5 ## both steady state currents of phases b and c and their changes are shifted by 2π / 3 and 4π / 3 with respect to the equation ( 1 ), respectively . there are relationships related to the phase of the generator as shown below . as seen from the equations ( 10 ) and ( 11 ), δi d and δi q consist of three components , respectively . by phasor representation , there can be compared with the above - described phasors related to the stabilization mechanism . it is to be noted that instantaneous values ( t representation ) are assumed to be equal to the imaginary part of phasor . an example of calculation is shown in fig5 ( a ) wherein phasors having been explained in relation to the stabilization mechanism and phasors derived from sub / super synchronous components are shown on the left and right sides , respectively . in the d e calculation , the stabilization phasor of the super synchronous component with respect to the lower frequency mode is given as follows . this phasor is added to the regulator output δe r as - δe st ( equation ( 7 )). the stabilized d e calculation result is shown in fig5 ( b ). - δi q - sp which is the phasor representation of - δi q - sp corresponds to the equation ( 14 ). it is to be noted that the phase of the phasor equivalent to the equation ( 14 ) is obtained by leading the phase of δi q - sp by π / 4 without using δi d - sp . in the sin function generator 14 in fig1 when θ g is substitued for ( θ g - π / 4 ), the phasor δi q - sp lead by a phase angle of + π / 4 . accordingly , it is sufficient that the phase difference φ p for adjustment for obtaining θ g of the generator from θ op of pll is substituted for φ p - π / 4 . thus , the calculation of δi d - sp becomes unnecessary . as a result , the calculation of the cos function corresponding to the sin function can be omitted . the adjustment phase φ p permits the phase adjustment of the stabilization phasor , but is basically a fixed angle . for example , for emtp , φ p is a constant as defined below . since the synchronous phase output θ op is coincident with the input phase θ in a steady state when the ac bus voltage of the converter is assumed to be a synchronous input of pll , the ac bus voltage e a is expressed as follows . on the other hand , when the phase angle of the generator is represented by θ g , the ac bus voltage e a is expressed , in accordance with emtp , as follows . ## equ6 ## by allowing the equations ( 15 ) and ( 16 ) to correspond to each other , φ p is calculated as follows . from θ g - π / 2 - δ s = θ op , φ p = π / 2 + δ s is obtained . in fig1 the quantity - δi q - sb derived from the sub synchronous component is taken as the stabilization signal over the higher frequency mode . the reason why its polarity in the adder 12 is taken in a manner opposite to that of the super synchronous component is that δi q - sb has substantially antiphase relationship with respect to δi q - sp in principle , ( fig5 ( a ) and 5 ( b ). fig5 ( a ) and 5 ( b ) show an example of calculation of lower frequency mode . such a tendency does not change for higher frequency mode . the reason why a quantity derived from the super synchronous component is used for the lower frequency mode and a quantity derived from the sub synchronous component is used for the higher frequency mode is as follows . by rewriting δθ g / δθ g for instantaneous value / phasor with ε / ε , these parameters are defined as follows . where ω m = 2πf m and f m is shaft torsional frequency . then , by applying the phasor relationship expressed as δθ g = ε to the above - mentioned equation ( 13 ), phasors i d ε and i q ε are defined . i d ε and i q ε are obtained by dividing respective terms of the right side of the equation obtained with individual d e calculation by ε . d e is calculated as the real part of δt e / δω as follows . as well known , δt e is first expressed as follows . ## equ7 ## assuming that the avr effect of the third term of the right side is negligible , when p = jω m ( pu ), the phasor representation of the equation ( 20 ) is as follows . δte =[ e . sub . fd -( x . sub . d - x . sub . q ( jω . sub . m )) i . sub . do ] δi . sub . q +( x . sub . q - x . sub . d ( jω . sub . m )) i . sub . qo · δi . sub . d ( 21 ) further , δi q and δi d are replaced with ε · i q ε and ε · i d ε using the equation ( 19 ), respectively . the phasor δω is expressed as follows . ## equ8 ## dividing equation ( 21 ) by equation ( 22 ) to obtain the real part d e , d e is expressed as follows . ## equ9 ## where * represents conjugate phasor , and a , b , c and d represent positive constants obtained from the generator constant and have the relationship expressed as a & gt ;& gt ; b , c & gt ;& gt ; d . accordingly , when phasors which respectively lag the phasors i d ε and i q ε by 90 degrees are prepared , d e is substantially proportional to the real parts thereof . the contribution to d e of the super / sub synchronous components is obtained as follows . the super / sub synchronous component is composed of the following two components . these components are expressed using phasors i d ε and i q ε as follows : ( a ) the contribution based on a change δi = δi d + jδi q of the generator current . ( b ) the contribution based on a change of the phase of the steady state value i o = i do + ji qo of the generator current . the sum of the above - mentioned ( a ) and ( b ) gives the sub / super synchronous components . when the above four vectors are resolved in directions of the phasors i d ε and i q ε , respectively , the contribution to d e can be seen from the equation ( 23 ). the above four phasors have been calculated in connection with respective stable / unstable examples of the shaft torsional oscillations of three frequencies of 8 . 33 , 16 . 67 and 20 . 0 hz , and the phasors contributing to d e & gt ; 0 have been studied . as a result , it has been that the following two frequencies have such a tendency . others have a small effect with respect to d e ( d e ≈ 0 ) or adversely contribute to d e & lt ; 0 . the reason why the super / sub synchronous components of the above ( a ) and ( b ) are used has been stated above . the advantages with the present invention are demonstrated by digital simulation based on emtp . when shaft torsional oscillations are excited due to a rapid deblock of 100 % of the dc transmission system , the comparison between off and on of the stabilization signal in connection with the shaft torque t 4 between a generator and the adjacent turbine which is not shown is shown in fig6 ( a ) and ( b ). in this example , the stabilization signals of fig1 are used . in the d e calculation , the effects with the single stabilization signal are calculated in connection with the corresponding shaft torsional frequency . further , the mechanical torsional system is opened to calculate d e in the open loop . on the other hand , the actual system has four shaft torsional mode frequencies in this example . since three stabilization signals are applied thereto , it is necessary to verify that attenuation effect in the closed loop including the mechanical system in the same manner as in the actual system . the whole attenuation tendency can be seen from fig6 when the stabilization signals are on , but attenuation of each mode cannot be seen . accordingly , fourier analysis of the waveforms are conducted to compare them . an example of this calculation is shown in table 1 . table 1______________________________________the effects of stabilization signalstimeoff onmode 1 2 3 1 2 3 note______________________________________1 10 . 5 % 6 . 3 9 . 0 7 . 5 % 4 . 5 3 . 0 time in seconds off : fig6 ( a ) 2 27 . 5 26 . 0 24 . 0 21 . 0 14 . 5 7 . 03 4 . 0 2 . 5 5 . 0 4 . 5 1 . 0 2 . 5 on : fig6 ( b ) 4 8 . 1 6 . 5 3 . 0 8 . 3 6 . 5 4 . 0______________________________________ table 1 shows a transient attenuation of a shaft torsional oscillation having a large amplitude at the earlier stage of disturbance when the stabilization signal is off . it is assumed that shaft torsional oscillations of the first and second modes last thereafter , i . e ., do not naturally attenuate . the reason is that the d e calculation results related thereto show d e ≦- 2 . 0 pu at the dc loading of 100 %, respectively . on the other hand , there is a tendency that all the modes attenuate in the application of three kinds of stabilization signals in fig1 . when the stabilization signals of the first and second modes are on and the stabilization signals of higher modes ( the third and fourth modes ) are off , shaft torsional oscillations of the first and second modes exhibit better attenuation effect rather than in table 1 . in contrast , shaft torsional oscillations of higher modes have poor attenuation effect and turn to lasting or increasing tendency . when stabilization signals of all modes are off , there appears natural attenuation tendency for higher modes ( table 1 ). this is assumed to result from the fact that the characteristic of d e essentially shows d e & gt ; 0 in higher frequency range . this is also assumed to originate from the fact that when stabilization signals of the first and second modes are applied , higher modes are caused to exhibit d e & lt ; 0 as a reaction thereof . accordingly , stabilization signals are required even for higher modes in order to obtain the attenuation tendency over the entire mode , resulting in the necessity of the configuration in fig1 . since there is a tendency that the stabilization signal of the higher mode interferes with the stability effect of the second mode , an on / off control of the higher frequency signal is assumed to be conducted . first , in the case of failure of the ac / dc system , when a shaft torsional oscillation is excited to much extent , the stabilization signal of the higher frequency mode is turned off for about 3 sec . at this time period , effective attenuation of the first and second modes is obtained by two stabilization signals of the first and second modes . for further enhancing the attenuation effect , addition of a δf signal is effective . when δf obtained from the ac bus voltage through a frequency sensor is used together with stabilization signals of the first and second modes , it is realized by the emtp simulation that the stabilization signals of the first and second modes attenuate below 2 % in about 3 sec . in contrast , when only δf is used , the attenuation effect of the first and second modes can be hardly anticipated . in this instance , δf is added in consideration of the polarity on the output side of the regulator without using bpf , etc . on the other hand , the higher mode occupies the greater part of residual ripples of the shaft torsional oscillation by the stabilization signal placed in off state after 3 sec . have passed . at this stage , by making use of the stabilization signal of the higher mode , the higher mode is caused to attenuate . for the first and second modes , they attenuate below about 2 % for the first 3 sec . and after the higher frequency signal is turned on the state of d e & gt ; 0 is maintained over all the mode frequencies . ( 1 ) there is less interference with respect to high response of the constant current control which is the basic control of the dc transmission . since a stabilization signal is obtained through a filter tuned to the shaft torsional oscillation frequency of the generator to add the stabilization signal to the output side of the regulator using an open loop , when the shaft torsional frequency is stabilized , the stabilization signal disappears . since a bandpass filter is provided in the signal path , there is less interference with respect to the basic control system by other frequencies . ( 2 ) conventional basic control system can be applied as it is . where there is the problem of the shaft torsional oscillation , it is possible to add the stabilization signal later . in general , mode frequencies of shaft torsional oscillations are known values for individual plants and the number of modes is not so many . since the apparatus according to the present invention provides optimum stabilization signals for low , medium and high frequency ranges of the shaft torsional oscillation frequency , it is sufficient to design these signals differently from the basic control to add them . accordingly , the countermeasure for the shaft torsional oscillation frequency can be completely independently taken with the conventional basic control system being as it is , resulting in small burden in control system design . ( 3 ) since the stabilization signal is constituted with easily available components at a convertor station , there is a little problem in practical use . the synchronous phase of pll is utilized as the generator phase θ g necessary for obtaining d , q axis components .