Description:FORM 2
THE PATENTS ACT, 1970
(39 of 1970)
&
The Patent Rules, 2003
COMPLETE SPECIFICATION
(See sections 10 & rule 13)
1. TITLE OF THE INVENTION
A CONTROL SYSTEM AND METHOD TO DAMPEN SSO IN VARIABLE SPEED PUMPED STORAGE UNITS CONNECTED WITH EXTRA HIGH VOLTAGE TRANSMISSION LINES

2. APPLICANT (S)
NAME NATIONALITY ADDRESS
DIVYASAMPARK IHUB ROORKEE FOR DEVICES MATERIALS AND TECHNOLOGY FOUNDATION IN Indian Institute of Technology Roorkee, Roorkee-247667, Uttarakhand, India.
3. PREAMBLE TO THE DESCRIPTION
COMPLETE SPECIFICATION

The following specification particularly describes the invention and the manner in which it is to be performed.

FIELD OF INVENTION:
[001] The present invention relates to the field of a control system for the rotor side converter (RSC) to dampen sub synchronous oscillations (SSO). More particularly present invention relates to control system for damping of sub-synchronous oscillation in variable speed hydro generating units connected with 765 kV series compensated transmission lines.
DESCRIPTION OF THE RELATED ART:
[002] Conventional hydrogenerators, mostly synchronous generators, are now employed in domestic and international hydropower facilities, with a limited number of standard asynchronous generators frequency range (50Hz).
[003] Doubly fed induction machine (DFIM) with partial rated megawatt (MW) power converters are widely preferred in large rated variable speed pumped storage power plant (VSPSPP) since it offers higher energy efficiency at wide variation in water head, reduced cost and grid stability. Partial rated power converters are adopted for rotor excitation of DFIM and it handles bulk excitation current (e.g. a typical 250 MW DFIM handles 11620 A). Due to the limited power handling capacity of semiconductor devices, parallel connected power converters are adopted. Any failures occur in the excitation system the operation of the generating unit is interrupted, as it is asynchronous in nature.
[004] SSR (Sub-Synchronous Resonance) is a phenomenon that occurs in electrical power networks when a turbine-interaction generator interacts with a long-distance series compensated transmission line. An electrical power system exists in a situation where electrical networks exchange energy. Mechanical system of the generator at frequencies lower than the typical frequency of the transmission line (50Hz).
[005] The induction generator effect, torsional interaction, and torque amplification are three types of sub-synchronous resonance. A steady state disturbance causes the first two categories, whereas transient disturbances induce the third.
[006] It is reported that shaft failure of rotor is due to Sub Synchronous Oscillation (SSO) of series compensated power system.
[007] Reference may be made to the following:
[008] Publication No. US2013027994 relates to a wind turbine controlled to damp sub synchronous resonance oscillations on a grid. The wind turbine includes rotor blades for turning by the wind, an electric generator rotatably coupled to the rotor blades, a power converter responsive to electricity generated by the electric generator, the power converter for converting the generated electricity to a frequency and voltage suitable for supply to the power grid, and the power converter for regulating voltage on the grid for damping the sub synchronous oscillations. Additionally, in one embodiment voltage regulation is supplemented by modulating real power to damp the sub synchronous oscillations.
[009] Publication No. WO2022122396 relates to a control system for controlling the operation of a doubly fed induction generator (DFIG) of an electrical power system, in particular of a wind turbine. A rotor side converter coupled to a rotor of the DFIG is controlled by the control system. The control system comprises an outer controller that is configured to generate a reference value for a control variable in accordance with which the operation of the DFIG is to be controlled and an inner controller that receives the reference value and provide feedback control of the rotor side converter. The inner controller is a state feedback controller that is configured to obtain at least one state of the power system or the power grid that is different from the control variable. The control structure of the state feedback controller is configured such that the control of the rotor side converter causes the electrical power system to act as a passive system at least in a predefined frequency range.
[010] Publication No. WO2020125920 relates to a method and system of sub synchronous oscillations and interactions damping integrated in in a rotor converter based on an adaptive state feedback controller with two spinning vectors, and a kalman filter whose parameters are optimized by minimizing maximum sensitivity under a constraint of positive damping for a plurality of sensible scenarios. The damping signal generated by the damping module is applied either to a power proportional integer controller or to a current proportional integer controller.
[011] Publication No. US2011109085 relates to a wind turbine for controlling power oscillations on a grid of a power system. The wind turbine comprises rotor blades for turning by the wind, an electric generator rotatably coupled to the rotor blades, a power converter responsive to electricity generated by the electric generator, the power converter for converting the generated electricity to a frequency and voltage suitable for supply to the power grid, and the power converter for regulating voltage on the grid supplemented by modulating real power for damping the power oscillations.
[012] Publication No. US2014246914 relates to a power distribution system comprising a point of common connection that receives electric power supplied by a first power generation system and a second generation system, with the second power generation system that comprises a renewable electric power generator; a transmission line operatively connected to the point of common connection for conducting the electric power between the point of common connection and an external AC electric network. The power distribution furthermore comprises a capacitive compensator connected in series with the transmission line to compensate for a reactive power component of the electric power conducted by the transmission line; and a shunt arranged flexible AC transmission system that mitigates a sub-synchronous resonance effect caused at least in part by the capacitive compensator. A flexible AC transmission system controller of the flexible AC transmission system comprises a damping effect on sub-synchronous oscillations included in the sub-synchronous resonance.
[013] Patent No. US9899941 relates to a damping system can include a sensor disposed to measure an amplitude of a speed ripple of a drive shaft of a generator, and can include a feed forward circuit connected to the sensor and to the generator. The feed forward circuit can determine a phase angle formed by a load resistance and a load capacitance driven by the generator, calculate a voltage signal value based on the amplitude and the phase angle, and can adjust a DC link voltage provided by the generator and across the load resistance according to the voltage signal value.
[014] Publication No. CN109995080 relates to a robust control method for suppressing the sub-synchronous oscillation of a grid connection system of doubly-fed wind turbines. The method comprises the following steps: establishing a state space description of a grid-connection system of doubly-fed wind turbines; establishing D-region stability constraints of hybrid H<2>/H; establishing a convex polytope model containing different wind power operation conditions; and D, solving a state feedback matrix by using a linear matrix inequality to form a sub-synchronous oscillation robust damping controller. Through the robust control method for suppressing the sub-synchronous oscillation of a grid connection system of doubly-fed wind turbines, sufficient damping can be provided for the sub-synchronous oscillation mode of the system, and the controller also has a good control effect when the wind power changes in a large range.
[015] Patent No. US10224854 relates to methods and systems to damp oscillations in a synchronous alternating current (AC) grid. Current may be received from the synchronous AC grid through a phase of an n-phase supply line. The current received from the synchronous AC grid is supplied to a phase of the synchronous motor. A sub-harmonic oscillation may be detected in the current received from the synchronous AC grid. The sub-harmonic oscillation may be damped by: shunting a portion of the current away from the phase of the synchronous motor during a first time period in an upper-half of the sub-harmonic oscillation, and/or supplying compensation current from a partial power converter to the phase of the synchronous motor during a second time period in a lower-half of the sub-harmonic oscillation.
[016] Publication No. CN110556840 relates to the technical field of control of gas turbine speed regulation systems, in particular to a damping control method and control system of a gas turbine generator set speed regulation system. The opening command value PCV of the output power of a gas turbine generator set is output by a speed regulation system, and the opening degree of a fuel valve is adjusted through an electro-hydraulic servo system so as to control the output power of the gas turbine generator. A correction curve function f (PCV) is added between the output of the opening command value PCV of the value by the speed regulation system and the input of the electro-hydraulic servo system, correction of the nonlinear relation between the fuel valve command and the fuel valve opening is achieved, and it is ensured that the fuel valve command and the generator output power are in a linear relation. Meanwhile, a damping controller is additionally arranged on the input signal side of the electro-hydraulic servo system, and ultralow-frequency power oscillation suppression of the gas turbine generator set is achieved.
[017] Patent No. US10790668 relates to a method for operating a wind turbine system, and associated system, provides real and reactive power to a grid. The wind turbine system includes a generator with a power converter and an integrated reactive power compensation device. A total reactive power demand (Qcmd) is made on the wind turbine system at a first grid state, and is allocated to generator reactive power (Qg) and compensation device reactive power (Qmvb). A first reactive power droop scheme is determined that includes a reactive power droop value applied to one or both of the control loops for (Qg) and (Qmvb) at the first grid state. Upon detection of a grid fault, the first reactive power droop scheme is changed to a second reactive power droop scheme by changing the reactive power droop values applied to one or both of the (Qg) and (Qmvb) control loops during recovery from the grid fault.
[018] Publication No. WO2011121041 relates to a torsional mode damping controller system is connected to a converter that drives a drive train including an electrical machine and a non-electrical machine. The controller system includes an input interface configured to receive measured data related to variables of the converter or the drive train and a controller connected to the input interface. The controller is configured to calculate at least one dynamic torque component along a section of a shaft of the drive train based on the measured data from the input interface, generate control data for a rectifier and an inverter of the converter for damping a torsional oscillation in the shaft of the drive train based on the at least one dynamic torque component, and send the control data to the rectifier and to the inverter for modulating an active power exchanged between the converter and the electrical machine.
[019] Publication No. CN102334276 relates to a device for regulating a double-fed asynchronous machine, preferably for a power generation plant, particularly for a wind turbine or hydroelectric power plant, comprising an indirect converter, which is connected to the double-fed asynchronous machine on the rotor side, wherein the indirect converter comprises a converter on the rotor side and a converter on the power-grid side, and at least one control element is provided for regulating the indirect converter, wherein at least one software- and/or hardware-based damping element having variable damping characteristics.; The invention further relates to a method for regulating a double-fed asynchronous machine using an indirect converter and a device according to the invention, wherein power-grid characteristics are determined, particularly measured, wherein hardware- and/or software-based damping elements having variable damping characteristics are provided, and the damping characteristics of at least one hardware- and/or software-based damping element are subsequently adapted. The invention further relates to a computer program, a computer program product, and a wind turbine or hydroelectric power plant for generating electrical energy using a double-fed asynchronous machine.
[020] Publication No. WO2010045964 relates to a protection system of a doubly-fed induction machine, wherein the system comprises a crowbar connected to the rotor, one DC-chopper provided in the DC-link, a converter controller and a separate protection device, which controls the DC-chopper and the crowbar. In order to protect the DC-chopper against excessive heat, the protection device counts the firing pulses of the DC-chopper or measures the operating time of DC-chopper, wherein the minimum switch-off time of the DC-chopper depends on the number of firing pulses or the amount of operating time in a previous operating cycle of the DC-chopper. The converter controller estimates a rotor current by using a machine model of the doubly-fed induction machine and measured values of the stator voltage, the stator current and the angle between stator and rotor voltage. The doubly-fed induction machine is switched off from the grid by the converter controller if a critical crowbar current is determined.
[021] Patent No. US7638983 relates to a controller of a grid coupled type doubly-fed induction generator having a multi-level converter topology, which can control the doubly-fed induction generator having a high voltage specification and can perform a fault ride-through function, an anti-islanding function and a grid voltage synchronization function required for a dispersed power generation facility. The controller makes a H-bridge multi-level converter generate a three-phase voltage waveform resulted from the structure that single-phase converters each being composed of a 2-leg IGBT are stacked in a serial manner, and controls a rotor current so as to make the rotor coil of the doubly-fed induction generator in charge of a slip power only. The boost converter is composed of a 3-leg IGBT and a boost inductor generating a direct current voltage of its source required for the H-bridge multi-level converter.
[022] Patent No. US8183704 relates to a variable speed wind turbine having a doubly fed induction generator (DFIG), includes an exciter machine mechanically coupled to the DFIG and a power converter placed between a rotor of the DFIG and the exciter machine. Thus, the power converter is not directly connected to the grid avoiding the introduction of undesired harmonic distortion and achieving a better power quality fed into the utility grid. Moreover, the variable speed wind turbine includes a power control and a pitch regulation.
[023] Publication No. EP2357483 relates to a method of sub synchronous resonance detection in electrical power systems with series capacitors. Voltage signals are measured on line and by using finding zero crossing points of discrete signal of measured voltage, positive and negative half cycles of the wave form of discrete signal of voltage are calculated in a computer device to which constant parameters are delivered by the user. The inventive method comprises the following actions: ¢ creating a demodulated signal (U Dem ) of voltage by adding the minimum value of negative half cycle of the wave form of discrete processed signal (U x) of voltage to the maximum value of positive half cycle of the wave form of discrete processed signal of voltage (U x) for time intervals having a signal length (T L), where (T L) is a constant parameter, delivered by the user, ¢ calculating a root mean square value (RMS) for demodulated signal of voltage U Dem and comparing it with the value of another constant parameter delivered by the user as the level of root mean square value (RMS Lev) and when the value of (RMS) is smaller than the value of (RMS Lev) it indicates that there is no sub synchronous resonance, and when the value of RMS is bigger than the value of (RMS Lev ), the presence of sub synchronous resonance is identified by the determination of voltage amplitude (A Fss) of sub synchronous resonance and/or frequency (f Fss) of sub synchronous resonance.
[024] The article entitled “SSR damping controller design and optimal placement in rotor-side and grid-side converters of series-compensated DFIG-based wind farm” by Hossein Ali Mohammadpour; Enrico Santi; IEEE Transactions on Sustainable Energy (Volume: 6, Issue: 2); April 2015 talks about the sub synchronous resonance (SSR) phenomena in a capacitive series-compensated DFIG-based wind farm. Using both modal analysis and time-domain simulation, it is shown that the DFIG wind farm is potentially unstable due to the SSR mode. In order to damp the SSR, the rotor-side converter (RSC) and grid-side converter (GSC) controllers of the DFIG are utilized. The objective is to design a simple proportional SSR damping controller (SSRDC) by properly choosing an optimum input control signal (ICS) to the SSRDC block, so that the SSR mode becomes stable without decreasing or destabilizing the other system modes. Moreover, an optimum point within the RSC and GSC controllers to insert the SSRDC is identified. Three different signals are tested as potential ICSs including rotor speed, line real power, and voltage across the series capacitor, and an optimum ICS is identified using residue-based analysis and root-locus method. Moreover, two methods are discussed in order to estimate the optimum ICS, without measuring it directly. The studied power system is a 100 MW DFIG-based wind farm connected to a series-compensated line whose parameters are taken from the IEEE first benchmark model (FBM) for computer simulation of the SSR. MATLAB/Simulink is used as a tool for modeling and designing the SSRDC, and power system computer aided design/electromagnetic transients including dc (PSCAD/EMTDC) is used to perform time-domain simulation for design process validation.
[025] The article entitled “SSCI damping controller design for series-compensated DFIG-based wind parks considering implementation challenges” by Mohsen Ghafouri; Ulas Karaagac; Jean Mahseredjian; Houshang Karimi; IEEE Transactions on Power Systems (Volume: 34, Issue: 4); July 2019 talks about the use of supplementary controllers for mitigating sub synchronous control interaction (SSCI) in doubly-fed induction generator based wind parks is quite promising due to their low investment costs. These SSCI damping controllers are typically designed and tested using an aggregated wind turbine (WT) model that represents the entire wind park (WP). However, no research has been reported on their implementations in a realistic WP. This paper, first presents various implementation schemes for a linear-quadratic regulator based SSCI damping controller, and discusses the corresponding practical challenges. Then, an implementation scheme that obviates the need for high rate data transfer between the WTs and the WP secondary control layer is proposed. In the proposed implementation, the SSCI damping controller receives only the WT outage information updates from the WP controller, hence it is not vulnerable to the variable communication network latency. The SSCI damping controller parameters are also modified when there is a change in WT outage information for the ultimate performance. The effectiveness of the proposed implementation scheme is confirmed with detailed electromagnetic transient simulations, considering different wind speeds at each WT and WT outages due to sudden decrease in wind speeds.
[026] In order to overcome above listed prior art, the present invention aims to provide a control system and method for the Rotor Side Converter (RSC) to dampen sub synchronous oscillations in variable speed hydro generating units connected with series compensated extra high voltage transmission lines.
OBJECTS OF THE INVENTION:
[027] The principal object of the present invention is to provide a control system and method for the Rotor Side Converter (RSC) to dampen sub synchronous oscillations in variable speed hydro generating units connected with series compensated extra high voltage transmission lines.
[028] Another object of the present invention is to provide a supplementary damping controller for Rotor Side Converter (RSC) to dampen SSR oscillations in series compensated transmission lines with variable speed hydro generating units.
[029] Yet another object of the present invention is to provide a rotor side damping controller (RSDC) in the rotor side converter (RSC) of variable speed pumped storage units with series compensated transmission line to dampen sub synchronous oscillation.
SUMMARY OF THE INVENTION:
[030] The present invention relates to a system and method for a control system to dampen SSO in rotor side control system (RSCS) of a variable speed hydropower plant. It relates to rotor side damping controller (RSDC) in the rotor side converter (RSC) of variable speed pumped storage units with series compensated transmission line to dampen sub synchronous oscillation. This is an efficient control system for RSDC in RSC of a variable speed hydro generating unit equipped with extra high voltage lines. The controller makes the damping of sub synchronous oscillation fast.
[031] The DFIM operation in hydro energy conversion system is distinct in view of (i) controllers involving with the hydro power unit contains three sections such as (a) hydro turbine controller, (b) rotor side converter controller, and (c) grid side converter controller, whereas research articles in the wind farm consider only RSC and GSC controllers for controlling power in the DFIM fed plants.
[032] (ii) In DFIM fed hydropower generation, parallel connected high power converters are placed in the rotor side (In case of Tehri DFIM fed hydro, five parallel connected converter are employed and each converter is rated as 2400 A, 3300 V) for ensuring variable speed operation. This parallel connected converter brings more challenges during sub synchronous control interaction.
[033] (iii) Phase-shift transformers (PSTs) have been designed in the DFIM fed hydro units to eliminate the lower-order harmonics and to suppress the circulating current in the parallel connected converter (In case of Tehri hydro, PSTs were arranged with two sets of groups: (i) one primary and three secondary windings (Yd1 (- 12⁰), d1 (-0⁰), d1 (12⁰)), and (ii) one primary and two secondary windings (Yd1 (-6⁰), d1 (6⁰))) [24], such transformer design is considered while analyzing SSCI, however PSTs may not be employed in wind power generation.
[034] Finally, (iv) a limited power rating (maximum rating of a wind generating unit is 8 MW) of wind power generation does not make any impact on the stability of the power grid if it gets shutdown due to resonance stability. However, in large-rated hydro generating unit, the power rating of DFIM is very high in comparison with wind power unit. This patent describes the control system to dampen SSO in a variable speed hydro generating unit.
BREIF DESCRIPTION OF THE INVENTION
[035] It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered for limiting of its scope, for the invention may admit to other equally effective embodiments.
[036] Figure 1 illustrates hydrological and electrical depiction of 250MW large scale DFIM drive with series compensated extra high voltage transmission line.
[037] Figure 2 shows system model of 250 MW large scale DFIM drive with series compensated 765 kV extra high voltage transmission line.
[038] Figure 3 shows series compensated EHV line with DFIM system experimental test rig.
[039] Figure 4 shows rotor current oscillations with and without damping controller.
[040] Figure 5 shows variation of power with compensation.
[041] Figure 6 shows hardware results of power oscillations.
DETAILED DESCRIPTION OF THE INVENTION:
[042] The present invention provides a control system and method to dampen SSO in the rotor side control system (RSCS) of a variable speed hydropower plant (HPP). It provides rotor side damping controller (RSDC) in the rotor side converter (RSC) of variable speed pumped storage units with a series compensated transmission line to dampen sub synchronous oscillation. This is an efficient controller for RSDC in RSC of a hydro generating unit equipped with a doubly fed induction machine (DFIM). The controller makes the damping of sub-synchronous oscillation fast.
[043] The control system to dampen SSO in rotor side control system (RSCS) of a variable speed hydropower plant (VSHPP) it comprises of a reservoir and turbine, with inlet valve and intake gates that can either be in a fully closed or fully open position to cut off the water supply to the turbine and penstock, Generator, with generator circuit breaker (GCB 4), Unit circuit breaker (UCB 6), Power transformer (7), excitation transformer (8), doubly fed induction machine (10), (DFIM) converter including grid side converter (11), rotor side converter, and extra high voltage transmission line (13) and grid (14) characterized in that DFIM converter control consists of rotor side and grid side control strategies separately, and electromagnetic torque and reactive components of DFIM are controlled through rotor side converter and DC-link voltage, and reactive power component of grid are controlled through grid side controller, guide vane, hydraulic governor control system with asynchronous generating and the grid wherein the control of the output/speed of the unit is by adjusting the position of guide vane opening and are kept in any position between a fully closed and nearly fully open position and regulate the flow of water to the runner to match the required output at the existing head using a closed-loop feedback controller.
[044] The present invention provides a control system and method to dampens SSO of a hydro generating unit equipped with a doubly fed induction machine (DFIM) through RSDC as shown in Fig. 1.
[045] A 250 MW DFIM with 765kV extra high voltage transmission line is modelled in MATLAB/Simulink based on Fig. 2 and simulated through time domain analysis. The speed of the test system is chosen as 214 rpm (0.93 p. u) considering sub synchronous oscillation and delivering active power and reactive power of 225 MW, and 18 MVAr, respectively.
[046] As an when series compensation applied to EHV transmission line, SSO Occurs with distorted voltage and current containing electrical natural frequency components are measured and fed to controller, then controller process the distorted information, the reference signal fed to the PWM pulse generator contains sub synchronous distortion this cause negative damping leading to SSO.
[047] This invention identifies the method to tuning RSC gain to dampen SSO. To verify the SSO mitigation methods, initially DFIM system is compensated with 50% level. Then the mitigation methods are applied to the DFIM system at 3s, and results is shown in Fig. 4. It is inferred that without mitigation methods the oscillation is carried in the rotor current and make the system into unstable mode if it is continuously existence in the system.
[048] In case of SSO mitigation methods, tuning of rotor side converter controller approach is performed in the experimentation and result is given in Fig. 5. To analyze the performance of the proposed SSO mitigation method, at first DFIM is operated with 50% series compensation level and its stator current is keep oscillating due to the nature of series compensation and when applying the proposed SSO mitigation method then oscillations and ripples are reduced and back to the original waveform.
[049] Here the stator terminal of DFIM is connected with series compensated EHV Transmission line to grid, and the rotor terminal of DFIM is connected through a parallel-connected power converter.
[050] Simplified Equation of converter is,
V_ca-V_ga=L (di_a)/dt+Ri_a
[051] Using Transformation
█(V_cαβ-V_gαβ=L (di_αβ)/dt+Ri_αβ@y ̅_αβ=y ̅_dq e^jωt@V ̅_cdq-V ̅_gdq=L didq/dt+(R-jω)i ̅dq)
[052] The transfer function as below
H(s)=k(T_s+1)/T_s ⋅1/(1+T_as )⋅1/(1+T_ds )⋅1/(R+LS)
[053] To Solve,
█(&1+Kp/((1+TaS)(1+Tds)(R+LS))=0@&=K_p (1+T_a )(1+T_d s)(R+LC)=0@&K_p+(1+S(T_a+T_d )+T_a T_α S^2 )(R+LS)=0@&■(&@&K_p+R[1+S(T_a+T_d )+T_d T_d S^2 ] )@+LS⋅(1+S(Ta+Td)+T_a T_d S^2 )=0@&K_p+R+R_s (T_a+T_d )+T_a T_d RS^2+LS+LS^2@&+(T_d+T_d )+LT_a T_d S^3=0@<_a T_d S^3+S^2 (LCT_d+T_d )+T_d T_d R.@&+S(R(T_a+T_d )+L)+1+R=0@&K_p+(1+T_as )⋅(1+T_dS )(R+LS)=0)
K_p+(1+T_ds)[(R+LS)+T_d RS+T_d LS^2 ]=0
█(K_p+R+LS+T_a RS+T_d LS^2+RT_a S+LT_a S^2@+T_a T_d RS^2+T_a T_d S^3=0)
T_d T_a LS^3+S^2 (T_d T_a ├ R+TdL+T_a L)
+S(L+TdR+RT_a )+K_p+R=0
[054] By Comparing,
a_0 s^3+a_1 s^2+a_2 s+a_3=0
[055] By RH criteria
s^3 a_0 a_2
s^2 a_1 □( ) a_3
s_1 (a_1⋅a_2-a_0 a_3)/a_1
s^0 a_3
[056] From M=0 a_1 a_2-a_0 a_3=0
[057] where a_0=T_a T_d L□( ) , a_1=(T_a T_d R+T_d L+TaL)
■(a_2=(L+T_a R+RT_a ) □( ) a_3=K_p+R@@)
■(@├ ∴" " T_a T_d R" +" T_a "L + " T_a )(L+T_d R+RT_a )@□( )-(TaT_d L)(K_p+R)=0)
[058] Solve for Z
█(&Z-(T_a T_dL )(K_p+R)=0@&├ Z-(T_a T_d L) K_p+T_aTdL )R=0@&K_p=(Z+(T_a T_d L))/((T_a T_d L) )@&∴a_1 S^2+a_3=0@&-aω^2-a_3=0@&ω=√(a_3/a_1 )=((K_p+R))/(T_d T_d R+T_a L+LT_d )@&T_I=2π/ω@&K_pc=0.45K_p@K_i=0.83T)
[059] For 250 MW machines,
█(&R=2.52mΩ,L=4.94mH@&C=18000μf.)
[060] A. Simulation Results
[061] In Fig. 4a. is the rotor current vs. time is shown. At t = 0.3 sec when series compensation is switched ON oscillation in rotor current is depicted in Fig. 4a. The rotor current vs time is shown in Fig. 4b. and illustrate damping of the rotor current oscillations at t = 0.3 sec when series compensation is switched ON.
[062] B. Experimentation Validation and Results
[063] An experimental setup having a 2.2 kW DFIM connected to 415V grid through series compensated transmission line, which is presented as inductors and capacitors have been setup in laboratory. The Fig. 3 shows laboratory rest rig. Switching pulses for the RSC are obtained from dSPACE MicroLabBox 1202. The switching frequency of 2.5 kHz and dead band of 6 μ sec is used for pulse generation.
[064] In case of SSO damping methods, tuning of rotor side converter controller approach is performed in the experimentation and result is given in Fig. 5. To analyze the performance of the proposed SSO mitigation method, at first DFIM is operated with 50% series compensation level and its stator current is keep oscillating due to the nature of series compensation and when applying the proposed SSO mitigation method then oscillations and ripples are reduced and back to the original waveform.
[065] The variation in the real power with and without compensation is shown in Fig. 6. In compensated systems, active power flow is increased and the power factor is much higher. In this, 50% compensation is given for the line and it is observed that the total real power delivered from the stator is increased between 3 and 3.6 min to 550 from 440W. It is found that when compensation is applied to the line, power flow rises and power transfer capacity improves, however SSO occurs.
[066] Numerous modifications and adaptations of the system of the present invention will be apparent to those skilled in the art, and thus it is intended by the appended claims to cover all such modifications and adaptations which fall within the true spirit and scope of this invention.
, Claims:WE CLAIM:
1. A control system to dampen Sub Synchronous oscillations (SSO) in rotor side control system (RSCS) of a variable speed pumped storage power plant (VSPSPP) comprises-
 a upper (1), lower (2) reservoir and pump turbine (3), with inlet valve and intake gates that can either be in a fully closed or fully open position to cutoff the water supply to the turbine and penstock,
 Generator, with generator circuit breaker (GCB 4),
 Stator short circuit breaker (SSCB 5),
 Unit circuit breaker (UCB 6),
 Power transformer (7),
 excitation transformer (8), and excitation circuit breaker (ECB 9),
 doubly fed induction machine (DFIM) (10),
 converter including grid side converter (11) and rotor side converter (12) characterized in that DFIM converter control consists of rotor side and grid side control strategies separately, and electromagnetic torque and reactive components of DFIM are controlled through rotor side controller and DC-link voltage, and reactive power component of grid are controlled through grid side controller guide vane, hydraulic governor control system with asynchronous generating and the grid wherein the control of the output/speed of the unit is by adjusting the position of guide vane opening and are kept in any position between a fully closed and nearly fully open position and regulate the flow of water to the runner to match the required output at the existing head using a closed-loop feedback controller.
 Extra high voltage (EHV) transmission line with series compensation (13), grid (14) and with asynchronous hydro generating unit is equipped with wherein the stator terminal of DFIM is connected with series compensated EHV Transmission line to grid, and the rotor terminal of DFIM is connected through a parallel-connected power converter.
2. The control system to dampen Sub Synchronous oscillations (SSO) in rotor side control system (RSCS) of a variable speed pumped storage power plant (VSPSPP, as claimed in claim 1, wherein the stator terminal of DFIM is connected with series compensated EHV Transmission line to grid , and the rotor terminal of DFIM is connected through a 2-channeled parallel-connected power converter (10 kW, IGBT modules).
3. The control system to dampen Sub Synchronous oscillations (SSO) in rotor side control system (RSCS) of a variable speed pumped storage power plant (VSPSPP, as claimed in claim 1, wherein the specified controller can be adjusted without the requirement for trial and error.