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This paper proposes a powerful subsynchronous component based (SSC) controller to mitigate the subsynchronous resonance (SSR) with statics synchronous series compensator (SSSC). The mitigation of SSR is achieved by increasing the network damping at those frequencies which are close to the torsional frequency of the turbine-generator shaft. The increase of network damping is done by the extraction of subsynchronous component of voltage and current from the measured signal of the system. From the knowledge of subsynchronous components, a series voltage is injected by SSSC into the transmission line to make the subsynchronous current to zero which is the main cause of turbine oscillations. To analyze the effectiveness of the proposed control scheme, IEEE first benchmark model has taken. The results show the accuracy of the proposed control scheme to mitigate the Torque amplification of SSR.

Series capacitor compensation has been extensively employed in power system to increase the power transfer capability of long HV and EHV lines, load sharing among parallel lines and enhance the steady state and transient stability limits [

SSR is an electric power system condition where the electrical network exchanges energy with a turbine generator at one or more of the natural frequencies of the combined system below the synchronous frequency of the system [

The fast development of modern power electronic devices led to the development of FACTs devices like TCSC, STATCOM and SSSC. A large number of methods and solutions have been addressed by the different authors to avoid the problem of SSR with the concern of FACTS devices [

The voltage sourced converter-based SSSC is essentially an ac voltage source which, with a constant dc voltage and fixed control inputs, would operate only at the selected output frequency, and its output impedance at other frequencies would theoretically be zero. The SSSC considered is a voltage source inverter, and is equipped with a proportional integrator controller (PI) that regulates the generator terminal voltage. In a practical SSSC, the voltage-sourced converter on the dc side is terminated by a finite energy storage capacitor to maintain the desired dc operating voltage. Thus the dc capacitor in effect interacts with the ac system via the operating switch array of the converter. This interaction may conceivably influence the subsynchronous behaviour of a practical SSSC [

This paper is organized as follows: a study system model i.e., IEEE first benchmark model with SSSC is introduced in Section 2. In Section 3, mathematical analysis is presented for extraction of subsynchronous component of voltage. This analysis will be helpful in determining the value of voltage injected in series to the line with SSSC. Consequently, in Section 4, the design of subsynchronous controller is depicted. Section 5 shows the parameters of the study system and the specifications of SSSC. In Section 6, simulation results obtained for IEEE first benchmark model with SSSC controller with three-phase fault. The Section 7 concludes the total work of the paper.

_{s} and the grid current is denoted by i respectively. The SSSC is modelled as a controlled ideal voltage source. The injected voltage is denoted by

To derive the subsynchronous component of the voltage at the generator terminals, consider the generic case of a synchronous generator connected to a transmission line.

The per-unit voltage at the generator terminals in αβ-plane as

where V_{s} is the amplitude of voltage at the generator terminals at rated speed, δ is phase displacement

where A is the amplitude of the oscillation and

The derivative of the rotor angle is given by

where,

The

The

When a small distribution is applied to the generator rotor, the resultant voltage will be constituted by the sum of three terms; they are fundamental frequency component, sub-synchronous component of frequency and super-synchronous component of frequency. The component of super-synchronous frequency is higher than the sub-synchronous frequency component. For super-synchronous frequency the network presents a small positive damping, thus the super synchronous voltage does not represent a risk for the power plant. Therefore super synchronous component of the voltage will not be taken in to account [

From Equations (6) and (7), the subsynchronous component of the measured voltage is given by

The grid voltage vector can be transformed in the synchronous reference plane as

where

nent is written as

When the generator rotor oscillates around its rated speed, the voltage at the terminals can be expressed in the synchronous dq co-ordinate system as

The frequency

In order to extract the sub-synchronous component from the measured signal, (11) can be rearranged so that

where indicated with p, the operator

pass filter (LPF) for the fundamental and for the subsynchronous component, respectively. Equation (14) can be written in the synchronous dq-frame as

Equations (13) and (15) can thus be combined together in order to extract the fundamental and the subsynchronous components of the measured voltage.

Consider the generator is modeled as an ideal voltage source behind the sub-transient inductance of the generator. To make the subsynchronous current to zero, the objective of the common control system is to produce and inject the subsynchronous component of the internal bus current/voltage by STATCOM/SSSC [

where^{ײ} are the resistance of the system upstream the SSSC, the leakage inductance of the transformer and the sub transient inductance of the generator, respectively. The current reference is

_{m}-coordinate systems using the transform angle

The system investigated for the study is the well-known IEEE first benchmark model. The system consists of 892.4 MVA turbine-generator connected to an infinite bus through radial series compensated line. The voltage and frequency are 539 KV and 60 Hz respectively. Program has been written to figure out turbine natural frequencies [

In this paper three-phase VSC based bridge is used for SSSC. The amount of power needed for mitigation of SSR is related to several factors (such as series-compensation level, fault duration and its location) that cannot be predicted accurately. As the sub-synchronous frequency component of voltage and current is low, the rating of SSSC is low (0.1%). The voltage rating is 8 KV (either DC source or capacitor). The power rating is 12 MVA. The results are obtained for active power 0.1 pu [

To know the effectiveness of the proposed control strategy to mitigate SSR due to Torque Amplification, the IEEE FBM with SSSC has been simulated using the Matlab-Simulink.

Network resistance | R_{L} | 0.0113 pu |
---|---|---|

Transformer reactance | X_{T} | 0.142 pu |

Transformation ratio | 22/539 KV | |

Line reactance | X_{L} | 0.50 pu |

Transmission line reactance | X_{sys} | 0.08 pu |

Reactance | Value [per unit] | Time constant | Value [sec] |
---|---|---|---|

0.130 | 4.3 | ||

1.79 | 0.032 | ||

0.169 | 0.85 | ||

0135 | 0.05 | ||

1.71 | |||

0.228 | |||

0.200 |

shown in

Inertia | H [s^{−1}] | Shaft section | Spring constant [pu∙T/rad] |
---|---|---|---|

HP turbine | 0.092897 | HP-IP | 19.303 |

IP turbine | 0.155589 | IP-LPA | 34.929 |

LPA turbine | 0.858670 | LPA-LPB | 52.038 |

LPB turbine | 0.884215 | LPB-GEN | 70.858 |

Generator | 0.868495 | ||

HP turbine | 0.092897 | HP-IP | 19.303 |

To avoid the shaft damage of torque amplification effect due to SSR, SSSC is connected.

In this Research work, an accurate SSC based control scheme is proposed to mitigate the oscillations due to SSR with SSSC. The SSSC is constituted by three-phase VSC connected in series with the power line. Based on the control scheme the SSR mitigation is obtained by increasing the network damping at those frequencies which are close to the natural mode frequencies of the turbine-generator shaft. In the control scheme the estimation of subsynchronous components are proposed and are used for SSR mitigation by injecting the voltage in series with the line by SSSC. It has been shown that SSR mitigation is achieved by injecting a low amount of voltage in the grid, leading to reduced power rating for the SSSC. Finally, simulation results have shown the effectiveness of the proposed control scheme.

Mohan P.Thakre,Vijay S.Kale,Koteswara RajuDhenuvakonda,Bhimrao S.Umre,Anjali S.Junghare, (2015) Study and Mitigation of Subsynchronous Oscillations with SSC Based SSSC. Journal of Power and Energy Engineering,03,33-43. doi: 10.4236/jpee.2015.39003