Simulation & Analysis Of Static Synchronous Series Compensator

With the increasing size and complexity of the transmission networks, the performance of the power systems decreases due to problems related with the load flow, power oscillations and voltage quality. Flexible AC Transmission (FACTS) offers effective schemes to meet these demands...  - Dr G D Kamalapur

A Static Synchronous Series Compensator (SSSC) is used to investigate the effect of FACTS device in controlling active and reactive powers as well as damping power system oscillations in transient mode. Simulation results obtained in two machine power systems show the efficacy of this compensator as one of the devices member in controlling power flows, achieving the desired value for active and reactive powers, and damping oscillations appropriately.

An SSSC is used to investigate the effect of this device in controlling active and reactive powers as well as damping power system oscillations in transient mode.

The SSSC equipped with a source of energy in the DC link can supply or absorb the reactive and active power to or from the line. Simulations have been done in MATLAB/SIMULINK environment.

The SSSC offers an alternative to conventional series capacitive line compensation. It is a solid-state voltage source that internally generates the desired compensating voltage independent of the line current. The voltage source nature of the SSSC provides the basis for its superior operating and performance characteristics not achievable by series capacitor type compensator: Internal reactive power generation and absorption without ac capacitors or reactors: control of reactive compensating voltage independent of the magnitude of the line current.

An SSSC is a member of FACTS family, which is connected in series with a power system, consisting of a solid state voltage source converter that generates a controllable alternating current voltage at fundamental frequency. When the injected voltage is kept in quadrature with the line current, it can be emulated as inductive or capacitive reactance – so as to influence the power flow through the transmission line. Primary purpose of an SSSC is to control power flow in steady state; it can also improve transient stability of a power system.

The proposed SSSC-based neuro-fuzzy controller provides efficient damping to power system oscillations and greatly improves the system voltage profile. The inter-area and local modes of power system oscillations are effectively damped by using this proposed SSSC controller.

The proposed stabilizers have been applied and tested on power system under severe disturbance and different loading conditions. It is also evident that the coordinated design of PSS and FACTS-based stabilizer provides great damping characteristics and enhances significantly the system stability compared to individual design. The simulation results show that FACTS devices improve the system stability, furthermore the SSSC-based stabilizer provide a better effectiveness than STATCOM-based stabilizer on damping power system oscillation.

The operation of the designed device is verified by a series of simulations in MATLAB environment and the obtained results proved to be satisfactory. The Total Harmonic Distortion studies performed both when SSSC is on and off shows that the harmonic content introduced to the line current is very low.

The voltage and current waveforms along with the instantaneous active and reactive power calculations reveal that the designed topology works satisfactorily. The compensation of the reactive power flow over the power line due to the power line inductance is compensated with the help of series injected voltage.

Modeling and control design of a converter used in an SSSC application, employing low frequency, fixed modulation index strategies, the magnitude of the output waveform is directly related to the DC-bus level by a fixed relation. DC-bus has to be varied to obtain the desired output voltage. A large signal model for the converter is derived in the paper, subsequently linearised to obtain a small signal model, which was used to propose a control strategy. The theory is then validated experimentally on a novel Voltage Sourced Converter configuration.

In emerging electrical power systems, due to increased loading or due to severe contingencies often lead to situations, where the system no longer remains in the secure operating region. Under these situations, it is primary objective of the operator to apply control action to bring the power system into the secure region. FACTS devices can play an important role in power system security enhancement. A real power flow performance index sensitivity based approach and line voltage distribution factor have been proposed to decide optimal location of TCSC and SSSC to enhance the security of the power system. The effectiveness of the proposed controller has been tested on modified IEEE 30 bus system using Power World Simulator.

An adaptive SSSC in single machine infinite bus system model consists of a voltage source. Magnitude and angle of this voltage source depends on the SSSC control parameters. The voltage source model of SSSC incorporated into the generator output power equation simplifies the dynamic Eigen value analysis of the system. This model can then be used to compare and determine the system’s dynamic behaviour, equipped with an adaptive SSSC.

A summary of different FACT controller schemes is given in table 1.

Static Synchronous Series Compensator (SSSC)

A static synchronous generator operated without an external electric energy source as a series compensator whose output voltage is in quadrature with, and controllable independently of, the line current for the purpose of increasing or decreasing the overall reactive voltage drop across the line and thereby controlling the transmitted electric power Fig.1. The SSSC may include transiently rated energy storage or energy absorbing devices to enhance the dynamic behaviour of the power system by additional temporary real power compensation, to increase or decrease momentarily, the overall resistive voltage drop across the line.

A SSSC consists of a coupling transformer, an inverter and a capacitor. The SSSC is series connected with a transmission line through the coupling transformer. In principle, the SSSC can generate and insert a series voltage, which can be regulated to change the impedance (precisely reactance) of the transmission line. In this way, the power flow of the transmission line, which the SSSC is connected with, can be controlled.

Figure 1: Basic SSSC circuit…

The SSSC can provide capacitive or inductive compensating voltage independent of the line current up to its specified current rating. The practical minimum line current is that at which the SSSC can still absorb enough real power from the line to replenish its losses. The VA rating of the SSSC is simply the product of the maximum line current (at which compensation is still desired) and the maximum series compensating voltage:

In impedance compensation mode, the SSSC is established to maintain the maximum rated capacitive or compensating reactance at any line current up to the rated maximum. In practical applications, for variable impedance type compensators, Imax may be separately defined for the rated maximum steady-state line current and for a specified short duration over current. The basic VA rating of the major power components of the SSSC must be rated for these currents and for the relevant maximum voltages.

Figure 2: SIMULINK Model without SSSC…

In many practical applications, only capacitive series line compensation is required. In these applications as well as in those which already use or plan to use series capacitors as part of the overall series compensation scheme, the SSSC may be combined cost effectively with a fixed capacitor, where an SSSC of 0.5 p.u. VA rating is combined with a fixed capacitor of 0.5 p.u. VAC rating to form a continuously controllable overall series compensator with a maximum compensating range of zero to 1.0 p.u. capacitive. This compensation scheme from the standpoints of major component (converter and fixed capacitor) ratings and operating losses is extremely advantageous, in spite of the fact that the fixed capacitor produces a compensating voltage that is proportional to the line current, and therefore, the controllable compensating voltage range of the overall compensator also becomes, to some degree, a function of the line current.

Objectives

The reactive shunt compensation is highly effective in maintaining the desired voltage profile along the transmission line interconnecting two busses of the AC system and providing support to the end voltage of radial lines in the face of increasing power demand. Thus, reactive shunt compensation, when applied at sufficiently close intervals along the line, could theoretically make it possible to transmit power up to thermal limit of the line, if a large enough angle between the two end voltages could be established.

However, shunt compensation is ineffective in controlling the actual transmitted power which, at a defined transmission voltage, is ultimately determined by the series line impedance and the angle between the end voltages of line.
It was always recognised that AC power transmission over long lines was primarily limited by the series reactive impedance of the line.

Series capacitive compensation was introduced decades ago to cancel a portion of the reactive line impedance and thereby increase the transmittable power. Subsequently, within the FACTS initiative, it has been demonstrated that variable series compensation is highly effective in both controlling power flow in the line and in improving stability.

Figure 3: SIMULINK model with SSSC…

Figure 4: SSSC subsystem…

Controllable series line compensation is a cornerstone of FACTS technology. It can be applied to achieve full utilisation of transmission assets by controlling the power flow in the lines, preventing loop flows and, with the use of fast controls, minimising the effect of system disturbances, thereby reducing traditional stability margin requirements.

In this section the basic approach of reactive series compensation will be reviewed to provide the necessary foundation for the treatment of power electronics based compensators.

The effect of series compensation on the basic factors, determining attainable maximal power transmission, voltage regulation, transmission efficiency will be examined.

Control attributes of SSSC

  • Current control
    • Damping oscillations
    • Transient and dynamic stability
    • Voltage stability
    • Fault current limiting

Figure 5: Voltage regulation with transmission line distance…

Figure 6: Sending end reactive power…

Figure 7: Receiving end reactive power…

Figure 8: THD with SSSC…

SSSC SIMULINK model

The SSSC is the versatile member of the FACTS family using power electronics to control power flow on power grids.

A SIMULINK model of SSSC consists of a coupling transformer, an inverter, and a capacitor, is series connected with a transmission line through the coupling transformer.

The load is assumed to be an inductive load in the order of MW/MVAr. The transmission line is the pi section transmission line.

The model is studied for a transmission line of pi varying distance from 400km to1400km, and results are obtained from fig 2 to 6, table II.

CONCLUSIONS

  • The main role of SSSC is controlling the active and reactive powers; besides these – it could fairly improve the transient oscillations of the system.
    • SSSC is capable of controlling the flow of power at a desired point on the transmission line. It injects a fast changing voltage in series with the line irrespective of the magnitude and phase of the current.
    • The capability of SSSC to exchange both reactive and active power makes it possible to compensate both the reactive and resistive line voltage drops and there by maintain a high effective X/R ratio for the line independently of the degree of series compensation. Thus, optimal power transmission (high active to reactive power ratio) can be attained even at a high degree of series compensation.
    • The reactive shunt compensation is highly effective in maintaining the desired voltage profile along the transmission line interconnecting two busses of the AC system and providing support to the end voltage of radial lines in the face of increasing power demand.
    • The Total Harmonic Distortion studies – performed under both the conditions keeping SSSC on and off – shows that the harmonic content introduced to the line current is very low, due to the utilisation of a multi-pulse inverter in the construction of the device, which inherently filters harmonics up to certain levels and thus enhances the output waveform quality.
    • Controllable series line compensation is applied to achieve full utilisation of transmission assets by controlling the power flow in the lines, preventing loop.
    • With the use of fast controls, minimising the effect of system disturbances, thereby reducing traditional stability margin requirements.
    • The non-capacitor like behaviour, the superior operating characteristics and application flexibility that SSSC offers effectively is a compensation for power flow control and system stability improvement.

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