Authors: Negma Iqbal, Aswathy Vidyasagar, Dr. John Chembukavu
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The consumption of power shows a steeply increasing behavior in the present scenario which demands the integration of various novel techniques to the existing grid. With the penetration of PV and wind energy units into the grid, the evolution from a generator dominated grid to an inverter dominated grid arises in the near future. The paper discusses the characterization of HVDC convertors with the behavior of SG emulating virtual inertia for frequency stabilization and proper power oscillation damping during transmission of renewable energy based power. The conventional control of HVDC converters is integrated with synchronous power controller with second order characteristics inside power control loop for proper damping essentialities. The functioning of HVDC link is enhanced by means of SMES coil based energy storage operation. The proposed formulation is applied to a four area model in IEEE 14 bus system incorporated with PV, wind and HVDC links. The dynamic behavioral improvements are tested by means of MATLAB simulation.
The development and swift expansion of AC systems into a scenario of multi-area interconnected power systems demands the rise in the presence of high power converter applications in future power grid which follow up with a very difficult issue which will affect various parts of the power grid and particularly the frequency stabilization and power oscillation issues in AC/DC systems . Automatic generation control facilitates various responsibilities like frequency restoration, optimal power dispatch and scheduled tie-line power flow in a multi-area power system .
Multi-area system may constitute AC or DC transmission lines in between various authority areas which perform scheduled tie-line power flow and appropriate support during abnormalities or contingencies . Even though AC transmission is very much implemented in the grid at present, HVDC links have gained a lot of attention in recent times for long distance transmission . HVDC has various beneficial advantages like frequency stability, maintain fast and bidirectional controllability, power oscillation damping (POD), better reliability and prevents global blackouts acting against cascaded disturbances -. Renewable energy resources based power generation has been a great contribution in the recent years. Integration of large off-shore wind farms, solar energy, pit head steam and hydroelectric power plants demand HVDC transmission as they are at a faraway place from the major power demand zone. The main reason for the latter is the economic feasibility of HVDC links in long distance transmission. Hence, the trend for HVDC connections and hybrid connections has gained an increasing attention as a preferred solution .
Conventional generators have the droop capability for providing inertia and respond against frequency fluctuations. The incorporation of renewable based energy systems, and HVDC links evolutes a convertor dominated grid from a generator dominated grid. A convertor based grid will lack sufficient inertia for stable power system operation -. The major issues include variation in scheduled tie-line power flow, oscillating power, complexity in frequency regulation. So, it is obvious that the present requirement is to model a system which emulates appropriate virtual inertia and implement the behavior of synchronous generators to inverters used in HVDC and RES based systems.
Efficient load frequency control (LFC) is a major concern for the stable operation of power system. There are various methods adopted for LFC in the modern systems with implementation of more functionality to conventional methods -. Coordination of HVDC transmissions and FACTS devices are very much useful for long-term dynamics control of modern systems -. There are various topologies for the virtual inertia emulation with diverse control methodologies -. Virtual inertia is emulated by highly innovated control strategy of power converters embedding the behaviors of synchronous machines to control part of converter systems to some extent -.
Emulation of inertia based on synthetic inertia method , frequency derivative criterion for HVDC systems - and Virtual Synchronous Generator (VSG) method  are some of the commonly discussed methods.
Corresponding signal for convertor controllers is the derivative of grid frequency in one of the methods which is a highly efficient criterion . The criteria based on frequency derivative for emulation of inertia are sensitive to the noise especially during measurement of frequency. Phase Locked Loop (PLL) is essentially used for proper assessment of the signals in most cases . Thus, any case of frequency deviation may give rise to amplification of noise signals. This may cause lose of locked condition of PLL which adversely affects the inverter operation.
The inertial response usually depends on the physical nature of various components, while in this paper enhanced method of emulating alternative inertia without this constraint is discussed. Synchronous Power Controller (SPC) is integrated with the usual control strategy of HVDC convertors for better operation. The control strategy proposes a novel technique in power converter control to behave itself similar to a SG with the capacity to emulate inertia without the limitations of a conventional generator. During the integration of the renewable energy sources into the power system, various instability issues are caused which can be power oscillations due to unstable generation from RES units. Damping of power oscillations is a major concern for the stable operation of the power grid. The HVDC convertor control is thus to be modeled so as to solve these instability concerns. The widespread use of renewable energy in the system necessitates the use an energy storage device (ESS) for reliable power system. Instantaneous and independent exchange of the active and reactive power in renewable energy based HVDC system is possible with energy storage devices . These energy storage devices help to mitigate the power quality issues arising due to the integration of renewable sources . Super-conducting magnetic energy storage (SMES) is a type of ESS which has a very high efficiency of more than 90%. SMES is a direct current device which can store energy of magnetic field in a superconducting coil .
The paper discusses a Virtual Synchronous Power (VSP) based control method for HVDC convertors of the multi-area interconnected power system incorporated with various renewable energy resources; namely solar and wind for power generation. The VSP based concept adopted in the paper was patented in the year of 2012 . The HVDC convertor control of the same system with conventional method of control and VSP based control is to be compared. The behavior of the system with VSP control after incorporating RES units is to be studied from simulation results. The performance of HVDC system with and without SMES coil as an energy storage device has been studied under fault conditions.
The main purpose of this paper is to suggest a novel move towards of power oscillation damping and frequency stability analysis in multi-area system, implementing the concepts of VSP and SMES based ESS in HVDC links. The significance of the VSP theory to provide additional capabilities like emulating the inertia and damping for frequency and power control improvements is another desire through the study in this paper. In case of higher order control designing of HVDC stations, the real time experimentation is really hard and thus this method proposes a very efficient pre-evaluation strategy for studying the dynamic behavior of the system . The significance of energy storage devices for HVDC links transmitting renewable energy based power is high in the current scenario. Hence a very efficient methodology for the same using SMES coil may be very helpful to cope up with the major power quality issues.
II. SYCHRONOUS POWER CONTROLLER
A. Electrical Princle of Operation
The interlinking of a synchronous generator to the grid can be represented using the circuit shown in fig. 1, where v represents the grid voltage, e is the internal induced emf of generator.
By programming the above equation in a digital controller, emulation of the electrical behavior of SG is possible. While, the implementation of the above equation straightforward will bring about issues due to the derivative term of measured current. This is due to presence of high frequency ripples, which will bring out issues during calculation of reference signals for the control of power converter. Hence, rearranging equation (1), results to generate equation below,
This alternative for equation (1) will be more realistic for physical modeling and performs stable, smoother and sensitivity to distortions is less. Thus, it represents the admittance model of the virtual generator shown in fig.2. The cost for extra filtering can be avoided using this model. There is an interior control loop for current in the model which helps to control power converters used in power generation applications. The model is based on the virtual admittance concept.
B. Electromechanical Princle of Operation
The electromechanical behavior of a synchronous machine is defined with the help of Swing equation. The difference between electrical and mechanical power is the reason for the rotor swing, which leads to inertial response. The immediate response during a frequency event due to rotor swing is by the inertial response by trying to mitigate the imbalance between the electrical supply and demand. This property of inertial response is due to the large rotating mass of the synchronous generators present in the power grid. The high penetration of renewable energy resources lead to a low inertia power grid. The paper discusses a novel method to implement the inertial response characteristics of a synchronous generator to the control scheme of an inverter, thus to compensate the absence of synchronous generator. While digitalizing the behavior of synchronous generator, it can also overcome drawbacks of conventional SG’s inherent oscillatory response. The values of inertia and damping are responsible for the inertial response in a synchronous generator which is due to its physical very large rotational mass. This limitation doesn’t occur in case of digitalized control system as the values of damping and inertia can be varied dynamically during perturbations. Swing equation may be mathematically expressed as,
As shown in fig. 3, any variation in the input power Pinp, product of vdc and idc and the power delivered by the converter Pout , product of v and i, is denoted as ?P, which is the input to the Power Loop Controller (PLC). The PLC will generate a relative value of frequency ?ω using equation (8) and (11). The values of ωn and ξ are generated executing the equations (9) and (10). The virtual inertia, J and the damping coefficient, k for the system are selected properly in order to emulate required amount of virtual inertia and damping. The value of rotating frequency of virtual rotor (ωr) is generated by adding up the values of relative frequency generated by PLC and synchronous frequency. The Voltage Controlled Oscillator (VCO) which acts as a simple oscillator is fed with the voltage and frequency signals to generate a three-phase virtual emf denoted as e. VCO implementation can be represented as below,
The generated emf voltage e and the grid voltage v is compared to generate the error v* which is fed to the virtual admittance block, where at the output generates current reference, i*. This is compared with the grid current i and this if fed to the standard current controller where PWM signals are generated fot the gate control of converter switches. The dynamic relationship between the input and output powers may be represented as,
The block diagram standard current controller of the HVDC convertor is represented in the fig. 4. The voltage and current of the receiving end bus or grid is monitored and is fed to generate control signals. The phase locked loop fed with the grid voltage will generate the required phase angle and frequency for control signal generation. The generation process is undergone with dq0-abc transformation and vice-versa where the phase angle is required. After the generation of Id* and Iq*, i.e. the d and q components of the reference signal, the required voltage signal to be fed to PWM generator is processed.
III. SUPERCONDUCTING ENERGY STORAGE DEVICE (SMES)
Superconducting energy storage device is made use of to store energy in the form of magnetic field. The electrical energy is being converted to magnetic energy, where excess of the DC current will circulate through the coil without any decay. This phenomenon is explained with the property of zero resistance to current flow of superconductors when the temperature goes below critical temperature.
The main benefits of using SMES compared to any other energy storage devices are high energy storage density, long application life time, high energy storage efficiency and little environmental pollution. The specification of SMES coil is expressed in rated power (W). The energy stored in the SMES coil may be expressed in terms of coil inductance, LSMES and DC current flowing through the coil ISMES as follows,
B. Operation of SMES Device
The load change in the system is accompanied by frequency deviation in the system which can cause power quality issue like voltage sag or voltage swell. The variation of the load and frequency has an inversely proportional relationship. The change in frequency is considered as the control signal for the operation of the SMES device. The SMES coil will be initially charged to a particular residual value.
When there is a negative change in the frequency, SMES supplies power to the system and it get discharged. While the positive change in frequency will make SMES coil to absorb power and get charged. When there is no change in the system frequency, the SMES remains at its standby mode of operation. The various modes of charging and discharging can be attained by means of the DC-DC chopper which will vary the DC voltage that comes across the SMES coil. Chopper has to maintain a duty cycle of 50% in order to attain a mode of standby operation. As shown in fig. 5, the control signal for the mode selection of SMES device is the ?f signal. KSMES is the gain of control loop, ?Id represent the deviation in inductor current, and Tdc is the time delay. The net deviation in the power, either absorbing or supplying power of SMES coil is ?PSMES.
IV. SYSTEM MODEL
The multi-area power system for the study is modeled in IEEE 14 bus test system . The IEEE 14 bus system is modeled in MATLAB using the bus datas and line datas available. The whole bus system is divided into four areas as shown in fig. 6. The Area 4 is assumed to be at a distant place from other areas while the power requirement in this area is high. Thus in order to attain a better transfer of power over long distance, a HVDC line is installed in between Area 3 and Area 4. The HVDC convertor control using conventional method is compared with that of synchronous power controller based strategy.
The Area 3 has high potential for the generation of power from solar energy and wind energy. HVDC being a good option for the transmission of wind generated power over long distance, there will be better performance. The synchronous generator in Area 3 is replaced and the power requirement is met with RES system comprising of wind and solar. The performance of the synchronous power controller based strategy is applied to the HVDC convertors for RES power transmission. The incorporation of the RES unit can lead to various power quality issues like voltage sag and voltage swell. To overcome the issues of power quality, SMES coil is integrated to the HVDC link. SMES coil has the ability to act according to fault conditions happening in the system by proper sharing of power with grid, choosing its exact operating mode. The schematic of the whole system is represented in figure 7.
V. SIMULATION RESULTS
The various ideas that were discussed in the previous sections have been simulated using MATLAB software. IEEE 14 bus system is modeled and simulated to obtain the stable result to ensure the test system credibility. The HVDC link is to be installed in between 8th and 14th bus of Area 3 and Area 4 respectively. The bus voltage and current of 8th and 14th bus in stable IEEE 14 bus system is shown in fig. 8. The various parameters of the system used to generate simulation results are given in Table 1.
The HVDC link is installed into the system and the convertor is being controlled by means of conventional control strategy. Then, the synchronous power control based method is incorporated with the conventional controller. The variations of the system based on both the methods can be studied by close analysis of the frequency deviation and power deviations curves plotted using simulink. The proper designing of firing angle is to be achieved.
The analysis of the frequency deviation and power oscillations are studied from the simulation results. The results of the both control strategies can be compared to understand the advantages of the latter. Fig. 9 and fig. 10 shows the variation of the power and frequency of the system when the convertors are being applied with conventional current control.
The better working of the synchronous power converter can be easily depicted from results shown in fig. 11 and fig.12. The frequency doesn’t reaches to the rated value in the first case, while in latter case; frequency is maintained at system frequency. In case of power deviations, the power oscillations are easily damped in the latter case. The better operation of this strategy based on proper feed backing of the frequency, power, current and voltage can make overall improvement in the operation of grid. The study of the graphs reveals requires inference.
The SPC strategy is best suitable for the case with AC grid incorporated with RES units. Hence, in order to analyze the performance of the system with RES, the synchronous generator in the bus 8 is incorporated with wind and solar generation. PV panels and wind together generates a power of 1MW. The variation in frequency and power deviations seems to be satisfactory in figures 13 and 14.
The wind, PV or any other renewable energy resource generates power in a varying nature. This is due to frequent variations in the driving source which is dependent on various environmental factors like temperature, pressure and other climatic variations. In case of such deviating power sources, it is very much necessary to incorporate very efficient control strategies for the coupling devices which connect them to the main grid. Hence VSP based control strategy can give better results.
The wind or PV or any other renewable energy resources has a frequently varying behavior throughout its operation period due to their dependence on natural factors. Thus we opt for the integration of a energy storage device to the system. The SMES coil is used as an energy storage device. To analyze the performance of SMES device, a fault is being applied to the transmission line connecting 8th and 14th bus at sample time 0.25 to 0.75sec. The performance is enhanced by proper control of SMES device.
The analysis of the result during the fault with and without the SMES device is depicted in figures 15 and 16. The SMES coil shifts to discharging mode during the fault and compensates the system power and maintains stable operation.
A novel advanced method to implement the emulation of virtual inertia based power in HVDC links in multi-area interconnected power systems is proposed. The performance of the system is analyzed by studying the characteristics of a four area system modeled in Matlab. The significance of the method in maintains the stability of a power system is shown with appropriate simulation diagrams, which depicts the performance of the system. The power deviations and frequency deviations of different cases are simulated and inferred the results. The variation of frequency beyond the allowable limits may make the system act even so worse with high penetration of solar and wind energy resources based energy into the power grid. SPC based AC/DC systems can be better during any kind of contingencies in the power system. The incorporation of SMES gives better results in case of any load change in the system. The fault situations are well dealt with proper selection mode of operation of SMES. The power quality issues are solved by proper sharing of power among SMES device and power grid. All together the system can be very much useful in future grids with high penetration of renewable energy resources which can solve the issue of low inertia. The practical implementation of the system requires proper and in depth designing which will be very complex.
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