Study of Power Grid Connection with an Unstable ...

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ScienceDirect Energy Procedia 56 (2014) 584 – 590

11th Eco-Energy and Materials Science and Engineering (11th EMSES)

Study of power grid connection with an unstable source from Elevator Energy Regenerative Unit (EERU) A.Noppakant1 B.Plangklang1 * Y. Paraken2 1

Department of Electrical Engineering, Faculty of Engineering, Rajamangala University of Technology Thanyaburi, Klong 6, Thanyaburi, 2

Pathumthani 12110 Department of Electrical Engineering, Faculty of Engineering, Thonburi University , Nongkanpul, Nongkhame, Bangkok 10160 E-mail: [email protected]

Abstract This paper presents a study of power grid connection with an unstable source from elevator energy regenerative unit (EERU). The investigated system used a Permanent Magnet Synchronous Motor (PMSM). The EERU is installed for this system to generate electricity according to the generative mode of the elevator which delivers power to the grid as an unstable power source. To meet standard specifications in term of power quality and safety for grid connection, the phase angle and the amplitude as well as the frequency of the grid voltage are considered for grid connection. Phase lock loop (PLL) algorithm is therefore very important and selected for grid synchronization. The study is done by simulation using Matlab/Simulink program. The simulation used phase lock loop to synchronize between two systems. The results of this simulation will lead to a proper design of the grid connection with EERU as an unstable power source.

Keywords: regenerative; grid connected inverter; phase lock loop(PLL).

* Corresponding author. Tel.: +66 2 549 3420; fax: +66 2 549 3422. E-mail address: [email protected]

1876-6102 © 2014 Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-review under responsibility of COE of Sustainalble Energy System, Rajamangala University of Technology Thanyaburi (RMUTT) doi:10.1016/j.egypro.2014.07.196

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1.

INTRODUCTION Regenerative system from elevator is connected between EERU and the grid, therefore it has many factors to take into consideration in term of standards. The factors that take into consideration are frequency and phase which have to be the same as the grid system at all time or called synchronization. EERU converts DC current to the grid as renewable energy into alternating current which performs like current source whereas the power from distribution transmission line performs like voltage. Therefore, in order to supply both current and voltage source into distribution system, it is necessary to have correct connection to the distribution system of power grid which requires EERU to generate the correct frequency and phase of the electric transmission system at all time, also called the synchronization. The synchronization voltage of the power supply is a three–phase, 380 volts. In this paper, the simulation of EERU system where the PLL is used to synchronize to grid will be presented

2.

THEORY AND IMPLEMENTATION 2.1 Phase lock loop A phase-locked loop or phase lock loop (PLL) is a control system that generates an output signal whose phase is related to the phase of an input signal. While there are several differing types, it is easy to initially visualize as an electronic circuit consisting of a variable frequency oscillator and a phase detector. The oscillator generates a periodic signal. The phase detector compares the phase of that signal with the phase of the input periodic signal and adjusts the oscillator to keep the phases matched. Bringing the output signal back toward the input signal for comparison is called a feedback loop since the output is 'fed back' toward the input forming a loop. Keeping the input and output phase in lock step also implies keeping the input and output frequencies the same. Consequently, in addition to synchronizing signals, a phase-locked loop can track an input frequency, or it can generate a frequency that is a multiple of the input frequency. These properties are used for computer clock synchronization, demodulation, and frequency synthesis, respectively. Phase-locked loops are widely employed in radio, telecommunications, computers and other electronic applications. They can be used to demodulate a signal, recover a signal from a noisy communication channel, generate a stable frequency at multiples of an input frequency (frequency synthesis), or distribute precisely timed clock pulses in digital logic circuits such as microprocessors. Since a single integrated circuit can provide a complete phase-locked-loop building block, the technique is widely used in modern electronic devices, with output frequencies from a fraction of a hertz up to many gigahertz.[2]

Fr

Tr (S)

Phase detector

PD ( Kd : V/rad )

To(S) N

Vd (S)

Vc (S) Loop filter

F(S)

Frequency divider

VCO ( Ko : rad/s/V )

Fo

To(S)

1/N

Fig. 1 Phase lock loop diagram

2.2 Inverter Three-phase inverters are used for variable-frequency drive applications and for high power applications such as HVDC power transmission. A basic three-phase inverter consists of three single-phase inverter switches each connected to one of the three load terminals. For the most basic control scheme, the operation of the three switches

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is coordinated so that one switch operates at each 60 degree point of the fundamental output waveform. This creates a line-to-line output waveform that has six steps. The six-step waveform has a zero-voltage step between the positive and negative sections of the square-wave such that the harmonics that are multiples of three are eliminated as described above. When carrier-based PWM techniques are applied to six-step waveforms, the basic overall shape, or envelope, of the waveform is retained so that the 3rd harmonic and its multiples are cancelled.

Fig 2. Structure of three phase inverter

3.

SIMULATION This paper demonstrated the simulation of the regenerative system which was connected to grid inverter to synchronize and interface with a grid system.

Fig 3. Block diagram of regeneration system

The block diagram of grid connected inverter system is given in Fig.3. The three phase full bridge inverter topology is used configuration as the three phase inverter system. IGBT are used as switching element which operated a frequency with a hysteresis band PWM technique is used in switches in each pair are turn ON and OFF simultaneously and output voltage varies between –Vdc and +Vdc. The control of this system applied for inverter consists of two control loops which control grid current and external voltage loop which control dc link voltage. A reference frame transformation abc to dq which transforms the grid current and voltage into d-q frame. [3-6] Base on designed by simulation of the regenerative system done by Matlab/Simulink, the system will generate an unstable characteristic of the source, then inverter H Bridge type including IGBT converts that energy for the system.

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Fig 4 Simulation of the regenerative system with MATLAB/Simulink

Fig. 4 shows the order of inverter, phase lock loop circuit, phase detector circuit, low pass filter circuit and voltage control oscillator become a part of this circuit. Phase lock loop plays an important role for grid connected system. They synchronized the output phase and frequency using different transformation. In three-phase system can be used d-q phase lock loop transformation with a design of loop filter. This paper presents the unstable energy which produced a small amount of energy. Then a small amount of current from the system has to synchronize to a distribution line that is larger than regenerative system will produce. Hysteresis controller become a important circuit ,they will derivative the load current and the reference current determines the switching time and frequency show in Fig 4.

Fig 5. Lowest voltage measure at DC link

Fig 6. Highest voltage measure at DC link

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Fig. 5 and Fig. 6, show that voltage with time domain of the system. Then the dc voltage from the synchronous motor become a generator and feed back to a buck boost converter. This research is separated into two cases. Fig.6. is a synchronous motor that becomes a generator normally then a voltage generated between 650-725 VDC and Fig. 5 is synchronous motor that becomes a generator as the elevator starting brake before reaching destination. Motor generates peak voltage at saturate time about 380-650 VDC and then feeds into ERU system.

Fig 7.Sin & Cos wave generate by PLL

Fig.7. PI controller gain varies to obtain zero phase error and to detect accurate inverter phase angle. Synchronization between output of inverter phase and grid phase is archived by locking PLL not only at zero crossing but at every instant of time between 0 to 360 degree.

Fig 8. Synchronization between reference grid voltage & PLL output voltage

Fig.8. shows synchronization between rotating reference frame PLL output voltage and grid voltage. The inverter is controlled in the order to feed active power to the grid, by using synchronous d-q transformation.

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Fig 9. Voltage of each phase from grid tie inverter

Fig 10. Current of each phase from grid tie inverter

Fig.9., Fig.10. show grid voltage and grid current. Inverter output frequency lock by PLL is 314 rad/sec which is 50 Hz. Results of PLL shows that when grid phase angle equals to inverter phase angle, the error at out of phase detector become zero and equal to reference and lock is set by PLL. Grid voltage from the three phase system connected to the inverter which generate PWM signal. The voltage at the output of inverter remains always constant in simulation is 315 volt. Grid current approve by Id component control the active current necessary to feed active power to the grid is 12.5 Amp. 4.

RESULTS

From the simulation program with Matlab/Simulink, it shows that the elevator motor can work as a generator in the regenerative mode. Regenerative Motor Driver system is using IGBT control which can control the energy can flow in both directions. The power will be fed to the ERU system in Fig. 5, Fig. 6, and then fed back to the grid into the distribution system. That means the grid synchronization plays important role for grid connected system. It synchronizes the output frequency and phase of grid voltage with grid current using different transformation. PLL cause one signal to track another one. It keeps an output signal synchronized with reference input signal in frequency and phase. PLL three phase system connected system can be implemented using d-q transformation. Another interesting point of this system is the unstable source the Hysteresis current controller is responsible for generating power back into the grid system. So this circuit creates a reference signal to drive the current that can flow into the system.

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CONCLUSION

The studies of the three-phase grid connected system to connect between unstable source from a regenerative system and distribution system by using a power supply from a synchronous motor that becomes a generator. The generator will produce the power be fed back to the grid system. The consideration has several important circuits to make them work property. The PLL is selected in this study and it is able to feed power to the grid, using synchronous d-q transformation. PLL is used to lock grid frequency and phase for synchronization. The phase detection of PLL is property done using d-q transformation in three phase system. . References [1] H. Inaba, S. T. Nara, H. Takahashi$M. Nakazato, "High speed elevators controlled by current source inverter system with sinusoidal input and output" Volume 28 , Issue 4 ,Jul/aug 1992 [2] Miss. Sangita R Nandurkar, Mrs. Mini Rajeev, “Design and simulation of three phase inverter for grid connected Photovoltaic system” : NCNTE 2012 [3]S.K. Chung , “Phase lock loop for grid connected 3 phase power conversion system,” IEE Proc. Electr. Power Application, Vol. 147,pp.213-219, May 2000 [4] Adrian Tim bus, Re mus Teodorescu and Frede Blaabjerg, “synchronization methods for 3 phase distributed power generation system-An overview & evolution,” pp.2474-2484,2005 [5] Franscisco D Freijedo, Jesus Dovel & Oscar Lopez, “Grid synchronization method for power converter,” pp.522-529,2009 [6] Guan Chyan Hsieh & James C Hung, “Phase Lock Loop Techniques A Survey, “ IEEE Transsaction on Industrial Electronics, Vol. 43, pp.50-60, December1999 [7] Boonyang Plangklang et al, Study of Generator Mode on Permanent Magnet synchronous Motor (PMSM) for Application on Elevator Energy Regenerative Unit (EERU), Elsevier & Science Direct, Energy Procedia 34 (2013), 382-389 [8] Thanit Phanprayoon and Boonyang Plangklang,”Study of Energy Saving from Elevator Energy Regenerative Unit(EERU)” 10th Eco-Energy and Materials Science and Engineering Symposium pp.105-108 December 58,2012 [9] Sittichai Kantawong, Akeratana Noppakant, Boonyang Plangklang,”Study of Generator Reaction on Permanent Magnet Synchronous Motor for Energy Regenerative Applications” 10th Eco-Energy and Materials Science and Engineering Symposium, December 5-8,2012