Post on 23-Apr-2020
Islanding control in grid connected photovoltaic system
1Mr. R.Ashokkumar,
2Dr.B.Indurani
1Assistant Professor Level II,Department of Electrical and Electronics Engineering, Bannari Amman Institute of Technology,
Erode,Tamilnadu,India
2Professor,Department of Electrical and Electronics Engineering, Bannari Amman Institute of Technology, Erode,
Tamilnadu,India
1ashokkumarr@bitsathy.ac.in,
2induranib@bitsathy.ac.in
Abstract— this paper discusses about the power quality issue termed voltage sag and its generation in a two stage photo
voltaic inverter. Hence the injection of reactive power control is necessary to maintain power system stability in generation
stations. In a grid connected PV system, inverter must regulate the reactive power to overcome the asso ciated problems
generated during synchronization of photovoltaic inverter to the grid. In this paper, 1 kW PV system is carried also basic control
strategy such as control of firing pulses for grid connected inverter is analyzed. Also in this study power quality disturbances are
studied and islanding condition is detected. In this method the monitoring of instantaneous voltage is not required to detect
islanding problem. Hence, an islanding detection using loop controller is implemented. During sudden changes in load or any
other transients in power system causes malfunctioning of the system. Therefore, voltage control algorithms are proposed to
overcome the islanding problem which detects the saturation of current in the outer voltage control loop. The entire 1kW photo
voltaic system is simulated and analyzed using MATLAB Simulink software.
Keywords: LVRT, MPPT, PWM, Reactive power control, Three Phase Inverter
I. INTRODUCTION
In recent days, in addition to generating power and distributing a quality power, need for green power has also grown.
Carbon emission being high in various countries, environmental safety is equally important in power generation. This made the
mankind to move towards renewable energy sources like solar, wind and the like. Though the e fficiency is low in solar, it has its
own advantages as it deals with free solar energy from the ultimate energy source. To get maximum power from the solar PV
system, various control techniques are used which are discussed in this paper. PV panels are ins talled as PV arrays and
connected to a DC-DC converter to regulate the DC power output from the panel. DC-DC converter is employed with Maximum
Power Point Tracking algorithm to extract more power. Then the regulated DC is converted to AC and supplied to t he grid using
AC-DC inverter. Block diagram for the system is given in Fig.1. In this paper, the inverter part with LVRT is discussed in detail.
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Fig.1. Block diagram of typical grid connected PV system
2. PV SYSTEM MODELLING
The study is done for 1kW system. PV modelling in MATLAB can be done by deriving the function from the equivalent
circuit of the PV cell. PV equivalent circuit is shown in Fig.2.
Fig.2. Equivalent Circuit of Photovoltaic cell
The relation between the terminal current It and voltage Vt is given by the equation (1).
….. (1)
Where, Io, Iph, I and V are saturation current, photo current, current and voltage of the module respectively. Vth = nNskT/q is
thermal voltage of the module; n, Ns, K, T and q are ideality factor, number of cells in series, man constant and electron charge
respectively. Expressing in terms of Short circuit Current and Voltage, the equation will be as shown below (2). Where, M ss and
Nss are number of shunt and series connected modules respectively.
…… (2)
.
Fig.3. I-V and P-V Characteristics of 1kW PV Panel
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2.1 DC-DC BOOST CONVERTER
DC-DC boost converters are used in PV systems, to boost the unregulated, constantly varying, and PV output to a
required voltage level. MPPT control technique is commonly used in all DC-DC converters. Maximum Power Point Tracking
algorithm can be derived from the expression that relates generated power and voltage, which is given below in equation (3).
……….. (3)
As far as DC-DC converter is concerned, it may be any of the maximum power point tracking (MPPT) algorithms, which
extracts maximum power from the PV array. MPPT controller can be used to generate switching pulse for DC-DC converter and
a constant terminal voltage can be maintained up to certain level of fluctuation in input. It is necessary to maintain a constant
output voltage irrespective of input irradiance and temperature. MPPT techniques have been discussed in [1]-[5] in detail. In this
paper, the simulations followed Perturb and Observe (P&O) Algorithm [6]. Flow chart of P&O algorithm is given in Fig.3.
Fig.3. Perturb & Observe algorithm
2.2 GRID INVERTER
In this study, a universal bridge type three-phase voltage source inverter (VSI) is used for the DC to AC conversion.
The VSI maintains a constant DC link voltage of 2kV. This inverter is responsible for providing active and reactive power to the
grid. PWM techniques are adopted in inverters to maintain constant output voltage irrespective of load. PWM is the process of
modifying the width of pulse with respect to carrier in order to reduce harmonic content of the inverter output, various types of
PWM is tested and adopted [7].
2.2.1 SINUSOIDAL PULSE WIDTH MODULATION
When voltage source inverter is switched in either 120 degree or 180 degree mode of operation, the output waveform
obtained will be a square wave. Hence, to acquire a sine wave, sinusoidal PWM is simple and efficient method. In this method,
a sine wave and triangular wave which acts as carrier are compared with comparator. The resultant pulse generated is the required
PWM signal. Basic principle of SPWM is shown in Fig.4.
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Fig.4. Sinusoidal PWM wave generation
There are certain considerations to be made, before going for SPWM. Let the amplitude of sine wave to be modula ted
is Am and the triangular carrier be Ac (refer Fig. 1). The modulation index (Am/Ac) has a greater impact on output voltage. Very
high carrier frequency has an advantage of reducing harmonic components due to the inductive nature. High frequency
switching also increases the switching losses in power electronic switches used in the inverter. For this reason, the carrier
frequency is chosen between 2 kHz to 15 kHz which is optimum to use. Similarly, for three phase inverters, it is mandatory to
maintain all three waveforms symmetrical. So, the ratio of sine wave frequency fm to the carrier frequency fc is chosen in integral
multiples of 3 (fm/fc= 3n, n ∈ N).
For a modulation index of 1, (i.e., Am/Ac = 1 or Am = Ac) width of the pulses will be low. If carrier amplitude is lower than
the modulation wave, then the modulation index goes beyond 1. In this case, some triangular spikes will not intersect with s ine
wave. So, very high pulse width is possible and an uneven distribution will be followed
Fig.5. Over modulation error
2.2.2 GENERATION OF FIRING PULSES
SPWM techniques are characterized by constant amplitude pulses with different duty cycles for each period. The width
of these pulses are modulated to obtain inverter output voltage control and to reduce its harmonic content
Sinusoidal pulse width modulation is the mostly used method in motor control and inverter application. In SPWM
technique three sine waves and a high frequency triangular carrier wave are used to generate PWM signal. Generally, three
sinusoidal waves are used for three phase inverter. The sinusoidal waves are called reference signal and they have 1200 phase
difference with each other. The frequency of these sinusoidal waves is chosen based on the required inverter output frequency
(50/60 Hz). The carrier triangular wave is usually a high frequency (in several KHz) wave. The switching signal is generated by
comparing the sinusoidal waves with the triangular wave [7]. The comparator gives out a pulse when sine voltage is greater than
the triangular voltage and this pulse is used to trigger the respective inverter switches. In order to avoid undefined switch ing
states and undefined AC output line voltages in the VSI, the switches of any leg in the inverter cannot be switched off
simultaneously. The phase outputs are mutually phase shifted by 1200 angles. The ratio between the triangular wave & sine
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wave must be an integer N, the number of voltage pulses per half-cycle, such that, 2N= fc/fs. Conventional SPWM signal
generation technique for three phase voltage source inverter is shown in
Fig. 5. Block diagram of SPWM generating control circuit for three phase PWM voltage source inverter
Fig.6. Amplitude modulation with 2 kHz
After generating the carrier wave, a comparator is used to obtain desired PWM signal. The comparator compares the
amplitude of carrier and voltage of each phase. Wherever the carrier amplitude is high, the output from comparator is high [8]. The
resultant waveform will be as shown in Fig.7.
Fig.7. Gate Pulses for grid inverter
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3. ISLANDING DETECTION
In the event of grid no avail-ability due to an upstream fault in the power system, it is likely that the utility-side breaker
trips under the action of grid-side control. Under such a scenario, the inverter continues to pump power into the local load,
resulting in islanding operation, sometimes islanding is desirable, but other times it is not. Islanding can create safety problems
for workers and equip-ments that are not under direct control of the utility [12]. Therefore, islanding condition should be
detected, and the inverter should be tripped. The inverter side breaker also needs to be opened to isolate the system from the grid.
This operating mode of a grid-connected (GC) inverter is known as anti-islanding.
Fig. 8. Islanding detection algorithm.
Operation in islanded condition of a GC inverter, i.e., in SA mode, can create safety problems for the workers in a plant
as the inverter may continue to energize some plant load even when grid power supply has failed. Therefore, SA mode of invert er
operation may not be allowed and the inverter has to trip. Let us say the system initially is in the GC mode [11]. If islanding
condition occurs, the system first detects it. [13]After islanding detection, the system goes into the SA mode if the operation in
the islanding mode is allowed. The system trips if the operation in the islanding mode is not allowed. When islanding has not
occurred, the system continues to check for low-voltage condition of the grid. As long as there is no low-voltage situation, the
system continues to operate in the GC mode.
Fig. 9. Control block diagram of outer voltage control loop
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Fig. 10. Control loop for design of inner current control loop design
As al-ready discussed, the PI controller of the outer voltage control loop eventually saturates in the GC mode. After
saturation, the boost converter operates with a current reference of Im p p , that comes from the MPPT controller of PV. In the GC
mode, the system always operates on current control with Im p p as the current reference [9]. Now, in the power system, islanding
condition would occur when the grid-side breaker gets open (may be due to some upstream fault). The controller of two-stage PV
inverter does not receive any direct signal from the breaker, as that is not physically at the same location, but the propose d
algorithm still detects that islanding has occurred. The proposed islanding detection algorithm requires that, due to the
difference between the output power of the PV at MPPT and the output power of the inverter, the load voltage has to change to
such a steady-state value that the PI controller will come out of saturation[10]. Here, we calculate the amount of power that would
to be drawn by the grid prior to islanding.
Fig. 11. Islanding detection
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4. SIMULATION RESULTS AND DISCUSSIONS
A 1kW grid connected system is simulated in MATLAB SIMULINK. The simulation circuit consists of an equivalent
solar module connected to the dc boost converter and wit the dc link it is connected to the inverter. The simulation off SPWM
technique is used to generate firing pulses for grid inverter. PI controller is used as feedback controller to provide the control
grid voltage. The grid voltage under fault condition without LVRT control is shown in Fig , 9. The grid voltage after including the
LVRT control, get regulated and the output is shown in Fig.12
Fig.12. Simulation model of 1kW grid connected solar PV system
Fig. 13. Three phase line voltage and current waveforms
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Fig.14. Grid Voltage under double line to ground fault
For testing the islanding detection algorithm, the system is initially allowed to operate in the GC mode for a small amount
of time till the outer voltage control loop reaches saturation. After the PI controller saturation, islanding detection algor ithm
starts working. Fig. 16 shows the simulation result of islanding detection. For creating the islanding condition, the grid -side
breaker is opened intentionally at 1 s (the outer voltage control loop is already saturated). As a consequence, at 1 s the gr id
current becomes zero.
5. CONCLUSION
In this paper, an integrated control algorithm for islanding detection and LVRT operation of a two -stage PV inverter is
proposed and verified. Islanding is detected from the status of current saturation at the output of the inverter vo ltage control
loop. The advantage of this method is that it is insensitive to disturbances as it does not depend upon the instantaneous value
of the grid voltage
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7. BIOGRAPHY
R.Ashok Kumar received his B.E Degree in Electrical and Electronics Engineering from Anna University, Chennai and his
master degeree M.E in Power Electronics and Drives from Anna University , Coimbatore. Currently he is working as
Assistant professor (Sr.G) in Bannari Amman Institute of Technology, Sathyamangalam. His area of research includes
renewable energy, power converters for renewable energy applications, solar photovoltaic grid connected system, power
quality issues and mitigation techniques.
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