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International Journal of Mechanical Engineering and Technology (IJMET) Volume 9, Issue 7, July 2018, pp. 579–585, Article ID: IJMET_09_07_061

Available online at http://www.iaeme.com/ijmet/issues.asp?JType=IJMET&VType=9&IType=7

ISSN Print: 0976-6340 and ISSN Online: 0976-6359

© IAEME Publication Scopus Indexed

THREE PHASE GRID CONNECTED PV SYSTEM

WITH LVRT CONTROL STRATEGY

T. MURALI KRISHNA

Assistant Professor, Dept. of EEE, CBIT, Gandipet, Hyderabad

DEEPAK GEHLOT

Manager, BHEL R&D, Hyderabad

N. ANUSHA

P.G. Student, Dept. of EEE, CBIT, Gandipet, Hyderabad

G. SURESH BABU

Professor, Dept. of EEE, CBIT, Gandipet, Hyderabad

ABSTRACT

In power system the transient short circuit faults result in low voltages in the lines.

Due to this, performance of the loads connected to the grid will be affected. In order

to maintain constant grid voltage during transient faults, reactive power must be

controlled accordingly.

In order to supply the required reactive power during dynamic fault condition,

conventionally Distribution Static Synchronous Compensator (D-STATCOM), Unified

power flow controller (UPFC), Unified Power Quality Conditioner (UPQC) etc. were

proposed. In order to reduce the installation and operating cost, Low Voltage Ride

through (LVRT) is one of the upcoming technique to supply the necessary reactive

power during low voltage condition. In this technique a Photo Voltaic (PV) system

should stay connected to the grid and injects reactive power in order to maintain

constant grid voltage during fault conditions. After the grid voltage recovers to its

nominal value, the PV system needs to provide real power as fast as possible, so as to

maintain real power balance which therefore helps the whole system recovery.

Keywords: PV system, LVRT, GRID, Short circuit faults.

Cite this Article: T. Murali Krishna, Deepak Gehlot, N. Anusha and G. Suresh Babu,

Three Phase Grid Connected PV System with LVRT Control Strategy, International

Journal of Mechanical Engineering and Technology, 9(7), 2018, pp. 579–585

http://www.iaeme.com/IJMET/issues.asp?JType=IJMET&VType=9&IType=7

Three Phase Grid Connected PV System with LVRT Control Strategy

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1. INTRODUCTION

In recent years, more efforts have been made on the integration of PV systems into the grid in

order to meet the imperative demand of a clean and reliable electricity generation. Electric

power generation through solar energy process is one of the more methods available at the

moment, during the operation of solar energy systems do not generate any greenhouse gas

pollution of the environment. The high penetration of PV energy into the power system has

resulted in power system operators revising the grid codes requirements for interconnection of

this type of generation [1].

With this increased penetration of Distributed Generation (DG) systems into grid, fault

tolerance of these DG systems became one of major concerns as the sudden tripping of DG

from grid during faults results in serious problems such as voltage flickers, power outages and

may even cause blackout. Hence high power DG systems must be considered as conventional

systems and they need to support grid even during fault conditions. [2].

A special focus in these requirements is drawn to the PV fault ride-through capability

(LVRT), which addressed primarily the design of the PV controller in such that PV is able to

remain connected to the network during abnormal operation condition as well as can

contribute to voltage support during and after the abnormal operation conditions[3-5].

According to the theory of instantaneous reactive power, the active and reactive currents

of inverter can be regulated by changing the amplitude and the phase of the output voltage of

the inverter. Based on this theory, the active power output and the reactive power

compensation (RPC) of the system are realized simultaneously at daylight. When the

insolation is weak or the PV modules are inoperative at night, the RPC feature of PV system

can still be used to improve the utilization factor of the system [6].

In grid-connected systems, with an electrolytic capacitor in the dc-link, the oscillations of

the dc-link voltage with Distributed generator facility (DGF) can deteriorate the capacitor

lifetime, and thus the entire system. The proposed Low-Voltage Ride- through (LVRT)

control strategy benefits from a reference current generation method, which can eliminate the

oscillations at the dc-link and in the active power during unbalanced voltage dips [7-8].

2. STRUCTURE OF GRID CONNECTED VOLTAGE SOURCE

CONVERTER

Grid connected converters find application in a wide variety of fields such as distributed

generation, Active power filters, UPF rectifiers, HVDC systems etc. These converters works

as inverters when the power flow is from the dc side to ac side and vice versa when they

operate as rectifiers. The control capabilities and the structure of these converters are very

much generic irrespective of the mode of operation, except the direction of power flow. The

general structure of grid connected voltage source converter (VSC) is shown in Figure 1.

Figure 1 General structure of control of grid connected converter

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A very important feature of grid side converter control is the grid synchronization. The

synchronization algorithm is able to detect the phase angle of grid voltage in order to

synchronize the delivered power. The purpose of this method is to synchronize the inverter

output current with the grid voltage, in order to obtain a unity power factor. The PLL is

shown in below Figure 2.

Figure 2 Basic structure for the SRF PLL system

Input is the three phase voltages measured on the grid side and the output is the tracked

phase angle. The PLL model is implemented in synchronous dq reference frame, where a Park

transformation is used. The phase-locking of the system is realized by adjusting the q-axis

voltage to zero or d–axis to zero. A PI controller is used for this purpose. By integrating the

sum between the PI output and the reference frequency the phase angle is obtained.

The phase angle θ is tracked by synchronizing the voltage space vector along q or d axis

in the Synchronous rotating reference frame. Here the voltage space vector is synchronized

with the q-axis.

3. LVRT CONTROL STRATEGY

The SEG system comprises photovoltaic (PV) arrays, a power conversion system, grid-side

inverter, a filter, and a three-phase grid. If the grid voltage drops in the solar energy

generation (SEG) system, reactive power is injected in the grid-side inverter by PV system. In

the SEG system, the LVRT control strategy with maximum power generation is widely used.

Using the LVRT control strategy, the q-axis current (Iq) required for injecting the reactive

power into the grid having a low voltage is determined based on the LVRT requirement

depending on the level of low voltage.

The rotating reference frame method, also called d-q control, is widely used in three-phase

systems. Rotating reference frame regulators have become industry standard in the field of

high-performance current-control methods. Vector control is performed entirely in the

rotating d-q coordinate system to make the controller side elegant for a wide range of

applications. The method for determining the injection quantity of the active and reactive

currents depends on the voltage drop ratio of the three-phase grid. The voltage-level (VLEVEL)

of the three-phase grid is calculated by the voltage-level calculation process using the three-

phase grid voltages Va, Vb and Vc. VLEVEL is classified into three parts depending on the

LVRT requirement of the grid-code regulations, and the method for determining the injection

quantity of the active and reactive currents is selected by each part as shown in Figure 3.

Figure 3 Method for determining the injection quantity of the active and reactive currents depending

on the voltage drop ratio of the 3-phase grid

Three Phase Grid Connected PV System with LVRT Control Strategy

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If VLEVEL is greater than 90% of the three-phase grid voltage under normal conditions, the

reactive current injected into the three-phase grid is zero, and the active current becomes the

reference current. If VLEVEL is greater than 50% but less than 90% of the three-phase grid

voltage, then reactive current to be injected into the three-phase grid is determined based on

the voltage drop ratio of the three-phase grid. In addition, the active current is calculated using

the reactive current and the rating current of the grid-connected ESS. If VLEVEL is less than

50% of the three-phase grid voltage, the reactive current injected into the three-phase grid is

the rating current of the grid-connected ESS, and the active current is zero.

4. RESULTS

The system consists of a 3-phase grid, battery and a LCL filter. An LCL filter is used to

interconnect an inverter to the utility grid in order to filter the harmonics produced by the

inverter. A PLL is used to synchronise the phase angle of the inverter with grid voltage. A

Closed loop vector current control is used to generate reference signals to the inverter.

Decoupling is used to independent control of active and reactive power. From the voltage

level Vd, the reference active current and the reference reactive current is controlled according

to the depth of voltage sags. The block diagram of 3-phase grid connected PV system with

LVRT is shown in Figure 4.

Figure 4 3-phase grid connected VSC with LVRT

Figure 5 Inverter voltages & currents at grid voltage 320V

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Figure 6 Inverter Active & Reactive power waveforms at grid voltage 320V

The Figure 5 & Figure 6 shows the Closed Loop Grid Connected PV System, inverter

voltage and current waveforms for a Grid voltage of 320V and DC voltage of 650V, Active

power of 500kW. Consider Base voltage as 320V, and Base power 500kVA. In normal

operating condition the Active power delivered is 500kW, and Reactive power is 0 kVAr.

Figure 7 Inverter voltages & currents at grid voltage 240V

Figure 8 Inverter Active & Reactive power waveforms at grid voltage 240V

The waveforms Figure 7 & Figure 8 shows, the Active power is 320kW, Reactive power

is 180kVAr during fault condition, for grid voltage 240V,

Figure 9 Inverter voltages with & without LVRT during normal operating condition.

Three Phase Grid Connected PV System with LVRT Control Strategy

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The inverter voltages with & without LVRT during normal operating condition is shown

in Figure 9. During normal operating condition, the inverter voltages are same in both the

circuits, with and without LVRT. The voltages are taken in per unit. The base value is 320V.

Figure 10 Inverter currents with & without LVRT during normal operating condition.

The inverter currents with & without LVRT during normal operating condition is shown

in Figure 10. During normal operating condition, the inverter currents are more without

LVRT and within the limit during LVRT. The excessive over currents through inverter are

controlled with LVRT.

Figure 11 Active power with & without LVRT during normal operating condition

The Active power with & without LVRT during normal operating condition is shown in

Figure 11. During normal operating condition, the active power delivered is 1.2 pu without

LVRT, and with LVRT the active power delivered is 1.6 pu. The active power delivered to

the load is more with LVRT.

Figure 12 Reactive power with & without LVRT during normal operating condition.

The Reactive power with & without LVRT during normal operating condition is shown in

Figure 12. During normal operating condition, the reactive power delivered is 1.8 pu, without

LVRT, and with LVRT the reactive power delivered is 0 pu. The Reactive power delivered to

the load is less with LVRT during healty condition.

5. CONCLUSION

The proposed control enables the PV system to generate a reactive power during fault to

support the grid. This helps the grid to maintain voltage stability during fault. Reactive power

is provided to support voltage recovery according to the depth of grid voltage sags. After the

grid voltage recovers to its nominal value, the PV system provides real power as fast as

possible, so as to maintain real power balance which therefore helps the whole system

recovery.

Decision Support Systems- An overview

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