Passive And Active Methods Of Islanding For PV Systems
Passive and Active Methods of Islanding for PV systemsTomáš Skočil, Oriol Gomis-Bellmunt, Daniel Montesinos-Miracle,Samuel Galceran-Arellano and Joan Rull-DuranCentre d’Innovació Tecnològica en Convertidors Estàtics i Accionaments (CITCEA-UPC)Departament d’Enginyeria Elèctrica, Universitat Politècnica de Catalunya,ETS d’Enginyeria Industrial de BarcelonaAv. Diagonal, 647, Pl. 2. 08028 Barcelona, SpainBarcelona, SpainPhone: 34 934016727Fax: 34 934017433Email: skocil.tomas@gmail.com, oriol.gomis@upc.edu, montesinos@citcea.upc.edu,galceran@citcea.upc.edu, rull@citcea.upc.eduURL: http://www.citcea.upc.eduKeywords Simulation , Control methods for electrical systems , Renewable energy systems , Pulse Width Modulation (PWM) , Modelling , Photovoltaic , Power supply , Protection device , Regulation , Alternative energy , Single phase system , Voltage Source Converter (VSC) .AbstractThis paper presents a review of some techniques for islanding detection, especially by using inverterbased DG applications and it also focuses on several islanding detection methods for a single-phasecurrent-control voltage inverter working with a PV system connected into the grid. It is deliberateda single-phase inverter with maximum power of 5 kW. Islanding detection methods are investigated,simulated and evaluated in MATLAB / SIMULINK.IntroductionAnti-islanding capability is an important requirement for distributed generators. It is necessary to detectwhen the system operates in an island and to disconnect it from the grid as soon as possible. Theisland can occur when a part of grid is electrically isolated from the power system but the part withisland is energized by distributed generators. The islanding detection is important for many reasons - apossibility to damage customer equipments and distributed generator, hazard for line-workers, islandingmay interfere with restoration of normal services for neighboring customers.The islanding detection is important for all distributed generation systems. Many algorithms have alreadybeen developed in last 10 years. All these methods can be divided into 4 categories [1], [2], [4]. passive inverter-resident methodsactive inverter-resident methodsactive methods not resident in the invertermethods based on the use of communications between the grid and PV inverterThis paper is only focused on passive and active inverter-resident methods. There are not shown neitherthe active methods not resident in the inverter because it needs more equipment or methods based on theuse of communication for their robustness and communication safety reasons. These 2 last methods arenot easy and cheap and it means another expenses.Passive detection methods [1], [4] have a large non-detection zone (NDZ) and that is not a significantanti-islanding protection. Two main system parameters frequency and voltage at the point of commoncoupling (to assign the size of NDZ) can change their values depending on a variable load connectedto the grid. If an inverter has the capability of over/under voltage protection and over/under frequency
protection, we say it has the basic islanding detection capability. Active detection methods should bealso used to decrease the size of NDZ. But the main disadvantage of active methods is the injecting ofa disturbance signal into the grid [2], [13]. For a faultless and correct use is better to combine moreislanding detection methods based on another working process [16]-[21].This report presents a review and simulations of the size of non-detection zone (over/under voltage andover/under frequency method), PLL based active method [6], [7], [13] and active frequency drift method[8], [12] for a single-phase inverter of active power about 5 kW. The same passive and active parametersfor simulations have been used and the same time when the island occurs and the same instant whendisturbance of active methods is injected. The tables comparing these methods are shown at the end ofthis paper.System configurationThe topology of the grid, the current-controlled inverter working with PV system and parallel RLC loadis shown in Figure 1. The node where all parts are connected is called the point of common coupling(PCC). Voltage and voltage frequency are measured in this node and used for islanding detection andalso as references for the control of inverter. For accurate control of inverter is also necessary to measurethe output inverter current.Figure 1: Voltage source inverter connected to local load and distribution networkThe inverter inductor is used for a smooth sine-wave of inverter current. For objective simulations itis also used the grid impedance between the PCC and the grid. R, L, C are resistance, inductance andcapacitance of parallel RLC load.Anti-islandingIslanding test conditionsIslanding of the grid connected generators (photovoltaic system) can occur when the part of utility systemhaving such generators is disconnected from the main grid and independent generators keep to energizethe isolated part. The system consists PV panels, an inverter, a local load (parallel RLC), a switch(breaker, fuse, .) and the grid. In this paper the distribution generation is considered to be in unitypower factor operation. This unity power factor condition combined with passive parameters of parallelRLC load and frequency in (1)-(4) consider the worst case for islanding detection when the active powerof load matches to output power of distribution generation. For simulations the worst case is used as testconditions according to UL1741, IEEE 929 and IEEE 1547 [9], [10], [16]:1. The power generated by DG should match the RLC load power, ΔP 0 and ΔQ 0.2. Resonant frequency of the RLC load is the same as grid line frequency ( f 50Hz).3. The quality factor Q f of RLC load is set to be 2.5. The quality factor is defined as that the reactivepower stored in L or C is Q f times the active power consumed in R.Under these conditions, when the grid is disconnected, the distributed generation and RLC load willresonate at nominal voltage and frequency to form an island, unless there is a mechanism to drive voltageat PCC or frequency out of their nominal range. Load definition can be represented as
f 1 (1)2π LCV2R PV2L 2π f Q f PQf PC 2π fV 2(2)(3)(4)whereR - effective load resistance [Ω], L - effective load inductance [H], C - effective load capacitance [F],P - active power [W], Q f - quality factor, f - grid frequency [Hz].The values of frequency and magnitude of the voltage at the PCC after grid disconnection (islandingcondition) depend heavily on the local load characteristic.Pload 2VPCCR 2Qload VPCC1 ωCωL(5) (6)Equations (5) and (6) describe the active and reactive power consumed by the RLC load. If the activepower demand of the load and active power production of PV system are not the same at the instant whenthe breaker opens, then the voltage at PCC must decrease or increase until PPV Pload . It is similar, if thereactive power demand of load and reactive power production are not matched at the time when the gridis disconnected. The frequency ω at PCC must change until QPV Qload . The mechanism, by whichthis happens, is that the PV inverter will seek a frequency at which the current-voltage phase angle of theload equals that of the PV system. Such voltage and frequency changes can be detected by over/undervoltage and over/under frequency relays.Difficulties appear when a load demand and PV generation are close. Then frequency or voltage changescan be insufficient to enable detection by PV inverter. It is the reason why it is necessary to developislanding techniques which can detect these cases when the powers of PV and load are closely matched.It is the aim of all islanding detection methods to reduce the non-detection zone near to zero.Anti-islanding methodsA. Over/under voltage and over/under frequency methodAll grid-connected inverters are required to have an over/under voltage and an over/under frequencyprotection. Limits when the inverter should be switched off can be different in each country. An exampleof values for most of countries in Europe is in Table I [15], [16].Table I: Voltage and frequency limitsValueFrequencyVoltageMinimumfmin 49HzVmin 0.88puMaximumfmax 51HzVmax 1.1puPower balance at PCC is given by equations (7) and (8). If ΔPload ΔPDG and if ΔQload ΔQDG thereis no active/reactive power difference between the PV system and the grid.Pload PDG ΔPQload QDG ΔQ(7)(8)The behavior of the system when the grid is disconnected depends on ΔP and ΔQ. If the resonantfrequency of RLC load is the same as grid line frequency, the linear load does not absorb reactive power.Active power is directly proportional to the voltage. After the disconnection of the grid, the active powerof the load is forced to be the same of the PV system, hence the grid voltage change into
V upper limitupuloCurrent - Controlled InverterPower sourceDCpowerfaultV lower limitvoltage measurementV faultfaultPV powerfaultf faultf upper limitfrequency measurementPCCupuloTripping protectionf lower limitSIN generatorPCC loadInverter Breakers2U iU gridV amplitudeAmp.GridZ gridcsinfreq.fn 1-fi0[V PCC]v -Figure 2: Topology of load, grid and inverter with required over/under voltage and over/under frequency detectionV K ·V(9)whereK PDG /Pload(10)If ΔPDG ΔPload there is an increase of the amplitude of the voltage and if ΔPDG ΔPload there is adecrease of the amplitude.A small ΔP results in an insufficient change in voltage amplitude and small ΔQ results in an inadequatechange in frequency to effectively disconnect the PV and prevent islanding. The probability of smallvalues of ΔP and ΔQ for the NDZ is significant and protection devices cannot detect an island reliably.In general, over/under voltage and over/under frequency devices alone are generally considered to beinsufficient anti-islanding protections. It is possible to calculate the NDZ from the mismatches of activeand reactive power and setting the threshold values for voltage amplitude and for frequency.B. PLL based active methodThe method is based on the modification of sine-wave current reference obtained from PLL. A sinusoidalsignal multiplied by distortion gain is injected to the inverter current angle. The inverter current angleand distorted signal are summed. From this new value of current angle the sine-wave current reference ismade. The sinusoidal signal injection can be done with positive or negative sign. The current referencefor the inverter must be synchronized with the voltage at PCC [13], [14].C. Active frequency driftIt is a method which can be easily implemented into a inverter with a microprocessor-based controllerand it adds no costs to such a system. Inverters give the output current slightly distorted into the utility atthe frequency slightly lower or higher than the voltage frequency at PCC. For example - during first halfof sinusoidal waveform of output inverter current is with frequency slightly higher than the frequency ofthe grid voltage is. When the inverter current reaches zero, it remains zero for time before the beginningof next half-cycle [8], [12].Simulation of anti-islanding detection methodModel passive and active parameters used for simulations are shown in table II. These parameters wereused purposely for comparing different detection methods.A. Over/under voltage and over/under frequency methodFigures 2 and 3 show the topology for detection under/over voltage and under/over frequency with inverter shutdown. Voltage at PCC can only change when the grid is disconnected and DC power andpower of load are not the same. If the load power is smaller then the voltage at PCC increases and ifthe load power is higher then voltage at PCC decreases. Voltage difference depends on the differencebetween these powers.Active power of parallel RLC load was changed for an assignment of NDZ for constant DC power ofPDC 2000 W without using another detection method. It was determined limit bounds of detection
Table II: Used model parameters for comparison of different islanding methodsParameterActive power of load [W]Resonance frequency of inverter load [Hz]Quality factor [-]Resistance of inverter load [Ω]Inductance of inverter load [mH]Capacitance of inverter load [μF]Cut-off time of breaker [s]Resistance of the grid [Ω]Inductance of the grid terNominal frequency [Hz]RMS voltage [V]Power of PV system [W]Resistance of smoothing inductor [Ω]Inductance of smoothing inductor [H]Limit of over voltage [V]Limit of under voltage [V]Limit of over frequency [Hz]Limit of under frequency [Hz]Value5023020000.15e-3253202.45149PLL voltage and frequency measurementfrequency measurement2frequencyV PCCvoltage1voltage measurementthetasinPID controllerCurrent reference calculationI refV PCCfaultPWMI ampI inv1fault0PV power2Power sourcevInverter -c2 [V DC]-PWM signal1sBreakerPV PCCi-PCC1PV-Figure 3: Inverter with PLL and current control loopcapability 1600 W (over voltage - 1.1VRMS ) and 2550 W (under voltage - 0.88VRMS ). If the load poweris between these values, the system is not able to detect an island operation. If the load power is out ofthe range, the islanding detection is very reliable. Plots for NDZ bounds 1600 W a 2550 W are in Figure4(a). The grid was connected until the instant t 0.8s (breaker opens in zero current). We can see thecurrent magnitudes corresponding to DC and load power. Grid current is the difference between currentof load and inverter current. Inverter current must be the same with load current after grid disconnection.When the islanding operation is detected, the inverter is switched off immediately. This time is about0.03 s. In the plot of voltage at PCC is obvious that voltage increases to the limit 1.1VRMS 253 V aftergrid disconnection for smaller load power than DC power of PV system and for higher load power thevoltage decreases until it reaches 0.88VRMS 202.4 V.Load active power of Pload 2000 W is used for a verification of under/over frequency detection. Values of load resonance frequency were changed. There exist 2 bounds for fo 49 Hz and fo 51 Hzbecause passive parameters of load determine the frequency reference for the control loop of inverter. (Ifresonance frequency is fo 50 Hz, load does not take reactive power when the grid is disconnected. Inmeasured frequency plot in Figure 4(b) we can see that the frequency decreases for fo 49 Hz after griddisconnection until it reaches f 49 Hz then the inverter is disconnected. For fo 51 Hz frequency ofvoltage at PCC increases after grid disconnection to f 51 Hz where the islanding operation is detectedand the inverter is switched off. The plots are shown in Figure 4(b).From these simulations it is possible to assign the bounds of non-detection zone which are shown inFigure 5. For all load parameters inside this zone it is impossible to detect an islanding operation. Whenwe use load power PDC 2000 W, we obtain detection limits:Pload 1600 WPload 2550 W PDC Pload 400 W PDC Pload 550 W 20% o f PDC27.5% o f PDC(11)(12)and for frequency:fo 49 Hzfo 51 Hz fn fo 1 Hz fn fo 1 Hz 2% o f fn2% o f fn(13)(14)
Current (Pload 2550 W)20II0grid-100.750.80.850.90.95I0.7400Inverter shutoff2000-2000.70.750.80.850.9Phasor of voltage at PCC (Pload0.95-200.6510.951Grid disconnection400Inverter shutoff00.70.750.80.850.9Phasor of voltage at PCC (Pload0.95Grid disconnectionVoltage [V]0.80.850.90.95110.80.850.90.95Grid r shutoff0.750.80.850.90.951Grid disconnection0.90.9510.951Inverter shutoff00.70.750.80.850.9oInverter shutoffGrid erter shutoff510.70.750.8Time [s](a) Load power 1600 W a 2550 W and f0 50 Hz0.85Frequency of voltage at PCC (f 51 Hz)50480.650.852Grid disconnection10.75-200-4000.6551Time [s]grid200oInverter shutoff0.85IVoltage at PCC (fo 51 Hz)00.7invIload-101-200-4000.65I0Frequency of voltage at PCC (f 49 Hz)10000.650.75200 2550 W)200Time [s]grid52Inverter shutoff1000.750.7Inverter shutoff10Voltage at PCC (fo 49 Hz)-200 1600 W)2000.7IGrid disconnectioninvIload-10300Grid disconnectionVoltage [V]0.9200-4000.6530000.650.85I0Frequency [Hz]-4000.650.8Voltage [V]Grid disconnection0.75Current (fo 51 Hz)20Inverter shutoff10Voltage at PCC (Pload 2550 W)Voltage [V]Voltage [V]400grid-10Voltage at PCC (Pload 1600 W)Grid disconnectioninvIload0-200.65120IVoltage [V]0.7Current (fo 49 Hz)Inverter shutoff10Frequency [Hz]-200.65Grid disconnectioninvIloadCurrent [A]Inverter shutoffCurrent [A]Current [A]Grid disconnection10Current [A]Current (Pload 1600 W)200.850.90.951Time [s](b) Load reson. freq. 49 Hz, 51 Hz and Pload 2000 WFigure 4: Over/under voltage and over/under frequency method - Q f 2.5, breaker opens at t 0.8 sThis detection method has a quite big zone where the detection is impossible, especially for active power.It is the reason why it is necessary to use another subsidiary methods.Figure 5: Non-detection zone for 0.88VRMS 202.4 V, 1.1VRMS 253 V, f 49 Hz and f 51 HzB. PLL based active methodIt was done a verification for load parameters PDC Pload 2000 W, f0 50 Hz, Q f 2.5 which is oneof the worst detectable islanding operations. The previous method cannot detect this case.For an assessment of limit from where it is possible to detect an islanding operation the distortion gainwas gradually increased from the lowest values. We obtained the distortion gain limit 0.2 when islandingoperation can be detected. The ability of detection is for the distortion gain higher than 0.2. On theother hand we need to have this value as small as it is possible. The higher is this value, the moredistorted is the sine wave obtained from current reference calculation. There the second harmonic isadded. Disturbances are invoked all the time also when the system does not work in islanding operation.PLL voltage and frequency measurementfrequency measurement1frequency[V PCC]V PCCvoltage2voltage measurementthetasindistortion[I amp]sinFigure 6: Topology for the PLL based active methodsqrt(2)[I ref]
Current (distortion 0.2)Grid disconnectionInverter shutoff10Voltage at PCC (distortion 0.2)400II0grid-10-200.60.70.8Grid disconnection0.92000-200-4000.610.7Phasor of voltage at PCC (distortion 0.2)200Grid disconnectionFrequency [Hz]Voltage [V]0.80.91Frequency of voltage at PCC (distortion 0.2)52250Inverter shutoff1501005000.60.70.80.9Grid disconnection5049480.610.7500Voltage [V]420.7Grid disconnectionInverter shutoff600.60.80.80.91Inverter voltage (distortion 0.2)8Grid disconnectionInverter shutoff51Theta (distortion 0.2)Θ [rad/s]Inverter shutoffinvIloadVoltage [V]Current [A]200.91Inverter shutoff0-5000.60.70.8Time [s]0.91Time [s]Figure 7: PLL based active method - distortion gain 0.2, PDC Pload 2000 W, f0 50 Hz, Q f 2.5, breakeropens t 0.8 s0.91Inverter current (distortion 0.2)Grid disconnectionInverter shutoffDisturbance100-10-200.60.70.80.9Grid disconnection0.7Inverter shutoff-1-20.92000.80.9Grid disconnectionInverter shutoff150Disturbance10000.61Grid disconnection80.70.8Grid disconnection0.90-2000.80.91-6000.6Grid disconnection1Inverter shutoffDisturbance490.70.80.91Inverter voltage (distortion 0.2)600Grid disconnectionInverter 61Inverter shutoff62000.851Theta (distortion 0.2)Inverter shutoffDisturbance0.7Frequency of voltage at PCC (distortion 0.2)52500.7Inverter shutoffDisturbance0-4000.61Grid disconnection-200250Inverter shutoffDisturbance400Disturbance00.8Voltage at PCC (distortion 0.2)200Phasor of voltage at PCC (distortion 0.2)-10600Voltage [V]Current [A]-200.61Igrid4000Inverter voltage (distortion 0.2)1-30.60.90-200.613Grid disconnection0.810Grid current (distortion 0.2)20.7Iload-10Load current (distortion 0.2)20Current [A]Current [A]20IinvInverter shutoffDistu
based DG applications and it also focuses on several islanding detection methods for a single-phase current-control voltage inverter working with a PV system connected into the grid. It is deliberated a single-phase inverter with maximum power of 5 kW. Islanding detection methods are in
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