PROBABILISTIC RISK ASSESSMENT OF ISLAND OPERATION OF GRID CONNECTED MULTI-INVERTER POWER PLANT

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1 Energy and the Environment (204) PROBABILISTIC RISK ASSESSMENT OF ISLAND OPERATION OF GRID CONNECTED MULTI-INVERTER POWER PLANT Mihovil Ivas, M.Sc.E.E. Telenerg d.o.o., Zagreb, Savska cesta 4/V, Phone: 0/677009, fax: 0/677002, Abstract: Island operation of distributed generation source without possibility of regulation of the generation power, with part of local network grid, is unwanted operation situation which can happen when one of the switching devices switches out and disconnects part of local grid, including distributed generation source, from the main grid supply. Such unwanted operation situation imposes danger for personnel safety, prevents the fault arc to extinguish during automatic re-closing dead time, allows switching onto unsyncronized part of network, and reduces power quality. Typical example for distributed generation source without regulation, which requires some kind of islanding protection, are power plants composed of grid connected inverter units, like photovoltaic power plants. Standardized islanding protections are integrated within inverters. Probability of island operation depends on balance of generation power and load power, where neither one of the values is constant. Both load power and generation power are independent variables, i.e. probability functions which can be derived from long-term measurements in operation. Real example of one of the largest solar power plants in Croatia is used for risk probability assesment of island operation and to justify the need for further research of this phenomenon and for investment in additional island protection. Key words: island operation, probabilstic risk analysis, distributed generation source, inverter, solar power plant, island protection. INTRODUCTION Island operation is a condition in which producing unit (distributed generation source) is feeding some of the consumers in separated part of the network which is disconnected from main power source (feed-in from grid side). Island operation of distributed source generally is acceptable when its main purpose is to feed consumers of the specific industrial grid or similar local network, while such sources in its normal operation work parallely with outher network in which they eventually feed the surpluss of the produced energy. Such sources allways have the posibility of regulation which allow maintaining of voltage and frequency in defined ranges. When distributed generation sources do not have regulation possibility, and this would be the case with all intermittent renewable sources which are being connected to the grid in growing numbers, islanding operation is unwanted. Paticular type of sources which are of interest to this work are multi-inverter power plants connected to medium voltage distribution networks.

2 94 M. Ivas, Probabilistic risk assessment of UNWANTED ISLAND OPERATION OF INVERTER-TYPE POWER PLANTS Island operation is defined as state of the producing unit in which it can safely feed partial load in disconnected part of network. Possibility of island operation means that source is having regulating systems for regulation of rotation speed, active power and excitation which allows for plant to be able to safely adapt to any amount of partial load larger of technical minimum for the particular producing unit. Such island operation must be possible to be maintained for several hours. During operation with partial load, producing unit must be capable to regulate changes in load amounting 0% of nominal active power []. Basic criteria for sustainable island operation of separated parts of alternate current electrical systems is that active and reactive power of generation and load must be balanced at any moment. From the requirement above it is clear that island operation of distributed source without possibility of regulation is unwanted. Common practice for network operators worldwide at the moment is not to allow such operation, and to request such sources to be disconnected in case of islanding conditions. Few main reasons for which such operation is unwanted are following: - there is a danger for maintenance and restoration personnel, not knowing that part of the network, although disconnected, is still powered, - voltage and frequency of islanded network part can slip out of defined range, power quality worsens, which adversely affect the loads and can result in damage of both customer and utility side equipment, - automatic re-closing of utility breaker may create a condition of asynchronous closure which can lead to serious damage of the equipment, - the very purpose of automatic re-closing of utility breaker, which is to clear the transient faults on the feeders, is not fulfilled, as fault arc will not exstinguish if it is fed from distributed generation source in island operation. 2. CONDITIONS FOR ISLAND OPERATION Immediately after switching out of switching device and disconnection from the main grid supply one or more distributed generation sources are in operation with local load separated from the rest of the electric system. Such island is non-regulated electrical system with unpredictable behavior, the cause of which is the mismatch of the generation power and load power and nonexistence of voltage and frequency regulation. Occurrence of the island operation on feeder having nominal load power much larger then maximum possible generation power on same feeder is practically impossible. Greater amount of power from distributed generation sources on network feeders increases the probability of lasting island operation. This probability depends on balance of generation power and load power, network conditions, regulation possibilities and applied protection methods. The most influencing factor is mismatch between load power and generation power, where neither one of the values is constant. Both load power and generation power are independent variables, i.e. probability functions which can be derived from long-term measurements during operation. If generation power and load power in disconnected system are approximately equal, there will not be necessary change in amplitude or frequency of the voltage at the point of connection to actuate any of installed over/under voltage and over/under frequency protection.

3 Energy and the Environment (204) NON-DETECTION OF ISLAND OPERATION Reliability of island detection methods can be defined by the size of the non-detection zone (NDZ), which is the range of mismatch of the active and reactive power (ΔP and ΔQ) in which island operation will not be detected. The non-detection zone is a consequence of a necessity to prevent unwanted and incorrect actuation of protection. It is necessary for island protection to be insensitive to some standard transient occurrences in electrical system, such is voltage dips, over voltages, harmonic distortions and changes in frequency. The ranges of acceptable voltage and frequency are standardized [2]. OF Q NDZ P UV OV UF Figure Graphical representation of non-detection zone The non-detection zone for standard over/under voltage and over/under frequency protection can be precisely calculated. For over/under voltage protection function non-detection zone is [3]: 2 2 V P V V P Vmin max DG () For over/under frequency protection function non-detection zone is [3]: 2 2 f Q f q q f min P DG fmax (2) Picture graphically shows non-detection zone for island operation, where mismatch ranges for active and reactive power (ΔP and ΔQ) for which island operation will not be detected are defined by settings of voltage and frequency protection functions (OV-overvoltage, UV-under voltage, OF-over frequency, UF-under frequency). Worst case for island operation detection is most certainly complete balance of generation power and load power in the disconnection moment, i.e. non existence of ΔP and ΔQ. 3. PROBABILITY OF ISLAND OPERATION To assess the probability of islanded operation of (inverter-type) distributed generation source connected to the grid in defined point and in defined time period, two parameters are needed: - probability of balanced active and reactive power of distributed source and load in part of local grid which forms an island, meaning that mismatch is within range of protection tolerances i.e. in the island operation non-detection zone,

4 96 M. Ivas, Probabilistic risk assessment of... - frequency of switching out of the switching device (circuit breaker, disconnecting switch) between main grid supply and part of the grid which forms an island. Frequency of operation of particular (or typical) switching device can be assessed (or known) from network operation statistical analysis, and consists of the frequency of protection caused switching out and operations conducted by operator. 4. PREVIOUS RESEARCH Certain research has been conducted related to this topic. Study was conducted to assess probability of island operation for the one unit inverter power plant connected to low voltage grid with mainly household consumers [4]. The study has determined that the probability of simultaneous occurrence of the balanced generation and load and switch opening event is practically zero. The conclusion of the research was that island operation phenomenon is not an obstacle for application of these types of distributer generation sources. On a real example of medium size solar power plant (00 kva) connected to low voltage grid by means of measurement it was determined that probability of occurrence of balanced power condition depend on the power factor on the output of the power source, in this case inverter power factor. If a power factor is, probability of balanced powers in this particular case was in range of 0-5 to 0-3, while with a power factor of 0,95, which matches the grid power factor, same probability was in range of 0-2 to 0 - [5]. The conclusion of the research was that risk of causing the accident as a consequence of unwanted island operation (in this case 6x0-2 per year) is not negligible and the recommendation was to set output power factor of inverters to. 5. METHOD FOR PROBABILISTIC RISK ASSESSMENT OF ISLAND OPERATION A simple models will be used to assess probability of island operation of the previously described distributed generation source. For any of the variables used here it is possible to develop more precise distribution models based on more data and thus get better results, but the method will be the same. For the general purpose, results derived with these models are derived to point out that there is a need for further research of this phenomenon. Important thing not taken into account for this analysis is reactive power balance. This would have significant impact to the calculation, but due to insufficient information the assumption is taken that majority of the reactive load is covered within load points (substations 20/0,4 kv) and the overall power factor in the MV grid is close to. This may not be the case in most cases. The power plant, when working in lower range or power, has a power factor of approximately 0,99. When it reaches 30% of nominal power the power factor is around. All further analysis is based only on active powers (of generation and load). 5.. Fault tree As explained previously, two basic events need to happen in order to cause island operation. Nevertheless, there are number of pairs of these events, depending on network configuration. The fault tree is shown on Picture 2. Explanation of the configuration influence can be understood from the Picture 7.

5 Energy and the Environment (204) island operation of distributed source OR AND AND AND balanced conditions in local grid # ( P 0, Q 0) disconnection of local grid # by switching device balanced conditions in local grid #2 ( P 0, Q 0) disconnection of local grid #2 by switching device Figure 2 Fault tree for island operation balanced conditions in local grid #n ( P 0, Q 0) disconnection of local grid #n by switching device 5.2. Generation model Average daily generation profile for the photovoltaic power plant (on yearly basis) can be represented by the graph shown on Picture 3. The photovoltaic plants are inverter-type sources. The interesting part of the graph is between 5 and 2 hours, as in the rest of the period there is no generation. This is important for load model representation. 0.8 p.u hours Figure 3 Average daily generation profile of the photovoltaic power plant 5.3. Load model Example of average daily load profile used for this analysis is represented by the graph shown on Picture 4 [6]. As stated previously, the period between 5 and 2 hours is interesting, as during the night there is no generation by power plant, and thus there is no probability for match of generation and load power. In further analysis, period between 2 and 5 hours will be excluded. 0.8 p.u hours Figure 4 Average daily load profile

6 98 M. Ivas, Probabilistic risk assessment of Frequency distribution of the variables For probability analysis the normal distribution (Gaussian function, equation (3)) is used to describe the frequency distribution for both of the continuous random variables (generation and load). It roughly describes daily generation and load profiles (as a possible development of the method, analysis on the hourly ranges could be conducted, and in that case the asymmetrical distribution functions should be used). 2 x µ 2 σ f( x) = e (3) σ 2 π In equation (3) x is a variable, μ is a mean or expectation, and σ is standard deviation. Generation is described by the following parameters: μ=0,5 p.u., σ=0,5 p.u., while the load (in the period of the day 5 to 2 hours) is described by the parameters: μ=0,5 p.u., σ=0, p.u. Graphs on Pictures 5 and 6 represent frequency distribution curves for generation and load power daily profiles (from 5 to 2 hours) from the Pictures 3 and 4. Horizontal axis is in p.u. values Figure 5 Frequency distribution curve for generation Figure 6 Frequency distribution curve for load 5.5. Probability calculation for island conditions Probability for the continuous random variable with normal frequency distribution to appear in the range a-b is calculated from equation (4): 2 x µ 2 σ b P = e dx (4) σ 2 π a To determine probability of island conditions in the local grid we have to calculate probability of balanced powers of generation and load, i.e. calculate probability on the whole range of possible power generation values whilst the load power values are in the range where the non-detection of island protection will happen, for each one of the power generation values. Equation (5) gives that probability: 2 2 x= P x m m max G G y= x+ P y + L 2 σ 2 σ G L Pic = e e dydx (5) σ 2 π σ 2 π x= 0 G y= x P L where x is generation power variable, y is load power variable, and function parameter subscripts are G for generation and L for load, and P ic is probability of island (balanced) conditions.

7 Energy and the Environment (204) Statistical operation data for switching equipment Statistical data for the switching devices can be collected from network operator databases. Either particular case study data for known device if data is available, or general data for the type of device can be used for assessing probability of operation (switching out) during defined time period Time period Time period used for this method for which the probabilities will be calculated is one year. It can however easily be modified to any other preferable time period. 6. SOLAR POWER PLANT KANFANAR EXAMPLE Photovoltaic power plant Kanfanar having nominal power of 999 kw (which is installed panels power, installed inverter power is 92 kw) is at the moment largest solar power plant in Croatia and first one connected to medium voltage grid operated by Croatian Electric Utility - Distribution Network Operator. It is connected to 20 kv grid of Distribution Area Elektroistra Pula. Producing unit is connected to its own substation S/S 0,4/20 kv in which the transformation of voltage from nominal "generator" voltage 0,4 kv (the plant has total of 76 inverters of 2 kw nominal power) to 20 kv nominal voltage is being done, and from there by cable connection it is connected to operator 20 kv substation where the point of connection to MV grid is formed. 6.. Possible connection configurations Principle scheme with possible network switching configurations, showing the two possible ways in which the power plant can be operating and feeding the grid is shown on Picture 7. Scheme shows possible switching points between main grid supply point (S/S 0/20 kv Vincent) and the power plant, where power plant can be disconnected from the network and thus create an islanded network with local load. It also shows nominal transformer loads for all substations connected to feeder points in between these two end points. The operator's preferred connection is via feeder bay "Rovinj", and the connection via feeder bay "Kanfanar 2" is reserve. The reserve connection shall be used minimally. S/S Vincent 0 kv 20 kv 3xS/S: 60 kva 3x000 kva 400 kva Feeder "Kanfanar 2" Feeder "Rovinj" xts: 400 kva 9xS/S: 630 kva, 400 kva, 250 kva, 250 kva, 250 kva, 630 kva, 00 kva, 00 kva, 00 kva, 00 kva, 00 kva, 50 kva, 50 kva, 400 kva, 00 kva, 260 kva, 60 kva, 00 kva, 630 kva xs/s: 50 kva PVP Kanfanar Pmax=92 kva HT circuit breaker disconnector switch network branch house transformer 3xS/S: 00 kva 00 kva 50 kva 3xS/S: 400 kva 400 kva 400 kva HT: 00 kva Figure 7 Substitute network configuration representation for possible PVP Kanfanar connectio

8 200 M. Ivas, Probabilistic risk assessment of Protection non-detection zone for islanded conditions In the following table the set parameters for over/under voltage and over/under frequency protection, integrated within inverters of the power plant are given, followed by the resulting non-detection zone ranges for islanded conditions [7]. Table Settings UF/UV OF/OV Frequency f min =47,5 Hz f max =5,5 Hz Voltage V min =0,88 p.u. V max =, p.u. P 7, 36% 29,3% P DG Q 2,% 6, 7% P DG (6) (7) 6.4. Statistical data for the switching equipment During the year of operation of the power plant, from the data gathered, it can be concluded that on the particular feeder supplied from S/S Vincent feeder bay "Rovinj", the successful and unsuccessful automatic re-closing of the circuit breaker happened at least 5 times [8]. During the relevant hours (5-2hours) it can be assumed that it will happen 0 times per year. For the operation of the disconnecting switches, for the purpose of this analysis, the number of one operation per year for each switch will be used, in lack of verified data. In further text, for the yearly number of operations of a switching device, symbol n (device) is used Restrictions for the calculation In this analysis, the probability of encountering the island operation will be calculated only for the connection of the power plant through feeder bay "Rovinj". The reserve connection through feeder bay "Kanfanar 2" will not be analyzed for purpose of this article, as its impact to final result can be neglected, due to the following two reasons: - as can be seen from the Picture 7, the load supplied through this feeder bay is much higher than the power of the power plant, only possible switching points to initiate the island operation are last two disconnection switches, and thus the possibility of encountering an island conditions is negligible compared with the first option, - the amount of time during the year in which power plant will be in operation through this connection is not known but is assumed negligible in comparing with the first option Calculation of the risk probability of encountering the island operation According to the method stated in previous chapters, the following Picture 8 shows the network configuration together with parameter data necessary for the calculation of the (yearly) risk probability of encountering the island operation for the Solar Power Plant Kanfanar.

9 Energy and the Environment (204) S/S Vincent Feeder "Rovinj" CB DS DS2 DS3 DS4 DS5 CB2 PVP Kanfanar Pmax=92 kva µ=456; =36,8 0 kv 20 kv Pinsr=250 kva Pinsr=200 kva Pinsr=00 kva PAB/=PBS/=550kVA;µ=775; =77,5 PBS0=PBS=300kVA;µ=650; =65 PBS2=PBS3=00kVA;µ=50; =5 PAB0/=0kVA;µ=0; =0 Figure 8 Substitute network configuration for probability calculation The probability for balanced power conditions can be calculated using equation (5), and the total probability takes into account statistical data for the switching devices (which would be the number of attempts in probability theory). The probability for the non-occurrence of the island is easier to calculate (neither of the islands on any switching point to happen, as can be seen from the Picture 2). Equation for the island probability (during one year) can thus be derived as: P = ( P ) ( P ) ( P ) ( P ) ( P ) ( P ) ( P ) (8) n CB n DS n 2 DS n 3 DS n 4 DS n 5 DS n ic CB ic DS ic DS ic DS ic DS ic DS ic CB2 CB When all parameters are entered into equation (8) the following is the result: x= 92 x 456 y=,29 x y =,29 = 2 36,8 x x y x 2 77,5 y , P = e e dydx = 0 36,8 2 π = 0,836 77,5 2 π e e dydx x y x x= 0 36,8 2 π y= 0,836 x65 2 π + (9) 2 2 x= 92 x 456 y=,29 x y , ( 0) = ( 0,65) ( 0,352) ( 0,0008) = 0,058 = 0,942 e e dydx 94% = 0 36,8 2 π = 0, π x y x Duration and (in)stability of island operation Continuous stable island operation with this particular generation source is not possible, but depending of the changing variables it can be undetected for as long as the varying variables are within the protection non-detection zone. Duration of the island operation is not considered by this analysis. Constant change of both generation power and load power will in certain amount of time, due to mismatch, cause the protections to recognize the loss of main source and eventually disconnect the plant. Some islanding protection tests have been conducted during the test operation of the PVP Kanfanar, when the island conditions have been simulated, and the results show that the island conditions have been recognized by the island protection within inverters, but only after certain amount of time has passed (542 ms) [9]. 7. CONCLUSION Probability of occurrence of forbidden island operation of power plant without regulation, assessed by the proposed method on a real example, has shown that in some particular cases risk for such situation is not neglectable. This is due to close match of nominal values for generation and load on a feeder. Suggestion is that configuration of the power plant network connection should be carefully reviewed in order to lower the risks, if there is no highly reliable protection.

10 202 M. Ivas, Probabilistic risk assessment of... REFERENCES [] Ministry of Economy, Labour and Entrepreneurship: Grid code for Electrical System (NN 36/06), Zagreb, April (in Croatian) [2] EN5060 Voltage characteristics of electricity supplied by public distribution systems [3] Teodorescu, R., Lissere, M., Rodriguez, P.: Grid converters for photovoltaic and wind power systems, John Willey & Sons Ltd, UK, 20. [4] Verhoeven, B.: Probability of Islanding in Utility Networks due to Grid Connected Photovoltaic Power Systems, Report IEA PVPS T5-07:2002, 2002 [5] Bruendlinger, R., Mayr, C., Causebrook, A., Dahmani, J., Nestle, D., Belhomme, R., Duvauchelle, C., Lefebvre, D.: State of the art solutions and new concepts for islanding protection, Project Dispower, Austria, 2006 [6] Hutter, S.: Distribution transformer load monitoring, CIRED, Umag, 200. (in Croatian) [7] Koncar - Electrical Engineering Institute inc.: PVP Kanfanar protection settings study, Zagreb, 203. (in Croatian) [8] PVP Kanfanar WebSCADA (events list), Telenerg ltd, 203. [9] Burul, I.; Damianic, M.; Lasic, M.: Commissioning and Trial Operation of Photovoltaic Power Station Kanfanar (999 kw), HRO CIGRÉ, Cavtat, 203. (in Croatian) [0] Ivas, M.: Island operation of grid connected multi-inverter distributed generation sources, doctoral qualification exam, Faculty of Electrical Engineering and Computing, University of Zagreb, 204. ( in Croatian) VJEROJATNOSNA PROCJENA RIZIKA OD POJAVE OTOČNOG RADA U POGONU ELEKTRANE SASTAVLJENE OD VIŠE MREŽNIH IZMJENJIVAČKIH JEDINICA Sažetak: Otočni pogon distribuiranog izvora koji nema mogućnost regulacije proizvodnje s dijelom lokalne mreže nepoželjan je pogonski slučaj koji se može dogoditi pri isklopu nekog od rastavnih uređaja kojim se odvoji dio lokalne mreže, uključujući distribuirani izvor, od vanjske mreže. Takav neželjeni pogonski slučaj predstavlja prijetnju sigurnosti radnika, otvara mogućnost napajanja luka kod kvara za vrijeme beznaponske pauze automatskog ponovnog uklopa i mogućnost nesinkronog uklapanja, a smanjena je i kvaliteta napona. Tipičan primjer distribuiranog izvora koji nema mogućnost regulacije te zahtijeva neki oblik zaštite od otočnog pogona, elektrane su sastavljene od mrežnih izmjenjivačkih jedinica, tipičan primjer kojih su fotonaponske elektrane. Standardizirane zaštite od otočnog pogona integrirane su unutar izmjenjivača. Vjerojatnost pojave otočnog rada ovisi o neuravnoteženosti proizvodnje i tereta, odzivu mreže, mogućnosti regulacije i odzivu primijenjenih metoda zaštite. Najvažniji je faktor razlika između snage proizvodnje i tereta, gdje niti jedna od veličina nije konstantna. I teret i proizvodnja su nezavisne varijable, tj. vjerojatnosne funkcije koje je moguće izvesti iz dugoročnih pogonskih mjerenja. Na stvarnom primjeru jedne od najvećih sunčanih elektrana u Hrvatskoj procijenit će se rizik pojave otočnog rada i opravdanost istraživanja ovog fenomena i ulaganja u dodatne zaštite. Ključne riječi: otočni pogon, vjerojatnosna analiza rizika, distribuirani izvori, izmjenjivač, sunčana elektrana, zaštita od otočnog pogona