Power Quality Analysis for Connecting a PV Park to the Power Network in Order to Obtain the Certificate of Conformity

Transcription

1 Power Quality Analysis for Connecting a PV Park to the Power Network in Order to Obtain the Certificate of Conformity Phd. Eng. Sorin NISTOR SFDEE Electrica Distributie Transilvania Sud SA Brasov, Romania nistorsorin2005@yahoo.com, Dr. Eng. Catalin MIHAI SFDEE Electrica Distributie Transilvania Sud SA Brasov, Romania catalin77@gmail.com Abstract Monitoring electrical quantities in different nodes of the Romanian electric grid, together with connecting renewable energy sources have highlighted the need to have indepth knowledge on the software used by the manufacturers of network analyzers for analyzing the monitored quantities. So that the information obtained corresponding to current standards and to ensure accurate data which is to be used as basis for decisions on assessment of power quality. The paper is focused on the analysis of the influence of a new connection to the distribution network the solar park Hoghiz taking into consideration the power quality indicators. The monitoring of the power quality indicators will be made by three network analyzers. The analysis of the measurements will indicate if the solar power producer can obtain the certificate of conformity, required in order to connect to the power system. Keywords power quality; solar power; certificate of conformity I. INTRODUCTION (Heading 1) Monitoring the power quality indicators and the analysis of their classification within the established limits, in each node of the grid and at the bus bars of each disturbance consumer, presents interest from the point of view of limiting the damage to consumers, as well as of the increase of the energy efficiency from the distribution system operator to the final user. EU policies on energy efficiency have established a number of regulations through directives and imposed measures to achieve the 20% target concerning the increase of energy efficiency by According to Directive 27/2012 [1], all EU countries are forced to use energy in a more efficient way, this being the reason for the rapid development of renewable energy sources (RES) in Romania. At the moment, both at a global and national level, there is registered a tendency of power supply for some consumers using solar energy as an unconventional and RES and thus, the appearance the of the small power producers. Solar energy solutions not only allow adjustment of electricity production to consumer needs, but also offer the opportunity to save money by valorization of any electricity surplus into the public grid. Prof. Dr. Ing. Calin Munteanu Dept. Electrotehnică şi Măsurări Facultatea de Inginerie Electrica Cluj Napoca, Romania Calin.Munteanu@et.utcluj.ro Photovoltaic sources are a new solution, compared to classical solutions, and hence the necessity to also be studied the mutual influence of the source and the system at the point of common coupling (PCC). As converting DC to AC lowers power quality, a set of conditions have to be met for connecting to the grid. In Romania, the National Regulatory Authority in the domain of Energy (ANRE) has developed and proposed through regulation, conditions for connecting RES in terms of power quality [2]. In order to achieve the monitoring of power quality indicators and verify whether the consumers fit in the limits of disturbance imposed by the regulations, a large number of electrical equipment s have been developed. The studies in this field [] analyze the problems of power quality in the low voltage network. The study is focused on voltage variations, unbalance, harmonics and their impact on customers, measurements being performed with the analyzer FLUKE 1760 during one month. The measurement results were analyzed, evaluated in comparison with to the SR EN standard. The study shows that during the measurement the standard has been violated, there were identified problems related to voltage unbalance, voltage fluctuation and harmonics. It was suggested to install compensating power and passive harmonic filters for reducing harmonic amplitude. Muscas in his paper [4], considers it necessary to conduct an assessment of the quality of both voltage and power in the supply network and to locate sources that can damage power quality. To achieve these goals, we need to define algorithms for reliable measurement with implementation of new methodologies of measurement, such as the use of appropriate devices necessary to obtain the expected information with high accuracy. The international standards which require for photovoltaic parks (PV) to be connected to the distribution network have continuously evolved worldwide in recent years. Through these requirements, PV must meet the voltage standards of each country. With the development of requirements for connecting renewable electricity to the distribution network, in paper [5] there is presented a study of power quality in a PV production facility in Spain. Voltages, sags and interruptions were 24

2 measured for one year in the region with the largest production capacity of PV installed in Spain. The events collected by the analyzer, show that the number of dips is 20 times higher than that of interruptions. These sags have an average depth less than 50 % and duration less than 500 ms, where for most of the present events present, a residual voltage is detected close to % with duration between 0 ms and 100 ms. The specific conditions of the electrical network, summing random disturbance caused by connected customers and continuous variation of electrical quantities requires analyzing monitoring equipment, knowledge of algorithms for calculating power quality indicators, the mode of acquisition and processing of obtained information in order to obtain reliable results, reproducible and consistent, regardless of the type of equipment. Accurate knowledge of the algorithm for calculating the power quality indicators is of interest especially because within the specialized technical literatures there are different definitions for the sizes they operate with. Quality, as in every industry, is not a static concept, the content of this concept ranging over time due to technological development and the evolution of social life, therefore, the requirements for power quality standard needs to be adapted continuously, continuously perfectible. Power quality analysis based on the quality indicators is done in correlation with the disturbance that can be introduced. In this paper we analyze the main concepts related the actual indicators of power quality and electrical quantities monitored according to European Standards. The analysis of electric power quality will be performed, for a new solar power plant in Hoghiz area. The case study indicates that the producer will be able to obtain the certificate of conformity required in order, to connect to the national electricity system. II. MEASURING TIME SPAN Conducted disturbances are of continuous or discrete type [6]. Steady state disturbances are present in each cycle of the alternative voltage and include slow voltage variations, harmonics, flicker and unbalance in the electrical network [7]. Detection and measurement is done by measuring their amplitude and determining the deviation from the nominal value of voltage or current. Discrete disturbances occur as isolated and independent events for which the date and time of occurrence are recorded. To this category belong gaps, surges of short duration, transient voltage impulse or oscillatory. Disturbances are detected by monitoring over the exceed of a certain threshold of normal values of voltage and current, threshold characterized by effective value (root mean square - RMS) or the peak value. The power quality evaluation of electricity is directly related to the identification of the type of disturbance., but these must be improved in terms of their reproducibility and accuracy. The number of samples over the period of reference voltage (for three-phase equipment typically is considered as a phase reference, phase A) is of particular interest, especially for harmonic analysis of power curves (in most cases, nodes of power grid voltage curves are slightly deformed) [8]. Using equipment with 64 samples per period (sampling frequency f e =.2 khz) is in generally insufficient to obtain information accurately in limits imposed by national standards [8]. Many of the current monitoring equipment using 256 samples per period (f e = 12.8 khz). The large number of data in the interval of tracking set their processing requires smaller time intervals. In this regard, the processing procedure adopted as the standard IEC - The International Electrotechnical Commission, taken as the national standard requires the use of the following time periods in order to provide comparison data [9]: Very short interval (T vs Short interval (T Long interval (T One day interval (T One week interval (T sh L D ) : s; ) : 10 min; ) : 1 h; Wk ) : 24 h; ) : 7 days. In general, processing the data obtained during a week provides sufficient information to characterize a consumer. The range of measurement is based on the capability of the device ( s to 10 min) and the length of follow-up. Evaluation of the disturbing quantities in medium and high voltage networks, in most cases, is made, based on information obtained in the secondary of the voltage and current measuring transformers. Knowledge insufficient of their characteristics can lead to information with a high degree of uncertainty. The main reasons that cause an error in the information obtained are: inadequate frequency characteristic; operation outside measuring transformers accuracy class; inappropriate choice of measuring transformers (especially those current in terms of the current in the primary circuit). Current transformers for measuring voltage and current, the magnetic circuit has a limited frequency band [9], leading to errors in the transfer of information on the higher level harmonics. Using capacitive voltage transformers (TECU) can cause significant errors in transferring information given practically only the limited transfer characteristic resonance frequency which was adjusted (in many cases in Romania there is used measuring equipment set for 47,5 Hz TECU). Thus, Fig. 1 shows a transfer characteristic determined experimentally for TECU 110 kv. Inappropriate transfer characteristic for transformers a measuring voltage leads, by measuring the secondary circuit of the transformer, to the wrong results especially for monitoring the flicker phenomenon. This type of transformer is not adequate to determinations of disturbance consumer bus bars. Monitoring consumer based on current curves leads to obtaining information with a high degree of uncertainty common in installations where the current transformers are not suitable for measuring power load. The information obtained when measuring transformers with 25

3 rated current being 10 times the current drawn currently by the consumer is heavily distorted in the nonlinear transformer magnetizing characteristic. SS tttt aaaaaaaaaaaaaa =. 1,1 2. UU aaaaaa. II aaaaaa ; UU aaaaaa = (UU AA + UU BB + UU CC ) II aaaaaa = (II AA + II BB + II CC ) SS tttt eeeeeeeeeeeeeeeeeeee =. UU ee. II ee ; UU ee = UU AA 2 + UU BB 2 + UU CC 2 (2) Fig. 1. Frequency characteristic of a transformer TECU 110 kv [10] At this point that requires the continuous evaluation and determination PQ there is proposed the replacement of the capacitive voltage transformers type (TECU) with voltage and current transformers according to standard IEC [11] whose variation of error is low, even if the voltage varies widely. Typical error curve with low loss voltage transformer is shown in Fig. 2. II ee = II AA 2 + II BB 2 + II CC 2 SS tttt ssssssssssssssss = SS 0 + SS + + SS ; where: A, B and C are phases of the three phase network, and 0, + and - refers to the zero sequence components, positive and respectively negative. Depending on the purpose of monitoring two different values can be displayed: 1. Total power factor: λλ ff tttttttttt = PP ff SS ff () adequate to analysis of losses in electrical networks; 2. Fundamental power factor: Fig.2. Typical error curve for voltage transformers with low losses [10] Also to ensure a correct reading and use more accurate information about the quality of electricity is required to use readers and data processing read using analyzers that use high sampling frequency, so high accuracy class. III. CALCULUS ALGORITHM The use of different definitions for the same electric quantity and the existence of proper monitoring equipment differently programmed for various calculations require a careful analysis of the information obtained through monitoring and correlation with the definition considered. A. Monitoring the power factor In calculating the total power factor (relative to the total three-phase apparent power, and harmonics corresponding to the three phases considered): λλ tttt = PP tttt (1) SS tttt may be considered following definitions of total apparent power [12]: SS tttt gggggggggggggggggg SS tttt gggggggggggggggggg SS tttt aaaaaaaaaaaaaaaa = SS AA + SS BB + SS CC ; = SS AA + SS BB + SS CC ; = SS AA + SS BB + SS CC ; λλ ff ffffffffffffffffffffff = PP ff1 SS ff1 (4) adequate to reactive power compensation analysis. Given that the power factor values calculated using relations (1 to 4) are different, it must specify the amount sought and appropriate setting of the measuring equipment. B. Monitoring unbalance Monitoring disturbances in the form of unbalance also requires clarification of the algorithm used in the development. According to IEC standards, negative unbalance factors kk ss and kk ss 0 are defined only for fundamental harmonics [1]: kk ss = UU UU + ; (5) kk 0 ss = UU0 UU + ; both for voltage curves and current curves. In equations (5), U + is the positive sequence component of the voltage, and the U - is component of the negative sequence voltage. In the specialty literature [14], [15] and at the development of monitoring equipment, the following definitions are encountered for unbalance factors: the unbalance voltage as deviation from the average; the unbalance factor as a deviation 26

4 from the phase A voltage, algebraic unbalance factor for voltage between phases (variant α); algebraic unbalance factor for voltage between phases (β version); unbalance factor of the phase voltages as voltage deviation from the mean. The analysis of the unbalance factor reveals that monitoring equipment programmed or set to a certain definition of the unbalance factor will send different information to the equipment programmed on another algorithm. Analysis of asymmetry of information on a particular node of the grid must be correlated with the type of equipment used and the algorithm used. C. Monitoring the distortion factor According to European norms [15], the distortion factor is defined as the ratio of the residue curve deforming U d and fundamental harmonic voltage or current. kk dd ffffffffffffffffffffff = UU dd UU (6) 1 Monitoring equipment currently used, however, calculated the distortion factor based on the RMS value: kk dd rrrrrr = UU dd UU eeee (7) Especially, for the electric current curves, which are usually more distorted than the voltage curves, the difference between the two factors of distortion is important, being necessary the specification of the implemented relation into the used monitoring device. D. Monitoring the flicker phenomenon According to the normative PE 142/80 of Romania [16], the phenomenon of flicker is defined and monitored based on the dose of flicker. This amount, although normal in Romania, still does not meet the standards IEC [8] which flicker is defined by the instantaneous values, flicker level (obtained on the basis of statistical processing of the instantaneous values) and Cumulative Probability Function (CPF). Currently for monitoring, a flicker meters used provides information processing algorithm based on IEC, but the results cannot be used to assess a consumer disturbance on the basis of normative PE 142/80. IV. CASE STUDY The power quality analysis is made at the point of common coupling (PCC), where photovoltaic generator inserts power in power system, by fitting analyzers for monitoring power quality indicators. The photovoltaic Park is located on an area of 0 ha with a production capacity of 17 MW installed. Measurements were performed during functional tests and the first tests were carried out by connecting the SEE through a transformer 0.4/20 kv to 20 kv overhead line (LEA) with direct injection to 110/20 kv Hoghiz connections station. Measurements were performed at the point of common coupling (PCC-1) through which is connected the photovoltaic generator to the power system, with the following equipment: network analyzer UMG 511 Janitza model, Mavowatt 0 POWER PLUS ALPHA electronic meter (which monitoring options several indicators of power quality). In figure is presented the wiring diagram of the three network analyzer UMG 511 Janitza model, Mavowatt 0 ALPHA POWER PLUS on PCC-1 to perform the necessary evidence to obtain the certificate of conformity issued by the distribution operator system SFDEE Electrical Distribution Transilvania Sud SA. Based on measurements the following has been found: the equivalent peak value of phase voltage is 2.5 V; equivalent value of peak current is 7A; current values of phases b and c are smaller: 2.5 A and 4 A, since all inverters do not work on these phases; waveform current curves based mostly on stage deformed due to malfunctioning of inverters and low-pass filters voltage mounted inside the solar park; currents are shifted before voltages (inverters operating as capacitive); THD U = 1.66% fits in (Ord. 28/2007 on the performance standard for electricity distribution service); THD I = %, the coefficient is not provided in the standard. Photovoltaic generator ~ Power inverter Hoghiz TG PCC-1 connections station PCC 0,4 kv 20 kv 110 kv 110 kv MAVOWAT 0 Trafo Trafo 0,4/22 kv 20/110 kv kwh JANITZA UMG 511 kwh kwh JANITZA UMG 511 MAVOWAT 0 Fig.. Connection diagram of apparatus in PCC From the network analyzer measurements, Mavowatt 0 at the output of the power inverter, measuring a period of 10 minutes shows that the phase of a sampling of 20 ms the following was observed: in figure 4 is showing the voltage curve is deformed, its maximum amplitude on phase a is tuned to 26.8 V when (t 09:51), and the minimum of 24.6 V at (t 09:52); Fig. 4. The waveform of the current phase a in the selected range Changing the voltage curve followed by immediate frequency variation are due to the use of inverters capable of implementing a fast automatic voltage adjustment by changing the command angle of the semiconductors that are built into inverters, voltage decreases caused a deficit of active power, responsible for variation in the sense frequency is lowered. 27

5 In Figure 5, the maximum frequency is Hz at moment (t 10:00), and the minimum is Hz at moment ( t 10:01). Fig. 5. The waveform of the frequency phase a in the range selected From the deferral period 1 14/09/2014 the following emerged: Voltages on the three phases are symmetrically out of phase; voltage and current values of the three phases have the following values: phase a: V, respectively 5.2 A; phase b: V respectively 1.8 A; phase c: V respectively 2.29 A, the low values of the currents from the phases b and c is due to the fact that all the inverters are not functioning; In the measurement period were not recorded variations of voltage (swells, surges) outside the thresholds set; There were current harmonics of rank 2 throughout the monitoring period; Value for THD U = 5 %; Value for THD I = %. Alpha PowerPlus electronic counter installed at the point of common coupling is able to register the electricity in both directions (is a two way counter). The meter was installed so as to record the photovoltaic generator power output. To achieve measurements with the electronic counter Alpha Power Plus the following thresholds were set to register values (fig.6): second current harmonic A for the three phases; THDU - 1% from the fundamental of all phases; THD I - 1% from the fundamental of all phases. Fig. 6. The harmonic spectrum for the three phases, recorded with the electronic counter Alpha Power Plus Because the electromagnetic radiation emitted by the sun, implicitly solar energy is variable in time, inverters with which the photovoltaic generators are equipped, are capable of operating at power factor 1, but when necessary, they can operate under both capacitive (generator of reactive power) when there is deficit, excess reactive power are low voltage in the network node injection is made or inductive (reactive power consumption) when there is reactive power excess and high voltage exists in the network node in which injection is made. At the same time, because the operation with power factor less than 0.92 is charged, at night and during periods when there is no electromagnetic radiation to produce energy, inverters that are installed in the photovoltaic park absorb from the distribution network active energy, transforming it into inductive reactive power, to be compensated the energy produced by the cable through which the power plant is connected at no load regime. Following measurements JANITZA UMG 511 power analyzer, according to SR EN test not passed flicker as shown in the table below: Long term flicker L1 Raport after EN Lower limit Higher limit Percentage Max value Average value Min value Total out Undercut Overcut Result % % 42.01% -- Failed Long term flicker L1 (7200) [CEF_BV_PFV_ HOGHIZ] Marker Higher limit The test performed using the analyzer Janitza did not pass to flicker so that the manufacturer is required to take the following measures: due to the variable RES and to maintain voltage in belt voltage is necessary to use a power quality 28

6 control system type Unified Power Quality Conditioner (UPQC) which includes a Dynamic voltage Restorer (DVR) circuit [20] to compensate for variations in voltage and providing a voltage across users set almost constant in normal operation and in disturbed regimes. Power quality control system type UPQC is able to: Maintains constant output voltage level for increases / decreases in input voltage widely; Compensation for disturbance caused by voltage variations and short interruptions; High speed response. The test is performed again and then the distribution operator may issue a certificate of conformity for the manufacturer. V. CONCLUSIONS After obtaining the certificate of conformity, it is recommended that system operators to continue studying the influence of disturbances in common connection point to highlight the effects of plant generating electricity with photovoltaic panels on the system. It must be studied also the disturbances influence produced in the system (dips, short circuit etc.) on photovoltaic equipment. Following the development of the study and also analysing the voltages and current curves of the three phases, we observe that: Voltage curves are very close, in terms of shape, to the fundamental sinusoid, complying with the curves guaranteed by the manufacturer of the inverters and with the rules that set the voltage waveform; Voltage curves for the three phases are symmetrical out of phase with each of these phase shifts and current curves indicating that during monitoring of photovoltaic power, the generator was operating in the capacitive mode; There were no variations in voltage (dips, swell) beyond the limits permitted or reverse movement and power factor decreases below the neutral (inverter manufacturer has guaranteed a unity power factor); Distortion coefficient for the voltage measured is set THDU = 5%, less than 8% (the limit allowed for low voltage-medium voltage, according to PE 14/94); There is a sharp deflection curves current due to malfunctioning of the composition inverters filters: THDI = %, but this value cannot be discussed due to lack of regulations for this indicator power quality; Nonoperation of two inverters connected to the phase s b and c are reflected in low values of current flowing in these two phases: the phase 1.8 A per phase b, 2.29 A per phase c, to the 5.2 A per phase a. By analyzing the harmonic levels, result an exceeding of the limit of compatibility (set of normative Romanian - PE 14/94) for harmonics of rank: 8, 10, 12, 14 and 15 per phase b, and 15 and 21 per phase c. REFERENCES [1] Energy Efficiency Directive 2012/27/EU of the European Parliament and of the Council of 25 October 2012 on energy efficiency, amending Directives 2009/125/EC and 2010/0/EU and repealing Directives 2004/8/EC and 2006/2/EC Text with EEA relevance [2] Ordinul 0/201 privind aprobarea Normei tehnice Condiţii tehnice de racordare la reţelele electrice de interes public pentru centralele electrice fotovoltaice, Monitorul oficial NR. 12, din 0 mai 201. [] R. Bodnar, A. Otcenasova, M. Regula, D. Szabo. "Measurement of power quality in low-voltage network." In ELEKTRO, 2014, pp IEEE, [4] C. Muscas, "Power quality monitoring in modern electric distribution systems." Instrumentation & Measurement Magazine, IEEE 1, no. 5 (2010): [5] A. Honrubia-Escribano,, A. Molina-Garcia, E. 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