Multifunctional Photovoltaic Inverter Systems – Energy Management and Improvement of Power Quality and Reliability in Industrial Environments

Dominik Geibel Institute for Solar Energy Systems, ISET e.V.

Division Systems Engineering and Grid Integration Koenigstor 59

34119 Kassel Germany dgeibel@iset.uni-kassel.de

Abstract -- A Multifunctional Photovoltaic Inverter System

providing additional functionalities is introduced. Beside the intrinsic injection of PV energy, improvement of Power Quality and Reliability (PQR), reactive power compensation, peak shaving and provision of ancillary services is conceivable. Such systems are of economical interest since the combination of two systems is less expensive and PV energy is priced at special tariffs in several countries.

Within a research project funded by the German government a prototype in the range of 100 kVA is being developed by ISET e.V.1 and SMA Solar Technology AG. Testing is performed in laboratory and at a test site in an industrial environment. This paper describes the technical realization of the system and provides results concerning practicability of the addressed additional functionalities in terms of simulations and measurements.

Keywords -- Ancillary Services, Industrial Application,

Inverters, Photovoltaic, Peak Shaving, Power Generation, Power Quality and Reliability, Reactive Power Compensation.

I. INTRODUCTION Development of renewable energy sources (RES) in past

and in future can be analysed very well regarding the German situation due to its leading role in this sector. Exemplary the aggregated capacity of RES installation in Germany from 1990 until 2008 is shown in Fig. 1.

Fig. 1 Aggregated capacity of RES installation in Germany from 1990 until 2008 [1]. Data: BMU, DEWI, BSW Solar. Proprietary diagram.

While in the early 90s mainly hydro power has a share on

renewable energy production, capacity of wind power plants

1 ISET e.V. will become Fraunhofer IWES (Fraunhofer Institute of Wind Energy and Energy Systems Technology) in 2009.

has been increasing since 1994 strongly. Number of photovoltaic (PV) and biomass power plants has been rising since 2004 very fast. In 2008 new PV power plants with a capacity of about 1.5 GWp were installed in Germany alone.

The fast growth of PV can be led back to subsidies given through special feed-in tariffs. Efforts in cost reduction have to be undertaken in order to reach grid parity. Therefore a new renewable energy act came into force in Germany in 2009. Feed-in tariffs for new installed PV plants are reduced from 8 % to 10 % per year. This has to result in lower prices for PV systems in order to be economical as attractive as nowadays. Another approach compensating the decrease of feed-in tariffs is a provision of a surplus value through additional functionalities. Especially in industrial environments these additional functionalities offer benefits for the customer. Economic studies have been carried out in [2]. This paper presents the technical realization of the proposed Multifunctional PV (Multi-PV) inverter system.

II. MULTIFUNCTIONAL PHOTOVOLTAIC INVERTER SYSTEMS The approach and the realization of the Multi-PV inverter

system developed in a research project [3] are described below.

A. Electrical concept and grid integration Fig. 2 shows the electrical concept of the Multi-PV

inverter system and the integration into an industrial environment.

Fig. 2 Concept and grid integration of the Multi-PV inverter system in an industrial environment.

The Multi-PV inverter system itself consists of a state of the art PV-inverter. Battery storage is connected via a DC/DC converter to the DC-link. The local sub-network is decoupled from the residual industrial and public grid through a decoupling inductor. A fast circuit breaker ensures a disconnection in case of disturbances in the superior grid.

Additional functionalities given in Table I are feasible through the before listed additional hardware extensions and the application of advanced control algorithms. Restrictions for the combination of several additional functionalities exist due to the used control methods and depend whether the decoupling inductor is used or bypassed.

TABLE I POSSIBLE FUNCTIONALITY OF THE MULTI-PV INVERTER SYSTEM DEPENDING

ON SYSTEM CONFIGURATION, MODIFIED FROM [4] Function With

decoupling inductor

Without decoupling inductor

Feed-in of PV energy yes yesLocal PQ Improvement yes noUPS Functionality yes yes/no2

Provision of Control Energy yes yesPeak Shaving yes yesReactive Power Supply/ Compensation at Point of Common Coupling (PCC)

no yes

Harmonics Compensation at PCC no yes

B. Technical realization and development process During the project duration a laboratory sample (Fig. 3,

left) and a prototype unit (Fig. 3, right) in the range of 100 kVA are set up. Both inverters are built by the SMA Solar Technology AG (SMA). The control of the laboratory sample is developed by ISET e. V. with a rapid-control-prototyping system [4]. A battery storage system (Fig. 4) with 192 single cells (384 V in total) and with an overall capacity of 800Ah is used.

Experiences collected through tests with the laboratory sample are applied for the development process of the prototype unit. In regard to a serial product the prototype unit is completely engineered by SMA.

Fig. 3 100 kVA laboratory sample (left figure) and prototype (right

figure) of the Multi-PV inverter developed by SMA Technology AG.

Fig. 4 Battery storage of the Multi-PV inverter system

2 Depending on the UPS classification according to EN 62040-3

The prototype system is tested in an industrial grid at a production facility of the project partner Hübner AG in Kassel, Germany. A PV plant with a nominal power of 104 kWp is run by the project partner Stadtwerke Kassel, a local utility, on the roof of the factory building. Since December 2008 the developed prototype system is in operation (Fig. 5). Several functionalities of Table I are performed during the testing period of one year.

Fig. 5 Test site for the prototype system with a 104 kWp PV-generator on the roof of the factory building of Hübner AG in Kassel, Germany.

Fig. 6 shows the interaction of the installed main

components at the test site. The operation of the prototyping system is managed through the superior operating control. A data acquisition and storage system provide measurement data of the industrial grid with a time resolution of 1 s. Further provision of metering data and synchronization with the local utility is ensured through this system.

The Multi-PV inverter has several basic operating modes (e.g. MPP-Tracking, injection of reactive power or UPS functionality) which can be set by the superior operating control via CAN-Bus. For realizing additional functionalities as reactive power compensation or peak shaving, the superior operating control has to give appropriate set-values to the Multi-PV inverter.

Also access to the SMA SUNNY WEBBOX from the superior operating control is possible in order to change parameters of the Multi-PV inverter. Other tasks of the superior operating control are the battery management and the operating times of the system, depending from the circumstances of the factory, e.g. shift operation.

An online access to the installed system at the test site with data exchange possibility is available.

Fig. 6 Assembly of the Multi-PV prototype system with data acquisition system, superior operating control and Multi-PV inverter.

III. IMPROVEMENT OF POWER QUALITY AND RELIABILITY This section describes the possibilities of the Multi-PV

inverter system to improve Power Quality and Reliability for sensitive loads in the sub-network. Three types of voltage disturbances are assessed in regard to Power Quality; firstly steady-state voltage variations, secondly transient voltage variations and thirdly voltage waveform distortions by means of harmonic content. In consideration of reliability the system is intermitted with grid faults. Advanced information can also be found in [5] and [6].

A. Steady-state voltage variations Voltage tolerance band is described in several norms, e.g.

DIN EN 50160 for public grids or DIN EN 61000-2-4 for industrial grids. In DIN EN 61000-2-4 several classes for different kinds of environments are defined. For example class 2 demands a voltage level of 230 VRMS ± 10%.

An advanced control algorithm of the Multi-PV inverter system and the usage of the decoupling inductor provide the possibility of keeping the voltage of the local sub-network at a pre-defined set-point. Therefore a defined injection of reactive power is required. The amount of reactive power depends on several parameters stated in equation (1)

),),cos(,,( GridDecnetsubnetsubMPVInv VLSPfQ −−= ϕ (1) with QInv the injected reactive power and PMPV the injected active power of the inverter, Ssub-net the apparent power and cos(φsub-net) the power factor of the local sub-network, LDec the size of the decoupling inductor and VGrid the actual grid voltage.

Grid voltage is varied slowly within 230 VRMS ± 10 % in order to test the proposed control method. Measurement results of phase 1 of the laboratory sample are given in Fig. 7. While the grid voltage is changed by a network simulator, the local sub-network voltage stays constant at the set-point of 230 V during the variations.

Fig. 7 Measured sub-network voltage during an applied grid voltage ramp of 230 VRMS ± 10 %.

The measurement results confirm that steady state voltage

variations can be compensated for sensitive loads connected to the local sub-network. The additional required reactive power mainly depends on the decoupling inductor for

comparable grid deviations. Smaller values of the decoupling inductor cause higher amounts of reactive power but lower resistive losses. If the inverter is not oversized this additional reactive power reduced the maximal PV energy which could be injected by the inverter in dependence of the grid voltage. Choosing the value of the decoupling inductor, above mentioned points has to be considered.

B. Transient voltage variations This section considers transient voltage variations with a

remaining voltage of at least 80 % of the nominal grid voltage. For voltage dips below this limit the UPS functionality secures sensitive loads in the sub-network.

During a voltage dip the Multi-PV inverter system injects a reactive current in order to damp the voltage variations. Once again the decoupling inductor has to be used. Several kinds of disturbances (85 % of nominal voltage, 70 ms or 100 ms duration and 0° or 90° phase angles) are systematically tested for the evaluation of voltage dip damping capability and the dependence on the size of the decoupling inductor. These voltage dips are carried out as one, two and three phase dips. Fig. 8 exemplarily gives measurement results of phase 3 of the sub-network voltage for three phase voltage dips. Additionally different active power injections (25 kW, 50 kW and 75 kW) of the Multi-PV inverter system and different load conditions (50 kW and 100 kW) for sensitive loads in the sub-network are performed. These are not marked different in Fig. 8.

Fig. 8: Reaction of load phase 3 at a different three phase voltage dips with 85% remaining nominal voltage for several decoupling inductors.

Although the voltage dips can not be compensated holistic

the dips are damped strongly. The strength of the damping depends on the decoupling inductor. Smaller sizes of the decoupling inductor result in deeper voltage dips. Minimal voltage sag of 205 VRMS for a decoupling inductor of 0.3 mH is measured. The differences between values of the decoupling inductor for 1.8 mH, 1.5 mH and 1.2 mH are slight, 215 VRMS can be achieved for these inductors at minimum, which reduce the sag about 57 %. Allowing a voltage deviation of ± 10% at most, a decoupling inductor of 0.6 mH is sufficient for considered voltage dips.

C. Voltage Quality Improvement Sensitive loads in an industrial grid can be disturbed by

powerful motors, fast changing loads or welding machines for example. Effort has to be undertaken in order to secure a proper function of these sensitive loads.

The Multi-PV inverter system topology allows improving the voltage profile for loads in the sub-network, since an inverter is able to provide a voltage profile with a low harmonic content. Additionally through the usage of the decoupling inductor a reduction of the harmonic content for the sub-network voltage is achieved. Higher values of the decoupling inductor’s size ensure a more effective damping.

Fig. 9 provides measurement results of the laboratory sample for the sub-network voltage if the grid voltage is disturbed by typical odd harmonics (3rd, 5th, 7th, 11th, 13th and 17th). Several decoupling inductors are tested.

Fig. 9 Harmonic reduction for various decoupling inductors for PMPV = 50 kW and Psub-net = 100 kW

The disturbed grid voltage reaches a total harmonic

distortion (THD) of 9.5 %. This corresponds to class 3 of the DIN EN 61000-2-4. The THD value is reduced to 7.11 % at minimal for an inductor of 0.3 mH and to 2.83 % at maximal for an inductor of 1.8 mH, which means an improvement to class 1 according to DIN EN 61000-2-4. All other tested inductor sizes (1.5 mH to 0.3 mH) lead to an enhancement from class 3 to class 2.

D. Improvement of Reliability The Multifunctional PV-inverter introduced within this

paper is not qualified to substitute a conventional UPS unit, but it provides the possibility having a more reliable network for customers with the primary aim using PV energy. This is valuable especially for regions with the combination of high solar radiation and weak networks due to the fact that in contrary to conventional UPS systems the power failure buffering time can be extended if additional energy from PV is available.

The Multi-PV inverter system disconnects from the interconnected grid for voltage dips deeper than 80 % of the nominal voltage. Depending from the control of the inverter, whether in current or voltage control mode, the voltage behavior during the transition time significantly is influenced.

The kind of control mainly depends whether the decoupling inductor is used (voltage control) or bypassed (current control).

The behavior of the laboratory sample is tested using a decoupling inductor of 0.6 mH. In order to determine the voltage decrease in the sub-network for grid faults, a three phase failure with 0% remaining network voltage has been performed in laboratory. Transient recording can be seen in Fig. 10. The minimal half cycle RMS voltage value of the sub-network load voltage during the fault is 60 % of the nominal voltage. The sub-network voltage reaches a RMS voltage value within 230 ±10 % after 50 ms at the latest. For a faster return an optimization of the control parameters has to be done.

Fig. 10 Measurements of the laboratory sample with a decoupling inductor of 0.6 mH

IV. REACTIVE POWER COMPENSATION IN INDUSTRIAL ENVIRONMENTS

In general the operator of an industrial grid has to keep a certain power factor at the Point of Common Coupling (PCC) against the local utility. Otherwise the reactive power amount above the agreed power factor is charged by the utility. For this reactive power compensation units are used in almost all industrial grids. According to the load profile of the industrial grid, for example dependent from shift operation or capacity utilization of the production facilities, these reactive power compensation units mostly does not working to full capacity.

If the reactive power compensation demand and the solar radiation profile are known, the reactive power compensation can be take over partly or substituted completely by the

Multi-PV inverter system. Detailed consideration has been carried out in [2], where in particular economic aspects are specified. Within this section the technical perspective of reactive power compensation is discussed by means of theoretical basis, simulations and measurement results of the prototype system at the test site

A. Theoretical basis for reactive power compensation The power factor PF is defined as

QP

SP

PFϕ

ϕsin

cos⋅

=== (2)

where P is the active power, Q the reactive power and S the apparent power of the system. Equation (2) can be solved to

PQ

=ϕtan (3)

which now only depends from the active and reactive power. Reactive power compensation is normally carried out at

the PCC between industrial grid and public grid. For calculating the actual power factor according to equation (3) measurement values of the active and reactive power at the PCC are sufficient, if no on-site generation units exist. Otherwise the active power contribution of these units also has to be taken into account for the power factor calculation. A typical structure of an industrial grid is shown in Fig. 11 with a PV-plant as on-site generation unit.

Fig. 11 Example of a typical electrical industrial grid with on-site generation, e.g. a PV-plant.

For the configuration of Fig. 11 the power factor is

calculated to

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

⎟⎟⎟

⎠

⎞

⎜⎜⎜

⎝

⎛=

⎟⎟⎟

⎠

⎞

⎜⎜⎜

⎝

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++

+=

LoadPPCCQ

MPVPTPTPTQTQ

PF arctancos21

21arctancos (4)

with QT1 and QT2 the reactive power at the transformers T1 and T2, PT1 and PT2 the active power at the transformers and PMPV the active power injection of the on-site generation unit, in this case a PV-inverter system. Hence QPCC and PLoad can be determined.

B. Control algorithm development process 1) Simulation Model Design

In order to develop a suitable control for the reactive power compensation and further additional operating modes, a model of the prototype system shown in Fig. 6 and of the

industrial grid of the test site are simulated with the help of Matlab/Simulink®

(cp. Fig. 12). Therefore recorded measurement data for QPCC and PLoad are used as input variables. The operating control is embedded as m-function in the model. For this development of new and improvement of existing control algorithms can be done slight and reliable.

Fig. 12 Schematically diagram of the simulation model

2) Control Algorithm for Reactive Power Compensation Aim of the reactive power compensation algorithm is to

guarantee a certain power factor at the PCC by injection of the required additional reactive power amount for compensation through the Multi-PV inverter system. Normally an accounting period of the utility lasts 15 minutes. Therefore the control has to keep the average power factor at the end of the time interval above a specified limit at least. The used control algorithm is shown in Fig. 13 in principle. The inputs for the control algorithm are measurement data of PLoad and QPCC as well as the set-point for the average power factor PFAVG,spnt.

Fig. 13 Principal algorithm for the reactive power compensation to a certain power factor in a 15 minute accounting interval.

The comparison of the actual average power factor

PFAVG,actual in the instantaneous 15 minute interval with the power factor set-point results in a factor given by the PI-controller for multiplication with QPCC,spnt, which is calculated according to equation (4). This set-point is compared with the actual reactive power at the PCC QPCC,actual and the control deviation is used as input for a proportional controller. The output QMPV,spnt is set-point for the Multi-PV inverter. This output is limited by the apparent power reserve dependent from the actual solar radiation. In case of doubt the injected reactive power is reduced due the more valuable PV energy. 3) Simulation results

Fig. 14 and Fig. 15 show simulation results using the control algorithm presented in Fig. 13. An overview of the average power factor for a 15 minute interval reached with and without compensation over a whole day is given in Fig. 14, while Fig. 15 presents the detailed behavior of the Multi-PV during a 15 minute interval.

Fig. 14 Simulation results of the power factor at the PCC with and without reactive power compensation using measurement data from 06/08/2009.

Fig. 15 Detail simulation results, 13:00 to 13:15 at 06/08/2009. The set-point for the power factor is chosen to 0.95. The

calculated power factor out of measurement data using no reactive power compensation has a significant number of 15 minutes time intervals where a power factor below the desired set-point of 0.95 is reached (green line in Fig. 14). Simulation results show that through the developed reactive power compensation operating mode the average power factor of a 15 minute interval is limited reliable to a minimum

average power factor of at least 0.95 (red line in Fig. 14). In Fig. 15 a detailed view at a 15 minute time interval is

given. Beside the curves of the average power factor in the actual interval with and without compensation, also the actual reactive power at the PCC is shown. Further the control value QMPV,spnt of the control algorithm given as set-point to the inverter control is provided. In this case QMPV,spnt has to be limited for some time during this interval due to a high PV energy injection which would exceed the total apparent power of the inverter system. Although the limitation due to the apparent power reserve occurs, the required power factor of 0.95 is reached at the end of the 15 minute interval in contrary to the simulation results using no reactive power compensation.

C. Measurement results of the prototype system at test site In order to test the developed control algorithm not only in

terms of simulation, the algorithm is implemented to the superior operating control of the prototype system. Fig. 16 and Fig. 17 show measurement results of the test site.

Fig. 16 presents the average power factor of a 15 minute interval, calculated out of measurement data. This 15 minute interval is synchronized with the accounting interval of the utility. The operating mode reactive power compensation is started at 06:15. Before the power factor is partly below the set-point of 0.95, after operating the system with reactive power compensation all 15 minute intervals reach an average power factor of at least 0.95.

Fig. 17 provides the possibility having a closer look on the behavior of the prototype system regarding reactive power injection for compensation purposes. Due to delay time for measurement data acquisition, set-point calculation for reactive power injection and communication with the inverter, dynamic changes of the reactive power at the PCC can not be compensated directly. This characteristic is accounted in the control algorithm in order to assure an average power factor equal or higher than the desired one at the end of a 15 minute interval.

Fig. 16 Measurement results of the average power factor of the test site from 07/13/2009. Reactive power compensation starts at 6:15.

Fig. 17 Measurement results of a 15 minute time interval for reactive power compensation with the Multi-PV prototype system.

Altogether these results prove the possibility of combining

the functionality of reactive power compensation with a PV-inverter by measurement results in an industrial environment. Effects due to the parallel operation between PV-injection and reactive power compensation as e.g. a limitation of the apparent power reserve are considered through the developed control algorithm.

V. PEAK SHAVING For companies with a high energy demand, costs for

electricity consist of demand charges and the price per kilowatt-hour. E.g. these tariffs are used for an annual consumption higher than 100,000 kWh in Germany.

The demand charge (€/kW/a) is usually determined by the highest average consumption of a 15 minutes interval within a year. These power peaks can be avoided using systematic peak shaving. Therefore demand charges are reduced. Peak power limitation can be achieved by demand-site management as well as by generation management. Fig. 18 shows exemplary the measured power consumption of the industrial grid at the test site for one day. The red colored areas would have to be shaved estimating a maximal average power consumption of 105 kW.

An approach for peak shaving using the Multi-PV inverter system is described. Here the energy stored in the battery is used for reducing peaks in the load profile. This energy is

additionally injected in the industrial grid in order to reduce the delivered energy from the utility. Since on the AC site of the inverter a differentiation between injected current from PV and energy from the battery can not be realized, a metering on the DC side has to be carried out.

Fig. 18 Measured load profile of the industrial grid at 10/22/2009.

A. Load profile analysis of the industrial grid An analysis of the load profile of the industrial grid has to

be carried out in order to determine a threshold for peak shaving. Therefore the metering data of the utility for the corresponding industrial grid is evaluated for one year. The metering data consists of values for the energy consumption in 15 minutes time intervals. Hence the average power consumption PLoad_AVG in a 15 minute time interval can be assigned.

For time intervals with higher average power consumption than the defined power consumption threshold, a covering of the difference has to be assured through on-site generation. Since the battery has a defined maximum number of load cycles and should only be loaded during the night, the selection of the peak shaving threshold depends strongly on the required energy for peak shaving purposes per day.

Fig. 19 illustrates the number of days, where peak shaving is required, separated into several power classes from 100 kW up to 135 kW in 5 kW steps within one year. Additionally the average and the maximum energy per day, required for peak shaving, are calculated. As stated before it is assumed that the battery system has a specified energy which can be used per day. The battery of the Multi-PV inverter system has a voltage of 384 V and a capacity of 800 Ah. The maximal energy of the battery for one cycle is calculated to 138.24 kWh assuming a C1 discharge. Due to the possibility of providing UPS functionality in parallel, only 75% of the battery should be used for peak shaving, which corresponds to 103.68 kWh. With this boundary conditions a peak-shaving threshold of PLoad_AVG = 105 kW is determined for actual evaluated measurement data.

Fig. 19 Number of days for each power class peak shaving is necessary as well as maximal and average energy per day required from battery storage.

B. Peak shaving control strategy Several factors have to be taken into account for the peak

shaving control strategy within the 15 minute interval. Since the load behavior from the actual time until the end of the time interval is unknown the control has not only to ensure a secure a peak shaving, but should also use as little energy from the battery as possible. Additionally the injection of PV energy should be reduced as little as possible due to the limited apparent power of the inverter.

Fig. 20 shows measurements of the load behavior at the test site within a 15 minute time interval where peak shaving is necessary if a threshold of PLoad_AVG = 105 kW is chosen. Additionally the calculated energy curve out of the measurements is given.

Fig. 20 Measured load curve and calculated energy in a 15 minute time interval

One possible basic control strategy is the injection of

battery energy whenever the accumulated energy in the 15 minute interval is higher than the set-point line. However this method implies risks especially for fluctuating load profiles.

VI. CONCLUSION The Multi-PV inverter system provides multiple additional

functionalities. The application of the system is suitable especially in industrial environments. The technical realization of most additional functionalities is described in this paper.

Power Quality and Reliability is improved for sensitive loads in the sub-network regarding voltage variations, harmonic disturbances and grid faults. A connection between the level of improvement and the size of the decoupling inductor is identified. Reactive power compensation can also be done by PV inverters. Verification is given through measurement results of the Multi-PV inverter system tested in an industrial grid at a test-site. Peak shaving reduces energy costs of the customer. Challenges for peak-shaving especially for the control strategy within a 15 minute interval are addressed.

The application of different additional functionalities has to be customized individually according to the needs of the customer. Beside a technical analysis always economic considerations have to be made in order to ensure a surplus value for the customer.

ACKNOWLEDGEMENT The author would like to thank the German Federal

Ministry of Environment, Nature Conversation and Nuclear Safety for funding the project “Multi-PV Multifunktionale Phototvoltaik Stromrichter – Optimierung von Industrienetzen und öffentlichen Netzen”(FKZ 0329943, see http://www.multi-pv.de).

Only the author is responsible for the content of this

publication.

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[3] Multi-PV consortium, http://www.multi-pv.de [4] D. Geibel, J. Jahn, R. Juchem: "Simulation model based control

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