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Virtual Synchronous Generator: An Element of Future Grids T. Vu Van, K. Visscher, Member, IEEE J. Diaz, V. Karapanos, A. Woyte, M. Albu, Senior Member, IEEE, J. Bozelie, T. Loix, Student Member, IEEE, D. Federenciuc, Member, IEEE
Abstract— The future power system integrated a large share of distributed generation (DG) may causes a reduction of the system rotational inertia, resulting in high frequency variations with any disturbances. An additional rotational inertia will be provided to the system if many DG units combined with relative small storage systems operate like virtual synchronous generators (VSG). This paper presents the demonstration approach for the VSGs in order to bring the VSG models developed in the laboratory into practice at two distribution test sites for both single- and three-phase applications. The results show promising results where changes in frequency and voltage are clearly counteracted by power flows from the VSG. In addition, VSG may provide a solution for hosting a large share of DG in future grids while maintaining system stability. Index Terms-Virtual Synchronous Generator, Demonstration, Power System Inertia, Micro-grid, Storage I. GLOSSARY
CHP DAQ DG DSO LV PV ROCOF SOC VSG
T
Combined Heat and Power Data Acquisition System Distributed Generation Distribution System Operator Low Voltage Photovoltaics Rate of Change of Frequency State of Charge Virtual Synchronous Machine II. INTRODUCTION
HE stability of power systems is traditionally achieved by regulating large synchronous machines with high inertia to accommodate every disturbance in power systems.
This work is a part of the VSYNC project funded by the European Commission under the FP6 framework with the contract No: FP6 – 038584 (www.vsync.eu). V.V. Thong and A. Woyte are with 3E, Vaartstraat 61, B-1000, Brussels, Belgium (
[email protected] ,
[email protected] ) K. Visscher is with Energy research Center of the Netherlands (ECN), The Netherlands (
[email protected] ) M. Albu is with the Politehnica University of Bucharest, Romania (
[email protected]) J. Diaz is with Ufe, Germany (
[email protected]) V.Karapanos is with the TU Delft, Netherlands (
[email protected]) J. Bozelie is with Liandon, The Netherlands (
[email protected]) T. Loix is with the KULeuven, Belgium (
[email protected]) D. Federenciuc is with Electrica, Romania (
[email protected])
With the increasing share of many small DG units usually interfaced by power electronics and thus having no synchronous inertia, the power generation from large synchronous generation units may be reduced. Some of the large synchronous generators will be shut down at low demand periods due to economic and technical reasons as they cannot operate at power values lower than some minimum power settings. This causes a reduction of the system inertia, resulting in high frequency variations in cases of any disturbances [1, 2]. A solution towards stabilizing such a power system within the limits of presently available system control strategies is to provide additional virtual rotational inertia. Principally, this can be attained by adding short-term energy storage to any DG unit together with an intelligent control of the power electronic interface to the grid. The DG unit will then operate like a virtual synchronous generator (VSG), exhibiting some of the desired properties of synchronous machines for short time intervals. The idea of a virtual synchronous machine is put into practice in the VSYNC project [3-6]. In order to successfully demonstrate the VSG concept, a significant part of work is allocated to the laboratory set-up and experimentation, including testing various control techniques operating on different types of short-term storage systems. Then, the field operation of large and small VSG systems is demonstrated [6,9]. This paper aims at describing in detail the demonstration approach for the VSG in order to bring the models developed from laboratory into practice. One demonstration site of 10 VSGs of 5 kW each in The Netherlands, and one demonstration site of one VSG of 100 kW in Romania are currently in operation. Testing activities are ongoing and some first results are presented. Finally, applications of VSG for future grids with a high share of DG are discussed. III. RATIONAL OF VIRTUAL SYNCHRONOUS GENERATOR Distributed generators are playing an important role in electricity generation. Besides the successful development of wind energy, photovoltaic generation is increasing significantly. EPIA (European Photovoltaic Industry Association) targets in 2020 to generate 12% of power demand in Europe from photovoltaic panels [11]. PV generation has no synchronous inertia by nature, thus leading to larger frequency swings as response to generation or load changes. According to Tuohy et al [12], as the Irish power system is
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weakly interconnected, the effect of increasing amounts of wind power on system inertia is greater than those can be seen on other systems of similar size with stronger interconnection. When the system inertia in isolated systems becomes lower, the frequency change will be faster if load or generation changes, resulting in a less stable grid. A method to increase system inertia is needed. The idea of VSG, although proven as feasible in simulations, is now on the verge of being demonstrated in field tests in Bronsbergen, The Netherlands and in Cheia, Romania. The most promising algorithms from lab experiments, approved for use in the field tests, suitable converter prototypes of VSG and short-term energy stores are used in the field test equipment in order to prove the VSG feasibility. The equipment of these two demonstration sites is installed and embedded into the existing distribution systems. In order not to interfere and harm the security of the local grids, careful testing before commissioning has been carried out. The VSG systems installed are monitored and data is collected via remote desktop web applications for further analysis and evaluation. IV. VSYNC PROJECT The VSYNC project consists of seven work packages ranging from realization with lab set-ups to real field test demonstrations (www.vsync.eu). VSYNC stands for “Virtual synchronous machines for frequency stabilization in future grids with a significant share of decentralized generation”. As demonstrations have to be prepared very well in order to be successful, a significant part of the work is allocated to research on the laboratory development and testing of VSG concepts with different kinds of short-term storage. The project is partly funded by the European Commission under the Framework Program 6. It is made of ten partners having core competences to the others: research office (ECN, Labein), distribution network operators (Electrica, Liandon), industry (Ufe), academic (K.U. Leuven, T.U. Delft, T.U. Eindhoven, Politehnica University of Bucharest), and a service provider (3E). It started in October 2007 and ends in October 2010. If successful, the project delivers prototypes of VSG's that are in the pre-market phase and can provide a cost effective solution to grid stability problems in areas where distributed generation is becoming significant. V. CONFIGURATION OF VIRTUAL SYNCHRONOUS GENERATORS 1) Small-size VSG Converter The small-size VSG converter is used for one-phase lowvoltage and power applications. The converter is built by VSYNC partner UfE, and is being tested at the Bronsbergen site in The Netherlands. It has a nominal power of 5 kW, AC voltage output of 230 V, and storage DC voltage input 48-60V (Fig. 1). An Ethernet connection for the VSG is available. This allows data retrieving from the DAQ (Data Acquisition System), access to some inverter status information and software and algorithm update via the internet.
The hardware platform has its own internal control unit. It is in charge of the control of the converter at low level (IGBTs switching, current/voltage measurement, over current/voltage protection, serial communication, etc.). Ho us e with pho to vo ltaic
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VRLA / Li-Ion batte rie s
48–60 Vdc
S OC
Dis tr. S ys Ope rato r
g rid V and f
Co ntro l s ig nal
high le v e l Control unit Co ntro l hard ware
DAQ s ys te m
Internet
Project partners
Fig. 1. Schematic of Ufe’s converter, 5 kW
The control of the inverter at high level is performed by other control unit where the VSG algorithm resides. Based on grid voltage and frequency, and battery state-of-charge, it is in charge of control the current (power) that go from/to the DC side (battery system) to/from the AC side (grid). The control algorithm will let the inverter behave as a VSG.
Fig. 2. Schema of Triphase’s converter, 100 kW
2) Medium-size VSG Converter For the three-phase VSG lab tests and field test, converter platforms were acquired from Triphase in Leuven, Belgium. The Triphase converter cabinet at test-site Cheia, Romania, contains two three-leg converters, allowing the use of one three-phase four-leg grid-connected AC-DC converter (with actively controlled neutral conductor), two bidirectional DCDC converters (hardware for Buck converter setup present in the cabinet) and one bidirectional DC-DC convertor (using the IGBTs and anti-parallel diodes of the break choppers of both converters), fit for connection of a DC source (Fig. 8). The grid –connected converter is rated at 90 kVA and the DC-DC converter at 30 kW. The DC-link voltage can be sustained through a 40kVA 400V-500V transformer connected to the grid feeding a diode rectifier coupled to the common DC-link.
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A so-called Target PC runs the model being tested, and directly controls the IGBT reference PWM signals. The model being tested is developed in the well-known Matlab-Simulink software (www.mathworks.com) on a so-called Engineering PC and then compiled and uploaded to the Target PC. Communication between the inverter, measurement sensors and contactors on one hand and the Target PC on the other hand is done using the EtherCAT field bus system, which uses Ethernet communication. The user works on the Engineering PC, which is connected to the Target PC via TCP Internet Protocol. The converter control scheme runs on the Target PC. The user can read out measurements, change control set-points and parameters, switch contactors, etc. by entering them on the Engineering PC. The Target PC interacts with the converter and associated hardware in the Triphase cabinet through the EtherCAT interface. One of the advantages of this setup is the built-in possibility of remote log-in, control and data logging of the platform.
- Easy to access of transport for equipment of a few tons; - Within constraints of budget and time schedule of the VSYNC project. Taking into consideration these criteria, a site at Bronsbergen in The Netherlands and a site in Cheia in Romania were chosen. 1) Bronsbergen Test Site This is a holiday park with about 208 houses. There are 148 cottages with PV roofs of over 3kW capacity each, 466 kWp in total. The whole system is supplied from a transformer to four feeders (Fig. 3). Peak load is 150 kW. Two large 200kW converters with a 375 KW lead-acid battery system (720V* 5Ah) are connected to the system. 10 VSGs of 5 kW each are installed at 10 houses in the system. A small three-phase VSG is also tested separately in this test site.
VI. STATE OF CHARGE OF STORAGE SYSTEM As the VSG algorithms are meant to both inject power to and absorb power from the grid, the nominal SOC (State of Charge) should be about 50%. The SOC should not cross a certain lower and upper boundary. For instance in case of lead-acid batteries, a nominal SOC of 50%, an upper boundary of 80% and a lower boundary of 20% may be chosen. In case of contingencies, there must be control measures preventing the SOC to rise above its upper level of sag beneath its lower level. In case of persistent power deficiency or surplus, a separate control algorithm must drive the averaged SOC back to its nominal value, irrespective of the exact value of the persistent power deficiency or surplus. From an economical point of view it should be stressed that storage capacity needed here is very limited as its only aim is to exchange power with the grid with the aim to diminish fast frequency changes. Therefore the averaged power is zero and for not too high frequency changes the SOC versus time curve reflects the actual frequency curve. It can be shown that the smallest possible size of VSG storage is proportional to the product of virtual inertia emulated and the maximum frequency deviation allowed [5]. The maximum virtual inertia is proportional to the nominal power of the VSG converter divided by the maximum rate of change of frequency allowed. Therefore, the virtual inertia in most cases may be chosen such that an economical solution for the storage capacity can be attained. VII. TEST SITES Site selection for field demonstration is important for successful tests. These are the criteria of site selection: - Can carry out as many test cases as possible without or with less harming to customers; - Compatible with the size of VSG. When the nominal power of the site is too big compared to the nominal VSG power, we cannot distinguish and see contributions from VSG power; - Can test with different control algorithms, functioning of VSG;
400 kVA MV
VSG
400/230
every 5 houses
VSG
VSG
Solar home
Fig. 3. Field test diagram in Bronsbergen
2) Cheia Test Site A test site at Cheia is chosen. It is located about 140 km North of Bucharest, and is accessible for transport of the demonstration equipment. The low voltage system is supplied by a 20 kV/0.4 kV transformer to seven feeders. The load is mainly residential. One VSG of 100 kW is connected to a reserved feeder at the low voltage side of the substation. An emulating DC voltage distributed generator (35 kW) is considered to install in this test site (the diode rectifier), interfaced with a converter. In addition, the distribution system operator Electrica owns the substation site and the 0.4kV system; and there are indoor and outdoor facilities for installing the equipment. It is a weak low voltage system with expected severe voltage variations at the point of common coupling (20/0.4kV substation where the VSG is connected); another issue concerns the security of the equipment for one year planned testing phase: it is ensured full surveillance and maintenance. Moreover, it is expected a strong development of the electrical network in the region and the VSG will be further needed. A short-term storage system (lead-acid rechargeable batteries) is added to the structure of the VSG. A detailed schema of the power section of the VSG in Cheia is presented in Fig. 4.
4 Overhead Line 20kV CHEIA
PTC ELECTRICA Distributie Muntenia Nord
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