Islanding Detection Methods for a Micro-Hydro Power ...

ELECTROMOTION 2009 – EPE Chapter ‘Electric Drives’ Joint Symposium, 1-3 July 2009, Lille, France

Islanding Detection Methods for a Micro-Hydro Power Station – Simulation and Experimental Results S. Breban1, 2, B. Robyns1 and M.M. Radulescu2 1

Laboratoire d’Electrotechnique et d’Electronique de Puissance (L2EP) de Lille Ecole des Hautes Etudes d’Ingénieur (HEI), Lille, France 2

Special Electric Machines and Light Electric Traction (SEMLET) Group Faculty of Electrical Engineering, Technical University of Cluj-Napoca, Cluj-Napoca, Romania Abstract – Distributed generation is becoming more and more a reality for the electric power system. One of the main problems encountered with this kind of generation is the potential formation of islands which could be feed by distributed generators even if the utility grid has failed. Many methods have been developed to detect this situation and they have been classified in three groups: passive methods, active methods and methods based on communication systems. This paper presents a micro-hydro power station, which is submitted to a power grid failure. Two active methods are tested in simulations and experiments in order to detect the islanding created.

I.

INTRODUCTION

The decision taken by the European Union to increase the production of electrical energy from renewable sources by 2020 presumes to reduce the use of coal, oil and natural gas in energy production processes, and replace them with renewable energy conversion systems as, for example, small hydropower plants or wind power units. This two energy sources have yet a great potential of development being the only capable to compete with the classical thermal power stations. Hydroelectricity is the most expanded renewable energy source over the world. Micro hydropower has a quite large potential of development because of the increasing interest in renewable energies and dispersed electrical generation. This type of hydroelectricity ranges from 0 to 10 MW in Europe where, without accounting for newly integrated countries, the microhydro capacity is over 11500 MW, representing 1.7 % in electricity production capacity and 10% of hydroelectric power. The European growth potential reaches about 6000 MW [1]. Micro hydropower stations are nowadays based on a fixedspeed synchronous machine or a squirrel-cage induction generator. In both configurations, no use is made of power electronic devices. In the first case, speed is necessary fixed; in the second one, speed may vary in a small range according to active power demand changes, if the station is grid-connected, or the additional capacitor and load equivalent impedance variations, if the asynchronous machine supplies a passive

network, i.e. the station is islanded. For both generators, the turbine rate of flow regulation allows supplying the necessary active power, and to control the frequency when the station is connected to isolated loads [2]. The electrical power system (EPS) is changing nowadays from a tree structure, with the big power plants at the bottom, to a net structure with many small distributed points of generation. The distributed generators (DG) are offering the possibility to combine dispersed generation with local energy usage and storage, reducing the energy losses produced along the transport and distribution lines. The islanded operation, as the main power grid drops, consist of one or several DG feeding distribution lines and local loads. This operation involves several problems with serious consequences. On the EPS side, safety measures should be adopted in order to protect the intervention personnel working to resolve the situation occurred. Moreover, a long duration of the islanded operation can produce damages in case of automatic re-closing of the utility protection devices. An islanded DG could get desynchronized during the stand alone period of operation, forcing the protections to fall again and being potentially dangerous for the electronic equipment due to the apparition of short-circuits at the moment of reconnection [3], [4]. II. SYSTEM UNDER STUDY In Fig. 1, the electromechanical structure of the proposed hydropower station is presented. This system has his origins in aeronautics, and was proposed to power the commercial aircrafts in [5]. When connected to power grid, the control strategy allows controlling the doubly-fed induction generator (DFIG) stator active and reactive power flow through PWM Converter 2; when feeding an isolated load, the DFIG stator rated voltage and frequency are controlled. It is to be noted that the DFIG operates in hyper-synchronism and also in hyposynchronism. The control strategy involving the PMSM permits to maintain the DC link voltage to desired value by means of PWM Converter 1.

Kaplan turbine Gear box

Shaft loads or grid

DFIG

PMSM

DC link

PWM

PWM

Converter

Converter

n° 1

n° 2

Fig. 1. Micro-hydro power system under study.

The micro-hydro power station is considered as a run-ofriver one, leading to the use of a Kaplan hydraulic turbine well suited for low water heads. The turbine drives a doubly-fed induction generator (DFIG) whose excitation is supplied on its rotor by a permanent-magnet synchronous machine (PMSM) mounted on the same shaft. Two back-to-back PWM power electronic converters, connected through a DC bus, carry out the electric link between the machines. Converter 1 controls the DC-link voltage, leading to the balance between the DFIGrotor active power and the PMSM one. Converter 2 is dedicated to the control of the DFIG, in order to achieve operation of this generator on isolated loads and on a power grid ([2], [6], [7]). It may be emphasized that the considered configuration is different from most common DFIGs, whose rotor windings are connected, ever through power electronic converters, to stator ones. This electromechanical set, when compared to classical structures used in micro hydroelectric power plants, features several interesting characteristics, i.e. it can operate in an autonomous way due to the PMSM allowing the DC-link capacitor stand-alone charging when the system starts; power electronic converters increase the plant control possibilities (e.g. additional capacitors used in squirrel-cage induction machines are no more necessary), and dynamics, as they replace mechanical controls; converter and PMSM rated powers are only around 30% of the plant rated power, representing the typical average slip power [8]. References [2] and [7] develop the structured model of the system connected to isolated loads, where the DFIG stator voltage and frequency are controlled.

guide vane one. According to these assumptions, hydro-power turbine behaviour may be taken into account by means of simplified static mechanical characteristics represented in Fig.2 for a fixed rate of flow. Turbine torque (Tt) vs. speed () characteristic is assumed to be a straight line. Torque becomes null for a rotating speed value e which is the runaway speed, i.e. speed when no-load torque is applied on the shaft. e is a turbine parameter, and a value of 1.8 times the turbine rated speed n is assumed [4]. Torque vs. speed characteristic equation under rated water flow and head is given next:

  Tt  Tn 1.8  n 

  , 

(1)

where subscript "n" is used for rated values. Mechanical power (Pmec) simplified characteristic is, consequently, a parabola. Taking into account the water wheel efficiency depending on the rate of flow and on the rotating speed, this power results from the hydro power (Phyd ) which is expressed as follows:

Phyd  ρgH q

(2)

Torque

Mechanical power

Hydropower turbine model As mentioned above, a Kaplan turbine is considered in this paper. It is referred to a fixed head and a constant water flow. It is assumed that water flow variations are very slow compared to the drive dynamics. The turbine model is a basic one, i.e. it includes neither blade pitch control nor upstream

0

n

e

Rotating speed

Fig.2. Hydro power turbine torque and mechanical power vs.rotating speed, for given water flow.

III. ISLANDING DETECTION For islanding detection several techniques have been developed: a) Passive techniques consisting in power grid parameters monitoring such as: amplitude, frequency and phase of the voltage. These three parameters are leading to three passive detection techniques amplitude and frequency deviations and voltage phase jump; b) Active techniques are characterized by a provoked variation either on amplitude, frequency or phase of a current in order to destabilize the islanded system and detect the power grid loss. This method is often associated to a passive technique which allows to disconnect the DG from the power grid; c) Using communication systems on the power lines is also another possibility to detect an islanding situation. In this case a low intensity signal it’s emitted from a transmission device via power lines. A receptor will detect the presence or absence of the signal on the side of the DG and thus detect the presence or the loss of the main power system. IV. SIMULATION RESULTS For our micro-hydro power system we have chosen the passive techniques i.e. amplitude and frequency deviations. The simulations where made in Matlab/Simulink environment. This micro-hydro power station is able to work connected to power grid and also to feed isolated loads. The unitary control procedure for both types of connections is allowing the smooth passage from power grid connection where active and reactive generated powers are controlled to isolated loads connection where DFIG stator voltage amplitude and frequency are controlled. The simulation procedure is the following: at t = 0 s, the micro-hydro power station is working at noload; at t = 1 s, the system is connected to the power network, references of active and reactive powers being: P=0 kW, Q= 0 kVar; at t = 2 s, active and reactive reference powers are: P=-1 kW, Q= 0 kVar; at t = 3 s, a power grid loss is detected and the micro-hydro system continues to feed the isolated loads, which are disconnected from the power network. Simulation results show that the micro-hydro power station is able to detect the islanding situations and it can continue to feed isolated loads only after separating from the power grid. In simulation the islanding conditions where recreated by cutting the power grid three phase voltages to zero. In this case the small power plant’s control structure continues to keep the active and reactive powers at the reference values. But, the power network its not imposing the voltage amplitude and frequency anymore, so this parameters are now deviating from the nominal ones. These deviations are detected and the small hydropower plant control structure is disconnecting from the

main power lines and is switching to the appropriate control procedure on isolated loads. The limits for the voltage amplitude where considered at ± 20% of the nominal value and for the voltage frequency ± 2Hz deviation from 50 Hz. As shown in Fig. 5, after the loss of the power grid, the DFIG stator rms voltage deviation (Fig. 6) is detected first. The DFIG stator frequency detection (Fig. 7) is slower, one of the reasons being the measuring time constant of this parameter. DC-link voltage (Fig. 8) is kept around the nominal value and the speed of the micro hydropower plant is varying in a wide range, assuring the stability of the power system.

Fig. 3. Water rate of flow

Fig. 4. Active and reactive powers generated by the micro-hidro power system

Fig. 5. Islanding detection using voltage and frequency deviations

Fig. 6. DFIG stator voltage amplitude deviation

Fig. 7. DFIG stator voltage frequency deviation

Fig. 8. DC-link voltage

V. EXPERIMENTAL RESULTS Fig. 10 shows the test bench conceived and built to validate the solution proposed in this paper. The micro-hydro power system is composed of a hydraulic turbine emulator, based on a torque-controlled DC machine, a PMSM and a 3 kW (4 poles) doubly-fed induction machine mechanically coupled to the DC

Fig. 9. Micro-hydro power-system rotating speed

machine and to the PMSM. Two converters make the link between the induction machine rotor and the PMSM. Power converter electronic switches are controlled by DSPACETM cards. As DFIG stator and rotor coils are star-connected with neutral point isolated, measuring two stator and two rotor currents is sufficient. Rotor currents are also useful to compute rotor fluxes. Stator instantaneous voltage value is also measured to calculate the active and reactive powers transmitted by the generator to isolated loads, to perform RMS value and frequency regulation. Tests are performed over an interval of 100 seconds. The micro-hydro power system is connected to the power grid whose line-to-line reference voltage is 225 V rms, i.e. 130 V phase voltage (Fig. 14). The active and reactive powers injected in the power network are controlled. The water rate of flow is considered constant for this interval of time because its variability has a time scale of hours or days. The experimental results are clearly showing the capability of the micro-hydro power system to control the active and reactive powers injected into the power network, to detect a power grid loss using two methods, and the capacity to automatically switch to the second control procedure and continue feeding isolated loads At the beginning, the micro-hydro power station is injecting into the power grid 600 W. The reactive power is kept to zero (Fig. 12). At about t = 32s, a power grid loss is induced. Immediately the two detection methods are indicating a power grid failure (Fig. 13), and the control system is disconnecting the power station from the grid, passing also to isolated mode control procedure. In this way, if a power grid failure appears the micro-hydro power station can (and is preferable) continue to feed some isolated loads with the total power inferior to the turbine mechanical power. Figs. 14 and 15 are showing the capacity of the power system to maintain the load amplitude and frequency voltage to the rated ones despite the sudden isolated loads increase (Fig. 12). On the DC-link side (Fig. 16), the PWM Converter 1, connected to PMSM stator, is keeping the DC-link voltage to the reference value, i.e. 200 V.

CAPTION Measurement

Hydro-power turbine emulator

Control

Shaft DCM

PMSM

DFIG

PWM

DS 1104

Isolated loads

PWM

CONVERTER 1

CONVERTER 2

DS 1103 Fig. 10. Block-diagram of the experimental test bench

Fig. 11. Water rate of flow Fig. 13. Islanding detection using voltage and frequency deviations

Fig. 12. Active and reactive powers generated by the micro-hidro power system

Grid

Fig. 14. DFIG stator voltage amplitude deviation

In Fig. 17, the rotating speed of the hydropower assembly is presented. It can be observed that DFIG is working on a really wide-speed spectrum, and has as an advantage the possibility to operate on any point of the turbine mechanical characteristic. VI. CONCLUSIONS

Fig. 15. DFIG stator voltage frequency deviation

In this work, two passive islanding detection techniques were presented. Their efficiency is tested with the help of simulations on Matlab/ Simulink environment and on test bench, using a 3 kW scale-model of an innovative micro-hydro power station. The results are demonstrating that these two methods are detecting a power grid loss. Following this event, the power station is disconnected from the grid, but continues to operate successfully on isolated loads by keeping their voltage and frequency at rated values. ACKNOWLEDGEMENTS This work was supported in part by the Romanian Ministry of Education and Research (Grant CNCSIS No. 421/2007) and ENSAM de Lille. The test bench development has benefited from the financial support of the Regional Council Nord-Pas de Calais, the European Union, the Technological Research National Center of Lille, Forclum Ingenierie Verquin, Innovelect and HEI. REFERENCES [1] [2]

Fig. 16. DC-link voltage

[3]

[4]

[5]

[6]

[7]

[8]

Fig. 17. Micro-hydro power-system rotating speed

[9]

Renewable energy barometer Eurobserv'ER ; www.energiesrenouvelables.org A. Ansel, B. Robyns, “Small hydroelectricity: from fixed to variable speed electromechanical drive”, Electromotion, vol.13, n°2, 2006. S. Gonzalez, R. Bonn, J. Ginn, “Removing Barriers to Utility Interconnected Photovoltaic Inverters”, 28th IEEE Photovoltaic Specialist Conference, Anchorage, AK, Sept. 15-22, 2000. H. Beltran, F. Gimeno, S. Gegui-Chilet, J. Torrelo, “Review of the Islanding Phenomenon Problem for Connection of Renewable Energy Systems”, International Conference on Renewable Energy and Power Quality – ICREPQ’06, 5 – 7 April 2006, Palma de Mallorca, Spain. F. Khatounian, E. Monmasson, F. Berthereau, E. Delaleau and J.P. Louis, “Control of a Doubly Fed Induction Generator for Aircraft Application,” IECON 2003, Roanoke, Virginia, United States, 2-6 november 2003, pp. 2709-2714 S. Breban, A. Ansel, M. Nasser, B. Robyns, M.M. Radulescu, “Experimental results for a variable speed small hydro power station feeding isolated loads or connected to power grid”, Proc. ACEMPELECTROMOTION 2007 Joint Conf., Bodrum, Turkey, pp. 760-765. S. Breban, M. Nasser, A. Ansel, C. Saudemont, B. Robyns, M. Radulescu, “Variable Speed Small Hydro Power Plant Connected to AC Grid or Isolated Loads”, EPE Journal, Vol. 17, No. 4, 2007, pp. 29 – 36. C.R. Kelber, W. Schumacher, “Adjustable-speed constant-frequency energy generation with doubly-fed induction machines,” European Conference Variable Speed in Small Hydro - VSSHy 2000, Grenoble, France. A. Dagoumas, A. Marinopoulos, G. Papagiannis, P. Dokopoulos, “Simulation of Small Hydro Generators in Islanding Operation in weak Distribution Networks”, ICEM 2006, September 2-5, 2006, Crete, Greece.