DYNAMIC BEHAVIOUR OF SINGLE-PHASE INDUCTION GENERATORS DURING DISCONNECTION AND RECONNECTION TO THE GRID J.Ramachandran 1 G.A. Putrus 2 1 Faculty of Engineering and Computing, Coventry University, UK j.ramachandran@coventry.ac.uk 2 School of Computing, Engineering and Information Science, Northumbria University, UK ghanim.putrus@unn.ac.uk Abstract The interest in renewable energy and Combined Heat and Power (CHP) is expected to lead to a continuous increase in Embedded Generation (EG) directly connected to the distribution network. In order to maximise the benefits from EG, it is expected that islanding operation will be allowed in the future. This paper discusses the dynamic behaviour of induction generators (widely used in EG) connected directly to the grid, during disconnection and reconnection to the grid. Direct connection of induction generators is used in EG, such as micro CHP and some wind turbines. Results showed that self-excited induction generators experience a serious voltage regulation problem when the load and speed change. Results from simulation and experimental analyses showed that it is necessary to control the real and reactive power in order to maintain the generator terminal voltage and to avoid high in-rush current during reconnection. Keywords: Micro-CHP, Induction Generator, islanding, Self-excitation, real and reactive power control, in-rush current 1 INTRODUCTION The worldwide interest in renewable energy and Combined Heat and Power (CHP) is expected to lead to a continuous increase in Embedded Generation (EG). Several countries anticipate that numerous domestic CHP would be installed in the near future and many of these use Induction Generators (IG) which are connected directly to the Low Voltage (LV) network [1]. In the UK, Small Scale EG (SSEG) systems are defined as generating units rated up to 16 A per phase and connected within a domestic or light commercial property [2]. This is equivalent to 3.68 kva for single-phase or 11 kva for three-phase systems. There are different technologies for application as μchp systems Fuel Cell, Internal Combustion (IC) Engine and Stirling Engine. Fuel cell systems generate d.c. voltage and are interconnected with the grid through inverters. In the case of IC engine and Stirling engine types, induction generators are usually used and are connected directly to the grid. Among these technologies, the Stirling engine technologies are now being marketed as replacements for conventional domestic central heating boilers [3]. The Stirling engine has the advantages of low noise, low maintenance and low emissions [4]. The main advantages of using induction generators over synchronous generators are low cost, simple construction and there is no need for a separate d.c. source for excitation. Since induction generator lacks separate field winding, it consumes reactive power from the grid. It is normal practice to provide capacitor compensation with the IG, usually about 1/3 of the machine capacity. According to the current UK regulations set by Engineering Recommendations G59 and G83, embedded generators (including SSEG) are not allowed to operate in islanding mode [2]. However, it is expected that in the future, islanding operation would be allowed to maximise the benefits from EG [5]. In order to facilitate the islanding operation, it is necessary to understand the behaviour of EG during the disconnection and reconnection to the grid. M.A. Ouhrouche et al., [6] showed that during disconnection from the grid, with input torque remains the same, induction generators have serious voltage regulation problem due to selfexcitation phenomenon. In micro-chp system based on Stirling engine (available in the market) the prime mover speed is usually constant. This paper further analyse the behaviour of small-scale IG, when the prime mover speed is constant. Simulation analysis and laboratory setup to represent the Stirling engine are used to study the behaviour of the IG. M.A. Al-saffar et al., [7] showed that by controlling the capacitance connected to the IG terminals, it is possible to maintain the terminal voltage of the self-excited induction generator (IG). In order to operate the IG in an islanding mode, methods to control the terminal voltage 16th PSCC, Glasgow, Scotland, July 14-18, 2008 Page 1
using reactive power compensation as well as load demand control has been proposed. The issue of high in-rush current when the islanded generator is connected back to the grid, is analysed and a method to overcome this problem has been proposed. The behaviour of induction generators during the disconnection from the grid is described in Section 2. The experimental setup adopted to study the behaviour of 1-phase IG during disconnection and reconnection to the grid is described in Section 3. Section 4 describes the usage of soft-starter to avoid the in-rush current during reconnection to the grid. Possible method to operate the IG in islanding mode and conclusion drawn from this study are presented in Sections 5 and 6. Figure 1 Induction generator terminal voltage (peak value) for the scenario 1 2 DISCONNECTION OF IG FROM THE GRID This section presents the behaviour of IG during disconnection from the grid and includes the following three scenarios: Scenario 1: Disconnection of IG without local load and 1/3 reactive power compensation Scenario 2: Disconnection of IG with local load of 1 kw and 1/3 of reactive power compensation Scenario 3: Disconnection of IG with local load and reactive power compensation equal to the machine rating. These scenarios have been modelled using MATLAB TM SimPowerSystem toolbox. For all these scenarios, a constant prime mover torque (representing a Stirling engine) has been assumed. The rating of the IG is 1.1 kw at 230 V. Scenario 1: For this scenario when the IG is disconnected from the grid, the only load on the IG is the capacitor banks that provide the reactive power compensation. Because of the presence of the capacitors, self-excitation phenomenon occurs during disconnection from the grid and high voltage level appears across the generator terminals, as shown in Figure 1. In this figure, the IG is disconnected from the grid at time t=1 Seconds. The nominal voltage of the machine is 325 V (peak). After disconnection, the terminal voltage increases to 4000 V (peak) within 0.25 Seconds. This is almost 12 times the nominal voltage rating. Figure 2 Speed of the induction generator, for the scenario 1 Figure 2 shows the speed of the induction generator before and after disconnection of IG from the grid. Before disconnection, the IG speed is slightly above the synchronous speed (1 p.u.). After disconnection at t = 1 Seconds, the speed of the machine increases and then settles to a new steady-state value. It is also observed that the final values of the IG terminal voltage and speed are dependant upon the value of the capacitance. If the value of the capacitance (usually is about 1/3 of the machine rating) is increased, then the time to reach the new steady-state point is relatively small and the final terminal voltage is slightly decreased (not significant change). Similarly, there will be a slight reduction in the steady-state speed of the IG after disconnection when the value of the capacitance is increased. If the capacitance value is too small, the IG cannot build up the voltage (as expected) and hence the terminal voltage collapses to zero value. 16th PSCC, Glasgow, Scotland, July 14-18, 2008 Page 2
This scenario clearly indicates that islanding of IGs without any load connected (no-load) causes very high voltage that can be very dangerous. Hence to operate the IG in islanding mode, it is necessary to control the load connected to the IG to avoid the no-load condition. Scenario 2: For this scenario when the IG is disconnected with 1kW resistive load connected at its terminals with a capacitor bank of 1/3 of machine rating. This scenario is the same as scenario 1, except that a load is connected to the IG. The behaviour of the IG in scenario 2 is modelled and the corresponding terminal voltage result is shown in Figure 3. Figure 4 Speed of the induction generator, for the scenario 2 500 400 300 Network Voltage IG terminal Voltage Peak Value of Voltage (V) 200 100 0-100 -200-300 -400 Figure 3 Induction generator terminal voltage (peak value), for the scenario 2 At the instant of disconnection, at time t = 1 Seconds, the terminal voltage slightly reduces and then it builds up to the value of 500 V (peak) and finally settles at a steady-state value of 425 V (peak). Even though the final terminal voltage value is higher than the nominal value, it does not increase as dangerously high as in the case of scenario 1. Figure 4 shows the speed of the IG before and after disconnecting the IG from the grid. Before disconnection, the speed of the IG is slightly higher than synchronous speed (1 p.u.) and then the speed increases after disconnection (at t = 1 Seconds) and settles at a new value (~1.8 p.u.). The frequency of the voltage at the network side and the voltage at the isolated IG side are analysed. The voltages at the network and IG sides, for scenario 2, are shown in Figure 5. -500 1.2 1.205 1.21 1.215 1.22 1.225 1.23 1.235 1.24 1.245 1.25 Time (S) Figure 5 Voltages at the network and IG sides, for scenario 2, after disconnection of the IG It is observed that the frequency of the IG voltage is 100 Hz, while the network frequency is 50 Hz. It is also observed that the frequency decreases with the increase in the capacitance value. This result is similar to the result obtained by Tamura et al [8]. This scenario indicates that the presence of the load reduces the dangerously high terminal voltage. However, the dynamic behaviour of the IG is governed by the amount of capacitance connected to provide the reactive power compensation. Scenario 3: From the previous scenarios, it is clear that no-load conditions can lead to very high voltages and when a load is connected the rise in the voltage is reduced. It is also found that the value of capacitance also affects the final steadystate terminal voltage and the time to reach the new operating point. 16th PSCC, Glasgow, Scotland, July 14-18, 2008 Page 3
In scenario 3, it is assumed that the load connected to the IG is equal to the machine rating (1.1 kw) and it is 100% compensated (capacitor supplies all the reactive power required by the IG). The terminal voltage of the IG before and after disconnection from the grid, obtained for this scenario is shown in Figure 6. The results obtained indicate that the value of the terminal voltage remains the same before and after the disconnection. It is also observed that the speed of the IG remains almost constant after disconnection, as shown in Figure 7. same as the network frequency. However, the IG voltage and grid side voltage are not in-phase. The frequency of the isolated IG is decreased as compared to scenario 2. As explained before, this is due to the increase in the capacitance value in scenario 3. Peak Value of Voltage (V) 400 300 200 100 0-100 -200 IG Terminal Voltage Network Voltage -300-400 1.2 1.21 1.22 1.23 1.24 1.25 1.26 1.27 1.28 1.29 1.3 Time Figure 8 Network voltage and IG terminal voltage, for scenario 3, after disconnection of IG Figure 6 Induction generator terminal voltage (peak value), for the scenario 3 Results from these scenarios and others considered in this work indicate that by controlling the load connected to the IG and the reactive power compensation provided, it is possible to keep the terminal voltage, frequency and speed of the machine constant. This will provide the means to operate the IG in islanding mode. 3. EXPERIMENTAL SETUP In a commercially available Stirling engine based Micro-CHP system, the primemover speed is usually constant. A laboratory experiment was setup to represent the Stirling engine and study the behaviour of the IG during disconnection and reconnection to the grid. Figure 7 Speed of the induction generator, for the scenario 3 The terminal voltage of the isolated IG and the network voltage, for scenario 3, are shown in Figure 8. In this scenario, the capacitance is providing 100% reactive power compensation. The frequency of the isolated IG is 50 Hz and is the A 500 W Induction Machine coupled with a d.c. motor is used to replicate the micro-chp system. The speed of the d.c. motor is kept constant using a speed control unit. The Laboratory setup used is shown in Figure 9. When the speed of the induction machine is made slightly higher than the synchronous speed, it operates as a generator. It is also observed that when the capacitor value is too small, the IG cannot maintain the voltage and this collapses immediately. 16th PSCC, Glasgow, Scotland, July 14-18, 2008 Page 4
Figure 10 shows the IG current before (during islanding) and after connection to the grid. It can be clearly seen that when the IG is reconnected to the grid, a high in-rush current, about 5 times the nominal current, is developed. 4. RECONNECTION WITH SOFT- STARTER Figure 9 Experimental setup to study the behaviour of Micro-CHP In islanding mode, the load connected to the IG can be resistive load or reactive load. Hence to maintain the terminal voltage during islanding condition, both the load and capacitors connected to the IG need to be controlled. Using the laboratory set up, the behaviour of the IG during reconnection to the grid is analysed. When the IG is connected back to the grid, a high in-rush current is developed. The-inrush occurs even when the magnitude of IG terminal voltage and frequency are maintained the same as the grid voltage which is due to the phase difference between the supply and IG voltages. When these non-synchronised voltage sources are connected, a high in-rush current is produced whose magnitude and duration depend on the phase difference at the time of connection. Depending upon this, the inrush current for the experimental set-up varied from 4-7 times the nominal current value. It is a common industrial practice to provide softstarting of motors to avoid in-rush current. In this work, a solid-state soft-starter using IGBTs was developed. The control circuit of the soft-starter produces a timed gate pulses to control the phaseangle of the IGBTs to maintain low current during reconnection. It is worth noting that the presence of capacitance during the reconnection can also cause in-rush current during reconnection using the soft starter. To avoid this in-rush current, capacitors need to be disconnected at the instant of reconnecting the IG to the grid. Figure 11 shows the current flow from the IG to the grid during reconnection, with the soft-starter (the capacitor is disconnected at that instant of reconnection). Using the proposed soft starter, the current gradually builds-up (as shown in Figure 11) and hence the high in-rush current is avoided. The nominal current rating of the machine is 3.12 A (peak). Using the soft-starter, the maximum peak current is maintained at only 3.8 A and the IG returns to the normal operating condition within 0.2 seconds. 15.000 10.000 5.000 Peak Value of Current (A) 0.000-5.000-10.000 Induction Machine connected to the id -15.000-20.000 0.000 0.200 0.400 0.600 0.800 1.000 1.200 1.400 Time (S) Current flow from Induction Machine without soft starter Figure 10 Current flow from IG to the grid during islanding and reconnection to the grid Figure 11 Current flow from IG to the grid, with soft starter, during reconnection to the grid 16th PSCC, Glasgow, Scotland, July 14-18, 2008 Page 5
5. DISCUSSION The results obtained show that the self-excited IG has serious voltage regulation problem when the load and/or speed changes. The dynamic behaviour of the IG during disconnection from the grid depends upon the amount of capacitors connected to the IG terminals. Simulation and laboratory results showed that the terminal voltage and frequency of the IG could be maintained fairly constant by controlling the IG load and excitation capacitance. When the induction generator is reconnected directly to the grid (from islanding mode), a high in-rush current of about 7 times the normal current is produced. Although the voltage and frequency at the IG and the grid sides are the same, an inrush current is produced because the two voltages are not synchronised. In order to reduce the in-rush current, the IG should be connected to the grid through a soft-starter. An experimental soft-starter using IGBTs is developed and tested in the laboratory. The softstarter is designed such that it disconnects the excitation capacitor at the instant of reconnection in order to avoid capacitor in-rush current. The capacitors are connected after the starting period is complete in order to improve the power factor of the machine. A schematic diagram to illustrate the proposed starter (and operation in islanding mode) is shown in Figure 12. During islanding mode (after the IG is disconnected from the grid), capacitor and load control is used to maintain the voltage constant. Figure 12 Schematic diagram for controlling IG for islanding operation 6. CONCLUSIONS In this paper, the dynamic behaviour of induction generator based micro-chp system during disconnection from the grid (for islanding operation) and reconnection is analysed. The results obtained show that IGs have serious voltage regulation problem during disconnection and high in-rush is developed during reconnection. A laboratory set up of an induction generator coupled with a d.c. motor (representing a Stirling engine where the speed remains constant) is used to analyse the behaviour of the IG during disconnection and reconnection to the grid. The results show that it is necessary to control the IG load and capacitance in order to maintain the voltage within limits and to avoid any in-rush currents. The principles of controlling IGs can be used in the concept of active management of distribution networks. ACKNOWLEDGEMENT Authors would like to acknowledge Dr. David Johnston, for his help in laboratory works. REFERENCES 1. McCarthy C., Renewable, CHP and electricity distribution networks: Challenges for the future, IEE/Ofgem Conference on CHP and Electricity Distribution Networks A strategic review, Sep 2002. 2. Engineering Recommendation G83/1, Recommendations for the connection of small-scale embedded generators (up to 16 A per phase) in parallel with public lowvoltage distribution networks, Energy Networks Association, September 2003. 3. A.D. Peacock and M. Newborough, Impact of Micro-Combined Heat and Power Systems on Energy Flows in the UK Electricity Supply Industry, Journal of Energy, Volume 31, Pages 1804 1818, 2006. 4. J. Harrison and S. Redford, Domestic CHP What are the potential Benefits? Report submitted to Energy Saving Trust by EA Technology Ltd., June 2001. 5. Duncan Botting, Technical Architecture Integration or Disintegration? The Future could be Distributed, A Tutorial on the UK Distribution Network how and why 16th PSCC, Glasgow, Scotland, July 14-18, 2008 Page 6
it is changing, 29 th June 2004, UMIST, Manchester Organised by IEE Power Systems and Equipment Professional Network. 6. M.A. Ouhrouche, X.D. Do, Q.M. Le and R. Chaine, EMTP Based Simulation of a Self-Excited Induction Generator After its Disconnection From the Grid, IEEE Transaction on Energy Conversion, Volume 13, Number 1, March 1998. 7. M.A. Al-Saffer, Eui-Cheol Nho and Thomas A. Lipo, Controlled Shunt Capacitor Self-Excited Induction Generator, the IEEE Industry Applications Conference, Volume 2, Page 1486 1490, October 1998. 8. J. Tamura, R. Nakamichi, C. Nakazawa and I. Chihara, Analysis of Isolated Self- Excited Induction Generator, The International Conference on Electrical Machines, August 2000, Finland, Page 519-522. 16th PSCC, Glasgow, Scotland, July 14-18, 2008 Page 7