| Abstract -We investigated a small isolated hybrid | | | | diesel accelerates when the input power is higher |
| power system that used two types of power | | | | than the electrical output power of the generator |
| generation; wind turbine and diesel generation. The | | | | (including losses). |
| interaction of diesel generation, the wind turbine, and | | | | The diesel decelerates when the input power is lower |
| the local load is complicated because both the load | | | | than the electrical output power of the generator |
| and the wind turbine fluctuate during the day. These | | | | (including losses). An oversized diesel engine does not |
| fluctuations create imbalances in power distribution | | | | have problems accelerating or decelerating, but an |
| (energy sources are not equal to energy sinks) that | | | | undersized diesel engine may create problems, during, |
| can affect the frequency and the voltage in the | | | | for example, the start-up of a wind turbine or large |
| power system. The addition of energy storage will | | | | compressor. Figure 5 illustrates a condition where the |
| help balance the distribution of power in the power | | | | diesel is undersized with respect to the load. The |
| network. For this paper, we studied the interaction | | | | genset frequency and the terminal voltage of the |
| among hybrid power system components and the | | | | wind turbine generator are shown on the top graph, |
| relative size of the components. We also show how | | | | and the real power of the diesel, wind turbine, water |
| the contribution of wind energy affects the entire | | | | pump, and local load are shown on the bottom graph. |
| power system and distribution and the role of energy | | | | At start-up, the wind turbine uses the smaller, 40-kW |
| storage under the transient conditions caused by load | | | | generator to motor up and bring the induction |
| changes and wind turbine startups. | | | | machine up to speed. Because the wind speed is low, |
| Index Terms - wind turbine, diesel generator, hybrid | | | | the wind turbine operates at low output power, and |
| power system, renewable energy, energy storage. | | | | the local load is set to 200 kW. The diesel engine has |
| | | | | a rated power of 400 kW. At t = 2 s, the wind |
| I. INTRODUCTION | | | | turbine is turned on. As we can see, the voltage dip |
| Windmills were used to pump water and mill grain, | | | | and the frequency dip are not very large, because |
| along with many other uses [1, 2, 3, 4]. | | | | the wind turbine is started using a smaller generator |
| Today, wind turbines are used for similar purposes | | | | |
| (i.e., water or oil pumping, battery charging, and utility | | | | Figure 5. Voltage, frequency, and power to |
| generation). One important aspect of wind turbine | | | | illustratean undersized diesel genset |
| applications, especially in an industrial environment, is | | | | At t = 10 s, the 80-kW water pump is started up. |
| that wind turbines generate electricity without | | | | The startup time for the water pump is longer than |
| creating pollution. Wind turbines are also well suited | | | | that of the wind turbine because the wind turbine is |
| for generating electricity in isolated places with no | | | | started when the rotor speed is close to the |
| connections to the utility grid [2,3,4]. However, in | | | | synchronous speed and the wind turbine also gets |
| isolated applications, especially very small applications, | | | | some help from the wind. The voltage drop is not |
| the power system components (sources and loads) | | | | very significant, but the frequency of the diesel |
| are limited, and the system networks are weak in | | | | drops about 3%. The diesel output power increases |
| many cases. Thus, any changes in the power input or | | | | to cover the real power needed, whereas the |
| output of one component may affect the rest of | | | | contribution from the wind turbine is insignificant |
| the system more dramatically than in a larger system | | | | because the wind is low. For a short time, the |
| where the smoothing effect of many components | | | | induction generator enters the motoring range |
| benefits the overall system. In this paper, we analyze | | | | between t = 10.8 s and t = 11.3 s. After the condition |
| a hybrid power system consisting of a wind turbine, | | | | is restored, at t = 14 s, the additional local load (300 |
| a diesel generator, a local load, and energy storage. | | | | kW noncritical) is turned on, bringing the total load to |
| We also present the impact of energy storage on | | | | 580 kW. Because the diesel can carry only up to 400 |
| the power system performance. The results and | | | | kW and the wind’s contribution is very small at |
| conclusions of this analysis apply to similar hybrid | | | | about 40 kW, the voltage and frequency start |
| power systems. | | | | decreasing, and the voltage and frequency sensors |
| | | | | detect the change. If the frequency drops below |
| | | | | 95% and the voltage drops below 90% for an |
| | | | | elapsed time of 0.5 s, the controller will drop the |
| II. SYSTEM CONFIGURATION | | | | additional load (300 kW) and keep the critical load |
| The system has two types of generation: the diesel | | | | (200 kW) to regain the voltage and frequency. After |
| generator and the wind turbine generator (Figure 1). | | | | the load is shed at t = 14.5 s, the frequency and |
| Theenergy storage can act as a load or as a | | | | voltage eventually return to normal. When the |
| generator depending on the need. The diesel | | | | frequency drops, the wind turbine’s power |
| generator provides smooth output power, whereas | | | | contribution suddenly jumps because of a sudden |
| the output power of a wind turbine depends on the | | | | increase of generating slip. Eventually, the genset |
| wind velocity. As the wind velocity varies, so does is | | | | frequency increases again for a short period and the |
| the power generation. For example, if the wind speed | | | | induction generator enters into the motoring condition |
| changes very smoothly, the output power of the | | | | (between t = 14.5 s and t = 15 s). This condition |
| wind turbine will also change very smoothly. On the | | | | worsens if the mechanical time constant of the wind |
| other hand, wind turbulence causes the output power | | | | turbine rotor (including the blade) is higher than the |
| to fluctuate. Figure 1 is a single line diagram that | | | | diesel genset time constant. In other words, the |
| represents the analyzed power system. The wind | | | | changing of the genset rotor speed is much faster |
| turbine has an induction generator with a capacity | | | | than the changing of the wind turbine rotor speed. |
| ranging from 40 kW to 225 kW. At low wind speeds, | | | | The response to the load change is shown by how |
| the generator operates at 900 rpm with a rated | | | | fast the governor corrects the frequency and how |
| capacity of 40 kW. At high wind speeds, the | | | | fast the generator’s field excitation control |
| generator speed is 1,200 rpm with a rated capacity | | | | reacts to the voltage changes. Undersized diesel |
| of 225 kW. We used 150 kW of energy storage as a | | | | engine with energy storage: As shown in the |
| buffer to operate as a load or a source depending on | | | | previous subsection, an undersized diesel engine |
| the need. This paper discusses only fixed-speed wind | | | | cannot supply all energy needed, and it must shed |
| turbine generation and does not cover variable-speed | | | | some of the non-critical load to retain power-system |
| wind turbine generation [5]. The diesel engine, which | | | | stability. To remedy this situation, a 150-kW energy |
| has a rated capacity of 400 kW, is operated in | | | | storage is installed to bring the combined output of |
| parallel with the wind turbine to supply the load. The | | | | the diesel genset and energy storage up to 550 kW. |
| local loads are mostly residential and light loads. Other | | | | Figure 6 shows the improved power system after |
| loads include water pumps, compressors, and heavy | | | | the energy storage is added. The same simulation is |
| equipment. An 80-kW water pump represents the | | | | performed except it is now equipped with an energy |
| transient condition of a heavy load. | | | | storage. There is a significant improvement in the |
| | | | | frequency regulation after the storage is installed to |
| Fig 1. One line diagram of power system | | | | stabilize the system. The non-critical load (300 kW) |
| | | | | survives even during low wind conditions. The |
| | | | | frequency dips during the wind turbine start-up and |
| III. COMPONENTS OF POWER SYSTEM | | | | the water pump start-up, and when the 300 kW load |
| The system we discuss in this paper consists of four | | | | non-critical load is switched, it is reduced dramatically. |
| major subsystems: a diesel generator, a wind turbine | | | | Obviously, the capability of the energy storage to |
| generator, heavy (industrial) loads, and energy | | | | deliver a large amount of power instantaneously plays |
| storage. In the power system network, the balance | | | | a major role in restoring the frequency of the power |
| of active power and reactive power must be | | | | system. An additional benefit is noticed in the system |
| maintained. The diesel-genset, then, must be able to | | | | voltage behavior of the wind turbine. Because the |
| keep the power balanced when the wind turbine or | | | | change in the frequency deviation presented to the |
| local load varies. This task is easy to accomplish | | | | wind turbine induction generator is small and smooth, |
| provided the diesel genset is sufficiently sized. | | | | the behavior of the stator current at the induction |
| Although they are important, we will not cover the | | | | generator is also smooth. Thus it reduces the Ldi/dt |
| details of the dynamic model for electric machines | | | | and overall voltage drop across the line. |
| used in the simulation. Many good textbooks are | | | | Oversized wind turbine: |
| available on this subject. | | | | When the wind power output exceeds the power |
| A. Diesel Generator | | | | required by the load, the synchronous generator of |
| In terms of an electrical system, a diesel generator | | | | the diesel genset becomes a synchronous motor that |
| can be represented as a prime mover and a | | | | tends to accelerate the rotor speed of the diesel |
| generator. Ideally, the prime mover is capable of | | | | engine. The excess energy from the wind power, |
| supplying any power demand up to rated power at | | | | then, tries to drive the diesel engine. Because the |
| constant frequency, and the synchronous generator | | | | diesel engine has only a small braking capability |
| connected to it must be able to keep the voltage | | | | resulting from engine compression, the frequency |
| constant at any load condition. Figure 2 is a block | | | | control can be lost when the extra power generated |
| diagram of the diesel generator. The diesel engine | | | | by the wind turbine is sufficiently high. |
| keeps the frequency constant by maintaining the | | | | |
| rotor speed constant via its governor. The | | | | Figure 6. Voltage, frequency, and power to |
| synchronous generator must control its output | | | | illustratean undersized diesel genset with storage |
| voltage by controlling the excitation current. Thus, as | | | | In Figure 7, the diesel generator has a rated power |
| a unit, the diesel generating system must be able to | | | | of 400 kW, the local load is initially set to 280 kW |
| control its frequency and its output voltage. The | | | | and at t = 4 s, and the local load is set to 100 kW. |
| inertia of the diesel genset, the sensitivity of the | | | | When the diesel is started, there is only a local load |
| governor, and the power capability of the diesel | | | | of 280 kW. The wind turbine is then started at t = 2 |
| engine all affect the diesel generator’s ability to | | | | s with a 225-kW induction machine. Although the |
| respond to frequency changes. The ability of the | | | | diesel genset is rated at only 400 kW and the wind |
| synchronous generator to control its voltage is | | | | turbine is started with a 225-kW induction machine, |
| affected by the field winding time constant, the | | | | the effect of wind turbine start-up on the power |
| availability of the direct current (DC) power to supply | | | | system is very mild, mostly because the induction |
| the field winding, and the response of the voltage | | | | machine current is limited by a soft start. A soft start |
| control regulation mechanism. | | | | is a device that limits starting current during start-up. |
| | | | | It consists of a pair of back-to-back thyristors |
| Figure 2. Diesel generator control block diagram | | | | installed in series with each phase of the motor |
| | | | | winding. Because the firing angle of the thyristor can |
| | | | | be controlled, the size of the starting current can be |
| B. Wind Turbine | | | | adjusted by controlling the firing angle of the |
| The main components of a wind turbine are the | | | | thyristors. As we can see (Figure 5), the same wind |
| rotor of the turbine, which is the prime mover, and | | | | turbine (225 kW) draws a starting power of 300 kW, |
| an induction generator. In general, the rotor is | | | | but after the soft start is installed (Figure 7), the |
| connected to the generator via a gearbox that | | | | power surge during start-up drops to about 100 kW. |
| matches the rotational speed. The simplest system | | | | After the wind turbine enters generating mode (at |
| uses a fixed-speed turbine. A fixed-speed turbine | | | | about t = 2.5 s), the local load (280 kW) is shared |
| must rely on the blade-stall condition to limit the | | | | between the diesel genset (55 kW) and the wind |
| output power when the winds are at high speed. | | | | turbine (225 kW). The voltage and frequency are |
| Note that, although the rotor speed of an induction | | | | maintained constant, and the diesel genset |
| generator varies with wind speed, the speed range is | | | | |
| within a 1% to 2% slip. On the other hand, the wind | | | | |
| speed variation may range from 5 m/s to 25 m/s; | | | | |
| thus, in terms of the wind turbine, the induction | | | | Figure 7. Voltage, rotor speed, and power of |
| generator operates at a relatively “fixed | | | | anoversize wind turbinegenerates only a small |
| speed” compared to the range of wind speed | | | | percentage of its rated load (about 13%). This makes |
| variation. | | | | a significant contribution to fuel savings from the |
| C. Induction Machines | | | | wind energy. At t = 4 s, the local load is reduced |
| Most electric machines used in industry as prime | | | | from 280 kW to 100 kW; the wind speed stays the |
| movers are induction motors. Two applications of | | | | same. As a result, the wind turbine tries to supply |
| induction machines in the power system network fall | | | | 225 kW, but the only load available is 100 kW. As a |
| within the scope of this study: one as the generator | | | | result, the synchronous generator of the diesel |
| on a wind turbine and the other as a motor driving | | | | genset turns into a motor (negative power), the |
| large pumps and compressors. By its nature, an | | | | governor loses its speed control, and frequency |
| induction machine is an inductive load. This machine | | | | runaway is triggered. This is an example of the wind |
| absorbs reactive power either as a motor or | | | | turbine being oversized compared to the local load. In |
| generator. The reactive power absorbed by the | | | | such a case, a dump load (water heater, water |
| induction machine comes from the line to which it is | | | | pump, battery charger, etc.) is usually deployed to |
| connected. In a hybrid power system, the reactive | | | | keep the diesel genset generating, which prevents it |
| power comes from the synchronous generator of | | | | from motoring. Minimum power generation of the |
| the diesel genset. In a wind turbine generator, a fixed | | | | diesel genset is usually pre-set (for example, |
| capacitor is usually installed to supply some of the | | | | 15%–40% of the rated load). If the generated |
| reactive power that the induction generator needs. | | | | power of the diesel genset is less than the preset |
| Figure 3 shows the equivalent circuit of an induction | | | | value, the dump load should be deployed. The dump |
| machine connected to a power system. The power | | | | load must be sized so that the diesel genset will |
| system is represented by infinite bus Es and the line | | | | always generate power above its minimum set point. |
| impedance is represented by reactance Xs. | | | | The dump loads are normally non-critical loads used |
| | | | | to store excess electrical energy in another form, |
| Figure 3. Equivalent circuit of an induction machine | | | | such as heat (water or space heater), electric charge |
| connected to power system | | | | (battery charging), or potential energy (water pump). |
| D. Various Loads | | | | Oversized wind turbine with energy storage: As |
| In the power system considered, there are two | | | | shown in the previous subsection, an oversized wind |
| major loads. The first is a large water pump | | | | turbine can drive the system into an unstable |
| representing a typical industrial load. The second is a | | | | condition because of the inability of the diesel engine |
| collection of loads for which the size and power | | | | to keep the frequency constant. An energy storage |
| factor can be programmed throughout the day to | | | | installed in the power system network is not only |
| represent a typical village load. The voltage at the | | | | useful to remedy the undersized diesel engine but |
| terminal of the load varies as a result of a voltage | | | | also for cases where there is an excess power |
| drop across the line impedance. The voltage drop | | | | produced by the wind turbine. Without energy |
| across the line impedance varies depending on the | | | | storage, the wind turbine can drive the synchronous |
| size of the current and the power factor of the load. | | | | machine into motoring region and the frequency |
| The terminal voltage for a wind turbine generator | | | | output will be out of control. With a power converter |
| (VS), as the output current of induction machine, | | | | to interface between the energy storage and the |
| varies from start-up to generating mode. During | | | | power network, the energy storage is capable of |
| start-up, voltage drops significantly at the terminal | | | | quickly absorbing excess power generated by the |
| voltage of the induction machine. The voltage drop | | | | wind turbine and hold the generator rotor speed |
| across line impedance is caused by the current surge | | | | from a runaway condition. As shown in Figure 8, the |
| during start-up. In addition, the phase angle of the | | | | frequency runaway can be prevented by using |
| stator current is very large and lagging. The | | | | energy storage to capture the excess power in the |
| combination of a poor power factor and a lagging, | | | | power network. |
| large current surge creates a voltage dip at the | | | | Figure 8. Voltage, rotor speed, and power of |
| terminal of the induction machine during start-up. | | | | anoversize wind turbine with energy storage |
| Thus, a start-up of short duration is preferable to a | | | | B. Case Study II: Charging the Storage Under Normal |
| prolonged one | | | | Condition |
| E. Energy Storage | | | | The energy storage will be charged only when there |
| The energy storage can be of different types (i.e. | | | | is an energy surplus from the wind and the required |
| flywheel, battery, hydrogen/fuel-cell, hydropower | | | | network load is very light. Because the governor of |
| etc.). In this paper, we assumed energy storage with | | | | the diesel generator will always maintain the |
| a power converter interface to the power network. | | | | frequency constant, the output power of the diesel |
| The power converter is connected to the energy | | | | generator is an indicator of the power within the |
| storage at one end. With variability of wind resource, | | | | system available to charge the energy storage. One |
| energy storage is an excellent contributor to the | | | | benefit of charging the energy storage during this |
| power system. The energy storage behaves like a | | | | condition is that the efficiency of the diesel engine is |
| large buffer to accommodate the unequal | | | | at its peak when it is operated near its rated power. |
| instantaneous energy in the power system. Ideally, at | | | | Thus, when a surplus of power is detected within the |
| any instant of time, there should be a zero net | | | | system, the energy storage will be charged and |
| exchange between the energy sources and the | | | | some energy will be stored within the system. The |
| energy sinks (both real and reactive power). If this | | | | amount of energy and the size of charging power |
| balance is not achieved, the voltage and frequency | | | | depend on the size of the surplus power. The |
| of the system changes to maintain equilibrium. At any | | | | charging process will be stopped when the energy |
| instant, the energy storage behaves either as an | | | | storage reaches its limit. Maximum charging current is |
| energy source or energy sink depending of the mode | | | | also limited by the energy storage and by the power |
| of operation. | | | | converter interface. Figures 9 shows the charging |
| Figure 4. Energy Storage control block diagram | | | | process. Initially there is enough wind speed to start |
| | | | | the wind turbine. The diesel generator is supplying a |
| It is assumed that the energy storage has a power | | | | constant load of 280 kW (power factor = 0.995 |
| converter interfacing the power network. Although it | | | | lagging) all the time. As the wind turbine generates full |
| is possible for the power converter to function as a | | | | power (225kW), the diesel governor redistributes the |
| reactive power compensator, the cost of a power | | | | load and there is a load sharing between the wind |
| converter is very expensive compared to other | | | | turbine and the diesel generator. As the transient |
| means of reactive power compensation currently | | | | settles out, it is shown that the diesel generator is |
| available in the market. Keep in mind that the size of | | | | contributing a very small amount of power to the |
| the power semiconductor in the power converter is | | | | load, thus the charging mechanism is started. The |
| limited by its current limit and its voltage limit. Thus, | | | | energy storage is charged slowly until it reaches its |
| minimizing the current passing through the power | | | | limit. |
| switches will minimize the current rating of the power | | | | |
| converter and will lower the cost. For this paper, we | | | | |
| only used the power converter to process real | | | | Figure 9. Real power flow in the power system |
| power in and out of the energy storage. Figure 4 | | | | |
| shows a block diagram of energy storage control | | | | In Figure 9, the charging of energy storage during |
| algorithm. It uses frequency deviation to indicate a | | | | normal condition is limited to 75 kW, which is about |
| real power imbalance in the system. The frequency | | | | 50% of the rated power of the capacitor. This limit |
| deviation is also used as the feedback to control the | | | | ensures that the power converter still has enough |
| energy storage output. If the load power demand is | | | | headroom to deliver or absorb power during an |
| higher than the power supply available, the frequency | | | | emergency. For example, if there is some loss of the |
| of the diesel generator will slowly drop. Other energy | | | | loads in the power systems, the energy storage |
| stored in the system includes the kinetic energy in | | | | must absorb the loads loss to avoid a sudden change |
| the turbine blades, the diesel generator inertia, and | | | | in frequency. Similarly, to compensate for a sudden |
| energy in the inductors and capacitors, etc. | | | | load increase to the power systems (e.g. the water |
| | | | | pump is started), the energy storage must release |
| F. Balance of Energy in the System | | | | energy to the power system to keep constant |
| In the isolated system we studied, the balance of | | | | frequency at the diesel generator. As shown in Figure |
| real and reactive power must always be maintained. | | | | 9, the real power used by the energy storage to |
| The balance of real power is maintained by the | | | | stabilize the frequency takes precedence over the |
| governor of the diesel generator. The balance of | | | | charging power used to charge the storage. This can |
| reactive power is maintained by the exciter of the | | | | be seen especially when the water pump is started |
| diesel’s synchronous generator. When the load | | | | at about t = 15 seconds. |
| demands more power than the diesel and the wind | | | | V. CONCLUSION |
| turbine can produce, and the diesel engine has | | | | After presenting an overview of the components of |
| reached its highest limit, as the loads continue to | | | | the power system under investigation, we described |
| increase, the governor of the diesel cannot push | | | | the operating characteristics of the components as |
| more power, and the rotor speed of the diesel will | | | | they relate to voltage and frequency variations in the |
| start to drop. The frequency of the generator will | | | | power network. The analysis shows the dynamic |
| then drop until balance is reached or the system | | | | interaction among the wind turbine, diesel engine, |
| collapses. The voltage in the system is also an | | | | large loads, and energy storage. It also demonstrates |
| indicator of the balance in the system. When the | | | | the dynamics of real power balance and how the |
| reactive power demand from the loads is higher than | | | | system is stabilized with the controlled energy |
| what can be provided by the diesel generator, the | | | | storage. The voltage regulation is very minimal and |
| capacitor, and other means of compensation, the | | | | the frequency regulation is controlled very closely. |
| system voltage will drop. Although the size of output | | | | The voltage regulation is controlled mostly by the |
| and input of the energy storage is adjustable, it is | | | | balance of reactive power in the system and the |
| limited by its ratings. For this paper, we assumed that | | | | time constant of the excitation system of the |
| the energy storage is capable of storing and | | | | generator. The frequency regulation depends on the |
| providing long-term energy to the power network to | | | | energy storage control, the size of the energy |
| maintain system balance. In reality, only a limited | | | | storage, the total inertia in the system (temporary |
| amount of energy can be stored. We will not discuss | | | | energy storage).Many technical solutions can be |
| energy analysis in detail in this paper. In practice, the | | | | implemented to remedy the shortcomings covered in |
| energy will be stored when the wind turbine | | | | this paper. However, as in any power generation |
| produces enough power and the diesel is operating | | | | system, the economic implications of the solutions |
| under light load. The actual loads are divided into | | | | must be carefully considered. |
| critical and non-critical loads. Critical loads are supplied | | | | REFERENCES |
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