ARTICLE Effect of Load Changes on Generator Set Stability

Study Case Effect of Load Changes on Generator Set Stability Michael Kurniawan (2018123456) University of Cincinnati, 2600 Clifton Ave, Cincinnati, OH 45221, United States michaelkurniawan2019@gmail.com Abstract A generator is an electrical device that functions to convert mechanical energy into electrical energy through the principle of electromagnetic induction. Generator sets have an important role as a backup power source to ensure continuity of energy supply in the event of an outage from the main grid. When the main electricity supply stops, the generator set will be activated to supply power to the existing load. However, the voltage generated by the generator set is not always stable because it is influenced by various factors, one of which is the characteristics of the load being served. The constantly changing load causes fluctuations in the performance of the generator set, which has an impact on changes in magnetic flux, excitation current, and output voltage. Therefore, the operational parameters of the generator set must remain within predetermined limits so that the system can function properly and the continuity of electricity supply is maintained. Keywords: Generator Set, Backup Power Generation, Electrical Energy, Electromagnetic Induction, Load Fluctuation INTRODUCTION The generator set used is a type of synchronous generator that is operated when there is a disruption in the main source to provide electrical energy for the load. The load received by the generator set is not always constant, but changes dynamically, so that its operational parameters also fluctuate. As a result of these changes, the generator set will adjust its performance to keep operating optimally, one of which is by adjusting the excitation current to maintain output voltage stability. LITERATURE REVIEW Voltage Fluctuations in Electric Power Systems Voltage in the power system network can experience changes due to several factors, including: A. Current flow in the network. MICHAEL KURNIAWAN 1 Study Case When current flows in the electric power network, there will be a voltage drop, so that the voltage received on the load side becomes lower. This is caused by the impedance of the conductor which consists of resistance and reactance. The reactance can be inductive or capacitive, depending on the dominant characteristics in the network. The amount of voltage drop that occurs can be explained through the following illustration. Figure 1. A simple circuit of an electrical power system Figure 1 description: 1. Voltage send (generator) 2. Voltage receive (load). From figure 1, the amount of voltage drop is: Description : ∆V = Voltage drop. I = current flowing in the conductor wire. Z = line wire impedance. Z = R + j X. With the voltage drop, the voltage supplied to the load will change, namely: Description : Vb = voltage on the load side. Vi = source voltage. From equation (2), it can be said that the voltage on the load side will be affected by the voltage drop on the line, which means it will be affected by the current flowing in the line. B. Load switching process. When switching or the process of connecting and disconnecting loads occurs, there are two main phenomena in the electrical system: 1. A sudden change in current, which causes a sudden drop in voltage. As a result, the voltage in the network fluctuates, especially when the load continues to change dynamically. 2. The formation of armature reaction in generators. MICHAEL KURNIAWAN 2 Study Case A synchronous generator, also known as an alternator, will experience current flow in its stator coils when under load. This current results in an armature reaction, where the flux generated by the load current (ϕA) interacts with the rotor field flux (ϕf), thus forming the resultant flux (ϕR). This interaction is referred to as the armature reaction, and its characteristics depend on the type of load connected to the generator. 1. The armature current (𝐼 ) is in phase with the induced emf (𝐸) → The type of load is pure resistance (resistive), so the armature flux (Φ𝐴 ) is perpendicular to the rotor field flux (Φ𝑓). This condition is depicted in Figure 2(a). 2. The armature current (𝐼) precedes the emf (𝐸) by an angle of 𝜃 → Capacitive type of load, so that the armature flux (Φ𝐴) lags behind by an angle (90 ∘ - 𝜃) to the rotor field flux (Φ𝑓). This condition is illustrated in Figure 2(b). 3. The armature current (𝐼 𝑎) precedes the emf (𝐸) by an angle of 90 ∘ → Pure capacitive load type, so the armature flux (Φ𝐴) amplifies the rotor field flux (Φ𝑓), which causes a greater magnetizing effect. This is shown in Figure 2(c). 4. The armature current (𝐼𝑎) lags 90∘ against the emf (𝐸) → Pure inductive load type, so the armature flux (Φ𝐴) weakens the rotor field flux (Φ𝑓), resulting in a demagnetizing effect. This condition is depicted in Figure 2(d). Figure 2. Anchor Reaction Based on Figure 2, it can be seen that the armature reaction in a synchronous generator is influenced by the type of load it serves, which depends on the phase angle between the armature current (𝐼𝑎) and the induced voltage (𝐸). This anchor reaction occurs due to the formation of two magnetic fields in the generator, namely the magnetic field in the field coil and the magnetic field in the armature coil. The magnetic field in the field coil is generated by the excitation current, while the magnetic field in the armature coil is formed due to the flow of load current in the coil. For example, a synchronous generator with two poles will produce two types of magnetic fields, as shown in the following figure: MICHAEL KURNIAWAN 3 Study Case Figure 3. Magnetic field in a generator. Description of figure 3: φA = load flux. φf = field fluctuation. Figure 3 shows the condition when the generator load is inductive, that is, the load contains both resistive and inductive components. In this condition, the direction of the magnetic field in the stator and rotor is opposite to the angle α. This α angle is an obtuse angle with a magnitude of more than 90°, which causes the resultant between the field flux and the armature flux to be smaller, as shown in the following figure. Figure 4. The resultant vector between φA and φf. The magnitude of the magnetic field in the form of resultant magnetic flux that will generate the generator output voltage, is : Description : φt = total generator fluction. φf = excitation field fluction. φA = armature induction generated by the load current. With the change in flux due to the load current, the generator voltage generation will also change. If the voltage state is taken at point A located in the armature coil, then: The effective value of EA is, Description : EA = voltage generated at point A in the armature coil. By assuming that the number of coil turns is constant, the voltage equation can be written as: MICHAEL KURNIAWAN 4 Study Case With the value of k is constant, namely : The frequency of the voltage produced by the generator depends on the rotor rotation speed, as expressed in the following equation: When a load is connected to the generator, the load current will produce flux opposite to the field flux, so that the field flux decreases. As a result, the rotor experiences an increase in mechanical load, causing the rotation speed to drop. This decrease in rotation will have an impact on the decrease in voltage frequency according to equation (7), which then causes the generator output voltage to decrease as stated in equation (5). In this condition, the generator is in a critical state and requires adjustment action. As a countermeasure, the generator will respond by opening the governor on the prime mover to restore the rotation speed to normal conditions. In addition, the excitation current will be increased to return the field flux to the initial value, so that the output voltage is stabilized again. Conversely, in the event of a sudden load shedding, the force holding the rotor decreases due to a decrease in load current, so the rotor speed increases. This increase in rotation causes an increase in the generator voltage frequency, which results in an increase in the output voltage. In this condition, the generator will respond by closing the governor, so that the prime mover speed is reduced and the voltage returns to normal conditions. C. Disturbances in the power system. Disturbances in the network that often occur are single-phase disturbances to the ground, where during disturbance conditions, the voltage of the disturbed phase will drop and the current will increase, while the voltage of the disturbed phase will remain normal if the neutral point is grounded. The magnitude of this fault current will cause the generator to experience heavy and unbalanced loads, so that the generator rotation will decrease, which will result in a decrease in electrical frequency. If equation (8) is fused with equation (7), the following equation will be obtained: MICHAEL KURNIAWAN 5 Study Case RESEARCH METHOD The research was conducted at the Electrical Machinery Laboratory of the University of Cincinnati School of Engineering, with a sample of a gasoline generator set with a power of 1500 watts. 1. Data requirements. In this study, generator data is required, which includes information from the generator nameplate and operational data. The data listed on the generator nameplate are as follows: - Operating voltage, phase and frequency. - Generator power. - Number of poles. - Rotation. - Maximum current. - Excitation current and voltage. - Work factor. In addition to the generator nameplate data, physical data from the generator is also needed, namely: - Rotor diameter - Rotor length - Pole gap width - Stator circle diameter - Thickness of stator core - Stator core length - Number of rotor turns - Number of stator windings For generator operational data, information on generator load, generator current, and generator excitation data within a certain time span is required. In addition, it is necessary to measure the generator stator resistance directly using a high-sensitivity measuring instrument, such as a high-precision digital Ohm-meter, to ensure the accuracy of the measurement results. 2. Generator Impedance. The impedance of the generator can be determined by using the known generator impedance percent, or it can also be done by calculation, which is as follows: The generator has a power coil on its stator side, where the coil has resistance and MICHAEL KURNIAWAN 6 Study Case reactance. The stator coil of the generator is analogous to a single-phase coil as follows: Figure 5. Analogy of the power coil on the generator. From the coil in Figure 5 above, the inductance of the coil can be determined, namely: N = number of turns of the coil. m = permiability of the coil core. A = cross-sectional area of the coil core. l = coil length. Tus the amount of reactance caused by the inductance of the generator is: phi = 3.14. (constant). f = electrical frequency = 50 Hz. L = coil inductance. So that equation (11) will be as follows: The impedance of the generator is: Then it will be obtained: 3. Generator Work Factor The value of the generator work factor (cosθ) can be determined using the following equation: MICHAEL KURNIAWAN 7 Study Case 4. Magnetic Flux Since the generator has an excitation system, the coil inductance will be directly related to the magnetic flux. As a result, the coil inductance value can be expressed as follows: The amount of flux (ϕ) on the excitation side of the generator depends on the excitation current, which is expressed in the following equation: Thus, equation (15) can be rewritten in the following form: Furthermore, the equation for generator reactance is obtained as follows: Where: N = Number of turns of the armature coil. Nf = Number of turns of the field coil. I = Anchor current. If = Field current (excitation). R = Field coil core reluctance 5. Magnetic Flux When a Shock Load Occurs When a shock load occurs, the load current increases suddenly, causing the opposing flux in the network to increase significantly. This results in a decrease in the flux generated by the excitation current. The amount of flux produced by the generator under no-load conditions can be expressed as follows: While the magnetic flux formed due to the load current is expressed as follows: With: 𝑁𝑠 = Number of stator windings 𝐼𝑏 = Load current 𝑅𝑠 = Stator core reluctance MICHAEL KURNIAWAN 8 Study Case ℓ𝑠 = Length of stator core track 𝐴𝑠 = Cross-sectional area of the stator core Thus, the total flux contained in the generator core can be calculated using the following equation: 6. Increase in Excitation Current During Shock Load Flux in the generator core increases according to equation (18). If the flux in the stator is converted to the rotor or exciter side, the amount of additional excitation current required to restore the generator output voltage to normal conditions can be calculated. The equation is as follows: If1 = Additional excitation current to overcome voltage drop due to load shock. 7. Voltage Dip When Shock Load Occurs When a load shock occurs, the current absorbed by the load increases suddenly, which causes a voltage drop at the generator output. Figure 6. Generator voltage dip occurrence This voltage drop is accompanied by a decrease in the frequency and rotation speed of the generator. This voltage dip condition can be described as follows: With: 𝐼𝑏 = Load current 𝑍𝑔 = Generator impedance MICHAEL KURNIAWAN 9 Study Case The generator output voltage per phase when a voltage dip occurs is expressed as: 𝑉𝑜𝑢𝑡 = Generator output voltage per phase during voltage drop 𝑉𝑛 = Nominal voltage of the generator per phase RESULTS AND DISCUSSION 1. Data. Generator set data as a research sample, namely generator nameplate and generator operational data. The generator nameplate data is as follows: - Operating voltage/power: 220 Volts / 1500 watts. - Phase / frequency / pole: 1 / 50 Hz / 4 - Round / pf: 1500 rpm / unity. - Nominal current: 7.2 Amperes. - Excitation voltage/current: 35 Volts / 0.8 amperes. In addition to the generator nameplate data, the workshop data of the generator is also given, namely: - Rotor diameter/length: 8.5 cm / 9.2 cm - Diameter / thickness of the stator core: 11.5 cm / 4.85 cm. - Stator length/air gap: 9.2 cm / 1.5 cm. - Number of stator/rotor windings: 184 / 32 For generator operational data, given as follows: The results of measuring the resistance of the generator coil, namely: Stator coil resistance: Rs = 24.63 Ω. Excitation coil resistance: Re = 2.86 Ω. Measurement results Excitation current without load: If = 0.15 Ampere. Measurement results of excitation current with 60% load: If(60) = 0.48 amperes. Measurement results of load current when the generator is operating: Ib(60) = 5.8 amperes. 2. Results of Genset Parameter Calculation during load shock. Based on the formulation in the research method, the calculation results are obtained in table 2, table 2, and table 3. MICHAEL KURNIAWAN 10 Study Case Table 1. Calculation results of load current and generator fluction. ∆ (Weber)  T (Weber) 0,72 0,00025939 0,00026879 20 1,44 0,00051880 0,00052820 3 30 2,16 0,00077830 0,00078770 4 40 2,88 0,00103760 0,00104700 5 50 3,60 0,00129720 0,00130660 6 60 4,32 0,00156600 0,00156600 7 70 5,04 0,00181600 0,00182540 8 80 5,76 0,00192512 0,00193452 If ∆If (ampere) No %load Ib (Ampere) 1 10 2 o (Weber) 0,00000940 Table 2. Calculation results of excitation current increase No Initial Load If0 (Ampere) Final Load Akhir(Ampere) 1 10% 0,15 0,49 2 20% 0,18 0,46 3 30% 0,21 4 40% 0,26 0,38 5 50% 0,31 0,33 6 60% 0,35 0,29 7 70% 0,42 0,22 8 80% 0,51 0,13 80 % 0,64 0,43 Table 3. Calculation results dip voltage (∆V). No Initial Load ∆V (Volt) Final Load ∆V (Volt) Selisih ∆V akhir (Volt) 1 10% 6,100 35,1 2 20% 10,90 30,3 3 30% 13,60 4 40% 18,00 23,2 5 50% 23,80 17,4 6 60% 29,50 11,7 7 70% 33,90 7,26 8 80% 41,20 0 MICHAEL KURNIAWAN 80 % 11 41,2 27,6 Study Case To perform the analysis, a provision is made as an assumption that the maximum load of the generator is 80% of its full load, because we are based on the generator efficiency of 80%. For a normally loaded generator of 60% of its full load, if the load fluctuation is 10% at each point, and applied to table 1, it can be seen that the total flux that occurs in the iron core will rise close to linear and is considered linear. This means that if there is a sudden shock to the load to 80%, the total induction will rise significantly. The increase in total reduction is accompanied by an increase in excitation current as shown in table 2. If the generator receives a sudden surge in load, i.e. from no-load condition directly to 80% load, then based on tables 1 and 2, a sudden increase in flux can be observed. This causes the generator flux to increase drastically from 0.00000940 Wb to 0.00193452 Wb, which results in a voltage drop of 41.2 Volts or about 18.73%. As a result, the generator output voltage drops to 178.8 Volts. This condition can cause other loads to not operate normally or even not function at all. To overcome this, the generator will respond by increasing the excitation current to restore the voltage to the normal level, which is 220 Volts. CONCLUSION 1. When there is a sudden load spike from 0% to 80%, the magnetic flux increases significantly to 0.00193452 Wb. This caused a voltage drop of 41.2 Volts, while the excitation current increased to 5.76 amperes to stabilize the voltage. 2. The voltage drop due to the load surge of 80% reached 41.2 Volts or 18.73%, so the generator output voltage dropped to 178.8 Volts. As a result, the generator cannot optimally serve the load because the voltage is below the permissible limit. Nonetheless, this drop is only momentary, as the generator immediately responds by increasing the excitation current to normalize the output voltage. However, there is a time lag in the voltage recovery process, so if the load is sensitive, it may trip. MICHAEL KURNIAWAN 12 Study Case REFERENCE MICHAEL KURNIAWAN 13