Page 1 Parallel Operation of Generators 1 Chapter 4: Parallel Operation of Generators In modern power systems isolated generators are very rare. Power systems are highly interconnected and many generators share the load. The first problem of an engineer is connecting a synchronous generator on an existing bus. Generator 1 Generator 2 System Load 3 phase switch Synchronizing lamps Figure 4-1 CONNECTING a GENERATOR to a BUS The above figure 4-1 illustrates a generator G1 which is already connected to a power grid under load. Generator 2 has to be connected or “brought on line” 1. The prime mover of the generator has to bring the speed of the shaft close to the rated speed of the generator. 2. The excitation of the generator has to be increased to give a no-load output voltage as close as possible to the existing bus voltage We want to create a phasor rotation for generator 2 output similar to the bus voltage phasor. 3. Observe the lights which are connected across the switches: they should beat, first get brighter and then dim as the phasors for generator and bus respectively shift. If the 3 lights beat concurrently, the phase sequence is correct, else if lights beat out of phase, one pair of phases should be reversed. 4. Adjust now the prime mover to slowly increase/decrease the speed of generator 2. One should observe a slow beat of the light brightness. 5. When the lamps beat slowly, the switches should be closed when the lights are extinguished (line-line voltage at minimum). Page 2 Parallel Operation of Generators 1 Chapter 4: Parallel Operation of Generators In modern power systems isolated generators are very rare. Power systems are highly interconnected and many generators share the load. The first problem of an engineer is connecting a synchronous generator on an existing bus. Generator 1 Generator 2 System Load 3 phase switch Synchronizing lamps Figure 4-1 CONNECTING a GENERATOR to a BUS The above figure 4-1 illustrates a generator G1 which is already connected to a power grid under load. Generator 2 has to be connected or “brought on line” 1. The prime mover of the generator has to bring the speed of the shaft close to the rated speed of the generator. 2. The excitation of the generator has to be increased to give a no-load output voltage as close as possible to the existing bus voltage We want to create a phasor rotation for generator 2 output similar to the bus voltage phasor. 3. Observe the lights which are connected across the switches: they should beat, first get brighter and then dim as the phasors for generator and bus respectively shift. If the 3 lights beat concurrently, the phase sequence is correct, else if lights beat out of phase, one pair of phases should be reversed. 4. Adjust now the prime mover to slowly increase/decrease the speed of generator 2. One should observe a slow beat of the light brightness. 5. When the lamps beat slowly, the switches should be closed when the lights are extinguished (line-line voltage at minimum). Parallel Operation of Generators 2 Recap: • Phase sequence must be the same • Voltages must have same magnitude • Frequency must be the same • Phasors must be aligned Note that in modern installations a ”synchroscope” is used. The synchroscope will instruct the governor of the prime mover to set the speed, and instruct the exciter to produce a voltage. When the phasors are detected within 5 degrees match, the synchroscope will close the switch. PARALLEL OPERATION of GENERATORS Figure 4-2 When the prime mover of a generator is set to deliver a certain power on the shaft, and the voltage is set to deliver that power to an electrical load, a certain operating point is reached [speed, Voltage, Power]. If the load increases, the generator speed (governor) will decrease (not enough power to move the shaft). Hence we can see the typical prime mover/governor characteristic. The characteristic starts at the “no load speed”, and droops. The droop rate is a parameter of the generator: rated load full load no P f f P f GD - = = ? ? [4-1] Since the power is related to the speed, a very useful formula is used as: ( ) sys nl p output f f S P - = [4-2] Where: S p is the slope of the curve in kW/Hz f nl is the no-load frequency of the generator f sys is the operating frequency of the system This shows that the power generated by a generator is a function of its frequency (or speed). Example: a single generator’s characteristic is 1MW/Hz and its no-load frequency is 61Hz. What is the load connected when the bus frequency is 60Hz? 60Hz 3600 62 500 Power(kW) Nominal F(Hz) Speed(rpm) No load speed (frq) Page 3 Parallel Operation of Generators 1 Chapter 4: Parallel Operation of Generators In modern power systems isolated generators are very rare. Power systems are highly interconnected and many generators share the load. The first problem of an engineer is connecting a synchronous generator on an existing bus. Generator 1 Generator 2 System Load 3 phase switch Synchronizing lamps Figure 4-1 CONNECTING a GENERATOR to a BUS The above figure 4-1 illustrates a generator G1 which is already connected to a power grid under load. Generator 2 has to be connected or “brought on line” 1. The prime mover of the generator has to bring the speed of the shaft close to the rated speed of the generator. 2. The excitation of the generator has to be increased to give a no-load output voltage as close as possible to the existing bus voltage We want to create a phasor rotation for generator 2 output similar to the bus voltage phasor. 3. Observe the lights which are connected across the switches: they should beat, first get brighter and then dim as the phasors for generator and bus respectively shift. If the 3 lights beat concurrently, the phase sequence is correct, else if lights beat out of phase, one pair of phases should be reversed. 4. Adjust now the prime mover to slowly increase/decrease the speed of generator 2. One should observe a slow beat of the light brightness. 5. When the lamps beat slowly, the switches should be closed when the lights are extinguished (line-line voltage at minimum). Parallel Operation of Generators 2 Recap: • Phase sequence must be the same • Voltages must have same magnitude • Frequency must be the same • Phasors must be aligned Note that in modern installations a ”synchroscope” is used. The synchroscope will instruct the governor of the prime mover to set the speed, and instruct the exciter to produce a voltage. When the phasors are detected within 5 degrees match, the synchroscope will close the switch. PARALLEL OPERATION of GENERATORS Figure 4-2 When the prime mover of a generator is set to deliver a certain power on the shaft, and the voltage is set to deliver that power to an electrical load, a certain operating point is reached [speed, Voltage, Power]. If the load increases, the generator speed (governor) will decrease (not enough power to move the shaft). Hence we can see the typical prime mover/governor characteristic. The characteristic starts at the “no load speed”, and droops. The droop rate is a parameter of the generator: rated load full load no P f f P f GD - = = ? ? [4-1] Since the power is related to the speed, a very useful formula is used as: ( ) sys nl p output f f S P - = [4-2] Where: S p is the slope of the curve in kW/Hz f nl is the no-load frequency of the generator f sys is the operating frequency of the system This shows that the power generated by a generator is a function of its frequency (or speed). Example: a single generator’s characteristic is 1MW/Hz and its no-load frequency is 61Hz. What is the load connected when the bus frequency is 60Hz? 60Hz 3600 62 500 Power(kW) Nominal F(Hz) Speed(rpm) No load speed (frq) Parallel Operation of Generators 3 The power generated therefore is: () kW Hz kW P output 1000 60 61 1 1000 = - = If one connects another 1000kW load to the bus what is the frequency drop? Hz Hz kW kW S P f f p nl sys 59 / 1000 2000 61 = ÷ ø ö ç è æ - = - = In order to bring the system frequency back to 60Hz: () 60 1 1000 2000 - = nl f Hz kW kW >> Hz f nl 62 = and the governor has to increase its no-load set point to 62Hz If two generator characteristics are shown, and they are connected in parallel on the same bus, they must have the same frequency of operation, hence the operating point. In figure4-3 we can see that Generator A delivers twice the power of generator B. PB PA frq Fixed Frequency Figure 4-3 In order to change the power in a generator for a given frequency of operation, one has to change the prime mover (change the value of the no-load frequency, or set point). Changing the governor will cause the characteristic to move up and down with the same slope. NOTE: if the governor and exciter are unchanged, any change of speed of one generator will cause a circulating current between the 2 machines in such a way as to oppose the change, hence it is called a “synchronizing torque”. This torque can be enormous and will always make sure that the machines are in synchronism (same frequency). Page 4 Parallel Operation of Generators 1 Chapter 4: Parallel Operation of Generators In modern power systems isolated generators are very rare. Power systems are highly interconnected and many generators share the load. The first problem of an engineer is connecting a synchronous generator on an existing bus. Generator 1 Generator 2 System Load 3 phase switch Synchronizing lamps Figure 4-1 CONNECTING a GENERATOR to a BUS The above figure 4-1 illustrates a generator G1 which is already connected to a power grid under load. Generator 2 has to be connected or “brought on line” 1. The prime mover of the generator has to bring the speed of the shaft close to the rated speed of the generator. 2. The excitation of the generator has to be increased to give a no-load output voltage as close as possible to the existing bus voltage We want to create a phasor rotation for generator 2 output similar to the bus voltage phasor. 3. Observe the lights which are connected across the switches: they should beat, first get brighter and then dim as the phasors for generator and bus respectively shift. If the 3 lights beat concurrently, the phase sequence is correct, else if lights beat out of phase, one pair of phases should be reversed. 4. Adjust now the prime mover to slowly increase/decrease the speed of generator 2. One should observe a slow beat of the light brightness. 5. When the lamps beat slowly, the switches should be closed when the lights are extinguished (line-line voltage at minimum). Parallel Operation of Generators 2 Recap: • Phase sequence must be the same • Voltages must have same magnitude • Frequency must be the same • Phasors must be aligned Note that in modern installations a ”synchroscope” is used. The synchroscope will instruct the governor of the prime mover to set the speed, and instruct the exciter to produce a voltage. When the phasors are detected within 5 degrees match, the synchroscope will close the switch. PARALLEL OPERATION of GENERATORS Figure 4-2 When the prime mover of a generator is set to deliver a certain power on the shaft, and the voltage is set to deliver that power to an electrical load, a certain operating point is reached [speed, Voltage, Power]. If the load increases, the generator speed (governor) will decrease (not enough power to move the shaft). Hence we can see the typical prime mover/governor characteristic. The characteristic starts at the “no load speed”, and droops. The droop rate is a parameter of the generator: rated load full load no P f f P f GD - = = ? ? [4-1] Since the power is related to the speed, a very useful formula is used as: ( ) sys nl p output f f S P - = [4-2] Where: S p is the slope of the curve in kW/Hz f nl is the no-load frequency of the generator f sys is the operating frequency of the system This shows that the power generated by a generator is a function of its frequency (or speed). Example: a single generator’s characteristic is 1MW/Hz and its no-load frequency is 61Hz. What is the load connected when the bus frequency is 60Hz? 60Hz 3600 62 500 Power(kW) Nominal F(Hz) Speed(rpm) No load speed (frq) Parallel Operation of Generators 3 The power generated therefore is: () kW Hz kW P output 1000 60 61 1 1000 = - = If one connects another 1000kW load to the bus what is the frequency drop? Hz Hz kW kW S P f f p nl sys 59 / 1000 2000 61 = ÷ ø ö ç è æ - = - = In order to bring the system frequency back to 60Hz: () 60 1 1000 2000 - = nl f Hz kW kW >> Hz f nl 62 = and the governor has to increase its no-load set point to 62Hz If two generator characteristics are shown, and they are connected in parallel on the same bus, they must have the same frequency of operation, hence the operating point. In figure4-3 we can see that Generator A delivers twice the power of generator B. PB PA frq Fixed Frequency Figure 4-3 In order to change the power in a generator for a given frequency of operation, one has to change the prime mover (change the value of the no-load frequency, or set point). Changing the governor will cause the characteristic to move up and down with the same slope. NOTE: if the governor and exciter are unchanged, any change of speed of one generator will cause a circulating current between the 2 machines in such a way as to oppose the change, hence it is called a “synchronizing torque”. This torque can be enormous and will always make sure that the machines are in synchronism (same frequency). Parallel Operation of Generators 4 CHANGES of OPERATING PARAMETERS Assume a generator is connected to an INFINITE BUS. This means that the bus has a CONSTANT FREQUENCY and a CONSTANT VOLTAGE. Furthermore it can absorb power (active and reactive) and can provide power (active and reactive) as needed. Assume an operating point (speed/excitation) of the governor and exciter which delivers an active power that is the power delivered by the prime mover (minus losses) wt = P mechanical power provided on the shaft If the excitation i f produces E so that the power factor is unity: E I jXI Pactive V Figure 4-4 The excitation remains constant and the prime mover increases the torque, hence the power output increases E jXI V I Pactive NEW Figure 4-5 It can be seen that as the power increases at the prime mover the internal angle increases and therefore I increases also. At the same time the current starts to lead, which means that the generator also provides excess of reactive power. If one wants to bring the power factor back (without touching the prime mover), one would have to decrease the excitation accordingly as shown in the next figure: Page 5 Parallel Operation of Generators 1 Chapter 4: Parallel Operation of Generators In modern power systems isolated generators are very rare. Power systems are highly interconnected and many generators share the load. The first problem of an engineer is connecting a synchronous generator on an existing bus. Generator 1 Generator 2 System Load 3 phase switch Synchronizing lamps Figure 4-1 CONNECTING a GENERATOR to a BUS The above figure 4-1 illustrates a generator G1 which is already connected to a power grid under load. Generator 2 has to be connected or “brought on line” 1. The prime mover of the generator has to bring the speed of the shaft close to the rated speed of the generator. 2. The excitation of the generator has to be increased to give a no-load output voltage as close as possible to the existing bus voltage We want to create a phasor rotation for generator 2 output similar to the bus voltage phasor. 3. Observe the lights which are connected across the switches: they should beat, first get brighter and then dim as the phasors for generator and bus respectively shift. If the 3 lights beat concurrently, the phase sequence is correct, else if lights beat out of phase, one pair of phases should be reversed. 4. Adjust now the prime mover to slowly increase/decrease the speed of generator 2. One should observe a slow beat of the light brightness. 5. When the lamps beat slowly, the switches should be closed when the lights are extinguished (line-line voltage at minimum). Parallel Operation of Generators 2 Recap: • Phase sequence must be the same • Voltages must have same magnitude • Frequency must be the same • Phasors must be aligned Note that in modern installations a ”synchroscope” is used. The synchroscope will instruct the governor of the prime mover to set the speed, and instruct the exciter to produce a voltage. When the phasors are detected within 5 degrees match, the synchroscope will close the switch. PARALLEL OPERATION of GENERATORS Figure 4-2 When the prime mover of a generator is set to deliver a certain power on the shaft, and the voltage is set to deliver that power to an electrical load, a certain operating point is reached [speed, Voltage, Power]. If the load increases, the generator speed (governor) will decrease (not enough power to move the shaft). Hence we can see the typical prime mover/governor characteristic. The characteristic starts at the “no load speed”, and droops. The droop rate is a parameter of the generator: rated load full load no P f f P f GD - = = ? ? [4-1] Since the power is related to the speed, a very useful formula is used as: ( ) sys nl p output f f S P - = [4-2] Where: S p is the slope of the curve in kW/Hz f nl is the no-load frequency of the generator f sys is the operating frequency of the system This shows that the power generated by a generator is a function of its frequency (or speed). Example: a single generator’s characteristic is 1MW/Hz and its no-load frequency is 61Hz. What is the load connected when the bus frequency is 60Hz? 60Hz 3600 62 500 Power(kW) Nominal F(Hz) Speed(rpm) No load speed (frq) Parallel Operation of Generators 3 The power generated therefore is: () kW Hz kW P output 1000 60 61 1 1000 = - = If one connects another 1000kW load to the bus what is the frequency drop? Hz Hz kW kW S P f f p nl sys 59 / 1000 2000 61 = ÷ ø ö ç è æ - = - = In order to bring the system frequency back to 60Hz: () 60 1 1000 2000 - = nl f Hz kW kW >> Hz f nl 62 = and the governor has to increase its no-load set point to 62Hz If two generator characteristics are shown, and they are connected in parallel on the same bus, they must have the same frequency of operation, hence the operating point. In figure4-3 we can see that Generator A delivers twice the power of generator B. PB PA frq Fixed Frequency Figure 4-3 In order to change the power in a generator for a given frequency of operation, one has to change the prime mover (change the value of the no-load frequency, or set point). Changing the governor will cause the characteristic to move up and down with the same slope. NOTE: if the governor and exciter are unchanged, any change of speed of one generator will cause a circulating current between the 2 machines in such a way as to oppose the change, hence it is called a “synchronizing torque”. This torque can be enormous and will always make sure that the machines are in synchronism (same frequency). Parallel Operation of Generators 4 CHANGES of OPERATING PARAMETERS Assume a generator is connected to an INFINITE BUS. This means that the bus has a CONSTANT FREQUENCY and a CONSTANT VOLTAGE. Furthermore it can absorb power (active and reactive) and can provide power (active and reactive) as needed. Assume an operating point (speed/excitation) of the governor and exciter which delivers an active power that is the power delivered by the prime mover (minus losses) wt = P mechanical power provided on the shaft If the excitation i f produces E so that the power factor is unity: E I jXI Pactive V Figure 4-4 The excitation remains constant and the prime mover increases the torque, hence the power output increases E jXI V I Pactive NEW Figure 4-5 It can be seen that as the power increases at the prime mover the internal angle increases and therefore I increases also. At the same time the current starts to lead, which means that the generator also provides excess of reactive power. If one wants to bring the power factor back (without touching the prime mover), one would have to decrease the excitation accordingly as shown in the next figure: Parallel Operation of Generators 5 E V Pactive NEW I jXI Figure 4-6 EXCITER CHARACTERISTIC In a generator connected to an infinite bus, one can see from the previous figures that the magnitude of the armature current varies extensively as the excitation-power operating point varies. It is important to make sure that the generator does not exceed the rated values during an operating point setting. Figure 4-7 illustrates this point. Assume an operation under unity power factor with a power of P0 and excitation E0, and rated current I0. The locus of the end of jXI must be on the circle as shown. V P0 E0 P1 jXI0 I1 E1 (1) (1') I0 Figure 4-7 If the governor changes to a new setting, say decrease its mechanical power, if one wants to maintain rated current, there would be 2 operating points (1) and (1’). (1) corresponds to a lagging current (inductive load), the other to a leading current. Hence the exciter has to assume the corresponding excitation to maintain stability. It is important to understand that the operating point of a generator has 2 control parameters: excitation (provided by the exciter) and real power (provided by the prime mover) = 2 degrees of freedom. However the operating point is also defined by the load as another degree of freedom: either PF, or magnitude of the current can be chosen.Read More

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