From the previous discussions in this forum, it is understood that the generator absorbs reactive power from the grid when the grid voltage is higher than generator voltage, and supplies reactive power to the grid when the grid voltage is lower than generator voltage. However, there is confusion about how reactive power exchange is managed when there is a generator transformer with an Under Load Tap Changer (ULTC).

Why is an ULTC necessary? What happens if you set the Automatic Voltage Regulator (AVR) to an unattainable set point to ensure that the generator supplies reactive power to the grid rather than absorbing it?

 

@Selk,

I'm no transmission and distribution expert, but ULTC's (as you call them; I've always heard them referred to as Load Tap Changers--meaning they can be operated when power is flowing through the contacts of the various taps (some tap changers have be isolated from power (both voltage and current) before the taps can be changed) allow for a wider range of response to power systems or captive loads that have large swings in voltage which could cause generator and transformer protective relays to operate and trip the machine or separate the synchronous generator from the power system.

Just about every application of a tap changer is unique, which means they can't all be generalized and stereotyped and considered similar. The application (what the needs of the situation at the site they will be installed and used) defines what they do and how they do it. Load Tap Changers (ULTCs) are expensive compared to "simple" tap changers (that have to be isolated when changing taps).

Have you used your preferred World Wide Web search engine to investigate tap changers, and load tap changers (or maybe ULTC)? I haven't seen a lot written here on Control.com about tap changers. And what I have seen reveals some unusual characterizations of them and how they are used or how they are to be used.

Power systems in different parts of the world, and at different large industrial sites, can and do have very different requirements for maintaining grid system voltage. Some systems/loads are very inductive in nature, and that can require extra VArs to be supplied to the grid in order to maintain system voltage. (I say "extra" because most AVRs (exciter; exciter regulator) only have a range of about 5% of rated generator terminal voltage unless they are very specially constructed (which costs a LOT of money), so using a tap changer or load tap changer is often an "inexpensive" way of extending the range of available voltage and VArs as system/load requirements change.) Some systems/loads are capacitive in nature, and can cause the relationship between the voltage and current sine waves to drift far apart from each other which affects grid stability, and tap changers and load tap changers can also help with that balance effort.

Again, I'm no expert on this (my expertise is mainly on the prime mover end of the generator, not the exciter end of the generator)--tap changers and load tap changers--and this is a VERY high-level view of how they work, and I have no other details or information to offer except to use your preferred search engine to get more information and details.

 

@Selk,

I'm no transmission and distribution expert, but ULTC's (as you call them; I've always heard them referred to as Load Tap Changers--meaning they can be operated when power is flowing through the contacts of the various taps (some tap changers have be isolated from power (both voltage and current) before the taps can be changed) allow for a wider range of response to power systems or captive loads that have large swings in voltage which could cause generator and transformer protective relays to operate and trip the machine or separate the synchronous generator from the power system.

Just about every application of a tap changer is unique, which means they can't all be generalized and stereotyped and considered similar. The application (what the needs of the situation at the site they will be installed and used) defines what they do and how they do it. Load Tap Changers (ULTCs) are expensive compared to "simple" tap changers (that have to be isolated when changing taps).

Have you used your preferred World Wide Web search engine to investigate tap changers, and load tap changers (or maybe ULTC)? I haven't seen a lot written here on Control.com about tap changers. And what I have seen reveals some unusual characterizations of them and how they are used or how they are to be used.

Power systems in different parts of the world, and at different large industrial sites, can and do have very different requirements for maintaining grid system voltage. Some systems/loads are very inductive in nature, and that can require extra VArs to be supplied to the grid in order to maintain system voltage. (I say "extra" because most AVRs (exciter; exciter regulator) only have a range of about 5% of rated generator terminal voltage unless they are very specially constructed (which costs a LOT of money), so using a tap changer or load tap changer is often an "inexpensive" way of extending the range of available voltage and VArs as system/load requirements change.) Some systems/loads are capacitive in nature, and can cause the relationship between the voltage and current sine waves to drift far apart from each other which affects grid stability, and tap changers and load tap changers can also help with that balance effort.

Again, I'm no expert on this (my expertise is mainly on the prime mover end of the generator, not the exciter end of the generator)--tap changers and load tap changers--and this is a VERY high-level view of how they work, and I have no other details or information to offer except to use your preferred search engine to get more information and details.

When the grid voltage is significantly higher than the generator terminal voltage while the generator is operating in PF/Var mode, what could occur?

When the grid voltage is significantly lower than the generator terminal voltage while the generator is operating in PF/Var mode, what could occur?

 

Briefly, PF/VAr control typically has upper and lower limits which can't usually be exceeded (this all depends on the manufacturer of the excitation system). So if the grid voltage is significantly higher than the PF/VAr reference (and limit) then the exciter isn't going to be able to respond to the excessive incoming VArs from the significantly high grid voltage. What may happen is the generator breaker and/or the machine may be tripped on underexcitation (which is there to protect against slipping a pole).

If the grid voltage is significantly lower than the PF/Var reference (and limit) then the exciter isn't going to be able to respond to reduce the excessive VArs from the generator. What may happen is the generator breaker and/or the machine may be tripped on overexcitation (which is there to protect the generator rotor from excessive heating due to excessive excitation current).

Now, put this all together with your knowledge of ULTCs (as you call them) and you can see that the "grid voltage" seen by the generator can be "manipulated" to appear to be different than it actually is--thereby increasing the range of the generator to be able to properly control excitation, remaining in the limits of the generator an excitation system and protecting the generator and the machine, and still meet the requirements of the grid when the grid voltage exceeds the limits of the machine/generator.

I have to say that there are a few of us who break with the strict definition of reactive power and consider VArs as being produced or consumed (in the same way watts/kW/MW are produced and consumed). It makes the basics of understanding AC (Alternating Current) power generation eaiser to grap in the beginning without all of the formulae and vector triangles and such. (For me and about three-fourths of my AC theory class we struggled mightily in our understanding of VArs when forced to learn it from the pure theoretical, mathematical perspective.) In our opinion being able to get our heads around complex things before getting all of the theory is much simpler especially when it's ALL new to us and those we are trying to help to find a way to understand what's happening as we increase the energy input to the prime mover of a synchronous generator (or decrease it), and when we increase excitation (or decrease it). Utilities bill large consumers of VArs for VAr-hours, just like they bill for watt-hours. Even residential and commercial customer rates include (hidden) charges for VArs--because someone has to deal with them and there ain't no such thing as a free lunch. Not for anyone. To me, if it's billable that means it costs money to produce or deal with and people want to make money--that's the name of the game (capitalism), isn't it?

 

Briefly, PF/VAr control typically has upper and lower limits which can't usually be exceeded (this all depends on the manufacturer of the excitation system). So if the grid voltage is significantly higher than the PF/VAr reference (and limit) then the exciter isn't going to be able to respond to the excessive incoming VArs from the significantly high grid voltage. What may happen is the generator breaker and/or the machine may be tripped on underexcitation (which is there to protect against slipping a pole).

If the grid voltage is significantly lower than the PF/Var reference (and limit) then the exciter isn't going to be able to respond to reduce the excessive VArs from the generator. What may happen is the generator breaker and/or the machine may be tripped on overexcitation (which is there to protect the generator rotor from excessive heating due to excessive excitation current).

Now, put this all together with your knowledge of ULTCs (as you call them) and you can see that the "grid voltage" seen by the generator can be "manipulated" to appear to be different than it actually is--thereby increasing the range of the generator to be able to properly control excitation, remaining in the limits of the generator an excitation system and protecting the generator and the machine, and still meet the requirements of the grid when the grid voltage exceeds the limits of the machine/generator.

I have to say that there are a few of us who break with the strict definition of reactive power and consider VArs as being produced or consumed (in the same way watts/kW/MW are produced and consumed). It makes the basics of understanding AC (Alternating Current) power generation eaiser to grap in the beginning without all of the formulae and vector triangles and such. (For me and about three-fourths of my AC theory class we struggled mightily in our understanding of VArs when forced to learn it from the pure theoretical, mathematical perspective.) In our opinion being able to get our heads around complex things before getting all of the theory is much simpler especially when it's ALL new to us and those we are trying to help to find a way to understand what's happening as we increase the energy input to the prime mover of a synchronous generator (or decrease it), and when we increase excitation (or decrease it). Utilities bill large consumers of VArs for VAr-hours, just like they bill for watt-hours. Even residential and commercial customer rates include (hidden) charges for VArs--because someone has to deal with them and there ain't no such thing as a free lunch. Not for anyone. To me, if it's billable that means it costs money to produce or deal with and people want to make money--that's the name of the game (capitalism), isn't it?

If both the Generator AVR and Transformer ULTC (OLTC) AVR are in Auto Voltage control mode, they would compete against each other to maintain control; building on this, I had a question, If the Generator AVR is set to PF/Var control mode while the Transformer ULTC (OLTC) AVR is in Voltage control mode, they won't compete. However, I was curious about what would happen in this scenario.
Here is my summary,

Scenario	Grid Voltage	AVR Mode	What happens?
1	Higher than Generator Terminal Voltage	Voltage controlled	The AVR will reduce the field to try to reduce the machine terminal voltage as it compensates for the rise in terminal voltage brought about by the rise in grid voltage; this may cause to hit the under-excitation limiter with the machine operating at a leading power factor as it absorbs reactive power from the grid.
2	Higher than Generator Terminal Voltage	PF/Var controlled	The generators operating in PF/Var mode see their reactive power decrease suddenly due to the rise in grid voltage. So the AVR increases excitation to bring the setpoint PF/Var back in line.
3	Lower than Generator Terminal Voltage	Voltage controlled	The AVR will increase the field to try to increase the machine terminal voltage as it compensates for the fall in terminal voltage brought about by the fall in grid voltage; this may cause to hit the over excitation limiter with the machine operating at a lagging power factor as it supplies reactive power to the grid.
4	Lower than Generator Terminal Voltage	PF/Var controlled	The generators operating in PF/Var mode see their reactive power increase suddenly due to the drop in grid voltage. So the AVR reduces excitation to bring the setpoint PF/Var back in line.