1. Microgrid Concepts and Distributed Generation Technologies
ECE 2795
Microgrid Concepts and Distributed Generation Technologies
Spring 2015
Week #10
© A. Kwasinski, 2014

2. Introduction Synchronous generators Input:
Mechanical power applied to the rotor shaft
Field excitation to create a magnetic field constant in magnitude and that rotates with the rotor.
Output:
P and Q (electric signal with a given frequency for v and i)
Field Excitation Q
© A. Kwasinski, 2014

3. Introduction Synchronous generators Open circuit voltage:
Magneto-motive force
(mmf)
© A. Kwasinski, 2014

4. Synchronous generators control
Effect of varying field excitation in synchronous generators:
When loaded there are two sources of excitation:
ac current in armature (stator)
dc current in field winding (rotor)
If the field current is enough to generate the necessary mmf, then no magnetizing current is necessary in the armature and the generator operates at unity power factor (Q = 0).
If the field current is not enough to generate the necessary mmf, then the armature needs to provide the additional mmf through a magnetizing current. Hence, it operates at an inductive power factor and it is said to be underexcited.
If the field current is more than enough to generate the necessary mmf, then the armature needs to provide an opposing mmf through a magnetizing current of opposing phase. Hence, it operates at a capacitive power factor and it is said to be overexcited.
© A. Kwasinski, 2014

5. Synchronous generators control
Relationship between reactive power and field excitation
The frequency depends on the rotor’s
speed. So frequency is controlled
through the mechanical power.
Pmec is increased to increase f
Pmec is decreased to decrease f
Field Excitation Q
© A. Kwasinski, 2014

6. Voltage and frequency control
The simplified equivalent circuit for a generator and its output equation is:
LOAD
Assumption: during short circuits or load changes E is constant
V is the output (terminal) voltage
Electric power provided to the load
© A. Kwasinski, 2014

7. Voltage and frequency control
It can be found that
Ideally, the electrical power equals the mechanical input power. The generator’s frequency depends dynamically on δ which, in turn, depends on the electrical power (=input mechanical power). So by changing the mechanical power, we can dynamically change the frequency.
Likewise, the reactive power controls the output voltage of the generator. When the reactive power increases the output voltage decreases.
Generator’s angular frequency
(Micro) Grid’s angular frequency
© A. Kwasinski, 2014

8. Voltage and frequency control
Droop control
It is an autonomous approach for controlling frequency and voltage amplitude of the generator and, eventually, the microgrid.
It takes advantage that real power controls frequency and that reactive power controls voltage
© A. Kwasinski, 2014

9. Voltage and frequency control
Droop control
Then a simple (e.g. PI) controller can be implemented. It considers a reference voltage and a reference frequency:
If the output voltage is different, the field excitation is changed (and, thus, changes Q and then V).
If the frequency is different, the prime mover torque is changed (and thus, changes P and then f).
© A. Kwasinski, 2014

10. Voltage and frequency control
Operation of a generator connected to a large grid
A large grid is seen as an infinite power bus. That is, it is like a generator in which
changes in real power do not cause changes in frequency
changes in reactive power do not originate changes in voltage
its droop control curves are horizontal lines
© A. Kwasinski, 2014

11. Voltage and frequency control
Operator of a generator connected to a large grid
When connected to the grid, the voltage amplitude and frequency is set by the grid.
In order to synchronize the oncoming generator, its frequency needs to be slightly higher than that of the grid, but all other variables need to be the same.
© A. Kwasinski, 2014

12. Higher commanded frequencies
Voltage and frequency control
Operator of a generator connected to a large grid
After the generator is paralleled to the grid then its output frequency and voltage will remain fixed and equal to the grid’s frequency and voltage, respectively.
Output power is controlled by attempting a change in frequency by controlling the prime mover’s torque. By “commanding” a decrease in frequency, the output power will increase.
A similar approach is followed with reactive power control, by controlling field excitation in an attempt to change output voltage.
Higher commanded frequencies
Higher power output
Operating frequency
No load droop line
© A. Kwasinski, 2014

13. A brief summary
In conventional ac grids, large machine inertia helps to maintain stability.
Since frequency needs to be regulated at a precise value, imbalances between electric and mechanical power may make the frequency to change. In order to avoid this issue, mechanical power applied to the generator rotor must follow load changes. If the mechanical power cannot follow the load alone (e.g. due to machine’s inertia), energy storage must be used to compensate for the difference. This is a situation often found in microgrids.
Reactive power is used to regulate voltage.
Droop control is an effective autonomous controller.
© A. Kwasinski, 2014

14. DC microgrids (droop control)
Consider a microturbine in a microgrid controlled by droop control.
Primary control:
Secondary control (voltage deviation compensation)
Depends on microgrid bus voltage
NOTE: Based on Guerrero et al “Hierarchical Control of Droop-Controlled AC and DC Microgrids—A General Approach Toward Standardization”
© A. Kwasinski, 2014

15. DC microgrids (droop control)
Tertiary control (associated with a grid tie):
Could be the input for a grid interface converter or the input for the distributed generation sources interface. The latter applies when there is a direct connection to a stiff grid because the stiff grid fixes the microgrid voltage. When there is a grid outage, the tertiary control is replaced by the secondary control. When the grid is present the secondary control is replaced by the tertiary control.
Depends on current to or from the grid
NOTE: Based on Guerrero et al “Hierarchical Control of Droop-Controlled AC and DC Microgrids—A General Approach Toward Standardization”
© A. Kwasinski, 2014

16. Secondary control
Tertiary control
© A. Kwasinski, 2014

17. DC microgrids (droop control)Ig
GRID
GIC
IμT
IμT IL
LOAD
Voltage range “to allow for power sharing and voltage regulation using droop control”
Set by the utility company
Droop slope (virtual dc output resistance)
© A. Kwasinski, 2014

18. DC microgrids (droop control)
IuT
IuT,1
IμT
IμT,2 IL IL
LOAD
LOAD
Voltage range “to allow for power sharing and voltage regulation using droop control”
IμT,1+IμT,2 = IL
DC bus voltage
IuT,1
IμT,2
© A. Kwasinski, 2014

19. DC microgrids (droop control)
IuT
IuT,1
IμT
IμT,2 IL IL
LOAD
LOAD
Voltage range “to allow for power sharing and voltage regulation using droop control”
IμT,1+IμT,2 = IL
When the load increases, current is shared between the two microturbines with the one with the highest capacity providing more current to the load
IμT,2
IuT,1
© A. Kwasinski, 2014

20. DC microgrids (droop control)
IuT
IuT,1
IμT
IμT,2 IL IL
LOAD
LOAD
Voltage range “to allow for power sharing and voltage regulation using droop control”
IμT,1+IμT,2 = IL
As the load increases, the voltage drops so current output from the microturbines can increase. Still, the microturbine with the highest capacity providing more current to the load
IuT,1
IμT,2
© A. Kwasinski, 2014

21. DC microgrids (droop control)Ig
GRID
GIC Ig
GRID
GIC Ig
GRID
GIC Ig
GRID
GIC
IuT
IuT,1
IμT
IμT,2 IL IL
LOAD
LOAD
Voltage range “to allow for power sharing and voltage regulation using droop control”
Ig+IμT,1+IμT,2 = IL
When the load increases even further the grid needs to provide the extra current in order to prevent voltage collapse
IuT,1
IμT,2 Ig
© A. Kwasinski, 2014

22. DC microgrids (droop control)Ig
GRID
GIC Ig
GRID
GIC Ig
GRID
GIC Ig
GRID
GIC
IuT
IuT,1
IμT
IμT,2 IL IL
LOAD
LOAD
Voltage range “to allow for power sharing and voltage regulation using droop control”
Ig+IμT,1+IμT,2 = IL
Current from the grid can be used to reduce the current from the microturbines and increase the dc bus voltage (see the voltage in the case with the same load in slide #19) Ig
IuT,1
IμT,2
© A. Kwasinski, 2014

23. DC microgrids (droop control)Ig
GRID
GIC Ig
GRID
GIC Ig
GRID
GIC Ig
GRID
GIC
IuT
IuT,1
IμT
IμT,2 IL IL
LOAD
LOAD
Voltage range “to allow for power sharing and voltage regulation using droop control”
Ig+IμT,1+IμT,2 = IL
When the load is light, extra power being generated by the microturbines can be injected back to the grid (see slide # 18)
IuT,1 Ig
IμT,2
© A. Kwasinski, 2014

24. DC microgrids (droop control)Ig
GRID
GIC Ig
GRID
GIC
IuT
IuT
IuT
IuT IL IL
LOAD
LOAD
Primary control is combined with a secondary control to compensate voltage deviations
Primary control is combined with a secondary control to compensate voltage deviations
Now, vref,NL can be adjusted with a δvref
Now, vref,NL can be adjusted with a δvref
© A. Kwasinski, 2014

25. DC microgrids (droop control)
IuT
IuT
IuT
IuT IL IL
LOAD
LOAD
Primary control is combined with a secondary control to compensate voltage deviations
Primary control is combined with a secondary control to compensate voltage deviations
IμT,1+IμT,2 = IL
Nominal
Adjusted with δvref
IuT,1
IμT,2
© A. Kwasinski, 2014

26. DC microgrids (droop control)
IuT
IuT
IuT
IuT IL IL
LOAD
LOAD
Primary control is combined with a secondary control to compensate voltage deviations
Primary control is combined with a secondary control to compensate voltage deviations
IμT,1+IμT,2 = IL
Notice that the currents are the same than in the case with no secondary control (slide #18) but now the voltage is kept at 380 V
IμT,2
IuT,1
© A. Kwasinski, 2014

27. Notice same δvref for both microturnines
DC microgrids (droop control)
IuT
IuT
IuT
IuT IL IL
LOAD
LOAD
Primary control is combined with a secondary control to compensate voltage deviations
Primary control is combined with a secondary control to compensate voltage deviations
Notice same δvref for both microturnines
IμT,1+IμT,2 = IL
IuT,1
IμT,2
© A. Kwasinski, 2014

28. Notice lower δvref than previous slide
DC microgrids (droop control) Ig
GRID
GIC Ig
GRID
GIC
IuT
IuT
IuT
IuT IL IL
LOAD
LOAD
Primary control is combined with a secondary control to compensate voltage deviations
Primary control is combined with a secondary control to compensate voltage deviations
Ig+IμT,1+IμT,2 = IL
Notice lower δvref than previous slide
IuT,1
IμT,2 Ig
© A. Kwasinski, 2014

29. DC microgrids (droop control)Ig
GRID
GIC Ig
GRID
GIC
IuT
IuT
IuT
IuT IL IL
LOAD
LOAD
Primary control is combined with a secondary control to compensate voltage deviations
Primary control is combined with a secondary control to compensate voltage deviations
Ig+IμT,1+IμT,2 = IL Ig
IμT,2
IuT,1
© A. Kwasinski, 2014

30. DC microgrids (droop control)Ig
GRID
GIC Ig
GRID
GIC
IuT
IuT
IuT
IuT IL IL
LOAD
LOAD
Primary control is combined with a secondary control to compensate voltage deviations
Primary control is combined with a secondary control to compensate voltage deviations
Ig+IμT,1+IμT,2 = IL Ig
IuT,1
IμT,2
© A. Kwasinski, 2014

31. DC microgrids (droop control)Ig
GRID
GIC
NOTE: Slide prepared by Prof. Dushan Boroyevich from VT
Paper: Boroyevich et al “Future Electronic Power Distribution Systems – A contemplative view –” Is Iw Ib IL
LOAD
Voltage range “to allow for power sharing and voltage regulation using the droop control”
Set by the utility company
Droop slope (virtual dc output resistance)
© A. Kwasinski, 2014

32. DC microgrids (droop control)
In the presence of constant-power loads, regulators in source converters cannot use PI controllers. From a static perspective, regulators designed for constant-power loads will make the source converter output characteristic to look like MPP trackers.
Battery interfaces have different characteristic depending on the state of charge of the batteries. For example, at the float voltage, the battery may take no current (if the state of charge is 100 %) or may take some current if the state of charge is less than 100 %.
© A. Kwasinski, 2014

33. DC microgrids (droop control)Ig
GRID
GIC Iw Ib
NOTE: Slide prepared by Prof. Dushan Boroyevich from VT
Paper: Boroyevich et al “Future Electronic Power Distribution Systems – A contemplative view –” Is IL
LOAD
Iw+Is+Ig+Ib = IL
Iw+Is+Ib = IL
Iw+Is+Ig = IL
Iw+Is+Ig = IL
Iw+Is= IL
0= IL Ig Iw Is Ig Ib Iw Iw Is Iw Iw Is Ib Is Is Ig
© A. Kwasinski, 2014

34. DC microgrids (droop control)
DC GRID Ig
IuT
IuT
IuT
IuT IL IL
LOAD
LOAD
Primary control is combined with a secondary control to compensate voltage deviations
Primary control is combined with a secondary control to compensate voltage deviations
With a stiff grid there is no limit to Ig
Ig is regulated by adjusting δvref
© A. Kwasinski, 2014

35. © A. Kwasinski, 2014

36. DC microgrids (droop control)
DC GRID Ig
IuT
IuT
IuT
IuT IL IL
LOAD
LOAD
Voltage is kept fixed by the stiff grid so no voltage regulation is necessary
Ig+IμT,1+IμT,2 = IL
IuT,1 Ig
IμT,2
© A. Kwasinski, 2014

37. DC microgrids (droop control)
DC GRID Ig Ig
IuT
IuT
IuT
IuT IL IL
LOAD
LOAD
Voltage is kept fixed by the stiff grid so no voltage regulation is necessary
Ig+IμT,1+IμT,2 = IL Ig
IuT,1
IμT,2
© A. Kwasinski, 2014

38. AC microgrids revisited (droop control)
Sources with a dc output or an ac output with a frequency different from that of the microgrid main bus need to use an inverter to be integrated into an ac microgrid. When implementing droop control, droop regulators are used to emulate the inertia of an ac machine.
Issues when implementing conventional droop control in ac systems with inverters:
Droop current-sharing methods are affected by harmonic content created by non-linear loads. These issues can be solved by distorting the voltage signal intentionally which leads to further issues.
Frequency is dependent on load levels in the same way that voltage levels depend on load levels. Also, frequency goals for two inverters with different capacity may be different. Frequency deviations dependant on load levels may lead to loss of synchronization when attempting to connect the microgrid directly to a main grid. Hence, it is only applicable to islanded operation and makes transition into grid connected operation complicated.
In islanded mode there is both frequency and voltage deviations leading to tradeoffs inherent to droop control in islanded mode.
Secondary controls have been proposed in order to solve these issues without the need for communication links.
© A. Kwasinski, 2014

39. Now secondary control depends on microgrid bus voltage and frequency
GP(s) and GQ(s) represent PI or P controllers.
ω*, E*, P* and Q* are reference signals, so when P=P*, ω=ω* and when Q=Q*, E=E*
Now tertiary control depends on real and reactive power flow from or to the grid
NOTE: Figure from Guerrero et al “Hierarchical Control of Droop-Controlled AC and DC Microgrids—A General Approach Toward Standardization”
© A. Kwasinski, 2014

40. Additional comments about droop controls in ac microgrids
It has been suggested by some researchers to consider energy stored in dc link capacitors (i.e. their voltage) of ac micrgrids with inverters interfacing sources and loads as analogous to rotating kinetic energy in ac power grids with generators directed connected to the distribution system.
Zs depends on the inverter circuit components and on how the inverter is controlled
© A. Kwasinski, 2014

41. Additional comments about droop controls in ac microgrids (Review)
Output real and reactive power of an inverter (or any source) equal
In conventional power grids XL>>RL, so, without considering ZS (and small angles
So, P relates to f and Q relates to V
In microgrids, it could be expected that XL<<RL, so, without considering ZS and small angles
So, droop relationships in microgrids may be inverted
© A. Kwasinski, 2014