1. Microgrid Concepts and Distributed Generation Technologies
ECE 2795
Microgrid Concepts and Distributed Generation Technologies
Spring 2015
Week #8
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2. Microgrids architectures and operation
dc vs. ac
The discussion refers to the system’s main bus. Remember the discussion in the first class about Edison’s electric system.
No frequency/phase control is necessary in dc microgrids.
From a general point of view dc systems are simpler to control.
Lack of a monitoring variable may complicate fault detection and autonomous controls implementation.
Since most distributed sources and energy storage devices have an inherently dc output, dc architectures are a more “natural” option for integration of such components.
Most modern loads inherently require a dc input. Even the “most classical” ac loads, induction motors, rely more on inherently dc input variable speed drives (VSDs) to achieve a more efficient and flexible operation.
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3. Microgrids architectures and operation
dc vs. ac
Availability: dc is several times more reliable than ac (NTT data from 30,000 systems [H. Ikebe, “Power Systems for Telecommunications in the IT Age,” in Proc. INTELEC 2003, pp. 1-8.])
Efficiency:
Efficiency gains in energy conversion interfaces makes dc systems 5 % to 7 % more efficient than ac systems.
dc powered VSDs are 5 % more efficient than equivalent ac powered VSDs because the rectification stage is avoided.
Dc systems tend to be more modular and scalable than ac systems because dc converters are easier to control and to parallel.
dc systems components tend to be more compact that equivalent ac ones because of higher efficiency and for not being frequency dependent.
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4. Microgrids architectures and operation
dc vs. ac
Modular design makes dc systems more flexible and easier to expand, allowing for a more effective capital investment management and a better planning of the entire facility power installation.
Well designed dc grids can achieve both hardware and operational cost savings over equivalent ac systems.
Power conditioning to improve quality tends to be simpler in dc systems.
Stability control tends to be simpler in dc systems.
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5. Microgrids architectures and operation
Stability issues
Stability issues are more prevalent in microgrids than in a large electric grid because power and energy ratings are much lower.
Analysis of stability issues in ac microgrids tend to follow the same concepts than in the main grid:
Voltage and frequency values need both to be regulated through active and reactive power control.
If sources are traditional generators with an ac output and are connected directly without power electronic interfaces, stability is controlled through the machine shaft’s torque and speed control.
In dc systems there is no reactive power interactions, which seems to suggest that there are no stability issues. System control seems to be oriented to voltage regulation only
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6. Conventional (ac) datacenters
Typical configuration:
Total power consumption: > 5 MW (distribution at 208V ac)
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7. Conventional (ac) datacenters
Data centers represent a noticeable fast increasing load.
Increasing power-related costs, likely to equal and exceed
information and communications technology equipment cost in the
near to mid-term future.
Servers are a dc load
860 W of equivalent coal power is needed to power a 100 W load
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8. New (dc) datacenters Use of 380 Vdc power distribution for:
Fewer conversion stages (higher efficiency)
Integration of local sources (and energy storage).
Reduced cable size
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9. Data centers efficiency dc vs. ac
A 380Vdc power distribution standard is currently under study by the IEC
Brian Fortenbery and Dennis P. Symanski, GBPF, 2010
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10. New distributed (dc) datacenters
Many “small” distributed data centers powered locally and with a coordinated operation
Energy is used more effectively.
Generation inefficiencies is energy that is not harvested (i.e. converted), contrary to inefficiencies in conventional power plants which represent power losses.
Latency may be a problem
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11. Utility dc distribution
Jonbok Bae, GBPF 2011
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12. dc Homes
dc in homes allows for a better integration of distributed generation, energy storage and dc loads.
With a variable speed drive air conditioners can be operated continuously and, hence, more efficiently (about 50%)
WIND GENERATOR
PV MODULES
LED LIGHTS (DC)
MAIN DC BUS
REFRIGERATOR (LOAD)
ENERGY STORAGE
ELECTRIC VEHICLE
AIR CONDITIONER
EPA 430-F
FUEL CELL
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13. ac microgrids stability
Main goal: be able to maintain frequency and voltage levels within a relatively tight ranges.
Stability issues can be considered at various time scales:
seconds to minutes (or more):
Need to continuously balance load + losses and generation with load and losses being unknown.
Generation ramp rates
This is a more challenging problem in microgrids due to the reduced stiffness when operating in islanded mode. General solutions involve the use of energy storage devices and/or load management (e.g. load shedding).
milliseconds to seconds:
Mostly a power quality issue that without a quick correction may usually lead to loss of stability
< milliseconds:
Usually originated in non-linear devices and loads.
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14. ac microgrids stability
Consider an ac microgrid with one ac generator and one load. Consider also a stiff grid with X>>R.
The simplified equivalent circuit for the generator and its output equation is:
From mechanics:
LOAD
Electric power provided to the load
Assumption: Infinite bus (simplifies the analysis but not true for micro-grids).
Also during shorts or load changes E is constant
Moment of inertia
electrical torque
angular acceleration
mechanical torque
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15. Mechanical equivalent of its electrical homologous variable
ac microgrids stability
If a synchronous reference frame is considered then
Swing equation:
where “p.u.” indicates per unit and
So if decreases and if increases
Mechanical equivalent of its electrical homologous variable
Synchronous speed
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16. 4) Because of rotor inertia increases up to here
Stability
Equal area criterion and analysis during faults or sudden load changes (particularly load increase). Let’s see the most critical case: a fault
After reaching δ(t) will oscillate until losses and the load damp oscillations and
If the generator looses stability because and the generator continues to accelerate.
4) Because of rotor inertia increases up to here
Both areas are equal
1) Initial condition
3) Fault is cleared here
2) During the fault pe = 0
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17. ac microgrids stability
In ac systems, real and reactive power needs to be controlled to maintain system 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, mechanical power applied to the generator rotor must follow load changes. If mechanical power cannot follow load, energy storage must be used to compensate for the difference.
Reactive power is used to regulate voltage.
Some autonomous control strategies will be
discussed in a future class.
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18. ac microgrids stability
Let’s review some assumptions:
Stiff grid.
In cables X>>R.
A stiff grid is a basic assumption for the application of the equal area criterion. However, microgrids cannot be generally considered a stiff grid.
A basic approach to address stability issues related to lack of stiffness is to add energy storage with a power electronic interface with fast controllers.
The amount of necessary stored energy is
Consumed electrical energy in the time interval under consideration
Maximum allowed frequency excursion
Microgrid inertia constant
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19. ac microgrids stability
Assumption:
In cables X>>R (implies long cables)
But in microgrids
In cables X<R (implies short cables)
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20. Most islanded microgrids
ac microgrids stability
Equations on the right assume that the phase difference δ between both voltages is small
Notice that conventional relationships between real power and frequency and reactive power and voltage in power grids are inverted for microgrids. This fact has important implications in terms of stability and control.
Loss of stability will be also observed in microgrids as voltage deviations.
Most islanded microgrids
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21. Microgrids architectures and operation
distributed and centralized architectures
Power systems with distributed architecture have their power distribution and conversion functions spread among converters and the distribution is divided among two or more circuits.
There are two basic structures in distributed architectures:
Parallel structures are used when the design focuses on improved availability.
Cascade structures are used to improve point-of-load regulation, reduce cost, and improve system efficiency. Hence, they have at least two conversion stages among three or more voltage levels.
Cascade structure
Parallel structure
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22. Microgrids architectures and operation
distributed and centralized architectures
The possibility of having different connection structures and different conversion stages makes distributed architectures more flexible than centralized architectures.
Hence, distributed architectures are the natural choice in systems requiring integration of a variety of energy sources with several different loads.
When power converters are modular, the distributed architecture allows the system capacity to expand gradually as the load increases over time.
Thus, distributed architectures have lower financial costs than equivalent centralized architectures.
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23. Microgrids architectures and operation
distributed and centralized architectures
Examples of distributed and centralized architectures can be found in telecommunications power plants (remember that telephony grids can be considered a low power dc grid).
Only (centralized) bus bars
Centralized architecture
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24. Microgrids architectures and operation
distributed and centralized architectures
Examples of distributed and centralized architectures can be found in telecommunications power plants (remember that telephony grids can be considered a low power dc grid).
Each cabinet with its own bus bars connected to its own battery string and loads. Then all cabinets’ bus bars are connected
Distributed architecture
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25. Power architectures topologies
Some examples of different power distribution architectures:
Radial
Ring
Ladder (analogous to breaker-and-a-half or Double breaker-double bus substation configurations)
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26. Dc microgrids stability
Stability issues (this problem is also observed with power electronic-based ac power distribution architectures but its analysis in ac is more complicated and out of the scope of this course).
Consider a cascade distributed architecture. The point-of-load (POL) converter tightly regulates the output voltage on the actual resistive load. If Vout is kept fixed regardless of the input voltage and R does not change, then the power dissipated in the load resistance PL is constant. If the POL converter is lossless its input power is constant so it acts as a constant-power load with input voltage and current related by
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27. Dc Microgrids Stability
In reality CPLs have the following form:
For the analysis we will assume that Vlim is close to zero.
Then the dynamic impedance is
Hence, CPLs introduce a destabilizing effect.
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28. Dc Microgrids Stability
Consider the following simplified system of a POL converter behaving like a CPL and a buck converter regulating the main bus voltage that equals the POL converter input.
Consider also the following circuit parameters: E = 400 V, D = 0.5, C = 1 mF, L = 0.5 mH, PL = 5 kW (the POL converter and load resistance are represented by this parameter).
The system will behave in two possible ways depending the initial conditions for the inductor current and capacitor voltage
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29. Dc Microgrids Stability
If the initial capacitor voltage is high enough the system’s state variables may oscillate. If the initial capacitor voltage is low enough and/or the power and inductance are also high enough and/or the capacitance is low enough, the inductor current will take very high values and the capacitor voltage will tend to zero.
iL(t)
iL(t)
vC(t)
vC(t)
vC(0) = 120 V
vC(0) = 50 V
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30. Dc Microgrids Stability
The phase portrait for a buck converter with a constant power load with a fixed duty cycle looks like this:
For all dc-dc converters it looks similar.
- Approximate for the separatrix:
- Necessary but not sufficient condition for oscillations:
Buck LRC with a PL = 5 kW, E = 400 V, L = 0.5 mH, C = 1 mF, D = 0.5.
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31. Dc Microgrids Stability
Regulating the output with a PI controller yields bad results:
Simulation results for an ideal buck converter with a PI controller both for a 100 W CPL (continuous trace) and a 2.25 Ω resistor (dashed trace); E = 24 V, L = 0.2 mH, PL = 100 W, C = 470 μF.
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32. Dc Microgrids Stability
Model for a buck converter with a CPL:
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33. Dc Microgrids Stability
Linearization yields that in order to achieve a stable regulation point two conservative conditions are:
where Ri equals the sum of RSD, RD(1-D), and RL.
Hence, stability improves if:
L is lower
C is higher
PL is lower
Ri is higher
R0 is lower (higher ohmic load)
(Predominant condition)
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34. Dc Microgrids Stability
Consider the following dc microgrid:
E1 = 400 V, E2 = 450 V, PL1 = 5 kW, PL3 = 10 kW, LLINE = 25 μH, RLINE = 9 mΩ, CDCPL = 1 mF, and buck LRCs with L = 0.5 mH, C = 1 mF, D1 = 0.5, and D2 = 0.45.
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35. Dc Microgrids Stability
With an open loop control (fixed duty cycles) the system shows again important oscillations, well distant of the desired dc behavior.
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36. 1 Ohm resistor added in parallel with the CPL
Dc Microgrids Stability
OPTION #1: add resistors in series with the circuit inductors or in parallel with the circuit capacitors.
Resistors damp the resonating
excess energy in the circuit
This solution is very inefficient.
A minimum resistive load is needed
1 Ohm resistor added in parallel with the CPL
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37. Dc Microgrids Stability
OPTION #2: Add filters, particularly capacitors.
Oscillations decrease with
increased capacitances
Since the oscillation
frequency is in the order of
hundreds of hertz, increasing
capacitance is expensive.
Large capacitors tend to be
unreliable.
60 mF capacitance placed at the CPL input and at the buck converters output.
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38. 1 F ultracapacitor located at the CPL input
Dc Microgrids Stability
OPTION #3: Bulk energy storage (primarily batteries) directly connected to the main bus.
Telecommunications power systems
is a typical example of this solution.
This solution tends to be expensive.
This solution is more suitable for
energy systems. For power systems,
such as microgrids, bulk energy
storage is not well suited.
Additional disadvantages in microgrids
are issues related with reliability, safety,
and protections when stacking several
battery cells in series to reach dc bus
voltages over 150 V. Inadequate cell
equalization is another disadvantage
(indirect connection does not work).
1 F ultracapacitor located at the CPL input
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39. PL3 dropping from 10 kW to 2.5 kW at t = 0.25 s
Dc Microgrids Stability
OPTION #4: Load shedding.
As PL decreases the oscillation
amplitude also decreases. .
This solution is not suitable for
critical mission loads.
This solution is equivalent to
load shedding in ac systems
PL3 dropping from 10 kW to 2.5 kW at t = 0.25 s
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40. Dc Microgrids Stability
OPTION #5: Linear controllers.
Linear controllers refer to PID-type of controllers in which output voltage regulation is achieved by creating a duty cycle signal by comparing the measured output voltage with a reference voltage and then passing that error signal through a PID-type controller.
PD controllers can stabilize constant-power loads. The controller adds damping without losses through virtual resistances embedded in the controller gains.
An additional integral action is used to provide line regulation and to compensate for internal losses.
In some situations (particularly with buck converters) a PI controller is enough. But, in general, stability is not ensured.
Advantages:
Simple
Cost effective
Disadvantages:
Stability is still not global
Derivative term create noise susceptibility.
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41. Dc Microgrids Stability
OPTION #5: Linear controllers.
PI controller
ki = 1
kp = 0.1
With PI controllers stability
is not insured and results
are poor.
vB1(t)
vB2(t)
vB3(t)
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42. Dc Microgrids Stability
OPTION #5: Linear controllers.
PID controller
ki =
kd =
kP = 0.5
Fast dynamics are
achieved thanks to the high
proportional gain. However,
this high gain can be
usually implemented only
in buck converters.
vB1(t)
vB2(t)
vB3(t)
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43. Dc Microgrids Stability
OPTION #5: Linear controllers.
PD controller
kp = 0.5
kd =
Fast dynamics are
achieved thanks to the high
proportional gain. However,
this high gain can be
usually implemented only
in buck converters.
vB1(t)
vB2(t)
vB3(t)
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44. Dc Microgrids Stability
OPTION #6: Geometric controls (boundary).
Geometric controls (e.g. hysteresis controllers) are based on event-triggered switching instead of time-dependent switching.
With an hysteresis control, the output voltage (or inductor current) is controlled to be between a band. Whenever the voltage (or inductor current) crosses the band’s boundaries a switch action is triggered (switch is closed or opened).
Advantages:
High performance (fast)
Global stabilization
Disadvantages:
Complicated output regulation: analysis may require to determine the trajectories. Overshoots caused by capacitances and inductances are difficult to control
Lack of a fixed switching frequency
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45. Dc Microgrids Stability
OPTION #6: Geometric controls
A line with a negative slope achieves an stable regulating point for all converters. Regulation is simple to implement.
Boost
Buck
Buck-boost
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46. Dc Microgrids Stability
OPTION #6: Geometric controls.
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47. Protections
Microgrids are more sensitive to power quality issues because of reduced stiffness and the typical values found in line impedances.
Faults need to be interrupt fast so that short circuits in a given circuit do not cause:
Significant voltage sags in neighboring circuits, and/or
make the system to lose stability.
To interrupt faults it is first necessary to detect faults quickly. However, except in the case of dc microgrids with batteries directly connected to a dc microgrid bus, fault currents are limited so they may be difficult to distinguish from a high-load condition. Also, their limited value may be too low to make protection devices to act quickly.
Inrush currents (e.g. caused by capacitors) also create power quality issues in dc microgrids but these tend to be issues that are easier to solve than fault protection.
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48. Protections
Consider as an example the power distribution system of a full electric ship (which can be considered as an islanded microgrid)
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49. Protections
Circuit protection: conventional approach based on switch gear. Issues:
Coordination
Fault current detection and interruption
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50. Protections
Circuit protection: based on power electronics or solid state circuit breaker.
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51. Dc systems faults management
In power electronic distributed architectures, faults may not be properly detected because, without a significant amount of stored energy directly connected to the system buses, short-circuit currents are limited to the converter maximum rated current plus the transitory current delivered by the output capacitor.
If the latter is not high enough, the protection device will not trip and the fault will not be cleared.
In this case, the converter will continue operation delivering the maximum rated current but with an output voltage significantly lower than the nominal value.
Consider the following situation
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52. Dc systems faults management
With C = 600 μF, the fault is not properly cleared and voltage collapse occurs for both loads.
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53. Dc systems faults management
To avoid the situation described above, the converter output capacitance has to be dimensioned to deliver enough energy to trip the protection element.
One approach is to calculate the capacitance based on the maximum allowed converter output voltage drop. However, this is a very conservative approach that often leads to high capacitance values.
Another option is to calculate the capacitance so that it can store at least enough energy to trip the protection device, such as a fuse.
Fuse-tripping process can be divided into two phases:
pre-arcing
Lasts for 90% of the entire process.
During this phase, current flows through the fuse, which heats up.
arcing
the fuse-conducting element melts and an arc is generated between the terminals. The arc resistance increases very rapidly, causing the current to drop and the voltage to increase. Eventually the arc is extinguished. At this point, the current is zero and the voltage equals the system voltage.
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54. Dc systems faults management
The energy during pre-arcing is
where TF is the total fault current clearing time, RF is the fuse resistance before melting, and IC,F is the limiting case capacitor current during the fault.
IC,F equals the fault current less the sum of the converter current limit and other circuit currents. For larger capacitances than the limit case, the converter current may not reach the rated limit value, so IC,F might be slightly higher than in the limit case.
.If a linear commutation is assumed, the portion of the arcing phase energy supplied by the capacitor is
Thus,
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55. Dc systems faults management
With VS = VF = 50 V, IC,F = 135 A, RF = 1 mΩ, and considering a typical value for TF of 0.1 s, the minimum value of C is 900 μF. If the previous system is simulated with C = 1mF, then
Ringing on R2 occurring when the fault is
cleared can be eliminated by adding a
decoupling capacitance next to R2
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56. Dc systems faults management
Additional simulation plots
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57. Series faults in ac systems
Series faults occur when a cable is severed or a circuit breaker is opened, or a fuse is blown…. Then an arc is observed between the two contacts where the circuit is being opened.
The arc is interrupted when the current is close to zero.
Due to cable inductances, voltage spikes are observed when the arc reignites.
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58. Series faults in ac systems
Visually, arcs in ac series faults are not very intense
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59. Series faults in dc systems
In dc arcs last longer (because there are no zero crossings for the current) but no voltage spikes are generated.
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60. Series faults in dc systems
Dc arcs last longer than ac ones, are much more intense and may damage the contacts.
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61. Solid state switches
DC currents are more difficult to interrupt than equivalent ac currents when using conventional switchgear (physical separation of contacts).
Proposed solution: solid state circuit breakers.
Solid state circuit breakers do not provide a physical disconnection, but such disconnection can be implemented by adding a conventional disconnect switch in series with the solid state circuit breaker. The conventional disconnect switch acts after the solid state switch interrupts the current.
Other issues with solid state circuit breakers:
ON-conduction losses
May fail as a short circuit (although conventional switches may also fail in this way)…. Solution: redundancy
For higher voltages, series connection of devices is necessary leading to coordination issues (“perfect” on-off action) and over-sizing to prevent device damage from excessive voltages during transients due to switching lack of coordination.
Advantages of solid state switches:
Allow for many ON-OFF cycles.
Contacts are not worn out (because there are no contacts).
Act quickly.
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62. Solid state switches Examples ac SCR ac IGBT (no redundancy) dc IGBT
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