ee 394j-10 distributed technologies

1 © Alexis Kwasinski, 2012

EE 394J-10 Distributed Technologies

Micro-grids architectures, stability and protections


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.



• 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.



• 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.


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 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


• Typical configuration:

•Total power consumption: > 5 MW (distribution at 208V ac)

Conventional (ac) datacentersConventional (ac) datacenters


Conventional (ac) datacentersConventional (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


• Use of 380 Vdc power distribution for:• Fewer conversion stages (higher efficiency)• Integration of local sources (and energy storage).• Reduced cable size

New (dc) datacentersNew (dc) datacenters


Data centers efficiency comparison dc vs. acData centers efficiency comparison dc vs. ac

Brian Fortenbery and Dennis P. Symanski, GBPF, 2010

• A 380Vdc power distribution standard is currently under study by the IEC


• 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.

New distributed (dc) datacentersNew distributed (dc) datacenters


Utility dc distributionUtility dc distribution

Jonbok Bae, GBPF 2011


FUEL CELL

ENERGY STORAGE

PV MODULES

AIR CONDITIONER

REFRIGERATOR (LOAD)

WIND GENERATOR

MAIN DC BUS

EPA 430-F-97-028

dc Homesdc 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%)

LED LIGHTS (DC)

ELECTRIC VEHICLE


ac microgrids stability• Consider an ac microgrid with one ac generator and one load.

• The simplified equivalent circuit for the generator and its output equation is:

• From mechanics:2

2 ( ) ( )mm e

dJ T t T tdt

LOAD

Assumption: Infinite bus (simplifies the analysis but not true for micro-grids).Also during shorts or load changes E is constant

Electric power provided to the load

. sineE VpX

Moment of inertiaangular

accelerationmechanical

torqueelectrical torque


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

#2 m

polesx x

( )m syn mt t

Synchronous speed

Mechanical equivalent of its electrical homologous

variable

2,0.5 m syn

rated

JH

S

2

. , . . , .2

2 ( )( ) ( ) ( )p u m p u e p usyn

H d tt p t p tdt

2,0.5 m syn

rated

JH

S

e mp p ( )te mp p ( )t e mp p ( )t


ac microgrids 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 will oscillate until losses and load damp oscillations and • If the generator looses stability.

1) Initial condition

0( )t ( )t

2) During the fault pe = 0

e mp p

e mp p

2

. sineE VpX

3) Fault is cleared here

2 3

Both areas are Both areas are equalequal

4) Because of rotor inertia

increases up to here

( )t2

( )t


ac microgrids stability• In ac systems, active 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 the future.


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.

Parallel structure

Cascade structure



• 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.



• 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).

Centralized architecture

Only (centralized) bus bars



• 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).

Distributed architecture

Each cabinet with its own bus bars connected to its own battery string and loads. Then all cabinets’ bus bars are connected


Power architectures topologies• Some examples of different power distribution architectures:

Ladder

RingRadial


Dc microgrids stability• Stability issues

• 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 ( )

( )LPv t

i t


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.

lim

lim

0 if ( )( )

if ( )( )

L

v t Vi t P v t V

v t

2

( ) 0( ) ( )

LPdv tZdi t i t


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


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.

vC(0) = 120 V vC(0) = 50 V

iL(t)

vC(t)

iL(t)

vC(t)


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.

Buck LRC with a PL = 5 kW, E = 400 V, L = 0.5 mH, C = 1 mF, D = 0.5.

2

( )CLL C

C L

CvPi E v

v LP

- Approximate for the separatrix:

2

( ) 1C L LL

C C

d t E v P Pi

L C v v

- Necessary but not sufficient condition for oscillations:


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.


• Model for a buck converter with a CPL:

( )( ) (1 ( ))( ),

with 0,

LS L D L D L L C

C CLL

C o

L C

diL q t E R i q t V i R i R v

dtdv vP

C idt v R

i v

Dc Microgrids Stability

22 2

1 1 1 0i iL L

o oo o

R RP PL R C L R C LCCV CV


• 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)

Dc Microgrids Stability

2 1L o i

o

CP V RL R

2 1 1L o

i o

P VR R

(Predominant condition)


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.


Dc Microgrids Stability• With an open loop control (fixed duty cycles) the system shows again important oscillations, well distant of the desired dc behavior.


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


Dc Microgrids Stability• OPTION #2: Add filters, particularly capacitors.

• Oscillations decrease with increased capacitances

• Since the oscillation frequency s in the order ofhundreds of hertz, increasingcapacitance is expensive.

•Large capacitors tend to beunreliable.

60 mF capacitance placed at the CPL input and at the buck

converters output.


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


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


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.



• PI controller

• ki = 1

• kp = 0.1

• With PI controllers stability is not insured and results are poor.

vB1(t)

vB2(t)

vB3(t)



• PID controller

• ki = 1 10-3

• kd = 50 10-6

• 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)



• PD controller

• kp = 0.5

• kd = 50 10-6

• 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)


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


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.

Buck

Boost

Buck-boost


Dc Microgrids Stability• OPTION #6: Geometric controls.


ProtectionsProtections• Consider as an example the power distribution system of a full electric ship (which can be considered as an islanded microgrid)


• Circuit protection: conventional approach based on switch gear. Issues:• Coordination• Fault current detection and interruption

ProtectionsProtections


• Circuit protection: based on power electronics or solid state circuit breaker.

ProtectionsProtections


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


Dc systems faults management• With C = 600 μF, the fault is not properly cleared and voltage collapse occurs for both loads.


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.

212C c FW Cv W


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,

2, ,

1 0.94F pa C F F FW I R T

, ,1 0.16F a C F F FW I V T

2, ,2

1 1 10.9 0.12 3C F F F C F F F

S

C I R T I V TV


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


Dc systems faults management• Additional simulation plots


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.


Series faults in ac systems

• Visually, arcs in ac series faults are not very intense


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.


Series faults in dc systems• Dc arcs last longer than ac ones, are much more intense and may damage the contacts.


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 incoordination.

• Advantages of solid state switches:• Allow for many ON-OFF cycles.• Contacts are not worn out (because there are no contacts).• Act quickly.


Solid state switches• Examples

ac IGBT (no redundancy)

dc IGBT

ac GTO


Solid state switches• Switch technologies: MOSFET, IGBT, Thyristors (SCR, GTO, GCT, MCT).• MOSFET: Intended for low power and voltage applications (in which semiconductor switches may not be competitive to conventional circuit breakers). Conduction losses are resistive and resistance increase with higher voltage ratings. For a given device the ON-resistance increases with current so in order to achieve low losses it may be needed to operate it below about 60% of its rated current.• IGBT: Voltage ratings higher than in MOSFETs. Losses are relatively higher than thyristor-based technologies but lower than MOSFETs as current increases. Control is simpler than in other technologies and limits the current in an inherently way (as oppose to thyristor-based devices in which the current is not limited by the gate voltage as it happens with transistor-based devices). A varistor is used to absorb energy and clamp the voltage during turn-off• SCR: Rated at higher voltages than other solutions and have lower conduction losses than other technologies in equivalent conditions. In principle it is not suitable for dc (how to turn it off?). Can still be used with force commutated circuits to reduce the current to zero fast but usually requires a capacitor (expensive at higher voltages and less reliable). Force commutation circuit increases the cost.


Solid state switches•GTO: Voltage ratings tend to be higher than in IGBTs. Limited turn-off capabilities (additional circuitry is required to implement a fast turn-off because although thyristor-based devices can conduct a given “large” current, they may be damaged when attempting to interrupt such current). A snubber may not be necessary provided that the circuit inductance is high enough to yield a low di/dt (i.e. snubber may not be necessary if the load includes a power transformer). However, when having multiple thyristors in series individual snubbers for each device may be needed in order to ensure equal voltage distribution among devices.• GCT (also known as Integrated Gate-Commutated Thyristor (IGCT)): Very similar to a GTO (the main difference is smaller cell size built with a p-n-p configuration on turn-off for the IGCT vs. a n-p-n-p configuration for GTO) and lower gate inductance). In equivalent conditions it has the lowest conduction losses of all of these technologies. High-cost and limited availability.• MCT: Similar to GTO, it has lower conduction losses than transistor-based technologies (IGBT and MOSFETs). Gate is controlled based on voltage instead of current as it is observed in the other thyristor technologies. The MCT requires a positive voltage applied to the gate in order to remain in the OFF condition. High-cost and limited availability.

ee 394j-10 distributed technologies

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