EE 394J-10 Distributed Technologies

1. EE 394J-10 Distributed Technologies Micro-grids architectures, stability and protections

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.

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.

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.

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 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

6. Conventional (ac) datacenters • Typical configuration: • Total power consumption: > 5 MW (distribution at 208V ac)

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

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

9. Data centers efficiency comparison dc vs. ac • A 380Vdc power distribution standard is currently under study by the IEC Brian Fortenbery and Dennis P. Symanski, GBPF, 2010

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.

11. Utility dc distribution Jonbok Bae, GBPF 2011

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-97-028 FUEL CELL

13. 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: 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

14. 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

15. 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. 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

16. 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.

17. 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

18. 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.

19. 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

20. 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

21. Power architectures topologies • Some examples of different power distribution architectures: Radial Ring Ladder

22. 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

23. 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.

24. 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

25. 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

26. 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.

27. 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.

28. Dc Microgrids Stability • Model for a buck converter with a CPL:

29. 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)

30. 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.

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

32. 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

33. Dc Microgrids Stability • OPTION #2: Add filters, particularly capacitors. • Oscillations decrease with • increased capacitances • Since the oscillation • frequency s 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.

34. 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

35. 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

36. 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.

37. 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)

38. Dc Microgrids Stability • OPTION #5: Linear controllers. • 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)

39. Dc Microgrids Stability • OPTION #5: Linear controllers. • 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)

40. 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

41. 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

42. Dc Microgrids Stability • OPTION #6: Geometric controls.

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

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

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

46. 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

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

48. 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.

49. 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,

50. 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