Fuel cell power system connection. Dynamics and Control of Distributed Power Systems. DC storage. DC/DC boost converter (1)

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1 Dynamics and Control of Distributed Power Systems Fuel cell power system connection Ian A. Hiskens University of Wisconsin-Madison ACC Workshop June 12, 2006 This topology is fairly standard, though there are numerous variations. The source can be fuel cells, microturbines, photovoltaics, variable speed combustion turbines,... DC storage DC/DC boost converter (1) Possibilities include batteries, ultra-capacitors, flywheels, Stabilizes the DC bus voltage. Provides a buffer between energy supplied from the source and energy delivered to the power system. If the storage is sufficiently large, then the source (fuel cell) is effectively decoupled from the power system. This adds expense and size though. Desire a control-based solution that minimizes storage. Storage plays a role similar to rotating inertia in traditional power plants. With the switch closed, the fuel cell voltage appears across the inductor. The inductor current (and hence stored energy) build up. Current backfeed from the capacitor is blocked by the diode. The inverter draws current (energy) from the capacitor, with consequent capacitor voltage reduction.

2 DC/DC boost converter (2) DC/DC boost converter (3) With the switch open, inductor current supplies inverter and capacitor. Energy is delivered from the inductor to the inverter and capacitor. Current into the capacitor builds up its voltage. The ripple on the inductor current (drawn from the fuel cell) is dependent upon the inductor size. Larger inductor => smaller current ripple. The ripple on the capacitor voltage (DC voltage seen by the inverter) is dependent upon the capacitor size. Larger capacitor => smaller voltage ripple. Inverter electrical relationships The inverter uses power electronic switches to synthesize AC voltage waveforms from a DC voltage source. Assume harmonics are negligible. The phase angle specifies the zero crossing relative to a central synchronous clock. RMS line-to-line voltage phasor (AC side) is RMS line current phasor (AC side) is Assume the inverter is lossless, so Inverter controls The inverter control variables are the modulation index, the voltage phase angle, and the frequency The modulation index has a maximum value. For a well designed inverter, operating under normal conditions, the maximum modulation index should not be encountered. However as inverter-based systems are driven harder, this may become a factor. The control objectives are (typically) to regulate the delivered active power and the terminal voltage.

3 systems Distributed generation Substation Transmission When all circuit breakers are closed, the distribution system can lean on the transmission system to balance supply with demand. In this case, distributed generators (fuel cells) can be set to deliver constant active power at unity power factor (zero reactive power.) Or controlled to some other desired reactive power setpoint. It doesn t matter how the loads behave, because all load changes are taken up by the transmission system. Customers Microgrid Demand Response system protection Microgrid concept Distributed generation causes difficulties for traditional protection schemes. systems usually have a radial (tree) structure, which ensures power flows from high voltage transmission system, through transformers (at the supply substation) and outwards along the distribution feeders. A fault in the distribution system causes high current flow. The high current will exceed the protection trip setting (if designed correctly.) A fuse (or circuit breaker) will trip, disconnecting the downstream section of the distribution system, clearing the fault, and causing minimum disruption to consumers. The current inflow provided by distributed generators alters this flow pattern. Faults become more difficult for protection to detect. Networked (mesh) distribution systems give higher reliability, but are even more difficult to protect because of inconsistent flow patterns. A microgrid is formed when a circuit breaker (CB) opens, isolating a section of the distribution system that includes generators and loads. The CB must also be able to reconnect/synchronize the microgrid to the main power system. Microgrids may not have consistent structure. In the example, either green CB may open. Substation Customers Transmission Microgrid Demand Response

4 Microgrid control issues For autonomous microgrid operation, distributed generators must provide frequency and voltage regulation. Generation must match load. Load control (tripping) will be required if the generation capability is significantly less than the load demand. P [pu] 1 P 2[pu] Frequency regulation (1) Output power versus frequency (droop characteristic.) Utility System Event: Transfer to Island A: : Grid B: Island 6 kw 54 kw F 1 P 1 42 kw 60 kw wire wire 75yd 25yd P 2 L1 L3 L4 L5 F 2 1 Frequency [Hz] Load kw 102 kw P min =0 Gr id Flow 42 kw 0.0 Per unit power base = 75 kva P max = 60 kw Frequency regulation (2) In an inverter-based system, AC voltage waveforms are synthesized by power electronic switches. Frequency is therefore totally arbitrary. Frequency measurement is given by a phase locked loop tracking zero crossings of the voltage waveform. Provides filtering. Voltage regulation Voltage regulation also requires a droop characteristic. Otherwise it is difficult to share reactive power. Simple example: If V 1 > V 2 reactive power will be generated by G 1 and absorbed by G 2. Generators can go to opposite limits, with the resulting current flow inducing unnecessary losses.

5 Droop characteristic Voltage droop Microgrid operation To achieve autonomous microgrid operation, DG controls should always operate with frequency and voltage droop characteristics. Frequency droop won t have any effect when grid connected, as the grid will hold the frequency effectively constant. When the microgrid separates from the main grid, generation/load imbalance will result in an altered frequency setpoint, and consequent adjustment of active power production. Load control If the load is too high after microgrid separation, the frequency will keep falling even after all generators have reached their maximum limits. This implies a need for under-frequency load shedding to trip excess load, and restore an achievable generation/load balance. Also, rate-of-change limitations on sources such as fuel cells place demands on energy storage and load control/scheduling. Microgrid reconnection (1) Reconnection of a microgrid to the main grid must take account of the different frequencies of the two grids. The frequency difference results in a beat frequency. The circuit breaker must close near the instant where the voltage difference across it is almost zero.

6 Microgrid reconnection (2) Inverter output regulation (1) Objectives: Deliver scheduled active power P gen (modulo frequency droop.) Regulate terminal voltage V 2 (modulo voltage droop characteristic.) Correct closing 27 early closing The inverter can synthesize voltage at any desired frequency. Note that angles describe the zero crossings of the respective voltages relative to some central clock that is dictating synchronous speed (nominal frequency.) But the inverter does not have (and does not need) access to that centralized information. The actual angle values cannot be measured directly. The angle difference is important though, as it determines the active power Inverter output regulation (2) The grid connection point AC (time varying) voltage is measured. The peak value is easily determined. A phase locked loop (PLL) is used to lock onto the zero crossings. Voltage magnitude regulation is achieved by Inverter output regulation (3) Active power is regulated by controlling the angle difference (the difference in zero crossings between the inverter terminal voltage and the grid connection voltage.) The PLL angle (zero crossing) is used as a (flitered) estimate of. Let Regulation is achieved by If generated power is less (greater) than the setpoint, increase (decrease) the angle difference.

7 Inverter output regulation (4) PLL tracking is developed from Inverter output regulation (5) The complete model is given by Second order is required to track time-varying. (This gives zero offset if follows a ramp, due to a constant frequency offset.) An estimate of the frequency is provided by, which can be used in the droop characteristic. However this PLL tracking scheme introduces undamped oscillations. Damped tracking is achieved with the modification State variables: Algebraic variables: Microgrid example Inverter connected to a switched resistor.