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Control and Protection Requirements for
Microgrids
Reza Iravani
(iravani@ecf.utoronto.ca)
Department of Electrical and Computer Engineering
University of Toronto
10 King's College Road
Toronto, OntarioCANADA
M5S 3G4
Montreal 2006 Symposium on Microgrids
June 23, 06
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Outline
Definitions
Microgrid Control
Microgrid Protection
A Signal Processing Module for Microgrid Integrated
Control and Protection Application Example
Study Results
Conclusions
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Definitions
Distributed Resource (DR) Unit:is either a distributed Generation (DG) unit, a Distributed Storage (DS) unit,
or any combination of DG and DS units that can be operated as either
a dispatchable or a non-dispatchable entity.
Dispatchable DR Unit:is a unit that its output instantaneous real-power and/or reactive-power
components are controllable, for a pre-specified time interval.
Microgrid:is a cluster of DR units and loads services by an electric power grid (usually
at a distribution voltage class), and capable of:
operation in a grid-connected mode,
operation in an islanded (autonomous) mode,
transition between grid-connected and islanded modes,
ride-through for each DR unit in either grid-connected and islanded
modes.
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Microgrid Control Functions:
Control of DR units (based on locally and/or remotely measured signals)- excitation and governor controls of Synchronous Machine (SM) based DR units
- direct- and quadrature-axis variables controls of VSC-coupled DR units, i.e.
- current-controlled VSC (CC-VSC)
- voltage-controlled VSC (VC-VSC)
- hybrid or hierarchical of VC-VSC and CC-VSC
- imposition of limits for each controller
Coordination of DR unit controls- Coordination of controllers of DR units for real-power management, for example
based on frequency-droop (and frequency restoration) method
- Coordination of controllers of DR units and reactive-power sources for voltage
control, for example based on (a) voltage-droop, (b) voltage regulation, (c) power
factor correction, (d) or any combination of these methods
- Coordination of limiters
A scenario that causes excursion of a microgrid out of the acceptable operational region,
should be counteracted by activating:
(a) either another set of controls, e.g. based on a discrete event control strategy
(b) or microgrid protection.
Coordination of controls requires fast detection capability.
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Microgrid Protection Functions
Protection against fast transient events which are beyond the speed of
response and/or range of operation of continues controls, for example:
surge arrester operation in response to transient over-voltages
fuse operation due to faults
fast current limiter operation subsequent to faults
Protection against dynamic events for which fast controls of a microgrid can
participate either to reduce the impacts or mitigate the phenomena, for example: over/under voltage protection
over/under frequency operation
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Microgrid Protection Functions (Continued)
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The latter case requires integration of protection and control functions
either through (i) change of control mode/strategy (e.g. CC-SVC to VC-VSC
mode), (ii) dynamic change of control parameters, (iii) activating auxiliary
controls, or (iv) a combination of (I) to (iii). Application example are to: enhance ride-through capability of DR units during microgrid faults,
enable fast islanding detection to prevent DR tripping,
provide ride-through capability subsequent to single-phase load
energization,
enable transition between grid-connected and islanded modes.
Integration of Control and protection functions requires
fast detection capability.
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7Block diagram representation of the Unified Three-phase Signal Processor (UTSP)
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DG unit: variable-speed, direct-drive wind unit
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field-controlled SG27 rpm, 84-pole690 Volt
Generator Technology:
IGBT switchtwo-level converter2400 Hz switching frequency
Converter Technology:
1000 kW Rated Power of Unit:
SG Grid
AC-DC
DC-DC
Converter
System
DC-AC
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(a) positive-sequence current controller and (b) positive-sequence voltage/current
resolver with embedded PLL
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(a) negative-sequence current controller and (b) negative-sequence
voltage/current resolver
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Block diagram of the converter PWM signal generator
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Control Protection platform of the DG Unit including UTSP
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Schematic diagram of the DG unit under UL1741 anti-islanding test condition
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Schematic diagram of the DG system illustrating positive- and
negative-sequence current injection for islanding detection
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UTSP and the test system signals under UL1741 test conditions
(a) PCC voltages, (b) grid currents,
(c,d) PCC instantaneous positive-, and negative- sequence voltages
(a)
(b)
(c)
(d)
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UTSP simulated output signals under UL1741 test conditions
(a) estimated frequency, (b,c) PCC estimated magnitudes of
positive-, and negative-sequence voltages
(a)
(b)
(c)
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UTSP output signals under UL1741 test conditions
(a) estimated frequency, (b,c) estimated magnitudes of
positive-, and negative-sequence PCC voltages
(a)
(b)
(c)
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Estimated magnitude of the negative-sequence of PCC voltage
for different levels of injected negative-sequence current
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Estimated magnitude of the negative-sequence of PCC
voltage for different SCR values
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UTSP output signals whenL changes from 95 to 105% of rated value
(a) estimated frequency, and (b,c) estimated magnitudes of positive-,
and negative-sequence PCC voltages
(a)
(b)
(c)
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Summary and Conclusions
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Operation of a DR unit in a microgrid requires coordinated control and
protection strategies that can handle:
- grid-connected mode of operation,
- islanded (autonomous) mode of operation,
- transition between grid-connected and islanded modes,- ride-through transients in grid-connected and islanded modes.
Coordination of control and protection of a DR unit requires fast and
reliable detection of type and severity of microgrid disturbances.
A detection method was presented and it its analog implementation and
digital hardware realization were illustrated.
Application of the detection method for islanding detection of a DG unit,
under UL1741test conditions, was demonstrated.
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