Microgrid topology for distribution feeders Alan McDonnell, - PowerPoint PPT Presentation

A new “Cut and Splice” design Microgrid topology for distribution feeders Alan McDonnell, President

Why a Microgrid? • The main purpose of this circuit topology is to be able to add significant amounts of distributed generation (DG) on to a distribution feeder and overcome interconnection permitting difficulties • The main obstacle is DG penetration depth limits • The penetration depth limits are related to limiting the negative effects of control, stability and protection of the existing grid that occur when DG is connected in the standard way

Review of standard, parallel interconnection

Fault Current Problems • One inherent problem with simply connecting DG in parallel with an existing feeder is that of fault current contribution, as can be seen in the following slide. • The existing grid is designed and rated based on known fault currents, which are a function of upstream source impedance. •Inverter based parallel connections can mitigate this down to a low level, but then may be prone to nuisance tripping

Fault current additions to existing protection devices

The destabilizing effect of too much DG • Another problem that is much harder to deal with is the fact that the existing grid, as a voltage source, may be influenced if DG penetration levels are too high. • Frequency control of the grid is achieved through throttle control of very large generators. DG sources are uncontrolled, which is not a problem as long as they are a small part of the overall grid. • Much effort is going into making control algorithms to overcome this problem

An example of an urban power grid from source to building

The Solution…. Start a new “microgrid”

Design Advantages • The main advantage to this design is that the DG and Storage Elements no longer need to be “interconnected” to the existing grid • By starting a new AC grid, un-synchronized to the main grid, the new grid simply appears as a load reduction, like adding a VFD to an existing motor. •This overcomes the need for upgrades to the existing grid to accommodate the effects of connecting generators or storage units

Controller Details; what’s inside the blue box?

Other Design Details • The “active rectifiers” that take power from the existing grid add no upstream fault current contribution or power quality distortion. •There are 2 separate DC buses, such that failure of one will not cause failure of the second. • Power can still be purchased from the main grid, when prices warrant or for back up in case of fuel disruption or DG failures.

Disadvantages • The capital cost of the power converters • Although energy can still be supplied from the grid, there will be an efficiency loss through the converters if grid power is used. • Reverse power flow is forbidden in order to bypass the interconnection process and limits, unless the grid operator wishes to allow it, but then all the rules of interconnection are required. •Downstream fault current sourcing capacity will be reduced, probably requiring new protection studies

Other Advantages • Fast installation or return to original distribution layout, since none of the existing distribution is removed, only cut and spliced. • UPS grade power for the whole microgrid, no need for momentary shut off during main grid loss • Power converters can be paralleled for redundant operation. With very reliable natural gas supply or energy storage, the microgrid can have very high reliability (many 9’s!)

The enabling technology • There now exists next generation power converters that have the capability for power levels of several MW, along with very high bandwidth control. • Using a neutral point clamp topology, and high switching frequency, low voltage IGBTs, the power converters can provide efficient, high quality, true 4-wire voltage source control. •High speed communications also add to the control capabilities

3-level inverter description

Unlimited expansion capability •Many microgrids can be added near each other, and energy can be shared between them without travelling through the existing grid. • Smaller DG sources can be added to run in parallel with the microgrid, with responsibility for penetration depth limits and safe operation now the responsibility of the microgrid, not the main grid. •The “Cut & Splice” can take place anywhere in the existing grid and can be used to extend existing feeders

An expanded future grid

Last features… • The microgrids can communicate with each other and to a larger “Smart Grid” controller to aggregate their benefits to the overall grid. •The “active rectifiers” that interface with the existing grid can be used to provide very high dynamic VAR current to the existing grid, allowing for greater use of the existing grid as well. • If necessary, the active rectifiers could also provide dynamic frequency stabilization of the existing grid, up to the real power limits of load, DG sources and energy storage capabilities.

One big happy family…

Alan McDonnell President Non-Synchronous Energy Electronics, LLC 94 Middlesex Rd. Merrimack, NH 03054 (603) 546-5785 www.nonsynchronous.com