Friday, December 3, 2010

Bolstering the Power Grid (2)

Shown is a trailer-based dynamic synchronous condenser, of the type used by the TVA. The compact size allows many of these HTS devices to be used in the grid system to avert power blackouts.

We continue where we left off, examining how voltage collapse can be prevented in a power grid using HTS technology. The point was made that capacitative elements inserted in a grid system can prevent or significantly limit voltage collapse because the current leads the voltage for capacitative reactances (X_C).

The primary problem with the U.S. power grid is the lack of volt-amp-reactive (VAR) dynamic compensation elements. Instead, an array of conventional methods are employed to give a rough equivalence to true VAR-compensation. If instead of this approach, HTS dynamic synchronous condensers (see image) were more generally used many advantages would accrue, including:

i) Large Q (imaginary) power output
ii) Rapid dynamic response
iii) Minimizing of switching transients
iv) Long term reliability from the stable operating temperature (of HTS coils)

To better grasp the HTS condenser’s remarkable abilities, a basic inductive physics set-up (often employed to teach physics students) is useful. We obtain a U-shaped magnet wrapped with n turns of conducting wire. Then the magnetic flux induced in the gap is proportional to nI (product of turns by the current) and inversely proportional to the gap length, x. Thus:

Flux ~ n I/ x

Note as x is decreased, the flux gets larger. In like manner, the closely spaced iron teeth in rotor and armature of a conventional motor are closely spaced precisely to enhance the flux by minimizing the gaps. Logically then, a machine with HTS rotor coils and no iron teeth has much greater effective gap between the armature coils and magnetic components in the core.

This lowers the flux, but this is compensated for by the fact the high current density HTS wire is so fine that many more turns n are allowed. This enables any HTS device to generate a much greater flux than its conventional counterpart.

When d.c. HTS rotor coils spin they generate a time –varying flux that induces an rms excitation voltage (V_e) according to Faraday’s law such that: V_e = - d(phi)/dt

Where 'phi' is the magnetic flux.

Meanwhile, the a.c. armature current I(a) induces a back emf (V_b) in the armature coils that is proportional to I(a) but out of phase with it. The proportionality constant is denoted as X_s the synchronous reactance. Then:

V_e = X_s I(a) - V_b

Now, the sum of the two voltages induced in the armature coil must equal the grid voltage: V_G, so the out of phase reactive armature current coupled to the grid is given by:

I(a) = X_s[V_e - V_G]

Note what happens above, say if the grid voltage V_G drops below the level set by V_e, say to V_e/10 or 0.01 (V_e). Then the HTS synchronous condenser injects capacitative current into the grid. If the converse holds, and V_G > V_e, the condenser injects inductive current. (Since recall the current lags behind the voltage). Most importantly, the magnitude of V_e can be adjusted in seconds by changing the HTS rotor coil current.

It is precisely the "hair-trigger" control over V_e that allows a dynamic response to the VAR-compensation needs of the grid.

Thankfully, first generation HTS wire-based dynamic sychronous condensers have already been field tested by the TVA (Tennessee Vallee Authority), and additional orders have been placed. This is good, but such technological enhancements need to be made throughout the U.S. grid to bring us up to date, as electrical power needs ramp up.

This will be especially important as global warming is exacerbated, with mean global temperatures` expected to increase by a further 3 degrees Celsius by 2100. (That is the average expected, it could be lower or higher). This means there'll be more prolonged heat waves and of greater intensity with higher night time temperatures - leading to a vastly higher draw down of power as millions in cities struggle to stay cool with their air conditioning. Unless grid capacity is majorly enhanced, along with its ability to deal with variable loads (from fault currents), surges ...this country is in for a lot of trouble.

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