Electricity: battery or capacitor charging or discharging – Capacitor charging or discharging
Reexamination Certificate
2000-11-09
2003-10-14
Tso, Edward H. (Department: 2838)
Electricity: battery or capacitor charging or discharging
Capacitor charging or discharging
Reexamination Certificate
active
06633154
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention generally relates to the use of polarized electrical charge storage devices in AC applications. In particular, the present invention relates to biasing polarized devices, such as polarized capacitors with a DC potential for uses in general AC applications.
BACKGROUND
Capacitors are used for a variety of purposes including energy storage, signal coupling, motor starting, power factor correction, voltage regulation, tuning, resonance and filtration. In series and shunt implementations, there are many operational advantages, both transient and steady state, for implementing capacitors in general AC networks.
Network efficiency is increased with power factor improvement, during transient conditions. Transient applications of series capacitors include voltage surge protection, motor starting, current limiting, switching operations and the like. Series capacitors can moderate the effects of AC network faults and other transient conditions. For example, low power factor transient currents are associated with magnetic inrush currents due to motor starting, transformer inrush and fault currents. Series capacitance improves the overall power factor and network voltage regulation during these transient conditions. Series capacitor banks also demonstrate a degree of current limiting due to the series impedance of the capacitor. This reduces fault currents and thus reduces generator, transformer, switchgear, bus and transmission line size requirements. The capacitor in series with the fault acts as a current limiting device. Tuned circuits composed of inductors and capacitors (LC circuits) are used for filtration. A high induction series version can dramatically increase network fault impedance by deliberately shorting out the capacitor bank. A series capacitor bank is typically coupled to a transformer. Transformer opposition to instantaneous current change combines with capacitor opposition to instantaneous voltage change. These characteristics lead to greater instantaneous network voltage stability as a result of increased use of series capacitor banks. Secondary effects include voltage surge protection, demand factor improvement and voltage regulation. Instantaneous power transfer efficiency can be improved with proper capacitor use. While these many series capacitor advantages are well known, and proven in the lab, unit cost and size requirements have prevented their general implementation.
AC network steady state characteristics are also improved through the incorporation of capacitors. High capacitance, series applications impress a low steady state AC voltage on the capacitor. This is helpful when electrical transfer devices are used in conjunction with series capacitor banks. Electrical wave distortion is similarly reduced with increasing capacitance. Steady state series capacitor applications include motor running, filtration, power factor correction, efficient power transfer, voltage boosting and the like. Series capacitor banks allow induction generators to power induction motors, by providing the required magnetizing [VARs] for both devices. This can also improve the power quality, while reducing the cost of electric grid alternative sources, emergency power supplies, mobile equipment and portable generators. Mechanical stress associated with bringing additional generation capacity, on line, to synchronous operation, can be moderated by the presence, of series capacitive coupling.
The two major capacitor categories are polar and non-polar. There are many realizations of each category. Due to their uni-directional, forward biasing requirements, polarized capacitors are mostly used in DC and small AC signal applications. Polarized capacitors are widely used in DC filtering applications, such as output stages of DC power supplies. Audible frequency (music) amplifiers use a DC biased polarized capacitor to couple signals. Conversely, non-polarized capacitors are generally useful in both DC and general AC applications. Unfortunately, non-polarized capacitors—especially in series applications—are not well suited for many AC and DC uses due to their limitations in size, capacitance, weight, efficiency, energy density and cost. The use of undersized non-polar capacitor banks causes significant current waveform distortion and a large voltage drop across the capacitor, which results in energy losses and poor AC voltage regulation at the AC load.
Conversely, polarized capacitors, as well as other polarized electric charge storage (PECS) devices, have a low cost per unit of capacitance, as well as smaller mass and dimensions, as compared with nonpolar capacitors. These characteristics favor their use over non-polarized capacitors. They also exhibit a relatively low series AC resistance at power frequencies. However, they may only be effectively operated with positive “forward” voltages relative to their positive and negative poles. A reversed voltage of any significant magnitude causes the capacitor to short, which usually results in an explosion that can be comparable to that of a hand grenade. In fact, with solid tantalum capacitors, this short circuit failure mode includes spontaneous combustion. Thus, polarized capacitors, for the most part, have not been amenable for general AC applications.
FIG. 1
models the normal operation of a polarized aluminum electrolytic capacitor as well as circuit operation in over-voltage and reverse bias voltage mode. The model consists of series inductor
101
, series resistor
102
, parallel resistor
103
, zener diode
104
and polarized capacitor
105
. Zener diode
104
models the forward and reverse shorting condition present when the impressed voltage exceeds a reverse bias voltage of 1.5 Volts or a forward bias condition of approximately 50 Volts over the rated working DC voltage (WVDC) of the capacitor. Inductor
101
is suitable for modeling the self-resonant frequency of the capacitor. The series resistor
102
models the (small, m&OHgr;) equivalent series resistance (ESR) measured in capacitor operation. The parallel resistor
103
models the (large, M&OHgr;) equivalent parallel resistance measured in capacitor DC leakage current phenomenon. In low frequency operation, forward biased voltage within the device working voltage conditions will allow signal current flow through the directional capacitor
105
. Reverse bias conditions will occasion a short through diode
104
.
The capacitor will suitably operate continuously between zero volts and the rated working DC voltage. A reverse bias voltage of up to about 1.5 volts DC to a rated forward bias surge voltage defines the outer limits of appropriate transient use of the capacitor. Capacitor operation outside this wider voltage envelope will cause short circuit conditions. There is typically a third, higher impulse voltage parameter. Excessive forward voltage on the capacitor will cause a reverse current flow through zener diode
104
. This electrical behavior is schematically modeled by depicting a zener diode
104
in parallel, but with opposite polarity alignment than the polar capacitor. Shorting through diode
104
, in either direction permits excessive current, heat buildup, which eventuates capacitor failure. This is why a single polarized capacitor fails in normal AC operation.
FIG. 2
depicts a simple circuit realization
250
that illustrates a typical prior art use of a DC biased polarized capacitor in a small AC signal coupling application. This circuitry is commonly used as a laboratory exercise for undergraduate analog electronic students and is employed in multi-stage amplifiers. Circuit
250
includes an AC signal source
255
superimposed upon a DC bias voltage source
260
, a capability of lab power supplies. The AC signal is coupled to the load
266
, while the DC bias voltage is blocked by and positively biases a polarized capacitor
262
. The capacitor and DC bias voltage are selected so that the superimposed AC and DC voltages are at all times within the proper voltage window. The AC source output section co
Nordstrom Erik R.
Tso Edward H.
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