Patent Description:
The quality of power distributed through modern electrical distribution systems continues to be an issue concerning operators of large systems. One such power quality problem is known as voltage flicker. Voltage flicker is a voltage dip that is of a magnitude sufficient to have an objectionable effect on other loads connected to the same circuit. The disturbance may be experienced as only blinking lights, but the magnitude and the frequency of the occurrences determine flicker's impact on system users.

<FIG> illustrates a common voltage flicker scenario. Flicker-producing loads <NUM> on system <NUM> are typically caused by large motors, welders, or arc-furnaces. These loads are characterized by high inrush currents of relatively short duration, as experienced in the starting of a motor. The motor's inrush current is typically of a low power factor, and causes a voltage dip of increasing magnitude along the feeder up to the point of the load's connection. This causes voltage flicker problems between the load <NUM> and the source <NUM>, which, when severe enough often leads to a user complaint <NUM>.

The distribution series capacitor <NUM> has long been recognized as a cost-effective solution to these types of flicker problems. Unfortunately, distribution-class electrical power lines equipped with a distribution series capacitor are subject to two distinct and damaging phenomena, ferroresonance involving transformers, and self-excitation of motors during starting. Ferroresonance is an often severe and rapidly building oscillatory overvoltage condition caused by system non-linearities that can appear when power transformer cores saturate. These non-linearities interact with the series capacitor to produce a low-frequency resonant condition, often in response to large inrush currents following breaker operations. Self-excitation of induction motors is a potentially damaging condition that can occur on the same system. The term "self-excitation" refers to sub-harmonic oscillations that may occur in an electric supply circuit that includes series capacitors. The sub-harmonic oscillations result from the interaction between the series capacitors and an induction motor when the motor is in the process of starting. These oscillations are typically characterized by motor starting problems and sustained overcurrent conditions.

When ferroresonance occurs, immediate action must be taken to prevent damage to other equipment. Ferroresonance is a rapidly occurring, high magnitude, and low frequency oscillation capable of reaching power system voltage levels of <NUM>-<NUM>% above normal for brief periods. When self-excitation occurs, low-frequency oscillations are produced as the motor starting sequence fails. The motor will search for the proper operating frequency, which will cause large current surges as the shaft acceleration alternates.

Power generation sites (e.g., thermal prime movers, induction generators, wind turbines, etc.) are often located very far from load centers. To enable the transmission of power over long distances, the use of series capacitors is often employed to raise the power limits of the resulting long transmission lines. The series capacitors can cause series-resonant oscillations, which have been known to cause damage to generator shafts. Damage could also be inflicted on wind turbine power transmission and control components.

The series-resonant oscillations occur at a sub-harmonic of the supply frequency (typically <NUM> in North America). This effect has become known as sub synchronous resonance (SSR). The most famous incident involving SSR occurred in <NUM> and again in <NUM> at the Mohave Generating Station in southern Nevada, USA. A generator experienced a gradually growing vibration that eventually led to a fracture of the shaft section between the generator and the rotating exciter. Investigations determined that an electrical resonance at <NUM> produced torque at <NUM> (the <NUM> compliment frequency), which was near coincident with the frequency of the second torsional vibration-mode of the turbine-generator at <NUM>. This interaction between the series capacitors and the torsional system is an example of subsynchronous resonance.

Wind turbines and wind farms are becoming increasingly popular and are being installed in greater numbers around the world. The best locations for wind farms are often located far from load centers. In addition, multiple wind farms may need to be connected to an existing electrical grid that may also connect to thermal generation stations (e.g., gas or steam turbines driving one or more generators).

The document by <NPL> describes the mitigation of SSR caused by an induction-generator effect as well as torsional interactions, in a series-compensated wind farm. The wind farm employing a self-excited induction generator is connected to the grid through a series-compensated line. A static VAR compensator (SVC) with a simple voltage regulator is provided to damp SSR when equipped with an SSR damping controller. Also, a thyristor-controlled series capacitor (TCSC) that operates to increase the power transfer capability of the transmission line also damps sub synchronous oscillations when provided with closed-loop current control.

The present invention resides in a system for compensating a power transmission grid as defined in the appended claims.

Various aspects and embodiments of the present invention will now be described in connection with the accompanying drawings, in which:.

The modern utility grid is evolving into a network that includes disparate generation sources located far from load centers. Multiple wind farms, solar power generating stations, and other non-conventional power sources are being connected to the existing power transmission lines. Different suppliers manufacture wind turbines and each supplier can manufacture their wind turbines with different operating characteristics. Solar power suffers the same problem. This wide variability in operating characteristics makes it difficult to connect these non-conventional power-generating sources to the existing transmission lines.

<FIG> illustrates a simplified example of one typical utility grid <NUM>. One or more non-conventional generating sources <NUM> can be connected to various parts of the grid through power transformers <NUM>. The non-conventional generating sources <NUM> can comprise various types of prime movers (e.g., wind turbines, wind farms, solar generating stations, etc.), and may be characterized by non-conventional electrical interfaces to the grid.

The non-conventional electrical interfaces may include induction generators or power electronic systems that can interact adversely with lightly damped series resonances in the transmission grid. The non-conventional generating sources <NUM> can comprise individual sources (e.g., a single wind turbine) or a group of sources (e.g., a wind farm comprising many turbines). Individual wind turbines may have power ratings of about <NUM> to about <NUM> MW or more, and wind farms may have a collective power rating of about <NUM> to about <NUM> MW or more. These ranges are for illustrative purposes only and may extend above or below the ranges given.

The grid <NUM> may also include one or more conventional generating sources <NUM> and one or more loads <NUM>. Conventional generating sources typically comprise synchronous machines and may have power ratings of about <NUM> MW to <NUM> MW or more per machine. An example of a conventional generating source is a gas or steam powered turbine that drives an electrical generator.

The series capacitors <NUM> are required in long transmission lines <NUM> to compensate for the inherent inductive reactance. The disadvantage to series compensation is that it creates lightly damped series resonances having frequency below the synchronous frequency (i.e., subsynchronous). The non-conventional generators <NUM> can interact with the lightly damped series resonances in the transmission lines <NUM> in a number of ways, which can cause damage to the non-conventional generating sources <NUM>.

The simplest form of non-conventional generation is a wind turbine using a simple induction generator. Radial transmission of large amounts of induction-generated electrical power through series-compensated lines is new to modern power systems because of the rise of wind power. With this new power source comes potential problems, among these problems is a specific sub synchronous phenomenon known as induction generator effect (IGE). The root cause of this effect is that induction machines appear as a negative resistance to electrical oscillations having frequency less than that induced by the rotor speed. When a series capacitor is added to the network, then the resulting sub synchronous series resonance with the inherent inductance of the network will be destabilized by the induction-generator negative resistance effect and can lead to an electrical instability.

Other types of non-conventional generation rely extensively on power electronics to convert power from the prime mover to the electrical characteristic needed by the grid. Power electronics inherently require several complex control algorithms operating at high speed to perform their function. Due to the high-speed nature of the algorithms, there will be substantial interaction with the subsychronous series resonance of the transmission grid that is created by series compensation. These control algorithms are designed based upon a simplified assumption of grid characteristics. It is impractical to design such algorithms to accommodate any arbitrary grid characteristic. Further, the details of these algorithms that govern the interaction phenomena vary with manufacturer and are typically considered highly proprietary.

A transmission line owner/operator may expend a large amount of labor and expense to individually tailor their transmission line to each disparate power source. Alternatively, the developers of each non-conventional power generating station must work in great detail with the vendor(s) of their generating equipment and with the vendor(s) of other non-conventional generating equipment to coordinate their operating characteristics to accommodate the transmission grid. Such coordination is not only extremely onerous and expensive to achieve, but is prohibited by existing regulations governing competitive generation markets.

An aspect of the present invention provides a transmission-compensation system that can couple multiple disparate generation sources to a common electrical grid, without the requirement for extensive coordination between generating stations, or requiring expensive and difficult efforts by the transmission system operator. Further aspects of the system of the present invention provide for damping subsychronous series resonance at the series capacitor location.

<FIG> illustrates an improved series compensation circuit according to one example not covered by the appended claims. The transmission line <NUM> is series compensated by series capacitor <NUM>.

However, a parallel damping circuit <NUM> is placed in parallel with capacitor <NUM>. The damping circuit can be broadly tuned to decrease or eliminate sub synchronous resonance caused by capacitor <NUM>. In addition, a switch <NUM> may also be placed in series with damping circuit <NUM>. The switch <NUM> can isolate the damping circuit in case of failure or for system maintenance. It is to be understood that a switch <NUM> could be placed on both sides of damping circuit <NUM> if desired.

<FIG> illustrates a schematic circuit diagram of one example of damping circuit <NUM>. The damping circuit is comprised of resistor <NUM>, capacitor <NUM> and inductor <NUM>. The damping circuit can be placed in parallel with the series compensation capacitor <NUM> or transmission line <NUM>. Switch <NUM> is optional and not shown in this example.

The resistor <NUM> damps the subsychnronous series resonance caused by capacitor <NUM>. A capacitor <NUM> and inductor <NUM> are connected in parallel, and in series with resistor <NUM>. The capacitor <NUM> and inductor <NUM> block current in resistor <NUM> at the synchronous frequency and reduce losses, which would have been attributed to resistor <NUM>. The passive components of the damping circuit are tuned for a broadband response, to compensate for all types of non-conventional power generating sources <NUM>, which may be connected to transmission line <NUM>.

<FIG> illustrates a schematic circuit diagram of a damping circuit <NUM>, according to an aspect of the present invention. The damping circuit <NUM> is comprised of resistor <NUM>, capacitor <NUM>, and inductor <NUM>. This damping circuit is placed in series with a series compensation capacitor <NUM>. The damping circuit may be placed in series with a series compensated or uncompensated transmission line <NUM>, and/or between the power transformer <NUM> of the non-conventional source <NUM> and the bus (on the higher and/or lower voltage side of this transformer), and/or between the neutral point of power transformer <NUM> and ground (one the higher and/or lower voltage side of this transformer).

The resistor <NUM> damps the sub synchronous series resonance caused by capacitor <NUM>. A capacitor <NUM> and inductor <NUM> bypass current around resistor <NUM> at the synchronous frequency and reduce losses, which would have been attributed to resistor <NUM>. The passive components of the damping circuit are tuned for a broadband response, to compensate for all types of nonconventional power generating sources <NUM>, which may be connected to transmission line <NUM>.

The bypass switch <NUM> can protect the damping circuit from system currents in case of failure or for system maintenance. It is to be understood that switch <NUM> could be a series of switches that not only bypasses damping circuit <NUM>, but also electrically isolates it from the system through a set of disconnects, bypass connections, and/or grounding blades.

<FIG> illustrates a simplified schematic of a utility grid incorporating aspects of the present invention. The damping circuit <NUM> is shown connected in parallel to some of the series capacitors <NUM>, which example is not covered by the appended claims.

The damping circuit <NUM> is shown connected in series to some of the series capacitors <NUM>, uncompensated lines <NUM>, between power transformers <NUM> and the higher or lower voltage buses, and between the transformer <NUM> neutral points and ground on the higher or lower voltage sides of those transformers. The damping circuits <NUM> and/or <NUM> may be a single or multiple (e.g., <NUM> or <NUM>) phase version of damping circuit <NUM> and/or <NUM>. Switch <NUM> (not shown in <FIG>) may also be included in damping circuit <NUM>. Switch <NUM> (not shown in <FIG>) is included in damping circuit <NUM>. The damping circuit(s) <NUM> and/or <NUM> protect(s) the non-conventional power generating sources <NUM> from damage due to subsynchronous resonance.

Many transmission lines are configured as <NUM>-phase lines, and the damping circuit of the present invention could be placed on one, two or all three phases. The passive components of the damping circuit could also be configured in a variety of ways. In some examples, not covered by the appended claims, a resistor is connected in series to a capacitor and inductor connected in parallel, or in series with an inductor, or in series with an inductor and capacitor. According to the invention. a resistor is connected in parallel with a series connected inductor and capacitor. In another example, not covered by the claims, the damping circuit could also be configured as a parallel connected resistor, inductor and capacitor.

The switch <NUM> could also be configured to switch to a backup damping circuit if the primary damping circuit <NUM> fails. In this embodiment, two or more damping circuits could be connected in parallel, but isolated via one or more switches. If a primary damping circuit failed, a local or remote control signal could be activated to operate one or more switches to disconnect the failed primary damping circuit, and switch in a secondary or backup damping circuit. The control of the switches could also be performed locally as well.

Claim 1:
A system for compensating a power transmission grid (<NUM>), comprising:
one or more non-conventional power generating sources (<NUM>) connected to said power transmission grid (<NUM>), the one or more non-conventional power generating sources (<NUM>) comprising one or more of a wind turbine, a wind farm and a solar power generating station;
one or more power transformers (<NUM>) connected to at least one or one or more non-conventional power generating sources (<NUM>);
at least one series compensation circuit (<NUM>) connected to at least a portion of said power transmission grid (<NUM>), said at least one series compensation circuit (<NUM>) compensating said power transmission grid (<NUM>); and
at least one damping circuit (<NUM>) comprising at least one resistor (<NUM>) connected in parallel with a series-connected capacitor (<NUM>) and inductor (<NUM>), and a bypass switch (<NUM>) connected in parallel with the at least one resistor (<NUM>) and with the series-connected capacitor (<NUM>) and inductor (<NUM>);
wherein, said at least one damping circuit (<NUM>) is tuned to decrease or eliminate sub synchronous resonance caused by said at least one series compensation circuit (<NUM>) on said power transmission grid (<NUM>); and
wherein said at least one damping circuit (<NUM>) is connected in series with said at least one series compensation circuit (<NUM>).