Reactive power: what it is; why it is important
There is a global trend toward adding dynamic reactive devices to serve urban load areas. A number of interrelated factors contribute to this increasing use of dynamic reactive devices, such as the following:
> Many urban areas are experiencing a growth in load levels. These urban areas also see less generation close to the load center. Either legacy generating units have been retiring or they are operating less, because they are less efficient than newer generators. Losing these generators reduces the amount of local power generation and dynamic var support available in the urban areas. As a result, urban areas often rely on increased power imports to serve their load.
> Increased power imports tend to reduce local transmission voltages, which, in turn, reduce the effectiveness of various devices that support the local voltages. In the extreme, this could become a kind of voltage spiral of death.
> In many cases, there is enough transmission thermal capability to deliver the needed imports, but there are increasingly frequent periods where voltage/var factors limit system operation.
> Public opposition to new transmission lines is also common.
> In many deregulated markets, there are limited means to require or encourage generator construction or to schedule power outside the normal generation dispatch market. In other markets, requiring generators to operate outside normal economic dispatch can be very costly.
These urban areas, however, often face an underappreciated risk associated with the addition of these complex dynamic reactive devices.
Reactive power and why it is important
Fundamentally, electrical power is developed, delivered, and consumed as voltage and current. In a simple direct current (DC) device (like a flashlight), the power (the brightness of the bulb) is the voltage times the current, which is measured in watts. Watts are a measure of the ability of a device to perform useful work. This is what is called real (and sometimes active) power because it can produce useful (real) work.
In alternating current (AC) systems—like all modern power systems—things become a bit more complicated. Common customer load devices—especially motors—have inherent characteristics that shift the relationship between current and voltage. This shift is measured in vars—from the French term Volt-Ampere reactive. Since much of the power system load is motors in factories, businesses, and homes, this becomes a significant issue for utilities. Transformers and transmission and distribution lines and cables also have characteristics that contribute to this shift.
Increasing var load reduces the ability of the system to deliver real power and perform useful work. In extreme cases, a high var load can shift the voltage and current so much that it reduces the power system’s delivery capability so that almost no active power can be delivered. There can also be other undesirable effects like low voltages and increased equipment heating and system losses.
While reactive power does not provide useful work, it is essential for AC transmission and distribution systems, motors, and many other types of customer loads. For motor loads, sufficient var levels are needed to avoid voltage sags that inhibit the conversion and flow of watts to meet load demand. Therefore, actual power systems require both real and reactive power to function properly.
Compensating for reactive loads
The shift of motors and other reactive loads can be offset using compensation devices. Reactive compensation commonly comes from three types of devices:
1) Capacitors are the largest source of compensating reactive power and are commonly used throughout the power system.
2) Synchronous condensers are a type of rotating machine—like a generator—but they do not produce real power, only reactive power. There are also other devices that use high-power electronics to rapidly control reactive power from large banks of capacitors.
3) Conventional generators, in addition to supplying real power, are an important source of reactive power.
Reactive power loads must be supplied either locally from customer-owned devices or from the system itself. It is almost universal for some of the reactive load to be compensated locally as power-factor correction. Power factor is a measure of the relation between real and reactive power. The power factor ranges from 0.0 to 1.0, where 1.0 means there is only real power (no reactive power); 0.0 means there is no real power, only reactive.
The difference between passive and dynamic reactive resources
While there are various differences in the hardware involved, the main distinction between passive and dynamic reactive resources is how quickly they can respond to the changing needs of the power system.
Passive reactive sources are devices with fixed var ratings, which can be connected or disconnected (switched) as system conditions change. The most common passive device is a capacitor. Capacitors can be permanently connected or switched throughout the day. The switching action can be made in a few seconds using automatic controls. (The response is much slower if it depends on manual actions by a system operator.)
While automated switching within a few seconds may seem fast, it is not fast enough for some situations to avoid voltage collapse (a blackout).
Dynamic reactive sources are active devices that can provide variable amounts of reactive power in a few milliseconds. Common examples are static var compensators (SVCs), static synchronous compensators (STATCOMs), synchronous condensers, and conventional generators. All are capable of rapid dynamic response. Some types of renewable generating sources are also capable of providing dynamic reactive supply using power electronics and other means.
KEMA recently completed a study where several dynamic reactive sources (SVCs) were needed to reach full output in 10 milliseconds (0.01 seconds). These static var devices then needed to continuously swing from maximum to minimum output to maintain stability within the first 10 seconds. Such a response is not possible with switched passive devices.
There is also a significant capital cost difference between passive and dynamic devices. Passive devices are about one-fourth the cost of dynamic devices. Therefore, the mix of dynamic and passive devices will have a significant impact on the cost of providing the necessary reactive compensation. A blend of both types of devices is most common. Specific performance factors and capital costs help determine the best mix for each situation.
David Korinek also contributed to this article, which is the first part of a three-part series for TECH Notes, a monthly publication that provides business and technical insights for secure transmission and distribution systems. Part 2 will discuss the problems associated with highly compensated systems. Part 3 will provide an overview of the solutions. Sign up to receive advance notification.
Learn about other project work by KEMA involving dynamic reactive compensation.
Image source: Wikipedia