Planning ahead for reactive power

March 5 2014 - Joe Zwirs

Whether one is talking about reactors or renewables, turbines or transmission systems, most of the attention goes onto making sure enough Megawatts (MWs) are generated to match the MWs being consumed at a given point in time. But in planning for future capacity, it is not just real power that matters, but complex power, that combination of real and reactive power that makes it possible to deliver the greatest quantity of real power over a transmission line to the end users. Without enough capacitive reactive power to offset the inductive reactive power of the transmission system, transmission capacity is reduced, voltage drops, power lines overheat, and blackouts can occur.

The electrical industry operates on a scale of decades. Some nuclear plants are undergoing certification to extend their lives to sixty years or beyond. Fossil and hydro plants built a century ago are still in operation. But over the next decade or two, we will see a huge shift in the way that power is generated and transmitted. Some countries are shutting down their coal and nuclear plants, while others are adding them at a rapid rate. An additional 70 GW of solar and wind came on line in 2013, with much more to come as utilities scramble to meet government renewables mandates.

One result of these changes is that instead of using local generating facilities which can provide both real and reactive power, the power is coming from solar panels, remote generating stations and wind farms. While this does reduce the local noise and emissions, it also removes the local generators which could provide the reactive power needed.

To maintain grid stability in the midst of these changes, it is essential to include reactive power as part of any generation and transmission master plan. To help those looking to create such a plan, here is some basic background data on what reactive power is, how it is generated, the effect it has on transmission and how one can ensure there is an adequate supply.

A Complex Problem

Reactive power is an unwanted but unavoidable part of AC electric power delivery systems that occurs due to the inherent nature of the components of these systems.

An electric power delivery system has two components of power:  real power (P) and reactive power (Q).  Real power does real work; it delivers energy from the generation source to the load.  Reactive power does no work but exists nevertheless, and must be carried between its sources and its loads by the electric power delivery system.  It has the effect of loading down the delivery system, reducing the capacity of the system to carry real power.  It can be viewed as an exchange of energy between elements of capacitance and inductive reactance that occurs naturally on alternating current systems. Real power and reactive power add vectorally as two sides of a 90-degree angle to equal complex power.

Real power on utility systems is usually measured in kilowatts or megawatts.  Reactive power is usually measured in kilovars (kVAr) or Megavars (MVAr), where “VAr” stands for “Volt Ampere reactive.”  Complex power is measured in kilo-volt amperes (kVA) or mega-volt amperes (MVA).

Reactive power results from the inductive reactance in the elements of an electric power system. Passing an electric current through elements of an electric power system causes the formation of electromagnetic fields. In addition, other elements of a power system, such as transmission and distribution lines, also have inductive reactance properties, although this inductive reactance is unwanted and unintended. Many electric system loads, especially motor loads, require reactive power which must be somehow created. Capacitive reactance is the opposite of inductive reactance in that it supplies reactive power to an electric power system.

The power factor of a load, and the current supplying it, is related to the relative amounts of real and reactive load.  Specifically, it is the cosine of the current resulting from the complex addition of the real and reactive current (or load) relative to the real axis.  If there is no reactive current, there is no reactive load and the cosine is 1; the power factor is called 100%, or unity.

Supplying Reactive Current

Real current must come from an electrical generation source to feed real load (turn the motor, light the lights, etc.). Similarly, reactive current, needed to supply reactive power, must be produced somewhere. When the generator is near the load, the same power generators that supply real current can provide the reactive current, but where long distances are involved, this approach will increase voltage drop and add to conductor heating. Heavy reactive current flow causes voltage to drop like a rock.

Fortunately, reactive current can be provided by sources other than the real power generators. Separate reactive current sources, located closer to reactive power loads, will reduce the reactive current carried over long distances by the delivery system. The reduced reactive current on the delivery system allows it to carry more real current which is what the utility sells to its customers. Voltage drop in the delivery system is also reduced, helping the utility to maintain its service voltage within required limits.

Separate reactive sources include capacitors and rotating machines set up to function as capacitors. The term “condenser” is generally applied to rotating machines that only supply reactive current. The sum of the real and inductive reactive load current and the capacitive reactive condenser or capacitor output is a lower overall current with a power factor closer to unity. 

Until the 1940’s, rotating machine condensers were widely used to supply reactive current and were located in utility substations, usually connected to the distribution voltage bus. They are dynamic sources in that their output can change quickly to match reactive power need. Since condensers are large rotating machines, they add stored energy, in the form of inertia, to the electric system. This property is useful in handling transient conditions such as temporary short circuits and momentary disruptions. This inertia may be especially useful with low inertia power sources such as photovoltaic cells and wind turbines.

Since the 1940’s, the major source of reactive power has been static capacitors connected to the utility’s primary distribution circuits or located in substations. Static capacitors are generally the choice over rotating machines for most applications due to their much lower cost and maintenance requirements. The downside is that their output is fixed, varying only by the number of discrete units on line.

The output of reactive power sources, whether rotating machines or static capacitors, must be varied as the load varies. Under fault conditions, or from a sudden loss of a major element (such as a transmission line) in the electric power system, increased reactive power may be needed suddenly in order to hold voltage to acceptable levels. Dynamic sources can respond immediately to system reactive power needs, while static capacitors, which are brought on line with switches or circuit breakers, are relatively slow to react.

By their nature, both ordinary switched capacitors and static var compensators, which are capacitors that are electronically switched with instantly-acting solid-state devices, experience severe output reduction under depressed voltage conditions since their output is a function of the square of the voltage at their terminals. Generators and synchronous condensers, on the other hand, provide dynamic response and their output holds up well under reduced voltage conditions. They also have short-term increased reactive output, something not possible with static capacitors.

New Opportunities for Condensers

With the introduction of large-scale wind turbine farms whose power output can vary widely and quickly as winds change, it is important to be able to react quickly to changing conditions. In addition to variations of the driving wind, the wind turbine generators themselves are built to be lightweight with low inertia, adding to the need for the inertial properties of synchronous condensers.

Using a rotating machine to supply reactive current, while more expensive, has the advantage of being infinitely variable within the capabilities of the machine, (or even in excess of rated capacity for short periods of time) and it does not require high voltage circuit breakers and switches to vary levels of reactive power. Once the rotating machine is spinning at a speed synchronous to the power delivery system, its reactive output can be continually varied as needed by adjusting the machine’s field control.

Maintenance tasks associated with rotating condensers are not proportional to machine size, so economics favor the use of larger machines. In many cases, the largest machines available are retired generators or intermittently used peaking units. A single generator at a retired coal or nuclear plant can supply hundreds of Megavars. And, since the generator has already been paid for, capital costs are much lower than the expense of adding static compensators. When planning to decommission a plant, or even one generator in a multi-unit facility, be sure to evaluate the benefit of using that generator as a synchronous condenser.

Using the generator of a peaking gas turbine generator as a synchronous condenser when real power (MW) is not required is another cost effective way of producing large quantities of reactive power. In this case, installing an overrunning clutch between the gas turbine and generator allows the generator to rotate in synchronism with the electric power system after the gas turbine is shut down.

This approach can be taken with both existing and new peaking GTs. When retrofitting an existing peaker with a gear between the turbine and generator, it is less expensive to attach the clutch to the gear, rather moving the gas turbine or generator to provide space for the clutch. With new units, adding the capability to operate as a synchronous condenser incurs little additional cost. In this case, the need for reactive power should be thoroughly investigated during the planning stage, so the unit can be installed in a way to take advantage of this low-cost source of reactive power, rather than having to retrofit it later.

 

(Joe Zwers is a freelance writer from Glendale, California specializing in engineering, business and technology.