As more wind and solar generation is added to the power grid and large coal and nuclear plants are retired, a reduction in reactive power is becoming a concern. Underutilized gas and steam turbines can be retrofitted to offer synchronous condensing capability, which can benefit both owners and grid operators.
The many gas turbines and combined cycle power plants deployed in the 1990s and 2000s were expected to enjoy life for many more years. Yet, changes in policy and competition from renewables have led to some units being mothballed. In other cases, utilization rates fall far short of those originally envisioned. Instead of combined cycle plants operating at baseload, they find themselves shifted over to peaking operation to rapidly come into action at times when renewable resources are insufficient to satisfy demand.
At times, these plants are hardly utilized at all. In Germany, for example, the Irsching Power Station was touted as a world record holder for efficiency when it installed new Siemens H-class units in 2011. But German energy policy has largely sidelined this combined cycle facility. It loses millions of euros every year. The utility has made several requests to close operation at one or more units as it receives little compensation to have them sitting idle as standby power.
As for peaking gas turbines, many are minimally used. Understandably, some utilities and independent power producers have been wondering what to do with these assets. Shuttering is an expensive option, both in terms of stranded assets and demolition costs.
A better alternative may be to retain these plants and repurpose the generators of idle turbines as synchronous condensers. Failure to do so could have serious negative repercussions for the grid. Why? Remove too many gas and steam turbines from service and disruption is inevitable. Power quality will drop, and a lack of inertia will lead to instability and potential blackouts.
Synchronous condenser conversions have been done successfully in many areas. In the U.S., there are a great many examples. The Los Angeles Department of Water and Power (LADWP) has four GE LMS100 gas turbines operating primarily as synchronous condensers but poised to provide generation when necessary. Numerous steam turbine generators have been repurposed throughout North America too, such as four 150-MW units at BC Hydro’s Burrard Generating Station near Vancouver, British Columbia, Canada. Around the world, there are similar synchronous condensing examples in countries such as the UK, Sweden, Germany, Mexico, Canada, Morocco, Kenya, South Africa, Iraq, Jordan, Bangladesh, New Zealand, Australia and Ecuador.
It is important to appreciate why such conversions should be done and how to go about it. Generators and motors provide inertia as they rotate at the same frequency as the electricity grid. Thus, the presence of gas and steam turbines acts as a buffer against power spikes and changes in frequency. But with wind and solar supplanting traditional energy sources, the grid is losing inertia. Wind doesn’t provide it, as frequency converters placed between the wind turbine and the electricity grid prevent the kinetic energy of their rotating mass from providing inertia.
During the peak evening period, frequency drops when people return from work and turn on air conditioning, heating, lighting, and appliances. At other periods of the day, frequency rises if there is an abundance of supply compared to demand. In other words, there is a constant balancing act taking place as system operators try to maintain frequency in the correct range (60 Hz for the U.S.).
If things get out of hand, utilities might have to conduct load shedding, that is, disconnecting neighborhoods from the grid to avoid damaging equipment and to keep the rest of the network in operation. In extreme cases, cascading outages can lead to a major blackout. The Northeast U.S. experience just such an event in 2003, leaving 60 million people without power. On a smaller scale, New York City and London endured blackouts during 2019.
The key concept to understand here is the difference between real power and reactive power. Real power (also known as effective power) delivers energy from the generation source to the load. It is measured in volts, amps, and watts. Reactive power, measured in volt-amperes reactive (Vars), does no actual work. It could be regarded as the form of electricity that creates or is stored in the magnetic field surrounding a piece of equipment.
Reactive power has several qualities. It can be positive or negative. Doubling the amount of power consumed, quadruples the amount of reactive power needed. Additionally, reactive power does not travel well. Long transmission lines operating at heavy loads consume Vars. Failure to address reactive power on these lines can lead to conductor heating and voltage failure. If the voltage sinks too low, the consequences can be electric system instability or collapse, motor damage, and electronic equipment failure. If the voltage goes too high, it can exceed the equipment’s insulation capabilities and lead to dangerous electric arcs.
Utilities solve this by placing reactive power compensating devices close to power loads to lower the reactive current demand on the transmission system. There are various static and dynamic reactive power options to choose from. Static devices are relatively low cost and are a standard part of utility operations. However, they are slow to respond, and their output drops when voltage drops. Dynamic reactive power sources can adjust output independently of the voltage level to prevent voltage collapse but are more expensive. The best approach, therefore, is generally a combination of static and dynamic sources.
Static devices include capacitor banks. They are reliable and easy to install but take up a lot of space. They only supply reactive power and cannot absorb it. Under rapidly increasing load and voltage drop, their effectiveness deteriorates. Relying solely on capacitors increases the chance for voltage collapses, as their output decreases with the square of the voltage.
Static Var Compensators (SVC) are another static approach. They consist of switches made of shunt capacitors and reactors connected by thyristors (solid-state semiconductor-based switches). They offer more voltage control than capacitors and can absorb and supply reactive power. However, their reactive power output varies according to the square of the voltage, so they struggle under voltage instability or voltage collapse.
Static Compensators (STATCOM) use power electronics to take their response time down to microseconds and to reduce space requirements. Vendors such as American Superconductor and S&C Electric offer large and highly capable STATCOMs. Pricing, though, is considerably more than their static cousins, as they are far more sophisticated.
Dynamic VAR Compensation
Synchronous condensers are a dynamic alternative to static devices. They offer immediate response to power fluctuation and can supply or absorb Vars to produce unity power factor. They absorb leading reactive power from the supply system when over-excited and add Vars when under-excited. They continually adjust their output level in tiny increments to smoothly balance the system, but are generally more expensive. A combination of cheaper static and more expensive dynamic Var compensation devices is typically advised.
Synchronous condensing can be done via a purpose-built electronic device synchronized to the grid such as those being installed for the Brindisi substation in southern Italy operated by Italian grid operator Terna. These GE synchronous condenser units will supply reactive power of up to +250/–125 MVar and 1,750 MW of grid inertia. They are being installed along the transmission system to produce or absorb reactive power to keep power flowing consistently. They regulate the energy parameters of the transmission network, generate or absorb reactive energy, regulate the voltage and improve the energy factor, and increase the overall inertia of the power grid via a flywheel system.
For those with idle or underutilized gas and steam turbines, however, it is often simpler and cheaper to convert them for use as a synchronous condenser. This is accomplished by installing a synchronous self-shifting (SSS) clutch between the turbine and the generator (Figure 1). In this scenario, the turbine brings the generator up to speed. Once the generator synchronizes with the grid, the turbine disconnects from the generator and shuts down. The generator uses grid power to keep spinning, constantly providing leading or lagging Vars as needed. The clutch disengages the prime mover and the generator when reactive power is needed. When real power is needed, the clutch automatically engages for power generation.
1. A synchronous self-shifting (SSS) clutch is a sophisticated freewheel gear-type clutch that engages and disengages automatically without the use of any external control system. Put simply, when the input side rotates faster than the output side, the SSS clutch will engage. Conversely, when the output side rotates faster than the input side, the clutch will disengage and overrun or freewheel. Courtesy: SSS Clutch
New Lease on Life for Aging Units
There is no shortage of aging gas turbines around the world. South and Central America, for example, host a great many models from the 1960s, 1970s, and 1980s. Even if no longer required for power generation, they can gain a new lease on life as a synchronous condenser.
One example is an aging peaking gas turbine generator in Mexico that was converted for synchronous condensing capability. This 50-year-old 14-MW Brown Boveri model BB-11L gas turbine generator is situated at CFE’s Universidad Power Station in Monterrey, Mexico. A clutch was retrofitted onto the unit (Figure 2) to provide reactive power for the CFE distribution transmission grid within the city of Monterrey. A new clutch replaced a shaft in the original machine. Neither the turbine nor the generator had to be moved. The new clutch was a spacer-type with an additional set of input gear coupling teeth. It acts as a flexible coupling between the gas turbine and the generator. The controls were upgraded by Sigrama Co. of Torreon, Mexico. CFE personnel performed most of the installation work to remove the solid shaft, install the clutch, and complete balance-of-plant conversion work.
2. A 14-MW Brown Boveri gas turbine generator at CFE’s Universidad Power Station in Monterrey, Mexico, is shown here being retrofitted with an SSS clutch to provide reactive power for the grid. Courtesy: SSS Clutch
There are many other examples of old or underutilized turbines that have been converted for dual purpose—generation and synchronous condensing. Instead of letting such units gather dust, they should be retrofitted to deliver reactive power. This is a vital factor in assuring the success of a future grid dominated by renewable generation sources. ■
—Morgan Hendry is president of SSS Clutch of New Castle, Delaware, a manufacturer of the large clutches that are used in synchronous condensers (SSSclutch.com).