Converting mobile GT gensets to synchronous condensers

Adequate reactive power supply is critical for grid voltage support, power factor correction, and increased power transfer capacity of transmission lines. As more renewable resources displace conventional fossil-fuel power-generation equipment to lower emissions, the grid loses rotational inertia and reactive power capacity, which negatively impacts system frequency and voltage profile and compromises power system stability.

A new white paper* from APR Energy addresses the importance of reactive power compensation for the power system and looks at various compensation solutions, in particular, APR Energy’s mobile gas turbine generators reconfigured as synchronous condensers.

By disconnecting the turbine, and operating the synchronous generator in an overexcited motoring mode, it behaves like a capacitor and provides reactive power to the grid.

This mode of operation proves particularly valuable when renewables penetration is increasing, providing the needed reactive power as well as rotational inertia and fault level support.

An increase in reactive power raises voltage, while a shortage of reactive power can have detrimental effects on voltage and potentially lead to a blackout such as that which occurred in the USA in 2003.

Figure 1 shows a water tank analogy for reactive power with the various reactive power sources and sinks and their impact on maintaining a steady voltage level.

Figure 1. Water tank analogy for reactive power and its impact on system voltage

Sources of reactive power

Reactive power sources can be classified as static and dynamic. Static MVAR sources, such as capacitor banks and transformer load tap changers, are typically embedded in the distribution system. Because these devices respond slowly to load changes, they do not provide a continuous source of reactive power. On the other hand, dynamic reactive power sources including rotating equipment, such as synchronous generators and synchronous condensers, and non-rotating (power electronics based) equipment, such as Static VAR Compensators (SVC), Static Compensators (STATCOM), D-VAR, and SuperVAR, are typically considered transmission service devices, which provide a fast response and continuous source of reactive power.

Capacitor banks have been used extensively to provide power factor correction and grid voltage support. However, as a reactive power source, the capacitor bank has shortcomings.

For example, reactive power from the capacitor is proportional to the square of the bus voltage to which the capacitor is connected. Consequently, during a voltage disturbance on the grid, the MVARs produced will experience an even bigger dip (ie, capacitors are least helpful when they are most needed).

Also, capacitor bank MVARs are fixed and can, at best, be changed in a pre-defined number of steps. The non-stepless behaviour of the capacitor bank can result in switching transients.

Furthermore, capacitors are static pieces of equipment without rotating mass. They provide no inertia support or low voltage ride through during grid disturbances.

Benefits of synchronous condensers

MVARs obtained from synchronous machines are dynamic in nature and controlled by the automatic voltage regulator (AVR) in the excitation system in a stepless manner, hence, no switching transients to produce/absorb MVARs. As a result, the synchronous machine provides better grid voltage support.

In addition, heavy, rotating synchronous machines store kinetic energy in the rotating shaft to provide critical inertia during grid disturbances. They also increase short-circuit levels at the point-of-connection to the grid, thus, enabling higher penetrations of renewables.

Purchased and installed as a stand-alone system on the transmission network, the synchronous condenser provides reactive power and voltage support.

Alternatively, retired power plants can be retrofitted and their synchronous generators operated as synchronous condensers.

The conversion of synchronous generators into synchronous motors makes the most of existing infrastructure, preserves the inertial support of the generator, and provides a dynamic source of reactive power to enhance power system reliability and security and enable high penetrations of renewable energy.

Turbine generator set as syncon

The option offered by APR Energy is operation of its mobile clutchless two-shaft aeroderivative GT based gensets as synchronous condensers.

In a clutchless two-shaft system (Figure 2), the gas generator (GG) part is on one shaft and the low-speed turbine (LSPT) driving the synchronous generator is on the other. This design facilitates dual operation modes, power generation and synchronous condensing.

Figure 2. Two shaft gas turbine as employed in APR Energy mobile genset

In power generation mode, the GG is fired to produce active and reactive power into the system. Fuel to the GG is shut down when switching from power generation to synchronous condensing mode. At this moment, the synchronous generator starts operating as a synchronous motor, spinning freely with a small parasitic load. However, the motor can produce higher than the rated reactive power into the bus under AVR control. This requires retrofitting of the control software to allow the generator to work in both modes.

The ability to produce increased levels of reactive power on demand from the same generator is an added value, which provides not only frequency, but voltage support to facilitate increased renewable penetration.

The synchronous condenser can be considered a “shock absorber” for dealing with grid disturbances. Left unattended, these disturbances can compromise grid stability. Thus, synchronous condensing technology serves as a kind of insurance policy that can be used when needed.

The key benefits can be summarised as follows:

• Synchronous condensers provide dynamic MVARs that boost voltage stability.
• Synchronous condensers Increase the short-circuit level at the point-of-connection to the grid.
• GT-based synchronous condensers can provide MW support (power generation mode) to supplement renewable power generation.
• Synchronous condensers do not require changes to the GT or generator core hardware power train, leveraging the reliable designs used in the genset fleet.
• LSPT and GG are based on low-friction roller and ball bearings to reduce parasitic auxiliary loads.
• No fuel is consumed during synchronous condenser mode as the fuel supply to the GG is shut down.
• Switching between power generation and synchronous condenser modes does not require the generator to be re-synchronised to the grid.

Table 1 compares the capabilities of the APR Energy GT-based synchronous condenser with a conventional synchronous condenser, while Table 2 shows start-up times for the APR Energy GT-based synchronous condenser.

The increase in MVARs upon switching the APR Energy GT-based mobile genset from power generation to synchronous condenser mode depends on the working power factor of the generator during power generation. Tables 3 and 4 show reactive power in both power generation and synchronous condenser modes, for 0.8 and 0.9 power factor values.

APR Energy uses various types of aeroderivative gas turbine gensets. Two examples are considered here and are referred to as Model 1 and Model 2, the difference being in the MVA ratings.

Table 3 shows the potential increase in reactive power output obtained upon conversion of the Model 1 turbine genset to synchronous condenser mode at 0.9 pf and 0.8 pf, while Table 4 shows the same data for the Model 2 gas turbine genset.

AC generators are typically run at 0.9 power factor. As can be seen, an increase of about 60% in reactive power capability can be obtained upon switching from power generation mode to synchronous condensing mode.

Technologies compared

Reactive power compensation options vary in terms of complexity, efficiency, and cost.

Table 5 compares various technologies. The synchronous condenser derived from an existing synchronous generator modified to operate in both power generation and synchronous condenser modes of course entails significantly less cost than a stand-alone synchronous condenser.