Efficiency gains in a coal-fired power plant via variable frequency drive upgrades

The 280 MW Rawhide Energy Station (Source: Platte River Power Authority).

By Jeff Harmon, Platte River Power Authority

By Brian Brigandi, Platte River Power Authority

By Nick Lumley, PE, Burns & McDonnell Engineering Co.

Heat rate is often the single most important factor in a coal-fired power plant’s economic and emissions profile. Expressed in Btu/kWh or GJ/kWh, heat rate is a measure of efficiency: the quantity of fuel burned to produce a unit of power. Heat rate is best (lowest) at rated plant load and increases as load declines, generally due to equipment being optimized for rated load operation.

In competitive power markets, units with low heat rates can profitably bid into the dispatch stack over a greater range of marginal pricing environments. Even must-run plants benefit from improved heat rates because they lead to better margins.

Minimum load is similarly important to the competitiveness of thermal plants. Low minimum load enables a plant to ‘ride though’ periods of low power demand and earn spinning reserve payments where available without accumulating damaging start-stop cycles.

Reducing heat rate is equivalent to reducing emissions and operating expenses and remains a goal of the power industry since its inception. However, the growing penetration of low-cost and prioritized variable renewable generation has made heat rate and minimum load more important than ever. Recognizing the benefit of improving these characteristics in the current market environment, Platte River Power Authority embarked on a program of improvements to the coal-fired Rawhide Unit 1 intended to reduce heat rate and minimum load.

The Rawhide Unit 1 VFD Conversion

The efficiency of rotodynamic machines, like centrifugal pumps and fans, depends on flow through the machine. For constant-speed machines, the best efficiency point is usually chosen for the flow at rated plant load, resulting in relatively poor efficiency at lower load. Conversion of constant speed machines to variable speed can offer high efficiency throughout the load range, saving megawatts of auxiliary power at low load.

Electronic variable frequency drives (VFD) are an ideal solution to convert centrifugal machines to variable speed operation. Large, medium-voltage VFDs can readily be matched to existing motors, even older non-inverter-duty motors, due to extensive, custom designed filtering. Even pumps and fans using hydraulic variable speed drives can benefit from a VFD conversion with hydraulic drive removal. Hydraulic drives are robust but their efficiency declines at lower transmitted speeds. In contrast, VFDs offer greater than 90% efficiency across the speed range and over 97% at full speed.

For the 280 MW Rawhide Unit 1, the 2x 5,000 hp boiler feedwater pumps (BFP), 2x 5,500 hp induced draft (ID) fans, and 2x 500 hp condensate pumps were found to be candidates for VFD conversion. The ID fans and BFPs had existing hydraulic drives; the condensate pumps were constant speed prior to conversion. The unit’s forced draft and primary air fans remain constant speed axial models with variable pitch vanes; variable-vane fans adapt vane pitch to flow, resulting in high intrinsic efficiency.

The conversion began with electrical engineering efforts to develop procurement specifications for the new VFDs and identify an installation location optimizing cable length and maintenance access. Siemens drives were selected for all pumps and fans and were custom designed for the electrical characteristics of each motor. No motor replacements were required. Fortunately, indoor laydown area underneath the steam turbine pedestal was available and large enough to install VFDs for each of the two BFPs and two ID fans. The smaller condensate pump VFDs were installed near the pumps (Figure 1 and Figure 2).

Removing hydraulic drives from the BFPs and ID fans constituted a fundamental redesign of the machines. Machinery modifications began with a rotordynamic study and subsequent shafting design by specialist subcontractor Electro-Mechanical Engineering Associates. The rotordynamic study involved a computational rotor model to evaluate vibration performance of candidate designs. For both the BFPs and ID fans, rigid shafts were found to have the best vibration performance (Figure 3).

During the rotordynamic analysis, it was found that the existing 3,550-rpm BFP motors, which were to be retained, had lateral critical speeds peaking at about 2,700 rpm. This critical speed is inherent to the motor’s design and cannot be removed.

To prevent operating the BFPs at damaging vibration amplitude, the shafts were designed with balance planes located along the antinodes of the lateral mode shape. Careful installation of balance weights, a specialist 3^rd party engineering and testing task, can offset some rotor imbalance and reduce vibration amplitudes. If potentially damaging vibrations were found to remain after balancing, the affected speed range could be rapidly bypassed using controls.

For the BFPs, existing severe-service recirculation valves were used to create artificial pump demand by shunting flow back to the deaerator to force speed out of the critical zone. Simply locking out certain speeds was not possible in this plant which uses BFP speed to control drum level.

Results of the VFD Conversion

Following conversion, Rawhide realized significant improvements as summarized in Table 1:

For context, heat rate improvement is 6% at minimum load and 1.5% at maximum load over pre-VFD performance. Lowering Rawhide’s full-load heat rate below 10,000 Btu/kWh was an achievement for a high-altitude subcritical unit. Rawhide has also realized improved furnace pressure and drum level control due to rapid VFD speed response.

The auxiliary load savings are a direct result of improved efficiency from VFDs, compared to hydraulic drives, and improvements in power factor. Reactive currents circulating in motors with low power factor, like induction motors at low load, dissipate real power as heat. The VFD conversion improves power factor on the auxiliary network, reducing heating. Moreover, the source of plant reactive power is the generator: better internal power factor makes more generator reactive power capability (MVARs) available to the grid.

Figure 4 shows the improvement in the power factor. Weighted mean power factor for all 6 motors could be as low as 0.70 prior to the VFD conversion. Following the project, the power factor is consistently about 0.97 across the load/speed range.

Trimming auxiliary load

Coal-fired power plants depend on large, high-power pumps and fans. These machines can easily draw a percentage of the generator’s gross output to cover auxiliary load. Because the efficiency of constant speed centrifugal pumps and fans depends on flow, converting existing constant speed machines to variable speed can reduce auxiliary load and improve heat rate throughout the load range.

Electronic VFDs are an ideal speed control mechanism for large machines, offering efficiencies exceeding 90% across the speed range and can be installed on existing motors. Even plants with existing hydraulic speed control can benefit from VFD conversions. Rotordynamic studies should be performed prior to a VFD conversion to explore means of reducing vibration, including possibly replacing motors.

Following its VFD conversion project, Rawhide Unit 1 saved over 4 MW of full load auxiliary power, setting records for its heat rate and dispatchable power, while simultaneously reducing CO₂ emissions per MWh.

Platte River Power Authority is a not-for-profit, community-owned public power utility that generates and delivers safe, reliable, environmentally responsible and financially sustainable energy and services to Estes Park, Fort Collins, Longmont and Loveland, Colorado. Platte River’s generation portfolio includes coal, wind, hydro, solar and gas resources. Rawhide Unit 1 is a 300-MW single-unit coal-fired power plant in northern Colorado.

Jeff Harmon is Rawhide’s Performance Engineer and project manager for the VFD conversion project.

Brian Brigandi is a Rawhide Plant Electrical Engineer and project electrical engineer for the VFD conversion project.

Nick Lumley, PE is the project’s lead mechanical engineer for Burns & McDonnell.

Related Articles

North Dakota coal plant now “fully circular,” owner says

New EPA air rule could cost $1.1b to implement

TransAlta completes coal phaseout at Canadian facilities

FirstEnergy coal plants seek OK for environmental compliance work