June 2009 Vol. 236 No. 6

Features

Static VAR Compensators (SVCs) Facilitate Electrification Of Pipeline Compressor Stations

Demands on pipeline infrastructure are growing. Modernizing the nation’s infrastructure requires boosting existing capacity and providing new lines where additional transport is required.

Modernization also requires mitigating voltage disturbances, emissions and noise and upgrading thousands of aging compressors.

Pipeline operators are increasingly looking to Static VAR Compensator (SVC) solutions to stabilize voltage disturbances stemming from the operation of large motors at compressor and pumping stations. SVCs help to maintain stable pipeline operation by eliminating power producer and customer-side voltage sags and flicker, providing a cost-effective alternative to building new infrastructure.

One of North America’s largest energy companies recently deployed four SVC solutions to modernize its distribution network and improve pumping station performance for a major crude oil pipeline. The pipeline deploying the solutions uses multiple motors rated at 4,000-5,000 horsepower at its pumping stations to transport more than 400,000 bpd. The SVCs eliminated the need for transmission system upgrades at each of the pumping stations that would have taken months to complete at a cost of millions of dollars.

Gas Transmission Retrofit

Most natural gas pipelines were originally designed to operate using gas-driven compressors at collection stations. Using some of the gas-fired compression equipment is becoming a challenge because of air pollution-permitting requirements, high maintenance and age. The transition to electrically driven compression required natural gas pipelines to develop entirely new skills. Pipeline operators today need to not only understand the functioning of medium voltage electric motors, but also the process of specifying, acquiring, installing and maintaining large motors.

Faced with the need to acquire this new set of engineering and operating skills, the pipeline industry is rising to meet the challenge. Reflected in the upgrade of older installations to electric technologies, the industry has gained a deeper appreciation for the increased efficiency and availability of electric drives, as well as for their simple maintenance. Operating economics are driving pipeline operators toward the use of higher capacity electric motors for compression and, at the same time, are reducing the number of compressor stations. Motors in the 2,000-4,000 hp range or larger, operating at 4,160 volts, are becoming commonplace at very large compressor stations, which increasingly are using 13 kV motors in the 8,000-15,000 hp range.

One advantage of these electrically driven compressors is the ability to increase pipeline operating pressures and throughput. That leads to lower unit costs. Flow control is more flexible and precise because the motor speed can be optimally adapted to the process.

Conversion to electrically driven compressors does bring one potential disadvantage. Starting large electric motors places a major strain on electric distribution circuits. Where the “voltage sag” that accompanies the start of a proposed large motor is too great, the electric utility supplying the power may immediately terminate electric service.

When this occurs, the size of the motor can be reduced. This, however, reduces operating flexibility and potential pipeline capacity. Alternatively, the local utility can be paid to upgrade the electrical service – frequently at a cost that is greater than the entire station upgrade. Finally, a very large variable frequency drive (VFD) can be used, albeit at a much higher cost than an SVC installation.

SVC Ensures Reliability

Previously, when pipelines were driven by natural gas-fired compressors, there was no need to have each compressor station located on a robust electric power system. As a result, most existing pipeline stations were sited without regard to electric power availability. Not surprisingly, the end results of this approach are spotty, with some stations directly abutting transmission lines, and others located miles away from significant sources of electric power.

Pipeline operators have found that a robust local electric grid is a pre-requisite to maximizing site compression capability. Medium-voltage SVC systems allow operators to increase motor size at any location without resorting to expensive and time-consuming utility upgrades. SVCs also eliminate the need to install VFD systems.

Technically, an SVC system provides cycle-by-cycle VAR (volt-amp-reactive) support for the compressor motor load. In particular, SVC technology protects large operating compressor motors against load sags of up to about 25%, provides increased motor torque under loaded operating conditions, and eliminates the reliability and cost problems associated with multiple “point” solutions such as motor starters and drivers. In short, using an SVC to mitigate local power system problems allows increased flexibility in updating existing compressor stations (or building new stations) and reduces utility charges and construction lead times.

With fully automatic controls and passive cooling technology, the SVC system contains no moving parts and requires no regular maintenance. It can run completely stand-alone or can be integrated with existing local and remote controls. SVC systems are compact and thus can be installed within a limited footprint and commissioned in a matter of days.

Field Experience

In natural gas compressor stations, an SVC solution has been used in the following common field applications:

• Boosting by up to six times the hp rating of a motor that can be used at a site;
• Siting multiple large pumps or compressors in one location, where each exceeded previous maximum starting size;
• Eliminating the need to provide new utility transmission service to serve an upgraded station;
• Eliminating utility complaints and power termination discussions for operators experiencing unacceptable sag and flicker complaints; and
• Avoiding the alternative of using multiple reduced voltage soft-starters.

In another project, an SVC was used to start an 8,000-hp motor at a major natural gas compressor station. When this project was initially proposed, the utility responded by requesting a multimillion-dollar, 18-month transmission upgrade. The SVC allowed the motor to be supplied with power by a 33 kV/13.8 kV distribution line, reducing project completion time to six months at a lower cost. After SVC start-up at this site, monitors documented a utility-side sag during motor start-up of 3%. According to customer personnel, the compressor compiled the best availability of any station on the line for several years after its final commissioning.

Another typical application is in a refined products pipeline, located in the north-central U.S. where, prior to an SVC installation, two existing motors, rated at 2,400 and 800 hp respectively, caused deep voltage sags of about 12% for nearly a minute. This was so disruptive to other customers on the circuit that the utility threatened disconnection. Employing an SVC in this application eliminated a $400,000 substation upgrade and post-installation monitoring documented a reduction in voltage sags to less than 3% during motor start-up.

Other examples of dealing with motors ranging in size from 600-14,000 hp across North America illustrate the same set of critical economic benefits for pipeline operators: ability to increase motor size, elimination of electric infrastructure costs and potential for significant reductions in construction timelines.

Key Components Of A Static VAR Compensator (SVC) System

SVC systems include thyristor valves, reactors, capacitors, a power transformer and an integrated control system. At the heart of the SVC is the anti-parallel thyristor valve. The state-of-the-art thyristor valve applies capacitors and/or reactors in single unit steps. This allows automatic adjustment of the system capacitance and inductance, thereby maintaining voltages with instantaneous response to system events. The electrically triggered thyristor (ETT) valves are contained in transformer oil for isolation and cooling. AMSC SVC systems are fully passive and contain no moving parts, eliminating the need for routine maintenance and boosting reliability.

By utilizing thyristor switched capacitors (TSC), the SVC provides stepped control with low losses and no harmonics. Each shunt capacitor in the SVC system is divided into a specified number of branches that are switched on and off via anti-parallel thyristor/diode switches. The thyristor/diode design lowers thermal losses and provides increased robustness. Since switching occurs during zero voltage across the thyristor, no switching transients are produced, hence the inherent high power quality potential of the unit.

Author:
Kerry N. Diehl is director of SVC business, AMSC Power Systems. He has more than 10 years of experience providing dynamic reactive VAR compensation for large motor operations. He holds a bachelor of science degree in chemical engineering from Lehigh University and a master of science degree in industrial administration from the Tepper School of Business at Carnegie-Mellon University. He joined AMSC in 2006 when Power Quality Systems, Inc. was acquired by American Superconductor.