Operation, reliability and maintenance of LCI VSD synchronous electric motors
LCI (Load Commutated Inverter) type VSD (variable speed drive) converter systems are used for synchronous electric motors.LCI type thyrist-or-frequency converters are among the most simple, cost effective and reliable power electronic systems on VSD market for variable-speed electric motor drivers particularly for medium and large size electric motors. These are among the best variable speed drivers for industrial and manufacturing machineries. Operation, reliability, and maintenance of LCI VSD synchronous electric motors are discussed.
LCI VSD CONVERTER SYSTEMS
The LCI VSD converter system is a proven technology with successful references. In identical basic configuration, and with similar power electronic devices, LCI converters are employed by power generation industry in wide range of power ratings in high-voltage DC (HVDC) transmission systems. Although, there are some differences between VSD converters and HVDC transmission systems; for instance, the DC link reactors. To avoid costly and space consuming externally-installed aircooled solutions, the directly watercooled and iron-cored reactor of smaller converters (similar technology) should be uprated to the DC link voltage of high voltage designs. Comprehensive type tests confirmed the suitability of such design for a wide range of applications including relatively high rated application.
However, there are some issues and challenges with LCI VSD systems. Load commutated inverter (LCI) drives are the source of voltage and current distortion (known as harmonics); this issue requires a special attention. Therefore, in most applications of LCI VSD systems, harmonic filters are needed.
As indications, the power factor of a plant load should usually be maintained between 0.85 (lagging) and 1.0 during steady-state operation scenarios and under all load and operational conditions. However, there are some challenges. Sometimes, there are some changes in electrical power supply system, such as transmission systems, after the placement of purchase order of main rotating machine trains including main VSD systems. Too often, the actual electrical grid of a plant is relatively weaker compared to theoretical grid in the design stage. This could be due to many reasons such as revised power supply arrangement, some modifications, etc. This feature can have some impacts on the size of the harmonic filter package. For example, in an industrial plant, an initial aboveground 8km power transmission line was changed to a 14km underground cable. This new power transmission arrangement caused many changes in VSD LCI systems and their harmonic filter configurations.
BASICS OF LCI VSD
A thyristor is a solid-state semiconductor device with four layers of alternating N-type and P-type material (or ‘silicon controlled rectifier’). A thyristor acts as a bi-stable switch, conducting when its gate receives a current trigger, and continues to conduct while it is forward biased, while the voltage across the device is not reversed. In other words, the thyristor is a four-layer, three terminal semiconducting devices, with each layer consisting of alternately N-type or P-type material (for example, P-N-P-N). The main terminals, labelled anode and cathode, are across the full four layers. The control terminal (called the gate) is attached to P-type material near to the cathode; this is a variant called an SCS (Silicon Controlled Switch) which brings all four layers out to terminals. Since modern thyristors can switch power on a wide scale power ratings (say below kW to multi MW), thyristor valves have become the heart of VSD converter systems or high-voltage direct current (HVDC) conversion systems either to or from alternating current.
RELIABILITY AND OPERATIONAL ISSUES OF LCI VSD
Reliability is a major consideration for VSD systems. In addition to the usual failure modes because of exceeding voltage, current or power ratings, thyristors have their own particular modes of failure, including: 1. Turn on “di/dt”: In this failure mode the rate of rise of on-state current after triggering is higher than that can be supported by the spreading speed of the active conduction area. 2. Forced commutation: In this mode, the transient peak reverse recovery current causes such a high voltage drop in the sub-cathode region that it exceeds the reverse breakdown voltage of the gate cathode diode junction. 3. Switch on “dv/dt”: In this mode the thyristor can be spuriously fired without trigger from the gate if the rate of rise of voltage anode to cathode is too high. Sometimes, plant specifications specify “n+1” redundancy for a thyristor. However, VSD system manufacturers usually argue that modern power thyristors are no more likely to fail than any other component in the power circuit which often used without spare. In other words, manufacturers usually advise a measurable increase in reliability cannot be obtained by this expensive scheme of “n+1” redundancy. Operating experiences over 2–3 decades have showed that the single-thyristor failure is a rare exception. Optical firing and checkback signals to and from the thyristor is handled by fast microprocessor equipment that allows both a serial data communication with the distant control and monitoring cubicles, and the exact identification of a faulty thyristor. Often “n+1” redundancy for thyristor is not implemented. However, this “n+1” scheme may be used for special VSD units with very high expected reliability.
To prevent impermissible “di/dt” values from endangering a thyristor, cable capacities to the electric motor and to converter transformer are usually compensated by toroidal cores slipped onto internal water-cooled busbars in the converter whereby previously used ferrite “donuts” are replaced by more efficient wound amorphous metal coils.
Combined heat losses of a variable-speed drive ( VSD) system, including the electric motor and frequency converter, are usually transferred by two separate closed-loop water-cooled systems with cooling banks. Except for the frequency converters with their integrated DC link reactors; the equipment is directly cooled by water. The high voltage potential inside the frequency converter requires a controlled cleanliness (conductivity) of the cooling medium; this mandates another closed-loop deionized water system which is linked to the primary cooling system via heat exchangers. Often, a sophisticated, modern compact plate-type heat-exchanger is employed to save space, obtain optimum costs and offer a good overall reliability. To maintain the availability of these cooling systems, their circulating pumps are redundant with automatic change-over.
The LCI system usually employs readily available disk-type thyristors (or silicon controlled rectifiers, SCRs) as solid state power switching elements. They are usually mounted in standardised equipment cubicles for installation at “Indoor”, or in purpose-built power centre modules at “Outdoor”.
TRANSFORMERS IN LCI SYSTEMS
For many VSD systems, to match the drive’s input voltage to the plant’s power line voltage, a transformer is usually required for the converter system. Great care should be taken for the sizing, selection, reliability,
acoustic insulation, and fabrication details of such transformers.
Generally, more care should be taken for transformers used in VSD converter systems rather than other transformers. Particularly, sizing, insulation, thermal design, and bi-concentric winding require special attention. A less known, but critical issue could be noise generation. Special measures should be taken to acoustically isolate the coil and core assembly from the oil tank to reduce structure-born noises, and to increase the impedance of the secondary windings as required limiting the possible short circuit current in the fuse-less thyristor frequency converter.
The voltage drop at plant’s high voltage distribution system during switching is usually of concern and such a voltage drop should be limited to certain values (say 2–3 percent). Series-connected inrush limiting resistors should thus be installed on the high voltage side. These are momentarily switched in by a separate circuit breaker for a certain time period (say one second) during transformer magnetisation.
Thermal considerations need special attention as many transformers and generally VSD systems were failed or experienced operational and reliability issues because of poor thermal management. In a case study for a VSD electric motor, a high voltage primary transformer was experienced several breakdowns. The analysis of oil from the transformer has demonstrated a premature oil ageing because of thermal issues due to improper initial sizing. The heat sizing of the equipment did not include an additional heating caused by harmonic currents. Therefore, as a lesson learned, firstly all effects such as harmonic effects, etc. should be considered and ample margins should be applied for the sizing of transformers with special attention to thermal loads. As an indication 20–35 percent margins should be considered on the thermal management. Many successfully operated transformers were designed with proper sizing margins and have been operated most of the time below their rated capacities (say below 85–90 percent); all these resulted in long trouble-free operation time.
LCI START-UP AND SHUTDOWN
The normal start procedure of a LCI VSD is: • The VSD system is energised. • The circuit breaker transformer is
switched on. • The harmonic filter gets the release
from the VSD control. • The circuit breaker filter is switched on. • The release is sent to the VSD control. • The electric motor starts the operation.
The normal stop procedure is the inverse sequence of the abovementioned. Regarding the regulation of supplying electric grid of a plant, it is useful to switch-on one VSD after the other. The switching operation of harmonic filter circuits and the start-up of VSDs have some short time transient effects and also some static voltage changing effects on the electrical grid. All these should be evaluated using proper analysis and studies.
EFFECTS OF GRID ON LCI HARMONIC FILTER
LCI (Load Commutated Inverter) VSD converters have been employed for many electric motor drivers in industrial and manufacturing plants. Harmonic filters are important parts of any LCI VSD system; a major issue is the effects of the grid (plant, etc.) and particularly other major loads on the operation and performance of these harmonic filters. The loads and characteristics of the electrical network of the plant which were known during the order of harmonic filters should have been considered on the filter design and its operation. A major problem is new loads in the plant; these are loads not defined at that time or changes in the plant after the order of major VSD systems.
In many plants, generated harmonics are excessive sometimes even during low fault level conditions. VSDs system operating parameters might change from the original rated or design ones, or the plant’s electrical network might be modified such as new consumers, etc.; therefore, harmonic filters are not as effective as they should be and consequently harmonics exceeding the specified limits. Considerations of possible future expansions, possible degradations and risk mitigations are important for reliability and long-term successful operation of VSDs.
All resonance cases associated to operating modes should be considered. This is not only about the plant; sometimes, a harmonic resonance case tends to occur when one of the power generators is off-line in the power source of the main electrical grid.
This can change the tuning of the electric system of the plant so that a resonance could occur near a specific harmonic.
Relatively long length and large sizes of some cables feeding to different loads in various units can increase their effects on VSD systems and overall network characteristics. The result could be a significant impact on natural frequencies of the overall electrical system of such a plant. Another observation is all possible operation modes should be considered for all these loads. For “N” cables that feed electric powers to “N” different loads, which can be connected to the same bus as the VSD (s), there are “2N” combinations of the different cables (including the case of no cables connected) which should be investigated for the reliability and safety of the overall electrical system of a plant. For example, in case of N=5 (five cables), 32 different combinations should be considered. For a very complex system, because there are a huge number of possible system configurations (for instance, for N=11: 2048 combinations), it is difficult to determine the levels of harmonic distortion for the various contingent conditions. Considering for some large plants, “N” is more than 20 or 25, and a printout of all of harmonic results for all possible combinations would make such a harmonic study report thousands of pages long. Obviously, it is impractical to look at each of these cases individually because there are too many. However, it is possible to automatically scan through all cases to determine worst case(s). Sometimes, it may be decided to report only critical cases. For example, critical cases could be topologies that have the mid-voltage bus ties open, or if closed having one transformer out of service.
Often a filter arm, known as cable damping filter arm, is recommended to be added to the filter package to deal with these cable load effects in a plant. The performance of a filter at lower order harmonic frequencies is usually similar for cases with or without cable damping filter arm. However, a cable damper filter arm can reduce the depth of resonance valleys for higher frequencies. Too often such a cable damper filter arm proposed by VSD system manufacturers. Complete and accurate cable transmission network is not often considered by VSD manufacturers; therefore, such a damper filter arm is recommended as mitigation. These damper filter arms are useful in some cases; but in many other cases, they just offer some slight beneficial effects. In other words, in many cases, this manufacturer proposed harmonic filter cannot have sensible effect on the reduction of harmonic values. A good advice could be maintaining damper high harmonic filter arm, but the system logic should be specified to continue operation even if this high-harmonic filter arm stops working.
Modifications could continue to occur in a plant, for instance, new consumers may be added, some others may be disconnected or there might be changes in electrical configurations. Such modifications could change experienced harmonic at the electric network or a VSD system; subsequently both harmonics on grid and the operation of machineries could be affected. Accordingly, a risk mitigation strategy is required to cater for possible changes or possible future expansions. Harmonic currents in each of harmonic filter arms should be monitored via communication links. This information could be used to monitor the harmonic situation and flag if additional harmonic filtering is required. Additional harmonic filters may be installed if harmonic levels are exceeded the specified limits (or values recommended by engineering practices) for safe and reliable operation.
Amin Almasi is a lead mechanical engineer in Australia. He is chartered professional engineer of Engineers Australia ( MIEAust CPEng – Mechanical) and IMechE (CEng MIMechE) in addition to a M.Sc. and B.Sc. in mechanical engineering and RPEQ (Registered Professional Engineer in Queensland). He specialises in mechanical equipment and machineries including centrifugal, screw and reciprocating compressors, gas turbines, steam turbines, engines, pumps, condition monitoring, reliability, as well as fire protection, power generation, water treatment, material handling and others. Almasi is an active member of Engineers Australia, IMechE, ASME, and SPE. He has authored more than 150 papers and articles dealing with rotating equipment, condition monitoring, fire protection, power generation, water treatment, material handling and reliability.