5.5.1 General
Several types of power electronic variable speed drive are available for propulsion drives.
By far the most popular is the multi level pulse width modulation (PWM) inverter but other types of drive are specified for some applications.
5.5.2 DC Drives
DC drives are the original variable speed drive. The DC drive usually consists of a silicon controlled rectifier driving a separately excited DC motor. They have the advantages of low cost, simplicity and compact size. Disadvantages include a more complex motor and limited power rating. There are still many vessels with DC drives in service and designers continue to specify DC propulsion in some vessel projects. When choosing to specify DC drives, it is important to note that they may be vulnerable to power system faults that cause misfire and commutation failure, leading to loss of all thrusters if supplied from a common power system.
Some types of drive will shut down and lockout a restart attempt until a local reset is applied, which has obvious implications for blackout recovery.
5.5.3 Cycloconverter Drive
The cycloconverter drives offer very high torque at low speed; typical applications include icebreakers and large ships. Disadvantages include poor speed range. Figure 96 shows the power component layout of a basic cycloconverter drive. Note that in practice, there are several different forms of cycloconverter.
INPUT TRANSFORMER
POSITIVE CONVERTER
GROUP
NEGATIVE CONVERTER
GROUP
M
Figure 96 – Cycloconverter drive
5.5.4 Synchroconverter Drive
The synchroconverter drive is a current-source load-commutated inverter (LCI) connected to a synchronous motor. This type of inverter is available to power ratings in excess of 50MW. The synchroconverter drive has the advantage of being electrically and mechanically simple. Characteristics include high starting torque, good voltage dip ride through and wide speed range. Figure 97 gives the basic power component layout of a LCI driver.
INPUT TRANSFORMER
M
SYNCHRONOUS MOTOR SMOOTHING INDUCTOR
THYRISTOR CONVERTERS
FORCED COMMUTATION FOR STARTING, NATURAL COMMUTATION WHEN RUNNING
Figure 97 – Synchroconverter drive
Note that good voltage dip ride through depends on the drive controls being well protected by UPS.
5.5.5 Voltage Source PWM Drives
Multilevel pulse width modulation (PWM) drives are now the industry standard for most electric propulsion applications. Figure 98 shows the basic power components of a PWM drive. PWM was developed to improve upon the current harmonics of six and twelve pulse fully controlled bridge output stages. Although the drive output voltage of simple PWM schemes is still a square wave, the mark space ratio is altered to simulate the effective area under an equivalent sinusoid. The effect is a near sinusoidal motor line current waveform with considerably fewer low order harmonics. Higher order harmonics may be increased but these are more easily filtered.
Lower harmonics in motor line current means smooth, quiet operation and a reduction in unwanted heating effects. Although the line current waveform of PWM drives is a major improvement the voltage waveform is still essentially a square wave. More advanced PWM drives use multiple step levels combined with pulse width modulation to improve the voltage waveform. When these drive output voltage waveforms are filtered, the result is a near sinusoidal voltage and current waveform.
Major drive manufacturers claim an overall efficiency of the order of 96% including the output filter. PWM drives offer many other advantages such as near constant power factor throughout the operating range; values in the region of 0.9 are typical. Many drives also offer sophisticated motor control algorithms, some of which use mathematical models of the motor. From a system protection standpoint, drive manufacturers offer short circuit proof drive output converters, which means that a thruster failure is handled at the drive itself and the upstream protection need not operate for this type of fault. Earth fault, thermal, over current and over voltage may also be offered as standards.
Wear and tear on circuit breakers may be reduced as motor starting and stopping is handled by the drive. Reduced arcing can also be expected.
All power electronic drives create harmonics on the system to which they are connected.
Generally speaking, the higher the order of harmonics, the more easily they are dealt with
and much may depend on the type of input stage specified. Six-pulse input rectifiers offer low cost with a penalty in terms of harmonic performance. Twelve-pulse rectifiers, supplied by drive transformers with star and delta secondary windings, are a standard way of improving upon this. Even better performance can be obtained by increasing the pulse number yet further, however, a cost penalty has to be accepted. Several manufacturers also offer drives with an active front end as another way of reducing supply side harmonics.
DC LINK
12 PULSE DIODE BRIDGE. OR THYRISTOR BRIDGE FOR REGENERATIVE APPLICATIONS
M
SYNCHRONOUS MOTOR
Figure 98 – Voltage source PWM drive
5.5.6 Ride Through Performance
Although power electronic variable speed drives have been in use in DP vessels for more than ten years, the significance of some of their features and flaws is only now being understood by the DP community – often as the result of investigations into DP incidents.
In many cases these features were well understood by the drive manufacturers but for some unknown reason the significance of these flaws and features was not communicated to the designers of the DP redundancy concept. One such feature is the ability (or lack of ability) of the variable speed drive to ride through a power system transient caused by the effect of clearing a fault elsewhere in the distribution system. This issue is also of great importance in the process and chemical industries where plant operators do not want critical parts of the process to trip every time there is dip in grid voltage.
Figure 99 shows a much simplified schematic of a variable speed AC thruster drive. Voltage source drives such as this will trip on severe voltage dips to protect themselves from the inrush current that follows power restoration. In recent years, drive manufacturers have addressed this issue by using the power of fast control systems to stop the drive consuming power during the voltage dip thus preventing its own internal voltage falling to dangerously low levels. However, these features are often not tested in practice and therefore the first real test is usually when the feature is called upon to operate in service. Had vessel owners and designers been more aware of these flaws and features, the arguments surrounding operation with busties open or closed may have been very different.
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Figure 99 – Variable speed thruster drive
Some designers back up the voltage dip ride through capability of the drive by providing automatic reconnection and restart. If the drive detects a significant power system disturbance (over/under voltage, over/under frequency) it will be disconnected but will continue to monitor the power supply until it determines that it is safe to automatically reconnect.