ECE 8830 - Electric Drives. Topic 19 : High Power AC Drive and Motor Applications Spring 2004. Introduction. The main interest of USN engineers related to AC drives is how to design ship propulsion systems of the future.
Topic 19: High Power AC Drive and Motor
The main interest of USN engineers related to AC drives is how to design ship propulsion systems of the future.
Several advances are taking place in motors, drives, and system configurations. Three papers presented in recent conferences related to this issue will be discussed - one each related to each of these three topics.
The three topics to be considered are as follows:
1) *High power superconducting magnet
2) *A low cost, low resolution encoder for
vector-controlled drive applications.
3) +The LHD-8 ship propulsion system.
*American Society of Naval Engineers (ASNE), Electric Machines Technology Symposium (EMTS), Philadelphia, PA, January 2004
+International Naval Engineer's Conference (INEC), Amsterdam, The Netherlands, March 2004.
The next evolutionary step in designing high power PM motors is to replace the high flux density NdFeB magnets with high TC superconducting magnets. The zero dc resistance offered by superconducting wires allows very high current densities to be conducted by the wires resulting in very high magnetic field generation. Furthermore, the associated zero wire resistance results in low synchronous reactance (although the transient and sub-transient reactances are similar to those for a conventional machine.)
In the late 1980’s/early 1990’s a new class of superconducting materials was developed based on rare-earth oxide materials. These materials displayed superconducting behavior at temperatures as high as ~100K (compared to conventional superconductors which displayed superconductivity at ~4K). This was technologically significant since liquid nitrogen could be used to cool the materials to superconducting temperatures rather than the much more expensive liquid helium.
One of these materials, barium strontium calcium copper oxide (BSCCO), has been developed to the point where wires of this material have reasonably high current-handling capability and have now been incorporated into motors for generating strong magnetic fields.
The paper that we review here describes a 5MW motor employing high temperature superconducting (HTS) wires for the field windings.
A cutaway drawing of the 5MW HTS motor is shown below:
The rotor assembly includes:
The stator assembly includes:
An external liquid He cryo-cooler module used to cool the field winding is located at the non-driven end of the shaft.
The field winding is conduction-cooled through the support structure. A torque tube transfers the torque from the cooled environment to the “warm” environment. The field winding contains 6 pole sets, each fabricated with BSCCO HTS wire. The pole sets and support structure are enclosed in an evacuated chamber to minimize radiative heat transfer to the HTS field coils.
An EM shield is located at the surface of the cryostat to:
The parameters for this motor are shown in the table below:
Superconducting magnet motors offer very high efficiency (see figure below) which results in the motor being approx. half the size and weight of a conventional motor of similar power and torque rating (see next slide).
Open circuit voltage is found to increase linearly with current for four speeds ranging between 60 rpm and 230 rpm as shown below:
The losses of the motor are seen to increase quadratically with line voltage (probably due to eddy currents) - see figure below:
The transient response of the motor to an instantaneous shorting of the stator terminals is shown below for a field current of 0.07 p.u. The time constant is found to match the motor specs.
Rather than using an expensive, high-precision encoder for an IM drive, a rotary encoder with only 64 pulses per revolution has been considered. Because of the low precision of the measurement, two different speed estimation methods have been used to estimate the rotor speed - one method used at high speeds (>200 rpm) and one for low speeds (<200 rpm). The measured speed contains some errors so a correction procedure is used to minimize this error.
The high-speed method is based on counting encoder pulses in a fixed time interval. The rotation angle is time period T is given by:
where n= # of pulses counted in time T,
and N= # of encoder pulses per revolution.
rotor speed r is simply given by:
The main problem with this approach is quantization error which can be reduced by increasing N (more expensive) or using digital filtering techniques (result in delays => not suitable for vector-controlled drives (at low speeds).
A different approach is used for low speeds.
For the low speed region, output pulses of a high frequency clock are counted between two consecutive encoder pulses. If the # of counted clock pulses is n and the frequency of the clock is fc , the rotor speed is given by:
This method also has a quantization error due to fc/n. This is only a problem at higher speeds but the approach is good at low speeds.
An error minimization method applicable at both low and high speeds is described in the paper.
Also, parameter sensitivity of the field orientation control and a self-tuning procedure are described in the paper as well as Matlab/Simulink simulation results.
The third paper describes a hybrid power system for a US Navy ship in which a steam-based power/propulsion system is being replaced with a hybrid gas turbine/auxiliary motors for propulsion and electric powered auxiliary systems.
These systems are designed for the LHD 8 ship which is in the Wasp class of amphibious assault ships (40,500 ton, 844 ft.) designed to support the USMC and perform amphibious assaults against defended shore positions.
The design goal for the shipboard power system was to replace two independent steam boilers and two 35,000 hp steam turbine engines. A GE 25,000 hp gas turbine engine was available and qualified for USN ship propulsion, but was deficient for this application. Also, gas turbines require ducting through the island structure of the ship. For these reasons (and the volume required for two gas turbine engines per shaft) made this approach unfeasible.
Recently, the GE LM25000+ gas turbine engine capable of delivering 35,000 hp has been developed and using one such engine per shaft, the ship propulsion requirements are met. Therefore, this was the technology that was incorporated into the LHD 8. In addition to the main gas turbine engines, an auxiliary propulsion motor (5,000 hp) driving a 2-stage reduction gear into a controllable pitch propeller was used (see next slide). Propulsion is provided by either the main gas turbine engine or the auxiliary motor - but not by both simultaneously.
Propulsion Gas Turbine
Main Thrust Bearing
Turning GearHybrid Ship Power System (cont’d)
FIGURE 3 - Conceptual diagram of shaft propulsion power train arrangement for LHD 8
FIGURE 4 - Sketch of shaft propulsion power train design concept for LHD 8
Several options were considered for the electric plant design for the LHD 8 which considered factors such as ship construction cost, life cycle cost, survivability, etc. The electrical load analysis is shown in the table below:
Table 1 - Predicted electrical load analysis summary with anticipated service life growth margin
A traditional USN 450V AC electric power distribution system was considered unsuitable for the system because of the limitations of USN 450 VAC circuit breakers at the expected power loads, and so it was decided to increase the system voltage to 4160 VAC. Since most ship loads are derived from 450 VAC, a power distribution system architecture for routing the 4160 VAC power, converting the 4160 VAC to 450 VAC, and then distributing the 450 VAC power, was required. An AC Zonal Electrical Distribution System (AC ZEDS) was selected (see paper for details).
For the auxiliary propulsion system (APS) 5 different motor types were considered. Their advantages/disadvantages are provided in the table below:
COMPARISON OF PROPULSION MOTOR TYPE OPTIONS
MOTOR RELATIVE RELATIVE RELATIVE
TYPES SIZE COST RISK
DC max dia near min min
AC SYNC near max dia near min min
AC INDUC near min dia min min
AC PM near min dia near max near max
AC SUPER-C min dia max max
A 5,000hp AC induction motor was selected. With the two auxiliary motors on the two shafts, a total of 10,000 hp was available for propulsion which could propel the ship at a steady state speed of 12 knots while at full power (70,000 hp) the ship could move at over 20 knots.
The next design issue related to the APS was how to start the ship only using the auxiliary motors.
The first approach considered was to get the shaft spinning using the gas turbine engine prior to switching to the auxiliary motor. However, this did not allow for completely independent operation of the two systems.
A variable speed drive (VSD) rather than a fixed speed drive approach was selected to allow full torque to be provided by the induction motor at start-up.
Having decided on a VSD approach, the particular VSD topology had to be selected. The range of design solutions is indicated in the table below:
A transformerless, 6-pulse rectifier requires substantial filtering in order to meet the stringent power quality requirements for naval shipboard use. Passive filtering takes up a lot of room and active filtering would need some development time. An alternative approach would be to use a PWM rectifier, but this too would require some development time.
For VSD designs incorporating transformers, 12-, 18-, and 24-pulse rectifiers are viable options and can provide the requisite power quality without additional filtering. However, the limited space in the machinery room could not accommodate a large single transformer or even two smaller transformers (and the additional required switchgear). Further work is needed to define the details of the VSD design.
The rest of the paper focuses on the concept of operations (CONOPS) description for the shipboard power system, including a conceptual layout of the machinery and a description of the power management priorities. The paper concludes with a description of the APS control system.