Alternators have a variety of features to help them maintain a steady voltage against step and transient load changes (motor starting). Such systems can also be used to provide high levels of current in the case of a short circuit for external protection equipment and control discrimination. Here we discuss how the Mecc Alte Alternator products meet these criteria.
In this Alternator testing video we compare three systems which are MAUX (Mecc Alte AUXiliary winding) PMG (permanent magnet excitation) and SHUNT (power direct from main stator). Each supplies power to the automatic voltage regulator (AVR). Tests were made on the 240kVA 60Hz ECO38 alternator as shown.
The standard AVR is fitted for these tests and is suitable for MAUX, PMG and SHUNT power supply (50Hz or 60Hz compatible, 60Hz in this test).
Transient tests show performances against short term load increases as you would see in starting motors. In real terms, the alternator will see a high kVA increase – such as 8-9x the motor running kVA on direct on line (DOL) starting, even if the engine sees a reduced kWm load with a low starting power factor.
MAUX/PMG may be classed as separately excited as power is not from the main stator of the alternator. So long as speed is maintained the power output is constant. In the case of SHUNT, the load step (motor start) will cause the voltage to dip. This also causes a dip in the power supply to the AVR and consequently the performances in SHUNT are less.
The MAUX/PMG can supply over 300% rated current, but there will be a consequential transient voltage dip (TVD) to consider. Using our published dip curves and/or Mecc Alte Sales Assistant sizing program http://www.meccalte.com/downloads/MeccAlte_Sales_Assistant.zip. You can select the size of alternator according to the load transient or step according to those dip curves. Typically, the industry standard is G2 regulation which is 20% TVD. This will equate to around 200% transient load on the alternator only achievable with MAUX or PMG fitted.
Here we test the machine with a 0% to 60% step load and 60% to 0% to see the voltage dip and rise. This will be the same for all systems as we are within the 100% capability of the machine and not invoking the transient overload ability of the MAUX/PMG systems.
As mentioned earlier the different systems have differing responses to a short circuit condition. In this test we switch in a full 3ph short circuit onto the alternator while running to see the sustained level of short circuit current.
With the MAUX powering the AVR we achieve 1109A short circuit current - 360% compared to the machines full Class H load amps!
With the PMG powering the AVR the result is very similar at 995A - 330% compared to the machines full Class H load amps!
With SHUNT there is no sustained short circuit current. The initial peak from 0.01s decays to a low level where MAUX and PMG capture this fall, lifting it to a useful sustained output.
In both MAUX & PMG systems Mecc Alte publish >300% current for up to 20 seconds. This allows power system integrators more scope in circuit breaker selection and discrimination. Peak current is determined by design and is used for the fast-acting part of breaker protection. The sustained part for overload protection and discrimination by circuit breakers and relays is better served by the MAUX/PMG systems! It is for this reason certain applications including marine always specify >300% sustained short circuit current!
UNDERSTANDING THE VALUE OF POWER