IEEE Electrification Magazine - June 2015 - 60

Volts

Voltages

11,200
11,100
11,000
10,900
10,800
10,700
10,600
10,500
10,400

A-B Vrms (max/min/val)

11,000

Volts

10,750
10,500
10,250

B-C Vrms (max/min/val)

Volts

10,000
11,100
11,000
10,900
10,800
10,700
10,600
10,500
10,400
10,300
10,200

C-A Vrms (max/min/val)
18:54:40
01/May/2013

18:54:45

18:54:50

Figure 12. The main busbar voltage transients due to the propulsion transformer's inrush current.

AVR Settings (Capability Limits)
As previously stated, AVRs are endowed with some limiters to avoid generator damage. In particular, the AVRs
must limit the voltage (equivalently the current) that they
apply on the excitation circuit of the synchronous
machine (the generator), with the aim of limiting the temperature that the excitation windings reach during this
operation. The simplest way to do that is to set two or
three time-based excitation voltage (or current) thresholds
(Figure 14). The rated current can be maintained at steady
state, whereas higher values can be reached in transient
conditions. The transient excitation boost is essential to
achieve an acceptable performance from the generator,
provided that these higher currents are cut off (by the
AVR) when they persist for too long. Thus, although it is
not the best-performing solution (the best practice is the
implementation of the entire machine's capability curve),
the time thresholds implemented in the simplest AVRs
are sufficient for the purpose.
A notable issue that can occur in excitation systems is
the incorrect setting of these thresholds. The excitation voltage (and current) for both no-load and rated-load generator's conditions are the only data that can be found in datasheets (in some cases not even these), so the thresholds
must be set accordingly to the competence of the personnel
who install the AVR. An error in a transient excitation

I E E E E l e c t r i f i c ati o n M agaz ine / j un e 2015

current threshold is dangerous due to the overtemperatures
that it causes, but has little effect on the machine's lifespan
because it lasts only a few seconds. A more harmful situation could arise when the steady-state current threshold is
set incorrectly. In this case, the generator could work for a
long time with an excitation current above the rated value,
causing a rise in the excitation winding temperature to values that shorten the machine's insulation lifespan.

System Integration
So far, this article has confirmed the complexity of the system. The IPS is an interdependent system in which every
component interacts with the others in multiple ways.
Attempting to control this complex and interconnected
system, which is MIMO, with an array of SISO controllers
could be a harsh matter. If the regulators are set without
paying attention to the interactions that could arise
between them and the rest of the system, some problems,
even serious, could occur. In fact, it is a common practice to
allocate the design of electrical machines, regulators, and
protection systems to different entities. These normally do
not communicate among themselves, and they do not utilize complete models of the ship's IPS or perform transient's
analysis and simulations (except for some special simplified cases). Moreover, the results of the design are often
tested only after installation on board. In this way, the

Table of Contents for the Digital Edition of IEEE Electrification Magazine - June 2015