1
Introduction

1.1 General

Shipboard power distribution systems have existed since the late nineteenth century. Only until recently, both commercial and naval ships have predominantly employed ungrounded power systems with nominal system voltages less than 1 kV. Ungrounded systems, equivalent to isolated systems (IEC), have a desirable feature in that the unintentional grounding of one conductor results in low ground fault currents and thus enables continued operation with a single ground fault. The ship's engineers can wait for a favorable time to find and clear the ground fault. For many years, ungrounded power systems served the maritime industry well.

However, over the past decades, the total electric load on many types of ships has risen, first due to the addition of heat loads that were previously served by steam prior to the adoption of diesel and gas turbine engines, and second due to the introduction of integrated electric propulsion in the form of an integrated power system (IPS) in the 1990s. In response to this growing load, ships started to employ increasingly higher power generation systems with nominal system voltages greater than 1 kV up to 13.8 kV. For these higher voltages, employing an ungrounded system is not recommended due to voltage stresses on system insulation and due to potential safety concerns. Instead, the use of a high‐resistance grounded (HRG) system has become prevalent for distribution systems with nominal system voltages above 1 kV and more recently has been employed in some systems with a nominal system voltage as low as 440 V. The desire to be able to easily integrate commercial equipment designed for shore‐based facilities has even resulted in some solidly grounded secondary distribution systems.

Additionally, power electronic conversion equipment, such as variable frequency drives (VFDs), has become prevalent in shipboard systems; VFDs enable motors and motor loads to operate at higher efficiencies, reduce inrush current, and increase displacement power factor. On the other hand, the integration of VFDs requires significant effort to ensure harmonic distortion and common mode (CM) currents and voltages are properly controlled. Controlling CM currents and voltages requires additional grounding considerations.

1.2 Grounding and Earthing Definitions

In terrestrial systems, the term “ground” (USA) or “earth” (IEC) refers to the voltage potential of the soil at a particular location and is used as a reference potential for measuring the voltage of other conductors. In some terrestrial power systems, the soil itself (at ground voltage potential) may be used as one of the conductors in the power circuit.

In shipboard systems, the term “ground” or “earth” refers to the voltage potential of the ocean. Since most ships have metallic hulls and structure in direct contact with the ocean, the hull and structure are also said to be at “ground” or “earth” potential. However, because of safety and corrosion concerns, the ship's hull and structure are not normally used as one of the conductors in the power circuit.

Bonding is the act of deliberately connecting exposed metal parts that are not designed to carry electrical currents under normal operation to the hull of the ship via a low‐impedance path. Bonding is primarily performed as a safety measure to prevent electrical shock to personnel caused by capacitively coupled voltages on the exposed metal parts, inductively coupled currents in exposed metal parts, or insulation failures. The impacts of electric currents on humans are discussed in Appendix C.

The connection of exposed metal parts to the chassis or frame of a piece of equipment is indicated on circuit diagrams by IEC Symbol 5020 (Figure 1.1f). A terminal, such as one connected to the chassis, that is intended to be connected to ground for the purpose of implementing bonding is called a protective earth (PE) terminal and is represented by IEC Symbol 5019 (Figure 1.1c). This book will additionally use the symbol depicted in Figure 1.1d to represent the connection of the terminal intended for bonding (IEC Symbol 5019) to the ship's hull. This symbol is used to represent a protective earth, and its incorporation into a design is called protective earthing.

Figure 1.1 Ground symbols: (a) IEC Symbol 5017, (b) IEC Symbol 5018, (c) IEC Symbol 5019, (d) Protective earth, (e) CM ground, and (f) IEC Symbol 5020.

The term “equipotential” means that two conductors have equal voltages with respect to a reference voltage. Two conductors that are electrically connected with a low impedance are equipotential. “Equipotential bonding” and “equipotential grounding” are equivalent to “bonding”; the conductors are at the same voltage as the hull of the ship.

Power distribution system grounding (or earthing (IEC)) is the act of deliberately inserting a solid connection or an impedance between a conductor or neutral of a power system and the hull of the ship. Grounding is usually done to limit conductor voltages with respect to the ship's hull, to provide a path for fault current in the case of a ground fault, and to provide a path for CM currents under normal operations. A ground fault is an unintentional electrical connection between a power system conductor and the ship's hull. A ground fault can be a “solid” ground fault with little resistance, or a fault, such as an arc fault, with a higher resistance.

One may also encounter IEC Symbol 5018 (Noiseless earth), depicted as Figure 1.1b; it is intended to represent a special grounding system for a particular application to minimize CM disturbance or noise on a grounding conductor. These special grounding systems are not covered by this book.

This book distinguishes between power distribution system grounding, protective earthing, and CM grounding. For this book, power distribution system grounding addresses ground currents at frequencies less than three times the fundamental frequency, while CM grounding addresses frequencies at or above three times the fundamental frequency. The two types of grounding do interact; this interaction must be accounted for in the design of each. IEC Symbol 5017 (Figure 1.1a) is used to indicate power system grounding. This book additionally uses the symbol depicted in Figure 1.1e to denote an intentional CM ground. Parasitic connections to ground participate in protective earthing (bonding), power system grounding circuits, and CM circuits; by convention, parasitic component connections to ground are displayed as IEC Symbol 5017. In summary, of the six ground symbols depicted in Figure 1.1, this book will use only the three depicted in Figure 1.2.

Figure 1.2 Ground symbols used in this book: (a) system ground, (b) protective earth, and (c) common mode ground.

Figure 1.3 Use of ground symbols.

Figure 1.3 depicts the use of the three different ground symbols in the case of a single‐phase transformer. The protective earth grounding (left) connects the exposed metallic structure and components of the transformer enclosure (not intended to carry current under normal operation) to ground through bonding. The CM ground (middle) is used to connect the transformer shield between the primary and secondary windings to ground; this shield is used to prevent capacitive coupling of CM currents between the primary and secondary windings. Finally, one of the conductors of the secondary winding is grounded (right) to form a system ground for a solidly grounded distribution system.

Electric current from the power system that, under normal conditions, flows through conductors that are not intended to carry power system current, such as conductors associated with protective earthing, is called objectionable current. Objectionable currents may arise when inappropriate connections are made between the power system and protective earthing conductors.

As defined by IEEE Std 45.1‐2023 (2023), the nominal system voltage is “the designated voltage for a power system used as a reference value for establishing other power quality measures. For direct current, single‐phase AC, and three‐phase AC systems, the nominal system voltage is measured line‐to‐line.” For AC systems, the nominal system voltage is expressed as a root‐mean‐square (rms) of the fundamental frequency component of the voltage waveform.

1.3 Common Mode Terminology

The neutral voltage of a set of conductors with respect to a reference voltage potential (typically ground) at any instant in time is the average value of the instantaneous conductor voltages with respect to the reference voltage. The neutral voltage is also called a CM voltage. Strictly speaking, the neutral voltage is a calculated quantity from the voltage measurements of multiple conductors and may not correspond to the voltage of any one conductor.

For a three‐phase system, the neutral voltage with respect to ground can be expressed as

where

vng, neutral‐to‐ground voltage

vag, vbg, vcg, phase‐to‐ground voltage

van, vbn, vcn, phase‐to‐neutral...