PROTECTION SYSTEM DESIGN

SHIP AND OFFSHORE VESSELS-AN OVERVIEW

Copyright Material IEEE Paper No. PCIC-2013-22

Moni Islam Chairman IEEE 45DOT President, M&R Global Trading 36 Madera Ct Kenner, LA 70065, USA Moni.islam@ieee.org

Kamal Garg, PE Supervisor, Protection Schweitzer Engineering Laboratories, Inc. 2350 NE Hopkins Court Pullman, WA 99163, USA kamal_garg@selinc.com

Pat Patel Development Manager Schweitzer Engineering Laboratories, Inc. 2350 NE Hopkins Court Pullman, WA 99163, USA pat_patel@selinc.com

Saurabh Shah Regional Manager Schweitzer Engineering Laboratories, Inc. 2350 NE Hopkins Court Pullman, WA 99163, USA saurabh_shah@selinc.com

Abstract—Offshore vessels and ships are islanded power

systems. Maintaining the operation during the adverse system

operating conditions is a challenge. This paper reviews the

concepts to improve the protection and operation using the

advanced system protection and new developments of the

simulating process.

Index Terms—Offshore, DP, common mode protection,

PMS, VFD, harmonics, blackout, modal analysis, load flow,

short circuit, relay coordination, real time digital simulations.

I. INTRODUCTION

Offshore vessels and ships are islanded power systems

operating in adverse system conditions. With the

advancement in technology in protection and controls, there is

a need to review the protection design and application for

ships and the Offshore Industry. This paper reviews some of

the concepts that can be applied to improve the protection

and control system for ships and the offshore industry

vessels. This paper will focus on the protection of generators,

transformers, drives, and motors as follows, in view of

reliability and redundancy:

a) System Design

b) Future System, Protection, and Control

c) Adjustable speed drive ASD Protection and

Challenges

d) PMS and Blackout Recovery

e) Common Mode Generator Protection

f) Design Verification

In most vessels, electromechanical relays containing single

function relays are installed. However, with the advancement

in technology digital relays can be employed more

economically with the added benefit of low maintenance front

panel indication and display, communications, event recording

and reporting, time synchronization, and high mean time

between failures (MTBF). The system depicted in Figure 1

shows an example of typical power generation and distribution

system with the digital relays. These relays are capable of

protection functions, as well as providing automation, control,

and monitoring functions. Many other functions which are

performed using programmable logic controllers (PLCs) can

also be integrated into these relays. In addition, functions like

blackout recovery and some

functions of the Power Management System (PMS) can

also be integrated and supported with minimal cost and effort

in digital relays.

A discussion of the details of design studies required for

load flow, short circuit, and coordination follows in addition to

and providing an insight into the properly designed protection

and control system. This paper also proposes that there is a

need for design verification using real time closed-loop digital

simulation before the system is implemented in the field.

II. SYSTEM DESIGN

The one-line of the system for an offshore vessel is shown

in Figure 1. A typical system includes four to six generators,

four to six thrusters, propulsion, drilling loads and hotel/local

loads [1]. A properly designed protection and control system

should be able to operate for the closed bus and open bus

operations in order to maintain the minimum system

requirements during fault conditions. The system design

should also meet the class requirement, such as DP2 and

DP3 [2][10]. For an accurate system design, proposed studies

are listed in the system design verification [3].

G1 G2 G3 G4

TH2 TH4

TF3 TF4

TD1 TD2

BUS 1A

11kV

BUS 1B

11kV

BUS 1C

11kV

BUS 1D

11kV

DRILL BUS A

DISTRIBUTION BUS

690/480V

To PMS

Panel

G1

Relay

G2

Relay

G3

Relay

G4

Relay

TH1

T1

RelayF1

Relay

F2

Relay

T3

Relay

T1

F5

Relay

F6

Relay

F7

Relay

F10

Relay

F4

Relay

9212kVA/

11kV

9212kVA/

11kV

9212kVA/

11kV

9212kVA/

11kV

T2 T4

4.5MW

3.3 kV

4.5MW

3.3 kV

4.5MW

3.3 kV

F9

Relay

F8

Relay

11/1.65kV

6MVA11/1.65kV

6MVA

TH3

F3

Relay

T3

4.5MW

3.3 kV

11/1.65kV

6MVA

11/1.65kV

6MVA

11/0.72/0.72kV

5MVA

11/0.72/0.72kV

5MVA 11/0.69/0.48kV

3.5MVA3.5MVA

TF1 TF2

11/0.69/0.48kV

3.5MVA

3.5MVA

T2

Relay

Figure 1: Example Protection System

III. INTELLIGENT DIGITAL RELAYS & DESIGN

The proposed solution includes protection for generators,

motors, and buses and will be able to detect any system

disturbance, and faults, using the properly designed and

coordinated protection system. The system also includes GPS

time synchronization, along with high resolution event

recording capability for the system operation analysis and

investigation.

A. Generator Protection

This solution can provide multiple protection functions in

one digital relay. Typical generator protection elements

included in a single digital relay may consist of the protection

elements shown in Figure 2.

52

46/50/

51G

24 27/59 81

49T 67G BF

50P 87

32 40 51V/C

60

LOP

Figure 2: Standard Features of the Generator Protection Relay

Generator protection protects the generator from the

internal faults. The 51V/C element is backup protection for the

external system faults if the faults are not cleared by the

primary protection elements. The Offshore Industry still uses

one protection relay. However, it is recommended that dual

protection be employed, similar to the protection applied in

other areas (i.e. generation, transmission, and distribution),

since the cost of new digital relays is very economical

compared to that of electromechanical relays.

B. Motor Protection

The digital relays can control, monitor, and protect induction motors, synchronous motors, ASD motors, and special applications such as high inertia motors. The motor thermal element provides integrated protection for all of the following motor operating conditions:

• Locked Rotor Starts

• Running Overload

• Unbalanced Current/Negative-Sequence Current Heating

• Repeated or Frequent Starts

The digital relay dynamically calculates motor slip to precisely track motor temperature using the thermal model. The rotor resistance changes depending on slip and generates heat

especially during starting when the current and slip are highest. By correctly calculating the rotor temperature, the thermal model reduces the time between starts. It also gives the motor more time to reach its rated speed before tripping. The advanced digital relays provide comprehensive metering and monitoring functions so that the motor can be maintained and serviced at regular intervals. The monitoring and reporting

functions offer the following:

• Motor Start Reports and Trends

• Load-Profile Monitoring

• Motor Operating Statistics

• Event reports (Four or 32 samples/cycle resolution)

• Sequential Events Recorder (SER) (As many as 1024 time-tagged, most recent input, output, and element transitions.

Typical motor protection elements included in a single digital

relay may consist of the protection elements shown in

Figure 3. The relays offer communications/integration support with multiple protocols and programmable automation features.

Figure 3: Standard Features of the Motor Protection Relay

C. Power Management & Blackout Recovery

The Power Management System (PMS) is the overall

control and monitoring system of the vessel [8][10]. The

main tasks include system configuration, maintaining

power generation and power factor (PF), avoiding

blackouts, start and stop generators, and supporting

blackout recovery. The proposed PMS is shown in

Figure 4. Protection relays provide the system critical

information to the PMS. The dual redundant PMS system

includes a generator control system (GCS), load sharing

processor (LSP), and front-end processors (FEPs) [4],[5].

Control command can be sent via the same

communications network either by controlling the loads or

by generation, as required. The key features of the LSP

and GCS are customized to meet the system requirements

of the end-user. This system is self-monitored, hence any

communications failure is reported in real time. In addition,

separate transducers, such as those existing in the present

system for metering and control functions, are not required

for the future system. New digital protection devices are

capable of providing all the information required for the

PMS system. The communications protocol can be either

DNP/Modbus® or IEC 61850 GOOSE. The main functions

of the PMS are:

• System operation/maintain power generation

• Load dependant start/stop

• Load sharing

• Frequency control

• Bus-tie breaker control

• Synchronization

• Generator and load breakers

• Power limiting thruster/drilling

• Blackout prevention

• Load shedding (underfrequency/overload)

• Harmonic monitoring

• Oscillation detection

• Data acquisition and event collection

• Graphical display (HMI) of the system

• Alarming

• System reconfiguration

Figure 4: Typical PMS Architecture

The blackout recovery will be programmed in the automation

controller and the digital intelligent relays. Most of the

information available in relays can be utilized to perform

blackout recovery logic. Figure 5 shows an example blackout

recovery system for Bus A. Similar logic can be applied to the

other buses. Bus-tie breaker, and any other system,

information is collected from the existing relays. The logic will

first detect the blackout conditions then based upon

generation in service during the bus recovery detection, a

blackout recovery sequence will be initiated. The critical loads

are auxiliary buses and thrusters, in order to maintain the

station.

D. Common Mode Generator Protection

Common modes of failure are defined as faults that affect the overall system operation and cause multiple redundant elements to react adversely. For normal operating conditions, all of the generators operate in parallel droop mode. In case of a fault on one generator exciter and governor, or any other common-mode fault, properly detecting and isolating the faulty generator from the system as soon as possible is desired [8] [10] [11]. It is also necessary to evaluate the response time of the controls (e.g., exciter and governor controls) before making decisions regarding any system isolation or islanding. Otherwise, undesirable system operations may result in additional faults or failures. This common-mode protection solution correctly detects and islands all of the common-mode faults, which are classified into the following categories:

• Governors

• Fuel or Actuator

• Exciters

Figure 5: System Blackout Recovery

Figure 6: Low kW and Droop Mode

Figure 6 shows the slope and three percent droop

characteristics for the generator prime mover operating in

parallel. Curve A is selected if the generators are operating

normally around 100 percent of load. Curve B is selected if

the unit normally operates around 50 percent of load. For this

analysis, Curve B is selected when the normal operating load

is 50 percent.

E. Small Signal Oscillations & Modal Analysis

Small signal oscillation monitoring and control is very

important for weak islanded systems i.e. offshore and drill

vessels. Without proper remedial actions, inter-area

oscillations can result in power system separation or major

blackouts. The solution is capable of monitoring these

oscillations. Usually local oscillation modes range in frequency

from 0.7 to 2 Hz system. Figure 7 shows the 60 Hz system

with subharmonics. It is proposed to monitor the system

oscillation and provide real time data for visualization and

analysis. [6] [7]

Figure 7: The 60 Hz Signal with Subharmonics

The real part, σ, gives the damping of the amplitude of the

oscillations and the imaginary component, ω, gives the

frequency of oscillations. Negative real part signifies that the

oscillations are decreasing, whereas the positive real part

indicates growing oscillations. The frequency, f, of oscillations

in hertz and damping ratio (ζ) is as given in the following

equation.

� ��

�� (1)

�� � ��

√������� ���� �

IV. ASD PROTECTION

ASD protection may include converter, inverter,

current-source inverter (CSI) and voltage-source inverter

(VSI), current-source converter and voltage-source converter,

and pulse-width modulation (PWM). ASDs, by definition,

operate over a variable frequency range. This is in contrast to

the fixed 50 or 60 Hz operation of across-the-line-operated

motors. A fixed frequency system can no longer be assumed.

The overload protection device must be able to operate

correctly at any allowed operating frequency of the motor.

Since the motor can be subjected to abnormal frequency

levels, overfrequency protection should be provided. Digital

intelligent relays are capable of operating on root-mean-

squared (rms) current measurement. ASDs also produce

significant harmonics in both current and voltage outputs. As a

result, the thermal model is more accurate for ASD operation

when true rms measurements are used instead of

fundamental frequency sequence components. True rms

measurements include both fundamental and harmonic

content.

A. Power Electronics (Drives Equipment)

The over- or under-voltages and voltage unbalances will be addressed in this section. Some drives may also include dc-link reactor overvoltage protection. Overcurrent protection is provided for the converter electronics and interconnected bus or wiring. Short-circuit protection is

provided by fuses in front of the thyristors. • dc Overvoltage

• dc Undervoltage – Loss-of-control power

• Over Temperature – This includes the rectifier and

inverter heat sinks as well as the enclosure

temperature

B. Motors, Propulsion and Thrusters

Motors should be provided with the same protection as constant speed motors of the same size. Motor protection should also include overfrequency and overvoltage, or overexcitation, protection. This protection is typically provided by the drive control system but could be provided by discreet or multifunction relays, as follows:

• Motor Ground Fault

• Motor Overcurrent

• Motor Overload I²t

• Motor Stall

• Motor Overspeed

• Current Unbalance

• Underload – May indicate a process malfunction and

will protect the machinery and the process in this fault

condition

• External Fault – An external relay input

When motors are applied to ASDs, certain operating characteristics of the motor are modified. The operating frequency impacts how the motor behaves during operation, both starting and running, as well as during abnormal operation and fault conditions. The areas that will be discussed are pertinent to the protection of the motor and drive system.

1) Reduced Frequency Operation Effect

The frequency of the source to the motor dictates the operating speed. At lower speed operation, the motor is not cooled as efficiently as it is at rated speed. Therefore, this must be taken into consideration with regard to motor thermal overload protection. Actual motor full load current (Full Load Amperes or FLA) is a function of the frequency, because lower FLA is drawn at lower frequency. The actual FLA must be used in the overload protection.

2) Harmonics

Harmonics in the motor current will cause additional heating in the motors and other connected elements. This additional heating needs to be considered when sizing and protecting the equipment. At near rated load, a typical value to accommodate the additional heating can be up to a 15 percent increase above the

fundamental heating effects.

3) Voltage & Dielectric Stress

State-of-the-art control algorithms used in ASDs keep the motor flux constant over the entire speed/frequency range. The voltage is proportional to the frequency

(V/f = constant) in the upper frequency range.

C. Input Transformer

The input transformer relay provides differential and

overcurrent protection. Based upon the transformer

connection, ground overcurrent and neutral overvoltage can

be enabled. A time overcurrent (51) element that operates on

the fundamental frequency (i.e., not rms) may be set with a

lower pickup, as it will not respond to the harmonic

components of the load current. An instantaneous ground

element (50N) and differential (87) element can be applied for

the input transformer. The feeder, 51, can provide

conventional time-delayed protection. For ASD drives, if using

capacitors for power factor correction, resonances can occur

which must be taken under consideration. Following is the

summary of protection proposed.

• Short-Circuit/Overcurrent – Fuse, CB, or OC relay

• Overload – Overcurrent protection with time delay

• Voltage Unbalance – Loss-of-input phase

• Ground Fault Overcurrent

• Direct current (dc) interconnection and protection

V. DESIGN VALIDATION

A properly designed and validated system with digital

multifunction relays using the validation tools, such as real-

time digital simulations can result in significant savings during

the on-site installation and commissioning [11]. This can be

used to develop proof of concept of design, leading to final

design. Figure 9 shows an example for the real-time digital

simulation, where the actual power system is modeled in real-

time software and the designed system is tested in a closed

loop. For the properly design protection system, the following

studies are required:

a. Load Flow

b. Short Circuit and Equipment Evaluation

c. Relay Coordination

d. Harmonic Analysis

e. Real-Time Digital Simulations

Load flow, short circuit, and equipment evaluation studies

will determine the proper size of cables, transformers, circuit

breakers, and trip devices. The properly designed system will

be able to operate for normal and abnormal operating system

conditions, and meet the system requirements. Properly

performed Coordination Studies will determine the correct

operation and fault clearing in a reasonable time. Figure 8

shows the coordination margin of more than 0.3 second

between the devices. For some system operation conditions,

some miscoordination may be tolerated in order to

accommodate either the system stability or the arc-flash

requirements. For new systems, arc–flash protection and

studies should also be performed.

Figure 8: Overcurrent Coordination

Figure 9: Real Time Test System and Closed Loop Test

VI. CONCLUSION

Offshore vessels and ships are islanded power systems.

Robust protection and control systems are required to operate

these vessels and ships under adverse system conditions.

This paper provides designs for a proposed future system

using the latest technology available. With the availability of

many new features and technologies, it is possible to integrate

many solutions in the protection relays and to improve the

system’s operability and reliability.

VII. REFERENCES

[1] C. Craig and M. Islam, “Integrated Power System

Design for Offshore Energy Vessels & Deep Water

Drilling Rigs,” proceedings of the 57th Annual Petroleum

and Chemical Industry Committee Technical

Conference, San Antonio, TX, September 2010.

[2] Maritime Safety Committee Circular 645, Guidelines for

Vessels With Dynamic Positioning Systems, June 1994.

[3] A. Jain and K. Garg, “System Planning and Protection

Engineering—An Overview,” proceedings of the 3rd

International Conference of Power Systems, Kharagpur,

India, December 2009.

[4] B. Cho, H. Kim, M. Almulla, and N. Seeley, “The

Application of a Redundant Load-Shedding System for

Islanded Power Plants,” proceedings of the 35th Annual

Western Protective Relay Conference, Spokane, WA,

October 2008.

[5] D. Miller, R. Schloss, S. Manson, S. Raghupathula, and

T. Maier, “PacifiCorp’s Jim Bridger RAS: A Dual Triple

Modular Redundant Case Study,” proceedings of the

11th Annual Western Power Delivery Automation

Conference, Spokane, WA, April 2009.

[6] E. O. Schweitzer, III, D. Whitehead, H. Altuve,

D. Tziouvaras, D. Costello, and D. Escobedo, “Line

Protection: Redundancy, Reliability, and Affordability,”

proceedings of the 37th Annual Western Protective

Relay Conference, Spokane, WA, October 2010.

[7] E. O. Schweitzer, III, D. Whitehead, A. Guzmán,

Y. Gong, and M. Donolo, “Advanced Real-Time

Synchrophasor Applications,” proceedings of the 35th

Annual Western Protective Relay Conference, Spokane,

WA, October 2008.

[8] L. Weingarth, S. Manson, S. Shah, and K. Garg, “Power

Management Systems for Offshore Vessels,”

proceedings of the Dynamic Positioning Conference,

Houston, TX, October 2009.

[9] P. Whatley, M. Lanier, L. Underwood, and S. Zocholl,

“Enhanced Motor Protection With the Slip-Dependent

Thermal Model: A Case Study,” proceedings of the 34th

Annual Western Protective Relay Conference, Spokane,

WA, October 2007.

[10] S. Shah and K. Garg, “DP Power Plant Open Bus

Redundancy With Reliable Closed Bus Operation,”

proceedings of the Dynamic Positioning Conference,

Houston, TX, October 2010.

[11] A. Al-Mulla, K. Garg, S. Manson, and A. El-Hamaky,

“Case Study: A Dual-Primary Redundant Automatic

Decoupling System for a Critical Petrochemical

Process,” proceedings of the 2009 PCIC Europe

Technical Conference, Barcelona, Spain, May 2009.

.

VIII. VITAE

Moni Islam -Marine electrical engineering subject matter expert consultant for commercial, Offshore Industry, and Office of Naval Research (ONR). Moni has forty years diversified experience with electrical power systems, particularly electrical propulsion systems. Chair-Central Coordinating Committee for developing IEEE-45 Standards, provides coordinating support for developing IEEE-45 DOT Standards for standardizing electrical design practice for the marine industry. Authored many technical papers on power system design architecture for the marine industry and authored “Handbook to IEEE-45, A Guide to Electrical Installation on Shipboard-2004”. Moni received his bachelor of electrical engineering (Valedictorian) in 1975 from Fort Schuyler, Maritime College, State University of New York.

Kamal Garg is a Protection Supervisor in the Engineering

Services division of Schweitzer Engineering Laboratories, Inc.

(SEL). He received his MSEE from Florida International

University and IIT, Roorkee, India, and a BSEE from KNIT,

Avadh University, India. Kamal worked for POWERGRID India

for seven years and B&V,USA for five years at various

positions before joining SEL, USA in 2006. He has experience

in protection system design, system planning, substation

design, operation, RAS schemes, Synchrophasors, testing,

and maintenance. Kamal is a licensed professional engineer

in six US states.

Pat Patel received his PhD in electrical engineering from the

University of Missouri-Columbia in 1971, after which he

served in the US Army for two years. He worked in various

positions for GE and CEGELEC(France) in the field of HVDC

and SVC systems and projects for 23 years. He has served

13 years at SEL as Manager and Product Engineer in

research and development and is currently actively involved in

industrial applications and product development.

Saurabh Shah is a Regional Manager in the Engineering

Services division of Schweitzer Engineering Laboratories, Inc.

(SEL). He received his AS (1991) and BS (1995) in computer

systems from Lewis-Clark State College. He has a broad

range of experience in the fields of power system operations,

protection, automation, and integrated systems. He has

served nearly 19 years at SEL, where he has worked in relay

testing, sales, business development, and engineering project

management.