Neutral Earthing Resistor (NER) in High Voltage Ships

 Neutral Earthing Resistor (NER) in High Voltage Ships

By Mobin Ahmed ETO, BP Marine Academy, CBD belapur

S O L A S R e g u l a t i o n :

C h a p t e r I I - 1 , P a r t D - E l e c t r i c a l I n s t a l l a t i o n s .

R e g u l a t i o n 4 5 P a r a 4 . 3 . 2 . 1

Introduction

Neutral Grounding Resistors are used to reduce problems such as insulation breakdown

caused by transient over-voltages produced by arcing ground faults in ungrounded systems

and damage to motors and switchgear caused by arcing in solidly grounded systems.

The two main methods of system neutral grounding are Low Resistance and High

Resistance. Low resistance NER system is not applicable to high voltage ships because the

ground fault current is 200A or 400A. This current being of a high value is not safe for

equipment and personnel safety. So, only High NER system is used in high voltage ships for

the safety of equipment and personnel.

The connection of the neutral point of a generator to the ship's hull via a neutral earthing

resistor (NER) serves several purposes.

1. Safety: It helps to limit the voltage to ground in the event of a single-phase ground

fault. By providing a path to ground through the NER, the voltage is limited, reducing

the risk of electric shock and damage to equipment.

2. Stability: It can help stabilize the system by providing a reference point for the

electrical system and limiting transient overvoltage.

3. Fault detection: The NER connection allows for the detection of ground faults,

which is important for maintenance and troubleshooting.

Overall, the connection of the neutral point to the ship's hull via a NER is an important

aspect of electrical safety and system stability on board a ship.

The Importance of Neutral Grounding Fault current and transient over-voltage events can

be costly in terms of network availability, equipment costs and compromised safety.

Interruption of electricity supply, considerable damage to equipment at the fault point,

premature ageing of equipment at other points on the system and a heightened safety risk

to personnel are all possible consequences of fault situations.

By installing NERs on the distribution system and controlling fault currents and transient

over-voltages, the following benefits can be realised:

• Elimination or reduction of physical damage to equipment

• Extended life of connected distribution equipment such as transformers

• Reduced operation and maintenance expenses

• Simplification and fast isolation and clearing of the original fault

• Improvement in network security and reduction in unplanned shutdowns

Methods Of Neutral Earthing

Depending on application and network, there are several methods of neutral earthing

including:

Low Resistance Grounding

Low resistance grounding is used in large MV/HV electrical networks where there is a high

level of capital equipment and network interruptions have a significant economic impact.

These NERs are generally sized to permit only 200A to 2500A of fault current to flow. The

allowed current level is enough to operate protective devices yet not enough to create

major damage at the fault point.

The system will trip in the case of a line-to-ground fault. The Neutral Grounding Resistor will

limit the ground fault to a maximum of 100 to 1000 A. Zero-sequence Current Transformers

and Ground Fault Relays will detect the fault and trip at 5 to 20% of the maximum ground

fault current. The Resistor is generally rated for 10 seconds with a maximum temperature

rise of 760 °C. The maximum ground fault current allowed by the resistor has to be large

enough to positively actuate the applied ground fault relay.

200 to 400 A rated Neutral Grounding Resistors are generally used in 6.9 kV to 34.5 kV

systems. 100 to 400 A rated Neutral Grounding Resistors are generally used in 2.4 to 4.16

kV systems.

Once the current rating is determined, the Resistance or Ohmic Value of the resistor is

calculated by dividing the Line to Neutral Voltage by the Current Rating.

i.e. for a 4.16 kV System Neutral Grounding Resistor rated at 400 A. The line to Neutral

Voltage will be 4.16 kV /√ (3) = 2400 V. The required resistance will be 2400 / 400 = 6

Ohms.

Solidly earthed grounding

Solidly earthed grounding NERs are typically used in LV applications of 600V or less and

connect the neutral point to earth. These systems reduce the problem of transient overvoltage

but do not limit the fault current during a fault event

High Resistance

High resistance grounding is used in commercial systems that require continuous operation

even after a fault occurs, such as continuous process industries. NERs typically reduce the

current to a low value, 10 Amps or less, without tripping the circuit breakers. Protection

schemes are activated during a fault event, allowing the system to quickly locate and clear a

fault or shut down the system in an orderly manner. No damage will occur at this current

level.

This system is applicable to high voltage ships. The system will alarm but not trip (as per

IMO requirement) in the case of a Line-to-Ground fault. It is recommended for systems

where power interruption resulting from single line-to-ground fault tripping is detrimental to

the process.

The Neutral Grounding Resistor will limit the ground fault to a maximum of 5A. Zerosequence

Current Transformers and Ground Fault Relays will detect the fault and alarm at

10 to 20% of the maximum ground fault current.

The resistor is rated for continuous duty with a maximum temperature rise of 375°C. The

maximum ground fault current allowed by the resistor must exceed the total capacitance to

ground charging current of the system and the vector sum of the system charging current

plus the resistor current shall not exceed 8 A.

Once the current rating is determined, the Resistance or Ohmic Value of the resistor is

calculated by dividing the Line to Neutral Voltage by the Current Rating i.e., for a 480 V

System Neutral Grounding Resistor rated at 5 A, the line to Neutral Voltage will be:

480 V /√ (3) = 277 V

The required resistance will be:

277 / 5 = 55.4 Ohms

Calculation Table of NER

Specifying an NER When designing and sizing an NER, the engineer must

consider these parameters:

1. Rated voltage: the line-to-neutral voltage.

2. Rated current: maximum current that will flow through resistor when it is cold.

3. Duty rating or time rating: length of time the NER must tolerate rated current.

4. Short time rating: normally 10 seconds or 60 seconds depending on design

parameters of the protection system.

5. Continuous rating: normally 10% of full load current for healthy system neutral

earthing resistor to be designed for continuous rating of 5% to 10% of full load

current (if required).

6. Insulation: specified based on line voltage.

7. Temperature rise: the maximum short time temperature rise for the resistive

element is 760°C, according to IEEE-32.

8. Element type: metallic resistors are usually specified over liquid as they do not

suffer from evaporation, freezing etc. and require no ancillary supply.

9. IP rating: ingress protection is typically specified at IP23 but higher ratings are

available for harsh environments, although this can significantly affect the cooling of

the unit.

10. Enclosure: usually available in stainless steel, mild steel or aluminium depending

on specification or application.

11. Ancillary items: vacuum contactors, current transformers, protection relays and

monitoring equipment are all optional extras.