High Voltage Grounding

High Voltage Grounding By Dr. Oladokun Sulaiman

Slide navigation • • • • •

Overview Types of grounding Fault interruption methods Marine system grounding Marine HV grounding

Grounding • • • • • • • •

The term grounding implies an intentional electrical connection to a reference conducting body, which may be earth, but more generally consists of a specific array of interconnected electrical conductors. The resulting circuit is often referred to by several terms, such as: ground plane, ground grid, mat or ground system. Grounding systems should be serviced as needed to ensure continued compliance with electrical and safety codes, and to maintain overall reliability of the facility electrical system. Action must be initiated and continued to remove, or reduce to a minimum, the causes of recurrent problem areas. Maintenance inspections should be performed at times which have the least affect on user activities The complexity of ground systems and the degree of performance expected from such systems is growing all the time. Maintenance or shop personnel are encouraged to become familiar with Article 250 of the National Electrical Code (NEC), which deals with grounding requirements and practices.

Factors which influence the choice of selecting system ground

• • • • • • •

voltage level of the power system, transient over voltage possibilities, types of equipment on the system, cost of equipment, required continuity of service, quality of system operating personnel and safety consideration including fire hazards.

Method of resistance grounding in neutral of three phase system • 150-175 ohm , 15A60secc on 4160v system, 3w • 0.55 ohm , 500A,for 10sec on 480v, 3 phase, 3w • Derived neutral with grounding resistor

Methods of solidly grounding the neutral of threephase systems

Earthing for ship • Distribution systems of ships are usually have their neutral points earthed to the ship’s hull through a resistor • The resistor in neutral line limits earth faults currents and protects equipment - Insulated Neutral System - Earthed Neutral System - Resistance earth Neutral System

Insulated Neutral to Earth • This system is totally insulated from the ship’s hull • This system maintains continuity of power supply to the equipment even in the event of single phasing fault. • This ensure power supply to critical equipment • The power supply to the equipment can disrupt only if two single phase faults occur simultaneously in two lines which is then equivalent to short circuiting faults • But such fault occur very rare

Insulated Earth Neutral

Gen

Load

Earth

Insulated Earth 3-Phase Gen

Earth

Load

Single Earth Fault no effect

Insulated Earth 3-Phase Gen

Earth

Load

Double Earth short Circuit

Neutral Earthed System • Earthed Neutral is done in 3-phase system for supply voltage of 3.3-6.6 kV and above • If a solidly earthed system is used for such high voltage then magnitude of earthed current will be extremely high which can damage the equipment • The earth current is limited by connecting a resistor in series between earth and neutral point • The earth current due to single phasing is limited not to exceed rated current at maximum load

Neutral Earth 1-Phase

Gen

Load

Earth

Fault

Neutral Earth 3-Phase

Gen

Earth

Load

Fault in Line

High voltage grounding for ships • The high voltage (e.g. 6.6 kV) installation covers the generation, main supply cables, switchgear, transformers, electric propulsion (if fitted) and a few large motors e.g. for side-thrusters and air conditioning compressors. • For all electrical equipment the key indicator to High Voltage Equipment voltage level which enhances the current flow. • Remember that an electric shock current as low as 15 mA can be fatal

High voltage grounding for ships • • • • •

The risk to people working in HV areas is greatly minimized by the diligent application of sensible general and company safety regulations and procedures. Personnel who are required to routinely test and maintain HV equipment should be trained in the necessary practical safety procedures and certified as qualified for this duty. Approved safety clothing, footwear, eye protection and hard that should be used where danger may arise from arcs, hot surfaces and high voltage etc. The access to HV switchboards and equipment must be strictly controlled using a permit-to-work scheme and isolation procedures together with live-line tests and earthing-down before any work is started. The electrical permit requirements and procedures are similar to permits used to control access in any hot-work situation, e.g. welding, cutting, burning etc. in a potentially hazardous area. All work to be carried out on HV Equipment is subject to an Electrical Permit to Work (EPTW)

EPTW Guidelines for the necessary declarations and procedures are : • Before work is commenced on HV equipment an EPTW must be issued. • Planned maintenance task carried out by the responsible person. EPTW Contains: -

Risk assessment Chief Electro technical Officer (CETO) or Chief Engineer Agreement on safety precautions and HV circuit has been isolated and earthed. - Suspension or completion of the designated work - Cancellation of the permit with a signature from the authorizing officer.

EPTW - A Permit-to-Work is usually valid only for 24 hours. Some marine and offshore companies will also require an associated electrical isolation Certificate to declare and record exactly where the circuit isolation and ear thing has been applied before the EPTW can be authorized. • A Sanction to- Test safety certificate may also be required when an electrical test (e.g. an electrical insulation test) is to be applied.

HV live-line testing components. • This is necessary as the circuit earth generally has to be removed during such testing. Before earthing-down the particular circuit or equipment declared in the EPTW • it must be tested and proved dead after disconnection and isolation. This can only be carried out by using an approved live- line tester as shown • The tester itself must be proven before and after such a test. • This is checked by connecting the tester to a known HV source (supplied either as a separate battery operated unit or included as an internal self-test facility). • Two people should always be together when working on HV equipment.

Earthing down • Before work can be allowed to commence on HV equipment it must be earthed to the hull for operator safety. • As an example, consider the earthing arrangements at an HV switchboard. • Here, the earthing-down method is of two types: 1- Circuit Earthing: • After disconnection from the live supply, an incoming or outgoing feeder cable is connected by a manually operated switch to connect all three conductors to earth. • This action then releases a permissive-key to allow the circuit breaker to be withdrawn to the TEST position. • The circuit breaker cannot be re-inserted until the earth has been removed and the key restored to its normal position

Earthing down 2- Bus-bar Earthing • When it is necessary to work on a section of the HV switchboard bus-bars, they must be isolated from all possible electrical sources. • This will include generator incomers, section or bus-tie breakers and transformers (which could back-feed) on that bus-bar section. • Earthing down is carried out at a bus-section breaker compartment after satisfying the permissive key exchanges. • In some installations the application of a bus-bar earth is by a special earthing circuit breaker which is temporarily inserted into the switchboard solely for the bus-bar earthing duty. • For extra confidence and operator safety, additional earthing can be connected local to the work task with approved portable earthing straps and an insulated extension tool, e.g. at the terminals of an HV motor

Bus-bar Earthling • Remember to always connect the common wire to earth first before connecting the other wires to the three phase connections. • When removing the earthling straps, always remove the earth connection last.