Switchgear and Protection of High Voltage Systems

Stewart Prince

Switchgear and Protection of High Voltage Systems DN43 35

May 2015

Introduction

In this report we are going to look at switchgear and protection of high voltage systems. We also need to consider continuity of supply and security of supply.

In regards to continuity of supply and security of supply, this required to ensure places like hospitals and prisons that require a constant supply of electrical energy without this there is chance of severe consequences. So systems are put in place to try and eliminate this.

We will also look at what protection is required, when fault occurs we require systems in place to isolate, protect and eliminate any unnecessary blackouts and also to prevent any further damage from occurring, if fault is detected and cleared we require equipment that turn the power back on without having engineers being called out and houses and building going without power.

They are two main regulations that solely deal with this, the electricity and work act 1989 under the provision of the health and safety act, and the Quality and Continuity Regulations (ESQR).

Contents Page

Section 1 The operation and main constructional features of a high voltage fuse.

Section 2 The operation and the main constructional features of two of the following circuit breakers: SF6 and Oil.

Section 3 The operation, application and the main constructional features of high voltage Isolator.

Section 4 The operation and the application of a distance protection scheme.

Section 5 The operation and the application of three items of embedded generation protection.

Section 6 Bibliography

Section 1 High voltage fuses

In the high voltage transmission and distribution networks we require devices that will be able cut the power as quickly as possible, so the high voltage fuse will act as the weak link this is connected to the circuit. The fuse will operate when an overcurrent is detected and when it detects an overvoltage.

The main type`s high voltage fuse is a HRC (high rupture capacity fuse) and dropout fuses which are used on overhead power lines.

A high voltage fuse is constructed with metal strip which has a low melting point, the fuse will placed across two terminals one to the circuit and the from the power supply, these high voltage fuses have a striker which will be propelled into the middle section of the fuse this has two benefits firstly it stop the power following to load and it will be visible to the maintenance person. The outer part of the fuse is constructed with a non-conducting material like glass or porcelain, the middle of fuse with have a material like sand which does not conduct electricity. When the fuse go`s this disconnect power whether it is a single/three phase supply it will stop all power, another feature of this fuse there is an arc quenching medium .(see below)

All fuses will come with the manufacturers rating, this will specified voltage/current once this is exceeded the fuse will blow and required to be replaced as these fuses are not rewire-able this one of its main safety precautions.

The second type of fuse is the drop-out fuse, when engineers require to work on overhead power these fuses will drop out of their housing and remain hinged to the lower contact therefore the power cannot be turned back on accidently therefore is also good to use for isolation.

Section 2 The operation and the main constructional features of two of the following circuit breakers: SF6 and Oil.

The circuit breaker is used to isolate the power out the system so maintenance can be carried out, the circuit breaker can be either operated manually or automatically by moving the two contacts away from each other but they can cause arcing to occur between the two contacts they are several systems currently available, this will protect an electrical circuits from damage under fault conditions. Circuit breakers differ from fuses mainly because the can be reset either automatically or manually, but they still protect electrical circuits from overload and short circuited fault conditions.

The circuit breakers are designed with two contacts one fixed contact and one moving contact, under normal working condition (no fault) the two contacts will remain closed and therefore allow operate.

The problem comes when the contacts are moved away from each other, this can cause an arc to be produced and any combustible material that are between the two contacts could become ignited and cause a fire resulting from damage and destruction, this could be air (which contains oxygen) or even water (which contains hydrogen and oxygen) in the correct amount to cause fire or even an explosion.

SF6 Circuit Breaker ( sulphur hexafluoride)

The operation of the SF6 circuit breaker is similar in operation as the air blast system, but instead of air the Sulphur hexafluoride (SF6) is used, sulphur hexafluoride gas is a non-flammable gas which is used in power circuit breakers, this gas is pumped into the space between the contacts of the switch which quenches the arc and also dissipates any heat given off, the contacts must be in a vacuum to allow this to happen as quickly as possible and under pressure. One main benefit of the SF6 system it can reduce the size circuit-breaker that is required compared to the oil filled circuit-breaker.

They are three main types of the SF6 circuit breaker available dependent upon the voltage level of the load/application they start from 245KV up to 800KV.

Sulphur hexafluoride gas is classed as an inert gas that does not react or undergo reactions under a given set of circumstances, this gas is not friendly to the environment so great care must be taken using this hence the reason for the totally sealed container it must be confined in, this also helps to establish a vacuum.

The SF6 circuit breaker must abide by British Standards (EN 62271-4:2013 – Handling Procedures of SF6 and its mixtures), (BS 5311 – High voltage alternating current circuit breaker) and (BS 5227 – A.C metal enclosed switchgear).

Oil Filled Circuit breaker

The operation of an oil filled circuit breaker this is one of the oldest types still in production today, as soon as the moving contacts start to move away from the fixed contact an arc is initiated between these two contacts, this arc is extremely hot so oil quenches the heat, at the same time ionized gas is also produced this will float to top of the oil, as there is a gap between the top oil the oil and lid there are pressure releasing valve located on lid so no build of gas will occur to dangerous levels.

The oil filled circuit breaker is usually made from strong metals like cast iron like a large tank, these make them very robust and weatherproof which is required like Britain. All of the contacts are submerged in mineral oil this oil is very vicious, extremely cheap, and easy to replace during regular maintenance, when sampling and testing of this oil will take place to look for any metal fillings and other dangerous materials and any signs of windings being scorched if any these signs are noticed and that they exceed what is expected the circuit breaker they need to investigated.

The oil filled circuit breakers must abide to British Standards (BS 5311 – High voltage alternating current circuit breaker), (BS 158 – insulating oil for transformers and circuit breakers) and (BS 5227 – A.C metal enclosed switchgear).

Section 3 The operation, application and the main constructional features of high voltage Isolator.

The operation of a high voltage isolator has two contacts one fixed and movable, on the movable contact there is a pivot joint to allow connection and disconnection possible, these are used in such places like substations, electrical power generation plant and they are others places where the devices can be found. The isolator allows us totally disconnect any device or circuit, this in turns allows engineers to work on high voltage equipment, these can also be locked off so power cannot be reinstated by accident while someone is working on high voltage equipment, this also stated in health and safety regulations.

High voltage isolators are made from a solid piece of copper. They are capable of handling large voltages and current.

These devices are not intended to be used as circuit breakers or fuses they do not have the ability to detect anything there only job is to either disconnect or connect electrical power.

The high voltage isolator must provide a safe means of Isolation of high voltage equipment and must adhere to the Electricity at work regulations – in a secure isolation.

Section 4 The operation and the application of a distance protection scheme.

Distance protection schemes work on the principle of ohmic method of detecting a fault, these devices are set equal distance apart from each other and they overlap one another to give us far better idea of exactly where the fault has occurred, so when fault occurs it does not trip every circuit breaker only trios the circuit breaker closest to it and therefore no unnecessary power outages will occur, this way we can prevent claims for loss power.

The current transformers (CT) are small magnetic devices which placed around the cable which in turns creates a small e.m.f (electromagnetic flux) these emf`s have known values so if value suddenly rises this will indicate fault and it will cause a relay to close/open.

Some systems nowadays will have an auto opening and auto closing circuit breaker so if the fault clears the power will flow back onto that line if the CT still detects a problem it will cause will trip the circuit breaker for that area and an engineer will be required to go and track down the fault on the line.

Section 5 Embedded Generation Protection

In high voltage power transmission and distribution we require many different systems to be located on lines, substations and local distribution network some look for power being out in-balances, some look for over-voltage and under-voltages, other will look at the frequency and the correct polarity is being maintained all the above can cause problems some more severe and protection must also to for energy spikes that can happen during thunderstorms.

Voltage and Frequency Protection Relay

These relays are constantly monitoring voltage and frequency which been transmitted if an over/under voltage detected and frequency has changed, we require somehow to detect this and stop this before it causes severe damage to equipment

Reverse Power Relay

In the transmission we usually have more than one electrical generator running in parallel, sometimes these generator are not located in the site so that the power being generated is not flowing from the bus bar back into the generator, this will cause severe damage to the generator or possibly to cause it to burst into flames so we install a reverse power relay this device under normal conditions the contacts are part from each other under fault conditions the top part of relay comes into contact with the second contact and cause the trip contact to be energised and the trip mechanism will activate and cut the power to it.

References

http://www.oocities.org/hemant_thermal/SFcircuitbreaker.htm

SF6 circuit breaker

http://electrical4u.com/oil-circuit-breaker-bulk-and-minimum-oil-circuit-breaker/

http://electrical-engineering-pics.blogspot.co.uk/2014/09/bulk-oil-circuit-breaker-bocb.html

Oil filled circuit breaker

http://engineeringagenda.com/agenda/2012/07/over-current-protection-relay/

Reverse power relay

Picture in section 3 of high voltage isolator and discussions with Morey Malyon and other class colleague’s material included within this report

City of Glasgow College notes