East Perth Power Station
Click here to download A Brief History of the Development of East Perth Power Station (.pdf). For a faster download, you may wish to simply download the two appendices which were written by a journalist from The West Australian in 1916:
Optimising Protection at the Customer’s Point of Connection
Click here to download Optimising Protection at the Customer’s Point of Connection (.doc).
From Protection that Coordinates to Protection that Protects
Historically, protection grading was carried out with the single objective of coordination. This required a rigid adherence to well-established rules that ensured relay operations could be coordinated in a sequential fashion. The result was, that for any given fault location, the amount of lost load would be minimised.
Power system operators were generally accepting of this approach. The penalty inherent with this highly coordinated approach is increasingly slow relay operating times towards the source of supply.
One utility power station engineer asked what went wrong with his protection when an apprentice was asked to retrieve an object from the top of an MCC. His weight had been sufficient to buckle the metal cladding to the extent that it made contact with the busbars. His records showed a 6 second clearance time. In fact, there was nothing ‘wrong’ with the protection. For a short circuit on the busbars, 6 seconds was the calculated operating time. At the time, such a long operating time was accepted as a non-negotiable outcome of the ‘normal’ protection coordination procedures.
A few years afterwards, the same utility was to pioneer the use of temporary protection settings as an arc flash hazard reduction technique. This was prior to the widespread use of electronic relaying. Each relay protecting a switchboard was modified for the application of a temporary setting which required the installation of three dedicated, transient overreach free, electromechanical highset elements.
Historically, the use of instantaneous elements on switchboard incomers was considered to be the antithesis of ‘good coordination’ because it could result in a simple feeder fault causing a loss of the entire switchboard. The more recent approach is to tolerate a nuisance trip to provide a better outcome in the event of a genuine busbar fault. A long duration busbar fault could destroy a switchboard and put personal safety at risk.
Protection coordination is, therefore, a balance between:
- Fast operating protection that minimises the risks to personnel and plant, and
- Slower protection that minimises the amount of load lost for a given fault.
Why did my Relay Trip?
‘Why did my relay trip?’, an easy question but one which might not have a simple answer.
With electromechanical protective relays, it was usually possible to establish why a relay tripped if you knew what flags had fallen and you had access to the settings applied on the front panel of the relay.
When a multifunction numeric relay trips, you need the software settings file. This is the key to understanding what the relay does and how it achieves its objectives.
This file holds the relay set points which are analogous to the settings on an electromechanical relay, and the relay logic. The relay logic condenses what would have been a number of discrete ‘all-or-nothing’ relays into logical equations. Understanding this logic will largely depend on how much effort the person writing it was prepared to invest in transparency and accountability. When that investment is high, the investigation can be a pleasure. When it is low, the investigations can be laborious and costly.
To facilitate understanding, relay logic needs to be supplemented with comments in the more complex logic fields and a separate document with a functional specification.
Figure 1: An electromagnetic relay
Figure 2: A numeric relay