Electric power utilities are required to continuously evaluate the reliability of their distribution system. Several reliability performance methods have been developed to measure outage duration and frequency, system availability, and response time. The most common distribution indices include the System Average Interruption Duration Index (SAIDI) and System Average Interruption Frequency Index (SAIFI). ABB's new protection method improves the reliability indices and thus saves the utility money by minimizing penalty fees and the other associated costs of power disruptions.
The role of protection relays in an electrical network is to protect people, circuits and equipment. When a protection relay detects a fault in the power system, it is most commonly set to trip a circuit breaker. At Helen Electricity Network, however, the protection relays scheme for earth-fault protection is to indicate selectively with an alarm that there is a fault, but no automatic tripping of the circuit breaker takes place.
ABB's new protection method has been developed to improve power system reliability and to deal with even the most challenging fault types, such as restriking earth-faults in compensated networks. This new protection functionality, known as multifrequency admittance-based earth-fault protection, minimizes the amount and consequences of unselective tripping. When an intermittent earth-fault occurs, it opens up the possibility for unselective tripping to take place. Unselective tripping can result in the disconnection of a healthy or too large part of the network and considerably prolong the fault location process, which is then registered as a decline in the reliability indices.
Helen Electricity Network continuously develops its network to ensure secure and disturbance-free power supply to its 375,000 customers in the Helsinki region. The company expressed an interest in testing the new and unique protection method in practice at their new underground substation, Ilmalantori, which is located under the Ilmala market square in the Finnish capital. ABB and the customer worked in close cooperation and performed a multitude of tests to gather data on the performance of the protection relays and the new earth-fault protection method.
The Ilmalantori underground substation. Photo courtesy of Helen Ltd.
"In our system, earth-faults trigger an alarm but there is no automatic disconnection. To speed-up the fault localization, correct and selective indication of a faulted feeder is essential. Based on practical experience, earth-faults in our network are often of a restriking type, which has been a very challenging fault type for the previous generation of numerical protection relays," explains Mika Loukkalahti, System Manager at Helen Electricity Network, and continues "The Relion 630 protection relay offers an all-in-one solution for all different earth-fault types. Setting value and protection function changes for adapting the protection scheme from compensated network to unearthed network operation are no longer needed. Our tests indicate that the new earth-fault function available in the relays is very reliable and it protects against even the most challenging fault types. This provides a great opportunity in future upgrade projects as we avoid the extra cost involved with using separate fault locator devices."
The 110/20 kV Ilmalantori substation was optimal for putting ABB's new earth-fault protection to the test, since the network supplied is centrally located in the country's most densely populated city and consists of underground cables in large scale. This large substation is supplied by two main transformers and UniGear ZS1 switchgear with 40 cubicles. In the switchgear there are double busbars, both of which are equipped with compensation coils (arc suppression coils) to limit the magnitude of fault current and hazardous voltages during an earth-fault.
Inside the substation: UniGear ZS1 switchgear with Relion protection relays. Photo courtesy of Helen Ltd.
The unique multifrequency admittance-based earth-fault protection method was first introduced last year with the new version of ABB's Relion 630 series protection and control relays. Since then the multifrequency admittance-based earth-fault protection has also been brought to the Relion product family's 615 and 620 series. Compared to traditional earth-fault protection methods, this new method significantly improves overall security and dependability of the protection system as it brings higher sensitivity and selectivity with only a minimum number of settings. One of the greatest benefits with this new protection method is the possibility of significant cost savings as separate fault type dedicated protection devices (such as transient fault detector devices) are no longer needed.