Lubomir Sevov

The Power of IEC 61850: Bus-Transfer and Load-Shedding Applications

The new communications technology and the newly developed IEC61850[1] standard for generic object oriented substation events (GOOSE) bring many advantages to the industrial protection and control applications. Some of the applications benefiting the most are the ones associated with the bus transfer and the load shedding schemes, together with more beneficial communication-assisted schemes, like zone-interlocking, fast bus trip, arc-flash reduction, etc. Some Intelligent electronic devices (IEDs) are equipped with more than one high-speed Ethernet channel to transmit/receive hundreds of discrete and analog values. This offers two very big advantages over the copper wired IEDs: the first - a single pair of network cable either copper of fiber, can substitute a big number of standard copper wires, and the second - using two or more network channels provide very good data exchange redundancy and hence higher reliability. The savings on substituting a big number of copper wires by a communication media like Ethernet copper twisted pair cable, or a pair of fiber optic, can be easily calculated.

Advanced bus transfer and load shedding applications with IEC61850

The new communications technology and the newly developed IEC61850 standard for generic object oriented substation events (GOOSE) bring many advantages to the industrial protection and control applications. Some of the applications benefiting the most are the ones associated with the bus transfer and the load shedding schemes, together with more beneficial communication-assisted schemes, like zone-interlocking, fast bus trip, arc-flash reduction, etc. Some Intelligent electronic devices (IEDs) are equipped with more than one high-speed Ethernet channel to transmit/receive hundreds of discrete and analog values. This offers two very big advantages over the copper wired IEDs: the first — a single pair of network cable either copper of fiber, can substitute a big number of standard copper wires, and the second — using two or more network channels provide very good data exchange redundancy and hence higher reliability. The savings on substituting a big number of copper wires by a communication media like Ethernet copper twisted pair cable, or a pair of fiber optic, can be easily calculated.

Differential protection for power transformers with non-standard phase shifts

The current differential protection is the most popular protection for transformers, providing good fault sensitivity, selectivity and security. Depending on the application, the transformer size, shape, winding connections vary. Some common power system applications require installation of two- or three-winding conventional power transformers or autotransformers, while others require phase shifting transformers (PST), Scott transformers, LeBlanc transformers, Zig-Zag grounding transformers, or converter transformers. Applying differential protection to conventional type power transformers with “standard” phase shifts of 30 degrees, or multiples of 30 degrees is trivial. However, applying this protection to transformers with non-standard phase shifts is challenging for the protection engineer. Current Transformers (CTs) installed in non-common locations adds even more to the complexity of applying the protection correctly. The later is associated with special CT — relay connections, correct computation of winding currents, phase angles, and selection of protective device supporting such applications. The paper provides essential knowledge on the transformer differential protections throughout theory and application examples. Current transformers and relay connections, as well as computation of transformer setup settings with standard and non-standard phase shift are covered.

Practical considerations of applying IEC61850 GOOSE based zone selective interlocking scheme in industrial applications

This paper gives an overview on the practical considerations when IEC61850 GOOSE messaging based Zone Selective Interlocking (ZSI) scheme is applied in industrial applications. Considerations are extended from normal instantaneous overcurrent (IOC) protection elements coordination, to timed overcurrent (TOC) protection elements coordination for both phase and ground protection, from one voltage level to multiple voltage levels, from ZSI schemes to fast bus tripping schemes. When transformer feeders are involved with a ZSI scheme, avoiding nuisance trip caused by transformer inrush current is a major concern that makes timed overcurrent protection to be used over IOC only. When motor feeders are involved with a ZSI scheme, motor lockout current need be considered, so timed overcurrent is also used instead of IOC element. The impact of different protection devices used in different motor circuits, breaker or contactor fuse combination is also discussed in the paper. Communication network plays an important role in IEC61850 GOOSE messaging based ZSI schemes. In consideration of ZSI blocking time, all influencing factors have to be taken into account. This includes breaker or contractor opening time, relay I/O response time and messages traveling time in the communication network. Reducing and separating network traffic by using advanced network technology is a key for the success. Communication network redundancy needs to be carefully considered and implemented as an important step in each stage of the project: in the early planning, the setup and the execution. Detailed arrangement and configuration of the switchgear and the motor control center is another factor to consider, in terms to accommodate the number of intelligent electronic devices (IEDs), which communicate using the technology available today.

Enhancing power transformer differential protection to improve security and dependability

Current differential principle is a well-known principle used for protection of transformers, motors, generators, buses, and any other type of power equipment with input and output current measurements. Further, the principle is used in developing percent differential protection, which can be programmed to the desired sensitivity for detecting in-zone faults and security during external faults. This protection dependability is usually achieved by modeling a differential-restraining characteristic with two regions, operating and nonoperating, and tracking the real differential restraint ratio during faults. Some external faults with high dc offset and high X/R system time constant would easily saturate the installed current transformers (CTs), which in return would cause high differential/restraint ratio above the preset characteristic into the operating region. In such cases, the differential protection would operate and cause unwanted transformer trip. This paper focuses on some enhancements applied to the differential principle of the main differential protection; it also defines guidance on how to setup the protection for better sensitivity and security. The paper is supported by fault cases, showing the improved security and dependability during internal/external faults with and without CT saturation.

Differential Protection in Low-Voltage Buses: An Exploration of Principles and Models

Current-differential principles are well known and commonly used for the protection of medium and large transformers, large motors, medium-voltage (MV) generators, MV and high-voltage buses, and any type of important power equipment with measurable input and output currents. However, is it practical to protect low-voltage (LV) distribution buses using differential protection? This article describes bus differential protection principles as well as interlocking principles for overcurrent protection. We discuss specific issues in applying differential protection in LV systems. Additionally, we present a concept of partial differential (PD) protection, which can be used in conjunction with zone-selective interlocking (ZSI) or as backup to traditional overcurrent protection to achieve high-speed and selective fault clearance. Additional concepts for the implementation of bus differential protection using networked data in LV systems are introduced.

Motor Reacceleration to Improve Process Uptime

Reacceleration is a method of automatically restarting motors after an unexpected deacceleration caused by system voltage events such as dips, outages, or bus transfers. Reacceleration schemes are designed to minimize process disruptions by rapid detection of supply loss, recovery/ monitoring of acceptable transient torque limits, and then automatic reclosure of the motor contactors. Depending on the connected load and minimum available fault current, the reacceleration may be “instantaneous” or a staged event designed to assure that the bus voltage is maintained at an acceptable level during the process restart. Poor coordination of the reacceleration process may cause further system outages if the nominal bus voltage drops below tolerable levels. Special consideration is required to prioritize loads, to maintain production, to prevent lifting of safety valves, and to avert equipment damage. Various processes differ in their ability to withstand temporary outages, from milliseconds (ms) to several seconds. Todays protection and control intelligent electronic devices (IEDs) provide settings, dedicated measurement, and timing circuits to allow these variances to be preprogrammed and activated based on outage duration and magnitude. Short-time outages within millisecond range combined with electrically held contactors provide the least complex situations for enabling a reacceleration system. More complex situations arise when motors are still decelerating and when the supply is restored, especially if the contactors were maintained closed during the outage. Certain more advanced IEDs also provide multiple reacceleration schemes that are automatically deployed based on longer outages—typically up to 30 min.

Enhanced overcurrent algorithm for performance under CT saturation in industrial applications

The computational capabilities of numerical relays allow the development of sophisticated overcurrent algorithms. These algorithms ensure dependable and fast operation of short circuit protection even under severe saturation of current transformers. This eliminates the need for impractical engineering of such applications, and increases confidence levels in cases with relatively well-rated CTs that do not experience extreme saturation but are exposed to currents much higher than the conventional 20 times rated.