Roger Hedding

Advanced Multi-terminal Line Current Differential Relaying and Applications

Current differential relaying has long been an alternative to directional comparison relaying, especially for short line applications. Current transformer performance has always been one of the limiting factors is using a current differential scheme. Summing the current transformer currents before entering the relay deepens the problem. With the advent of multi-terminal current differential relays, individual currents can be kept isolated throughout the differential process, mitigating the problem. This paper describes differential relaying techniques and shows examples of how keeping currents separate helps solve real application problems

Protection of battery energy storage systems

In todays power systems, growing demand, aging infrastructure, system constraints as well as the increasing renewable energy portfolio have increased the need for utilities to find new ways to manage their system and increase reliability. One area that is surfacing as a potential solution to this is what is commonly referred as the “holy grail” of the industry or energy storage. The utility industry is the only industry that does not have a common warehouse or inventory of the product they produce. When a customer turns on a light switch, the power is generated immediately. To store this power in a warehouse has not been done economically in the past.

Protection and control system impacts from The Digital World

Technology has changed significantly from over the past 30 years and will continue advance enabling more benefits from The Digital World. The early adoption of microprocessor relays started the era into The Digital World. Along with their significant advantages, they also introduced our world to software and communicating devices to the realm of Cyber Threats in our changing environment. The introduction of the IEC 61850 station and process bus standards for substations has provided a platform that all manufactures can develop upon to achieve the overall goal of interoperability. John Burgers visionary ideas are being realized with the technology available today. In addition to the interoperability benefits, footprint of primary switchgear reduction using sensors (NCIT) replacing conventional measuring transformers and breaker controls allows a much safer work environment and a massive reduction of cabling by going from a lot of copper cables to a few fiber optic communication cables. As for the challenges presented by the cyber threats, the industry must embrace modern device capabilities to deter, delay and detect the bad guys. Let us not forget that the multifunction relay is today the source of information that can enable higher level systems to be proactive in the overall power system stability. Most importantly, the “R” in NERC means reliability so while CIP standards might drive organizations to shutdown communication access to the substation information, it is so crucial that the substation data be accessible to higher level systems.

Application experience with line current differential relays on three terminal series compensated lines

In order to increase the power transfer capability of transmission lines a series capacitance is added to the line. Addition of this series capacitance reduces the net reactance of the line thereby increasing the power transfer. Series capacitors introduce a number of problems in protecting transmission lines due to the fact that the state of the series capacitor (whether it is inserted or bypassed) can change dynamically by gaps flashing or metal oxide varistor operation during high current conditions. The problem is further compounded by the amount of series compensation applied to the line, and the location of the series capacitors on the line. Add to this that the line is a three terminal line and one realizes the complex issue Bonneville Power Administration had to resolve to protection several transmission lines. This paper will describe several three terminal lines series compensation that BPA protection engineers were charged with protecting. Some were heavily loaded. One of the lines also has four reactors to extinguish the secondary arc during single pole open and allow for successful reclose. Some transmission lines had near 100% compensation.

Review of unit protection schemes for auto-transformers

An auto-transformer is a power transformer in which at least two windings have a common part [1]. For standard auto-transformer design this common part is the common winding. Typically auto-transformers are used to interconnect two electrical networks with similar voltage levels (e.g. system intertie transformer). In practice auto-transformer tertiary delta winding is normally included. It serves to limit generation of third harmonic voltages caused by magnetizing currents and to lower zero sequence impedance for five-limb core construction. Standard practice is to size the tertiary delta winding for at least one third of the rated total through power of the auto-transformer. This is done in order to achieve adequate short-circuit withstand strength of the delta winding during earth fault in the HV systems. The application of the unit protection schemes for auto-transformers is somewhat special because such scheme can be arranged in a number of different ways: ▸ Based on autotransformer ampere-turn balance ▸ Based on First Kirchhoff Law between galvanic interconnected parts ▸ Based on zero-sequence currents (restricted earth-fault protection) ▸ Dedicated unit protection schemes for tertiary delta winding Most commonly used auto-transformer unit protection schemes will be described in this document. Advantages and disadvantages of every unit protection scheme will also be discussed.

Some old and new thoughts on phase angle regulator protection

“Smart Grid” can be defined as getting the power to the customer the smartest way possible with the least amount of losses. One of the ways to control the power flow over the transmission line is to insert a phase angle regulating transformer (PAR) in the line. PARs have long been dreaded by the protection engineer as a very complex device with many different currents that make it tedious at best to develop a complete protection scheme. The problem is compounded by currents changing with tap position on the load tap changer. This paper will review the theory and application of phase angle regulating transformers. Then review the classical protection given PARs. finally, suggesting new approaches to the protection problem made easier by the use of microprocessor based transformer differential relays.

Protecting transmission lines terminated into transformers

Transmission lines that are terminated into transformers are applications where the line and transformer cannot be separated by a circuit breaker and are therefore in the same zone of protection. These applications may be addressed with either current differential or line distance protection. In either case the implementation of separate transformer differential protection isolating the protection zone is recommended. The line protection application needs to consider the instrument transformer locations and transformer winding connections at the transformer end. This paper will discuss the application considerations for both line differential and distance schemes for lines terminated into transformers and provide application guidelines.

Transformer tank rupture - A protection engineer's challenge

According to a recent draft IEEE PC 57.156 Guide for Tank Rupture Mitigation of Liquid Immersed Power Transformers and Reactors published by the IEEE PES Transformer committee “There are combinations of arc energy, location, and duration which overwhelm the capacity of the standard pressure relief to protect the tank. It has also been demonstrated, theoretically and from field tests, that for low impedance faults with energy levels susceptible to cause the rupture of the tank, the rate of pressure rise is too high and the Pressure Relief Device (PRD) have, at best, little effect in preventing the rupture.” Later in the document, in Annex B, titled “Other Items Relating to Prevention of and Responses to Arcing Rupture”, under the heading Transformer and System Design Considerations it states. “Inclusion of an appropriately rated protective relay may allow for prompt isolation from the energy source of the fault.” The ball seems to be in the protective relay engineers court.

Subsynchronous oscillation detection using microprocessor relays

This paper will review the Subsynchronous oscillation phenomena and the interactions between the electrical power system and the mechanical turbine generator system. Past relays used to detect SSO will be discussed, and a new detection technique will be described which can be easily incorporated into a microprocessor relay. Finally, some results will be given showing the application and field experience in several power systems.

The IEEE PES Power System Relaying Committee what is it? What's happening there? Why is it important to you?

The Power System Relaying Committee (PSRC) is one of the 17 technical committees of the Power and Energy Society (PES) reporting to the PES Technical Council. The PSRC was established over 75 years ago as the repository for the standards and application guides pertaining to protective relays used in our industry. Lots of changes have come to the industry since the PSRCs inception and PSRC is changing to meet those needs. This paper will discuss a brief history of the PSRC, its organization, where it fits into the Standards body, what its doing currently, how its evolving to meet your needs, and you can help.