INTRODUCTION

Members of CIGRE B5.16 Working Group: Zoran Gajic, Andr Quaresma dos Santos, Hans-Werner Funk, Kenneth Opskar, Jose Miguel Yarza Narro, Damien Tholomier, Ljubomir Kojovic, Phil Beaumont, Juergen Westerfeld

The article is organized into three sections.
Section 1 presents general practices for the selection of busbar protection (BBP) designs and describes centralized and de-centralized digital (numerical) busbar protection solutions

Section 2 describes common features of modern busbar protections. Busbar protection schemes utilize sophisticated algorithms to provide reliable performance during substantial CT saturation, resulting in low CT requirements.  A disconnector replica implemented in relay software eliminates the need for switching in CT secondary circuits and trip circuits. Protection functions such as breaker failure protection, end fault protection, and overcurrent feeder protection can be implemented into the BBP. Built-in self-supervision improves BBP reliability. Communication enables remote access to relevant information available within the BBP. Other features presented include disturbance recording (oscillography) and an event list that provides improved evaluation of BBP protection operation

Section 3 addresses advanced features of modern busbar protection designs. Feasibility of using new types of current sensors for current measurements is presented. Also described is integration of other protection functions such as feeder protection within the individual bay units of a de-centralized BBP arrangement. Furthermore, the impact of IEC 61850 on busbar protection design and operation is considered describing the use of GOOSE messages and their influence on scheme design, as well as blocking of auto-reclosing after BBP operation. Finally, issues regarding the integration of BBP schemes into modern substations using an IEC 61850 process bus approach conclude the discussion

1. Factors Influencing BBP Selection and Implementation

Today, electric power companies (utilities) worldwide, driven by deregulation and increased competition, have changed the way they operate. Power plants and lines are becoming loaded up to thermal and stability limits. Existing power plants are expected to operate up to and beyond the end of their original design life. Corrective event-based repair replaces preventive maintenance. Considering these changes, power system protection and control face new technical and economic challenges.
Modern secondary systems play an important role in satisfying the above requirements for lower investment and operational cost without compromising system reliability.

To assure power system integrity during fault conditions, one of the most important requirements is reliable performance of power system busbar relay protection. This requirement is further emphasized by the fact that an incorrect operation of busbar protection will result in loss of all connected lines, power transformers, and generators, which may lead to a power system blackout.
Reliable performance of the busbar protection system must be preserved for both In-Zone and Out-Zone faults. This is a challenging task since high fault currents may exist at the substation, making it difficult, or even impossible to avoid saturation of conventional iron-core CT.

Most busbar protection systems operate on a differential principle by comparing input and output currents. If a CT saturates, then a false differential current will be derived by the relay. Busbar protection schemes implemented in modern digital multifunction relays are designed to tolerate substantial CT saturation, while providing high-speed operation for In-Zone faults (dependability). Relays are designed to reliably operate in the presence of distorted waveforms, or prior to CT saturation (time-to-saturation). High-speed busbar protection operation is required since bus faults may result in large fault currents endangering the entire substation due to the high dynamic forces and thermal stresses experienced.

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For external Out-Zone faults (security), the protection scheme must remain stable for all types of faults for the time needed to clear the fault. Manufacturers use different algorithms to achieve relay stability during CT saturation. While both security and dependability are important requirements for busbar protection, the preference is always given to security.

Four key issues (reliability, operability, maintainability, and cost) need to be addressed when designing a substation and selecting a busbar configuration. At EHV/HV levels, solutions that provide a high degree of reliability may be justified. A modern busbar protection system is able to dynamically update the bus topology and has design flexibility to protect all existing bus arrangements.

In general, the main requirements for busbar protection include:

• Security - probability of an unwanted protection operation for through faults (Out-Zone faults) is low
• Dependability - probability that the protection will not operate for a fault on the bus (In-Zone faults) is low
• Speed - high-speed operation is needed to limit equipment damage, and to preserve system transient stability
• Sensitivity - to detect and clear high resistive faults
• Selectivity - to minimize the power outage and to ensure continued operation of the healthy parts of the power system

All these requirements are interrelated and therefore, it is not possible to satisfy one without affecting the other. The design solution should meet the requirements that correspond to the importance of the substation within the network and the layout of the substation.

Differential Protection Systems

Differential Protection Systems
Two main designs used for high voltage busbar protection are high impedance and low impedance differential protection systems.

• High impedance differential protection systems have been in use for over 50 years. The protection system consists of CTs whose secondary windings are connected in parallel and to one high impedance voltage relay. High impedance protection responds to a voltage across the relay. All CTs must be well-matched, have equal ratios, and have low secondary leakage impedance. The major disadvantage is the need for dedicated CT cores. When used for re configurable buses, the switching of CT secondary currents may affect the performance of the protection and increase the cost. In addition, this solution requires voltage limiting varistors. New microprocessor-based high impedance relays operate on the same principle as traditional designs. However, they also provide functions such as sequence of events, disturbance recording, and communication
• Low impedance differential protection systems employ digital relays. The CT inputs are connected to individual channels. The relay derives differential signals by executing protection algorithms. These solutions allow the use of CTs with different ratios since CT matching is performed inside the relay. The same CT core can be used by different protection relays. In addition to the operating quantity, low impedance differential protection systems derive a stabilizing quantity and apply a percent (biased) characteristic in order to ensure the stability of the scheme

Modern relays, in addition to the percentage characteristic, typically have implemented sophisticated algorithms to cope with severe CT saturation. Some relays are designed to make decisions before the CT saturates. For modern digital busbar protection schemes, a time-to-saturation of 2-3 ms can be sufficient to stabilize the protection in case of external faults, requiring small over-dimensioning factors of the CT. Typical operating times are one cycle or less.
Other advantages include the integrated functions described in Section 2.

Modern Busbar Protection System Designs
Digital low impedance busbar differential protection systems may be designed as centralized or decentralized.
Centralized systems require all signals to be transferred from all bays tothe central unit where a single relay performs the protectionfunction (Figure 8). The amount of cabling is approximately the same as in conventional solutions. A reduction in wiring can only be achieved if additional protection functions are gathered into a busbar protection relay such as circuit breaker failure (CBF)protection. The advantage of digital protection equipment is simplicity in developing protection functions andflexible arrangements of input/output signals. Furthermore, the digital technique allows a fast and easy connection to substation automation systems, providing fast fault analysis and monitoring. Today, centralized busbar protection can be found mainly in substations where old conventional protection was retrofitted and the cables are still in good condition.

Decentralized systems include dedicated bay protection panels located close to the bay protection devices (Figure 9), which only require wiring over short distances, such as to start CBF protection and to block auto-reclosing. Signals between the bay panels and the central unit have been transmitted using fiber-optic cables, resulting in reduced cabling and cost savings.
The bay protection panel may be designed to look the same, which results in a cost saving and a reduction in the time required for testing, commissioning, and maintenance. Further savings can be made by integrating more functions such as feeder (bay) protection functions. Decentralized busbar protection systems are mainly used in new and refurbished substations.

2. Modern Relay Features

This section includes a brief description of some of the main features that characterize modern BBP relays.

• Dynamic Bus Replica. Digital low impedance busbar protection schemes are well-suited for complex busbar arrangements. They provide dynamic bus replica by virtual CT secondary current switching in software, without the need to perform physical current switching. Simultaneously, trip circuits are adjusted accordingly
• CT Requirements. Theoretically, while it is possible to dimension a CT so that it will not saturate under any fault conditions, it would result in an unreasonably large CT. Busbar protection schemes implemented in modern digital multifunction relays can tolerate substantial CT saturation, while providing reliable performance, resulting in low CT requirements. However, it is necessary to determine the CT time-to-saturation to verify that the requirements for proper CT selection are met. CIGRE Report provides guidelines for CT selection for different fault conditions and time-to-saturation requirements
• Protection Function Integration. Modern BBP relays include features such as CBF, end fault protection to cover dead zones in feeder bays and blind spot logic to cover dead zones in bus coupler bays
• Self-Monitoring Functions. These functions include monitoring of the relay hardware and software such as supervision of voltages (supply and internal), relay output circuits, memory modules, and watchdog
• External-Monitoring Functions. These functions include monitoring of the relay environment such as status of associated isolators and circuit breakers, and the CT and trip circuit integrity
• Bay Unit or Communication Failure. With distributed arrangements, serial links between bay units and the central unit are continuously monitored
• Service Values. Modern digital busbar relays have a human-machine interface (HMI) to provide information about the present status of the BBP and substation. A busbar relay may be used as a small monitoring system
• Virtual Testing. For commissioning and factory acceptance test purposes, modern BBP relays can effectively be virtually tested. For instance, the relay can be interfaced to an external PC that runs software that simulates binary input status, which eliminates a need for other external test equipment
• Disturbance Recording and Event List. Provides information from analogue and digital inputs of BBP and all events generated by the BBP itself
• Remote Access. Communication enables remote access to BBP relays. This is convenient for relay setting, substation monitoring, and disturbance and event record collection
• Signaling and Alarming.  BBP relays interfaced to the substation automation system or the remote terminal unit (RTU) can provide detailed information about the event, such as the faulty bus section and phases involved in the fault

Latest Advances in BBP
Electronic Current Transformers

Non-conventional current transformers are part of a new generation of current sensors, defined by Standards and as electronic current transformers (ECT). They have a wide operating range and may have a metering accuracy that allows the use of the same device for both metering and protection. However, ECTs are low power sensors and cannot be directly interconnected with conventional equipment. They need microprocessor-based equipment designed to accept signals from ECTs such as Rogowski coils, low power iron-core current sensors, and optical current sensors.

• Rogowski coil output signal is a scaled time derivative di/dt of the primary current. To use such signals with phasor-based protective relays, signal processing is required to extract the power frequency signal. This may be achieved integrating the Rogowski coil output signal, or using a non-integrated Rogowski coil output signal. The integrated output voltage is proportional to, and in phase with, the measured current. Signal integration may be performed within the relay (by using analogue circuitry or digital signal processing techniques) or immediately at the coil location. When using a non-integrated signal, it needs to be scaled by magnitude and shifted by 900. Connections to relays can be via wires or through fiber-optic cables
• Low power iron-core current sensors have similar designs to conventional CTs, but employ a minimized iron core, resulting in a reduced size and weight. To obtain an output voltage directly proportional to the current, a resistor is connected internally across the output terminals.  Because of the iron core, they can saturate in a similar manner to a conventional CT, which must be considered when selecting these sensors
• Optical current sensors operate on the principle of the Faraday rotation effect using a monochromatic light source. Current flowing in a conductor creates a magnetic field, which rotates the plane of polarization of the light travelling in optical fibers, encircling the conductor proportionally to the current flowing in the enclosed conductor. The interface between the sensor and the electronic module in the control/relay room is over fiber-optic cables
• Applications for Busbar Protection. The ECT output signal is a voltage and requires appropriately designed relays. For differential protection of busbars, ECTs are connected in a voltage-differential circuit. Differential voltage can be obtained by analogue or digital summing of secondary voltages of all ECTs protecting a bus. Designs that use analogue signal-summing need only one relay and one input channel to connect the differential voltage.  Designs that use digital signal-summing inside the relay may use one relay with multiple channels or dedicated relays interfaced by communications
As of now, BBP systems based on ECT are not readily available.
 

Integration of Other Protection Schemes into Modern BBP
In bay units of a modern de-centralized BBP arrangement additional protection/control functions can be integrated and used as main or back-up protection. These functions may include line distance protection, transformer differential protection, CBF protection, auto-reclosing, and synchrocheck. Benefits include significant cost reductions in hardware, as well as time saved during engineering, commissioning, and maintenance.

Impact of IEC 61850 Standard
The IEC 61850 standard consists of ten major sections that standardize communication networks and systems in substations to allow interoperability of devices from different manufacturers. IEDs connected to the substation local area network (LAN) can exchange information with the substation control system or with each other. For BBP interoperability, the IEC 61850 standard specifies only Logical Node Class PDIF, which is common for all types of differential protection. For example, BBP application issues such as splitting and/or merging of Zones are not covered by this standard at this time.  

IEC 61850-8-1 (station bus) standardizes communication services between IEDs and the substation control system that can also be applied to BBP.  Information that can be provided to the supervisory system may include analogue measurement data (bay-wise and zone-wise quantities), alarms, disturbance recording, and event list.

Generic object oriented substation events (GOOSE) messages can be used to exchange information between different IEDs and BBP such as CBF starting, auto-reclose blocking, bay inter-tripping, and primary apparatus status. Another application can be for blocking/releasing signals for simple busbar blocking schemes.

IEC 61850-9-2 (process bus) defines communication between merging units (that interface instrument transformers and process bus) and IEDs. With this approach, IEDs connected to the process bus receive sample analogue values (currents and voltages). However, as of now, there are no readily available BBP relays that can operate based on data available on the process bus. 

Biographies

Zoran Gajic received his MSEE with honors from the University of Belgrade, Serbia and PhD in electrical engineering from Lund University, Sweden. Since 1993 he has held various engineering positions in the area of power system protection and control within the ABB Group of companies. Currently he is Global Product Manager for Transformer & Generator Protection with ABB SA Products in Sweden. Dr. Gajic is a member of Cigré and PES/IEEE. Zoran has published many technical papers and has several patents. His main areas of focus are applications of protection relays, computer applications for protection and control, development of protection algorithms for numerical relays and power system simulations.

Sidebars:
To assure power system integrity during fault conditions, one of the most important requirements is reliable performance of power system busbar relay protection.

Modern relays, in addition to the percentage characteristic, typically have implemented sophisticated algorithms
to cope with severe CT saturation.

Some modern relays are designed to make decisions before the CT saturates.

GOOSE messages can be
used to exchange information between different IEDs and BBP such as auto-reclose blocking, bay inter-tripping, etc.

In bay units of a modern de-centralized BBP arrangement additional protection/control functions can be used as
main or back-up protection.

The IEC 61850 standard consists of ten major sections that standardize communication networks and systems in substations to allow interoperability of devices from different manufacturers.