Generator Stator Ground Fault Protection

Steve Turner, Arizona Public Service Company Columns, Relay Column, Spring 2022 Columns

Generator faults are usually serious and can lead to significant repair costs and long outage times. PART I: ANALYSIS Part 1 demonstrates how to analyze a stator ground fault for a large combustion turbine generator. The faulted phase and fault location are determined using fault voltage recorded by the numerical generator protection relay installed for this generator. This information is …

Analysis of Generator Protection Operations

Steve Turner, Arizona Public Service Company Columns, Relay Column, Winter 2021 Columns

This article analyzes several protection operations for large combustion turbine generators. Generator protection trips tend to be rare, so any major event is always of interest. EVENT 1: CLOSE-IN THREE-PHASE FAULT A large gas combustion turbine generator experienced a close-in three-phase fault during a rainstorm. The generator terminal voltage is rated 13.8 kV line-to-line, and the machine is high-impedance grounded. The …

Programming Numerical Relays to Alarm

Steve Turner, Arizona Public Service Company Columns, Fall 2021 Columns, Relay Column

Numerical protection relays offer the ability to alarm for many abnormal operating conditions, including the health of the relay. Below is a list of typical conditions to alarm for due to an abnormal condition: Overexcitation Phase undervoltage Loss of potential or blown fuse(s) Relay firmware or hardware failure Relay access Over or under station battery DC voltage Trip coil monitoring …

Best Practices for Setting Transformer Differential Protection CT Compensation

Steve Turner, Arizona Public Service Company Columns, Relay Column, Summer 2021 Columns

Historically, CT connections compensated for the phase shift across a transformer when applying differential protection prior to the advent of numerical transformer protection relays. For example, a delta-wye transformer would have wye connected CTs on the delta side and delta connected CTs on the wye side. The delta connected CTs have the same connection as the transformer delta connected winding …

Measuring Field Winding Insulation Resistance

Steve Turner, Arizona Public Service Company Columns, Relay Column, Spring 2021 Columns

Multi-function numerical generator protection relays can calculate the insulation resistance of a synchronous generator field winding to ground. The calculation can be performed on an energized or de-energized generator. The particular relay discussed in this article uses a separate module located close to the rotor slip rings and brushes to monitor the field. The close proximity of the module to …

Combustion Turbine Generator Trip Analysis

Steve Turner, Arizona Public Service Company Columns, Relay Column, Winter 2020 Columns

Relay personnel responsible for testing protective relays are often called upon to troubleshoot and gather system data such as settings and event recordings when a major event occurs (for this example, a large generator trip). If only electromechanical relays are installed and no digital fault recorder is available, it can be difficult to determine the root cause of the trip …

Effect of CT Saturation on Transformer Differential Protection

Steve Turner, Arizona Public Service Company Columns, Fall 2020 Columns, Relay Column

This article demonstrates how CT saturation can adversely affect transformer differential protection and how to easily mitigate its effects via inbuilt relay functions. The solution is to provide calculations and simulation for a three-phase fault on the low side of the transformer just external to the CTs. Application The application is for a three-phase delta-wye connected transformer serving station load …

Archiving Protective Relay Settings

Steve Turner, Arizona Public Service Company Columns, Relay Column, Summer 2020 Columns

Electric utilities have vast numbers of protective relays in their fleets, including generation, transmission, and distribution systems. It is very important to maintain an accurate database that stores the actual in-service settings for each and every relay. Off-the-shelf software already exists for storing and keeping track of all relay settings (Figure 1). It is vital to download the relay settings …

Aligning CT Polarity for Multi-Current Input Differential Relays

Steve Turner, Arizona Public Service Company Columns, Relay Column, Spring 2020 Columns

Differential relays operate on three or more sets of three-phase current inputs. These relays can be older electromechanical relays with current transformers (CTs) connected in parallel to the three-phase current inputs or newer numerical relays that have more than two sets of three-phase current inputs. Note that it is poor practice to connect CTs in parallel to a single three-phase …

Functional Testing for a Sudden Pressure Relay

Steve Turner, Arizona Public Service Company Columns, Relay Column, Winter 2019 Columns

This article presents a functional test procedure for a sudden pressure relay. The sudden pressure relay trips when it detects a sudden increase in gas pressure due to a fault internal to the transformer tank. Recommendations for mounting the relay and a procedure for post-analysis following field trips are also provided. The sudden pressure relay (Figure 1) offers three main …