Stanley E. Zocholl

Mathematical models for current, voltage, and coupling capacitor voltage transformers

This paper reviews a number of mathematical models used to represent the nonlinear behavior of the magnetic core of instrument transformers. Models of instrument transformers using these core representations are presented. The transient response of the instrument transformer is compared to actual test results recorded in the laboratory. The paper provides practical guidelines as to which of the physical elements of instrument transformers are important to model during transient studies and which elements could be ignored without sacrificing the accuracy of the simulation results. The electromagnetic transients program (EMTP) data files used to generate the models are also provided in the appendix to help new EMTP users model instrument transformers for evaluation of high-speed protective relaying systems.

A Current-Based Solution for Transformer Differential Protection Part II: Relay Description and Evaluation

This paper describes a new approach for transformer differential protection that ensures security for extemal faults, inrush, and overexcitation conditions and provides dependability for internal faults. This approach combines harmonic restraint and blocking methods with a wave shape recognition technique. We compare in the paper the behavior of some traditional transformer protection methods to that of the new method for real cases of magnetizing inrush conditions.

On the protection of thermal processes

Standards specify an equation for the time-current characteristic of a thermal relay. When the equation is published without reference to its derivation, it may be mistaken for the characteristic of an inverse-time overcurrent relay which if implemented would provide only marginal protection. This paper explains the ambiguity introduced in the standard and addresses thermal protection from the perspective of a thermal model that provides optimum protection. The paper employs MATLAB simulations to compare the dynamics of the thermal model with that of an overcurrent implementation during cyclic overloads.

Considerations for using high-impedance or low-impedance relays for bus differential protection

Two drastically different types of differential relays, one with a single set of very high-impedance inputs and another with multiple sets of low-impedance inputs, are available for bus differential protection. Protection engineers often question which type to choose for their bus protection applications. This paper discusses for each type of relay the concepts used, the pros and cons, and the importance that current transformer selection plays in applications.

Primary high-current testing of relays with low ratio current transformers

This paper will serve as one of few references describing primary high-current testing of protective relays using low ratio current transformers. The tests show the limitations of Fourier and cosine filters used in microprocessor relays that extract the fundamental phasors and eliminate harmonics. The tests validate the operation of a cosine-peak adaptive filter designed to cope with the highly distorted saturated waveforms produced by the low ratio CTs subjected to high current. The details of relay operation are shown in unfiltered event records of the test cases. A report is given on the results of primary high-current tests of overcurrent, motor, and distance relays using low ratio CTs. The test currents ranging from 6 kA to 50 kA were used with current transformers with ratios of 50:5, 300:5, and 600:5. The paper compares the internal unfiltered event records with MATLAB simulations of the same cases.

Detecting broken rotor bars with zero-setting protection

Broken rotor bars in induction motors can be dependably detected by analyzing the current signatures under sufficient motor load conditions. Detection becomes less dependable under light motor load conditions. There are also cases in which tolerable motor operating conditions generate current signatures similar to those of motors with broken rotor bars. These cases may present security concerns when the detection element is set to trip the motor and to send alarms. In this paper, we aim to achieve the following: show how broken rotor bars cause characteristic current signatures; show how to detect broken rotor bars with a zero-setting protection element, which uses the current signature method; use cases with different motor operating and fault conditions to analyze the performance of the zero-setting broken bar protection element; identify cases when the current signature method is dependable and cases when security is a concern, and present solutions to address security concerns.

Tutorial: From the Steinmetz Model to the Protection of High-Inertia Drives

The thermal limitations of induction motors are specified by thermal limit curves that are plots of the limiting temperature of the rotor and stator in units of I2t. This paper discusses the thermal protection provided by rotor and stator thermal models defined by the thermal limit curves and supporting motor data. The thermal model is the time-discrete form of the differential equation for temperature rise due to current and is derived from fundamental principles as shown in the Annex. In the rotor model, voltage and current are used to derive the slip-dependent I2r watts that permit the safe starting of high-inertia drive motors.

Optimizing Motor Thermal Models

The thermal limitations of induction motors are specified by thermal limit curves that are plots of the limiting temperature of the rotor and stator in units of l2t. This paper discusses the thermal protection provided by rotor and stator thermal models defined by the thermal limit curves and supporting motor data. The thermal model is the time-discrete form of the differential equation for temperature rise due to current and is derived from fundamental principles as shown in the appendix. The rotor model derives the slip-dependent l2r watts using voltage and current that permit the safe starting of high-inertia drive motors. The performance of the models is shown in constant and cyclic load tests

Standard CT accuracy ratings in metal-clad switchgear

This paper shows the consequences of low accuracy ratings and analyzes the relay response for fault currents in the range of 25 kA to 50 kA, using the CT accuracy rating from the relay data acquisition. To ensure proper relaying performance, the user should make a careful analysis of CT performance considering the relaying requirements for the specific short circuit current and the secondary circuit impedances. The careful analysis of CT performance for high magnitude offset fault requires computer simulation of the relay, as well as the CT, and cannot be carried out by manual calculation.

Understanding service factor, thermal models, and overloads

Induction motors require thermal protection to prevent overheating to cyclic as well as steady state overloads. This paper discusses the problem that occurred while a customer was testing the motor relay that trip prematurely when subjected to cyclic loads that do not overheat the motor and the characteristic curves are analyzed.