SEL-751 Relay Logic

The SEL-751 Relay Logic component, shown in Table 1, is a Schematic Editor library block from the Protection part, of the Microgrid library. It implements the following protection elements: Overcurrent, Time-Overcurrent, Overvoltage, and Undervoltage with the Trip/Close and Reclose Supervision Logic. The model is parametrized using two SEL-751 configuration files (Group settings and Logic settings). Initially, if the files are not loaded, the SEL-751 Relay Logic component is parametrized with the SEL-751 Factory Default values. Please refer to the SEL 751's instruction manual for the Factory Default values.

Table 1. SEL-751 Relay Logic component in the Schematic Editor core library

component	component dialog window	component parameters
SEL-751 Relay Logic		Property tabs: 1 - General 2 - Configuration 3 - Extras

Protection functions

The SEL-751 model includes the following protection elements:

• Instantaneous/Definite-Time Overcurrent Elements
• Time-Overcurrent Elements
• Voltage Elements
• Frequency protection
• Rate of change of frequency protection

Phase overcurrent protection operates on the maximum phase current, neutral overcurrent protection operates on the neutral current, ground overcurrent protection operates on the sum of the three phase currents, and negative-sequence overcurrent protection operates on three times the negative-sequence current.

One level of inverse Time-Overcurrent Elements (TOC) is available for A-, B-, C-phases and negative-sequence (Q) overcurrent. Two levels of inverse time-overcurrent elements are available for maximum phase (P), neutral (N), and residual (R) overcurrent. Five U.S. and five IEC inverse characteristics are supported. Each element can be torque controlled (enabled/disabled) through the use of appropriate SELogic control equations (when 51P1TC := IN301 the 51P1 element is operational only if IN401 is asserted).

The phase elements operate on phase A current, phase B current, and phase C current, respectively. The maximum phase inverse time-overcurrent protection operates on the maximum of the three phase currents, neutral inverse time-overcurrent protection operates on neutral current and ground inverse time-overcurrent protection operates on the sum of the three phase currents and negative-sequence overcurrent protection operates on three times the negative-sequence current.

The settings necessary to set the inverse time overcurrent elements are listed in Table 3. The exact Setting Names, according to the element and level of interest, could be derived by replacing x with “A”, “B”, “C”, “P1”, “P2”, “Q”, “N1”, “N2”, “G1”, or “G2”.

The current transformer (CT) ratio settings configure the relay to accurately scale measured current values. The current transformer ratio settings are listed in Table 4.

When the SEL-751 model is configured to use phase-to-phase connected voltage transformers (VT), setting DELTA_Y = DELTA, the relay provides two levels of phase-to-phase overvoltage and undervoltage elements. When the SEL-751 model is configured to use phase-to-neutral connected VTs, setting DELTA_Y = WYE, the relay provides two levels of phase-to-neutral, phase-to-phase overvoltage, and undervoltage elements. Two levels of negative-sequence overvoltage elements are available regardless of the voltage transformers connections. Two levels of zero-sequence overvoltage elements are available when the voltage inputs are connected in wye configuration (DELTA_Y := WYE).

When the setting “DELTA_Y” is set as “DELTA”, then phase to phase voltages are used as operating voltages. When this tap is set as “WYE”, then the phase to ground voltages are used as operating voltages. The phase undervoltage protections operate on the minimum phase operating voltage, the phase overvoltage protections operate on the maximum phase operating voltage. The zero-sequence voltage elements operate on zero-sequence voltage, while the negative-sequence voltage protection operates on three times the negative-sequence voltage.

The necessary settings for the voltage elements are listed in Table 5. The exact Setting Names, according to the element and level of interest, could be derived by replacing x with “P”, “PP”, “G”, or “Q”, and replacing n with “1” or “2”.

The voltage configuration settings configure the relay voltage inputs to correctly measure and scale the voltage signals. The supported voltage settings are listed in Table 6. For additional information and examples, refer to the SEL-751 instruction manual.

The relay provides six levels of over- and underfrequency elements. Each element has independent trip level and time-delay settings, and can be torque controlled using appropriate SELogic control equations. Individual elements operate as an overfrequency protection when the trip level setting is greater than the nominal frequency setting. When the trip level setting for an element is less than the nominal frequency setting, it operates as an underfrequency protection.

The settings necessary to set over- and underfrequency elements are listed in Table 7. The exact Setting Names, according to the level of interest, could be derived by replacing n with a number between "1" and "6".

Four levels of rate of change of frequency elements (81R) are available. There are three settings that are common for all four levels of protection: E81R, 81RVSUP, and 81RISUP. E81R is used to enable the number of elements you want to use. Voltage and current supervision are specified with the 81RVSUP and 81RISUP settings, respectively. If these settings are specified, a minimum positive-sequence voltage and/or current are necessary for operation of this protection. If these settings are set to OFF, this protection can operate without current and/or voltage supervision. The element is also supervised by the Relay Word FREQTRK, which ensures that the relay is tracking and measuring the system frequency.

Other settings necessary to set 81R elements are listed in Table 8. The exact Setting Names, according to the level of interest, could be derived by replacing n with "1", "2", "3", or "4".

SEL-751 Relay Logic inputs and outputs

The SEL-751 Relay Logic component has 3 inputs - Relay input from the User Interface (ui_out), Relay analog input (A_in), Relay digital input (D_in) - and 2 outputs Relay digital output (D_out) and Relay output to the User Interface (ui_in).

ui_out is the vectorized input which consists of the 4 elements which are described in Table 9 and where signals orders are given. These signals are used to send basic manual commands to the relay. These are digital signals implemented in a positive logic, asserted when high and deasserted when low.

A_in is the vectorized input which consists of the 20 elements which are described in Table 10 and where signal orders are given. These are analog signals which are equal to the measured voltages or currents. All these inputs can be easily obtained from the Three-Phase Meter component that is also part of the Microgrid library. Please refer to the SEL-751 Example model.

D_in is the vectorized input which consists of the 20 elements. Supported digital inputs are given in Table 11 along with the signal order. These are digital signals which can be used for digital feedback signals (from contactor for example) or used as external logic that can influence the internal protection functions. This can be defined from the imported group settings file using the SELogic equations definition in the desired protection functions.

D_out is the vectorized output which consists of the 20 elements. Supported digital outputs are given in Table 12 along with the signal order. Relay digital outputs are programmable and their functionality is defined from the relay logic settings file using SELogic equations for the desired digital output.

ui_in is the vectorized output which consists of the 227 elements which are separated in three parts: digital outputs that control user Target LEDs, analog outputs with measurements and Relay Word Bits (RWB) digital outputs.

The first 20 elements are digital outputs that control the Target LEDs. All Target LEDs are fixed pre-programmed with a SEL-751 Default settings. Supported LED digital outputs are given in Table 13 with the corresponding signals' position in the ui_in vectorized output.

The next 40 elements are reserved for analog signals of measured voltages and currents. Supported analog measurements are given in Table 14 with the corresponding signals' position in the ui_in vectorized output.

The final 167 elements of the ui_in output are Relay Word Bits (RWB) digital outputs. These RWBs are calculated from the implemented protection functions and also can be used for defining the SELogic equations in the input configuration files. If some RWB is used but is not supported in the model, it will be automatically replaced with 0 and reported in the message log of Schematic Editor. Supported RWBs are given in Table 15 with their position in the ui_in vectorized output.

In the Example, the SEL-751 Relay Logic component is surrounded by helping subsystems where inputs and outputs are also defined. This is shown in Figure 1. The UI subsystem contains just a small and representative part of the available SEL-751 Relay Logic signals. The selection is done using Bus Selector components and numbers from the upper tables. These subsystems can be freely changed according to the specific application or reused in the other models. This whole structure is located in the wrapping SEL-751 Subsystem in order to keep everything in one place. For the more information, please refer to the provided SEL-751 Example model.

Component dialogue box and parameters

The SEL-751 Relay Logic component dialogue box consists of four tabs for specifying parameters of the component.

Tab: "1 - General"

In this component tab, general parameters of the SEL-751 Relay Logic component can be specified.

The secondary voltage input configuration is the parameter that defines what type of voltage inputs are used for the relay. When Standard is selected, 300 V rated inputs are presented on the relay; if LEA is selected, 8 V inputs are presented instead. This setting influences how the measured voltages are scaled internally. The PTR parameter is valid for the standard input. For LEA inputs, the LEA_R parameter is used instead of the PTR (see Table 6). Please refer to the SEL-751 instruction manual for additional information.

Tab: "2 - Configuration"

In this component tab, parameters that are related to the Configuration of the SEL-751 can be specified.

Tab: "3 - Extras"

In this component tab, execution rate parameters can be specified.

The faster execution rate is the rate at which RMS values are calculated using a one-cycle cosine filter. By default this value is set to 625 us, which implies 32 samples per power system cycle (50 Hz) and it matches the real SEL-751 Relay. In this model, the Faster execution rate value can be changed in order to adapt the component to the other execution rates in the model if necessary. The cosine filter will work with a different number of samples depending on the execution rate. The minimum Faster execution rate is 500 us. The Slower execution rate is the rate at which protection functions are executed, which is set to four times faster than the power system cycle by default. The Slower execution rate can also be changed and adapted to other execution rates in the model. Changing the execution rates of the SEL-751 Relay Logic can produce small differences in dynamic responses relative to the default values.

Example

Overall behavior and control methodologies can be better understood with the use of the given example:

Model name: sel 751.tse

SCADA interface: sel 751.cus

Path: /examples/models/microgrid/SEL-751