The function can be enabled and disabled with the Operation setting. The corresponding parameter values are "On" and "Off".
The operation of DPHxPDOC can be described using a module diagram. All the modules in the diagram are explained in the next sections.
Directional calculation
| Polarizing quantity | Description |
|---|---|
| Pos. seq. volt | Positive sequence voltage |
| Neg. seq. volt | Negative sequence voltage |
| Self pol | Self polarization |
| Cross pol | Cross polarization |
The directional operation can be selected with the Directional mode setting. The user can select either "Non-directional", "Forward" or "Reverse" operation. By setting the value of Allow Non Dir to "True", the non-directional operation is allowed when the directional information is invalid.
The Characteristic angle setting is used to turn the directional characteristic. The value of Characteristic angle should be chosen in such a way that all the faults in the operating direction are seen in the operating zone and all the faults in the opposite direction are seen in the non-operating zone. The value of Characteristic angle depends on the network configuration.
Reliable operation requires both the operating and polarizing quantities to exceed certain minimum amplitude levels. The minimum amplitude level for the operating quantity (current) is set with the Min operate current setting. The minimum amplitude level for the polarizing quantity (voltage) is set with the Min operate voltage setting. If the amplitude level of the operating quantity or polarizing quantity is below the set level, the direction information of the corresponding phase is set to "Unknown".
The polarizing quantity validity can remain valid even if the amplitude of the polarizing quantity falls below the value of the Min operate voltage setting. In this case, the directional information is provided by a special memory function for a time defined with the Voltage Mem time setting.
DPHxPDOC is provided with a memory function to secure a reliable and correct directional protection relay operation in case of a close short circuit or an earth fault characterized by an extremely low voltage. At sudden loss of the polarization quantity, the angle difference is calculated on the basis of a fictive voltage. The fictive voltage is calculated using the positive phase sequence voltage measured before the fault occurred, assuming that the voltage is not affected by the fault. The memory function enables the function to operate up to a maximum of three seconds after a total loss of voltage. This time can be set with the Voltage Mem time setting. The voltage memory cannot be used for the "Negative sequence voltage" polarization because it is not possible to substitute the positive sequence voltage for negative sequence voltage without knowing the network unsymmetry level. This is the reason why the fictive voltage angle and corresponding direction information are frozen immediately for this polarization mode when the need for a voltage memory arises and these are kept frozen until the time set with Voltage Mem time elapses.
- The fictive voltage is discarded after Voltage Mem time
- The phase angle cannot be reliably measured before the fault situation.
DPHxPDOC can be forced to the non-directional operation with the NON_DIR input. When the NON_DIR input is active, DPHxPDOC operates as a non-directional overcurrent protection, regardless of the Directional mode setting.
Level detector
The measured phase currents are compared phasewise to the set Start value. If the measured value exceeds the set Start value, the level detector reports the exceeding of the value to the phase selection logic. If the ENA_MULT input is active, the Start value setting is multiplied by the Start value Mult setting.
The start value multiplication is normally done when the inrush detection function (INRPHAR) is connected to the ENA_MULT input.
Phase selection logic
If the fault criteria are fulfilled in the level detector and the directional calculation, the phase selection logic detects the phase or phases in which the measured current exceeds the setting. If the phase information matches the Num of start phases setting, the phase selection logic activates the timer module.
Timer
Once activated, the timer activates the START output. Depending on the value of the Operating curve type setting, the time characteristics are according to DT or IDMT. When the operation timer has reached the value of Operate delay time in the DT mode or the maximum value defined by the inverse time curve, the OPERATE output is activated.
When the user-programmable IDMT curve is selected, the operation time characteristics are defined by the parameters Curve parameter A, Curve parameter B, Curve parameter C, Curve parameter D and Curve parameter E.
If a drop-off situation happens, that is, a fault suddenly disappears before the operate delay is exceeded, the timer reset state is activated. The functionality of the timer in the reset state depends on the combination of the Operating curve type, Type of reset curve and Reset delay time settings. When the DT characteristic is selected, the reset timer runs until the set Reset delay time value is exceeded. When the IDMT curves are selected, the Type of reset curve setting can be set to "Immediate", "Def time reset" or "Inverse reset". The reset curve type "Immediate" causes an immediate reset. With the reset curve type "Def time reset", the reset time depends on the Reset delay time setting. With the reset curve type "Inverse reset", the reset time depends on the current during the drop-off situation and the value of START_DUR. The START output is deactivated when the reset timer has elapsed.
The setting Time multiplier is used for scaling the IDMT operate and reset times.
The setting parameter Minimum operate time defines the minimum desired operate time for IDMT. The setting is applicable only when the IDMT curves are used.
The timer calculates the start duration value START_DUR, which indicates the percentage ratio of the start situation and the set operating time. The value is available in the monitored data view.
Blocking logic
There are three operation modes in the blocking function. The operation modes are controlled by the BLOCK input and the global setting in which selects the blocking mode. The BLOCK input can be controlled by a binary input, a horizontal communication input or an internal signal of the protection relay's program. The influence of the BLOCK signal activation is preselected with the global setting Blocking mode.
The Blocking mode setting has three blocking methods. In the "Freeze timer" mode, the operation timer is frozen to the prevailing value, but the OPERATE output is not deactivated when blocking is activated. In the "Block all" mode, the whole function is blocked and the timers are reset. In the "Block OPERATE output" mode, the function operates normally but the OPERATE output is not activated.