4-20
Protection and Control
BE1-951
67 Directional Overcurrent Element
The 67 element provides directional supervision for control of the overcurrent tripping elements. The BE1-
951 uses four polarization methods to provide directional control for the overcurrent tripping elements. The
polarization methods are Internal to the 951 and each method has designated bits: One for forward and one
for reverse direction. Combined, these eight bits are referred to as the directional status byte and are used
to control the various overcurrent elements. The polarization methods are as follows.
•
Positive Sequence Polarization – Compares Pos Seq Volts (V1) to Pos Seq Current (I1)
•
Negative Sequence Polarization – Compares Neg Seq Volts (V2) to Neg Seq Current (I2)
•
Zero Sequence Voltage Polarization – Compares Zero Seq Volts (V0) to Zero Seq Current (I0)
•
Zero Sequence Current Polarization – Compares optional ground CT input (IG) to 3I0 (IN)
Relay connections that define forward and reverse are provided in Section 12,
Installation.
Positive sequence polarization: Is used to determine direction for three-phase faults. Under these
conditions, very little negative or zero sequence quantities are present, making the other polarization
methods unreliable for this fault condition. For close-in faults the relay will also need to depend on memory
voltage to determine direction (see below). Positive sequence bits are used to supervise only the phase
over current elements.
To provide memory, the positive sequence voltage is stored continuously until a fault occurs. Memory
voltage is used when the positive sequence voltage falls below the minimum acceptable level of 12 volts.
Due to minute errors in the sample rate and variations in the power system, the memory voltage becomes
less accurate over time. Conservatively, the BE1-951 can maintain memory voltage accuracy to less than
5° error for approximately one second. This should be adequate, as close in faults are expected to trip in
very short time intervals.
Negative sequence polarization: Is used to test directionally for all fault types, except three phase faults.
Negative sequence bits are used to supervise phase, neutral, and negative sequence over current elements.
With load flow and low fault currents it is possible for the positive sequence bits to be set at the same time
negative sequence bits are true. Under these conditions, the negative sequence bits have priority and the
positive sequence bits are cleared.
Zero Sequence Voltage Polarization: Is used to test directionally for ground faults and is used to supervise
only the neutral overcurrent elements.
Zero Sequence Current Polarization: Is also used to test directionally for ground faults and is used to
supervise the neutral overcurrent elements.
Polarization Summary for tripping elements is as follows.
•
Phase Element:
Positive Sequence; Negative Sequence.
•
Negative Sequence Element:
Negative Sequence
•
Neutral Element:
Negative Sequence; Zero Sequence Volt; Zero Sequence
Currrent.
The neutral overcurrent elements can be supervised by various polarization methods. This is necessary
depending on the application and fault conditions applied to relay. In many instances the zero sequence
voltage may be too low to measure during a fault, making the zero sequence polarization unreliable. A
similar condition can occur with the negative sequence voltage, although it is less likely. Under these
conditions a user may need to use current polarization or dual polarization to provide reliable directional
tripping.
The S#-67N command allows the user to select the polarization method and quantity for the neutral
elements on a per settings group basis. The command accepts Q, V, and I parameters to determine the
polarization method, and VO, VX, IN, IG reference and operate quantities for zero sequence voltage
polarizing. All of the following are valid parameters for the S#-67N command, or through HMI screen
5.1.10.2.
Содержание BE1-951
Страница 157: ...BESTlogic Programmable Logic BE1 951 7 2 Figure 7 1 BESTlogic Function Blocks Courtesy of NationalSwitchgear com...
Страница 238: ...BE1 951 Human Machine Interface 10 5 Figure 10 5 Metering Menu Branch Structure Courtesy of NationalSwitchgear com...
Страница 239: ...10 6 Human Machine Interface BE1 951 Figure 10 6 Reports Menu Branch Structure Courtesy of NationalSwitchgear com...
Страница 270: ...12 2 Installation BE1 951 Figure 12 1 BE1 951 S1 Case Dimensions Courtesy of NationalSwitchgear com...
Страница 274: ...12 6 Installation BE1 951 Figure 12 5 BE1 951 H1 Case Dimensions Courtesy of NationalSwitchgear com...
Страница 283: ...BE1 951 Installation 12 15 Figure 12 16 Typical AC Connections Courtesy of NationalSwitchgear com...
Страница 394: ...A 18 Appendix BE1 951 Figure A 14 Time Characteristic Curve B Very Inverse 99 1376 Courtesy of NationalSwitchgear com...
Страница 396: ...A 20 Appendix BE1 951 Figure A 16 Time Characteristic Curve G Long Inverse 99 1622 Courtesy of NationalSwitchgear com...
Страница 397: ...BE1 951 Appendix A 21 Figure A 17 46 Time Characteristic Curve Courtesy of NationalSwitchgear com...