Basler Electric BE1-11t - Manual (Page 49)

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Overexcitation (24) Protection
The overexcitation (24) element monitors the volts per hertz ratio and protects transformers and
generators from the adverse effects of excessive heating resulting from overexcitation. Overexcitation
exists whenever the per unit volts per hertz exceeds the equipment design limitations.
Element logic connections are made on the BESTlogic
™Plus screen in BESTCOMSPlus® and element
operational settings are configured on the Overexcitation (24) settings screen in BESTCOMSPlus. A
summary of the logic inputs and outputs and operational settings appears at the end of this chapter.
BESTCOMSPlus Navigation Path: Settings Explorer, Protection, Voltage, Overexcitation (24)
HMI Navigation Path: Settings Explorer, Protection, Settings Group x (where x = 0 to 3), Voltage
Protection, Overexcitation 24
Element Operation
Overexcitation occurs when a generator or transformer magnetic core becomes saturated. When this
happens, stray flux is induced in non-laminated components, causing overheating. The BE1-11t detects
overexcitation conditions with a volts per hertz element that consists of one alarm setting, one inverse
time characteristic with selectable exponents (3 sets of time curves), and two definite-time characteristics.
This allows the user to select an individual inverse-time characteristic, a composite characteristic with
inverse time, and one or two definite-time elements, or a dual-level, definite-time element.
The inverse time characteristic closely approximates the heating characteristic of the protected equipment
as overexcitation increases. A linear reset characteristic provides for the decreasing (cooling) condition.
The overexcitation element responds to the magnitude of voltage versus frequency where the measured
voltage is phase to phase and includes the phase with the frequency measurement element.
Sensing Configuration
The pickup settings determine the V/Hz pickup level. The measured V/Hz is always calculated as the
measured voltage divided by the sensed system frequency. The measured phase depends on the
sensing voltage setting. The 24 element monitors VAB for both 3-wire and 4-wire connections. Thus, the
setting is in VPP/Hz for VT connection = 3W, 4W, AB, BC, CA and VPN/Hz for VT connection = AN, BN,
CN. For more information, refer to the
Configuration chapter.
Nominal voltage for the BE1-11t is defined as a phase-to-neutral quantity. Nominal V/Hz depends on the
sensing voltage (VT) connection, nominal voltage, and nominal frequency settings. Nominal V/Hz is
calculated as the nominal voltage divided by nominal frequency. For VT connections equal to 3W, 4W,
AB, BC, CA, the nominal voltage (phase-neutral value) must be converted to a phase-phase value by
multiplying by the square root of 3. No additional conversion is required for VT connections equal to AN,
BN, or CN.
For 3W, 4W, AB, BC, or CA phase to phase sensing connections:
Frequency Nominal
3 V
V/Hz
Frequency Measured
V Measured
V/Hz
Nominal
Nominal
Phase - Phase
Measured
∗
= =
Equation 1. V/Hz Measured (3W, 4W, AB, BC, or CA)
For AN, BN, or CN phase to neutral sensing connections:
Frequency Nominal
V
V/Hz
Frequency Measured
V Measured
V/Hz
Nominal
Nominal
-Neutral Phase
Measured
= =
Equation 2. V/Hz Measured (AN, BN, or CN)
BE1-11t Overexcitation (24) Protection