Siemens MJ-4A Installation And Operation Manual page 53

Reverse power flow is indicated when the real power is
negative and the real component of current is greater than
1% of the full scale current. To avoid excessive switching
between Forward and Reverse Operation (defined below),
the microprocessor delays five seconds after the occur-
rence of a current flow reversal before switching from for-
ward to reverse, and vice-versa.
As an example, assume current is flowing in the forward
direction. When load current magnitude drops through
zero and increases to a magnitude that exceeds 1% rated
current in the reverse direction, the microprocessor delays
five seconds and then considers reverse power flow to be
in effect. Reverse power flow remains in effect until cur-
rent direction becomes forward once again and its magni-
tude exceeds 1% of rated current in the positive direction
for five seconds. At that time, the microprocessor consid-
ers forward power flow conditions to have resumed.
Power flow direction is indicated on many of the metering
screens. "F" indicates forward power flow, "R" indicates
reverse power flow. "F" is shown when: kW > 0 and
|Ireal| > 1% (of full scale) for 5 seconds minimum. "R" is
shown when: kW < 0 and | Ireal | > 1% (of full scale) for 5
seconds minimum. If the real current magnitude is below
the 1% threshold, the last known power flow direction
remains in effect. If the real current magnitude is below
the 1% threshold after power up or unit reset, the MJ-4
defaults to forward power flow operation.
It is possible for Reverse Power Flow conditions to occur
in regulators with no source-side voltage input (e.g., no
"Source-Side PT" for Siemens Inverted Regulators). In
this case, the MJ-4 uses the calculated source-side volt-
age for performing reverse power flow regulation. See
sections 5.1 and 5.2 for details on source-side voltage cal-
culation.
When determining power flow direction, the system takes
into account the current-to-voltage phase relationship
associated with the system as specified in the <CONFIG-
URE> Menu (System, Utility Polarity, and I [current] Shift
parameters).
6.5.2 Forward/Reverse Operation
The MJ-4 Control Panel accommodates power flow rever-
sals by alternating between "Forward" and "Reverse"
modes. Because the microprocessor monitors (or derives)
the voltage on both sides of the regulator, it can reverse
modes by simply reversing its definition of "source" and
"load. "
• In Forward Operation mode, the microprocessor
senses the "L " Bushing (or derives the "L " Bushing
voltage from the "S" Bushing voltage and the knowl-
edge of the tap position) to determine load voltage
and controls the tap changer on the basis of the For-
ward setpoints (Voltage, Bandwidth, Time Delay, Line
Drop Compensation). During Forward Operations,
"forward" Meter data is displayed.
• In Reverse Operation mode, the microprocessor
senses the "S" Bushing (or derives the "S" Bushing
Siemens Energy, Inc.
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MJ-4A & MJ-4B Control Panel Automatic Mode
voltage from the "L " Bushing voltage and the knowl-
edge of the tap position) to determine load voltage
and controls the tap changer on the basis of the
Reverse setpoints (Voltage, Bandwidth, Time Delay,
Line Drop Compensation).
During Reverse Operations, "reverse" Meter data is dis-
played. The following parameters determine whether the
tap change motor is energized to raise the tap position or
to lower it:
1. Power flow direction,
2. Forward/reverse operation,
3. Out-of-band status,
4. Power flow mode (see below for details).
Tables 6.3 and 6.4 on the next page summarize the tap
change direction for each Power Flow mode
6.5.3 Current Bias and Bias Percent
This option is only in effect when the power flow mode is
Bi-Directional. For this special operation, the Bias percent
must be greater than the I Threshold%. Typically I Thre-
hold % should be set to 0%.
The operation of this feature is described below, catego-
rized into direction and magnitude of current flow.
Current is greater than the Bias percent (no change in prior
functionality):
• Forward Power Flow: tap changes will occur based on
the Load bushing voltage.
• Reverse Power Flow: tap changes will occur based on
the Source bushing voltage.
Current is less than the Bias Percent (new functionality):
• None: no tap changes will occur.
• Fwd: tap changes will occur based on Load bushing
voltage.
• Rev: tap changes will occur based on Source bushing
voltage.
• NeutT: tap changer will run to Neutral Tap.
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