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| Illustration 1 | g03886291 |
Load add and load shed are complementary features that work together to add and shed loads while the EMCP 4.4 SCP is running. For further information refer to Application and Installation Guide, LEBE0040, "EMCP 4.4 Supervisory Control". The following sections outline these two features.
The load add feature on the EMCP 4.4 SCP will automatically add loads in a priority-based manner. This feature occurs when the generating capacity of the system is sufficient to power all connected loads.
The load add feature consists of an initial conditional load add period followed by a load sensitive load add period. The load add feature enters the load sensitive load add period when the conditional load add timer expires.
The supervisory control panel can be configured to add or remove all loads when the EMCP 4.4 SCP does not have a run signal.
The conditional load add timer starts when a unit detects bus voltage. The conditional load add timer is still used even if the load sense load demand (LSLD) is disabled on the generator system sets based on the last setting of the timer.
The EMCP 4.4 SCP does not need to wait for all available generator sets to have synchronized onto the bus before the unit starts to add loads.
Load steps are added based on priority. The highest priority will have the lowest number. Priorities are assigned using "Load Add Priority for Load Step N (setpoint)".
If multiple load add steps are programmed to the same priority, the lower number load step has priority over the higher number load step. Although not required, the recommended action is to program each step for the matching priority.
A load step is added if the following are true:
- Capacity headroom is greater than the load step rating of the load step (both kW and kVAR)
- The "Load Add Minimum Generator Load Step Time Delay (setpoint)" (triggered for the previous load step added) has expired.
- The "Load Shed Latch Enable for Load Step N (setpoint)" is not enabled for that load step
- The "Load Add Priority for Load Step N (setpoint)" number is less than or equal to the "Load Add Generator Supplies Maximum Load Step Priority" number
The "Load Add Minimum Generator Load Step Time Delay" starts to time when any load step is added. Minimum delay is still enforced between successive non-zero load add steps when the capacity is such that two or more load steps may be applied. If the load step is defined as rated 0 kW and 0 kVAR, then this time is not applied and the load step may be added immediately after the previous.
Note: A "Load Add Minimum Utility Load Step Time Delay" (setpoint) should be used for load steps above the "Load Add Generator Supplied Maximum Load Step Priority" (setpoint) number.
Once the "Load Add Minimum Generator Load Step Time Delay" starts to time, a timer starts against the "Load Add Duration for Load Step N" (setpoint) time. This timer tracks the setpoint time expiration when conditional load add is ended and transitions to load sensitive load add.
Note: The "Load Add Duration for Load Step N" time is not used to space out loads during the conditional load add.
Note: Anytime the load shed timers are timing, all load add timers are paused. Anytime a load step is shed, all load add timers are reset.
Load sensitive load add is active when the conditional load add timer expires and when a "Load Add Duration for Load Step N" time (that started while the conditions load add period was still active) expires.
The load shed feature on the EMCP 4.4 SCP will automatically take loads offline. Loads are taken offline based on priority if the available generator sets capacity is not sufficient to power all connected loads. This feature works with the load add feature.
When the kW or percent kVAR load of the generator set system exceeds the appropriate load shed threshold setpoint, the associated load shed timer will begin to all time. If the percentage of the generator set system drops below the threshold, the timer is not immediately reset. The timer will count backwards towards zero. If the load shed timer expires, the last load step that was added will be shed.
Manual load control is done via the display only. The display has a screen that allows users to select load steps to be added manually. The display also has a screen that allows users to select load steps to be shed manually.
A user can select to add load steps beyond the current available system capacity.
Note: The EMCP 4.4 SCP will shed the load step when the load step is added and the load shed threshold conditions are satisfied.
Load steps will be latched or unlatched based on the setpoint of each step. Any steps that are latched stay inactive unless unlatched by one of the methods specified below.
The methods for unlatching or resetting of a latched load step are the following.
- The latched load step is manually reset via the soft key on the manual control display screen. A single key press of the reset latch soft key will only reset the lowest latched priority number.
- The latched load step is manually added with the soft key on the manual control display screen. A single key press of the add load step soft key will only add the next priority number.
- Activation of a digital input programmed for "Load Shed Reset Command"
- Removal of RUN signal
- User configures the load step latch enable for load step "N" to disable then back to enable.
- Power cycle the EMCP 4.4 SCP
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| Illustration 2 | g03891785 |
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Load bus overview screen | |
The "Load Bus Overview" screen displays the status of each unit.
Selecting "Page Right" using the F4 (2) key shows the next set of eight units. "Page Left" will be shown for the F3 (3) key once the "Page Right" selection has been used to navigate to the additional set of units.
Use the left arrow (10) or the right arrow (7) to highlight the desired unit. The information for the unit will be displayed below on the same screen.
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| Illustration 3 | g03891788 |
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Load bus overview help screen | |
Select "HELP" using the F1 (5) key. Illustration 3 is then displayed.
The "Load Bus Overview Help" screen defines the status symbols seen on the "Load Bus Overview" display.
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| Illustration 4 | g03891790 |
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Load priority detail | |
Once the desired unit has been highlighted, select using the "OK" (8) key to display the "Load Priority" detail screen seen in illustration 4.
To return to the "Load Bus Overview" screen, press the F1 (5) key.
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| Illustration 5 | g03891805 |
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(11) Load bus control screen with step add status information
(12) Load bus control screen with step shed status indication (13) Load bus control screen with step add completion status | |
From the "Load Bus Overview" display, select the F2 (4) key to access the "Load Bus Control" display.
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| Illustration 6 | g03891806 |
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Manual load control screen | |
The "Manual Load Control" screen may be accessed through the "Load Bus Control" menu by selecting the F2 (4) key.
From this screen, the user can add a load step by selecting the F1 (5) key.
The user can also shed a load step by selecting the F2 (4) key.
A reset latched load step will be shown as an inactive load step. A single button press of the F3 (3) key will reset the current lowest priority numbered latched step. Holding the F3 (3) key will not reset more than a single latched load step.
Note: The user can manually shed a load step currently being added. The user can also manually add a load step currently being shed
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| Illustration 7 | g03891815 |
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Custom text example | |
Custom text associated with each load step will provide a short one line description of each load step on the display. This text is helpful both in seeing which load step is about to come on and when assigning priority to each load step.
If programmed, the custom text string will appear on the load priority detail screen as shown in illustration 1. If the user has not provided a custom text string, the text that appears on the load priority detail screen says "Load Step #xx". The "xx" represents the load step number. Refer to Systems Operation, Troubleshooting, Testing and Adjusting, UENR1210, "EMCP 4.3/4.4", Custom Text String Programming for more information