GE Industrial Solutions POWER LEADER PMCS Network Architecture Guide - Manual (Page 31)

Power Management Control System
Chapter 2 – Network Design
21
2–7 Performance Recommendations
Although a PMCS network will function as long as all the
rules in the previous section are followed, you can enhance
performance by considering the following
recommendations for Modbus, commnet.
The Ideal Network
Theoretically, a single Modbus IED or 40 commnet IEDs
distributed across five Modbus Concentrators (one IED
per commnet segment) yields maximum performance.
Naturally, in the real world few networks will fall into this
precise configuration. To extract maximum performance
from the PMCS, follow the performance recommendations
below.
Modbus performance recommendations
1. Use multiple RS-485 networks if possible, depending
on the RS-485 connection at the host. If using an
eight-port RS-485 card or an Ethernet Gateway (four
ports) for connection to the network, you can improve
performance by using the full number of ports
available, rather than burdening a single RS-485 port.
Distributing the IEDs across all available RS-485 ports
permits the communications load to be distributed
rather than asking a single network to carry the full
load.
2. Divide the IED loads evenly when distributing IEDs
across multiple RS-485 networks.
3. Pay careful attention to Modbus Rule 2, regarding the
communication speeds of IEDs on a given network.
Although a network will function with mixed IEDs, its
communication speed will be dragged down to the
lowest common denominator. Thus, a single 2400-
baud IED forces the entire network to communicate at
2400 baud, regardless of the presence of any 19.2-
kbaud IEDs.
Commnet performance recommendations
1. Modbus is preferred over commnet where either
protocol is available. This is the case for the POWER
LEADER Electronic Power Meter and the POWER
LEADER MDP Overcurrent Relay. Each of these IEDs
offers a Modbus communications option.
2. Minimize the number of commnet IEDs per segment.
The Modbus Concentrator is a polling IED, meaning
that it queries each commnet segment continuously
and in order asking for information. It then stores the
information until it is asked by the PMCS to transmit
its data to the host. If the number of commnet IEDs is
unevenly distributed, the Concentrator takes longer
than necessary to poll each segment.
3. Keep data-intensity in mind when connecting more
than eight commnet IEDs to a single Modbus
Concentrator. If it is necessary to connect more than
eight commnet IEDs to a single Modbus Concentrator,
one or more segments will be supporting multiple
IEDs. Keep the following list in mind when selecting
which IEDs to double up on the same commnet
segment. The best choices for doubling up are listed
first.
1. Spectra MicroVersaTrip trip unit
2. Enhanced MicroVersaTrip-C and -D trip units
3. POWER LEADER MDP Overcurrent Relay
4. Spectra Electronic Control Module
5. POWER LEADER Electronic Power Meter
6. POWER LEADER Meter
IEDs at the top of the list are less data-intensive and
easier for the Concentrator to poll. IEDs at the bottom
of the list are very data-intensive and, if possible,
should be given their own commnet segment. In other
words, if you must put multiple IEDs on a segment, it is
preferable to put low-demand IEDs together on a
segment and try to keep high-demand IEDs on their
own segments. Try to distribute the high-demand
IEDs across the available segments, keeping the
number of high-demand IEDs per segment evenly
distributed.
2–8 Addressing the IEDs
Each IED attached to a PMCS network must have a unique
address. Prior to installing any wiring, you should plan the
addresses for the IEDs to avoid any conflicts. Keep in mind
these important points when assigning network addresses.
• Keep a table of IED names and addresses to avoid
conflicts and to help with host configuration. Table 5
summarizes Modbus addressing considerations,
based on the following rules.
- Modbus Concentrator addresses must be in the
range of 1 to 32.
- Modbus-native IEDs other than the Concentrator
may occupy any address from 1 to 247.
- Commnet IEDs must have Modbus-equivalent
addresses in the range of 33 to 247.
- The host PC does not require an address due to the
master-slave organization of the PMCS.