Sub-metering with Modbus power meters: choosing the meter and CTs

Sub-metering means putting a meter on each circuit you want to account for: a production line, a tenant, a chiller, a floor. Most problems come from current transformers (CTs) that do not suit the current or the conductor, and from the link that brings the readings to a controller or SCADA system. This article covers the choices for sub-metering with Modbus power meters: meter type, CT rating and inner diameter, accuracy, wiring, Modbus RTU or Modbus TCP, and which ICP DAS PM series fit.

Single-phase, three-phase or multi-circuit

A list of circuits, their supply type and maximum current decides the meter type.

• Single-phase: the PM-3112 measures two single-phase loops and the PM-3114 four, in 1P2W wiring with voltage inputs up to 300 V and CTs up to 200 A.
• Three-phase: the PM-3133 series handles 3P4W, 3P3W, 1P3W and 1P2W wiring at 10 to 500 V. In 1P2W-1CT mode one meter can also measure three single-phase circuits.
• Many circuits in one board: the PM-4324 series measures up to 8 three-phase or 24 single-phase circuits, or a mix, through 24 CT inputs. It is intended for multi-tenant buildings, offices and shopping centres.

Sizing current transformers: rating and inner diameter

The CT rating should cover the highest normal operating current of the circuit. The inner diameter must fit around the conductor including its insulation, or around the busbar. Place one CT around each phase conductor, never around a complete multi-core cable: the currents in the cores cancel out and the meter reads close to zero.

The PM-3133, PM-2133D, PM-5133 and PM-4324 series are supplied with matched CTs, and the order code suffix shows which. Not every suffix exists in every series.

These CTs have 4 m leads. For large conductors, the PM-3133-RCT series uses flexible Rogowski coils: 500 A (55 mm), 1000 A (80 mm), 2000 A (105 mm) and 4000 A (185 mm), each measuring from 5 A.

Reusing existing CTs

Where the switchboard already has CTs with a 1 A or 5 A secondary, the PM-3033 accepts them directly; note their ratio, for example 400/5 A, as it is needed to scale the readings. For 333 mV CTs, use the PM-3133P, PM-2133DP or PM-4324P, which come without CTs and do not support Rogowski coils. The matching CA-SCTxxP split-core CTs cover 5 to 400 A with inner diameters of 10 to 36 mm; the CT ratio must be set correctly in the meter.

Accuracy: reading the figures

ICP DAS states active power accuracy at power factor 1:

• Better than 0.5% for the PM-3112, PM-3114, PM-3133, PM-4324 and PM-5133 with their supplied CTs, and for the PM-3033 (above 0.5 A input current for the PM-3033 and PM-5133).
• Better than 2% for the Rogowski versions, above 50 A for the 500 A and 1000 A coils and above 200 A for the larger ones.
• Approximately 2 to 5% for the 333 mV models, depending on the CT.

Choose the smallest CT rating that covers the maximum current; an oversized CT spends most of its time at the bottom of its range. Rogowski coils suit large incomers, not lightly loaded feeders. If readings are used to invoice tenants, check which meter approval national rules require.

Wiring and installation

• CT direction: mount each CT in the direction marked on it. The three-phase models measure bi-directional energy, so a reversed CT shows up as exported energy rather than as an obvious fault.
• Phase assignment: each CT must sit on the same phase as its voltage input, or power and power factor will be wrong.
• Auxiliary supply: the PM-3112, PM-3114 and PM-3133 need 12 to 48 VDC, with Power over Ethernet as an alternative on the PM-3133-MTCP. The PM-4324 and PM-2133D take 85 to 264 VAC.
• Outputs: many models have two Form A power relays (5 A at 250 VAC) for alarms.

Voltage connections and CTs in a distribution board must be installed by a qualified electrician.

Modbus RTU or Modbus TCP

The standard versions use Modbus RTU on RS-485. One twisted-pair bus runs from meter to meter, each meter has its own address, and the bus is terminated at both ends. Baud rates from 9600 to 115200 bit/s are supported.

Versions with the suffix -MTCP have a 10/100 Mbit/s Ethernet port with Modbus TCP, which avoids a separate bus where meters are spread out and Ethernet is already in place. Several series also come with CANopen (suffix -CPS).

Choosing a Modbus power meter from the PM series

All of them measure voltage, current, power, energy, power factor and frequency, updated every second. Where no PLC or SCADA system reads the meters, a PMC or PMD power meter concentrator can poll them and log the data.

Send technical sales a list of circuits with voltages, maximum currents and cable sizes, and we will propose suitable meters and CTs.