Solar string and DC UPS monitoring

The problem

Photovoltaic plants — from 75 kW distributed generation to multi-MW utility-scale — suffer from invisible losses on individual strings. A module with a microcrack, an oxidized MC4 connector or accumulated dirt degrades string power without triggering any inverter alarm, which continues reporting "OK" while the customer loses generation. In DC UPS for critical missions (data center, hospital, sensitive industrial process), silent failure of a battery string between scheduled tests is the dangerous equivalent. In both cases, the operator needs continuous per-string visibility, with granularity that makes mismatch detectable in hours — not in quarterly O&M cycles — and without violating the isolation voltage of the DC bus.

Golden rule: the inverter reports the aggregate; the problem lives in the string.

Typical topology

Small and medium PV plants typically have 4 to 12 strings in parallel connected to a combiner box, then to the inverter. String voltage in standard conditions (STC) ranges from 400 to 1000 VDC depending on the design. DC UPS have battery strings (lead-acid or LiFePO4) in parallel forming a 220 to 480 VDC bus.

PV strings (8x example)
   |    |    |    |    |    |    |    |
   v    v    v    v    v    v    v    v
+---+--+---+--+---+--+---+--+---+--+---+
|       shunts on each string         |
+---+--+---+--+---+--+---+--+---+--+--+
   |    |    |    |    |    |    |    |
   +----+----+----+----+----+----+----+
        |
        v
   DC combiner + disconnect switch
        |
        v
+----------------+
| PV inverter or |
| DC UPS         |
+----------------+

Sensing:
+--------------+
| AEM-60DC8 x N| <-- 100:1 divider for string voltage
+--------------+
       |
       v
   Modbus RTU --> Gateway --> SCADA / generation portal

For string voltages above 200 VDC, all voltage channels use a precision resistive divider with class ≤0.5%.

Golden rule: each shunt in the combiner box is a visibility point — saving on a shunt is saving on future diagnostics.

What to measure and why

In PV plants and DC UPS, eight quantities are priorities:

Quantity	Justification
Per-string current (A)	Comparison between parallel strings detects mismatch, shading, dirt and localized degradation.
Per-string voltage (V)	Confirms operation at maximum power point; STC drop indicates open-circuit module.
DC bus voltage (V)	Combiner output; inverter or UPS input.
Total bus current (A)	Sum of strings; confirms topology.
Ambient temperature (deg C)	Reference for normalization — PV current varies with irradiance and temperature.
Module temperature (deg C)	Via sensor attached to a reference module; voltage coefficient is negative with temperature.
Irradiance (W/m²)	Via pyranometer with 4–20 mA output; input for PR (performance ratio) calculation.
Bus ground isolation (kΩ)	Detects DC cabling deterioration; mandatory under IEC 62548.

For DC UPS, irradiance and module temperature are replaced by charge/discharge current and per-battery-string temperature. Other quantities remain applicable.

Golden rule: measuring current without normalizing by irradiance triggers false alarms every morning.

Sizing — how many AEM-60DC8?

Formula:

N_AEM = ceil( (2 × N_strings + bus_channels + environmental_channels) / 8 )

Where 2 channels per string are voltage and current.

Simplified example for PV plant with 6 strings at 500 VDC:

• 6 strings × 2 channels = 12 channels
• Bus: 1 voltage + 1 current + 1 isolation = 3 channels
• Environmental: 1 irradiance + 1 ambient temp + 1 module temp = 3 channels
• Total: 18 channels → 3 AEM-60DC8 (with 6 free channels for expansion)

Simplified example for 380 VDC DC UPS with 4 battery strings:

• 4 strings × 2 channels = 8 channels
• Bus: 1 voltage + 1 total current + 1 isolation = 3 channels
• Temperatures: 2 channels
• Total: 13 channels → 2 AEM-60DC8

Golden rule: size counting future inverters and UPS — expansion without spare hardware requires reinstall.

Recommended setpoints and alarms

Variable	Warning	Critical	Automatic action
PV string current vs. average (%)	<85 or >115	<75 or >125	Notify O&M with string identification
PV string voltage STC normalized (%)	<90	<80	Module degradation alert
DC UPS bus voltage (V)	±5% nominal	±10% nominal	Flag UPS off-spec
Battery string discharge current (A)	>0.1·C10 sustained	>0.3·C10 sustained	Start event log
PV module temperature (deg C)	>65	>75	Monitor operating limit
Battery string temperature (deg C)	>35	>45	Reduce float current
Ground isolation (kΩ)	<100	<50	Block operation (on UPS)
Performance Ratio (PR daily)	<80	<70	Investigate plant

Limits are written to holding registers protected by firmware v1.03 Secure by Design.

Golden rule: alarms proportional to average (not absolute) eliminate noise in PV current.

SCADA / PLC integration

The AEM-60DC8 operates as a Modbus RTU Slave. Recommended baudrate 19200 bps for small plants and 38400 bps for plants with 6+ AEM-60DC8 on the same bus.

Ignition: use the Solar module to create UDTs per string with attributes voltage, current, irradiance_norm_power and performance_ratio. History in Tag Historian with 5-year retention for seasonal analysis.

Elipse E3: the IODriver-Modbus driver integrates the AEM-60DC8 units and the pyranometer on the same gateway. ECMA script-calculated tags produce PR and mismatch detection.

AVEVA Insight (or System Platform): for distributed plants, the cloud connector pushes data from each plant for comparative analysis between sites — a relevant differentiator in portfolios with 20+ plants.

For SaaS generation portals (PVcase, Solar-Log, Meteocontrol), the gateway converts Modbus RTU to Modbus TCP or MQTT. Keep the AEM-60DC8 behind an industrial firewall.

See whitepaper WP01 for Modbus RTU over Secure by Design details.

Golden rule: SaaS generation portal is integration, not a substitute for local SCADA — design for both.

Regulatory compliance

PV and DC UPS systems follow specific standards:

• ABNT NBR 16690 (electrical installations for PV arrays)
• ABNT NBR 16149 (grid-connected PV systems — requirements)
• IEC 62548 (Photovoltaic arrays — design requirements), including isolation monitoring requirements
• IEC 61724-1 (Photovoltaic system performance monitoring)
• IEC 62040-1 and IEC 62040-3 (UPS — safety and performance) for DC UPS
• IEEE 1547-2018 (interconnection of distributed energy resources)
• IEC 62443-4-2 SL2 as target/in progress for the cybersecurity layer
• ANEEL REN 1,000/2021 and updates of distributed generation in Brazil

The AEM-60DC8 forensic telemetry contributes to the performance report required in IEC 61724-1 Class A.

Golden rule: PV standards evolve quickly — check the current ABNT and IEC editions before closing a project.

Illustrative case

Simplified example: a 1.2 MWp solar plant in central-west Brazil showed gradual PR drop from 84% to 78% over 14 months. The inverter continued reporting normal operation and the OEM SaaS portal did not flag any alarm. The cause was only found after string-level monitoring was installed: 3 of 18 strings had 12 to 18% lower current than the average under identical conditions.

Implementation: 5 AEM-60DC8 reading 18 string currents, 18 string voltages, irradiance, ambient temperature and module temperature. Integration via Modbus TCP gateway to local Elipse Power SCADA.

Result: field visual analysis confirmed oxidized MC4 connectors on two strings and a string with one low-current module from a microcrack. Replacement of 2 connector sets and 1 module restored PR to 83%. The instrumentation cost was recovered in 7 months from additional generation.

Golden rule: per-string measurement is the difference between auditing the plant and guessing where the problem is.

Deployment checklist

1. Survey topology (number of strings, STC voltage, combiner configuration).
2. Specify 100:1 class ≤0.5% resistive dividers for strings >200 VDC.
3. Select shunts compatible with string Isc with 25% headroom.
4. Add a class B (minimum) pyranometer with 4–20 mA output.
5. Compute N_AEM and provide 2 reserve channels per unit.
6. Place AEM-60DC8 inside the combiner box or dedicated cubicle.
7. 24 AWG shielded twisted-pair cable, shield grounded at a single point.
8. Configure baudrate 38400 bps in plants with 6+ units.
9. Validate 147 holding registers and enable forensic telemetry.
10. Document baseline on a clear day, near the solstice, as reference.

Golden rule: baseline on a clear day near the solstice is the only valid reference for the whole year.

FAQ

See the FAQ block in the frontmatter of this page.