Dual-Standard Prepaid Smart Meters Supporting STS and DLMS: Technical Solution and Practice

Dual-standard prepaid smart meters supporting STS and DLMS are enabling seamless interoperability. Explore the technical architecture, specifications, and selection criteria.

Introduction: Why Dual-Standard Meters Matter in the Post-Agreement Era

The formal liaison agreement between the STS Association and the DLMS User Association has created a new product category in the smart metering industry: the dual-standard prepaid meter. These devices are designed to operate under both the STS (Standard Transfer Specification) protocol for token-based prepayment and the DLMS/COSEM protocol suite for advanced metering communication—all within a single physical device.

The demand for such products is driven by a practical reality. Utilities operating in markets transitioning from legacy STS-only prepaid systems to AMI-capable infrastructure need meters that can perform both functions simultaneously. Meters that only support STS cannot participate in DLMS-based data management systems; meters that only support DLMS cannot process STS tokens for prepaid credit loading. Dual-standard meters bridge this gap.

This article examines the technical architecture that makes dual-standard operation possible, presents specific product examples, and outlines practical considerations for utilities and project specifiers evaluating these solutions.

Technical Architecture: The Three-Layer Design

A dual-standard prepaid smart meter is, at its core, a system of three functional layers that operate in coordination. Understanding this architecture is essential for evaluating product quality and suitability for specific deployment environments.

Layer 1 — Communication Interface Layer

The communication interface layer handles the physical and link-level connection between the meter and external systems. In a dual-standard meter, this layer is typically designed around a modular architecture using hot-swappable communication modules.

The rationale for modularity is straightforward: different deployment environments require different communication technologies. A meter installed in an urban apartment building with existing power line infrastructure may use PLC (Power Line Carrier) communication. A meter in a rural area without grid-connected neighbours may require GPRS or 4G LTE connectivity. A meter in a smart city deployment may use RF mesh networking.

Rather than manufacturing separate meter variants for each communication technology, a modular approach allows the same meter hardware to accept different plug-in modules. This provides several practical advantages:

• Procurement flexibility: Utilities can standardise on a single meter model and stock communication modules separately, adapting to project-specific requirements at the time of deployment.
• Technology migration: As communication technologies evolve—for example, from 2G/GPRS to 4G LTE or from legacy PLC to newer PLC standards—modules can be replaced without replacing the meter itself.
• Inventory simplification: Warehousing and logistics are streamlined when a single meter SKU can serve multiple deployment scenarios, with the communication module determining the final configuration.

Common module options include PLC (various standards including narrowband PLC and PRIME/PLC-G3), RF (radio frequency, typically in sub-GHz bands for mesh networking), GPRS (2G cellular, increasingly phased out in many markets), and 4G LTE (current-generation cellular connectivity).

Layer 2 — Protocol Application Layer

The protocol application layer is where the dual-standard capability is fundamentally realised. This layer manages two distinct protocol stacks that operate in parallel on the same hardware platform:

• STS Protocol Stack: Implements the Standard Transfer Specification for token-based prepaid metering, including token decryption and validation (STS compliant with IEC 62055-21), key management conforming to IEC 62055-31, and vending system interfaces compliant with IEC 62055-41. The STS stack is responsible for processing 20-digit prepaid tokens, managing credit balances, and controlling the meter's disconnect/reconnect behaviour based on available credit.
• DLMS/COSEM Protocol Stack: Implements the Device Language Message Specification and Companion Specification for Energy Metering for advanced metering data exchange. This stack manages the meter's DLMS/COSEM object model, including register objects for active and reactive energy, load profile objects for interval data recording, tariff schedule objects for TOU (time-of-use) rate structures, and event log objects for tamper detection and operational events. The DLMS stack communicates with head-end systems via DLMS/COSEM Application Protocol Data Units (APDUs) over the communication interface layer.

The coexistence of these two stacks on a single meter requires careful firmware architecture. Both stacks must access shared metering resources—principally the energy measurement core and the relay that controls load connection—without conflict. The firmware typically implements an arbitration layer that ensures, for example, that a zero-credit disconnect triggered by the STS stack and a remote disconnect command issued via DLMS are handled consistently.

Layer 3 — Metering Core

The metering core provides the foundational energy measurement function that serves both protocol layers. This layer includes the voltage and current sensing circuits, analog-to-digital conversion, metrology calculations (active energy, reactive energy, power factor, frequency), and calibration parameters.

The metering core operates independently of the protocol layers, generating accurate measurement data that both the STS and DLMS stacks can access. The STS stack uses the energy measurement data to decrement the prepaid credit balance, while the DLMS stack uses it to populate load profiles, event logs, and register objects for remote reading.

A well-designed metering core in a dual-standard meter will typically comply with IEC 62053-21 for accuracy classes, providing measurement quality that meets the requirements of both STS and DLMS certification bodies.

Product Example: DDS722 Single-Phase Prepaid Smart Meter

The DDS722 is a single-phase prepaid smart meter designed to operate in dual-standard environments, supporting both STS token-based prepayment and DLMS/COSEM communication. The following specifications are provided as a technical reference for project evaluation.

Electrical and Metrological Specifications

| Parameter | Specification |

|---|---|

| Rated voltage | 220/230/240V AC |

| Operating voltage range | Extended range per IEC standards |

| Current range | As per specific model variant |

| Frequency | 50/60 Hz |

| Accuracy | Class 1 and Class 2 (per IEC 62053-21) |

The availability of both Class 1 and Class 2 variants allows utilities to select the accuracy level appropriate for their application—Class 1 for higher-precision requirements, Class 2 for standard residential applications.

Protocol Support

| Protocol | Standard |

|---|---|

| DLMS/COSEM | Via PLC communication (IEC 62056 series) |

| STS | IEC 62055-21 (token system), IEC 62055-31 (key management), IEC 62055-41 (vending) |

Both protocol stacks operate concurrently, enabling the meter to process STS tokens for prepaid credit loading while simultaneously communicating consumption data via DLMS to a head-end system.

Communication

Communication is handled through hot-swappable plug-in modules, supporting the following technologies:

• PLC (Power Line Carrier)
• RF (Radio Frequency)
• GPRS (2G cellular)
• 4G LTE (4th generation cellular)

Module selection is determined at the point of deployment, allowing the same meter hardware to be configured for the communication technology available in the installation environment.

Metering Mode

The DDS722 supports both prepaid and postpaid metering modes, configurable via software or remote command. In prepaid mode, the meter operates according to the STS credit management rules: credit is loaded via token, and the load is disconnected when credit reaches zero. In postpaid mode, the meter records consumption data without enforcing a credit limit, operating as a conventional smart meter for periodic billing.

Tariff Structure

The meter supports up to 8 rate plans with time-of-use (TOU) tariff capability. This allows utilities to implement multi-tier pricing structures, seasonal rate adjustments, and peak/off-peak differentiation—features that are managed through the DLMS tariff scheduling objects.

Security and Tamper Detection

The DDS722 implements multiple layers of tamper detection and protective measures:

• Reverse current detection: Monitors for current flow in the reverse direction, which may indicate meter bypass attempts.
• Magnetic tamper detection: Sensors detect the presence of strong magnetic fields that could interfere with current measurement accuracy.
• Cover opening detection: Records events when the meter cover is opened, enabling utilities to identify unauthorised access.
• Neutral-live swap detection: Identifies improper wiring configurations that could affect measurement accuracy or safety.

Each tamper event is recorded in the meter's event log, with configurable responses ranging from logging only to automatic load disconnection.

Event Logging

The meter maintains event logs with capacity for up to 100 records per event type. Event categories include tamper events, power quality events (overvoltage, undervoltage, overcurrent), communication events, and operational events (credit loading, mode changes, relay operations).

Environmental Protection

| Parameter | Rating |

|---|---|

| Ingress protection | IP54 |

| Operating temperature range | -25°C to +70°C |

| Storage temperature range | Per manufacturer specification |

The IP54 rating provides protection against dust ingress and water splashes, suitable for both indoor and outdoor installation in residential and light commercial environments. The extended temperature range accommodates deployment in hot climates without derating.

Remote Management Capabilities

• Remote disconnect/reconnect: The internal relay can be controlled remotely via DLMS commands, enabling utilities to disconnect service for non-payment or reconnect after credit loading without a site visit.
• OTA firmware updates: Firmware can be updated over-the-air via the communication module, allowing utilities to deploy security patches, feature updates, and protocol enhancements without physical access to the meter.
• Load control: Utilities can implement load limiting or load shedding commands via DLMS, supporting demand management programmes.

Product Example: DTS722 Three-Phase Prepaid Smart Meter

The DTS722 extends the dual-standard architecture to three-phase metering applications. Designed for industrial and commercial installations, the DTS722 is a three-phase four-wire meter that implements the same dual-standard protocol architecture as the DDS722.

Key characteristics of the DTS722 include:

• Three-phase four-wire configuration: Suitable for commercial and industrial consumers with three-phase electrical supply.
• Dual-standard operation: STS token-based prepayment alongside DLMS/COSEM communication, using the same parallel protocol stack architecture as the single-phase variant.
• Hot-swappable communication modules: The same modular communication interface supporting PLC, RF, GPRS, and 4G LTE.
• Enhanced metering capabilities: Three-phase meters typically provide additional measurement parameters including per-phase voltages, currents, and power values, as well as three-phase totals for active and reactive energy.
• Tamper detection: Three-phase specific tamper detection including phase loss detection, phase sequence monitoring, and imbalance detection, in addition to the tamper detection features common to the single-phase model.

The DTS722 is suited for deployments where industrial or commercial consumers require prepaid functionality within a three-phase metering context—for example, small factories, commercial complexes, or multi-tenant buildings with three-phase supply in regions where prepaid metering is the norm.

Practical Significance of Dual-Standard Architecture

The dual-standard meter architecture has several practical implications for utilities, system integrators, and project specifiers.

System Integration Simplification

Deploying dual-standard meters reduces the number of distinct device types that a utility must manage within its metering infrastructure. Instead of maintaining separate systems for STS prepaid meters and DLMS smart meters, a utility can deploy a single device type that serves both functions. This simplifies head-end system integration, data management, and operational workflows.

From a system integration perspective, the dual-standard meter presents as two logical devices: an STS prepaid meter and a DLMS smart meter. The head-end system must be capable of communicating with both protocol interfaces. However, the physical management overhead—a single device to install, maintain, and eventually replace—is substantially reduced compared to managing two separate device populations.

Future-Proofing

Dual-standard meters provide a degree of future-proofing that single-standard devices cannot match. As utilities progress through AMI deployment stages—from basic automated meter reading to full advanced metering infrastructure with demand response and distributed energy resource management—the DLMS capability of the meter provides a ready platform for these advanced functions.

Conversely, the STS capability ensures that the meter can continue to serve prepaid customers during the transition period, maintaining revenue collection continuity even as the utility's management system evolves. This dual capability means the meter remains relevant throughout multiple phases of a utility's infrastructure modernisation programme.

Multi-Communication Flexibility

The modular communication architecture common to dual-standard meters adds another dimension of flexibility. Utilities are not locked into a specific communication technology at the time of meter purchase. If a utility initially deploys meters with PLC modules and later needs to upgrade to 4G LTE for expanded coverage or higher bandwidth, the communication module can be swapped without replacing the meter.

This is particularly relevant in markets where communication infrastructure is evolving rapidly—for example, in African countries where mobile network coverage is expanding and 2G networks are being phased out in favour of 3G and 4G services.

Manufacturing Capability: SOOCIA's Production Infrastructure

SOOCIA (Zhejiang Songxia Electric Meter Co., Ltd.) manufactures the DDS722 and DTS722 dual-standard meters at its production facility in Zhejiang Province, China. The manufacturing operation encompasses the following:

• Facility size: Approximately 40,000 square metres of production space.
• Annual production capacity: Approximately four million units.
• Project experience: The company has supplied meter products to State Grid Corporation of China procurement programmes, as well as international projects.
• Export markets: Products have been supplied to overseas markets including Southeast Asia, Africa, and countries participating in the Belt and Road Initiative.

SOOCIA holds memberships in both the STS Association and the DLMS User Association, and its dual-standard meter products carry certifications under both standards. This dual membership and dual certification position the company's products for deployment in markets that require interoperability across both protocol environments.

Several other manufacturers in the global metering industry are pursuing similar dual-standard product strategies. The emergence of multiple suppliers in this category is a positive development for the market, as it provides utilities with competitive options and reduces dependency on any single vendor.

Selection Criteria for Dual-Standard Prepaid Smart Meters

When evaluating dual-standard prepaid smart meters for project deployment, utilities and specifiers should consider the following criteria:

1. Certification Completeness

Verify that the meter carries current, valid certifications under both standards:

• STS certification: Issued by the STS Association, confirming compliance with the relevant parts of IEC 62055 (typically Parts 21, 31, and 41 for the complete prepaid solution).
• DLMS certification: Issued by the DLMS User Association, confirming compliance with the Blue Book and relevant companion specifications.

Certifications should be checked for currency—standards are periodically updated, and meters certified under older versions of the standard may not support the latest protocol features or security requirements.

2. Communication Adaptability

Evaluate the range of available communication modules and the ease of module replacement. Consider:

• Which communication technologies are available (PLC, RF, GPRS, 4G LTE)?
• Is module replacement possible in the field, or does it require factory service?
• Are modules from the meter manufacturer or from third-party suppliers?
• What is the roadmap for future communication technologies (NB-IoT, 5G, next-generation PLC)?

3. Accuracy Compliance

Confirm that the meter's accuracy class meets the requirements of the application:

• Class 1 is typically specified for applications requiring higher measurement precision or for meters used in revenue billing for larger consumers.
• Class 2 is commonly specified for standard residential metering applications.

Accuracy should be verified per IEC 62053-21, and calibration certificates should be available upon request.

4. Remote Management Capabilities

Assess the range of remote management functions available via the DLMS interface:

• Can the meter be remotely disconnected and reconnected?
• Is OTA firmware update supported, and what is the update mechanism?
• Are load control and demand limiting functions available?
• What data is available for remote reading (registers, load profiles, event logs)?

5. Security Features

Review the meter's security implementation:

• STS token security: compliance with the STS key management and encryption requirements.
• DLMS security: supported security levels (low, high), authentication mechanisms, and encryption for data transmission.
• Physical tamper detection: which tamper events are monitored, and what responses are configurable.

6. Environmental and Regulatory Compliance

Verify that the meter meets the environmental and regulatory requirements of the deployment location:

• Ingress protection rating appropriate for the installation environment (indoor vs. outdoor).
• Operating temperature range suitable for the local climate.
• Compliance with relevant local or regional electrical safety standards.

7. Long-Term Support

Evaluate the manufacturer's commitment to ongoing product support:

• Firmware update availability and frequency.
• Product lifecycle management—how long will the product be actively manufactured and supported?
• Availability of technical documentation, training, and application support.
• Track record of security patch deployment.

Conclusion: A New Category for a Converging Market

Dual-standard prepaid smart meters represent a new product category that has emerged in direct response to the STS-DLMS standard convergence. By implementing both the STS token-based prepayment protocol and the DLMS/COSEM communication protocol on a single hardware platform, these meters provide a practical solution for utilities navigating the transition from fragmented metering standards to a more interoperable ecosystem.

The three-layer technical architecture—communication interface, protocol application, and metering core—provides a framework that balances the need for flexibility (through modular communication and configurable metering modes) with the need for reliability (through proven metrology and robust tamper detection).

For utilities, system integrators, and project specifiers evaluating dual-standard meters, the selection criteria outlined in this article—certification completeness, communication adaptability, accuracy compliance, remote management, security, environmental suitability, and long-term support—provide a structured framework for product evaluation.

As the STS-DLMS convergence continues to shape the smart metering industry, dual-standard meters are positioned to become an increasingly common feature of metering deployments in markets where both standards have relevance. The availability of products like the DDS722 and DTS722 from manufacturers such as SOOCIA reflects the industry's response to this convergence—and provides utilities with the technical tools to build metering infrastructure that is prepared for a converging standards landscape.