IEC 61850 to IEC 104 and DNP3 mapping in the gateway

What the gateway does

Inside the substation, the IEDs speak IEC 61850: each data point has a hierarchical name (logical device, logical node, data object and attribute), detailed quality and a time tag. The control center, on the other hand, usually speaks IEC 60870-5-104 or DNP3 (IEEE 1815), where each data point is a numeric address of a defined type.

The gateway or RTU sits in between. It is an MMS client of the IEDs, receives their reports and publishes them as an IEC 104 server or a DNP3 outstation. For each signal it keeps a mapping rule: where it comes from, which address it goes to, with which type and with what handling of quality and time.

The data point changes its name at each hop

An example with the position of a breaker:

IEC 61850 names are self-describing: an engineer reads the path and knows what it is. IEC 104 and DNP3 addresses say nothing by themselves: their meaning lives in the signal list agreed with the control center.

Data type

• Single and double points. A breaker position is a double point in both worlds (open, closed, intermediate, invalid). An alarm is a single point.
• Measurands. In IEC 61850 they are analog values with scale and unit; in IEC 104 you choose between normalized, scaled or floating-point values, and in DNP3 among the analog input variations. The choice affects resolution and deadband.
• Counters. Energy and operation counts are mapped as integrated totals.
• With or without a time tag. IEC 104 has types with a time tag (CP56Time2a, seven octets) and without one. Types with a time tag are used for events the operator must put in order.

Quality

IEC 61850 reports quality with validity (good, invalid, questionable), details (overflow, out of range, failure, oscillatory, inconsistent) and source (process or substituted), plus the test and operator-blocked flags. IEC 104 has a shorter quality descriptor (invalid, not topical, substituted, blocked, overflow) and DNP3 has its flags (online, restart, communication lost, local or remote forced, among others).

The gateway decides how each combination is translated. Two decisions matter:

• What happens if the IED stops communicating. The right behavior is for the gateway to flag that IED’s signals as not topical or communication lost, so the operator does not see an old state as valid.
• What happens with a data point in test mode. A data point flagged as test in IEC 61850 must not reach the control center as if it were real.

Time tag

The time tag should come from the IED that detected the change, not from the gateway that forwarded it. If the gateway applies its own time, the sequence of events loses accuracy. The time zone also matters, as does the IED, gateway and control center sharing the same time source.

Commands

In IEC 61850, each controllable object has a control model: direct operate or select before operate (SBO), with normal or enhanced security. In IEC 104, the command carries a qualifier that distinguishes select from execute, and the response comes back with the activation confirmation and activation termination causes of transmission. In DNP3, select and operate are function codes 3 and 4, with direct operate as an alternative.

The gateway translates each step:

If the IED rejects the command because of an interlock, local mode or the maximum time between select and execute, the gateway must carry that rejection to the control center with the right cause. A command rejected without explanation is one of the most common faults: see command rejected from SCADA.

What is lost in translation

• The self-describing name: in IEC 104 and DNP3 only an address remains.
• Part of the quality: IEC 61850 details are summarized in a few bits.
• The IED mode and behavior (on, blocked, test), unless they are mapped as additional signals.
• Time resolution, when the time tag moves to a millisecond format.
• The structure: a logical node with several data objects becomes a flat list of addresses.

Mapping references

• IEC TS 61850-80-1, Edition 2 of 2016: guidance for exchanging IEC 61850 model information over IEC 60870-5-101 and 104, with a focus on the gateway. A third edition is in development.
• IEEE 1815.1-2015: information exchange between IEC 61850 and DNP3 networks, in both directions, including the gateway architecture and the description of the mapping in SCL and in the DNP3 device profile.

These standards give the general rules. The actual table for each project is set by the utility or the owner of the control center.

Gateway configuration with Tiza Group

Mapping is where integration succeeds or fails: a badly translated quality or a gateway time tag in place of the relay’s goes unnoticed until the first real event. We configure IEDs, switches and gateways remotely through our remote relay and communications configuration service, with a backup before and after every change. Our team configures and tests gateways and RTUs against the control center’s signal list, with point-to-point tests, as part of our SCADA and gateway integration service, in medium-, high- and extra-high-voltage substations and industry, in Colombia, or elsewhere on request. You can review the reports and datasets in your SCD with our free IEC 61850 SCL viewer tool. Write to us with the protocol and the control center of your project.

Related guides

• IEC 61850 to the control center: TR 61850-90-2
• Redundant gateway and dual links to the control center
• Migrating from 103, 101 or Modbus to IEC 61850
• Substation communications by voltage level: MV, HV, EHV

References

• IEC TS 61850-80-1:2016, guideline to exchanging information from a CDC-based data model using IEC 60870-5-101 or IEC 60870-5-104.
• IEEE 1815.1-2015, Standard for Exchanging Information Between Networks Implementing IEC 61850 and IEEE Std 1815 (DNP3).
• IEC 60870-5-104, Edition 2 of 2006 with its Amendment 1 of 2016.
• IEEE 1815 (DNP3).
• IEC 61850-7-2 and IEC 61850-8-1: services, control model and mapping to MMS.

Frequently asked questions