The Business Case for IEC 61850

Author: Alexander Apostolov, USA

Remote Testing

Maintenance testing is something that needs to be done, regardless of the different challenges that may be faced by the crew:

  • Long distance between the substation and the base of the testing team
  • Difficult terrain with bad roads
  • Difficult weather conditions
  • Requirements for reduction of outage time because of maintenance

One of the benefits of IEC 61850 based digital substations is that all devices (PAC IEDs, substation computers and test devices) are connected to the substation communications network. If there are testing tools that are connected to the network in the substation on a permanent basis, it becomes possible to perform the tests from a remote location. (see Figure 9)

Outages for Testing of Protection IEDs
Maintenance testing in conventional hardwired schemes requires the isolation of the tested protection device which is performed typically using a test switch connecting the test device to the test object. In many cases this requires an outage, due to the unavailability of the protection device. At the distribution feeders typically there is only a single protection IED, which means that during the testing if the feeder is still in service the protection will be provided by backup functions, for example in the transformer protection IED. This will result in extended fault clearing times and may result in the shutdown of many DERs connected to the distribution system.

The virtual isolation features in IEC 61850 allow the isolation of functions, sub-functions or even a single function element (for example an overcurrent step as shown in Figure 10). In this case all remaining protection functions are active which improves the reliability of the system, reduces the fault clearing time and eliminates the need for an outage - all very significant benefits compared to the traditional testing practices.

CT Saturation
IEC 61850 process bus based applications offer some important advantages over conventional hard wired analog circuits. The first very important one is the significant reduction in the cost of the system due to the fact that multiple copper cables are replaced with a small number of fiber optic cables.

Using an IEC 61850 process bus based on standalone merging units (SAMUs) also results in the practical elimination of CT saturation because of the elimination of the current leads resistance. Traditionally the CT knee-point voltage is a function of the resistance of the different components of the current circuit:

VK = f (RCT, RL, RRP)
where:
VK = Required CT knee-point voltage (volts)
RCT = Resistance of the current transformer secondary winding (ohms)
RL = Resistance of a single lead from relay to current transformer (ohms)
RRP = Impedance of a relay phase current input

In some cases RL is multiplied by 2 and plays a key role in determining the CT requirements.
In this case the CT secondary is connected to the phase current inputs of the Merging Units and RL is practically equal to zero.

The knee-voltage then will be only dependent on
VK = f (RCT, RRP)

The impedance of the merging unit current inputs RRP is very small, thus resulting in the significant reduction in the possibility for CT saturation and all associated with it protection issues.
CT saturation is completely eliminated when using non-conventional instrument transformers, because there is no CT circuit.

High Penetration of DERs
When a short circuit fault occurs on a transmission line connected to a substation with DERs at the distribution level, the voltage drop caused by the fault needs to be considered in the analysis of the performance of the DERs and their ability to ride through the fault. When the fault is in Zone 2 of the protected transmission line (especially on shorter lines) the time delayed trip will depend on the time delay setting which may be in the range of 300 - 400 msec. Such a delayed trip will result in the duration of the voltage sag experienced by a DER in the tripping area of the ride-through characteristic. An accelerated protection scheme can significantly reduce the fault clearing time and bring it within the stay connected area of the characteristic.

The challenge for the implementation of accelerated transmission line protection schemes is that they require a communications channel which, if it is a dedicated one, will require additional costs.
IEC 61850 routable GOOSE (R-GOOSE) messages (see Figure 11) are a technology that can help us achieve these goals without the need for additional investments.

A Very Strong Business Case
Based on the analysis of all use cases described above it is clear that IEC 61850, especially when fully implemented in digital substations with non-conventional instrument transformers and state of the art disconnecting breakers, offers significant savings in time and money. At the same time the risks are minimal, and they have to do with the fact that it is a new technology that requires training in order to apply it properly and use it efficiently.  

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