ENEL Smart Grid Projects - DER Management Applications

by Pietro Tumino, Giorgio Di Lembo and Gianpatrizio Bianco, Enel Distribuzione, Italy

Introduction

Historically the existing MV network was designed for mono-directional power flows, the line voltage profile was a monotone decreasing profile. Today this is not true anymore, furthermore DER causes uncertainty about the power generated, problems to protection, control and automation systems and network congestions.
Due to these problems there is a limited capacity to integrate Distributed Energy Resources (DER). Enel is working in a lot of projects aimed at finding the best way to integrate DERs in electrical networks and to use them as a resource for the network.


Enel Projects
The Figure on the first page shows Enel projects related to Smart Grids.

Functional and architectural developments
In all of these projects Enel has developed a lot of new functionalities in order to go towards the Smart Grids. The main of them can be seen in Table 1.

Voltage control
The massive introduction of distributed generators changes the constraints in hosting capacity calculation. In fact the hosting capacity of a MV network, with a high presence of DG, is not usually limited by conductors capability but by voltage increase caused by the distributed generators.
Historically and up to now, the voltage control was done by modulating the On Load Tap Changers (OLTC) of each HV/MV transformer by means of a method called “current compound.” The goal was assuring a good voltage level at both ends of the feeders. This method works until the voltage profile is monotone decreasing, but with the introduction of DG a different approach is required.

The new voltage control technique is based on two main points:

  • MV Bus bar voltage setting, based on electrical calculation made by a central DMS (Distribution Management System) and on the measurements coming from the network
  • Decentralized voltage control in corresponding of DG injection points, based on local measurement and reactive energy absorption of the generator

DMS evaluates the optimal voltage setting at the bus bar level each 10 minutes; in case of overvoltage due to a power injection from a distributed generator, a local control provides to ask the generator to absorb reactive power at a specific power factor.. If the local control may not be sufficient to restore the voltage levels, the control center could ask the closer generators to modulate the reactive power in order to “help” the voltage reduction. In extreme situations the control center could, also, ask the generator that causes the overvoltage to curtail the active power generation. (see Figure 1 and Figure 2)

Anti-Islanding + ∆f
The scope of the interface protection (IPR) installed in correspondence of the private power plant is to disconnect the generators from the network to avoid:

  • Unwanted Islanding supply conditions in case of the loss of the mains (DGs keep on supplying portions of network even if the main network is down)
  • Giving energy to a fault occurred on the MV or LV line where the client is connected
  • Damages of the generation equipment in case of automatic or manual reclosing of the distributor line breaker, with phase discordance between with the main network and the generator

IPR detects the tripping conditions measuring voltage and frequency variations due to islanding working situations. According to the Italian TSO Annex A70, the IPR is able to shift the range of frequency threshold between a restrictive range (49,7 - 50,3 Hz) and a permissive range (47,5 – 51,5 Hz), depending on local fault conditions that cause a relevant perturbation on the parameters.

Despite these countermeasures, in absence of faults, if the generated power is very close to the load, a balance condition may be present in a portion of the network. In this case an islanding condition may occur with negligible variations of voltage and/or frequency (they may remain inside the no trip area of the IPR).

This may happen at different levels:

  • On MV busbars of a HV/MV substation (no P/Q flows through the HV/MV TR)
  • In correspondence of a MV feeder

In a MV section of a feeder, automatically isolated by the automation system in case of a ground fault; Enel Remote Control System detects each potential situation of islanding and avoids possible problems sending explicit command of remote disconnection to all generators of the island, according to the recommendations stated in the annex A.70 of the Italian Grid code (issued by Italian TSO). (see Figure 3)

Let?s start with organization in protection testing