Overview of the Impact of Renewables on Distribution Protection and Control

By David G. Hart, M. Amin Zamani and Ashok Gopalakrishnan

Efforts focused on grid resiliency, smart grid initiatives, and clean energy are driving the growing penetration of renewable energy resources such as photovoltaic (PV) systems, wind turbines, and fuel cells. Despite their advantages, distributed energy resources (DERs) inevitably challenge the traditional operating principles of distribution systems. More specifically, a unique set of protection and control (P&C) challenges is introduced to utilities as DERs proliferate.

Overview of DER Issues

Traditionally, distribution circuits have been designed with substations providing power to dispersed customer loads. The substation transformer is typically Delta-connected on the high voltage side and Wye-grounded on the low voltage side to distribution feeders. Since the power flows in one direction from the substation to the customer, the protection system typically consists of phase and ground overcurrent relays, fuses, and feeder reclosers. Fuse curves and overcurrent relay curves are selected to ensure protection coordination, assuming that the fault current is uni-directional from the substation. Additionally, load break sectionalizers may be utilized in conjunction with reclosers to reduce the area impacted by a fault on the feeder. The distribution system voltage is controlled by load tap changers (LTCs), capacitor banks, and/or voltage regulators (VRs). The algorithms used to control these devices can be as simple as time-based control assuming that the load current flows from the substation to loads. With the integration of DERs into the electric grid, these conventional schemes for distribution feeder protection and control are challenged.

The impact of the DERs on distribution feeder protection and control depend on several primary and secondary system factors. Some key factors are:

• Amount of DER power output in relation to the feeder loading (i.e., DER penetration level);
• Type of DER generation: while synchronous generators can provide sustained fault currents, induction generators can supply fault currents for a relatively shorter time (order of cycles). More importantly, power electronic inverters may only be able to provide minimal fault currents near typical load current values (typically around 1.2 to 2 per unit);
• DER Interconnection transformer to the distribution grid can have numerous winding configurations. For example, the interconnection transformer can be Delta/Delta, Delta/Wye, Delta/Wye-Grounded, Wye-Grounded/Delta, or other permutations of winding connections. The interconnection transformer configuration impacts the type of protection that should be used. In particular, the impact of ground currents from grounded banks and the impact of overvoltages for ungrounded banks need to be considered;
• Size of the DER (aggregate DER output rating);
• Location of DER (substation vs feeder end);
• Variability/Intermittency of DER output (e.g., PV system at night may have minimal impact as opposed to peak daylight time); and
• Availability of communications for protection and control systems; and of communication-enabled intelligent electronic devices, e.g. relays, recloser controls, sectionalizer controls, capacitor bank controllers, etc.

Since the introduction of DERs compromises the radial structure of the distribution grid, they can affect system protection and control. The presence of DERs manifests itself as (i) change in the fault current range of feeders, (ii) multi-directional power flow in the system, and (iii) change in the fault current seen by protective devices in series. The impact on traditional protection systems is seen in the: protection miscoordination issues, including protection blinding and nuisance fuse blowing; sympathetic tripping on adjacent feeders due to DER fault current contributions; misoperation of sectionalizers due to the presence of voltage on the feeder from DERs; ineffective operation of LTC, VR, and capacitor controls due to the reverse power flow from the DER to the substation; misoperation of reverse-power and/or non-directional protection functions; exceeding equipment rating due to higher fault current; and auto-reclosing failure.

These emerging challenges have motivated protection engineers to devise new protection schemes for distribution systems embedding DERs.

Potential Solutions

To overcome the aforementioned protection issues in distribution networks with high penetration of DERs, industry experts and researchers are trying to employ alternative protection solutions. The proposed protection solutions can broadly be categorized into three groups.

Conventional relays with customized protection logic are methods that offer an economic option. They may not provide effective protection coordination for all DER configurations. Examples include voltage-based protection, symmetrical-components-based protection, directional overcurrent relay, etc.

Communication-based protection schemes, although effective, are normally considered a costly solution. Examples include differential protection and directional comparison protection.

An adaptive protection option modifies the protective response to a change in system conditions in a timely manner by means of external signals. It does not need a high-speed communications, but requires advanced studies to define relay settings for each system condition. Further, the relays should have communication capabilities with flexible settings.

Use of high-speed communication systems and advanced microprocessor-based relays can enable one to come up with a generic solution. However, an engineering solution needs to consider both the technical and economic aspect of the designed protection scheme. To that end, it is essential to study the behaviour of the distribution system and its interaction with the DER, particularly in a highly-penetrated environment, prior to the design of the protection system.

Contributors