Demystifying Modeling of Distributed Energy (PART-3)

Lets begin with the 'why' before getting on to the how. Firstly the penetration of Distributed Energy Resources or DER (read more about DER here) is increasing all over the globe. With the increasing penetration the traditional operation of the grid, with huge (MW scale) generation plants located far away from load centers, long transmission lines and distribution system, is no longer possible. Now we have rooftop solar plants, electric vehicles with batteries, demand response mechanism which can all potentially alter the status quo. Most significant aspect is that when the distribution system was designed, we did not think about rooftop solar plants, or electric vehicles. In summary- modelling is important from the perspective of power system planning and operation.

Next thing to remember is that DER modelling is important from the perspective of both the distribution system and the bulk power system. DER, by virtue of being a resource in the distribution system has a definite impact on distribution system. However, there is also much hue and cry about the potential impact of DER penetration on the bulk power system. In case you are wondering what this bulk power system is, it is the term used to describe the generation and the transmission system together.

One of the key areas that is being researched is the potential impact of DER on the ability of the power system to do its basic function i.e. supply power without interruptions. Some of the potential impacts of DER on the reliability of the power system , as identified by experts is as follows:

• Visibility- Lack of DER data and its implications for the operation, planning, and design of the bulk power system;
• Coordination between resources connected to the bulk power system and DERs;
• The effect of DER daily generation profiles on system unit commitment and ramping needs; and
• The effect of distribution connected variable PV and wind output on day-ahead load forecasts

But there are benefits from DER also. As DER penetration increases, benefits to the bulk power system also increase. For example, by providing power close to the customer, DERs can serve to reduce grid losses and reduce system peak load. (a little bit more detail on DER benefits is here)

Modeling DER

As DER penetration increases, the interaction between the bulk and the distribution system changes. Hence it is imperative to model DER to study both the systems. The most common power system study is perhaps the load flow model . The load flow models with DER can help determine the impacts to bus voltages and changes in power flows across the bulk power system following a contingency, such as loss of a Solar PV generation plant.

The primary difference between a bulk power system and a distribution system study is in the underlying assumptions. Typical load flow studies are performed for bulk power systems and assume a balanced three phase system. On the other hand distribution models don't assume that the system in balance and often portray the individual phases. It makes sense as often loads in distribution are single phase and correspondingly DER installations could be single phase as well.

Now the feeder with DER can be modeled in three ways as shown above. (1) as a detailed model of the feeder which is a composite load-generation model- Example an hourly load and Solar generation model from actual measurements and insolation data.

(2) as the aggregated generation of the DER which is independent of the aggregated load at the feeder connection. Example: the same load as a single load in the substation and aggregated the solar PV units as a single solar PV installation interconnected in the substation.

(3)as net demand/load-such that the generation from the DER is balanced by the load of customers. Example-both load and solar PV installations were netted as one load located in the substation.

As you would expect, the complexity and computation requirement of the models listed above are in the descending order. The net demand load modelling is easier and it the common industry modelling practice. . Nevertheless, the impact of DER on the feeder load is only fully captured in the detailed model and partially captured in the aggregated models. The netting of DERs with load may result in inaccurate estimates of power flowing on the system. The typical net demand/load model can't help study a high PV penetration system shown below. Hence, an aggregated load PV model as shown below is recommended.

A little bit more for the power system enthusiasts

A few more details on the modeling of DER is added for the three major bulk power system planning studies:

1.Steady-state load flow studies

Load flow study determines the real and reactive power flows for network expansion planning or voltage stability studies. Steady-state power flow calculations only require a standard generator model. This could be a simplistic voltage or current source models with voltage control loops which is appropriate for steady-state analysis under normal conditions of voltage and frequency.

2. Steady-state short-circuit studies

Short-circuit calculations determine the short-circuit power levels for equipment rating and voltage sag propagation analysis. Steady-state short-circuit studies require appropriate DER models that would adequately represent the short-circuit contribution from DER. Remember that inverter-based DER are current and power limited sources. A current limited Norton equivalent with control loops that adequately model the response under abnormal conditions of voltage is required. Additionally, the short-circuit contribution of DER depends significantly on the performance specified by interconnection requirements, such as trip and ride-through requirements.

3. Dynamic Studies

Power system dynamic studies relevant to DER is either a disturbance ride-through analysis or a transient stability analysis. The former helps to determine the frequency and voltage stability after a fault considering the amount of DER power that may be tripped during the disturbance. Transient stability analysis on the other hand to determine transient stability during and following faults with consideration of fast reactive support from DER that may improve the transient response of the overall system.

Modeling of DER in dynamic studies requires a solid understanding of DER performance based on both specifications of technology and interconnection requirements. Interconnection requirements or the also performance requirements is specified by the regulators depending on their interconnection voltage level, size of the system, technology type etc. The IEEE Standard 1547 and California Rule 21 are the two most popular standards for interconnection which specifies the requirements such as fault-ride through. (read more about IEEE 1547 and Rule 21 here)

Demystifying the PVD1 and CMPLDWG models

One of the best in class PV models is PVD1. It is a simplified equivalent model for distributed PV systems behind a single equivalent distribution feeder impedance. It is a highly reduced, almost algebraic, model to represent distributed PV systems in stability studies. It includes active power control, reactive power control, and protective functions and can account for partial tripping of distribution connected PV systems without the need to represent the distribution feeders explicitly. It can also consider the evolving mix of DER with and and without ride-through capabilities. The PVD1 model is shown below.

Besides modeling of generation, proper representation of load is important. Hence the CMPLDWG or the Composite Load Model is integrated with distributed PV as shown in image below. These models are important as they have helped in detailed distribution-level analysis such as determining the amount of DER that would trip off-line.

Concluding thoughts

The ability to accurately model the power system is vital given increasing penetration of DER in an already complex power grid. System modeling is critical for power grid operations and planning to ensure reliable operation. All components of the system must be represented in the models, either directly or in an aggregated way, to provide useful and accurate results. One of the ways to overcome the barriers for higher DER penetration is to incorporate accurate modeling DER in system planning and operating studies. A little bit of knowledge can go a long way here.

Related articles

• Part 1: The DER-DRE conundrum
• Part 2: The DER story...continued
• Part 4: Value of Distributed Energy

References

NERC documentation on DER modeling