# Proving a Concept of Flexible Under-Frequency Load Shedding with Hardware-in-the-Loop Testing

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## Abstract

**:**

## 1. Introduction

## 2. Methods

#### 2.1. Innovative Use of Rate-of-Change-of-Frequency

_{LIM}). For this purpose, an assumption is made that if none of the frequency control/protection functionalities intervene, the f(t) variation will follow the same trend as indicated by the respective RoCoF(t) measurement at the same instant. The local nature of the EPS frequency dynamics that follows an active power imbalance ΔP incident means that in a selected point in time, the calculated M(t) is different in every busbar in the system (due to the oscillating nature of the EPS [20]). In Figure 1a, the blue and yellow curves depict the EPS frequency response to two unequal ΔP incidents (ΔP

_{1}and ΔP

_{2}respectively) assumed to have taken place in the same busbar. At moment t = t

_{1}, respective RoCoF(t

_{1}) indicates different M(t

_{1}) values, depending on the underlying ΔP incident. It is clear from Figure 1a that ΔP

_{1}caused a less serious frequency excursion compared to ΔP

_{2}. This manifests in a larger M(t

_{1}) value, an indicator of having more time before f

_{LIM}is reached.

_{LIM}) in the lower left part of the diagram (blue curve) or (ii) far away from the ordinate axis in the right-hand side of the diagram (yellow curve). This conclusion makes one able to define a new, double-criteria UFLS tripping function for each UFLS stage. The first criterion remains identical to conventional UFLS, being the violation of a predefined frequency threshold f

_{thr}. The second criterion, on the other hand, is a violation of a predefined stability threshold M

_{thr}and is monitored independently of the first. Only when both tripping criteria are simultaneously fulfilled, the trip signal is generated. A simplified logical diagram of the process is presented in Figure 2 since the detailed version is a subject to an international patent application submitted by the University of Ljubljana.

_{thr}criterion not being violated simultaneously in different locations. This can be seen as if one is dealing with a much larger number of UFLS stages, which can eventually manifest as a fine-tuning of UFLS (further explained in the continuation of this paper).

#### 2.2. Intelligent Electronic Device

_{S}), high accuracy and short response times, input/output (I/O) ports and real-time processing. By modifying the external quantities according to the needs of the innovative UFLS method, we obtained a fully functional IED device that is ready for deployment.

#### 2.3. Rate-of-Change-of-Frequency Filtering

_{S}. To decrease the noise, we could lower F

_{S}, but this would lead to a slower response of the UFLS system depending on RoCoF. This is why it is of utmost importance to find the optimal response of the UFLS system while still getting meaningful values for RoCoF. In Figure 4, the 200-ms-long windowing function was added to the RoCoF calculation (to both the PMU-IED and the PMU-COMM) that averages all samples (moving average) in the window. Averaging reduces the system response capabilities since, with a longer window, it passes only the DC component in its limit in the frequency domain. As seen in Figure 4, the moving average filter greatly improves the RoCoF reported by both devices. On the other hand, this is achieved on account of having a higher output delay (around half of the window length, i.e., ≈100 ms, which can be noticed as a shift of the PMU-IED and PMU-COMM curves with respect to the PMU-RTDS), which is also reflected in the f(t)-M(t) diagram (Figure 4b).

_{S}(50 Hz) and the noise reduction is related to the square root of points used in the window [23].

## 3. Experimental Setup

#### 3.1. HIL Setup—Overview

#### 3.1.1. Real-Time Digital Simulator

#### 3.1.2. Omicron CMS-156 Amplifier

#### 3.2. HIL Setup—Intelligent Electronic Device

_{LIM}and all threshold values (f

_{thr}and M

_{thr}) can be provided separately, together with window widths for filtering described in Figure 7.

#### 3.3. HIL Setup—Electric Power System

_{x}-UFLS where subscript “x” represents the number of the corresponding busbar. The seventh partition, however, represents the remaining aggregation of all the consumers not included in the UFLS scheme (L

_{x}-S). Second, an infinite power source was additionally introduced to the model, busbar 4, to be more specific. Its disconnection was considered the main event causing active power deficit conditions in the newly formed island. With this, many different power conditions can be simulated by changing the steady-state production of all three generating units prior to the main event. An infinite power source in all cases supplies/consumes enough power to meet the power balance in a steady-state. Once disconnected, an EPS island is formed with a certain imbalance between the production and the consumption of active and reactive powers. In this paper, circumstances with a lack of active power were simulated, which cause an EPS frequency to decay and consequently trigger UFLS.

_{5}on busbar 5 and was able to disconnect loads L

_{5}-UFLS according to Table 1. The other two load busbars (6 and 8) were monitored and controlled with a PMU-RTDS component within a simulated environment, fed by voltages U

_{6}and U

_{8}, respectively.

## 4. Results

_{thr}are taken from the Slovenian Grid code [27], and additional frequency stability margin thresholds M

_{thr}are selected with a one-second span between them since the analysis showed there are no special differences when selected otherwise.

_{LIM}.

_{thr}and M

_{thr}values are set to same values for all three devices participating in a UFLS scheme, the variety of calculated M(t) values is much higher across the network than the variety of calculated f(t) values. With three load busses, we therefore introduced three ‘’substages’’ for each of the UFLS stages, which can be seen as fine-tuning power imbalance with UFLS. Furthermore, in systems with more load busses, even finer tuning could be achieved due to those inherently introduced substages.

_{LIM}and, consequently, the M criterion of the fifth stage would have been triggered.

_{thr}) and it is vital to be aware that this is also the case with innovative UFLS. The initial concern with the innovative UFLS scheme was related to extreme scenarios, in which the curve on the f(t)-M(t) plane is expected to make a rather abrupt jump from the upper right corner (steady-state conditions) to the upper left corner of the diagram once power imbalance appears. The delayed recognition of such critical conditions due to two additional moving average filters (Figure 7) could, therefore, translate into f

_{thr}of a certain stage being violated before the corresponding M

_{thr}. This would in turn be observed as shedding at lower frequency compared to a conventional UFLS scheme. However, our testing proved that even when dealing with an extreme initial RoCoF $(\cong $−10 Hz/s), the M

_{thr}criterion is still violated sufficiently prior to the f

_{thr}criterion. In Table 2, the actual frequency at which the M

_{thr}was met is given for each UFLS stage. Evidently, M criteria of all UFLS stages were met before the frequency even reached the f

_{thr}value of the first UFLS stage (0.483 Hz margin in a worst-case scenario—see the last column in Table 2). This proves that the innovative UFLS scheme would respond to such an extreme event identically as the conventional scheme, with no additional time delay.

## 5. Conclusions

## 6. Patents

## Author Contributions

## Funding

## Conflicts of Interest

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**Figure 1.**EPS frequency f(t) response to different ΔP incidents (

**a**) and the corresponding frequency versus frequency stability margin diagram (

**b**).

**Figure 3.**The RoCoF(t) report of three applied sources (PMU-IED, PMU-COMM and PMU-RTDS) after an RTDS simulation of an active power deficit (

**a**) and a corresponding frequency versus frequency stability margin diagram (

**b**).

**Figure 4.**The improvement in reported RoCoF(t) after applying a moving average filter to the PMU-IED and the PMU-COMM (

**a**), and a corresponding frequency versus frequency stability margin diagram (

**b**).

**Figure 5.**The RoCoF(t) report of three applied sources (PMU-IED, PMU-COMM and PMU-RTDS) after the RTDS simulation of an active power deficit and three UFLS interventions (

**a**) and a corresponding frequency versus frequency stability margin diagram (

**b**).

**Figure 6.**The improvement in reported RoCoF dynamics after applying a median filter to the PMU-IED and the PMU-COMM (

**a**), a corresponding frequency versus frequency stability margin diagram (

**b**).

**Figure 9.**A single-line diagram of a IEEE 9-bus test system [3], used for the RTDS HIL testing.

**Figure 10.**Overview of HIL results extracted from the entire set of 66 simulated cases; the total amount of disconnected consumption in percentages (

**a**) and recorded RoCoF after the last UFLS intervention (

**b**).

**Figure 11.**A time-domain response of the EPS frequency f(t) (

**a**) and RoCoF(t) (

**b**), corresponding to simulation case 49.

**Table 1.**Conventional UFLS setting (frequency thresholds f

_{thr}) along with supplemented frequency stability margin (M

_{thr}) thresholds.

UFLS Stage Number | ${\mathit{f}}_{\mathbf{t}\mathbf{h}\mathbf{r}}\text{}\left[\mathbf{Hz}\right]$ | ${\mathit{M}}_{\mathbf{t}\mathbf{h}\mathbf{r}}\text{}\left[\mathbf{s}\right]$ | EPS Load Decrease [%] |
---|---|---|---|

1. | 49.0 | 6.0 | 10 |

2. | 48.8 | 5.0 | 10 |

3. | 48.6 | 4.0 | 10 |

4. | 48.4 | 3.0 | 10 |

5. | 48.2 | 2.0 | 10 |

6. | 48.1 | 1.0 | 5 |

UFLS Stage Number | ${\mathit{f}}_{\mathbf{t}\mathbf{h}\mathbf{r}}\text{}\left[\mathbf{Hz}\right]$ | Frequency at Which M_{thr} Criterion Is Met [Hz] | Margin [Hz] |
---|---|---|---|

1. | 49.0 | 49.4830 | 0.4830 |

2. | 48.8 | 49.4830 | 0.6830 |

3. | 48.6 | 49.4623 | 0.8623 |

4. | 48.4 | 49.4623 | 1.0623 |

5. | 48.2 | 49.4364 | 1.2364 |

6. | 48.1 | 49.3415 | 1.2415 |

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**MDPI and ACS Style**

Sodin, D.; Ilievska, R.; Čampa, A.; Smolnikar, M.; Rudez, U. Proving a Concept of Flexible Under-Frequency Load Shedding with Hardware-in-the-Loop Testing. *Energies* **2020**, *13*, 3607.
https://doi.org/10.3390/en13143607

**AMA Style**

Sodin D, Ilievska R, Čampa A, Smolnikar M, Rudez U. Proving a Concept of Flexible Under-Frequency Load Shedding with Hardware-in-the-Loop Testing. *Energies*. 2020; 13(14):3607.
https://doi.org/10.3390/en13143607

**Chicago/Turabian Style**

Sodin, Denis, Rajne Ilievska, Andrej Čampa, Miha Smolnikar, and Urban Rudez. 2020. "Proving a Concept of Flexible Under-Frequency Load Shedding with Hardware-in-the-Loop Testing" *Energies* 13, no. 14: 3607.
https://doi.org/10.3390/en13143607