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Load Shedding is the Power System’s Safe Guard

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Dr. Amal Khashab's picture
Expert Independent Consultant ,Electric Power Systems Engineering Free lancer

Summary Full Academic Qualification by obtaining B.Sc. (1971), M.Sc. (1980) and Ph.D. (1991) of Electric Power Engineering. Active continuous education by participating in long periods of...

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Load Shedding is the Power System’s Safe Guard

Power System Operation and Control

- In an electric power system of N buses, every bus has been defined electrically by four variables: active power P, reactive power Q, the voltage V, and the frequency f . P and Q are independent variables depending on the generation or loading quantities , while V and f are dependent variables.

- Normally, the power system balances at steady state with the following operating conditions:

(1) Equality constraints representing the load flow equations :

Σ P generation = Σ P demand = Σ P load + Σ P losses

Σ Q generation = Σ Q demand = Σ Q load + Σ Q losses

(2) Inequality constraints representing the allowable range of acceptable operation of the buses frequency and voltage:

(f0-Δ f ) ≤ f ≤ (f0+Δ f)

(V0-Δ V) ≤ V ≤ (V0+Δ V)

- The normal operation of power system may witnesses continuous small and / or large variations, that affect the values of frequency and voltages among its buses. Hitting the limits as mentioned in the above inequalities necessitate proper actions to maintain them again. These could be centralized or decentralized actions. Long lasting power system analysis proved that frequency variation is related to active power P variation, while the voltage variation is related to reactive power Q variation.

Power system Frequency control

The frequency control is performed in the following procedures:

1. Governor control of generator, also by load control to keep balance between P generation and P demand (First control).

2. Transmission system operator command generator disconnect by frequency rise above its higher limit (Second control).

3. Automatically load shedding (Third control).

When the available power generation is insufficient to meet the system demand, the system frequency will drop. When that drop hits the lowest predetermined value(f0-Δf), the load shedding is activated according to the above mentioned priorities.

Normally, the load shedding is activated in several stages. In each stage, a predefined amount of the load is disconnected and the shedding of the load is continued till the normal frequency (i.e. the power balance) is restore. The figures below show that using small load shedding blocks can be an effective way to the prevention of over-shedding.

              

As an illustrative example The Egyptian transmission system operator automatically performs the load shedding as shown in the table below. It shows the relationship between the frequency drop and the load shedding. According to the table, 60% of the load is disconnected from the grid when the frequency drops to 48.7Hz.

Load shedding for frequency support Frequency (Hertz)

Frequency (Hertz)

Required Load Dropping

49.2

6% of Original Load

49.1

3% of Original Load

49.0

4% of Original Load

48.9

7% of Original Load

48.8

20% of Original Load

48.7

20% of Original Load

48.65

15% of Original Load

Power system voltage control

The voltage control is performed in the following procedures:

1. Terminal voltage adjustment of synchronous capacitor and synchronous generator

2. Static synchronous compensator.

3. Phase modifying equipment.

4. Tap change of transformer.

Sometimes for appropriate power flow and load requirements, the active power equality has been met while the reactive power equality has not been met. This will lead the voltage at specific locations in the network is lower than the minimum predetermined value (V0-Δ V). Centralized power system voltage control is performed according to the mentioned above procedures.

In case of the resultant will not sufficient to bring the voltage buses to the value (V0-Δ V), decentralized reactive load shedding has to be activated, to maintain the reactive power equality again. Usually distribution feeders equipped with definite time relays, that triggered to open when its voltage is lower than (V0-Δ V) for few seconds. A typical phases chosen aimed at decentralized relay are:

1. Once monitored bus voltages drop to 90% or lesser of standard, trip 5% of the load for a minimum of 3.5 seconds.

2. When bus voltages drop to 92% or lesser, trip 5% additional load for 5.0 seconds.

3. When bus voltages drop to 92% or lesser, trip 5% additional load for 8.0 seconds.

It is worth to mention that certain critical customers cannot be dropped from load despite the help it may present to the system. Critical customers include hospitals or customers that would lose lots of revenue from being dropped.

The figure below shows long-term voltage variation with proposed corrective actions procedures.

Conclusion

The under frequency load shedding and the under voltage load shedding schemes, can be considered as special protection or wide area protection systems that attempt to minimize the impact of continuous variation of loading conditions and prevent blackout in power systems. Both types of load shedding should have shedding algorithms defining the methods by which several loads will be switched-off (and switched-on again) automatically to keep the power consumption below a defined security level.

Reference:

- Power Sector Cooperation Planning , Survey in Arab Republic of Egypt , Final Report , Japan International Cooperation Agency (JICA), Japan International Cooperation Agency (JICA) , Tokyo Electric Power Services Co., Ltd. (TEPSCO) ,October 2018.

 

 

 

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Dr. Amal Khashab's picture
Thank Dr. Amal for the Post!
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