Please enable JavaScript.
Coggle requires JavaScript to display documents.
Proper Setting of Under-frequency Load Shedding Relays in Industrial Plants
Proper Setting of Under-frequency Load Shedding Relays in Industrial Plants
ABSTRACT
A new algorithm is proposed based on transient stability studies
Utilizes a comprehensive transient stability module to find out the unwanted events
The most reliable tools in active power generation deficiency conditions :check:
The relays are sets in
two ways
:
The power system are preserves the electricity in service for critical loads.
Frequency is maintained in permissible limits.
INTRODUCTION
To enhance system performance accordingly when the dynamic response of the system is investigated
To suggest an effective load shedding scheme against severe frequency decay conditions.
A fast & proper load shedding design can halt frequency drop and prevent system instability by means of load reduction & decrease in demand
The transient analysis is performed to obtain realistic and applicable result.
Algorithm include
two main parts
that are performed automatically in T/S module of PASHA software
Events which decrease system frequency significantly are detected
Underfrequency load shedding relay steps are set
CASE STUDY
B. Load Shedding scheme
TABLE II LOAD SHEDDING STEP SETTINGS
steps f step (Hz) df/dt step(Hz/s) Shed Loads
1st step 49.3 -0.491 XB, NC
2nd step 49 -0.262 OC
3rd step 48.7 -0.526 YB
TABLE I CONTINGENCIES WHICH CAUSE SEVERE FREQUENCY DECAY
Event fmin fstl
Gen-1 outage 43.3614 43.5286
Line B-C outage 39.8693 40.0895
Gen-2 outage 38.9544 39.1898
Gen-3 outage 38.9544 39.1898
Line A-C outage 34.0846 34.7555
C. Simulation Results
For contingencies 1 and 2, 1st step of load shedding
operates and solves the generation-load unbalance. Figs. 2 and
3 show frequency change after these scenarios respectively.
Simulation of scenarios which results in frequency decay
verifies proper load shedding performance in case of frequency
decline
Fig. 4 shows the operation of 1st and 2nd load shedding
scheme in case of Gen-2 or Gen-3 outage scenarios.
If 2nd load shedding step doesn't operate in these scenarios, frequency decreases below the minimum allowed frequency (Fig. 5).
Operation of three steps of load shedding is required to save the power plant in worst event (Fig. 6) otherwise if 3rd step doesn't operate, the system becomes unstable
A. Description
Bandar Abbas refinery plant composed of:
2 gas turbines unites, each one with a generating capacity of 30MW that in normal condition delivers 28MW
1 steam turbine generation unit, with a generating capacity of 24MW
Protective under frequency relays of generation units are set in 48 Hz with 1000 (ms) time delay and their over frequency protective relays trip generation units in 52 Hz.
Should be ensured that load shedding operation doesn't increase frequency over 52 Hz.
Total demand of industrial facility is up to 93.6 MVA.
PROPOSED ALGORITHM FOR DESIGNING LOAD SHEDDING SCHEME
A. Finding Severe Contingencies
Events which cause frequency deviation are automatically determined by the algorithm
The obtained contingencies in the previous stage are sorted according to their minimum frequencies
The algorithm reads required data
B. Setting of Load Shedding Steps
The amount of load that is removed by the 1st step is specified in this stage.
Finding the optimal combination from 'search space' is carried out in this stage.
Rate of change of frequency setting for 1st step is calculated in this stage.
After setting of the 1st step 'contigencies list' will be updated.
REMARKABLE CONSIDERATION FOR LOAD SHEDDING DESIGN
The under frequency load shedding relays are set so that to prevent tripping of the generation units by their corresponding under frequency protective relays.
The rate of change of frequency (ROCOF) will be used in addition to frequency values and it used to shed the loads as quick enough for those events that are severe frequency declines.
The proper mathematical models of the power system components and their parameters have to be recognized to determine severe contingencies by the transient stability simulations.
Loads are classified according to their importance and signifance which are vital, semi-vital and non-vital.
CONCLUSION
Load shedding scheme is programmed so that frequency decay is arrested before tripping of operating generation units and electricity provision is limited to critical loads.
Proposed algorithm offers an
appropriate underfrequency load shedding relay setting based on transient stability studies.
Severe contingencies which lead to undesired frequency drop are detected.
The effectiveness of proposed
algorithm is confirmed by transient stability simulations.