04-02-2012, 03:37 PM
RISK ANALYSIS OF ISLANDING OF PHOTOVOLTAIC POWER SYSTEMS PROTECTION
photovoltaic_power_systems_protection.doc (Size: 112 KB / Downloads: 32)
INTRODUCTION
Under stable steady state operation, all synchronous machines in the grid operate at synchronous speed. Frequency is the measure of load/generation balance. Prevailing frequency of synchronous generators and the Grid depends on matching between (Total MW Load on the Grid plus Losses) and the (Total MW Generation) at that time. Excess Load causes frequency drop. Excess Generation causes frequency rise. Hence frequency is a major control parameter.
Primary frequency control is by governor – control of turbine speed to maintain constant rated frequency of generator unit.
WHAT IS NETWORK ISLANDED
When system frequency starts falling due to overloads or fault, there is a possibility of cascade tripping of turbine-generator units in the entire regional grid and the total National Grid. To avoid this and maintain save the Grid, the Network is Islanded into separate Islands. Frequency relays between adjacent islands measure and monitor df/dt and f such that during faster rate of fall of frequency, the Network is divided into separate islands, each having certain generation and load. Load shedding is carried out in each island. Thereby each island is saved from loss of synchronism and after the disappearance of the distaurbance, the islands are reconnected and the original Network is restored.
LOAD SHEDDING:
When generators get overloaded beyond the maximum mechanical power input, it becomes necessary to interrupt some load to save the system from loss of stability. This process is called load shedding.
In majority of power systems, load shedding is automatically performed because the time available is insufficient for manual operation. For automatic load shedding, the overloads should be sensed by relaying in suitable form. During overloads beyond maximum mechanical input, the frequency of generators or part of system decays proportional to the generator inertia and amount of overload.
PROBABILITY OF ISLANDING
Islanding of inverter-connected PV-generator systems means “any situation where the source of power from the network operator’s distribution system is disconnected from the network section in which the generator is connected, and one or more inverters maintain a supply to that section of the distribution system”.
Load/Generation Match
The probability of load/generation match conditions being met at any time is a function of the load/generation match criteria. The match criterion depends on the degree of closeness required to be considered as “matched”. It also depends on the parameters that need to be considered to be matched, such as load/generation current, real power and reactive power.