Safety System for Switch Board and Generators

Apart From Direct Temperature Measurement Of The Stator Windings And The Internal Air, The Protection Of A Generator Is Largely Based On The Sensing Of Current And Voltage From Current Transformers (CT) And Voltage Transformers (VT). The Number And Type Of Protective Relay Functions Increase With The Generator KVA Rating And Voltage Level. Protective Relays Are Electromagnetic (traditional) Or Electronic (increasingly More Common) Which Are Mounted On The Generator Front Panel Of The Main Switchboard. Some Protective Functions May Be Grouped Together Within A Single Relay Case. Settings For Level And Time-delay Must Be Periodically Checked By Injecting Currents And/or Voltages Directly Into The Relay (usually Via A Special Multi-pole Socket Adjacent To The Relay And Internally Wired To It). 

Some Typical Relay Types Employed For Generator Protection Are Outlined In The Figure On The Left.
i) OCIT (Over Current Inverse Time Relay)
The Over Current Inverse Time Relay Monitors General Balanced Overloading And Has Current/time Setting Determined By The Overall Protective Discrimination Scheme.

ii) OC (INST) (instantaneous Trip)
Instantaneous Trip To Protect Against Extremely High Overcurrent Caused By A Short-circuit Fault.

iii) NPS (Negative Phase Sequence)
A Negative Phase Sequence Relay Determines The Amount Of Unbalance In The Stator Currents That Is An Indirect Measure Of The Generator Stator And Rotor Temperature. A Relatively Small Degree Of Unbalance Causes A Significant Temperature Rise So The NPS Current Setting Is Low At Around 0.2.ln.

iv) DIFF (Differential Current)
This Is A Differential Measurement Of Current At Each End Of A Stator Phase Winding. This Comparison Of Current Is To Detect An Internal Fault In The Stator Windings That May Be Caused By Partially Short-circuited Coil Turns And/or Earth Faults.
Current Settings For This Very Serious Fault Are Very Low E.g. About 0.1.In. 

v) DEL (Earth Leakage)
An Earth Leakage Relay (sometimes Called Zero Phase Sequence) Detects An Earth Fault Current Returning Back Through The Earthed Neutral Connection. In A Ship's HV Generator System The Earth Fault Current Is Limited By A High Impedance NER (neutral Earthing Resistor) Or Earthing Transformer So The Pick-up Current Setting Is Very Low, E.g. 1-5A With A Time Delay Of 0.1- 0.5 S.

vi) UV /OV (Under Voltage And Over Voltage)
Under Voltage And Over Voltage Functions Are Monitored By These Relays With Settings Of Around 0.8. Un And 1.2. Un Respectively (Un = Rated Voltage) With Time Delays Of About 2s. An Overvoltage Function May Not Be Required In Many Protection Schemes.

vii) UP/OF (Under & Over Frequency)
Under And Over Frequency Settings Are Typically 58 Hz And 62 Hz For A 60 Hz System.

viii) LO (Lock Out Or Trip)
This Is The Master Lock Out Or Trip/hand-reset Relay Responsible For Tripping The Generator Circuit Breaker. Its Action Is Instantaneous When Triggered By A Protective Relay. It Can Also Be Used To Trip The Generator Prime-mover And Initiate Generator Field Suppression Together With The Signalling Of An Alarm.

ix)    RP (Reverse Power)
Generators Intended To Operate In Parallel Must Have Reverse Power Protection (RP).
A Reverse Power Relay Monitors The Direction Of Power Flowing Between The Generator And The Load. If A Prime-mover Failure Occurred The Generator Would Act As A Motor. The Reverse Power Relay Detects This Fault And Acts To Trip The Generator Circuit Breaker.
          The Pick-up Power Level Setting And Time-delay Setting Are Adjustable And Are Pre-set To Suit The Prime Mover. If The Prime Mover Is A Turbine, Very Little Power Is Absorbed When Motoring And A Reverse-power Pick-up Setting Of 2-3% Is Usual. If The Prime Mover Is A Diesel Then A Setting Range Of 5-15% Is Usually Adopted. A Time Delay Range Of About 0.5-3 S Is Usual.
          The RP Relay Operation Is Easily Checked During A Generator Changeover. The Outgoing Generator Is Gradually Throttled Down So That It Motors Causing The Reverse Power Relay To Trip Its Generator Circuit Breaker.

x)Preferential Trip
To Maintain Generator Operation During An Overload, A Preferential Load Shedding Arrangement Is Employed. This Is Achieved By A Special Overload Relay, Called A 'preference Trip Relay'.
         If A Generator Overload Develops, The Preference Trip Sets An Alarm And Acts To Trip Selected Non-essential Loads. This Reduces The Generator Load So That It May Continue To Supply Essential Circuits.
          Each Generator Will Have Its Own Circuit Breaker That Is Typically High Set At 150% With A 20 Seconds Delay.
In Addition, Each Generator Has Its Own Preference Overload Trip, This Being Low Set Generally At 110% Current, Instantaneous Operation. 

If A Generator Overload Condition Develops, Its Preference Overload Trip Will Operate To Energise The Timing Relay. The Timing Relay Then Operates To Disconnect Non-essential Services In A Definite Order At Set Time Intervals, E.g.
1st Trip - air Conditioning And Ventilation - 5 Seconds
2nd Trip - refrigerated Cargo Plant - 10 Seconds
3rd Trip - Deck Equipment - 15 Seconds

This Order Of Tripping Obviously Varies With The Ship Type. When Sufficient Non-essential Load Has Been Disconnected, The Preference Overload Trip Resets And No Further Load Is Disconnected. 
          Once A Machinery Has Tripped, Its Switch Will Have To Be Reset.The Generator Preference Trip System Can Also Be Initiated By Low Generator Frequency Or By Low Speed At The Generator Prime Mover. 

In Many Cases The Preference Trip Protection Is Incorporated In A Combined Electronic Relay That Also Monitors Generator Overcurrent And Reverse Power. 

To Maintain The Preference Relay Trip Settings As Originally Specified They Must Be Periodically Tested By Calibrated Current Injection. Preferential Load Shedding, Generator Scheduling And Load Sharing Is Usually Part Of An Overall Power Management System (PMS) Under Computer Control.