GO3 - Syn Interconnection

Grid Operation

Part 3

System Interconnection

Synchronous

Better Start With PART-1

Performance of grid system was encouraging. Found to be favorable for its stiffness with increased system bias and inertia. Interconnection with neighboring system is beneficial to both the systems. It facilitates power assistance to each other during distress condition. Also Government of India undertaking like NPC, NTPC, NHPC had established pooled power plants in the region.  Interconnection were also necessary to draw share of power from such plants. So interconnections with neighboring systems were established. But with this operation criteria had changed.

Review of water supply system will high light the issue.

Water supply system setup is modified accordingly. There are two tanks with its inflow and outflow lines as in the figure. Both the tanks are connected at bottom by a pipe line.

Initially interconnecting line valve IC is kept closed. In this condition both the systems are independent and behave like standalone system as discussed earlier. Water level in tank A is exclusively regulated by control valve CGA and CLA and it has is no influence on water level in tank B and vice a versa.

Initially inflow through control valves CGA and outflow through control valves CLA are adjusted that water level in tank A is maintained 50%. Similarly control valve CGB and CLB are also adjusted that water level in tank B is also maintained at 50%.

Now opening of valve IC on interconnecting line has no effect on water level in any tank.  Now there is passage for water to flow in either direction but there is no water flow in any direction because of equal level in both the tanks.

Make some change in tank A only. Control valve CLA is opened to increase outflow. As usual, water level in A drops because outflow is more than inflow.

Consequently, head HGA will increase and inflow through CGA increases. Also head HLA will decrease and outflow through CLA decreases.

But water level in B is same as before. So water flows from high level in B to low level in A. Now level in B also drops. Consequently, head HGB will increase and inflow through CGB increases. Head HLB will decrease and outflow through CLB decreases.

Tank B inflow has increased and outflow has decreased. Means there is surplus inflow that will flow to A to makeup the shortage.

This will continue till total inflow matches total outflow. In this case level drop is less than independent working.

In other case increase of inflow in tank A causes rise of level. So inflow will decrease with decrease of HGA. Outflow increase with increase of HLA. Water will flow from tank A with higher level to tank B at lower level. So level in B also increase. Inflow in B will decrease and outflow increase. Ultimately stabilize at little higher level.

Similarly level stabilize with auto control when increase/decrease of inflow/outflow in any tank.

Three observations

  1. Disturbance in one tank reflected in other tank.
  2. Equal level is maintained in both tanks with simultaneous rise/drop.
  3. Water flow from one tank to other.  

1.       Grid system also operates in similar manner.

Consider two power systems A and B having interconnection as shown in the figure

Both the systems are running independently with ideal operating condition having balanced load and generation at normal frequency of 50 Hz. There would not be any power flow when these systems are connected through tie line.

Let load X MW increased in system A after interconnected. System frequency will drop due to shortage of total generation against total demand. Drop in frequency is common in both the systems as they are interconnected. Generation in system A as well as in system B will increase by governor action due to drop in frequency. Similarly power demand by loads in system A as well in system B will reduce with drop in frequency due to frequency dependant loads. Consequently system B will have surplus generation.  So power flow from system B to system A through tie line. Shortage of generation occurred in system A due to rise of load is partially met by gain in own system and balance by power import from system B. Whole grid will stabilize at little lower frequency. Of course drop of frequency will be smaller compared to standalone system because of auto corrective action from both the system.

System frequency will rise during loss of load in system A. Generation in system A as well as in B will reduced by governor action due to rise in frequency. Similarly power demand by loads in system A as well in system B will increase with rise in frequency due to frequency dependant loads. Power will flow from system A to B and grid will stabilize at little higher frequency.

Similarly frequency is controlled during add/drop of demand/generation in any system.

Three effects during mismatch in any system.

  1. Effect of mismatch in one system is experience by other system also.
  2. Rise/drop of frequency is throughout the grid.
  3. Extra power flow between the systems. 

1.     Consider systems A and B were operating with following data

System A has 15000MW Demand, 15000MW Generation and 600MW Bias

System B has 10000MW Demand, 10000MW Generation and 400MW Bias

System is operating at 49.90 Hz

Only system A has 300 MW loss of generation.

Combined bias = 600 + 400 = 1000 MW

Drop of frequency 300/1000 = 00.30 Hz

Gain in system A = 00.30 × 600 = 180 MW

Gain in system B = 00.30 × 400 = 120 MW

This is surplus in system B and will flow to system A

System A had shortage of 300 MW

Gain in system A itself is 180 MW

Net shortage = 300 – 180 = 120 MW

This is received from system B

So both systems are stable at 49.90 – 00.30 = 49.60 Hz

Only system frequency has to be monitored and controlled in standalone system. Governor was actively regulating generation to maintain frequency within limit. But two parameters have to be monitored and controlled in case of interconnected grid system.

Variation of demand or generation in one or both the system has following effect.

  1. Frequency variation. >  Depends on overall surplus/deficit of generation.
  2. Tie line flow variation. >  Depends on defaulting system.

1.      Control centers for this purpose were established. These are known as Load Dispatch Centers. Along with above two primary functions, center has other responsibility of economic power dispatch, system security and others. So center was authorized to instruct the power stations and load centers to operate as required for power system operation with above objectives. The center has direct communication with all power stations and load centers and with such center in other power systems.

Following thumb rule was useful for corrective action.

Sr.

Frequency

Tie-line Flow

Default system

1

High

Import

Other System

2

High

Export

Own System

3

Low

Import

Own System

4

Low

Export

Other System

 Of course this is not perfect criterion but generally works well.

In case of 1 and 4, other system LDC is contacted for corrective actions.

In case of 2

1.       Release load if any restriction on any type of load.

2.      Reduce generation. >  costly first.

In case of 3

1.       Pick up generation. > costly last.

2.      Restrict load as per decided priority.

This was working well during normal load variations and small disturbances. But it is emergency during sudden loss of generation due to tripping of generator or loss of load due to tripping of trunk line to big area. System condition may be severe due to critical frequency and/or critical tie line flow and/or critical loading on network (line/s or transformer/s) in any system. So concern system operators have to act quickly to reinstate normal stable condition at the earliest. Manual control has its own limitations. Automatic control system is the solution.

Load Frequency Controller (LFC)

LFC is the device that calculates Area Control Error.

Area Control Error ACE = ΔPi + BΔF

Where

Pi = Power Import deviation = Pa - Ps

B  = System Bias = Power/Hz

Fd = Frequency deviation = Fs – Fa

Pa = Actual Power Import

Ps = Schedule Power Import

Fs = Schedule Frequency

Fa = Actual Frequency

Figure of ACE  indicates severity of default condition.

Positive ACE indicate generation short of demand.

Negatives ACE indicate over generation than demand.

All tie line flow and frequency data is fed directly on line to the controller. Also system bias, schedule frequency and schedule tie line power are set manually. Application of LFC may be as operators guide. Or it can have online remote control to regulate generation of assigned power plants.

Effectiveness of such control depends on installation of such device by all the systems in the grid. Some state had installed such control but was not successful due to followings.

·         This was not installed by any other system in the group.

·         There was frequent up/down of generation. Such variations were detrimental to thermal generator sets. Thermal generating sets have limited range for on line regulation.

Interconnected systems can be operated in one of the three modes with mutual consents.

Flat Frequency Control.

Control of frequency is primary task.

Suitable for alike systems where in both systems act to corrects frequency.

Flat Tie Line Flow Control.

Suitable for small system connected to large system. Large system controls frequency whereas small system adjusts tie line flow.


Tie Line Bias Frequency Control.

Tie line flow is regulated in accordance with frequency. Area control error will be zero provided system bias setting is correct.

Regional Grid

Grid system has advantage of stiffness. Interconnected systems are stable against any eventuality in any system in the group. Larger the group better is the performance. So many systems were interconnected to form large group up to regional level. Regional grid has multiple interconnections with different systems. There may be more than one tie line between any pair of system. It is also possible that particular pair of systems may not have any direct tie line but they are interconnected through other systems. There is   pooled generations in different systems.

Tie line flow in two systems was bilateral. Export of one system is import for other system. But it is different in grid system. Each system has to derive tie line flow on net transfer base as under.

Schedule for power draw is algebraic sum of its share from various ISGS in the grid plus any bilateral agreement for import or export from/to other source.

Actual power drawn is algebraic sum of power import/export on all tie lines plus injection by ISGS within the system.

Meanwhile regional load dispatch center were functional helping in interstate coordination. System control continued to be manual and there was wide variation in frequency and tie line flows. Generation on all units drops including cheap sources by governor action whenever frequency increases. Import from grid increase and had to pay for it at higher rate. Frequency rise were from very low to better level due to load shedding by one or other constituent. Similarly frequency drop with resumption of such loads and costly generation was picked up due to governor. Thermal stations were subjected to continuous load up down with frequency and each time readjustment manually. Such operation had adverse effect on power plants. This is when no ABT. This needed to get rid of the nuisance.

Solution was as in our water supply system as under. 

The inflow pipes which were up to bottom of the tank under the water were cut short as shown. Now pipes were out of the water and free of back pressure related to water level in the tank. Hence there is no effect on inflow due to change in level in the tank.

Generator governor control set for full load with no droop so that generation has no effect of change in frequency. Ultimately frequency control is as shown in the chart hereunder.

 


  Part-4